PROTOTYPE DEVELOPMENT AND EXPERIMENTAL STUDY OF HIGH SPEED SINTERING PROCESS PARAMETERS. By ALEXANDER THOMAS HURST

Transcription

1 PROTOTYPE DEVELOPMENT AND EXPERIMENTAL STUDY OF HIGH SPEED SINTERING PROCESS PARAMETERS By ALEXANDER THOMAS HURST A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN MECHANICAL ENGINEERING WASHINGTON STATE UNIVERSITY School of Engineering and Computer Science, Vancouver MAY 2017 Copyright by ALEXANDER THOMAS HURST, 2017 All Rights Reserved

2 Copyright by ALEXANDER THOMAS HURST, 2017 All Rights Reserved

3 To the Faculty of Washington State University: The members of the Committee appointed to examine the thesis of ALEXANDER THOMAS HURST find it satisfactory and recommend that it be accepted. Dae-Wook (Dave) Kim, Ph.D., Chair Hua Tan, Ph.D., Co-Chair Yoon-Jo Kim, Ph.D. ii

4 ACKNOWLEDGMENT This research project would not have been possible without the help of my co-advisors Dr. Dave Kim and Dr. Hua Tan. Thank you for your guidance. I owe tremendous gratitude to Huicong Jiang and Brett Merritt for their work on this project as well. I would like to thank Chad Swanson and the WSU-Vancouver engineering staff who supported me throughout my time at WSU-Vancouver. To my family and friends, I am forever grateful to have you in my life. You helped me through this more than I can put into words. iii

5 PROTOTYPE DEVELOPMENT AND EXPERIMENTAL STUDY OF HIGH SPEED SINTERING PROCESS PARAMETERS Abstract by Alexander Thomas Hurst, M.S. Washington State University May 2017 Co-chair: Dae-Wook (Dave) Kim, Hua Tan High speed sintering (HSS) has emerged in recent years as a new powder-based additive manufacturing process similar to selective laser sintering (SLS). Instead of using a high-power laser to selectively sinter desired cross-sections, HSS uses carbon black ink to selectively absorb infrared radiation energy from a low-cost infrared source. To study the HSS process, a prototype system was built using low-cost, open source designs. A combination of manual and automated control was implemented for this prototype system. The primary goal of this research is to study HSS parameters for the prototype system and their effects on printed PA2200 nylon specimens. The volume of ink deposited on a powder surface area is investigated using a high-speed imaging system. By measuring the ink drop volume and drop ejection rate, the ink volume is calculated. The surface roughness of sintered PA2200 specimens is investigated using optical microscopy. Finally, the HSS prototype system is evaluated and used as a guiding influence for future system design recommendations. iv

6 TABLE OF CONTENTS ACKNOWLEDGMENT... iii ABSTRACT... iv TABLE OF CONTENTS... v LIST OF TABLES... xi LIST OF FIGURES... xiii CHAPTER 1: INTRODUCTION Polymers Manufacturing of Polymer Stuctures Traditional Manufacturing Methods Injection Molding Additive Manufacturing of Polymers Analysis of Plastics Melt-Flow Rate Differential Scanning Calorimetry v

7 1.4 Summary of Modern-Day Plastic Manufacturing CHAPTER 2: LITERATURE REVIEW Additive Manufacturing as a Viable Manufacturing Method Benefits and Limitations of Additive Manufacturing Cost Analysis of Polymer AM Methods and Injection Molding Manufacturing Thermoplastic Structures via High-Speed Sintering The High-Speed Sintering Process Materials for HSS Degree of Particle Melt Greyscale and Print Density Sintering Parameters: Sintering Speed, Powder Temperature, and Flow Agent CHAPTER 3: OBJECTIVES CHAPTER 4: PROTOTYPE DEVELOPMENT HSS Prototype System Design and Realization Prototype Overview XY-Motion Frame Inkjet Microcontroller Heated Powder Bed Infrared Lamp vi

8 4.1.6 Thermoplastic Powder Carbon Black Ink Powder leveling Auxiliary Infrared Lamp Furnace Ventilation Prototype Motion Control via Manually Activated Automated Routines X-Y Inkjet Printing Control Circuit Front End-stop Left End-stop Sintering Stroke Powder Bed Motion Control Circuit (Z-Axis) CHAPTER 5: EXPERIMENTAL METHODS Experimental Methods on Ink Drop Volume and Firing Rate Measurement High Speed Imaging Experimental Set-Up Inkshield and Pulse Delay Settings Droplet Volume Measurements Drop Firing Rate Measurement Using High Speed Imaging Frame Rate Ink Density Calculation vii

9 5.2 Experimental Methods on Powder Bed Temperature Investigation Temperature Measurement Experimental Data Acquisition (DAQ) Set-Up PA2200 Temperature Gradient Measurement Procedure Heated Powder Bed Temperature Regulation Set-Up and Procedure Experimental Methods on Single Layer Sintering Study Ink Deposition Geometry Single Layer Specimen Production Procedure Single Layer Specimen Image and Physical Property Collection Methods Experimental Methods on Two-Layer Rectangular Specimen Production Two-Layer Specimen Production Parameters Two-Layer Specimen Production Procedure Specimen Quality Assessment Methods CHAPTER 6: RESULTS & DISCUSSION Ink Drop Measurements and Ink Density Calculation Ink Drop Volume Measured via MATLAB Edge Detection Program Ink Drop Ejection Rate from a Single Nozzle Using Every-Other-Nozzle Ejection Pattern Ink Density Calculation Summary of Drop Volume and Ink Density Study viii

10 6.2 Heated Powder Bed Temperature Study Characterization of PA2200 Temperature Gradient Temperature Regulation with Surface Heating Lamp and Sintering Strokes Summary of Powder Bed Temperature Studies Single Layer HSS Study As-Printed Specimen Geometry Sintered Specimens Sintered Single Layer Specimen Mass Sintered Specimen Thickness Summary of Single Layer Study Two-Layer HSS Study Two-layer Specimen Mass Two-layer Specimen Thickness Two-layer Specimen Surface Roughness via Optical Measuring Microscope Summary of Two-Layer Specimen Study HSS Prototype Evaluation and Future Design Recommendations HSS Prototype Evaluation Future Design Recommendations ix

11 CHAPTER 7: CONCLUSIONS BIBLIOGRAPHY APPENDIX x

12 LIST OF TABLES Table 2.1: Tool cost and cost per part for injection molding (copied from [20]) Table 2.2: Costs for parts made via SL (copied from [20]) Table 2.3: Costs for parts made via FDM (copied from [20]) Table 2.4: Costs for parts made via SLS (copied from [20]) Table 2.5: Mechanical properties of tensile samples made via HSS with varying carbon black wt % (*Data obtained from [23] Table 2.6: HSS build parameters for tensile test specimens (copied from [25]) Table 2.7: Greyscale levels used in Nylon 12 print density study [30] Table 2.8: Tensile specimen HSS build parameters [30] Table 2.9: greyscale values and corresponding derived degree of particle melt [30] Table 2.10: HSS machine parameters for tensile specimen fabrication [31] Table 2.11: TPE 210-S material properties [32] Table 2.12: HSS build parameters for Norazman et al (copied from [31]) Table 2.13: Build parameters for investigation of lamp speed at two bed temperatures and constant powder composition [31] Table 2.14: Build parameters for investigation of bed temperature at two lamp speeds and constant powder composition [31] Table 2.15: Build parameters for investigation of flow agent at two lamp speeds and two bed temperatures [32] Table 2.16: Comparison of HSS, LS, and injection molding part mechanical properties [32] Table 4.1: Hexadecimal symbols and their corresponding binary numbers/sequences [42] xi

13 Table 4.2: Thermal properties of PA2200 [58] Table 5.1: Machine parameters for single layer sintering study Table 5.2: Two-layer specimen production parameters Table 6.1: Drop ejection frequency data from Phantom 310 high-speed imaging set-up Table 6.2: Ink density calculations using the solutions from equations Table 6.3: Thermal properties of materials in HSS heat transfer Table 6.4: Boundary temperatures during sintering exposures Table 6.4: As-printed ink geometries at five print speeds before sintering with 1/32in scale ( mm) above each image Table 6.5: Sintered ink patterns with 1/32in scale ( mm) above each image Table A.1: Full list of factors in Hopkinson et al cost analysis (copied from [20]) Table A.2: Mechanical property data reported by Ellis et al for tensile specimens fabricated via SLS and HSS [25] xii

14 LIST OF FIGURES Figure 1.1: Amorphous and Crystalline Macromolecular Polymer Structure (copied from [3])... 2 Figure 1.2: Diagram of an injection molding machine (copied from [4])... 4 Figure 1.3: Warping due to non-uniform wall thickness in injection molding (copied from [4]).. 4 Figure 1.4: Boss design to prevent sinks (copied from [4])... 5 Figure 1.5: Boss strengthening technique (copied from [4])... 6 Figure 1.6: Diagram of a FDM process (copied from [6])... 8 Figure 1.7: Diagram of SL process (copied from [9])... 9 Figure 1.8: Diagram of SLS process (copied from [12]) Figure 1.9: Melt-flow index machine and diagram of internal components (copied from [16]).. 13 Figure 1.10: An example DSC curve for Nylon 12 (copied from [18]) Figure 1.11: Thermoplastics materials and their compatibility with SLS production methods (copied from [19]) Figure 2.1: Small lever part (copied from [20]) Figure 2.2: Medium-sized cover part (copied from [20]) Figure 2.3: cost comparison for the small lever arm by four different processes (copied from [20]) Figure 2.4: Cross-sectional view of the sintering method in HSS (copied from [23]) Figure 2.5: The effects of carbon black and radiation intensity on sintering time for Duraform nylon 12 (copied from [23]) Figure 2.6: SEM images of sintered parts from first experimental set-up (copied from [23]) xiii

15 Figure 2.7: SLS roller system modified with mounted inkjet print-head and IR lamp (copied from [24]) Figure 2.8: SLS machine layout with HSS modifications (copied from [24]) Figure 2.9: Ultimate tensile strength and elongation at break for DuraForm PA (copied from [25]) Figure 2.10: Young s modulus for DuraForm PA tensile test specimens (copied from [25]) Figure 2.11: Ultimate tensile strength and elongation at break for DuraForm HST specimens (copied from [25]) Figure 2.12: Young s modulus for DuraForm HST10 specimens (copied from [25]) Figure 2.13: Elongation at break for TPE 210-S specimens (copied from [25]) Figure 2.14: Young s modulus for TPE 210-S specimens (copied from [25]) Figure 2.15: DSC chart with two-peak region (copied from [28]) Figure 2.16: Two-peak region relationship to melted and un-melted particle regions in a part (copied from [28]) Figure 2.17: Position of two-peak measurements using DSC data curve (copied from [28]) Figure 2.18: Dithered patterns with variable densities, numbers at the top represent greyscale values where 0 is all black and 227 is light grey (copied from [30]) Figure 2.19: Crystallinity vs greyscale level (copied from [30]) Figure 2.20: Greyscale levels (converted to dithered pattern) for each set of tensile specimens (copied from [31]) Figure 2.21: DSC scan of virgin TPE210-S and mixed with 0.2wt% Cab-O-Sil (copied from [32]) Figure 4.1: HSS prototype system xiv

16 Figure 4.2: Makeblock XY Plotter (copied from [33]) Figure 4.3: Inkjet microcontroller and inkjet print-head mounted to the moving carriage Figure 4.4: Inkjet print-head mounted onto the carriage using a custom 3D printed attachment (orange part) Figure 4.5: Front view of the prototype assembly with the modified leg attachments highlighted by red boxes, MDF powder bed in center Figure 4.6: Inkjet print-head clearance over the powder surface Figure 4.7: Aluminum attachment arms for the infrared lamp, mounted onto the cross-axis Figure 4.8: Inkshield with inkjet cartridge (copied from [36]) Figure 4.9: Diagram of the thermal inkjet mechanism (copied from [39]) Figure 4.10: Diagram of Inkshield ejection events (copied from [40]) Figure 4.11: Diagram of an SLS heated processing chamber (copied from [43]) Figure 4.12: Solidworks sketch of heated powder bed assembly (cross-sectional view) Figure 4.13: Internal bed components (left) and MDF frame (right) Figure 4.14: Threaded nut (outlined by red box) press fit into MDF layer Figure 4.15: Power resistor heat bed design by Kevin Smith (copied from [53]) Figure 4.16: Raidmax RX-380K power supply (copied from [54]) Figure 4.17: Halogen SR-Spot infrared heating lamp (copied from [55]) Figure 4.18: Manual powder leveling Figure 4.19: Auxiliary heat lamp positioned above the powder bed Figure 4.20: Furnace (left) and aluminum baking tin with powder (right) Figure 4.21: HSS assembly with ventilation hood highlighted by red box Figure 4.22: Arduino Mega 2560 (copied from [64]) xv

17 Figure 4.23: Minimal wiring diagram for the DRV8825 to a microcontroller and stepper motor (copied from [66]) Figure 4.24: X-Y motion control circuit consisting of an Arduino Mega and two DRV8825 stepper drivers Figure 4.25: Micro-switch used to trigger inkjet deposition (red-box) Figure 4.27: Front end-stop Figure 4.28: Left end-stop Figure 4.29: Micro-switch used to trigger sintering stroke Figure 4.30: Second Arduino Mega and third DRV8825 stepper driver used for powder bed z- movement Figure 4.31: Micro switches for z-movement down (red box on the left) and for z-movement up (red box on the right) Figure 5.1: Thor-labs camera set-up with fixed inkjet print-head and led aligned with the camera lens Figure 5.2: Alignment of LED and camera, perpendicular to inkjet nozzle array Figure 5.3: Phantom 310 camera imaging set-up with AmScope LED positioned on the opposite side of inkjet nozzle array from the camera lens Figure 5.4: DAQ set-up with 3-wire RTD placed on aluminum plate (red box) Figure 5.5: RTD placement secured with kapton tape Figure 5.6: LabVIEW program to convert RTD resistance to temperature and write data to an Excel file Figure 5.7: Non-contact temperature measurement of powder surface Figure 5.8: Example of x-direction Z-profile (blue line) xvi

18 Figure 5.9: Example of y-direction Z-profile (blue line) Figure 6.1: Example of MATLAB edge detection for drop head Figure 6.2: Example of drop tail edge detection Figure 6.3: Final processed image frame showing the number of drops measured and average drop volume Figure 6.4: Final processed image frame showing the number of drops measured and average drop volume for regularized measurements Figure 6.5: Graph of low and high ink densities as a function of print speed Figure 6.5: Powder bed aluminum plate temperature study Figure 6.6: Measurement of boundary temperatures in powder bed during temperature regulation study Figure 6.7: Temperature measurements during fourteen sintering strokes and temperature regulation Figure 6.8: Effect of print speed on black sintered mass Figure 6.9: Effect of print speed on excess sintered mass Figure 6.10: Effect of print speed and ink ejection pattern on single layer specimen thickness 122 Figure 6.11: Effect of print speed and sintering speed on sintered black mass Figure 6.12: Effect of print and sinter speed on excess white mass Figure 6.13: Thickness measurement data for two-layer sintered specimens Figure 6.14: Specimen surface variation parallel to ink deposition direction for mm/s sinter speed Figure 6.15: Specimen surface variation perpendicular to ink deposition direction for mm/s sinter speed xvii

19 Figure 6.16: Specimen surface variation parallel to ink deposition direction for 53.2 mm/s sinter speed Figure 6.17: Specimen surface variation perpendicular to ink deposition direction for 53.2 mm/s sinter speed Figure 6.18: Specimen surface variation parallel to ink deposition direction for mm/s sinter speed Figure 6.19: Specimen surface variation perpendicular to ink deposition direction for mm/s sinter speed Figure 6.20: Effect of print speed and sinter speed on mean surface variation parallel to ink deposition direction Figure 6.21: Effect of print speed and sinter speed on max variation height parallel to ink deposition direction Figure 6.22: Effect of print speed and sinter speed on mean surface variation perpendicular to ink deposition direction Figure 6.23: Effect of print speed and sinter speed on max variation height perpendicular to ink deposition direction Figure 6.24: Three-dimensional surface topography of specimen produced with 26.6 mm/s print speed and mm/s sinter speed Figure 6.25: Three-dimensional surface topography of specimen produced with mm/s print speed and mm/s sinter speed Figure 6.26: Three-dimensional surface topography of specimen produced with 39.9 mm/s print speed and mm/s sinter speed xviii

20 Figure 6.27: Three-dimensional surface topography of specimen produced with 26.6 mm/s print speed and 53.2 mm/s sinter speed Figure 6.28: Three-dimensional surface topography of specimen produced with mm/s print speed and 53.2 mm/s sinter speed Figure 6.29: Three-dimensional surface topography of specimen produced with 39.9 mm/s print speed and 53.2 mm/s sinter speed Figure 6.30: Three-dimensional surface topography of specimen produced with 26.6 mm/s print speed and mm/s sinter speed Figure 6.31: Three-dimensional surface topography of specimen produced with mm/s print speed and mm/s sinter speed Figure 6.32: Three-dimensional surface topography of specimen produced with 39.9 mm/s print speed and mm/s sinter speed Figure A.1: Ultimate Tensile Strength vs Greyscale (copied from [30]) Figure A.2: Elongation at Break vs Greyscale (copied from [30]) Figure A.3: Young s Modulus vs Greyscale [30] Figure A.4: Density vs Greyscale (copied from [30]) Figure A.5: Density vs Greyscale (copied from [31]) Figure A.6: UTS vs Greyscale (copied from [31]) Figure A.7: Young s Modulus vs Greyscale (copied from [31]) Figure A.8: EaB vs Greyscale (copied from [31]) Figure A.9: Effect of IR lamp speed on EaB at two bed temperatures (copied from [32]) Figure A.10: Effect of lamp speed on UTS at two bed temperatures (copied from [32]) xix

21 Figure A.11: Effect of lamp speed on Young s Modulus at two bed temperatures (copied from [32]) Figure A.12: Effect of bed temperature on EaB at two lamp speeds (copied from [32]) Figure A.13: Effect of bed temperature on UTS at two lamp speeds (copied from [32]) Figure A.14: Effect of bed temperature on Young s Modulus at two lamp speeds (copied from [32]) Figure A.15: Effect of flow agent on EaB at different lamp speeds and bed temperatures (copied from [32]) Figure A.16: Effect of flow agent on UTS at different lamp speeds and bed temperatures (copied from [32]) Figure A.17: Effect of flow agent on Young s Modulus at different lamp speeds and bed temperatures (copied from [32]) Figure A.18: Unwanted melting of PA2200 due to poor plate temperature regulation xx

22 CHAPTER 1: INTRODUCTION 1.1 Polymers Polymers are defined as molecules composed of many repeating subunits whose atoms are linked to each other through primary, usually covalent, bonds [1]. These low-molecular-weight subunits are called monomers and they are essentially the building blocks of polymers. The properties of a polymer are largely dependent on the monomers it is comprised of and the way in which they bond to each other. The chemical process through which monomers are linked together is referred to as polymerization [1]. Polymer chain structure can be amorphous or crystalline. Amorphous polymer has random chain entanglement and is characterized by its glass transition temperature (TG). This is a transition from rigid to mobile molecular structure occurring over a small temperature range of 10 degrees or less [2]. Below TG, the structure is rigid like glass. Above TG, amorphous polymer is elastic like rubber and becomes increasingly viscous as heat is added. Crystalline polymer has highly ordered chain structure and is characterized by its crystallization temperature (TC) and melting temperature (TM). Below TC, the structure is ordered and rigid. Above TM, polymer crystals melt and overall the material is fluid-like [2]. A crystalline polymer is more accurately called semi-crystalline because polymer chains cannot achieve 100% crystallinity [1]. Instead, they exist in a state of partial crystallinity with both amorphous and crystalline regions. Figure 1.1 shows a macromolecular diagram of these regions in a semicrystalline thermoplastic. 1

23 Figure 1.1: Amorphous and Crystalline Macromolecular Polymer Structure (copied from [3]) The degree of crystallinity in each semi-crystalline polymer is dependent upon its structure and processing factors, such as the rate of cooling and deformation prior to or during crystallization [2]. There are three categories of polymers: elastomers, thermosets, and thermoplastics. Elastomers can be deformed to a high degree and recover their original dimensions [1]. This elastic behavior is allowed by low polymer cross-link density and long polymer molecules. Elastomers have a low TG, which allows them to be elastic at room temperature. Thermosets are rigid and amorphous, with a high degree of polymer chain cross-linking. Once processed and formed, thermosets degrade rather than become fluid when additional heat is applied [1]. However, they are more resistant to high temperatures than thermoplastics and exhibit better mechanical strength than both polymer counterparts. Epoxy resins, polyester, and phenolic resins are examples of common thermosets. Applications for thermosets include fiber-reinforced composites, dental filings, and polymer coatings. 2

24 In contrast with thermosets, thermoplastics can undergo multiple cycles of heating and cooling without sustaining damage. Polymer cross-links in a thermoplastic are weaker compared to thermosets. Thus, less heat energy is required to break these cross-links and allows them to be repeatedly processed. It is this recyclability that makes thermoplastics the most widely used polymer material in commercial products like food and beverage containers. 1.2 Manufacturing of Polymer Stuctures Traditional Manufacturing Methods There are a variety if processing methods for polymer materials. Some of the more prominent processing methods include: extrusion, calendaring, injection molding, blow molding, rotational molding, compression molding, casting, and thermoforming. Each manufacturing method is designed to produce specific part geometries. Extrusion is used to produce films, tubes, and other products with uniform cross-section [2]. Thermoforming is used to shape a deformable polymer preform against a mold. The processing techniques which involve the filling of a mold cavity include casting, compression molding, transfer molding, and injection molding Injection Molding Injection molding is the most widely used manufacturing process for plastics. It is compatible with thermoplastics, thermosets, and elastomers. Complex geometries can be produced through intricate mold design. Figure 1.2 shows the general set-up for an injection molding machine. Polymer material is fed into a barrel heater, heated and mixed, then forced into a metal cavity (mold) where it cools and hardens. 3

25 Figure 1.2: Diagram of an injection molding machine (copied from [4]) Complex geometries are possible via injection molding but there are some limitations. An example of this is variable wall thicknesses, which can result in warping due to different cooling rates (Figure 1.3). Figure 1.3: Warping due to non-uniform wall thickness in injection molding (copied from [4]) 4

26 There are established methods for reducing part warp such as the use of a rib to isolate a boss and prevent sink marks in the primary wall (Figure 1.4). Figure 1.4: Boss design to prevent sinks (copied from [4]) While these techniques reduce warping, and adjust for non-uniform wall thicknesses, they are limited in their tolerances. A few examples of these techniques are the use of connecting ribs (Figure 1.5) can be used to isolate a boss from the nominal wall to eliminate sink marks and gussets can be used to reinforce a boss and reduce the possibility of warping. In Figure 1.5, the boss wall thickness should be less than 60% nominal wall thickness and rib thickness should be 50-60% nominal wall thickness to avoid warping [4]. 5

27 Figure 1.5: Boss strengthening technique (copied from [4]) Molds can be made of aluminum, hardened steel, pre-hardened steel, and/or berylliumcopper alloy [4]. Aluminum is the economical option; hardened steels have a longer lifespan but are more expensive. Beryllium-copper alloy is used for fast heat dissipation and high heat areas. The mold design stage is critical to cost-effective injection molding [5]. Mold design is a complex step that requires many considerations including clamping force, heating temperature, and injection speed [4]. These process parameters determine manufacturing cycle time and the overall maintenance and support costs. Injection molding is the most cost-effective manufacturing process for large quantities of polymer parts, but under certain production volumes the costs from mold design and fabrication become prohibitive to the overall cost per part. Regardless, injection molding is the most common manufacturing method for polymer parts due to its high production rate and low cost per part. Mold design and fabrication can be detrimental to the cost per part, so much so that cheaper alternative manufacturing methods could be adopted in certain cases. Certain design considerations such as those needed to prevent warping in injection molding, may not be required in other manufacturing methods. 6

28 1.2.3 Additive Manufacturing of Polymers Additive manufacturing (AM) is generally defined as the process of building a threedimensional object by combining layer upon layer of material from 3D model data. Most AM processes use polymer build material, but there are some that can process metals, ceramics, and composites. AM offers advantages over traditional polymer processing methods. Further discussion of these advantages is presented in Ch 2. AM technologies utilize built in computer hardware and specialized software. This software is used to slice and position a 3D CAD model, then determine the required toolpaths for the entire build. These toolpaths control the movement of the printing mechanism, which varies depending on the AM technology. Fused deposition modeling (FDM) was invented and patented in 1989 by Scott S. Crump, co-founder of Stratasys. The key components of an FDM machine are shown in Figure

29 Figure 1.6: Diagram of a FDM process (copied from [6]) Polymer filament is extruded through heated nozzles and deposited in the desired crosssectional geometry for each layer. Extrusion occurs as the nozzle mechanism follows the toolpaths determined by the computer software. To print more complex geometries, secondary support material is used as a substrate for the build material. Support material is used for more complex geometries where there are overhang angles greater than 45 degrees. However, the use of support material requires more than one extrusion nozzle or the changing of filament by hand, and it adds to the overall build time for a part. There are many different material options for the filament including both thermoplastics and thermosets, but thermoplastics are more commonly used. Some thermoplastic materials for FDM include ABS (acrylonitrile butadiene styrene), PC (Polycarbonate) and Nylon [7]. FDM is 8

30 typically used for prototype development due to its low machine cost and relatively simple design. The disadvantages of FDM are long production time and poor final part mechanical properties. The production time is limited by the rate of extrusion, which in turn limits how fast the extrusion nozzle can execute its toolpath. Support material adds additional build time to this process. Today s production grade FDM machines can produce parts within an accuracy of +/ mm (+/ inch)[8]. Stereolithography (SLA) uses light sensitive polymer as build material. The build platform is submerged in a vat/pool of this photopolymer and a laser is used to selectively activate a photopolymerization reaction in the resin material (Figure 1.7). Figure 1.7: Diagram of SL process (copied from [9]) 9

31 The photopolymerization reaction is also called curing, in which the structural properties of the resin change when exposed to certain wavelengths of light (often in the ultraviolet or infrared electromagnetic spectrum). This structural hardening is due to the cross-linking of polymer chains. SLA has better resolution (+/ mm or +/ inch) [10] compared to FDM. However, it is limited in compatible materials, print time, and final material properties. Selective laser sintering (SLS) uses a high-power laser to sinter powder particles layer by layer to generate the final part geometry. Sintering is defined as the process of heating a powder material into a single coalesced mass without melting (liquefaction) [11]. The operational principle and mechanical components for this powder-based AM method is shown in Figure 1.8. There are two powder beds: a feed bed and a print bed. The feed bed moves up (z-direction) by one layer thickness and the print bed moves down by the same distance. The roller system then pushes the powder extending above the rim of the feed bed over into the print bed. This is how powder is deposited on the print bed for each layer. Note that the un-sintered powder material serves as support for the printed part. 10

32 Figure 1.8: Diagram of SLS process (copied from [12]) Production grade SLS machines use a CO2 laser with power in the range of W [13]. Due to the high power of the laser, this AM method offers a broader range of compatible materials and decreased print time. SLS is compatible with polymer, ceramic, glass, and metal powders [14]. SLS produces superior polymer parts compared to other additive manufacturing methods. They are highly durable and can be used for real-world testing as opposed to simple prototyping. The quality and strength of SLS processed parts rival those made from injection molding. Another advantage of SLS is its ability to make highly complex geometries including mechanical joints and snap fits [14]. 11

33 1.3 Analysis of Plastics For polymer processing methods, it is important to fully understand the behavior of polymer material at elevated temperatures. There are methods of polymer analysis that can characterize a polymer material s physical properties and transitions at elevated temperatures, such as melt-flow rate (MFR) and differential scanning calorimetry (DSC) Melt-Flow Rate Melt flow rate is a measurement of a melted thermoplastic and how easily it flows [15]. This is expressed as g/10 min in SI units. Alternately, the volume flow rate (VFR) (cm 3 /10min) can also be used [15]. A small thermoplastic sample (granule, flake, powder) is heated to a viscous state in a heated barrel and extruded through a capillary die. A piston loaded with dead weights forces the melted plastic material through a capillary die [16]. After enough sample material has been extruded, it is removed and weighed or the volume is measured by the machine [16]. Figure 1.9 shows a melt-flow index machine and corresponding diagram of internal components. 12

34 Figure 1.9: Melt-flow index machine and diagram of internal components (copied from [16]) material. The die size, barrel temperature, and piston force are set per standards for each tested Differential Scanning Calorimetry Differential scanning calorimetry is a thermal analysis method that measures the change in a material s heat capacity (CP) with temperature [17]. Heat capacity is the amount of energy a unit of matter can hold. All materials show an increase in heat capacity with temperature and the heat capacity of a molten material is higher than when it is solid [17]. Heat capacity is expressed in the following unit forms: J/g, J/Mol, or calories/g. In DSC analysis, the heat capacity is monitored (as changes in heat flow) while a sample of known mass is heated or cooled (Figure 13

35 1.10). DSC analysis detects transitions in a material such as melting temperature, glass transition, phase changes, and curing [17]. Heat Flow Peak Figure 1.10: An example DSC curve for Nylon 12 (copied from [18]) The material transitions in a polymer show up as peaks on a DSC curve, where there is an abrupt change in heat flow. Figure 1.10 shows an example DSC curve for Nylon 12. Nylon is a semi-crystalline polymer so it undergoes melting and crystallization. The peak pointing downward marks the melting transition for Nylon 12. Negative heat flow means the DSC machine is supplying heat to the material sample. Liquid material can hold more heat than when it s solid. Melting is then an endothermic transition. DSC analysis allows for more information to be gathered from a sample than just its melting temperature. It also provides the onset of melt and peak temperature. The peak temperature corresponds to complete melting and the energy 14

36 needed for melting to occur. This is called the enthalpy of a transition and it is closely related to the crystallinity of a material [17]. Conversely, the second peak points upwards and marks the crystallization transition for Nylon 12, since crystallization is an exothermic transition from an amorphous state to crystalline state. 1.4 Summary of Modern-Day Plastic Manufacturing Today, polymer material is widely used due to its highly customizable material properties. Plastics can be tailored to fit specific applications. There are many traditional methods to manufacture plastic parts, but injection molding dominates this group of technologies. In recent years, additive manufacturing has seen considerable development as a viable manufacturing method within the right circumstances. Today s AM technologies can produce end-use parts at a lower cost per part. Of the established AM techniques, SLS has shown the greatest potential as a competitive manufacturing method. Companies using SLS technology, such as Stratasys, have a wide selection of material options. However, these are all thermoplastic materials. Figure 1.11 shows a graphic of thermoplastic materials that can be processed via SLS, as well as thermoplastics that cannot. 15

37 Figure 1.11: Thermoplastics materials and their compatibility with SLS production methods (copied from [19]) Nylon is the most widely featured material due to its superior properties, which make it useful in both engineering and performance applications. The following materials listed in Figure 1.11: PA 6, PA 66, PA 11, and PA 12, are all Nylon relatives featuring slightly different monomer structures. 16

38 CHAPTER 2: LITERATURE REVIEW 2.1 Additive Manufacturing as a Viable Manufacturing Method The capabilities of AM technologies to produce single part geometries is well known. There are few AM methods used to manufacture greater quantity of parts. This section discusses the manufacturing capabilities of AM technologies and how this compares to traditional injection molding Benefits and Limitations of Additive Manufacturing AM technology offers some advantages over traditional plastic manufacturing methods like injection molding. No tooling is required, meaning the printing mechanism doesn t need to be changed or modified for different printed geometries. A new product design can be put into AM production immediately without waiting for tooling. In addition, AM parts can be made in or near their final destination. There is minimal waste in AM, which reduces the cost of materials and waste disposal. Lastly, AM allows geometric freedoms such as variable wall thicknesses and zero draft, which are not possible with injection molding [20]. These advantages allow considerable freedom in product design and manufacturing lead time. At the small-scale production level, additive manufacturing is used commonly for prototype production. AM prototype parts are useful for checking fit and illustrative purposes. However, their mechanical properties are typically not suitable for end use. The most suitable products for AM are small parts with high geometric complexity to be made in small volumes [21]. 17

39 The current limitations of AM are printed part mechanical properties and high machine cost. The mechanical properties of AM parts have been improved in recent years and can be further improved through additional research and development. An example of this is studying the effects of process parameters on final mechanical properties to identify how these parameters can be adjusted to optimize mechanical properties. To determine whether AM technologies can be competitive manufacturing methods, the costs for each AM method should be compared with injection molding, the most prominent polymer processing method Cost Analysis of Polymer AM Methods and Injection Molding Dr. Neil Hopkinson is a Reader in the Wolfson School of Mechanical and Manufacturing Engineering at Loughborough University, UK specializing in additive manufacturing technologies [22]. Hopkinson et al performed cost analysis [20] [21] of injection molding and three AM methods: FDM, SL, SLS. The findings from these studies will be discussed in this section. The costs for producing parts via AM were separated into machine costs, labor costs, and material costs. A detailed breakdown of these costs and how they were calculated can be found in Appendix A, Figure A.1. Two different parts were used in this cost comparison: a small lever arm (Figure 2.1) and medium sized cover part (Figure 2.2). These were chosen due to their different sizes and complexity. 18

40 Figure 2.1: Small lever part (copied from [20]) Figure 2.2: Medium-sized cover part (copied from [20]) The unit cost to produce these parts was compared using injection molding and the three AM methods listed previously. It should also be noted that the parts used in this study were designed for molding manufacture. Material properties, surface finish, and the accuracy of parts were not considered. Injection molding yielded similar unit costs for both parts, despite the difference in size. The reason for this is attributed to material choice. The lever was made from polycarbonate and 19

41 the cover from polypropylene, which is a cheaper material [20]. The calculated tool cost and unit cost per part is shown in Table 2.1. Table 2.1: Tool cost and cost per part for injection molding (copied from [20]) It is important to note that the tool cost far outweighed the unit cost for each part produced. For low production volumes, this will result in high total cost per part. For SL, the unit cost for the lever was roughly 1/10th the unit cost for the cover (Table 2.2). The material used for each part was epoxy and constituted around 30% of the cost. Machine cost made up 70%, with labor costs being negligible [20]. Table 2.2: Costs for parts made via SL (copied from [20]) 20

42 The material choice for the FDM parts was ABS, a common thermoplastic polymer. The calculated costs for both parts produced through FDM are shown in Table 2.3. Table 2.3: Costs for parts made via FDM (copied from [20]) The total cost per part for FDM is slightly lower than SL for both parts. The total cost per lever was about 1/10th the total cost per cover. Material costs were about 40% of total cost per part and machine cost was about 60%, again labor costs were not considered. Cost analysis for SLS was performed only on the lever, using nylon as the material choice. Machine costs for SLS were lower because the machine can build a greater number of parts by stacking them vertically in the powder print bed and the build speed is higher [20]. The calculated costs for SLS lever production are shown in Table

43 Table 2.4: Costs for parts made via SLS (copied from [20]) Note that machine cost makes up about 23% of the total cost per part and material cost is about 74%. For each AM method, the cost to produce the lever was substantially less than the cost for the cover. From this it is concluded that AM methods are best suited to produce smaller parts [20]. A graph comparing the cost per part for injection molding and the three AM methods is shown in Figure

44 Figure 2.3: cost comparison for the small lever arm by four different processes (copied from [20]) From Figure 2.3 injection molding, at low production volumes, was more expensive than the AM methods due to tooling cost. The intersection of the injection molding line with each AM line marks the points at which the cost per part for injection molding is the same as AM methods. For SL, FDM, and SLS these points were 5500, 6500, and parts respectively. SLS appears to be the most competitive among AM methods with the lowest cost per part and highest breakeven production volume with injection molding. Hopkinson et al s cost analysis confirmed the viability of AM as a competitive production method for polymer parts. Furthermore, SLS was identified as the most competitive AM method. The primary sources of cost for AM were identified, with machine cost playing a major part in overall cost per part. From this it was concluded that reducing the machine cost of 23

45 SLS could significantly reduce the cost per part and would result in the most competitive AM method with injection molding [20]. 2.2 Manufacturing Thermoplastic Structures via High-Speed Sintering Hopkinson et al s cost analysis of AM methods and injection molding showed that SLS was the best candidate for a competitive manufacturing method. If the machine cost could be reduced and the production throughput increased, a far more competitive additive manufacturing process for medium to high production volumes would be possible [21] The High-Speed Sintering Process High speed sintering (HSS) was introduced by Hopkinson et al as a cheaper and faster adaption of SLS technology. The expensive laser was removed and a new sintering mechanism was introduced. This new sintering process utilized an infrared energy source and radiation absorbing material (RAM) [23] to vary the absorbance of incoming radiant energy on a powder surface. Areas where RAM is deposited absorb sufficient energy to reach the powder material s melting temperature while leaving the remaining areas below the melting temperature. Figure 2.4 shows a cross-sectional diagram of the sintering mechanism in HSS. The entire powder bed surface is exposed to IR radiation, but only the areas covered by RAM are sintered. This is repeated layer upon layer until the desired 3D object has been generated. 24

46 Figure 2.4: Cross-sectional view of the sintering method in HSS (copied from [23]) In their early research on HSS, Hopkinson et al used Duraform nylon 12 powder mixed with carbon black powder (RAM) and a sintering mask to make tensile test samples [23]. The sintering mask proved to be troublesome and was abandoned in future designs. However, the impact of carbon black on the sintering time was observed. This confirmed the ability of carbon black to absorb IR radiation more readily than the nylon powder. Figure 2.5 shows the effects of carbon black percent weight on sintering time at two different IR lamp intensities [23]. 25

47 Figure 2.5: The effects of carbon black and radiation intensity on sintering time for Duraform nylon 12 (copied from [23]) The trends for both intensity curves in Figure 2.5 shows that sintering time is decreasing with increasing carbon black wt %. Both curves flatten out towards 1 wt %, indicating a point will be reached where increasing carbon black wt % will not reduce sintering time. Figure 2.5 also shows that increasing infra-red radiation intensity increases the sintering rate [23]. Hopkinson et al suspected that particles of carbon black heat up and transfer thermal energy to surrounding particles of nylon by conduction. The radiation energy absorbed then causes the nylon particles to reach melting temperature and sinter without fully melting [23]. SEM images of sintered nylon particles are shown in Figure 2.6 (a) and (b). 26

48 Figure 2.6: SEM images of sintered parts from first experimental set-up (copied from [23]) Hopkinson collected SEM images for samples prepared using the first experimental desktop set-up [23]. Figure 2.6 (a) is at 50x magnification and Figure 6 (b) is at 250x. A high degree of necking between nylon particles can be seen and the small light colored particles present on the surface of the larger nylon particles are predicted to be carbon particles of size D50-2µm [23]. However, this still needed to be confirmed using EDX analysis. There was no method for pre-heating the powder used in Hopkinson et al s early tensile sample experimental design. Warping in the printed samples was observed and this made building multiple layer samples very difficult [23]. The tensile samples were tested according to BS EN ISO 527-2:1996 standards and mechanical properties determined from tensile testing are shown in Table 2.5. The mechanical properties of parts produced by HSS in this study were superior to comparable SLS parts. When comparing the two different sets of HSS data, it was observed that the largest carbon black wt % did not produce the best mechanical properties. For 2 wt % carbon black, the Young s Modulus increased while both the ultimate tensile strength (UTS) and elongation at break (EaB) decreased. These results and the trends observed in Figure 2.5 confirm that between 1 and 2 wt 27

49 % there is a composition of carbon black that no longer increases mechanical properties and can in fact become detrimental to final mechanical properties. Table 2.5: Mechanical properties of tensile samples made via HSS with varying carbon black wt % (*Data obtained from [23] Process Composition of carbon black (weight %) Young s Modulus (MPa) UTS (MPa) EaB (%) SLS* HSS HSS Hopkinson et al later released another paper discussing additional research and development of HSS. Previously Hopkinson et al mixed carbon black powder with nylon powder and used a mask to selectively sinter Duraform nylon [23]. In this next iteration of HSS experimental design Hopkinson et al selectively printed RAM onto the powder bed surface [24]. RAM, in this case, was changed to black ink containing carbon black pigment. This could easily be deposited onto the powder substrate using an inkjet print-head. An inkjet print-head and IR lamp were attached to the roller system in an SLS machine (Figure 2.7 and Figure 2.8). 28

50 Figure 2.7: SLS roller system modified with mounted inkjet print-head and IR lamp (copied from [24]) Figure 2.8: SLS machine layout with HSS modifications (copied from [24]) 29

51 The following is an outline of Hopkinson et al s manufacturing process using the described testrig, which was still being optimized at the time of this study: 1. A layer of nylon powder is deposited onto the build area (from left to right). 2. Using RAM, a 2D cross-sectional profile is printed onto the main powder bed 3. Following the print-head, the IR lamp exposes the 2D RAM profile as the roller traverses the bed (from right to left). 4. The RAM absorbs the IR radiation and thermal energy is transferred to nearby nylon particles, reaching melt temperature and initiating viscous sintering. 5. The desired 2D profile has now been sintered. 6. Another layer of powder is deposited onto the build area (from right to left) and the process is repeated until the required geometry has been manufactured. This printing process was automated using C programming to control the roller position, powder bed heights, the inkjet printing system, and IR lamp exposure time [24]. C programming is a powerful system programming language that largely influenced the more modern C++ programming language. Hopkinson et al report one of the main issues during HSS process development was the hardness of excess powder sintered to the printed part. This affected part removal and required post-processing for excess material removal. An investigation was performed to optimize settings for the part bed temperature and IR power levels. Hopkinson et al were not sure if the excess powder sintering was due to the IR power or the part bed temperature. Based on the results of this investigation they conclude that the IR power and part bed temperature both 30

52 influence the hardness of excess powder. Hopkinson et al state the need to balance IR power and bed temperature to prevent increased hardness of excess powder [24] Materials for HSS Ellis et al [25] studied materials to be used for HSS. To identify ideal materials for HSS, Ellis et al consider the desired material characteristics for HSS. First, semi-crystalline polymers for HSS should have a large processing window [25]. This process window is the difference between the polymer crystallization temperature and its melting temperature. Second, the crystallization temperature should be less than the melting temperature [25]. Once sintering has occurred it is important to prevent a semi-crystalline polymer layer from cooling below its crystallization temperature. If it drops below the crystallization temperature the polymer will solidify (crystallize), inducing thermal stresses and warp causing the AM build to fail at that layer [25]. Third, an ideal material for HSS should have a narrow melt temperature range and low melt viscosity, meaning it can be quickly heated to a level of fluidity for good fusion without excess energy being added [26]. 3 different materials were studied by Ellis et al: DuraForm PA, DuraForm HST (High Strength and Temperature), TPE 210-S. These materials are SLS grade Nylon 12 powders. DuraForm PA is a form of Nylon 12 and DuraForm HST is a filled Nylon 12 composite powder with mechanical properties tailored for high strength and temperature (HST). TPE 210-S is a thermoplastic elastomer widely processed by injection molding and is used in both automotive and household appliance industries. For each material, six type IV ASTM standard tensile test specimens were fabricated via HSS. Tensile tests were performed per ASTM D standard test method for tensile 31

53 properties of plastics. The materials studied and tensile specimen build parameters are shown in Table 2.6. Table 2.6: HSS build parameters for tensile test specimens (copied from [25]) Ultimate tensile strength, Young s modulus, and elongation at break were recorded for each specimen. The resulting mechanical property for each material is shown in Figures In these figures the mechanical property data for HSS specimens is compared with SLS tensile specimen mechanical properties. Figure 2.9: Ultimate tensile strength and elongation at break for DuraForm PA (copied from [25]) 32

54 Figure 2.10: Young s modulus for DuraForm PA tensile test specimens (copied from [25]) Figures 2.9 and 2.10 show the mechanical property comparisons for DuraForm PA. HSS produced better UTS and EaB compared to SLS. However, the Young s Modulus of HSS DuraForm PA specimens was less than that of the SLS counterpart. Figure 2.11: Ultimate tensile strength and elongation at break for DuraForm HST specimens (copied from [25]) 33

55 Figure 2.12: Young s modulus for DuraForm HST10 specimens (copied from [25]) The UTS for DuraForm HST10 HSS specimens was less than SLS specimens (Figure 2.11). Similarly, Young s Modulus was also less for HSS specimens (Figure 2.12). Figure 2.13: Elongation at break for TPE 210-S specimens (copied from [25]) 34

56 Figure 2.14: Young s modulus for TPE 210-S specimens (copied from [25]) For TPE 210-S specimens, EaB for SLS was 110% and 265% for HSS. Young s Modulus was nearly the same between HSS and SLS. The reported data in numerical form for this study can be found in the Appendix, Table A.2. Ellis et al conclude from their material investigation that HSS is capable of processing SLS grade polymeric powders. Elongation at break for TPE elastomer was markedly improved over SLS with nearly double the elongation percent. Ellis et al showed that these three SLS grade materials were compatible with HSS processing and even outperformed SLS in final mechanical properties Degree of Particle Melt SLS research on degree of particle melt (DPM) provides critical information for build parameters and their effects on final mechanical properties in HSS fabricated parts. Some research on SLS [26] has shown that the mechanical properties of a part are largely determined by the degree of particle melt. If powder particles are not provided with sufficient energy, they will not melt fully and poor particle fusion is the result. It is already known that the presence of 35

57 un-melted particles in injection molding leads to high residual stresses and reduced mechanical properties [27]. Majewski et al found that changes in SLS build parameters can cause variations in DPM. Furthermore, a higher energy input results in greater DPM. Additional research by Majewski et al [28] showed that percentage crystallinity has a significant effect on tensile strength and elongation at break, where decreasing % crystallinity leads to an increase in DPM and mechanical properties. This trend was observed until full particle melt occurred. [28]. DPM was calculated using % crystallinity and Equation 1, derived by Majewski et al Equation 1 CT is the total crystallinity of the sample, CP is the crystallinity of the powder (0.47), and CMR is the crystallinity of the melted and recrystallized material (0.25). DSC analysis was used to derive the total crystallinity of a sample. Figure 2.15 shows a sample DSC chart. Majewski et al analyzed the two-peak region of the DSC chart, which has been related to the melted and un-melted regions in a sintered polymer material. 36

58 Twin-Peak Region Figure 2.15: DSC chart with two-peak region (copied from [28]) 37

59 Figure 2.16: Two-peak region relationship to melted and un-melted particle regions in a part (copied from [28]) The area under the curve for the two-peak region (Figure 2.17) was derived (mj) and using the sample s mass, the heat of melting (J/g) was calculated. The % crystallinity for the sample was then calculated by comparing this heat of melting value to that of 100% crystalline sample. Gogolewski et al [29] determined that the heat of melting for 100% crystalline nylon 12 was J/g. 38

60 Figure 2.17: Position of two-peak measurements using DSC data curve (copied from [28]) The relationship shown by the research discussed in this section indicates that by optimizing the degree of particle melt, the final mechanical properties of a sintered polymer will also be optimized. This relationship will be brought into discussion in the following sections Greyscale and Print Density The volume of carbon black ink in a 2D cross-section directly effects the amount of thermal energy absorbed and distributed to the polymer material in HSS. Noble et al [30] studied ink print density, aiming to characterize the relationship between print density (ink deposition density) and the mechanical properties of HSS parts. Previous HSS research had used the maximum amount of print density. Noble et al proposed using greyscale and/or dithering patterns 39

61 to control print density. Greyscale is most commonly seen in inkjet printing when using black and white settings. Greyscale is a range of monochromatic shades from pure white to pure black. Each pixel is in 8-bit (each bit can be either 0 or 1), meaning each pixel has 2 8 = 256 possible brightness levels. The greyscale value (0 256) indicates the brightness/shade level of a sampled pixel. These brightness levels are obtained by varying the ink droplet volume. The inkjet print-head used in Noble et al s HSS assembly was only capable of a single droplet size or 1-bit, which prevented the use of true greyscale. A 1-bit printing method forming a dithered pattern was used to re-create a greyscale like print density. Figure 2.18 shows Noble et al s conversion of a greyscale level to a dithered pattern. It should be noted that each greyscale image has a uniform pixel brightness value, such that the greyscale number above represents the brightness of every pixel in the image. Figure 2.18: Dithered patterns with variable densities, numbers at the top represent greyscale values where 0 is all black and 227 is light grey (copied from [30]) 40

62 Noble et al used these dithered greyscale patterns to study the impact of print density on final mechanical properties of HSS tensile specimens. Nylon 12 and TPE 210-S were used in two print density studies [30] [31]. Using 100% virgin Nylon 12, six tensile test specimens were fabricated for each greyscale level shown in Table 2.1 and the HSS machine parameters for all tensile specimens are shown in Table 2.2. Table 2.7: Greyscale levels used in Nylon 12 print density study [30] Greyscale Levels Table 2.8: Tensile specimen HSS build parameters [30] Build Bed ( C) Build Overhead ( C) Feed Bed ( C) Feed Overhead ( C) Preheat Stroke mm/s) Sintering Stroke mm/s) Tensile specimens were produced with a layer thickness of 0.1mm (100 µm). Tensile specimens were tested per ASTM D standards at an extrusion rate of 5mm/min. The resulting mechanical property data can be found in the Appendix (Figures A.1-A.4). This data shows that print density has significant effects on the mechanical properties of HSS parts. In general, the mechanical properties increased with decreasing greyscale until about 140 greyscale level, where decreasing greyscale represents increasing print density. Below 140 greyscale, UTS and Young s Modulus decreased with decreasing greyscale. This trend was different for EaB below 140, generally it appears to level out. Density continued to increase with 41

63 decreasing greyscale, but mostly leveled out at about 140 greyscale. This suggests that DPM was nearly complete at 140 greyscale and further thermal energy helped to fill any pores that were still present. The increase of mechanical properties as greyscale decreased shows that mechanical properties of specimens were enhanced with increasing density of ink. However, it appears that 140 greyscale level is an optimal ink density where additional ink becomes detrimental to some final mechanical properties like UTS and Young s Modulus. Noble et al concluded that increasing ink density resulted in greater absorption of thermal energy and enhanced mechanical properties due to increased particle melt [30]. Furthermore, the decrease observed in UTS and Young s Modulus below 140 greyscale level is due to the degradation of Nylon 12 after full particle melt was achieved. DSC analysis was performed for each set of specimens using a Perkin Elmer DSC 8500 machine in combination with Pyris DSC analysis software [30]. The crystallinity and degree of particle melt were calculated using Equation 1 and the methods established by Majewski et al [28]. The calculated % crystallinity and DPM for specimens based on DSC data can be seen in Figures 2.19 and

64 Figure 2.19: Crystallinity vs greyscale level (copied from [30]) Table 2.9: greyscale values and corresponding derived degree of particle melt [30] Greyscale Level Derived DPM (%) Between 170 and 142 greyscale levels, the DPM appears to be complete. This corresponded well with the mechanical property results from tensile testing. The fact that the DPM continues to rise above 100% is illogical. This was determined to be a fundamental limitation of Equation 1, where it is only valid for crystallinity values above the crystallinity of 43

65 the melted and recrystallized region (25%). From this it was determined that Equation 1 should be used as an indicator of where DPM is complete, not as an exact calculation method [30]. Ellis et al studied the effects of print density (greyscale) on mechanical properties of 100% virgin TPE210-S tensile specimens [31]. TPE210-S has great potential for applications in the automotive and aerospace industries where high elasticity and durability is desired. Six tensile specimens were fabricated for each greyscale level in Figure Figure 2.20: Greyscale levels (converted to dithered pattern) for each set of tensile specimens (copied from [31]) Table 2.10: HSS machine parameters for tensile specimen fabrication [31] Build Bed ( C) Build Overhead ( C) Feed Bed ( C) Feed Overhead ( C) Preheat Stroke mm/s) Sintering Stroke mm/s) Tensile tests were performed on each specimen to determine Young s Modulus, EaB, and UTS [31]. Additional rectangular samples were printed for density and DSC testing. Mechanical property data from tensile testing is shown in the Appendix (Figures A.5-A.8). Like the results 44

66 from Noble et al, mechanical properties were optimized within a range of greyscale values. This critical greyscale range was between 57 and 113. Compared to Noble et al s work, Ellis et al reported a lower critical greyscale range for optimized material properties. This was due to the difference in build material. TPE210-S does not require as much energy as Nylon 12 to sinter due to its lower melting temperature Sintering Parameters: Sintering Speed, Powder Temperature, and Flow Agent Norazman et al [32] studied the effects of IR lamp speed, powder bed temperature, and powder composition on the mechanical properties of HSS processed TPE 210-S. Table 2.11 lists some material properties for TPE210-S. Table 2.11: TPE 210-S material properties [32] Properties Value Average particle size (D50) 85 µm Melting Point 178 C Tensile Modulus 8 MPa Elongation at break (un-infiltrated) 110% Shore hardness (un-infiltrated) 40 ASTM Type IV tensile test specimens were manufactured for this study. See Table 2.6 for a list of HSS build parameters used to make these tensile specimens. 45

67 Table 2.12: HSS build parameters for Norazman et al (copied from [31]) Tensile specimens were tested per ASTM D638 standard test method. Table 2.7 shows the build parameters for an investigation of changing IR lamp speed at two powder bed temperatures and constant powder composition. Table 2.13: Build parameters for investigation of lamp speed at two bed temperatures and constant powder composition [31] Build No. Powder ratio Build Bed Overhead Lamp Speed (mm/s) Temperature ( C) 1 50% Virgin 50% Recycled % Virgin % Recycled 70 46

68 The tensile data results for this investigation can be found in the Appendix, Figures A.9- A.11. The maximum values for EaB, UTS, and Young s Modulus were observed at the lowest lamp speed (50 mm/s) and higher bed temperature (100 C). Another investigation was performed to observe the effect of bed temperature at two lamp speeds and constant powder composition. The build parameters are shown in Table 2.8 below. Table 2.14: Build parameters for investigation of bed temperature at two lamp speeds and constant powder composition [31] Build No. Powder ratio Lamp Speed (mm/s) Build Bed Overhead Temperature ( C) 1 50% Virgin 50% Recycled % Virgin % Recycled Refer to the Appendix (Figures A.12-A.14) for tensile data results. The maximum values for EaB and UTS were observed at the highest powder bed temperature (110 C) and fastest lamp speed (70 mm/s). Maximum Young s Modulus occurred in specimens manufactured at 100 C and the slower lamp speed 60 mm/s. The final part of Norazman et al s study investigated the effect of flow agent (fumed silica) at two lamp speeds and two bed temperatures. Table 2.9 shows the build parameters for 47

69 this investigation. Note that the best sets of parameters in the two previous investigations were used. Table 2.15: Build parameters for investigation of flow agent at two lamp speeds and two bed temperatures [32] Build No. Build Bed Overhead Temperature ( C) Lamp Speed (mm/s) Flow Agent Level (% by weight to virgin TPE) Figures A.15-A.17 in the Appendix show the tensile data collected for parts using fumed silica compared to 100% virgin powder parts. The maximum tensile property values were observed at 0.2 wt % silica. A steady decrease in these properties was observed with additional flow agent (greater than 0.2 wt %). Norazman et al used Frenkel s Model of Sintering to explain this phenomenon. Frenkel s model relates particle viscosity to predicted sintering rate. Norazman et al speculate that fumed silica promotes fluidity and reduces the viscosity of the TPE210-S mixture. A lower viscosity resulted in lower sintering time and lower energy input. Tensile properties largely depend on the amount of heat energy input. This lower energy input resulted in inferior properties. The mechanical properties of HSS parts produced in this study were compared to SLS and conventional injection molding in Table

70 Table 2.16: Comparison of HSS, LS, and injection molding part mechanical properties [32] Process % by weight of fumed silica mix EaB (%) Young s Modulus (MPa) UTS (MPa) HSS HSS LS N/A Injection Molding Thermal analysis was performed using a double furnace Perkin Elmer DSC8500 and per ASTM D3418 standard test method. Figure 2.21 shows the resulting DSC curves for 100% virgin TPE210-S and virgin TPS210-S mixed with 0.2 wt % Cab-O-Sil. Figure 2.21: DSC scan of virgin TPE210-S and mixed with 0.2wt% Cab-O-Sil (copied from [32]) 49

71 Both curves are nearly identical in shape, with phase transitions occurring at constant temperatures: TGlass = 105 C, TMelt = 140 C, and TCrystallization = 50 C. A shift of about 5 mw was observed for the TPE mix line compared to the virgin TPE line. The TPE mix line showed sharper melt (TM2) and crystallization peaks (TC2). Norazman et al suggest that the addition of flow agent increased the heat of fusion and the amount of energy released during crystallization [32]. Furthermore, the presence of silica atoms in between TPE210 atoms may cause them to pack closer together and subsequently strengthen intermolecular bonds. These strengthened bonds would require more energy to break them, which matches well with what was observed in the data. Lastly, the difference between TG2 and TM2 (process window) is larger for TPE210 mixed with silica. Norazman et al conclude that the addition of flow agent changed the physical properties and enhanced thermal properties of TPE210-S parts [32]. 50

72 CHAPTER 3: OBJECTIVES This research project is intended to develop foundational knowledge of high speed sintering and expand on information reported in existing HSS literature. The first aspect of this research project is to design and implement an HSS machine design. Due to cost and time constraints for this project, an HSS prototype was designed and built using low-cost, open source solutions. Critical HSS parameters have been identified through the work of Hopkinson et al and are used as a guiding influence in this research. The capabilities of the HSS prototype are evaluated and used to make recommendations for improved HSS machine design. The second aspect of this research project is an investigation of critical HSS elements using PA2200 nylon. These HSS parameters include: ink deposition (density), powder bed temperature, and sintering lamp speed. The volume of ink deposited on a power substrate directly effects the properties of the final printed part in HSS. Inkjet drop volume was measured with an experimental high speed imaging set-up and the ink volume was calculated as a function of print speed. The effect of ink deposition density on sintered PA2200 nylon was evaluated based on sintered mass, thickness, and surface roughness. The amount of sintering time necessary for quality sintering is also investigated, in conjunction with ink deposition. The objectives of this study are summarized as follows: 1) Design and build an HSS prototype utilizing the machine characteristics observed in SLS technology and research from Hopkinson et al. 2) Investigate ink deposition, powder temperature, and sintering time to identify their effects on HSS processing. 51

73 3) Identify areas in HSS for further investigation. 4) Evaluate prototype capabilities and make design recommendations for future machine iterations. 52

74 CHAPTER 4: PROTOTYPE DEVELOPMENT 4.1 HSS Prototype System Design and Realization Prototype Overview The printing concept and operating principle of HSS has been established by Hopkinson et al [23][24]. To conduct experimental study on HSS, a prototype system was designed and realized. The end result of the HSS prototype build is shown in Figure 4.1. The fume hood and furnace on the far side of the picture are auxiliary components to the set-up. The system should include an x-y motion frame, inkjet microcontroller, a heated powder bed, and an infrared lamp. A more detailed description of the realization process and each component will be provided in the following sections. Fume Hood Furnace IR Lamp Inkjet Controller X-Y Frame Heated Powder Bed Figure 4.1: HSS prototype system 53

75 4.1.2 XY-Motion Frame Traditional inkjet printing operates in the x-y plane when it deposits ink on a substrate. HSS requires inkjet deposition onto a powder substrate. To control inkjet deposition in this manner, a controlling system must be used to eject ink from the print-head as well as move the print-head in the x-y plane. In traditional inkjet printers, there are specialized circuit boards and software that perform these functions. However, this technology is difficult to remove and implement into another application. A simple and low-cost option for x-y motion control is an x- y plotter, which is typically used as a drawing machine. A Makeblock XY Plotter was purchased and implemented as the frame of the HSS build (Figure 4.2). More detail on the x-y controlling system will be discussed in section 4.2. Figure 4.2: Makeblock XY Plotter (copied from [33]) The blue anodized aluminum frame seen in Figure 4.2 has a 310mm x 390mm (12.2 in x in) working area. The pencil holding assembly was removed and the carriage that moves along the cross-axis was modified to hold an inkjet print-head and an inkjet microcontroller 54

76 (Figure 4.3 and 4.4). Further detail on this inkjet microcontroller will be provided in section Figure 4.3: Inkjet microcontroller and inkjet print-head mounted to the moving carriage Figure 4.4: Inkjet print-head mounted onto the carriage using a custom 3D printed attachment (orange part) 55

77 The orange attachment used to mount the inkjet print-head to the x-y carriage is a custom design which was designed in Solidworks and fabricated using a fused deposition modeling (FDM) 3D printer, using polylactic acid (PLA) as the material. The x-y frame was modified to increase the height of the frame by 178 mm (7 inches). This allowed enough clearance for a powder bed with dimensions: mm length (7.5 inch length) by mm width (7.5 inch width) by 165 mm (6.5 inch height). Figure 4.5: Front view of the prototype assembly with the modified leg attachments highlighted by red boxes, MDF powder bed in center Four aluminum 6061 frame legs were fabricated and attached to replace the small blue legs that come with the x-y plotter kit (Figure 4.5). To secure the overall assembly, the aluminum legs were mounted into slots cut into a 25.4 mm (1 inch) thick plywood base. This improved the rigidity of the frame and prevented it from moving/sliding on the lab countertop. An important consideration when designing the aluminum leg attachments was the clearance of the inkjet print-head over the powder surface (top of the MDF box in Figure 4.5). With the new aluminum 56

78 legs in place, the clearance of the inkjet over the powder surface was approximately 2.54 mm (0.1 inches), as shown in Figure 4.6 below. Figure 4.6: Inkjet print-head clearance over the powder surface The last modification made to the x-y frame were attachment arms for the infrared lamp, to mount the IR lamp onto the moving cross-axis. Using mm (0.125 inch) thick aluminum 6061 plates, two attachment arms were fabricated and they hold the IR lamp approximately 25.4 mm (1 inch) from the powder surface. The attachment arms can be seen in Figure 4.7 below, highlighted by red boxes. 57

79 Figure 4.7: Aluminum attachment arms for the infrared lamp, mounted onto the cross-axis Inkjet Microcontroller Inkjet deposition required control and use of an inkjet print-head. Commercial printheads are sold as individual print-heads without the controlling system. The controlling systems in commercial inkjet printers are not easily removed and used in other applications. There are some industrial inkjet print-heads made by Xaar [34] and Trident [35] that could be purchased and implemented into an HSS design. These feature robust inkjet print-heads with controlling systems. However, they are high in cost. A low-cost and easily implemented solution for inkjet deposition was an Arduino based microcontroller designed and sold by Nicholas C. Lewis [36] called the Inkshield, which is an open source microcontroller attachment. The Inkshield and corresponding inkjet cartridge assembly are shown in Figure

80 Figure 4.8: Inkshield with inkjet cartridge (copied from [36]) An open-source Arduino library supporting the Inkshield was available for download on GitHub [37]. The Inkshield allows an Arduino to control an HP C6602 thermal inkjet cartridge, which features a drop resolution of 96 dots per inch (dpi) and average drop volume of 160 pl [38]. Figure 4.9 shows a diagram and operating principle of the thermal inkjet. 59

81 Figure 4.9: Diagram of the thermal inkjet mechanism (copied from [39]) Each nozzle in the C6602A cartridge ejects a drop when a 6 µs electric pulse is sent to the thin-film resistors inside each nozzle reservoir. These pulses of electricity cause the thin-film resistors to heat-up quickly and vaporize ink in the reservoir. This forms a vapor bubble that grows quickly until it exerts enough force to push a drop of ink through the nozzle orifice. After drop ejection, the ink reservoir needs to re-fill with ink before another drop can be ejected. Based on the Inkshield theory, drop ejection events are staggered such that each nozzle fires 0.5 µs after the nozzle before it and has an 800µs delay until it fires again to avoid burning out the nozzle. A diagram showing the staggered timing of these events is shown in Figure

82 Figure 4.10: Diagram of Inkshield ejection events (copied from [40]) The Inkshield plugs directly into an Arduino Uno [41]. The Arduino Uno is a microcontroller featuring input and output pins and a programmable logic chip. It can be programmed to control the inkjet cartridge via the Inkshield. The Arduino Uno sends electric pulse signals through several of its output pins to the Inkshield, which increases the voltage and sends electric pulses to each nozzle on the print-head. The Inkshield is essentially a communications relay between the Arduino microcontroller and the C6602A print-head. The programming language for the Arduino is C/C++. After downloading and importing the Inkshield library, the Arduino can be programmed using specialized C/C++ command functions. The special command functions tell the Inkshield where to send electric pulses and when. A hexadecimal number is inserted into a command function to tell the Inkshield which nozzles to fire and in what order. Hexadecimals are used to represent longer binary sequences, where binary digits are either 0 or 1. Table 4.1 shows the sixteen hexadecimals and the binary numbers/sequences that they represent. For example, the hexadecimal FFF is equal to the binary sequence , where F =

83 Table 4.1: Hexadecimal symbols and their corresponding binary numbers/sequences [42] Decimal Hexadecimal Binary A B C D E F 1111 In the case of the Inkshield, a twelve-digit binary number represents the firing pattern for all 12 nozzles. A zero means no drop is ejected and a 1 means a drop is ejected. Using the previous example, the hexadecimal FFF or binary represents all 12 nozzles ejecting a drop of ink. The Inkshield was programmed to eject drops in three different patterns in coordination with the x-y printing motion. More detail on this will be provided on section Heated Powder Bed In HSS, polymer material needs to be held at a sufficiently high temperature to allow for quick sintering IR exposure. The ideal heating method for polymer material in HSS is the same as SLS, which features many of the same processes and parameters. SLS machines use a heated 62

84 process chamber [43], in which the surface of the powder bed is heated by infrared heaters (Figure 4.11) and the powder bed is insulated to prevent heat loss in the powder bed. Figure 4.11: Diagram of an SLS heated processing chamber (copied from [43]) Building a heated process chamber is a complex and expensive process. It requires an enclosed environment with temperature feedback and automated temperature control. A faster and less complicated alternative for powder bed heating is to provide heat from the bottom an insulated print bed and eliminate the enclosure/temperature control system. A heated powder bed was designed and realized in the WSU-Vancouver machine shop, incorporating open-source designs [44][45]. The main components of the heated powder bed are listed below: mm by mm by mm (6 inch by 6 inch by 1/8 inch) aluminum 1100 plate [46] four aluminum housed 5 ohm resistors [47] 63

85 12 AWG copper wire [48] NEMA 17 stepper with leadscrew and nut [49] mm diameter peek rod [50] MDF frame/internal layers [51]. Aluminum 1100 was selected for its high thermal conductivity. This is ideal for transferring heat from the resistors to the polymer powder. MDF was selected because it is low cost and has low thermal conductivity. Figure 4.12 show a Solidworks labeled assembly of the powder bed components and the realized powder bed is shown in Figure Aluminum 1100 Plate MDF Frame 5Ω Resistor PEEK Rod MDF Frame Stepper Motor MDF Traveling Layer Leadscrew MDF Bottom Figure 4.12: Solidworks sketch of heated powder bed assembly (cross-sectional view) 64

86 Figure 4.13: Internal bed components (left) and MDF frame (right) The MDF frame (ride side of Figure 4.13) slides down over the internal bed components (left side of Figure 4.12). The middle MDF layer is referred to as the traveling layer because this is where the vertical movement is generated via a threaded nut. This threaded nut is press fit into the bottom side of the traveling MDF layer (Figure 4.14). 65

87 Figure 4.14: Threaded nut (outlined by red box) press fit into MDF layer When the stepper motor turns the leadscrew, the MDF frame prevents the internal components from rotating and drives the traveling layer up or down inside the frame. More detail on the circuit controlling the stepper motor for the powder bed will be provided in section 4.2. Figure 4.14 also shows the resistive circuit consisting of four resistors wired in parallel, mounted to the bottom face of the aluminum plate. To facilitate heat conduction to the plate from the resistors, silver heatsink paste [52] was applied to the sides of the resistors that are in contact with the bottom of the aluminum plate. The resistive heating circuit used in the powder bed was developed from an open-source design [53]. This design used twelve aluminum housed 1Ω resistors wired together as shown in Figure

88 Figure 4.15: Power resistor heat bed design by Kevin Smith (copied from [53]) There are four rows of three resistors, where each set of three is wired in series. Each of the four rows is wired in parallel with each other. Each row of three resistors has a total resistance of 3Ω (RRow,Series = 1Ω + 1Ω + 1Ω = 3Ω). The total resistance of the circuit is 0.75Ω (RTotal,Parallel,Circuit = [4(1/3 Ω)] -1 = 0.75Ω). This heated bed design was tested with a voltage supply of 12V. At 12V and a circuit resistance of 0.75Ω, the current draw is 16 Amps (I = V/R = 12/0.75 = 16). This heated bed design was reported to have reached 110 C at 12V. Using this design, a new power resistor circuit was developed to increase the resistance of the circuit and generate more heat from the resistors. This was realized by using four aluminum housed 5Ω resistors wired in parallel. The resistance of this new circuit is 1.25Ω (RTotal = [4(1/5Ω)] -1 = 1.25 Ω). To provide 12V to the resistive heating circuit a computer power supply (Figure 4.16) was implemented. This power supply has a +12V line and GND that were cut and soldered to the resistive circuit leads. 67

89 Figure 4.16: Raidmax RX-380K power supply (copied from [54]) The goal of this heated powder bed was to keep PA2200 powder above its crystallization temperature of approximately 138 C while remaining below its melting temperature of 186 C during operation. More details on PA2200 will be provided in section in section Infrared Lamp Hopkinson et al used a 2 kw IR lamp to sinter polymer material [23][24]. This power was too high for WSU laboratory safety regulatinos, so the infrared source used in this prototype system is a 1 kw halogen short wave heating lamp (Figure 4.17) [55]. The lamp has physical dimensions mm long and mm wide (18 in x 6 in). Power to the lamp is toggled manually via a 60-minute timer dial located at the top of the lamp. 68

90 Figure 4.17: Halogen SR-Spot infrared heating lamp (copied from [55]) Thermoplastic Powder The material used in this investigation was PA2200, a polyamide material in the Nylon 12 family, which was donated by Ultimate 3D, Hillsboro, OR. A selective laser sintering (SLS) manufacturing company. The donated powder material was overflow from SLS machine operation, meaning it is un-sintered powder pushed off during the leveling step for each layer. There are reported effects of aging in re-used overflow powder [56]. This material was used with the understanding that there could be some aging in the powder since it had been exposed to a heated SLS environment. The reported average particle size for the PA2200 powder is approximately 50 µm [57]. PA 2200 is a semi-crystalline thermoplastic polymer. Thus, this material has a melting temperature range, crystallization temperature range, and glass temperature range. Table 4.2 provides some of the thermal properties of PA2200. The glass transition is not precisely defined 69

91 for this material. The crystallization temperature is approximately 138ºC and the melting temperature is approximately 184ºC. Table 4.2: Thermal properties of PA2200 [58] Property Value Melting Temperature 184 C Crystallization Temperature 138 C Specific Heat Thermal Conductivity Ignition Temperature 2.35 J/gK W/mK >350 C Carbon Black Ink The HP C6602A inkjet cartridge comes with a dye based ink inside of it. This does not work in HSS because it does not contain carbon black pigment. Hopkinson et al showed that carbon black pigment is required for adequate IR absorption and thermal transfer in HSS [23][24]. Thus, ink from another inkjet cartridge (HP 21 black inkjet cartridge [59]) containing carbon black pigment was transplanted into the HP C6602A print-head. This was accomplished by first removing the top panel of both inkjet cartridges. Then the ink and sponge in the C6602A were discarded and replaced with the sponge and ink from the donor HP 21 cartridge. Once the carbon black ink was successfully transplanted, the lid was secured back onto the top of the C6602A cartridge duct tape. 70

92 4.1.8 Powder leveling Powder leveling in SLS machines is an automated process because it needs to be consistent for each layer. However, automation was not considered in this HSS system due to time and cost constraints. Powder leveling was performed manually using a solid mm diameter aluminum rod each layer. Leveling was performed by holding the rod firmly down to the top side of the MDF frame and sliding the rod across the top of the powder bed (Figure 4.16). Figure 4.18: Manual powder leveling The leveling rod needed to be kept above 140 C in a furnace, otherwise it would cool the powder surface during leveling. This required the use of thermally protective gloves for handling during leveling. 71

93 4.1.9 Auxiliary Infrared Lamp After some experimental testing, an additional heat source was introduced to heat the powder surface. This heat source is a 125-Watt incandescent heat lamp [60]. The auxiliary lamp was positioned directly over the powder bed, approximately 140 mm (5.5 inches) from the powder surface. This is the closest distance the lamp can be without obstructing the movement of the sintering lamp. The auxiliary lamp is held in position using a swing lamp arm [61] and can be adjusted to move the lamp closer to the powder surface when greater surface heating is required (Figure 4.19). Figure 4.19: Auxiliary heat lamp positioned above the powder bed 72

94 Furnace During HSS operation, PA 2200 powder needs to be added to the print bed for any multiple layer prints. The powder added to the bed for each layer is called feed powder. The feed powder needs be heated to the same temperature as the powder bed to prevent cooling of a previously sintered cross-section. Feed powder was pre-heated in a box furnace (Figure 4.20, left) [62] using an aluminum baking tray to hold the powder (Figure 4.20, right) [63]. Figure 4.20: Furnace (left) and aluminum baking tin with powder (right) The aluminum leveling rod was also placed in this furnace to keep it at the same temperature as the feed powder Ventilation To reduce the risk of explosion or fire, the assembly was placed under an exhaust hood to reduce the amount of airborne powder particles. 73

95 Figure 4.21: HSS assembly with ventilation hood highlighted by red box 4.2 Prototype Motion Control via Manually Activated Automated Routines A fully automated HSS system would have required a greater amount of time and money to realize. However, certain steps in the HSS process, like ink deposition and sintering, need to be automated. The implemented solution was a set of manually controlled automated routines executed via Arduino microcontrollers. This was used for inkjet deposition, sintering, and powder bed z-movement X-Y Inkjet Printing Control Circuit Some initial testing was performed to try and use the controlling circuit board that came with the x-y plotter. However, this proved to be difficult. The controlling circuit board included with the x-y plotter kit was removed and replaced with a control circuit using an Arduino Mega microcontroller [64]. The x-y plotter came with two stepper motors [65] that were implemented in x-y printing. These are rated for 12V and 1.7 amps per phase. The stepper motors are 74

96 controlled using an Arduino Mega and two DRV8825 stepper drivers [66]. Figure 4.22 shows an image of the Arduino Mega model used in this system. Figure 4.22: Arduino Mega 2560 (copied from [64]) The Arduino Mega is a larger version of the Arduino Uno featuring additional input/output pins. The DRV8825 stepper driver connects to a microcontroller, external 12V power supply, and stepper motor. Figure 4.23 shows a simplified wiring diagram for the DRV8825 stepper driver. 75

97 Figure 4.23: Minimal wiring diagram for the DRV8825 to a microcontroller and stepper motor (copied from [66]) The stepper driver requires power from the microcontroller to operate its logic chip. The external power source provides power to the stepper motor. Connecting the four wires to the stepper motor in the right order is important, otherwise the motor will not operate correctly. The realized x-y printing control circuit is shown in Figure 4.24 below. 76

98 Figure 4.24: X-Y motion control circuit consisting of an Arduino Mega and two DRV8825 stepper drivers The Arduino Mega is programmed to communicate with the stepper drivers through 15 of its input/output pins (7 pins for each stepper driver and a common 5V pin). The 5V pin on the Arduino Mega connects to both drivers. Two GROUND pins from the Arduino connect to the GROUND pin on each driver. This completes the connections required to power the logic chips on the stepper drivers. The following pins on the stepper drivers each connect to their own digital input/output pin on the Arduino: ENABLE, M0, M1, M2, M3, STEP, and DIR (direction). The ENABLE controls whether the motor moves or not. M0-M3 are used to generate micro-step resolutions for the stepper motor. There are six micro-step resolutions: 1/1, ½, ¼, 1/8, 1/16, 1/32. The benefits of micro-stepping are improved movement accuracy and reduced vibration/noise. External power 77

99 to both stepper motors is provided by a benchtop power supply [67]. This was programmed to limit power to 1.3 amps at 12 volts. Four micro switches [68] were included with the x-y plotter. These micro switches have two wires, a ground and a positive wire. These two wires connect to a ground and digital input pin on the Arduino Mega. When one of these switches is pressed, it sends a signal to the Arduino and triggers a programmed routine to execute. The switch controlling inkjet deposition is named the ink switch (Figure 4.25). When pressed, a programmed inkjet printing routine is triggered to execute. Figure 4.25: Micro-switch used to trigger inkjet deposition (red-box) 78

100 The inkjet printing routine moves the inkjet print-head from the zero position (Figure 4.26) to the print location and deposit ink. The inkjet carriage must be moved to the zero position manually each time. Left End-stop Front End-stop Figure 4.26: Zero position with inkjet carriage moved to left end-stop and front end-stop The end-stops are pieces from the original x-y plotter that are fixed in place using two screws and two nuts each. Close-up images of the front and left end-stops are shown in Figures 4.27 and 4.28 below. 79

101 Figure 4.27: Front end-stop Figure 4.28: Left end-stop 80

102 The Inkshield was programmed to fire the inkjet nozzles in a desired pattern and was treated as a switch in the controlling circuit. To be more specific, the Arduino Mega is programmed to send signals to the Inkshield during its x-y routine to control when the ink is deposited and when it is not. This allowed for the printing motion and ink deposition to be synchronized. The following hexadecimal parameters represent three drop ejection patterns used in this experimental study: FFF = (all nozzles), AAA = (every other nozzle), and 000 = (no nozzles). More detail on the x-y printing motion will be provided in section 5.3 when the specific ink print geometry is discussed Sintering Stroke A separate automation routine was used to move the IR lamp across the powder bed at a programmed speed. To do this, a third micro switch (Figure 4.29) was implemented and used to trigger movement of the cross-axis for an infrared sintering stroke. This switch (named IR switch) was connected to a digital input pin and ground on the Arduino Mega. 81

103 Figure 4.29: Micro-switch used to trigger sintering stroke In addition to pressing this switch for the sintering stroke, the operator must also turn the timer dial on top of the IR lamp to turn the lamp power on and off. Thus, the sintering step in this HSS system requires manual pressing of the IR switch and turning of the IR lamp dial Powder Bed Motion Control Circuit (Z-Axis) To move the level of the powder down by a layer thickness as small as 150 µm, a third micro-stepping driver and control circuit was required. A second Arduino Mega and a third DRV8825 driver were implemented to control the movement of the powder bed stepper motor (Figure 4.30). Adding a second Arduino Mega simplified wiring and power distribution to the third stepper motor and driver. 82

104 Figure 4.30: Second Arduino Mega and third DRV8825 stepper driver used for powder bed z- movement Two more micro switches (Figure 4.31) were used to control the powder bed z- movement. One switch (Z-down) triggers the Arduino circuit to move the bed level down by a single layer thickness. The other switch (Z-up) triggers the bed level to move up at a faster rate for quick re-adjustment. 83

105 Figure 4.31: Micro switches for z-movement down (red box on the left) and for z-movement up (red box on the right) 84

106 CHAPTER 5: EXPERIMENTAL METHODS 5.1 Experimental Methods on Ink Drop Volume and Firing Rate Measurement Ink droplets ejected from an inkjet are very small with drop volumes less than 200 pl and ink ejection speeds can be as high as 3 khz [38]. Due to the small size and fast rate of ejection, capturing images of ink droplets is difficult and requires specialized high speed imaging. This section introduces the experimental approach to measure ink drop volume and the firing rate High Speed Imaging Experimental Set-Up Two high-speed imaging set-ups were developed and realized by an undergraduate mechanical engineering student, Brett Merritt, at WSU Vancouver. The high speed imaging setups were used to capture images of ink droplets exiting an HP C6602A thermal inkjet print-head. The first set-up (Figure 5.1 and Figure 5.2) was used to capture images for drop volume measurements using a MATLAB edge detection program. The equipment used in this set-up included: a universal LED controller [69], Berkeley pulse delay generator [70], Thor-labs 340M- GE camera [71], and an HP C6602A thermal inkjet print-head controlled by the Arduino Inkshield. The 340M-GE camera and LED components can gather high resolution images due to precise shutter/exposure timing. 85

107 LED Controller Inkjet Print-head Delay Generator LED Thor-labs Camera Figure 5.1: Thor-labs camera set-up with fixed inkjet print-head and led aligned with the camera lens Figure 5.2: Alignment of LED and camera, perpendicular to inkjet nozzle array 86

108 A second high-speed imaging set-up featured a different camera (Phantom Miro 310 [72]) capable of faster frame rates with reduced resolution. The pulse delay generator and LED controller from the first imaging set-up were also replaced with an AmScope constant LED source [73]. This second imaging set-up was used for precisely measuring the firing rate of the inkjet, again using MATLAB. Figure 5.3: Phantom 310 camera imaging set-up with AmScope LED positioned on the opposite side of inkjet nozzle array from the camera lens Inkshield and Pulse Delay Settings The Inkshield was programmed to fire the HP C6602a print-head in an alternating every other nozzle pattern. This means every other nozzle out of 12 (6 total) fire once and alternate with the other 6 nozzles that didn t fire. In the Thor-labs camera set-up, the pulse delay generator was set to 47 pulses per second. Each pulse from the delay generator sends a signal to both the strobe LED and the camera, causing the strobe to flash and the camera shutter to open in synchronized timing. Thus, the Thor-labs camera captured images at a rate of 47 frames/second. 87

109 In the Phantom 310 camera set-up, the frame rate was set to 500,000 frames per second. The AmScope LED source was constantly on and set to approximately 1.5W power level Droplet Volume Measurements Images obtained via the Thor-labs high speed imaging set-up were cropped with a 0.01 mm microscope calibration scale [74] using ImageJ to set a global pixel distance for the images. A MATLAB program was developed by Huicong Jiang, a mechanical engineering graduate student at WSU Vancouver, to measure ink drop diameters and calculate the average drop volume. This program uses edge detection to find drops in each image frame by distinguishing between white and black pixels. Each image shows an array of drops at different stages of their firing events (staggered). A drop is measured when it reaches a specific stage after ejection. This stage is characterized by a large drop head (round), long tail (detached from the nozzle), and a series of droplets following the tail and head. All three of these constitute a single drop. The detected drop area is split into many slices, where each slice is a pixel thick. A single slice has a cross-sectional area that can be approximated as a circle and a volume that can be approximated as a short cylinder. These are added together to calculate a single droplet volume. All calculated drop volumes were then averaged to output the final average drop volume. Due to limitations of image resolution, a second set of volume data was measured with regularized edge detection. The purpose of this was to compensate for the lack of a sharply defined drop edge. The regularization was applied during the pixel slice diameter measurements. 0.5 pixels were subtracted from each pixel slice diameter and the drop volumes were calculated. 88

110 5.1.4 Drop Firing Rate Measurement Using High Speed Imaging Frame Rate Images collected via the Phantom 310 camera set-up were processed through a different Matlab edge detection program developed by Brett Merritt. Each image frame was cropped to show only a small window where drops from a single nozzle pass through. The images were converted to binary, where every pixel is either black or white. When converted to binary, each pixel has a value of 1 (white) or 0 (black). Next, the binary images were inverted so that a value of 1 is black and 0 is white. The black pixels were added for each frame and stored in Matlab. As drops enter and leave the cropped frame, the black pixel area values fluctuate. A black pixel area threshold was set. When this black pixel area threshold was exceeded, the number of frames between was calculated in Matlab. The frames between drop thresholds were converted to time (time between drop ejection) and averaged along with standard deviation. The ejection rate per second (Hz) was calculated from the measured time between drop ejections Ink Density Calculation The average drop volumes and firing frequency results were used to calculate the volume of ink deposited in a known surface area. At a known travel speed and ejection rate, the number of drops deposited in surface area can be calculated using Equations 2-5. Inkjet travel speed Distance traveled = Ink ejection time Equation 2 Ink ejection time Ejection rate = Total ink drops ejected Equation 3 Total ink drops ejected Average drop volume = Total ink volume Equation 4 Total ink volume Surface area = Ink volume per area Equation 5 89

111 5.2 Experimental Methods on Powder Bed Temperature Investigation The resistive heating circuit for the powder bed needed to be tested to confirm its temperature performance. A preliminary experimental testing stage was established to print PA2200 samples using the HSS prototype system Temperature Measurement Experimental Data Acquisition (DAQ) Set-Up The temperature of the aluminum 1100 plate was monitored over time as power was supplied to the resistive heating circuit mounted to the bottom side of the plate. Temperature data for the top of the aluminum print bed plate was acquired using a data acquisition (DAQ) NI USB-6251 board [75], an SCC-RTD01 module [76], resistive temperature detector (RTD) [77], and a signal receiver/conditioner (SC-2345) [78] at a collection rate of 10 data-points-persecond. Figure 5.4 shows this experimental set-up with the HSS prototype, DAQ system, laptop for data collection, and RTD placement. Figure 5.5 shows the precise location of the RTD sensor on the aluminum plate of the powder bed. 90

112 DAQ System RTD Sensor Lab Laptop Figure 5.4: DAQ set-up with 3-wire RTD placed on aluminum plate (red box) Figure 5.5: RTD placement secured with kapton tape 91

113 The RTD temperature sensor is a class A, 3-wire RTD (OMEGA SA1-RTD) with selfadhesive backing. The DAQ system set-up with LabVIEW can collect resistance changes in the RTD and convert these resistances to temperature values. The LabVIEW program used in this temperature study is shown in Figure 5.6. Figure 5.6: LabVIEW program to convert RTD resistance to temperature and write data to an Excel file PA2200 Temperature Gradient Measurement Procedure Two sets of temperature data for the aluminum plate were gathered using the DAQ system, LabVIEW program, and RTD set-up. One set of temperature data had no powder in the print bed and the other had 2.67 mm of powder filling the print bed Heated Powder Bed Temperature Regulation Set-Up and Procedure The temperature performance of the resistive heating circuit showed that it needed to be regulated during HSS operation. A second temperature study was performed using the same DAQ system, LabVIEW program, and RTD set-up on the aluminum plate. However, this study included IR sintering strokes, a 125 Watt surface heating lamp, surface temperature 92

114 measurements, and regulated resistive circuit power. This HSS set-up was the final operational system later used to print PA2200 specimens. The IR lamp sintering speed was set to mm/s and the powder depth was 2.53 mm (0.1 inches). The powder surface temperature was measured with an infrared thermometer (Extech [79]) as displayed in Figure 5.7. Figure 5.7: Non-contact temperature measurement of powder surface Non-contact temperature measurements were recorded manually every 30 seconds during pre-heat time. Once the plate or powder surface temperature was sufficiently high, the number of surface temperature measurements was increased to collect more data points during the sintering strokes. 93

115 Power to the resistive heating circuit was regulated by turning it off for 10 seconds and then back on for 1 minute. The power was turned off before each sintering stroke and back on 10 seconds later. Fourteen sintering strokes were performed and the time between sintering strokes was about 1 minute. 5.3 Experimental Methods on Single Layer Sintering Study Before printing multiple layer specimens, a single layer sintering study was performed to observe the effect of different ink densities and two deposition patterns on final sintered PA2200 layers. The temperatures of the aluminum plate and powder surface were adjusted based on the results from the temperature studies outlined in Ink Deposition Geometry The print geometry for this study was a rectangular pattern with dimensions: 50.8 mm length and 12.7 mm width (2 in x 0.5 in). The print swath of the 12-nozzle array is mm (0.125) [38]. Dividing the width of the rectangular pattern by the print swath gives the number of passes need to print the width of the geometry (12.7mm 3.175mm = 4). The Arduino in the x-y control circuit was programmed to move from the zero position to the print location and deposit four parallel ink swaths. Once the inkjet has traveled to the print location (5000 steps x- direction and 3000 steps y-direction), the Arduino tells the Inkshield to begin ejecting ink as the inkjet travels 50.8 mm (2 inches). Then the Inkshield is told to stop firing ink as the inkjet moves perpendicularly into position to print another ink swath next to the previous one. The inkjet then travels opposite of its first print direction as it prints ink again for another 50.8 mm (2 in). Again, 94

116 the inkjet stops ejecting ink as it moves perpendicularly for another print swath. The routine is then repeated a second time to realize the final print geometry Single Layer Specimen Production Procedure Five print speeds and two ink ejection patterns were used to print the rectangular geometry from The following was the procedure for preparing single layer specimens: 1. Set initial powder depth 2.26 mm (0.089 inches) 2. Deposit PA2200 powder into powder bed and level using aluminum rod 3. Position the auxiliary 125 Watt lamp directly over the powder bed, about mm (4 5 inches) from the surface 4. Turn on power to heated powder bed (Raidmax power supply) and auxiliary lamp 5. Measure surface temperature at or near print location with IR thermometer until target surface temperature of 140ºC is reached (above TCrystallization) 6. Manually move the inkjet carriage to the zero position (front and left end-stops) 7. Click ink switch to deposit ink pattern 8. Turn heated powder bed off via Raidmax power supply (10 seconds) 9. Position the sintering lamp on the near side of the powder bed (zero position) 10. Turn on sintering lamp power (via timer on top), click IR switch, turn off sinter lamp when movement is complete 11. Measure powder surface temperature as the powder bed cools 12. Remove specimen once surface temperature below 40 C 95

117 A sintering speed of 53.2 mm/s was used for all specimens, which translates to approximately 5 seconds of IR exposure. Table 5.1 shows the travel speeds tested and nozzle firing patterns. Table 5.1: Machine parameters for single layer sintering study Parameter Value Print Speeds 13.3, 19.95, 26.6, 33.25, 39.9 (mm/s) Sintering Speed (IR Exposure Time) 53.2 (mm/s) Target Powder Surface Temperature 140ºC Single Layer Specimen Image and Physical Property Collection Methods Images were collected for each printed geomtetry before and after sintering, using the 12 MP camera on an iphone 7 [80]. Images collected before sintering were evaluated for uniform ink coverage and deposition accuracy. Images collected after sintering were evaluated for uniformity of sintering. The thickness of each sample was measured using a Mitutoyo micrometer [81]. Each sample thickness was measured six times and averaged. The final sintered mass of each sample was also measured with a Symmetry PA 120 analytical balance [82], before and after excess sintered material was removed. 5.4 Experimental Methods on Two-Layer Rectangular Specimen Production The results from 5.2 and 5.3 were used to optimize the HSS system and operational procedure for printing multiple layers of PA2200. A multi-layer experiment was performed to generate nine two-layer rectangular specimens using the same print geometry as the single layer sintering study (section 5.3.1) Two-Layer Specimen Production Parameters The production parameters for the two-layer specimens are listed in Table 5.2 below. 96

118 Table 5.2: Two-layer specimen production parameters Parameter Value Layer Thickness 200 µm Print Speeds 26.6, 33.25, 39.9 (mm/s) Sintering Speed (IR Exposure Time) 46.55, 53.2, (mm/s) Target Powder Surface Temperature 150ºC Two-Layer Specimen Production Procedure The operational procedure for producing two-layer specimens required some preliminary set-up steps. The experiment preparation steps were: 1. Set initial powder depth mm (0.085 inches) 2. Deposit initial powder into powder bed and level using aluminum rod 3. Position the auxiliary 125 Watt lamp directly over the powder bed, about mm (4-5 inches) from the surface 4. Turn on power to furnace (set to 285ºF or C), heated powder bed (Raidmax power supply), and auxiliary lamp 5. Measure surface temperature at or near print location with IR thermometer until target surface temp of 150ºC is reached Once the powder surface temperature reached 150 C, the experiment procedure was initiated. The following was the experimental specimen production procedure: 1. Manually move the inkjet carriage to the zero position (front and left end-stops) 2. Click ink switch to deposit ink pattern 3. Turn heated powder bed off via Raidmax power supply (10 seconds) 4. Position the sintering lamp on the near side of the powder bed (zero position) 97

119 5. Turn on sintering lamp power (via timer on top), click IR switch, turn sintering lamp power off after movement is complete 6. Turn heated powder bed back on after sintering stroke (after 10 s) 7. Move the inkjet carriage to the left end-stop and push the cross-axis all the way to the back of the assembly 8. Position auxiliary lamp 3-4 inches from the printed pattern (move closer to powder surface than its passive position) 9. Click z-down switch for powder bed 10. Open furnace, take out feed powder, close furnace, sprinkle powder on near side of printed pattern 11. Place feed tray back into furnace, take the aluminum rod out of the furnace and level the powder bed, place back into furnace 12. Measure powder temperature on top of printed pattern with IR thermometer a. If temperature is between 145 C and 165 C, continue b. If temperature is below 145 C, move auxiliary lamp 2-3 inches from powder surface and wait 30 seconds while monitoring temperature with IR thermometer i. Once temperature is 150 C, move auxiliary lamp 4-5 inches above powder surface, turn power off, and continue c. If temperature is above 165 C, wait 20 seconds and continue 13. Repeat Specimen Quality Assessment Methods The HSS processed specimens quality was assessed in terms of the following parameters: mass, thickness, and surface roughness. The mass of each two-layer specimen was 98

120 measured and recorded using a Symmetry PA 120 analytical balance, both before and after excess sintered material was removed. The thickness of each specimen was measured using a Mitutoyo micrometer. A total of six thickness measurement were made for each specimen and the average thickness was calculated. Surface roughness was investigated using a Nikon MM-400 measurement system [83]. This optical microscope system can collect composite images consisting of multiple images taken at different focus depths. The distance between images (step) was set to 15 µm and each composite image consists of layers. Using NIS-Elements software, the surface topography of each sample was measured using the enhanced depth field function. This outputs the change in Z distance with respect to a specified direction that is set manually. An example of how the Z- profile data line was set for each composite image is shown for both x and y-directions in Figure 5.8 and 5.9 below. Each data point was subtracted from the average z height, resulting in surface roughness data with respect to the designated dimension. For each specimen, the z-profile was collected for both the x and y directions. 99

121 Figure 5.8: Example of x-direction Z-profile (blue line) Figure 5.9: Example of y-direction Z-profile (blue line) 100

122 CHAPTER 6: RESULTS & DISCUSSION 6.1 Ink Drop Measurements and Ink Density Calculation Ink Drop Volume Measured via MATLAB Edge Detection Program The edge detection program in MATLAB proved to be effective at detecting ink drop edges. Examples of the edge detection for a drop head and tail are shown in Figures 6.1 and 6.2 below. While the edge of the drop head in Figure 6.1 was detected, the resolution of the image captured by the Thor-labs camera makes the drop head edge poorly defined. This occurred for all measured drops. Figure 6.1: Example of MATLAB edge detection for drop head 101

123 Figure 6.2: Example of drop tail edge detection The distance of a single pixel in the Thor-labs images and both figures above is 3.43 µm. The diameter of an average drop head is approximately 61 µm. Using this value and approximating the drop head as a sphere, the volume of the drop head would be about picolitres. To show the sensitivity of volume calculations at this scale, assume the diameter of the drop head was a single pixel smaller (57.57 µm). Performing the same volume calculation produces a result of 100 pl. The difference of a single pixel when measuring the drop diameter results in a volume difference of roughly 20 pl or 20%. Thus, the volumes calculated from the MATLAB program are an approximation limited by the resolution of the image at such a small scale. 102

124 Figure 6.3: Final processed image frame showing the number of drops measured and average drop volume The average drop volume calculation (Figure 6.3) for ink drop images collected using the Thor-labs camera set-up is about 217 pl. Since it is clear the image resolution limits the accuracy of this calculation, the drop measurements were regularized to compensate. In this case, regularizing the measurements is simply removing 0.5 pixels from the measured drop diameter for each pixel slice. The average drop volume using this regularized diameter is about 182 pl (Figure 6.4). 103

125 Figure 6.4: Final processed image frame showing the number of drops measured and average drop volume for regularized measurements The drop volume results for this study are a range of pl. The reported drop volume for the C602A ink cartridge was 160 pl [38]. The difference in drop volumes is likely due to different ink viscosity. Black pigment based ink was transplanted into the C6602A ink cartridge, which comes with a different dye based ink inside it. The thermal inkjet ejects a drop of ink by forming a vapor bubble inside the print-head that can push ink through the nozzle orifice. Based on the difference in drop volume between the two inks, it can be logically concluded that ink composition affected the drop volume for the C6602A thermal inkjet. Changing ink composition changes the physical properties in a way that affects its behavior in the thermal inkjet. Existing research has shown that lower ink viscosity causes larger drop volume for a thermal inkjet [84]. 104

126 Since the experimental drop volumes are greater than the reported drop volume for the C6602A cartridge, the carbon black ink must have a lower viscosity than the dye based ink. However, the viscosity of both ink compositions should be measured and compared to confirm that this is true. Further study can be performed on ink viscosity and its effects on both ink deposition and the HSS process. Altering the composition of the pigment based ink would allow for an ideal ink recipe to be established and for HSS part customization. For example, adding more isopropyl alcohol would increase ink viscosity. This would result in a larger drop volume and effect the distribution of ink on a substrate. The interaction between picolitre sized ink drops and a powder substrate are not well understood. This is an area of research that should be studied to better understand how this interaction affects the HSS process. For example, the size of an ink drop may affect how it is absorbed by a powder substrate. This would affect ink penetration and thermal distribution during sintering. The absorption could also be affected by changing the powder particle size and geometry (uniform rather than non-uniform). The size and number of capillary voids (air) would change and result in different capillary forces pulling ink into the powder substrate Ink Drop Ejection Rate from a Single Nozzle Using Every-Other-Nozzle Ejection Pattern Three sets of images were collected using the Phantom 310 camera set-up. These images were separated into three videos. In each video, there are about 180 droplets. Table 6.1 shows the average drop ejection frequency for each set of images. 105

127 Table 6.1: Drop ejection frequency data from Phantom 310 high-speed imaging set-up Data Set Frequency (Hz) Standard Deviation (Hz) The standard deviation for the third data set is much greater than the other two because the nozzle failed to eject a drop once during imaging. Other than this outlier, the frequency data is reasonably consistent, with an average drop ejection frequency of Hz (drops per second). Figure 4.10 in section shows the theoretical ejection timing for the Inkshield, where a single nozzle ejects a drop approximately every 806 µs. This converts to a theoretical ejection frequency of 1240 Hz. Based on the experimental data, the Inkshield ejects droplets from a single nozzle at a lower frequency than predicted, about 301 Hz less. Blazincic et al [85] discusses the physics of the thermal inkjet. The ejection frequency of a thermal inkjet is dependent on how quickly the ink reservoir refills after drop ejection. This in turn depends on the reservoir geometry and ink s physical properties, such as viscosity. The theoretical ejection frequency for the Inkshield did not consider a change in ink composition. The experimental ejection frequency for the Inkshield shows that drop ejection frequency has likely been reduced due to the change in ink composition. This could also be due to an unknown error in the Inkshield. Further investigation is required to confirm the cause of reduced drop ejection frequency. 106

128 6.1.3 Ink Density Calculation The experimental HSS research for this project used the same print geometry, a 50.8 mm long by 12.7 mm wide (2 in x 0.5 in) rectangular pattern. The ink densities calculated here are for the surface area of this geometry. Based on the description of the inkjet printing routine in section 5.3.1, the distance traveled by the inkjet while it deposits ink is calculated to be mm (4 x 50.8 mm). Table 6.2 provides the solutions to equations 2-5 (section 5.1.5) for each print speed used in this experimental study and the every-other-nozzle ejection pattern. The solutions for number of drops is rounded to the nearest whole drop. Inkjet Print Speed (mm/s) Table 6.2: Ink density calculations using the solutions from equations 2-5 Total Ink Print Time (s) Number of Drops Ejected Total Ink Volume (nanoliter, nl) Ink Density (nl/mm 2 ) The total ink volume and ink density are calculated as a range of values due to the range of drop volumes ( pl) resulting from Figure 6.5 shows the relationship between the calculated ink densities and print speed. The low values of the ink density range and the high end of the ink density range are graphed as separate curves. 107

129 Ink Density (nl/mm 2 ) Effect of Print Speed on Ink Density Low Ink Density High Ink Density Print Speed (mm/s) Figure 6.5: Graph of low and high ink densities as a function of print speed Ink density decreases with increasing print speed at a non-linear rate. As print speed increases the ink density decreases at a slower rate. The range of ink density values at a given print speed appears to narrow with increasing print speed. Slower print speed results in greater total number of ink drops, which increases the effect of the drop volume range. These ink densities will be brought into further discussion in the following sections Summary of Drop Volume and Ink Density Study The high-speed imaging systems used in this study were effective at capturing images of inkjet droplets for measurement via MATLAB. The drop volume results show a larger experimental volume compared to the reported drop volume for the HP C6602A print-head. This is due to a change in ink composition when pigment-based ink was transplanted into the HP C6602A cartridge. Furthermore, the drop ejection frequency is lower than the frequency indicated by the Inkshield theory. Again, this can be explained by the change in ink. The Thor- 108

130 labs high-speed imaging system should be improved to acquire better resolution images and produce more precise drop volume calculations. Understanding the behavior of ink ejection is critical to the HSS process because it directly effects the printed part. Calculating the exact volume of ink deposited in each surface area allows for identification of how much thermal energy is absorbed and transferred per unit volume of ink. 6.2 Heated Powder Bed Temperature Study The process window for PA2200 is the range of temperatures above its crystallization temperature (138 C) and melting temperature (184 C). To maintain the powder temperature within this 46 C range, the temperature performance of the powder bed was investigated. To better characterize PA2200 in the HSS process, the heated bed system can be looked at in terms of heat transfer. Accurately modeling each heat transfer phenomenon in the HSS system would be very complex, so some assumptions are made to simplify the heat transfer in the system. The aluminum 1100 plate conducts heat to the PA2200 powder. In this case the PA2200 powder has an effective thermal conductivity which is based on the volumetric fraction of air and solid PA2200 material. Heat is transferred from the surface of the powder/air compact via conduction (PA2200 to air molecules) and convection (mass transfer of air molecules carrying heat energy away). The boundary temperatures of interest in the HSS powder bed are the aluminum plate- PA2200 boundary (plate temperature, TPlate) and PA2200-air boundary (surface temperature, TSurface). The thermal conductivity of PA2200 (Table 6.3) is low compared to aluminum In the HSS powder bed this means the PA2200 acts as an insulator for heat transferred from the 109

131 aluminum plate. It also means that TPlate will be greater than TSurface due to the temperature gradient that forms through an insulating material. Table 6.3: Thermal properties of materials in HSS heat transfer Material Thermal Conductivity Specific Heat (J/gK) (W/mK) PA C Aluminum From the thermal properties listed in Table 6.3, the thermal conductivity of air is about 4 times lower than PA2200. Therefore, the presence of air in the PA2200 powder substrate makes the effective thermal conductivity lower than that of bulk PA2200. Calculating the effective thermal conductivity in this case is complex. For example, the heat flow (Watts) is unknown in the aluminum 1100 plate. Furthermore, the ratio of solid PA2200 material to air is also unknown. For the purposes of this experimental system, the temperature gradient through a known depth of PA2200 is measured over time to adjust temperature control for printing multiple layers of PA Characterization of PA2200 Temperature Gradient The collected data is graphed with the calculated average temperature difference between the data curves (Figure 6.5). 110

132 Temperature ( C) Effect of PA2200 Powder on Plate Temperature Average ΔT: C Depth of PA2200: 2.67 mm No powder Time (minutes) Figure 6.5: Powder bed aluminum plate temperature study The temperature increases faster from 0-2 minutes compared to 2-9 minutes for both curves. From 0-2 minutes, the temperature of the plate with PA2200 powder increased at a slower rate than the plate without PA2200. This is due to the insulating effect of the polymer powder. The insulating effect of PA2200 powder is clear in Figure 6.5. On average, the plate temperature is approximately 14.3 C less with 2.67 mm (0.1 in) of PA2200 powder. It is concluded from this evidence that the depth of PA2200 effects the time required to heat the powder bed. The heat output from the resistive heating circuit without PA2200 powder continues to increase after 9 minutes. This indicates the power to the resistive circuit needs to be regulated to prevent continued increase of heat output. The temperature data for the curve with PA2200 stops at about 6.5 minutes because the LabVIEW program paused data acquisition during the 111

133 experiment. Since the temperature curve appears to be leveling out at where data stops, the experiment was not repeated. The temperature gradient will need to be controlled when maintaining the target powder surface temperature. This is more difficult when heating the powder bed from the bottom. As polymer powder is added to the print bed and the depth increases, the temperature gradient through the powder will increase. The temperature gradient can be reduced by heating the surface of the powder with another heat source Temperature Regulation with Surface Heating Lamp and Sintering Strokes The temperature data in Figure 6.6 shows that the temperature gradient through 2.67 mm of PA2200 powder grows as the temperature of the aluminum plate and powder increases. After 24 minutes, the plate and powder surface temperatures are about 174 C and133.9 C respectively. The temperature gradient is calculated using Equation 6. ΔT = TPlate TSurface = 174 C C = 40.5 C Equation 6 Even though the surface temperature was not at 138 C (TCrystallization), the plate temperature was sufficiently high that sintering strokes were initiated. This was a precaution to prevent melting during sintering. If the plate temperature was not regulated, it continued to increase in temperature until melting of PA2200 powder occurred throughout the powder bed (Figure A.18). 112

134 Regulated Heating With 125 Watt Auxiliary Lamp and mm/s Sintering Speed Temperature ( C) RTD Plate Temperature PA2200 Surface Temperature Time (Seconds) Figure 6.6: Measurement of boundary temperatures in powder bed during temperature regulation study The increases in temperature due to sintering are clear in Figure 6.6, as well as the effect of regulating the power to the resistive heating circuit. The plate temperature was successfully kept below the melting temperature of 184 C. To identify the temperature behavior at the plate and powder surface, an isolated data set from about 23 to 42 minutes is shown in Figure 6.7 below. 113

135 Temperature ( C) 190 Effect of Sintering on Powder Boundary Temperatures Time (Seconds) RTD Plate Temperature PA2200 Surface Temperature Figure 6.7: Temperature measurements during fourteen sintering strokes and temperature regulation The maximum plate and powder surface temperatures are C and C respectively. These maximum temperatures occur at different times. Maximum surface temperature occurs after the seventh sintering stroke (31 minutes), while the maximum plate temperature occurs after the last sintering stroke (38 minutes). Since the surface temperature was measured by hand with an IR thermometer, the temperature after each sintering stroke is slightly inaccurate as there was a short amount of time between the end of the sintering exposure and when the measurement was taken. The maximum surface temperature after the seventh sintering stoke is most likely due to a quicker temperature reading after sintering. The changes in temperature due to each sintering stroke are averaged in Table

136 Boundary Table 6.4: Boundary temperatures during sintering exposures Average Maximum Temperature ( C) Average Minimum Temperature ( C) Aluminum Plate Powder Surface Average ΔT after Sintering( C) The average ΔT due to sintering exposure indicates theat the powder bed temperature should be adjusted to maintain PA2200 in its process window while preventing melt at the plate where the temperature is highest. Since the plate temperature increases by 3.37 C after a sintering stroke, the plate temperature can be allowed to reach about 177 C without melt occurring because of sintering exposure. This would shift the temperature gradient such that the powder surface temperature would be higher. The average minimum powder surface temperature was below TCrystallization (138 C). To print multiple layer specimens this needs to be increased so that a sintered layer does not solidify before the print is complete Summary of Powder Bed Temperature Studies The depth of PA2200 powder in the print bed affects the temperature gradient that forms, in which the temperature at the aluminum plate is higher than the powder surface temperature. Sintering experiments show that the plate temperature can be allowed to reach 177 C without melt occurring after sintering exposure. The powder surface temperature needs to be maintained at a higher temperature to prevent solidification a sintered layer. 6.3 Single Layer HSS Study The Inkshield can be programmed to eject ink drops in a desired pattern, but it was unclear how to accomplish a greyscale deposition pattern. The goal of this single layer study was 115

137 to use two ink ejection patterns at five print speeds and evaluate which combination should be used for multiple layer specimen production As-Printed Specimen Geometry The images collected of each as-printed geometry are shown in Table 6.4. The presence of white uncovered lines through all printed geometries can be seen. Each time this occurred, there were three of these white lines. Table 6.4: As-printed ink geometries at five print speeds before sintering with 1/32in scale ( mm) above each image Print Speed (mm/s) 13.3 Alternating-Every-Other-Nozzle Pattern All-Nozzle Pattern

138 Since there are four nozzle swaths in each printed geometry, there are three boundary lines between ink swaths in each printed geometry. This indicates that the white uncovered lines form at these print swath boundaries. The white lines are not sharply defined, rather they appear to spread lightly onto the previous print swath. This suggests that the inkjet displaces PA2200 powder during deposition and pushes some of it over onto a previously printed ink swath. This happens for each subsequent pass of the inkjet. The extent to which this powder displacement occurs varies between each pattern and print speed. Comparing the two ink ejection patterns, the white uncovered powder lines are more prominent in the all-nozzle pattern. This pattern deposits more ink than the alternating-everyother-nozzle pattern at each print speed. This displaces more PA2200, forming larger uncovered white lines through the print geometry. Ink splatter was also observed in geometries printed with the all-nozzle pattern. This splatter is visible outside of the print geometry boundary. At print speeds and 39.9 mm/s this splatter was minimal. At print speeds 13.3, 19.95, and 26.6 mm/s the splatter is easily seen. Most of the splatter occurs at the end of the printed geometry where the inkjet moves into position to start another pass. For 13.3 and mm/s there is noticeable splatter along the length of the pattern as well. The print speed is slow enough in these cases that ink is being deposited onto already placed ink, causing a splashing effect that displaces ink outside of the desired area. The alternating-every-other nozzle pattern shows reduced powder displacement at all five print speeds. The shade of black in each pattern varies with print speed, where the slowest print speed (13.3 mm/s) had the darkest ink pattern and the fastest print speed (39.9 mm/s) had the 117

139 lightest. There is some ink splatter and uncovered white lines observed at the slower print speeds of 13.3 and mm/s, but this is minimal compared to the other all-nozzle ejection pattern Sintered Specimens Images of each ink geometry after sintering are shown in Table 6.5. The uncovered white lines observed in Table 6.4 remained after sintering IR exposure. Table 6.5: Sintered ink patterns with 1/32in scale ( mm) above each image Print Speed (mm/s) 13.3 Alternating-Every-Other-Nozzle Pattern (Sintered) All-Nozzle Pattern (Sintered) All printed geometries were sintered at the same speed (53.2 mm/s), so the variations between sintered specimens are due to the variations in ink deposition. The effect of the white line in the all-nozzle patterns is observed as an uneven surface where the ink lines were deeper 118

140 than the white lines, forming furrows in the layer. For additional layer adhesion, this would not work well. The alternating-every-other-nozzle pattern shows better uniformity of sintering compared to the all-nozzle pattern. The white lines that were most prominent in 13.3 and mm/s print speeds remained after sintering. The print speed showing the most uniform ink coverage and sintering is 39.9 mm/s. The three fastest print speeds and the alternating-every-other-nozzle pattern exhibit the best sintering results out of all single layer specimens Sintered Single Layer Specimen Mass After sintering exposure, all single layer specimens had excess white PA2200 powder partially sintered to their sides and bottom. The mass of each sintered specimen was measured before and after removing this excess white powder. The mass of single layer specimens after removing excess PA2200 is shows in Figure 119

141 Mass (g) Specimen Mass After Excess White Mass Removal Alternate Every Other Nozzle All Nozzle Print Speed (mm/s) Figure 6.8: Effect of print speed on black sintered mass The effect of print speed on the black sintered mass of single layer specimens is unclear from the data in Figure 6.8. Some trends can be observed but there ware outliers in both data curves. The alternating-every-other-nozzle pattern shows a general trend of decreasing sintered mass with increasing print speed. The mass at 26.6 mm/s print speed is an outlier in this case because it has less mass than the other four specimens. The all-nozzle pattern shows a decrease in sintered mass when comparing the slowest and fastest print speeds. However, print speed mm/s is greater in mass than 13.3 mm/s print speed. The mass at mm/s is less than 39.9 mm/s so it also breaks the expected trend. The mass of PA2200 removed from single layer specimens in shown in Figure

142 Mass (g) Excess White Mass Removed Alternate Every Other All Nozzle Print Speed (mm/s) Figure 6.9: Effect of print speed on excess sintered mass The effect of print speed on the mass of excess sintered PA2200 for single layer specimens isn t clear for the all-nozzle pattern in Figure 6.9. For the alternating-every-othernozzle pattern there is a trend of increasing sintered mass with print speed. However, this is counterintuitive to what would be predicted. Increasing print speed correlates to decreasing ink density and less thermal energy absorption, so there should be a decrease in sintered mass with increasing print speed. The data in Figures 6.8 and 6.9 is inconsistent and difficult to characterize. The variation in specimen mass and excess sintered PA2200 mass is most likely due to variations in the powder bed temperature. If the plate temperature was slightly different between specimens, then the transfer of heat during sintering would be slightly different. 121

143 Thickness (mm) Sintered Specimen Thickness Single layer thickness indicates the extent of ink penetration into the PA2200 powder substrate. The thickness data for each single layer specimen is shown in Figure 6.10 below. The thickness of single layer specimens ranges from 0.35 mm to 0.45 mm. The all-nozzle pattern shows inconsistent trends due to outliers at and 26.6 mm/s print speeds. There is a decrease in specimen thickness from 13.3 to 39.9 mm/s but more data should be gathered to confirm this trend Effect of Print Speed and Ink Ejection Pattern on Specimen Thickness Alternate Every Other Nozzle All Nozzle Print Speed (mm/s) Figure 6.10: Effect of print speed and ink ejection pattern on single layer specimen thickness 122

144 The alternating-every-other nozzle pattern also has two outliers, in this case at print speeds of 26.6 and mm/s. There is a decrease in specimen thickness from the slowest to fastest print speed but more data should be acquired to confirm the validity of this trend Summary of Single Layer Study The alternating-every-other-nozzle pattern is the ideal drop ejection pattern because it causes the least powder displacement and ink splatter, while also having more uniform coverage of the powder substrate. This directly translates to uniform sintering. Among the print speeds tested, the print speeds that showed the best sintering results were 26.6, 33.25, and 39.9 mm/s. 6.5 Two-Layer HSS Study The ideal ink drop ejection pattern and print speeds identified in 6.4 were used to print two-layer specimens. Additional sintering speeds were introduced to vary IR sintering exposure Two-layer Specimen Mass The mass of two-layer specimens indicates how much thermal energy was absorbed and distributed during specimen production. With the additional of a second layer of ink and PA2200, there is a greater amount of absorbed thermal energy. The mass of each specimen was measured before and after removing excess PA2200. Figure 6.11 shows the mass data after excess powder removal. The removed powder and sintered black layer are referred to as the white mass and black mass respectively. 123

145 Mass (g) Effect of Print Speed and Sinter Speed on Specimen Mass After Excess Powder Removal mm/s 53.2 mm/s Print Speed (mm/s) Figure 6.11: Effect of print speed and sintering speed on sintered black mass There is a decrease in mass with increasing print speed for both and 53.2 mm/s sinter speeds. All masses for the 53.2 mm/s sinter speed were less than the corresponding values for mm/s sinter speed. This indicates a trend of decreasing mass with increasing sinter speed. The third sintering speed (59.85 mm/s) is an outlier in Figure At mm/s print speed, the mass is greater than the mass at 26.6 mm/s print speed. This does not follow the trend of decreasing mass with increasing print speed. In addition, the mass at and 39.9 mm/s prints speed are greater than the same mass data points for the 53.2 mm/s sinter line. This does not follow the trend of decreasing mass with increasing sinter speed. 124

146 Mass (g) Figure 6.12 shows the mass of PA2200 removed from each specimen. Looking at the slowest and fastest print speeds for each sinter speed, there is an increase in mass removed with increasing print speed. Effect of Print Speed and Sinter Speed on Excess Mass Removed mm/s 53.2 mm/s mm/s Print Speed (mm/s) Figure 6.12: Effect of print and sinter speed on excess white mass In general, the trends in Figure 6.12 are difficult to decipher because there is no consistency among the excess mass data. The inconsistency observed in the mass of excess sintered PA2200 is most likely due to inconsistent powder bed temperature Two-layer Specimen Thickness The thickness of two-layer specimens indicates how two sintered PA2200 layer fuse together. Since the second layer thickness was set to 0.2 mm (200 µm) and the single layer thicknesses were greater than 0.3 (300 µm), the total thickness of the two-layer specimens is expected to be greater than 0.5 mm (500 µm). As shown on Figure 6.13 with the thickness data 125

147 Thickness (mm) collected for two-layer specimens. Most process parameters do not produce the desired 0.5 mm thickness except the combination of 39.9 mm/s print speed and mm/s sinter speed. The sinter speed of mm/s results in undersized thickness, ranging from 0.47 mm to 0.43 mm. The increase of specimen thickness with sintering speed does not make sense. The specimens should get thinner as sintering speed increases. This implies that the powder bed temperature is not the same for each specimen. If the powder bed is a few degrees greater in temperature than intended, the thermal energy during sintering could transfer deeper into the powder and cause a thicker specimen result. Effect of Print Speed and Sinter Speed on Specimen Thickness mm/s 53.2 mm/s mm/s Print Speed (mm/s) Figure 6.13: Thickness measurement data for two-layer sintered specimens Like the mass data, the thickness data for two-layer specimens shows few clear trends (Figure 6.13). For all three sinter speeds, the thickness of specimens at the slowest print speed 126

148 (26.6 mm/s) is greater than at the fastest print speed (39.9 mm/s). Overall, the thickness of twolayer specimens ranges from 0.35 mm (350 µm) to 0.64 mm (640 µm). The three specimens at mm/s sinter speed are well below the expected thickness, approximately 0.2 mm thinner. This could be an error during z-movement where the powder bed level did not move the full layer thickness. Inconsistent powder bed temperature is most likely the reason why the thickness data is not consistent Two-layer Specimen Surface Roughness via Optical Measuring Microscope The surface roughness of two-layer specimens is important to understanding how HSS process parameters such as printing speed and sintering speed alters the surface quality of each specimen. For example, specimens printed with greater ink density and slower sintering speed absorb more thermal energy. Both x and y-direction surface variation were measured for each specimen. The x-direction is parallel to the ink deposition direction. The y-direction is perpendicular to the ink deposition direction. Figures 6.14 and 6.15 show the surface roughness data for specimens produced with mm/s sintering speed. The x-direction surface variation for 39.9 mm/s print speed has greater frequency of peaks and valleys (about every 0.25 mm) compared to the other two print speeds. The peaks and valleys observed at this print speed are sharp with amplitudes of approximately 25 µm. This suggests that less sintering occurred because of lower ink density at 39.9 mm/s print speed. The frequencies of peaks and valleys for the other two print speeds (26.6 mm/s and mm/s) are similarly irregular. 127

149 Z Height (µm) 100 Surface Variation in the X-Direction for mm/s Sinter Speed X Distance (mm) Print Speed: 26.6 mm/s Print Speed: mm/s Print Speed: 39.9 mm/s Figure 6.14: Specimen surface variation parallel to ink deposition direction for mm/s sinter speed Comparing Figures 6.14 and 6.15, the y-direction shows greater variation frequency and amplitude at mm/s print speed. The 26.6 print speed is similar in roughness for both x and y-directions. This could be a result of greater print density, which should absorb greater thermal energy during sintering. Greater degree of sintering reduces the difference in roughness between x and y-direction. The amplitude of roughness in the y-direction appears greater compared to the x-direction at the mm/s sinter speed. 128

150 Z Height (µm) 100 Surface Variation in Y-Direction for mm/s Sinter Speed Y Distance (mm) Print Speed: 26.6 mm/s Print Speed: mm/s Print Speed: 39.9 mm/s Figure 6.15: Specimen surface variation perpendicular to ink deposition direction for mm/s sinter speed Surface variation data for specimen produced with 53.2 mm/s sintering speed is shown in Figures 6.16 and 6.17 below. Overall the surface roughness for all print speeds appears greater in frequency compared to the slower sintering speed of mm/s. This can be explained by the a decrease in radiative thermal energy, which causes less sintering to occur and great surface variation frequency. 129

151 Z Height (µm) 100 Surface Variation in the X-Direction at 53.2 mm/s Sinter Speed X Distance (mm) Print Speed: 26.6 mm/s Print Speed: mm/s Print Speed: 39.9 mm/s Figure 6.16: Specimen surface variation parallel to ink deposition direction for 53.2 mm/s sinter speed The slowest print speed (26.6 mm/s) shows the most frequent variation (about 0.2 mm between peaks and valleys) compared to the faster print speeds. This is true for both x and y- directions. A deep valley occurs at 1.75 mm (26.6 mm/s print speed), which is most likely a defect in the surface. The y-direction data in Figure 6.17 shows greater peak and valley amplitudes compared to the x-direction (Figure 6.16). The valleys could correspond to spaces in between ink nozzle lines in the specimen. Optical microscopy might be able to confirm this. 130

152 Z Height (µm) Surface Variation in Y-Direction for 53.2 mm/s Sinter Speed Print Speed: 26.6 mm/s Print Speed: mm/s Print Speed: 39.9 mm/s Y Distance (mm) Figure 6.17: Specimen surface variation perpendicular to ink deposition direction for 53.2 mm/s sinter speed The prints speed showing the highest surface variation frequency in the y-direction is 26.6 mm/s. Ink density is greater at this speed out of the three, but it is unclear how this affects the variation frequency in this case. The third set of x and y-direction surface variation data at a sintering speed of mm/s is shown in Figures 6.18 and 6.19 below. Compared to the two slower sintering speeds, the surface variation at mm/s sinter speed is greater in both x and y-directions. This follows the trend of increasing surface variation with increasing sinter speed. 131

153 Z Height (µm) Surface Variation in the X-Direction for mm/s Sinter Speed Print Speed: 26.6 mm/s Print Speed: mm/s Print Speed: 39.9 mm/s X Distance (mm) Figure 6.18: Specimen surface variation parallel to ink deposition direction for mm/s sinter speed The surface variation for 39.9 mm/s print speed shows more frequency of variation compared to the two slower print speeds. This is more pronounced in the y-direction (Figure 6.19). The two slowest print speeds are very similar in frequency and amplitude. This applies to both the x and y-direction data. Since mm/s is the fastest sinter speed and the surface roughness of specimens produced at this sinter speed show greater variation frequency and amplitude, the amount degree of sintering that occurred is likely less compared to the specimens produced with slower sinter speeds. 132

154 Z Height (µm) Surface Variation in Y-Direction for mm/s Sinter Speed Print Speed: 26.6 mm/s Print Speed: mm/s Print Speed: 39.9 mm/s Y Distance (mm) Figure 6.19: Specimen surface variation perpendicular to ink deposition direction for mm/s sinter speed The effect of print speed on the surface variation in Figures 6.18 and 6.19 is not clear because all three specimens show high variation compared to the other specimens discussed previously. The average x-direction surface variation is calculated by finding the absolute value of each z-height and then the average (Figure 6.20). The average surface variation is represented by the symbol Ra. 133

155 Average Surface Variation in the X-Direction R a 10 5 Sinter Speed: mm/s Sinter Speed: 53.2 mm/s Sinter Speed: mm/s Print Speed (mm/s) Figure 6.20: Effect of print speed and sinter speed on mean surface variation parallel to ink deposition direction Figure 6.20 shows an increase in Ra with increasing sinter speed. This trend is also observed in the surface roughness data. For both and mm/s sinter speeds, these is a decrease in Ra with increasing print speed. However, there is an outlier at mm/s print speed where Ra is lower than the 39.9 mm/s print speed. For 53.2 mm/s sinter speed the values of Ra are about the same for each print speed. The max z-height variation in the x-direction is calculated by subtracting the minimum z- height from the maximum z-height (Figure 6.21). 134

156 Maximum Variation Height in X Direction R y Sinter Speed: mm/s Sinter Speed: 53.2 mm/s Sinter Speed: mm/s Print Speed (mm/s) Figure 6.21: Effect of print speed and sinter speed on max variation height parallel to ink deposition direction There is in increase in Ry with increasing sinter speed in Figure This was observed in the surface variation data as well. At mm/s sinter speed the values of Ry are nearly the same for each print speed. For both 53.2 and mm/s sinter speeds there is a decrease in Ry with increasing print speed. However, at mm/s print speed the values of Ry do not follow this trend and are less than Ry at 39.9 mm/s print speed. Average surface variation in the y-direction is displayed in Figure This data is less clear because of the 53.2 mm/s sinter line, which does not follow the trend of increasing surface variation with sinter speed. At 26.6 mm/s print speed the variation is greater than all other data points. 135

157 Average Surface Variation in Y Direction R a Sinter Speed: mm/s 5 Sinter Speed: 53.2 mm/s Sinter Speed: mm/s Print Speed (mm/s) Figure 6.22: Effect of print speed and sinter speed on mean surface variation perpendicular to ink deposition direction There is an increase in Ra with increasing print speed from the slowest to fastest print speeds for both and mm/s sinter speeds. This could mean that with less ink and faster sintering exposure, there is less thermal energy absorbed and the ink lines did not spread across the surface as much. Maximum surface variation in the y-direction is shown in Figure Again, the 53.2 mm/s sinter speed is an outlier in this data because it breaks the expected trend that was observed in the average variation data. Additional specimens should be produced to confirm whether this sinter speed is an outlier or not. 136

158 Maximum Variation Height in Y Direction R y Sinter Speed: mm/s Sinter Speed: 53.2 mm/s Sinter Speed: mm/s Print Speed (mm/s) Figure 6.23: Effect of print speed and sinter speed on max variation height perpendicular to ink deposition direction From to mm/s sinter speeds there is an increase in Ry for all print speeds. The 53.2 mm/s sinter speed has greater Ry than the other two curves at 26.6 and mm/s prints speed. The surface variation data has shown so trends among the produced specimens. Generally, this is an increase in surface variation and amplitude with sintering speed. The effect of print speed is less clear, but the slowest print speed (highest ink density) shows less variation on average. Looking at optical microscope images of the two-layer specimen s surfaces should provide additional evidence confirming or disproving these trends. Figures show three-dimensional surface topographies of the specimen surfaces obtained from the Nikon optical measuring microscope. 137

159 At the three print speeds shown in Figures , the specimen surfaces show no ink lines. There appears to be less surface variation in the 26.6 mm/s print speed specimen. Figure 6.24: Three-dimensional surface topography of specimen produced with 26.6 mm/s print speed and mm/s sinter speed Figure 6.25: Three-dimensional surface topography of specimen produced with mm/s print speed and mm/s sinter speed 138

160 Figure 6.26: Three-dimensional surface topography of specimen produced with 39.9 mm/s print speed and mm/s sinter speed At 53.2 mm/s sinter speed (Figures ) the ink lines are visible at all three print speeds. These ink lines are likely due to limitations a limitation the Inkshield and C6602A printhead to uniformly cover a substrate with ink. A more robust inkjet print-head and controlling system may be required to solve this issue. Figure 6.27: Three-dimensional surface topography of specimen produced with 26.6 mm/s print speed and 53.2 mm/s sinter speed Ink distribution after sintering is seen at mm/s print speed. This is when the ink lines spread out and are less defined. The three-dimensional topography (Figure 6.28) smoother with less variation across the surface. 139

161 Figure 6.28: Three-dimensional surface topography of specimen produced with mm/s print speed and 53.2 mm/s sinter speed Figure 6.29: Three-dimensional surface topography of specimen produced with 39.9 mm/s print speed and 53.2 mm/s sinter speed The fastest sinter speed (59.85 mm/s) shows the greatest surface variation. Figures show that ink lines are not well distributed and this is likely due to less sintering. It is clear the surface variation for these specimens is greater than specimens produced with lower sinter speeds. 140

162 Figure 6.30: Three-dimensional surface topography of specimen produced with 26.6 mm/s print speed and mm/s sinter speed Figure 6.31: Three-dimensional surface topography of specimen produced with mm/s print speed and mm/s sinter speed 141

163 Figure 6.32: Three-dimensional surface topography of specimen produced with 39.9 mm/s print speed and mm/s sinter speed Summary of Two-Layer Specimen Study Two-layer specimens were produced using the HSS prototype system, but the mass and thickness data showed that inconsistent powder bed temperature may have caused irregular sintering results for layer-to-layer fusion. Surface roughness data indicates that as sinter speed is increased, the surface variation also increases. Three-dimensional surface images confirmed that as sinter speed increased, ink lines did not spread across the specimen surface during sintering. Sintering speed appears to have the most prominent effect on sintering in HSS. 6.6 HSS Prototype Evaluation and Future Design Recommendations HSS Prototype Evaluation As an experimental system for learning about HSS and building foundational understanding of its parameters, the HSS prototype system was a success. However, true additive manufacturing requires the ability to print many layers of material into a three-dimensional geometry. The HSS prototype system was unable to produce consistent, multiple layer PA2200 specimens. This was most likely due to poor powder bed temperature control. The depth of 142

164 PA2200 powder was about 2.6 mm (0.1 inch). At this shallow of depth, the printed geometry is more sensitive to differences in the aluminum plate temperature. Using more powder was not possible due to the temperature gradient that forms through the PA2200 powder. The primary heat source is from the bottom of the powder bed. The surface of the powder is difficult to control using this set-up. There is considerable heat loss in the current HSS prototype system that causes warping of sintered layers. When the powder bed is set to output more heat to compensate for this heat loss, melting of PA2200 throughout the powder bed occurs during HSS operation. Another source of inconsistency is the manual aspects of the current HSS prototype. Transferring feed powder from the furnace to the powder bed is a source of inconsistency because the feed powder loses heat during this process. Powder leveling can be a sensitive step in HSS, especially when the temperature of the printed part and powder bed is not high enough. When this occurs, the printed geometry is easily displaced on the powder surface during leveling because it has solidified. It should be noted that some of the difficulty in temperature control for the HSS prototype system had to do with the material being used. PA2200 has higher processing temperatures and a narrow processing window due to its crystallization temperature. There are other polymer materials, such as PMMA (polymethylmethacrylate), that have lower processing temperatures and do not crystallize. These materials may be easier to process with the current HSS system. 143

165 6.6.2 Future Design Recommendations Based on the evaluation of the current HSS prototype system, there are several key modifications that can be realized to improve its ability to print consistent, multiple layer specimens with a variety of polymer powder materials. To improve the temperature control of the system, the heat loss should be minimized. Adding powder beds with their own heating elements to serve as feed beds for the main print bed would reduce the heat loss during powder transfer and leveling. Furthermore, an enclosure for the system would reduce heat loss and increase the air temperature above the powder surfaces. Automating the HSS system is important for consistent results. One aspect of automation would be temperature control. Ideally, the temperature of the HSS system should be monitored during operation and used to control the heat input to the system. If the temperature is too low, then the heat input is increased and vice versa. This could be accomplished using IR temperature sensors, thermistors (or RTD), and microcontrollers. The microcontroller could control power to heating elements in the system via power relays, based on the input from the temperature sensors. Using both IR temperature sensors and physical contact temperature sensors (thermistor, RTD) allows the system to monitor both the powder surface temperatures and internal bed temperatures. Another aspect of automation is ink deposition. A more robust inkjet system is required for complex print geometries and consistent ink deposition. Having controlling system and software that can do this would allow for precise adjustment of the print geometry and ink distribution on the powder substrate. 144

166 CHAPTER 7: CONCLUSIONS The purpose of this study was to develop a high-speed sintering (HSS) prototype system and perform experimental study on HSS parameters. The volume and frequency of ink drops ejected from an HP C6602A thermal inkjet were investigated and used to calculate ink density in units of nl/mm 2. The temperature performance of the resistive heating circuit for the powder bed was studied and evaluated for multiple layer specimen production. One and two-layer specimens were produced using the HSS prototype system and their physical properties were analyzed. The conclusions drawn from the results of these investigations are as follows: 1. The volume and frequency of ink droplets ejected from a thermal inkjet print-head is affected by the physical properties of the ink inside it. 2. Comparing the all-nozzle and alternating-every-other-nozzle patterns shows that uniform ink coverage generates better sintering uniformity rather than ink density. In other words, it is more important how the ink is distributed rather than how much. 3. One and two-layer PA2200 specimens show some inconsistencies in mass and thickness. This is most likely a result of inconsistent powder bed temperature. Further investigation of powder bed temperature and its impact on the mass and thickness of sintered PA2200 specimens is required. 4. Sintering speed appears to have a direct effect on the surface roughness of sintered PA2200 specimens. The surface roughness increased with increasing IR sinter travel speed. This translates to less IR radiation exposure and less sintering of PA2200. Further study of the degree of sintering in PA2200 specimens is required. 145

167 5. The HSS prototype system can be improved to increase the consistency of specimen production by automating ink deposition, powder leveling, and temperature control. Furthermore, reducing heat loss in the system is critical to maintaining target processing temperatures. 146

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178 APPENDIX Table A.1: Full list of factors in Hopkinson et al cost analysis (copied from [20]) 157

179 Table A.2: Mechanical property data reported by Ellis et al for tensile specimens fabricated via SLS and HSS [25] Material Process Young s Modulus EaB (%) UTS (MPa) (MPa) DuraformPA HSS SLS DuraformHST HSS SLS TPE 210-S HSS Approximately 265 Not Reported 8100 SLS Approximately Not Reported Figure A.1: Ultimate Tensile Strength vs Greyscale (copied from [30]) 158

180 Figure A.2: Elongation at Break vs Greyscale (copied from [30]) Figure A.3: Young s Modulus vs Greyscale [30] 159

181 Figure A.4: Density vs Greyscale (copied from [30]) Figure A.5: Density vs Greyscale (copied from [31]) 160

182 Figure A.6: UTS vs Greyscale (copied from [31]) Figure A.7: Young s Modulus vs Greyscale (copied from [31]) 161

183 Figure A.8: EaB vs Greyscale (copied from [31]) Figure A.9: Effect of IR lamp speed on EaB at two bed temperatures (copied from [32]) 162

184 Figure A.10: Effect of lamp speed on UTS at two bed temperatures (copied from [32]) Figure A.11: Effect of lamp speed on Young s Modulus at two bed temperatures (copied from [32]) 163

185 Figure A.12: Effect of bed temperature on EaB at two lamp speeds (copied from [32]) Figure A.13: Effect of bed temperature on UTS at two lamp speeds (copied from [32]) 164

186 Figure A.14: Effect of bed temperature on Young s Modulus at two lamp speeds (copied from [32]) Figure A.15: Effect of flow agent on EaB at different lamp speeds and bed temperatures (copied from [32]) 165

187 Figure A.16: Effect of flow agent on UTS at different lamp speeds and bed temperatures (copied from [32]) Figure A.17: Effect of flow agent on Young s Modulus at different lamp speeds and bed temperatures (copied from [32]) 166

188 Figure A.18: Unwanted melting of PA2200 due to poor plate temperature regulation 167