Selective Inhibition Sintering: Advancements & Systematic Research approaches for the process development

Transcription

1 Selective Inhibition Sintering: Advancements & Systematic approaches for the process development Sagar M Baligidad 1, K Elangovan 2 and S Gopi 3 1 scholar, Visvesvaraya Technological University, Belgaum, Karnataka, India. 2 head and Associate Professor, Cambridge Institute of technology, Bangalore, Karnataka, India. 3 Associate Professor, CMR Institute of Technology, Bangalore, Karnataka, India. Abstract SIS system is a new emerging Rapid Prototyping (RP) technique. Commercial rapid prototyping process like STL and electron beam melting produces the parts by infusion of laser energy. But in SIS process, parts are built by fusion of polymer powder particles in the part s body and preventing joining powder particles at the part boundary. Fundamental of SIS processes were investigated by several researchers. The polymer based SIS process has numerous advantages over the existing RP techniques. This paper discusses the major factor encounter in the development of polymer based SIS process, material selection, optimization of various input parameters, software and hardware design and development, fanatical path generation, application of response surface methodology which has led to optimum performance, factors affecting surface quality, strength of part and dimensional accuracy. Keywords: Rapid prototyping, NC path generation, selective inhibition sintering, powder metallurgy and material selection etc. INTRODUCTION Since 1970 several rapid prototyping techniques were developed, some of which are commercialized and have directly being used in medical application, manufacturing industries and even in art. SLA (Sterolithography), SLS (Selective Laser Sintering), FDM (Fused Deposition Modeling) and LOM (Laminated Object Manufacturing) are the best known commercial RP system. In recent times, research on powder based Additive Manufacturing (AM). New approaches SIS have been developed to produce parts of high quality with polymers, ceramics and metals. SIS is a new emerging AM technique in which parts are built by joining polymer powder particles through sintering in the part s body and by sintering inhibition at selected powder areas at the part boundary. Sintering inhibition at the part boundary is performed by means of wetting the selected powder. In SIS process, indigenously available sintering inhibitors like potassium iodide, salt and other high temperature materials are spread on the first layer based upon the NC tool path generation from a CAD data. The inhibitors are sprayed on the periphery or outer edge of the component under consideration. Inhibitor at the edges of polymer Polymer surface Each layer will be sintered (applying temperature below melting point uniformly) and subsequently cooled for some time. Then, another layer of polymer will be adding and inhibitor will take place. Continuous heating and cooling process occurs and the process continues until the computer sends the NC code generated for the specific CAD part to the nozzle system to manufacture the part. Each RP system has its own way of material processing technique; they use different polymer material to develop physical parts, because each polymer materials possess different properties. Process development of each RP system revealed a set of factors essential to develop functional system. A set of interdisciplinary approaches are required to develop each RP system because of the interplay between activities and simultaneous study. Therefore several series of trials were studies and conducted simultaneously. A laboratory level machine was build to do initial research studies (figure 1). The machine includes the following parts. 1. Source tank unit is required to store fine powder of thermoplastic materials. 2. Builder tank unit is required, on which building of model takes place by receiving the powder from source tank. 3. Collector tank unit: the doctor blade pushes the excess powder to the collector tank from the build tank. 4. Layer thickness adjuster / Z movement unit: By providing appropriate values in layers and thickness the stepper motor moves the powder present in the source tank. At the same time the build tank will moves downwards to build the powder. The amount lowering movement sets the layer thickness in the final part. 5. Inhibitor dispenser system and carrier unit / X-Y movement unit: Dispenser carrier unit will moves X-Y direction at a time to create the required profile at Interpolation speed. The barrel and the hosepipe are connected to the control unit, which sprays liquid for inhibition on the plastic powder. 6. Dispenser control panel: It controls the amount of liquid used for inhibiting at the required pressure. By using pressure setter and pressure adjuster knobs, adjust the required air pressure to get a well defined profile in build tank at standard speed. 7. Material spreader / doctor blade and carrier: This will spreads the powder uniformly from source tank and excess powder will moves to collector tank. This will work with function of pneumatic system. When the X-carrier unit reaches the source tank after 7058

2 number of rotations over in Z-movement, doctor blade will come down and spread the powder uniformly over the build tank from the source tank. 8. Drop-on-demand nozzle for printing the inhibitor liquid. 9. Heater coil and carrier: The heater coil is used to sinter the thermoplastic powder particles and the carrier will moves the heater coil at set speed. When dispenser system stops the spraying, the heater coil is switch on and moves on the build tank and it will switch off automatically at the end build tank. 10. Air compressor: Air compressor will give sufficient air pressure to blade spreads the powder uniformly over the build tank and dispenser unit sprays a jet of inhibiting liquid to outline the desired shape. RESEARCH METHODOLOGY To develop a new RP system, ground work is required to understand the factors to be considered in order to develop the functional system, and is classified into three categories: Experimental research, developmental research and analytical research. These research activities are dependent to each other (figure 2). Simultaneously theoretical study, hardware and software, experimental study and optimization need to be conducted. Experimental research would be the key factor to evaluation, optimize and validate the related factors and experiments. Each research studies about approaches are explained in detail in this paper. towards design and implementation of machine NC path generation and hatch path generation are discussed in developmental research. Experimental research focuses towards selection of appropriate polymer and inhibitor combination for the process. Analytical research discus about the major goals like surface quality of the model, strength and accuracy of the model are need to be achieved while designing SIS system and are organized in different levels. Analytical Experimental Developmental Feasibility study Factor Identification Machine Parameter setting Machine Development & Modification Figure 1: The SIS machine Cause-Effect Analysis Response Surface Methodology Part Fabrication Proper Material Selection Model property improvement Machine NC path generator algorithm development and modification Software development WORKING PRINCIPLE OF SIS PROCESS It includes the following steps: 1) Before starting the process, initial set up is required that includes setting up the inhibitor tank pressure and printer voltage. 2) Powder from the source tank pushes upward with a desired magnitude. Simultaneously build tank moves down with a desired magnitude. The desired downward movement sets the layer thickness. 3) Doctor blade will carries the powder from source tank and spreads over the build tank. Doctor blade rotates in clockwise direction and move along X axis direction and it pushes the access powder to the collector tank. 4) Dispenser unit moves inhibitor nozzle over the powder bed. Dispenser unit moves according to the print pattern, is the machine path file generated by the NC path generator. 5) After plotting the inhibitor on the powder layer, the heat source is moves along X direction and its temperature spreads over the layer. Due to heat source movement, creates the sintering of powder and unsintering pattern at part s boundary or inhibitor place. Step 1-4 is repeated until the part is completed. 6) Part is removed from the build tank and sent to the post processing, including cleaning. Figure 2: General R&D methodology for SIS System Developmental approaches Design of the machine, development of the machine and software development are the major activities in the development approaches. Control system will work based on NC path instruction generated by the CAD/CAM software. Operational elements of SIS System-Polymer processing mechanism In a typical RP process, thermoplastics are shaped or processed at their melting temperature (T m). The semi crystalline and amorphous polymers show a distinct glass transition temperature (T g). To ensure processing of all the chosen polymers, the SIS system being developed should be able to cover the range of 50 C-265 C.The first phase of the development envisages rapid heating through tungsten halogen lamps. Sintering through these tungsten halogen lamps allows attainment of full power in less than one second. Based on the experience of previous research work, it is expected that the achievable magnitudes of power density are adequately high enough (20-40 W/cm 2 ) to facilitate the planned experiments. For ensuring the efficient utilization of the available power wavelength of the emitted IR rays and the absorption spectrum of the polymer are to be properly 7059

3 matched. The present research studies the feasibility of rapid prototyping of the parts through selective heating sintering of Nylon-6, Nylon-66, Mineral filled Nylon-6, Glass filled Nylon, HDPE, LDPE and Polystyrene. In case of amide (nylon) group C=O stretch occurs at approximately cm -1. N-H stretch in primary amides gives two bands near 3350 and 3180 cm -1 i.e. short wave IR. Bending occurs around cm -1 for primary and secondary amides. In case of aromatic (polystyrene) group the C=C absorption occurs above 3000 cm -1. The C-H absorptions occur between 3100 and 2850 cm -1.The O-H or N-H absorptions occur between 3200 and 3600 cm -1. This shows that an alcohol, N-H containing amine or amide, or carboxylic acid absorption occurs. The C-O absorptions occur between 1080 and 1300 cm -1. SIS uses polymer powders and quickly fuses the entire layers of parts in a single operation. Infrared binds polymer powders and forms thin layer by surface melting and adhesion. The driving force for the sintering process is the reduction in surface free energy of the particle and this is achieved by diffusion transport of the material from the centers particle to the particle-particle necks. As the individual particles fuse, the surface area of the particles decreases and this leads to the free energy reduction in the system leading to rapid sintering. The process is repeated continuously to build layer upon layer to get the desired shape Development of table top system In order to perform experimental studies on SIS process a suitable bench top system has to be developed. The multi nozzle system(two nozzles can deliver the inhibitor from the source tanks). These nozzles deposites the inhibitor material in a XY plane. Built platform mvoes in Z-axis direction, which determines the thickness of the layer (addition of material in layer by layer). The build platform should accommodate to build parts of 150mm x 150mm x 150mm. The build patform should have margins in all sides so as to remove the part easily. To perform heating on the layer the previously developed system can be incorporated and the designed heater moved along each layer to achieve uniform temperature distribution during the sintering process. (layer thickness : 0.5 mm to 1mm). The above mentioned operations are programmed in such a way that, the controller unit sends the sequence of commonds without much time delay. Suitable machine path algorithm which includes the binary path and heating pattern will be degined for the complex object. Slicing algotithm will be generated to slice each layer of required thickness. These algorithm are coded with the help of any language and embeded with the commercial software for NC tool path generation. The proposed algorithm is designed to develop the tool path for complicated parts directly from the available CAD data. Machine path generation SIS system has its own specifications. Formation of part boundary, method of part filling and separation from its surrounding materials are defined by NC path pattern for each layer. Commercial available software s are not able to generate appropriate machine path. Thus a new NC machine path generator system was developed. System reads only.stl file with in ASCII format and works in two steps. In first step generates slices at each increment of Z by intersecting the XY plane with the facets within the part. The second step uses all individual unsorted intersection lines in each Z increment to form the contour. Experimental approach In any of the powder based RP process, experimental research approach begins with feasibility study between polymer powder materials and available inhibitors, chemical reaction between them and validation of the initial concepts developed. Series of experiments conducted for to design, implement and to improve fabricated part property. They include experiments with machine parameter variation (e.g., heater temperature, layer thickness, and printer feed rate), post processing material variation (e.g., adhesive and wax), and process steps variation (e.g., bulk sintering). Primary settings for factors in conducted experiments in this were assigned through the one-factor at-a-time method. Set of experiments were conducted to evaluate and optimize the system, software, hardware and polymer materials used is as follows. 1. Basic set up of 30x30x0.8mm size of square ceramic tray is used to hold a desired layer of polymer powder. For sintering process, a suitable heating filament being used. 2. The selected inhibitors selected had to be companionable with available desktop printer technologies. 3. The estimated sintering temperature and time to produce sintered layers of changeable thickness will be consequently determined. 4. Test iterations need to be carried out for several liquid inhibitors combinations. Several trials were conducted by introducing droplets of inhibitor onto a powder layer and then exposing the powder layer to heat. 5. Experimental trials will also be conducted for inhibitor mixtures like water and alcohol, lubricating oils, salt solutions, and silicone.. 6. Based on the above experimental results, inhibitors which results in sharp, well-defined edges are preferred to use for SIS process. 7. Effects of temperature and time on sintering are needs to be investigating by conducting series of experiments for variable thickness of polymer powders. Factors distressing the sintering time are the polymer material, layer thickness, temperature of the heat source, and distance of the heat source from the powder surface are optimized. Based on the experimental findings if any changes required in the software and hardware parts, repetitive experiments will be conducted, analyzed and optimized to achieve effective performance. Also, the parts required to fabricate for SIS system were analyzed and optimized by using statistical quality techniques. 7060

4 Polymer powder selection Complex objects are easily manufactured by sintering of polymer powders and series of experiments were conducted on wide materials, at the beginning initial experiments were conducted with three major polymer powders are polycarbonate, polyamide, and polystyrene. Each polymer powder results different desired properties thus several experiments were conducted on all types to observe their behavior during the inhibition process and sintering. Some polymers possess glass transition temperature near to melting temperature. Consolidation of these polymers happens above melting temperature T m because they have high order molecular structure with sharp melt point. A polymer remains hard until they reach melting temperature and then rapidly reaches viscous liquid. However each polymer requires the appropriate setting. Some polymers shrink and deform due to heat during inhibition process which effects part strength and quality. Therefore several experiments were conducted on different polymers and results are presented below. 1. Polycarbonate: More heat is required for sintering and possesses poor layer attachments, poor strength and rough surface finish. 2. Polyamide: It has glass transition temperature near to the melting point temperature, thus it results improper fusion between the layers. 3. Polystyrene: posses appropriate layer fusion and form sharp edges. Even at multiple heating and cooling process, it does not affects the part strength. Thus work is completely focused on this material. Inhibitor selection Inhibitor prevents the powder particles from sintering at the part s boundary. Inhibition of sintering can be achieved laying droplets at boundaries. Several possible liquids are tried to achieve powder inhibition. Some inhibitors penetrate the polymer powder shows an impact event, in which inhibitor cuts the powder layer results displacement of particles to such an extent that they won t be adhere to each other. Such inhibitors possess poor resolution of fine features in the part. Inhibitors like silicon and salt water droplets enter the powder without disturbing the surface and prevent adhesion of adjacent particles during sintering. After layer sintering and liquid evaporation, salt crystals remains in the powder voids and preventing the particles from fusing during sintering process. Salt is then washed away quickly by dipping the part into water. Trial with water is also carried, but small amount of alcohol is added to reduce the surface tension results proper resolution at fine feature, enable the easy removal of part from the surrounding material. Water cools the powder during heating process via evaporation and preventing the powder particle surface melting. Trial on hydrogen peroxide as inhibitor was successful with polystyrene polymer powder but at low temperature. By producing chemical species that is resistant of sintering which avoids the fusion of powder particles. But polystyrene material is highly reactive with oxygen at high temperature thus trial with hydrogen peroxide was conducted at low temperature. Potassium iodide is also has been used as inhibitor with polystyrene material and used for part fabrication experiments. Analytical approach Analytical approach towards the development of SIS system is to recognize and controls the factors affecting them. There are different mile stones to recognize and control factors affecting which are summarized into different s. The major mile stone or 1 is to develop high quality functional SIS system. It depends on the factors like surface quality of the part, part strength and part accuracy. The factor like good surface quality of the part is achieved by maintaining inhibition on the selected area, strength between powder particles and strong attachment between layers. Part strength depends on attachment between powder particles and powder layers. Part accuracy depends on part deformation, sintering thickness and layer shift. Potential factors are identified and validated to reach the major goal in terms by achieving surface quality, strength and accuracy of the part. Control variables of all the potential factors are understood, identified and classified. For the development of rapid prototyping polymers using infrared sintering process there are factors such as, heater temperature, layer thickness, and powder feed rate. For SIS process improvement and optimization RSM (Response surface methodology) is a very effective method. The main objective of RSM is to optimize the response. It uses mathematical and statistical techniques to modeling and analysis of the factors influences the response of interest. Hierarchy of the factors which influences the SIS process is as shown in figure 3. Top level 2 nd 3 rd 4 th 5 th Proper inhibition at the selected area Developing New SIS system Part Surface Quality Part Strength Part accuracy process variable affecting inhibition Strong bonding between powder particles process variables affecting penetration Strong bond between layers process variables affecting powder compaction Minimal Part deformation Theoretical-experimental studies Uniform sintered thickness process variables affecting sintering Minimal layer shifting process variables affecting printing Figure 3: Milestone hierarchy plot for SIS process Fabrication of physical parts Several 3D parts are have been successfully fabricated by developed SIS process. Figure 4 illustrates some 3D fabricated parts with variety of complexity in shape and design. 7061

5 [6] Myers, R. Montgomery, D., 1995, Response Surface Methodology, John Wiley & sons, Inc. Figure 4: Parts produced by SIS process CONCLUSIONS on the SIS process begins with feasibility study tests. Since then there have been extensive attempt and evolution on the development of the SIS process. In this paper, investigations on the influence of major issues in developing a SIS process were presented and a systematic R&D approaches required is offered. The developed SIS machine is capable of producing functional parts out of polymer powder. This machine is capable to process with various polymer materials and ability to produce the parts with good quality, higher accuracy, speed and low cost. The software required for generation of machine path is specially designed and developed. After continuous research and experiments it was determined that parts produced by the combination of polystyrene and potassium iodide results best approach among many combination. REFERENCES [1] Asiabanpour, B. et.al, 2003, An experimental study of factors affecting the Selective Inhibition of Sintering process, [2] Asiabanpour, B. et.al, 2003, Advancements in the Selective Inhibition of Sintering, International Symposium on Solid Free from Fabrication, Austin, TX. [3] Asiabanpour, B. et.al, 2003, Performance factors in the Selective Inhibition of Sintering process, IIE Conference, Portland, OR. [4] Asiabanpour, B.,, 2003, A new memory efficient tool path generation method for applying very large STL files in vector-by-vector rapid prototyping processes Computer and Industrial Engineering, San Francisco, CA. [5] Khoshnevis, B., Asiabanpour, B., Mojdeh, M., and Palmer, K., 2003, SIS-A New SFF Method Based on Powder Sintering, Rapid Prototyping Journal, V. 1, N. 1, PP