Proceedings of the ASME 2015 International Mechanical Engineering Congress and Exposition IMECE2015 November 13-19, 2015, Houston, Texas IMECE2015-52634 EXPERIMENTAL STUDY OF MECHANICAL PROPERTIES OF ADDITIVELY MANUFACTURED ABS PLASTIC AS A FUNCTION OF LAYER PARAMETERS Todd Letcher South Dakota State University Mechanical Engineering Department Brookings, SD, USA Behzad Rankouhi South Dakota State University Mechanical Engineering Department Brookings, SD, USA ABSTRACT Sina Javadpour South Dakota State University Mechanical Engineering Department Brookings, SD, USA INTRODUCTION In this study, a preliminary effort was undertaken to represent the mechanical properties of a 3D printed specimen as a function of layer number, thickness and raster orientation by investigating the correlation between the mechanical properties of parts manufactured out of ABS using Fused Filament Fabrication (FFF) with a commercially available 3D printer, Makerbot Replicator 2x, and the printing parameters, such as layer thickness and raster orientation, were considered. Specimen were printed at raster orientation angles of 0, 45 and 90. Layer thickness of 0.2 mm was chosen to print specimens from a single layer to 35 layers. Samples were tested using an MTS Universal Testing Machine with extensometer to determine mechanical strength characteristics such as modulus of elasticity, ultimate tensile strength, maximum force and maximum elongation as the number of layers increased. Results showed that 0 raster orientation yields the highest mechanical properties compared to 45 and 90 at each individual layer. A linear relationship was found between the number of layers and the maximum force for all three orientations, in other words, maximum force required to break specimens linearly increased as the number of layers increased. The results also found the elastic modulus and maximum stress to increase as the number of layers increased up to almost 12 layers. For samples with more than 12 layers, the elastic modulus and maximum stress still increased, but at a much slower rate. These results can help software developers, mechanical designers and engineers reduce manufacturing time, material usage and cost by eliminating unnecessary layers that do not increase the ultimate stress of the material by improving material properties due to the addition of layers. In contrast to conventional subtractive manufacturing methods (removing layers of material to reach the desired shape), Additive Manufacturing (AM) is the technology of making objects directly from a Computer Aided Design (CAD) model by adding a layer of material at a time. There have been many advancements in AM technology since it was developed in 1980s, however it was not until the appearance of desktop 3D printers in the early 2000s and the emergence of the term 3D printing in the early 2010s that the technology became popular among public and tech-enthusiasts as well, making AM one of the most rapidly developing technologies in the world. The market for additive manufacturing, consisting of all AM products and services worldwide, grew at a compound annual growth rate (CAGR) of 35.2% to $4.1 billion in 2014. The industry expanded by more than $1 billion in 2014, with 49 manufacturers producing and selling industrial-grade AM machines. The CAGR over the past three years (2012 2014) was 33.8% [1]. Now that this technology is boldly entering space manufacturing era by introducing the first zero-g printer from Made In SPACE in 2014 [2], the future of 3D-printing looks more promising than ever before. Although the tremendous growth rate of this technology in recent years, have led to enhancements in areas such as hardware and software, yet, the standardization of the mechanical properties of the manufactured parts has remained a major concern. Various factors that play a role in changing the mechanical properties of the final product, are one of the key reasons. Figure 1 shows the significant affecting factors in 3D-printing process that affects the properties of the final product. Furthermore, commercially available printers and filaments have not reached their full potential yet and experiments are still being conducted to improve their capabilities and performance [3-5]. 1
Figure 1. Affecting parameters on the strength of the final product All specimens were printed on a consumer level 3dprinter, the MakerBot Replicator 2x. Custom printing profiles were used to control the slicing/printing software which allowed printing in a single specified raster orientations for the entire specimen. Single raster orientations were used to eliminate possible variability between samples with different number of layers (for instance, a 3 layer sample with a 0, a 45, and a 90 layer vs a 4 layer sample with two 0, a 45, and a 90 layer). Each specimen was printed individually at the same position on the printing bed in order to produce all specimen as similarly as possible. 204 specimens were printed and tested for this study. Four specimens were considered for each layer number. Table 1 shows the list of test samples for each orientation. Figure 2 shows the tested raster orientations and their definitions. For all specimens, one shell was used on the perimeter in order to minimize its effects on the material properties of the specimen. Moreover the inside of the specimens were printed with 100% infill at each specified raster orientation. When using 100% infill, toolpaths for overlapping filament layers could be influential on part strength. However, in this study, default settings on the Makerbot software were used and overlapping was not considered as an independent variable. The layer height for all tested specimens was 0.2 mm. For example, the thinnest sample with one layer is 0.2 mm thick while the thickest sample with 35 layers is 7 mm thick. The ABS material was extruded at 230 C with the heated bed surface at 115 C, except for one and two layer specimens which were printed with heated bed surface at 70 C. Table 2 shows the details of print parameters. All specimens were printed with the same generic brand of ABS filament from two 1-kg spools purchased together. Specimens were printed according to ASTM D638, [12]. However, after running the test for a few samples with 0 raster orientation, the geometry of the specimens, commonly referred to as dogbones, caused premature failures. This was more significant for thin specimens in 0 orientation. The specific shape is designed to yield the best result in tensile test since it tunnels the stress region to the smallest cross sectional area. However, in this particular case due to the nature of FFF Acrylonitrile Butadiene Styrene (ABS) and Polylactic Acid (PLA) are the two most common thermoplastic polymers used in desktop 3D printing. Easy accessibility, low cost, diversity in appearance and relative reliability are some of the advantages of these popular feedstock. These materials have shown that they can be considered as mechanically functional substitutes due to their adequate mechanical properties such as tensile strength and elastic modulus [6]. However, reliable information on the correlation between the final ABS or PLA product and printing parameters is limited and often scattered throughout the literature. The effect of printing parameters such as infill density, infill pattern, raster orientation, etc. on the strength of the 3Dprinted material, both ABS and PLA, have been investigated and reported in various journals and conferences [7-10]. The present study aims to find the relationship between the number of layers, raster orientation and mechanical properties of 3D-printed material using experimental data of over 200 tested specimens. The authors hope that the result of this study will help 3Dprinting software developers, printer manufacturers, hobbyists, startups and designers, improve their products by eliminating unnecessary layers that do not increase the ultimate stress of the material by improving material properties due to the addition of layers. EXPERIMENTAL PROCEDURES Mechanical tensile testing was conducted on specimens that were 3d-printed out of Acrylonitrile Butadiene Styrene (ABS), which is the most popular material among public users. Each structural unit of ABS is responsible for one of its properties, it gains its heat resistance from acrylonitrile, rigidity from styrene and impact strength from Butadiene [11]. ABS is a thermoplastic polymer, a material which becomes moldable in a specific temperature and solidifies upon cooling, makes it a perfect candidate for fused deposition modeling. 2
method, the dogbone shape has a drawback. Raster termination near the fillet radius causes stress concentration which in turn leads to premature failure, Figure 3, [9]. As an attempt to remedy this, rectangular version of the ASTM D638 specimen were used. Figure 4 shows the final geometry of the specimen used for tensile testing. Each specimen was individually measured (thickness and width) at several locations throughout the test section and the minimum measured value was chosen. Tensile testing of the 3d-printed specimen was conducted on an MTS Insight machine with an MTS 1kN load cell, for the first eight layers and 5KN load cell for 9 layers and above. Built in LVDTs were used to measure displacement, while an MTS extensometer (Model 634.31F-24) with a gauge length of 20mm was used to measure strain for specimens with 6 layers and higher. For specimens with Table 1. Test Samples Number of layers Total thickness (mm) Number of specimens 1 0.2 4 2 0.4 4 3 0.6 4 4 0.8 4 5 1 4 6 1.2 4 7 1.4 4 8 1.6 4 9 1.8 4 10 2 4 11 2.2 4 12 2.4 4 15 3 4 20 4 4 25 5 4 30 6 4 35 7 4 Total per orientation 68 Table 2. Print Parameters Layers 1 Layers Parameter and 2 3 to 35 Layer height (mm) 0.2 0.2 Feed rate (mm/sec) 100 150 Extruder temperature ( C) 230 230 Bed temperature ( C) 70 115 Number of shells 1 1 Infill density (%) 100 100 less than 6 layers, the distance along the specimen between the grips was considered as the initial gauge length for measuring the strain. All tests were conducted at room temperature (approximately 20 C). The test procedure was carried out according to ASTM D638 Standard Test Methods for Tensile Properties of Plastics [12]. The MTS wedge grips were displaced at a rate of 5 mm/min with data (force, grip displacement and strain) collected at 100 Hz. Figure 5 shows the testing setup for tensile testing of the 3d-printed specimen. Four specimens were tested at each of the three raster orientations in this study. Figure 3. Stress concentration of ASTM D638 specimen due to raster termination near the fillet radius in 0 orientation Figure 4. Tensile specimen dimensions (mm) for 35 layers Figure 2. Raster orientation directions from the top veiw, 0, 45 and 90 3
0 Raster Orientation Number of Layers 1 2 3 4 5 6 7 8 9 10 11 12 15 20 25 30 35 Figure 5. Tensile testing setup RESULTS AND DISCUSSION Ultimate Stress 32.196 30.320 33.664 32.904 32.155 35.370 35.720 35.462 36.717 37.215 37.712 37.468 34.604 38.674 38.014 38.666 39.376 Maximum Force (N) 91.382 185.756 301.183 385.875 474.529 601.352 696.734 803.519 932.743 1038.163 1153.544 1233.049 1426.237 2075.201 2534.638 3077.112 3640.457 Elastic Modulus Elongation at Break 1818.402 1754.304 1812.932 1799.954 1750.634 1915.869 2107.340 2127.875 2055.722 2148.163 2093.861 2106.170 2123.866 2175.927 2160.027 2054.345 2221.300 (%) 3.558 7.475 10.947 3.746 5.759 11.262 5.290 8.975 21.802 13.526 22.66 13.992 1.846 4.877 13.152 10.541 3.89 delamination for 45 and 90 respectively which show the influence of layer bond on the strength of the material. Microscopic inspection specifies that noticeable air gaps between the shell and raster, unlike thin specimens, seem to have insignificant effect on the failure of the material for thicker specimens, as indicated in previous studies, [11]. Microscopic views of the fracture surface also suggest that stress concentration caused by the two types of air gaps (between layers and between toolpaths) had the highest impact on specimens with 45 raster orientation and lowest on specimens with 0 raster orientation. As shown in Figure 11 (0 orientation), the air gap pattern is regular, while Figure 13 (45 orientation), indicates irregularity in such a way that compromises the structural integrity of the sample. This interesting finding will be studied further in future work. The relationship between the ultimate stress of the specimens and layer number is shown in Figure 6. Layer number seems to have the least effect on the ultimate stress of specimens in 0 raster orientation in contrast with 45 and 90 since it can mostly be attributed to individual raster strength rather than layer strength. Specimens began to experience a constant ultimate stress after passing almost 12 layers, regardless of their raster orientation. The same observation can be made in case of the elastic modulus. Characterization of this particular behavior can be the basis of future work. As the number of layers increased, the required fracture force also increased linearly, as indicated by Four specimens were tested for each layer number at each orientation. The averaged results for each test at each orientation are shown in Tables 3, 4 and 5. Representative curves for each orientation are presented in Figures 6 to 9. ABS parts in 0 raster orientation have previously been reported to have tensile strength varying from 10 to 18 MPa and Elastic Moduli from 1000 to 1700 MPa for various parameters [6]. According to the results of this study it is evident that the specimens with 0 raster orientation are the strongest of the three orientations. On the other hand, specimens with 90 raster orientation are the weakest in terms of mechanical properties. This can be explained by considering raster delamination and orientation during tension. Raster delamination had the least impact on specimens strength in 0 raster orientation since each raster was pulled along its longitudinal axis, causing tensile failure of individual fiber, while in specimens with 90 raster orientation force was exerted perpendicular to raster longitudinal axis resulting in highest impact on the final tensile strength of the material. In this case, layer adhesion along with the shell number in specimens with 90 raster orientation significantly affect the tensile strength since loads were taken only by the fusion bonding between raster and layers. Microscopic inspection of the fracture surface at all three orientations for 35 layer samples also support this hypothesis. As indicated in figure 10, the fracture cross section of a specimen at 0 raster orientation shows the least correlation with layer adhesion while Figures 12, 13 and 14 indicate Table 3. 0 raster orientation test results 4
45 40 Ultimate Stress, ave 35 30 25 20 15 90 Orientation 10 45 Orientation 5 0 Orientation 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Number of Layers Figure 6. Representative of ultimate stress of specimens in three raster orientation Elongation at Break, ave (%) 25 20 15 10 5 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Number of Layers 90 Orientation 45 Orientation 0 Orientation Figure 7. Representative of elongation at break of specimens in three raster orientation 4000 Maximum Force, ave (N) 3500 3000 2500 2000 1500 1000 500 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Number of Layers 90 Orientation 45 Orientation 0 Orientation Figure 8. Representative of maximum force of specimens in three raster orientation 5
2500 Elastic Modulus, ave 2000 1500 1000 500 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Number of Layers Figure 9. Representative of elastic modulus of specimens in three raster orientation 90 Orientation 45 Orientation 0 Orientation Table 4. 45 raster orientation test results Number of Layers Ultimate Stress 45 Raster Orientation Maximum Force (N) Elastic Modulus Elongation at Break (%) 1 9.154 35.466 1219.457 1.527 2 21.446 150.904 1561.523 1.814 3 23.837 235.668 1665.530 2.164 4 28.860 362.095 1755.157 3.268 5 29.989 459.611 1822.434 2.717 6 30.598 557.600 1801.063 3.207 7 30.790 637.904 1796.458 3.434 8 31.292 743.238 1979.902 5.677 9 32.214 839.881 2021.104 2.718 10 32.798 945.913 2091.998 5.354 11 34.297 1055.885 2131.873 7.898 12 35.466 1165.330 2224.844 9.286 15 34.110 1391.164 2195.359 3.239 20 35.826 1926.203 2193.063 10.334 25 36.144 2374.271 2230.451 3.412 30 35.746 2869.600 2128.044 6.47 35 35.824 3280.856 2157.18 4.744 Table 5. 90 raster orientation test results Number of Layers Ultimate Stress 90 Raster Orientation Maximum Force (N) Elastic Modulus Elongation at Break (%) 1 3.881 17.831 550.339 2.494 2 12.098 82.336 1264.198 1.156 3 22.041 200.685 1519.477 2.215 4 26.398 306.071 1606.552 2.549 5 27.962 394.576 1682.075 2.821 6 28.237 494.656 2111.643 1.878 7 28.689 558.825 2125.273 2.830 8 30.280 666.854 2153.903 2.741 9 30.471 756.209 2196.069 6.183 10 31.322 840.607 2045.592 2.833 11 31.748 930.660 2103.010 4.023 12 31.328 1020.539 2133.834 3.830 15 31.787 1267.723 2193.749 2.060 20 31.640 1699.517 2172.094 2.099 25 31.685 2098.166 2140.855 1.409 30 32.526 2567.022 2072.947 7.242 35 31.603 2920.332 2076.136 3.395 figure 8. For instance, the required force to break a specimen with 30 layers is three times the required force to break a specimen with 10 layers. The maximum elongation at break had very inconsistent results as is shown in Figured 7. On average, the 0 orientation samples had the longest elongation. The 45 and 90 orientation samples had relatively lower elongations than the 0, but neither was conclusively different from the other. 6
Figure 10. Fracture surface of a 35 layer specimen at 0 raster orientation (20x) Figure 12. Fracture surface of a 35 layer specimen at 45 raster orientation (20x) Figure 11. Fracture surface of a 35 layer specimen at 0 raster orientation (50x) Figure 13. Fracture surface of a 35 layer specimen at 45 raster orientation (50x) changes at 12 layers. Another noteworthy result to be studied in the future is determining quantitative values for the air gaps and their relationship with the stress concentration factor that develops due to these gaps. Furthermore, alternating orientations and different layer thicknesses should be tested to conclude whether the results from this study can be applied to the raster orientations generally used to define 3d printing toolpaths. CONCLUSION AND FUTURE WORK The relationship between layer number, raster orientation and mechanical properties of ABS printed specimens using a commercially available 3D-printer, Makerbot Replicator 2X, was investigated. Tensile testing of 204 specimens shown that 0 raster orientation yields the highest strength at each layer number compared to 90 and 45 raster orientations. It is also concluded that specimens experienced lower increase rate of maximum stress after 12 layers regardless of their raster orientation. The same conclusion is also true for elastic modulus of specimens with 12 layers and higher. Future work will focus on determining the reasons why the behavior of the samples ACKNOWLEDGMENTS The authors would like to thank the Materials Evaluation and Testing Lab (METLAB) and the Mechanical 7
Engineering Department at South Dakota State University for the use of testing equipment. [11] Sood, Anoop Kumar, R. K. Ohdar, and S. S. Mahapatra. "Parametric appraisal of mechanical property of fused deposition modelling processed parts."materials & Design 31.1 (2010): 287-295. [12] ASTM Standard D638, 2012, Standard Test Method for Tensile Properties of Plastics, ASTM International, West Conshohocken, PA, 2012. Figure 14. Fracture surface of a 35 layer specimen at 90 raster orientation (50x) REFERENCES [1] Wohler Associates Annual Report 2015. http://www.wohlersassociates.com/2015report.htm. [2] Snyder, Michael, Jason Dunn, and Eddie Gonzalez. "The effects of microgravity on extrusion based additive manufacturing." Proceedings of the AIAA SPACE Conference and Exposition. 2013. [3] http://www.orbitalcomposites.com [4] http://www.arevolabs.com/ [5] http://www.madeinspace.us/ [6] Tymrak, B. M., M. Kreiger, and J. M. Pearce. "Mechanical properties of components fabricated with open-source 3-D printers under realistic environmental conditions." Materials & Design 58 (2014): 242-246. [7] What is the influence of infill %, layer height and infill pattern on my 3D prints? http://my3dmatter.com/influence-infill-layer-heightpattern/#more-95 (visited 28/3/2015) [8] Li, L., et al. "Composite modeling and analysis for fabrication of FDM prototypes with locally controlled properties." Journal of manufacturing processes 4.2 (2002): 129-141. [9] Ahn, Sung-Hoon, et al. "Anisotropic material properties of fused deposition modeling ABS." Rapid Prototyping Journal 8.4 (2002): 248-257. [10] Letcher, Todd, and Megan Waytashek. "Material Property Testing of 3D-Printed Specimen in PLA on an Entry-Level 3D Printer." ASME 2014 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2014. 8