Selective Laser Sintering Processing Behavior of Polyamide Powders

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1 Selective Laser Sintering Processing Behavior of Polyamide Powders Yuanyuan Wang, Christina M. DiNapoli, Gabby A. Tofig, Ross W. Cunningham and Raymond A. Pearson, Lehigh University, Bethlehem, PA Abstract Selective laser sintering (SLS) is a rapid developing additive manufacturing process. It produces parts by selectively sintering powder together in a layer-by-layer mode. SLS processing behavior were investigated with a desktop printer for commercial polyamide/carbon black (PA/CB) powders and a fabricated PA12/silica nanocomposite powder. By systematically increasing thermal and laser energy received by powder, low laser power (2 W) was sufficient for successfully sintering PA11/CB, PA12/CB and PA12/silica powders. PA11/CB exhibited a wider processing window for part bed temperature than PA12/CB. Printed PA11/CB parts yielded well and elongated up to 65%, while PA12/CB parts broke before yielding. Both were of ultimate tensile strength above 50 MPa. An in-house prepared PA12/silica nanocomposite powder tolerated higher bed temperature than powder without silica in it. Incorporation of silica nanoparticles into SLS powder brought comparable tensile strength and elongation at break to parts printed without silica in the powder while tensile modulus was noticeably increased. Finally, DSC is a useful tool to evaluate degree of powder melting during SLS. Introduction Additive manufacturing (3D printing), contrasted with traditional subtractive manufacturing, refers to processes and technologies that build end parts layer by layer according to pre-designed 3D models. With this method, one could directly print out very complex parts traditionally hard or impossible to manufacture. Based on different raw material forms and fusing techniques, several 3D printing processes were invented and developed. Selective laser sintering (SLS) being one of them is a process employing laser power to selectively melt powder material layer-by-layer on a heated powder bed. A relatively wide selection of powder materials from metals to polymers can be sintered this way. Polyamides like PA11 and PA12 are commonly used. A powerful SLS machine is equipped with high laser power (often CO 2 laser), large build envelope and able to reach high laser scanning speed. Laser power of tens to hundred watts is typical in commercial SLS machines. [1] But with maturation of SLS technique, some SLS printers with low laser power and small build envelope become available. In this work, we investigated SLS processing behavior and limitations of PA11 and PA12 powders using a desktop SLS printer with a very low-powered laser (2 W) and small build envelope ( cm 3 ). SLS with a fairly weak laser usually calls for low laser scanning speed and high operating temperature since the laser is only powerful enough to elevate powder bed s surface temperature for 2~3 C to melting temperature. With above-mentioned printer, another point of interest in this work is to evaluate an in-house-prepared nanocomposite SLS powder composed of PA12 and silica nanoparticles as filler. Nano-sized fillers like carbon fibers, clay platelets and inorganic nanoparticles are sometimes added or incorporated into SLS powders to enhance physical and mechanical properties of printed parts. [2-4] Addition of these fillers often has an effect on processing behavior of SLS powders. For example, carbon black (CB) additives can increase thermal and electrical conductivity of SLS powder, facilitating absorption and conduction of laser energy. [5] It was shown that silica nanoparticles (23 nm diam.) at 5 vol.% doubled fracture toughness of cured epoxy by increasing its flaw tolerance. [6] Thus similar mechanism and effect were expected when PA12/silica nanocomposite powder was fabricated and used in SLS process. But addition of silica nanoparticles inevitably alters processing behavior of SLS powder due to higher energy demand for fusing powder together. To address this issue, re-optimization of process parameters is often required. In some cases, too high a silica load might even cause unsuccessful SLS no matter what parameter combination is chosen. [7] Experimental SLS printer used in this work was commercially available (Sintratec, Switzerland) equipped with a 2 W blue diode (445 nm) laser. SLS powder manufactured by the same company was PA12/CB composite powder. Typical processing parameters were listed in Table 1. Throughout this work, parameters varied were laser speed, chamber temperature and bed temperature only. Type V tensile specimens (ASTM D638-14) were printed and tested on a universal testing machine (Instron 5567). Thermal transitions were examined by differential scanning calorimetry (DSC Q2000, TA Instruments). PA11/CB composite powder (Rilsan Invent Black, Arkema) and neat PA12 powder (DuraForm PA, 3D Systems) were also used. All SLS powders were of fairly smooth shapes and Poisson-like distribution peaked at around 50 µm. Nano-sized carbon black particles were distributed evenly at PA powder surfaces (Figure 1). PA12/silica nanocomposite powder was fabricated from neat PA12 powder and colloidal silica (22 nm diam., Ludox TM-40, Grace) via a wet mixing method developed at Loughborough University, England. [8] Uniform SPE ANTEC Anaheim 2017 / 112

2 monolayer coverage of silica nanoparticles on PA12 powder was observed in two bottom pictures in Figure 1. This composite powder was then dry-mixed with recycled PA12/CB powder to prepare a 1 wt.% silica-loaded SLS powder ready for printing. Table 1. SLS process parameters. Parameter PA11/CB PA12/CB Layer Thickness, µm Hatch Spacing, µm Laser Speed, mm/s Chamber T, C Bed T, C result in presence of unmolten particles inside parts, weakening their mechanical properties. Sometimes parts won t be completed at all because significant warping might happen to printed layers. Figure 2 is a demonstration of theoretical processing window of PA11/CB and neat PA12 powders with their DSC curves. The temperature range between onset of crystallization and onset of first melt (melt peak of original powder) is theoretical operating window of part bed temperature. [1] Figure 2. Processing windows marked on DSC curves of PA11/CB and neat PA12 powders Figure 1. SEM images of SLS powder surfaces. (a. PA11/CB; b. PA12/CB; c. PA12; d. PA12/silica; e. carbon black nanoparticles on PA11/CB; f. silica nanoparticles on PA12/silica) Results and Discussion I. Processing Windows of SLS Powders For successful SLS, powder bed temperature should be kept in a certain processing window (or operating window). Bed temperature is usually 2~3 C lower than melting temperature of SLS powder so that scanning laser is able to give a final rise of temperature until powder melts. But if bed temperature is too high, part shape accuracy will be partially or completely sacrificed because powder around the part also melts and then sticks to the part. Powder bed might be caked together after SLS so printed part might be irretrievable. On the other hand, if bed temperature is too low, insufficient sintering will In Figure 2, SLS powders were heated from 25 C to 250 C, then cooled down to 25 C and heated up again at 10 C /min. First melt peak (labeled in Figure 2) indicated melting of original virgin powder. Second melt peak indicated melting of sample that had already undergone first melt and recrystallization. Second melt peaks for PA11/CB and neat PA12 are C and C respectively. For all SLS powders used in this work, second melt peak were at a lower temperature than first melt peak possibly due to lower crystallinity of recrystallized powder. For PA11/CB, processing window was approximately 173~185 C. For neat PA12, it was 154~168 C. Figure 3 is a close-up comparison of first melt peak and crystallization peak between PA11/CB and PA12/CB powders. PA11/CB had a wider processing window than PA12/CB. PA12/CB exhibited a processing window of 162~172 C. It is worthwhile to notice that PA12/CB had a narrower window than neat PA12 (154~168 C, Figure 2) because crystallization peak of PA12/CB shifted to an 8 C higher temperature. This is common phenomenon for nucleation-controlled polymer crystallization under heterogeneous nucleating conditions. The presence of carbon black nanoparticles facilitated nucleating process thus nucleation set on at a higher temperature and crystallization likely finished sooner than homogeneous polymer crystallization. [9] Meanwhile, onset of melt peak of PA12/CB shifted to a higher temperature too, but to an extent smaller than shift of crystallization peak. SPE ANTEC Anaheim 2017 / 113

3 temperature was melt peak of well-sintered parts while the small peak at higher temperature was melt peak of unmolten cores of SLS powder. Height of core peak directly relates to fraction of unmolten particle cores present in printed parts. [8] Even if printed under optimal conditions for this SLS system, both PA11/CB and PA12/CB parts showed a small core peak, indicating incomplete melting. These small core peaks will disappear on second heating curve because unmolten cores completely melt after first heating cycle. Tensile specimens were printed with PA11/CB and PA12/CB powders at both x-axis and z-axis build orientations. Results were listed in Table 3. Typical stressstrain curves were in Figure 5. Figure 3. DSC curves of PA11/CB and PA12/CB powders Table 2. Thermal transitions of SLS powders and parts 1st Melting Crystallizing Sample Peak T/ C Peak T/ C PA11/CB PA11/CB Parts Neat PA PA12/CB Recycled PA12/CB PA12/CB Parts Above discussion was on theoretical processing window so far. Depending on actual SLS printer in use, processing window could be even narrower because: (1) the ability of laser to raise powder temperature is limited by laser power and laser scanning speed. For less powerful laser, bed temperature has to be kept as high in the processing window as possible. (2) Stability and uniformity of bed temperature, being never 100% perfect, depends on temperature control system. For less high-end printer, ±1 C of fluctuation is not unusual. With the printer used in this work, given that laser speed was kept as low as possible for good sintering, a slight decrease in bed temperature can adversely affect part mechanical properties. For PA11/CB, compared to optimized conditions, a 6 C lower bed temperature caused 50% lower elongation at break and 11% lower UTS. Optimized bed temperature means highest bed temperature possible for successful SLS. For PA12/CB, a 3 C lower bed temperature caused 40% lower elongation at break and 14% lower UTS. Therefore, processing window in reality is much narrower than in theoretical case. II. SLS with PA11/CB and PA12/CB powders A series of SLS with PA11/CB and PA12/CB powders were conducted to find out the optimal process parameters. DSC curves of cutoff pieces of obtained parts were charted in Figure 4. There were clearly two melt peaks for each curve. The prominent peak at lower Figure 4. DSC curves of PA11/CB and PA12/CB parts Figure 5. Tensile results of PA11/CB and PA12/CB parts (tensile test direction: x-axis) For both x-axis and z-axis prints of PA11/CB and PA12/CB in this work, ultimate tensile strength (UTS) was comparable to that from injection-molded specimens while elongation at break was smaller especially for z-axis prints, a common phenomenon in SLS. [10] Z-axis prints differed little from x-axis ones regarding to UTS but there was a significant decrease in elongation at break. Reason SPE ANTEC Anaheim 2017 / 114

4 for this could be low flaw tolerance of z-axis prints because their SLS layers were perpendicular to tensile strain so one failed layer during tensile test could cause fracture and break of sample. This is not the case for x- axis samples. It was also observed that failure mode differed between PA11/CB and PA12/CB specimens. PA11/CB ones exhibited full plasticity failure. Sample fracture occurred long after yielding and the onset of necking at UTS. Ductile tearing was present at fracture. PA12/CB prints exhibited fracture limited ductility. Sample fracture occurred after the onset of strain hardening and typically before the onset of significant necking. Little ductile tearing was observed at fracture. Table 3. Tensile results of PA11/CB and PA12/CB parts Build avg. UTS avg. Elongation Material Axis /MPa at Break /% PA11/CB X PA11/CB Z PA12/CB X PA12/CB Z III. SLS with PA12/silica Nanocomposite Powder Figure 6. Tensile results of PA12/silica parts printed at different energy levels Owing to nature of SLS powders and printer used in this work, results in section II suggested possible further improvement of mechanical properties for PA12/CB. Two considerations inspired research work in this section. First, by introducing silica nanoparticles into SLS powder to form a PA12/silica nanocomposite powder, printed parts might show better flaw tolerance and fracture toughness as in research of epoxy toughened by silica nanoparticles. [6] Second, DSC results of virgin and recycled PA12/CB powders showed a 3 C higher melting temperature of recycled powder than virgin one. Recycled SLS powder underwent warming-up and cooling-off during and after SLS process but was neither molten nor caked. Thermal history during SLS process might act like annealing thus increase polymer crystallinity and/or molecular weight, elevating melting temperature of recycled powder. A heightened melt point of SLS powder might be desirable because it could compensate for low laser power by operation at a higher bed temperature without caking the powder. It could mitigate negative effect of bed temperature fluctuations. Based on these two considerations, a novel PA12/silica nanocomposite powder was fabricated by firstly distributing silica nanoparticles onto PA12 powder then mixing with recycled PA12/CB powder. Final silica load was 1 wt.%. Table 4. Process parameters and mechanical properties of the best parts printed under different energy levels (print orientation: x-axis; tensile test direction: x-axis) Level 1 Level 2 Level 3 Level 4 Laser Speed, mm/s Chamber T, C Bed T, C UTS, MPa Elongation at Break, % Tensile Modulus, GPa During SLS process, sintering powder receives both laser energy and thermal energy. Total energy thus is a function of laser parameters and temperature environment given that other process parameters remain the same. In Figure 6 laser speed, bed temperature and chamber temperature were systematically adjusted (Table 4) so that parts were printed under heightened energy levels from 1 to 4. DSC curves of printed parts showed that decreasing core peak height was an indicator of increasing energy input (Figure 7). As total energy received by sintering powder increased with combination of parameter adjustments, UTS, elongation at break and tensile modulus increased accordingly. At energy level 4, this novel PA12/silica nanocomposite powder withstood a bed temperature of 167 C for effective powder sintering without bed caking, while virgin PA12/CB powder only withstood 163 C at its highest. Except for elongation at break, UTS and tensile modulus of PA12/silica printed at energy level 4 were comparable to injection-molded parts [10], especially tensile modulus. Introduction of silica nanoparticles improved part modulus while slightly lowered elongation at break, which is relatively not easy to improve significantly in SLS. Figure 8 is a comparison between parts printed in x- axis and z-axis at energy level 4 in a same batch. Similar trend to results in Figure 5 was observed. With all other parameters fixed, z-axis prints can have comparable tensile strength and modulus to x-axis prints from PA12/silica. But elongation at break decreased for z-axis prints. Although silica nanoparticles were introduced into SPE ANTEC Anaheim 2017 / 115

5 SLS powder aiming to enhance fracture toughness, noticeable improvement regarding to tensile strength and elongation at break were not observed. However, tensile modulus of prints from PA12/silica was 31% higher than neat PA12 and 17% higher than PA12/CB, potentially indicating toughening effect of silica nanoparticles. provided by SLS powder manufacturers. With the less powerful printer used in this work, after careful optimization of process parameters, we were able to obtain parts with better mechanical properties than manufacture ones. For PA12/CB prints, UTS and elongation at break were enhanced. For PA12/silica prints, former two properties remained enhanced while tensile modulus was noticeably increased by incorporation of silica nanoparticles into SLS powder. Conclusions Figure 7. DSC curves of PA12/silica parts printed at different energy levels Processing behavior of PA11/CB, PA12/CB and PA12/silica powders were investigated. It was affected by actual powder composition and printer conditions. Addition of nano-sized fillers (carbon black or silica nanoparticles) made processing window of part bed temperature narrower. After optimization of SLS process parameters, well-sintered parts with tensile properties better than manufacturer data were obtained. PA11/CB and PA12/CB parts had UTS around 50 MPa and elongation at break up to 65% and 16% respectively. Addition of silica nanoparticles into SLS powder increased tensile modulus of printed parts. In all cases in this work, parts printed in z-axis build orientation maintained comparable UTS to x-axis ones but elongated shorter at break of specimen. Acknowledgements The authors acknowledge financial support from America Makes and Pennsylvania Department of Community and Economic Development (PA-DCED). References Figure 8. Tensile results of PA12/silica parts printed in different build orientations Table 5. Mechanical properties of PA12 parts Elongation Tensile UTS SLS Material at Break Modulus /MPa /% /GPa Neat PA12 a PA12/CB a 40±3 5±3 N/A PA12/CB b PA12/silica b a. Manufacturer data; b. data in this work Table 5 gave mechanical property data of prints from neat PA12 and PA12/CB according to data sheets 1. D. L. Bourell et al., Physics Procedia, 56, 147 (2014). 2. C. Yan et al., Composites Science and Technology, 71, 1834 (2011). 3. J. Kim, T.S. Creasy, Polymer Testing, 23, 629 (2004). 4. Y. Chunze et al., Journal of Reinforced Plastics and Composites, 00, (2008). 5. S. R. Athreya, K. Kalaitzidou and S. Das, Composites Science and Technology, 71, 506 (2011). 6. P. Dittanet and R. A. Pearson, Polymer, 54, 1832 (2013). 7. H. Chunga and S. Dasb, Materials Science and Engineering A, 487, 251 (2008). 8. C. Majewski, H. Zarringhalam, and N. Hopkinson, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 222, 1055 (2008). 9. S. H. Kim, S. H. Ahn and T. Hirai, Polymer, 44, 5625 (2003). 10. E. Moeskops et al., Solid Freeform Fabrication Proceedings, Austin USA (2004) SPE ANTEC Anaheim 2017 / 116