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The process and performance comparison of polyamide 12 manufactured by multi jet fusion and selective laser sintering
Journal of Manufacturing Processes 47 (2019) 419–426
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
The process and performance comparison of polyamide 12 manufactured by multi jet fusion and selective laser sintering Zhiyao Xu, Yue Wang, Dingdi Wu, K. Prem Ananth, Jiaming Bai
T
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Shenzhen Key Laboratory for Additive Manufacturing of High-performance Materials, Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen 518055, China
ARTICLE INFO
ABSTRACT
Keywords: Additive manufacturing Selective laser sintering Multi jet fusion Polyamide 12
This study is dedicated to comparing the processes of selective laser sintering (SLS) and multi jet fusion (MJF), and evaluates the morphologies, thermal and mechanical properties of the polyamide 12 (PA12) parts fabricated by these two technologies. The results showed that the PA12 powders used by these two technologies both had approximately elliptical shapes, similar particle sizes, distributions and sintering windows. For the printed parts, the surface roughness of the MJF where detailing agent was applied on was significantly better than the roughness of SLS parts. During the processing, with the powerful instant heating capability of laser, the degree of particle melting in SLS was higher than that of MJF. The mechanical properties of the MJF parts was relatively lower than that of SLS, which could be explained by the lower density of the MJF parts. However, the printing speed of MJF was almost 10 times than that of SLS's by the reason of different processing principles.
1. Introduction Additive manufacturing (AM), also named as 3D printing, is a manufacturing technology which apace produce models or products. This technology bases on the method of accurately accumulating materials to produce parts. With AM, a very complex part can be generated from CAD data without the assistant of moulds [1–3]. AM has been widely used in various sectors, such as biomedical, aerospace, automobile, and industrial design [4]. Based on the states of the raw materials, AM technologies can be divided into three main categories: solid-based, liquid-based and powder-based AM. For example: Stereo Lithography Apparatus (SLA), whose raw material is photosensitive resin, is a liquid based AM technique [5]; Fused Deposition Modelling (FDM) which use plastic wire material as the raw material is a solid based AM process [6]; Using plastic powder as the raw material, SLS is a powder based AM technology which has been largely applied in different industry sectors [7]. SLS was firstly developed by Dr. C.R. Deckard at the University of Texas Austin in 1989 [8]. With the unceasingly improvement of the technology, nowadays SLS has become a mature and widely used material forming technology [9–13]. SLS has a number of advantages [14]. Firstly, during the SLS process, no support structures or basement are required, where the unused powders support the parts. Secondly, additives, such as binders, initiators and catalyst, are not necessary, which
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means that the parts are more likely to be used in medical area because additives may bring toxicity. Thirdly, although the resolution of SLS is not as good as some AM techniques, such as SLA, the mechanical property of SLS parts is generally better. In addition, the resolution problem can be solved by the improvement of the laser system. Furthermore, in principle, SLS technology is not restricted by materials, and most powder materials can be used in SLS if the laser power and wavelength match the requirements of the sintering material [14]. However, this kind of laser-based technologies, or point-based technology (SLS or SLA), has an inevitable disadvantage. Due to the accumulated way of "point → line → face (slice) → body", the processing speed is restricted by this complicated procedure. In recent years, in order to speed up printing while maintaining the performance of printed parts, HP developed a brand-new AM technology named MJF [15,16]. MJF is a powder-based AM technology whose process is "face (slice) → body", which means that, compared with SLS, the processing speed of the MJF fundamentally rises. MJF uses infrared lamps as the heat source to fuse the plastic powders to predefined 3D geometries. Up to now, the major raw polymer powder used in MJF is PA12, which is also widely used for SLS and other powder-based AM. PA12 is a thermoplastic materials with high mechanical strength and low melt viscosity. As a semi-crystalline polymer, when it is heated to over glass transition temperature, due to the existence of crystal portion which
Corresponding author. E-mail address: [email protected] (J. Bai).
https://doi.org/10.1016/j.jmapro.2019.07.014 Received 23 October 2018; Received in revised form 19 June 2019; Accepted 8 July 2019 Available online 26 October 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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prevents the movement of amorphous portion, PA12 will not soften as temperature rises. When PA12 is heated to melting temperature, crystal will be liquefied to molten state with low melt viscosity. Therefore, in the AM forming process, using PA12 will get a high sintering rate, and the printed parts can achieve a high compactness [17]. As two important and similar powder 3D printing technologies, the technical characteristics and performances of SLS and MJF should be well understood for the researchers and industry users. The performance of the printed part is an important factor for the potential applications of each technology, and the technical characteristics are more interested to the researchers and process developers. The technology with higher potential needs to be focused and developed in order to quickly achieve technological and application advancement. In this work, the SLS and MJF powder based AM technologies are investigated and compared comprehensively. The PA12 powders were processed by both technologies with optimised processing parameters. The morphologies, surface properties, thermal and mechanical properties of the PA12 powders and printed specimens are compared and analysed.
Table 1 The built-in SLS parameters. Parameters Laser power Laser scan speed Laser scan spacing Powder bed temperature Layer thickness
ED =
P v×t×s
16 W 1500 mm/s 0.25 mm 167 °C 0.1 mm
(1)
According to Table 1, the energy density equals to 0.4267 J/mm3 for the SLS process in this study, and the mechanical testing specimens were flat printed. In MJF, powder cartridges are inserted into the processing station firstly to load the powder into building unit. Then the consumable powders are equably tiled by a roller to form a thin layer of powders and pre-heated by a fixed set of infrared lamps over the building platform. Then the scanner which contains one set of nozzles and two sets of unclosed infrared lamps on both sides scans over this layer to jet fusing agent and detailing agent [15]. The jetted area is the cross-section of the printed part. Fusing agent, which is sprayed into the interior of the cross-section, is used for speeding up the energy absorption of the PA12 powder from the infrared lamps. The detailing agent, which is sprayed to the contour of the part cross-section, is used for separating parts from powders and reducing surface roughness. After the infrared heating, the powders with fusing agent ink melt, crystallize and integrate. Then the building platform drops a small distance and a new powder layer is tiled by the roller. The printing process of the MJF is shown in Fig. 2. After processing, the powder tank is returned to the processing station to recycle the powders and carry out the post-processing of the printed parts. The available print volume is 380 × 284 × 380 mm, which equals to the volume of the powder tank in the building unit. The processing parameters of the MJF are shown in Table 2. The mechanical testing specimens also were flat printed.
2. Experiment 2.1. Machine and materials The SLS experiments were conducted on a FORMIGA P 110 system (EOS, Germany), which is equipped with a CO2 laser, which has a wavelength of 10.6 μm, a maximum power of 30 W, and the diameter of the focused laser beam is less than 0.5 mm. The MJF experiments were carried by Jet Fusion 3D 4200 (HP, US) which contains a movable building unit, powder processing station and printer. Both SLS and MJF use PA12 as the raw materials powders, and according to the specification of the datasheet, the bulk densities of two kinds of powders are 0.440 g/cm3 and 0.425 g/cm3 respectively. 2.2. Sample preparation by SLS and MJF SLS uses laser as the heat source to fuse the plastic powders to the predefined 3D geometries [7]. The CAD data of the part is sliced to several layers, and each slice is a scanning area. Consumable powders are equably tiled and pre-heated on the building platform to form a thin layer, and the laser scans over this layer, heats the PA12 powders to molten state. After one layer of scanning, the building platform drops a small distance that is called layer thickness and a new powder layer is tiled by the roller. This process is then repeated and the parts is fabricated layer-by-layer (Fig. 1). The bulit-in factory processing parameters of SLS are shown in Table 1. Energy density (ED) is the most important parameter in the SLS process, which is determined by laser power (P), scan speed (v), layer thickness (t) and scan space (s) [19]:
2.3. Characterisations The particle size distributions of PA12 powders were analysed by a laser particle size analyser (Mastersizer 3000E, Malvern Instruments). The dimensions of the printed specimens were measured by calliper and spiral micrometre. The surface roughness was measured by a surface roughness measuring instrument (SV-400, Mitutoyo). The micro morphologies of the raw PA12 powder and the specimen surfaces were observed using scanning electron microscope (SEM, Zeiss Merlin). The
Fig. 1. Schematic profile of SLS.
Fig. 2. Schematic illustration of MJF. 420
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Table 2 The MJF parameters. Parameters Effective building area Building speed Layer thickness
380 mm × 284 mm 10 s/layer 0.08 mm
specimens and powders were sputter coated with platinum under vacuum condition for 240 s to conducting. The differential scanning calorimeter (DSC, TA Instruments) and the corresponding software were used to measure the thermal properties of powders and printed specimens. The samples were heated from 25 to 200℃ with a constant rate of 5℃/min. Samples were held at 200℃ for one minute, then cooled to 25℃ with the same rate [20]. The tensile properties of the SLS and MJF specimens were tested under ambient conditions through universal testing machine (CMT4304, MTS-SANS) at a constant tensile speed of 5 mm/min. Standard tensile test specimens were designed with reference to ISO 527-2. For each data, the average value was taken from at least three tests. The impact properties were tested through Charpy impact test according to ISO 179.
Fig. 4. Particle size distribution of MJF and SLS PA12 powder. Table 3 The volume median diameter of the MJF and SLS powders (unit: μm).
3. Results and discussion 3.1. Powders In the polymer powder 3D printing process, the spherical shapes and smooth surfaces of the powder materials will ensure a good flowability in powder feeding, and the accuracy of the printed parts. The SEM shown in Fig. 3 revealed the particle geometry and surface morphologies of the PA12 powders used by SLS and MJF. It can be seen that both MJF and SLS powders had approximately elliptical shapes and rough surfaces, which could be further developed by improving the sphericity and surface qualities. In the powder bed fusion process, suitable particle size and distribution are very important for the powder processability and the mechanical properties of the printed parts. Goodridge et al. revealed that the most promising size range for polymer powder bed additive manufacturing was from around 45 to 90 μm [21]. The laser diffraction analysis shown in Fig. 4 and Table 3 revealed the particle size distributions, which were extremely close for MJF and SLS powders. The average particle sizes were 54.76 μm for MJF and 57.24 μm for SLS, which were in the suggested particle size distribution range for polymer powder bed additive manufacturing.
Type
MJF
SLS
Dv(10) Dv(50) Dv(90)
34.4 55.4 87.0
36.6 58.6 90.8
designed and the printed parts by SLS and MJF. It can be seen that the sample width were well controlled for both SLS and MJF. However the dimensional accuracy of the SLS parts on the vertical direction was not as good as MJF parts, which might be due to the exceeded layer-overlapping during the SLS process as the result of the strong laser energy input. Embossment on edges, as shown in Fig. 5b, obviously existed for both SLS and MJF parts in this study. The main cause of embossment in both SLS and MJF can be speculated as the residual stress caused by temperature gradient mechanism (TGM) [22]. As shown in Fig. 6, when powders were heated, the temperature difference between the high temperature as-heated layer and the underlying of the part caused a temperature gradient. Because of detailing agent, the temperature gradient on edges would be distinct. The expansion of the heated layer is limited by the previous layers, so elastic and plastic compression successively occurred, and the part bended. When the part was cooled, the last layer shrank and the plastic compression section was tensed. Afterwards, the part bended in the opposite direction and the bottom surface restored to nearly flat. Another potential cause is possibly due to the shrinkages difference of the block's inside and the outer wall connected to the powders, as shown in Fig. 7. Voids always exist in powder bed. During the crystallization, as the void had been occupied by melted PA12, the vertical
3.2. Dimensional accuracy To meet the requirement of various industry applications, the dimensional accuracy is very important for the printed components.Table 4 shows the dimension comparison between the
Fig. 3. SEM morphology of (a) MJF and (b) SLS PA12 powder. 421
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Table 4 The average dimensions of printed specimens. Type
width(w) (mm)
thickness(t) (mm)
side width(sw) (mm)
section area (mm2)
embossment(e) (mm)
Design MJF SLS
10 9.93 ± 0.02 10.00 ± 0.02
4 3.93 ± 0.01 4.24 ± 0.01
20 19.95 ± 0.02 20.05 ± 0.03
40 39.04 ± 0.12 42.34 ± 0.10
– ∼0.17 ∼0.07
shrinkage would occur more clearly at the internal of the printed parts. At the outer wall of the printed parts, due to the obstruction of unmelted powders which were adhered by outer wall of parts, the vertical shrinkage would occur less clearly. Apart from the reason above mentioned, the higher embossment of MJF was possibly due to the effect of the detailing agent, but the specific mechanism should be studied more deeply.
printed parts are shown in Fig. 9. During the melting processes, the MJF parts showed two obvious melting peaks, which illustrates that there were two phases in the parts. The two phases phenomenon could also been seen in the melting curve of the SLS parts, though the second phase had a tiny dent in the DSC curve of SLS parts. Zarringhalam et al. [23] revealed that the two distinct melt peaks correspond to the melted outer section and un-melted core of the PA12 powder respectively. In the SLS processing, due to powerful instant heating capability of laser, most portion of the PA12 powder was melted except a little un-melt core, so the existence of SLS un-melt core is extremely inconspicuous in the DSC curve of the SLS parts. Table 6 lists melting enthalpy ( Hm ), crystallization enthalpy ( Hc ), initial and peaking melting, core peaking melting temperature (Tim, Tpm, Tcpm ), initial and peaking crystallization temperature (Tic , Tpc ) from the DSC curves. Sintering window (Ws ) is the difference between Tim and Tic , which is the reflection of sintering processability. The crystallinity is the ratio between Hm and the melting enthalpy of an 0 entirely crystalline specimen ( Hm ). The calculation formula is the following Eq. (2) [24]:
3.3. Surface quality The surfaces quality of printed parts were examined by SEM, which are shown in Fig. 8. The morphologies of top (Fig. 8c) and bottom (Fig. 8d) surface of the SLS parts were similar, whose PA12 particles and sintering necks were apparent meanwhile some voids could be seen on the surface. The surface particles which almost did not melt were adhered by the melt PA12 layer. The top and bottom surface roughness of the SLS were very similar around 14 μm, shown in Table 5. For another, the MJF specimen exhibited a denser surface (Fig. 8a and b) without obvious pores and particles compared with SLS parts. Both of MJF surfaces were tight, but there was a large amount of upwarp features on the top surface possibly due to the lack of detailing agent, which could be the reason of roughness difference (Table 5). Although the specification of HP only shows that the detailing agent was used on edge of cross-section, the fact that detailing agent was also possibly used on bottom surface can be speculated by the roughness difference.
Xc =
Hm 0
Hm
× 100% 0
(2)
Where Hm is taken to be 209.3 J/g [25]. According to the datasheet of VESTAMID [18], both extrusion and injection moulding PA12 had a melting temperature of 178 ℃, which was in the melting temperature intervals of the MJF and SLS PA12 materials. Ws is the range of the powder bed temperature (Tb ). In general, a higher Ws value means that the controllability and precessability
3.4. Thermal properties The DSC of heating/cooling cycle of the raw PA12 powders and
Fig. 5. (a) Model and (b) cross section sketch of printed tensile test specimens; (c) MJF and (d) SLS printed tensile test parts. 422
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Fig. 6. TGM inducing residual stress in MJF.
Fig. 7. The microscopic process of MJF at part edge.
Fig. 8. The SEM images of MJF specimens: (a) top; (b) bottom surface, and the SLS specimens: (c) top; (d) bottom surface.
caused by lower crystallinity, which means that the parts' dimensions are close to designed values [14]. Moreover, for the same polymer, higher crystallinity could leads brittleness of the parts [27], which match the lower impact properties of the MJF parts.
Table 5 Surface roughness of printed tensile test specimens. Type
Ra(top)(μm)
Ra(botom)(μm)
MJF SLS
15.58 ± 0.59 14.62 ± 0.45
6.31 ± 0.43 14.40 ± 1.06
3.5. Mechanical properties The mechanical properties of the MJF, SLS printed and injection moulding, extrusion parts are compared in Table 7. The stress-strain curves of the special data are shown in Fig. 10, where the strain hardening phenomena did not occur. The properties of the SLS parts were notably more excellent than the MJF. The SLS specimens exhibited an outstanding elongation at break of 31.55%, high Young's
of the sintering process is better [26]. In Table 6, the Ws of MJF raw PA12 powder was slightly greater than the Ws of SLS raw PA12 powder, so MJF raw PA12 powders would have a better controllability and processability. Crystallinity is another important parameter to evaluate the sintering processability. Generally, lower volume shrinkage is 423
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Fig. 10. Stress-strain curves of the SLS and MJF specimens.
Modulus of 1.76 GPa and impact toughness of 0.34 J/cm2, which were nearly 2 times, 1.24 times and 1.3 times of the MJF specimens respectively. The tensile strength of the MJF parts (40.10 MPa) was close but still slightly lower than the SLS parts of 43.61 MPa. However, in addition to the Young’s Modulus of SLS parts, the mechanical properties of MJF and SLS parts were lower than extrusion and injection moulding parts. The SEM images of fracture surfaces of the MJF and SLS components are shown in Fig. 11. It can be found that the spherical voids existed on the fracture surface of both SLS and MJF samples without distinct PA12 particle. The PA12 parts densities tested by Archimedes principle was 0.93 g/cm3 for MJF and 0.99 g/cm3 for SLS severally, which means that the porosity of MJF parts was higher than that of SLS parts. The ductility and modulus are especially sensitive to the internal voids of the solid components. The speed of crack and failure would rise because of the existence of inherent pores where stress concentration occurs. Higher density normally results in good mechanical property, which should be the main reason for the SLS part’s better mechanical properties.
Fig. 9. The DSC thermograms during the heating/cooling cycle of the (a) MJF part, powder and (b) SLS part, powder. The peak temperature of melted outer section is called peaking melting temperature, and the peak temperature of unmelted core is called core peaking melting temperature.
3.6. Technique comparison SLS and MJF have many similarities and distinctions. The most similarity is they are both powder-based processes, which means that the techniques of SLS's powder system, such as the powder roller, powder recycle and build platform, are similar to MJF's. Second, both printing processes belong to sintering, in which the powder is heated to fully or partially melt and then cooled to semi-crystallize solids. Whereas, the detailed processing methods have huge differences. In SLS, the heating power source is the laser, which can increase the temperature rapidly without the assistance of any reagents. In MJF, the heating source is the infrared light and the powder deposited with fusing agent ink is heated by the infrared lamps, which are fixed on the top of the building chamber and scanner in the printer respectively. Instant heating effect of infrared lamp is not as good as laser, so the processing temperature of MJF is presumed to be lower than SLS’s. During the SLS process, the laser scans line-by-line and the printed area
Table 6 Heating/cooling cycle characteristics of the raw PA12 and printed parts. Sample
Hm (J/g)
Hc (J/g) Tim (℃) Tpm (℃)
MJF raw PA12 82.587
39.989
173.577 185.387
Tcpm (℃)
–
Ws (℃) X c (%)
17.505 39.46
Tic (℃) Tpc (℃)
156.072 151.177
MJF parts
SLS raw PA12
SLS parts
62.8
74.477
51.014
171.823 179.417
170.992 183.895
171.205 179.737
155.81 150.893
156.564 151.435
53.937
37.951
186.118
–
– 30
15.182 35.58
159.301 155.288
41.954
187.173
– 24.37
Table 7 The average mechanical properties of MJF, SLS, injection moulding (IM) and extrusion (E) specimens. Type
Tensile Strength (MPa)
Elongation at Break (%)
Young's Modulus (GPa)
Impact Toughness (J/cm2)
Density (g/cm3)
MJF SLS IM [18] E [18]
40.10 ± 1.49 43.61 ± 0.46 53 46
17.45 ± 3.87 31.55 ± 2.93 > 50 > 50
1.42 ± 0.04 1.76 ± 0.02 1.7 1.6
0.26 ± 0.01 0.34 ± 0.03 – –
0.93 ± 0.08 0.99 ± 0.04 1.03 1.02
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Fig. 11. The SEM fracture surface images of (a) MJF, (b) SLS.
is the laser scanning area. In MJF, infrared lamps scans the whole building platform, and the sintered area is the region where the inks are applied, which sharply and fundamentally raised the printing speed. According to the data sheet, the printing speed of the MJF under standard mode is about 10 s per layer which equals to 10,792 mm2/s; however, the maximum print speed for the SLS is just 5 m/ s × 0.25 mm = 1250 mm2/s, whose maximum laser scan speed is 5 m/ s. MJF shows great promotion of the printing speed compared to SLS, however as discussed in the previous section, the mechanical properties of the MJF parts was not as good as SLS. In terms of the mechanical properties, the MJF printed parts showed some inferior compared to the SLS parts. SLS has been well developed for several decades, and the processing parameters for the PA12 is mature and well optimised. As an emerging technology, the MJF process and the processing parameters for different materials are still under developing. Furthermore, based on the authors’ knowledge, the PA12 powders, which are almost the same for SLS and MJF, have been processed by SLS for a long time. As a new 3D printing process, a specific or exclusive powder materials for the MJF process should be developed, which could further unlock the potential of the MJF process. The excellent printing speed reveals the potential of MJF in the fast growing AM field, and the following aspects could be focused to further improve the industry applications of this technology. Firstly, the printing materials for the MJF should be developed and expanded. Currently, similar to SLS, the PA12 and its composites are the main materials used for MJF. As a unique technology, the powder materials should be investigated specifically for MJF. Secondly the processing parameters should be continuously optimized during the material development. Thirdly, functional ink, such as nanofiller ink, can be developed and applied to facilitate the printing process and improve the properties of the MJF parts.
4) The mechanical properties of the SLS specimens were higher than MJF specimens, especially Young's modulus and elongation at break, which could be due to the higher density of the SLS specimens. 5) Compared with SLS, the printing speed of MJF sharply and fundamentally raised due to the innovative face-sintering processing method. It is believed that materials and process development will further push the industry applications of MJF. Acknowledgements The study was supported National Natural Science Foundation of China (Grant No. 51805239) and the Shenzhen Key Laboratory for Additive Manufacturing of High-performance Materials (Grant No: ZDSYS201703031748354). References [1] Holmes Larry Jr. R, Riddick Jaret C. Research summary of an additive manufacturing technology for the fabrication of 3D composites with tailored internal structure. JOM 2014;66:270–4. https://doi.org/10.1007/s11837-013-0828-4. [2] Jacobs Paul F. Stereolithography and other RP&M technologies-from rapid prototyping to rapid tooling. Dearborn: Society of Manufacturing Engineers; 1996. [3] Melchels Ferry PW, Domingos Marco AN, Klein Travis J, Malda Jos, Bartolo Paulo J, Hutmacherad Dietmar W. Additive manufacturing of tissues and organs. Prog Polym Sci 2012;37(8):1079–104. https://doi.org/10.1016/j.progpolymsci.2011.11.007. [4] Ngo Tuan D, Kashani Alireza, Imbalzano Gabriele, Nguyen Kate TQ, Hui David. Additive manufacturing (3D printing): a review of materials, methods, applications and challenges. Compos Part B Eng 2018;143:172–96. https://doi.org/10.1016/j. compositesb.2018.02.012. [5] Melchels FP, Feijen J, Grijpma DW. A review on stereolithography and its applications in biomedical engineering. Biomaterials 2010;31:6121–30. https://doi.org/ 10.1016/j.biomaterials.2010.04.050. [6] Zein Iwan, Hutmacher Dietmar W, Tan Kim Cheng, Teoh Swee Hin. Fused deposition modeling of novel scaffold architectures for tissue engineering applications. Biomaterials 2002;23:1169–85. https://doi.org/10.1016/S0142-9612(01)00232-0. [7] Kruth J-P, Mercelis P, Vaerenbergh J Van, Froyen L, Rombouts M. Binding mechanisms in selective laser sintering and selective laser melting. Rapid Prototyp J 2005;11:26–36. https://doi.org/10.1108/13552540510573365. [8] Carl R. Deckard, inventor; University of Texas System, assignee. Method and apparatus for producing parts by selective sintering. United States patent US 4863538A. 1989 Sep 5. [9] Olakanmi EO, Cochrane RF, Dalgarno KW. A review on selective laser sintering/ melting (SLS/SLM) of aluminium alloy powders: processing, microstructure, and properties. Prog Mater Sci 2015;74:401–77. https://doi.org/10.1016/j.pmatsci. 2015.03.002. [10] Shirazi Seyed Farid Seyed, Gharehkhani Samira, Mehrali Mehdi, Yarmand Hooman, Metselaar Hendrik Simon Cornelis, Kadri Nahrizul Adib, et al. A review on powderbased additive manufacturing for tissue engineering: selective laser sintering and inkjet 3D printing. Sci Technol Adv Mater 2015;16:033502. https://doi.org/10. 1088/1468-6996/16/3/033502. [11] Yuan Shangqin, Shen Fei, Bai Jiaming, Chua Chee Kai, Wei Jun, Zhou Kun. 3D soft auxetic lattice structures fabricated by selective laser sintering: TPU powder evaluation and process optimization. Mater Des 2017;120:317–27. https://doi.org/10. 1016/j.matdes.2017.01.098. [12] Yuan Shangqin, Bai Jiaming, Chua Chee Kai, Wei Jun, Zhou Kun. Material evaluation and process optimization of CNT-Coated polymer powders for selective laser sintering. Polymers 2016;8:370. https://doi.org/10.3390/polym8100370. [13] Bai Jiaming, Goodridge Ruth D, Yuan Shangqin, Zhou Kun, Chua Chee Kai, Wei Jun. Thermal influence of CNT on the polyamide 12 nanocomposite for selective laser sintering. Molecules 2015;20:19041–50. https://doi.org/10.3390/
4. Conclusions This study investigated the process and parts performance of PA12 parts fabricated by SLS and MJF. Specimens were printed to evaluate the morphology, mechanical properties and thermal characteristic of the PA12 parts. The major findings include: 1) Both PA12 powder used by SLS and MJF were near ellipsoid geometry. The particle size and distribution of the two types were close. But the MJF powder had higher crystallinity and wider sintering window. 2) The top surface roughness of SLS and MJF parts were extremely close, but the bottom surface roughness of MJF part, was distinctly better than SLS part bottom’s, which might be due to the applying of detailing agent. 3) Due to the powerful instant heating capability of laser, the unmelted core of SLS was distinctly smaller than MJF. The crystallinity of SLS parts was higher than MJF, which lead better impact property and dimension accuracy. 425
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
The process and performance comparison of polyamide 12 manufactured by multi jet fusion and selective laser sintering Zhiyao Xu, Yue Wang, Dingdi Wu, K. Prem Ananth, Jiaming Bai
T
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Shenzhen Key Laboratory for Additive Manufacturing of High-performance Materials, Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen 518055, China
ARTICLE INFO
ABSTRACT
Keywords: Additive manufacturing Selective laser sintering Multi jet fusion Polyamide 12
This study is dedicated to comparing the processes of selective laser sintering (SLS) and multi jet fusion (MJF), and evaluates the morphologies, thermal and mechanical properties of the polyamide 12 (PA12) parts fabricated by these two technologies. The results showed that the PA12 powders used by these two technologies both had approximately elliptical shapes, similar particle sizes, distributions and sintering windows. For the printed parts, the surface roughness of the MJF where detailing agent was applied on was significantly better than the roughness of SLS parts. During the processing, with the powerful instant heating capability of laser, the degree of particle melting in SLS was higher than that of MJF. The mechanical properties of the MJF parts was relatively lower than that of SLS, which could be explained by the lower density of the MJF parts. However, the printing speed of MJF was almost 10 times than that of SLS's by the reason of different processing principles.
1. Introduction Additive manufacturing (AM), also named as 3D printing, is a manufacturing technology which apace produce models or products. This technology bases on the method of accurately accumulating materials to produce parts. With AM, a very complex part can be generated from CAD data without the assistant of moulds [1–3]. AM has been widely used in various sectors, such as biomedical, aerospace, automobile, and industrial design [4]. Based on the states of the raw materials, AM technologies can be divided into three main categories: solid-based, liquid-based and powder-based AM. For example: Stereo Lithography Apparatus (SLA), whose raw material is photosensitive resin, is a liquid based AM technique [5]; Fused Deposition Modelling (FDM) which use plastic wire material as the raw material is a solid based AM process [6]; Using plastic powder as the raw material, SLS is a powder based AM technology which has been largely applied in different industry sectors [7]. SLS was firstly developed by Dr. C.R. Deckard at the University of Texas Austin in 1989 [8]. With the unceasingly improvement of the technology, nowadays SLS has become a mature and widely used material forming technology [9–13]. SLS has a number of advantages [14]. Firstly, during the SLS process, no support structures or basement are required, where the unused powders support the parts. Secondly, additives, such as binders, initiators and catalyst, are not necessary, which
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means that the parts are more likely to be used in medical area because additives may bring toxicity. Thirdly, although the resolution of SLS is not as good as some AM techniques, such as SLA, the mechanical property of SLS parts is generally better. In addition, the resolution problem can be solved by the improvement of the laser system. Furthermore, in principle, SLS technology is not restricted by materials, and most powder materials can be used in SLS if the laser power and wavelength match the requirements of the sintering material [14]. However, this kind of laser-based technologies, or point-based technology (SLS or SLA), has an inevitable disadvantage. Due to the accumulated way of "point → line → face (slice) → body", the processing speed is restricted by this complicated procedure. In recent years, in order to speed up printing while maintaining the performance of printed parts, HP developed a brand-new AM technology named MJF [15,16]. MJF is a powder-based AM technology whose process is "face (slice) → body", which means that, compared with SLS, the processing speed of the MJF fundamentally rises. MJF uses infrared lamps as the heat source to fuse the plastic powders to predefined 3D geometries. Up to now, the major raw polymer powder used in MJF is PA12, which is also widely used for SLS and other powder-based AM. PA12 is a thermoplastic materials with high mechanical strength and low melt viscosity. As a semi-crystalline polymer, when it is heated to over glass transition temperature, due to the existence of crystal portion which
Corresponding author. E-mail address: [email protected] (J. Bai).
https://doi.org/10.1016/j.jmapro.2019.07.014 Received 23 October 2018; Received in revised form 19 June 2019; Accepted 8 July 2019 Available online 26 October 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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prevents the movement of amorphous portion, PA12 will not soften as temperature rises. When PA12 is heated to melting temperature, crystal will be liquefied to molten state with low melt viscosity. Therefore, in the AM forming process, using PA12 will get a high sintering rate, and the printed parts can achieve a high compactness [17]. As two important and similar powder 3D printing technologies, the technical characteristics and performances of SLS and MJF should be well understood for the researchers and industry users. The performance of the printed part is an important factor for the potential applications of each technology, and the technical characteristics are more interested to the researchers and process developers. The technology with higher potential needs to be focused and developed in order to quickly achieve technological and application advancement. In this work, the SLS and MJF powder based AM technologies are investigated and compared comprehensively. The PA12 powders were processed by both technologies with optimised processing parameters. The morphologies, surface properties, thermal and mechanical properties of the PA12 powders and printed specimens are compared and analysed.
Table 1 The built-in SLS parameters. Parameters Laser power Laser scan speed Laser scan spacing Powder bed temperature Layer thickness
ED =
P v×t×s
16 W 1500 mm/s 0.25 mm 167 °C 0.1 mm
(1)
According to Table 1, the energy density equals to 0.4267 J/mm3 for the SLS process in this study, and the mechanical testing specimens were flat printed. In MJF, powder cartridges are inserted into the processing station firstly to load the powder into building unit. Then the consumable powders are equably tiled by a roller to form a thin layer of powders and pre-heated by a fixed set of infrared lamps over the building platform. Then the scanner which contains one set of nozzles and two sets of unclosed infrared lamps on both sides scans over this layer to jet fusing agent and detailing agent [15]. The jetted area is the cross-section of the printed part. Fusing agent, which is sprayed into the interior of the cross-section, is used for speeding up the energy absorption of the PA12 powder from the infrared lamps. The detailing agent, which is sprayed to the contour of the part cross-section, is used for separating parts from powders and reducing surface roughness. After the infrared heating, the powders with fusing agent ink melt, crystallize and integrate. Then the building platform drops a small distance and a new powder layer is tiled by the roller. The printing process of the MJF is shown in Fig. 2. After processing, the powder tank is returned to the processing station to recycle the powders and carry out the post-processing of the printed parts. The available print volume is 380 × 284 × 380 mm, which equals to the volume of the powder tank in the building unit. The processing parameters of the MJF are shown in Table 2. The mechanical testing specimens also were flat printed.
2. Experiment 2.1. Machine and materials The SLS experiments were conducted on a FORMIGA P 110 system (EOS, Germany), which is equipped with a CO2 laser, which has a wavelength of 10.6 μm, a maximum power of 30 W, and the diameter of the focused laser beam is less than 0.5 mm. The MJF experiments were carried by Jet Fusion 3D 4200 (HP, US) which contains a movable building unit, powder processing station and printer. Both SLS and MJF use PA12 as the raw materials powders, and according to the specification of the datasheet, the bulk densities of two kinds of powders are 0.440 g/cm3 and 0.425 g/cm3 respectively. 2.2. Sample preparation by SLS and MJF SLS uses laser as the heat source to fuse the plastic powders to the predefined 3D geometries [7]. The CAD data of the part is sliced to several layers, and each slice is a scanning area. Consumable powders are equably tiled and pre-heated on the building platform to form a thin layer, and the laser scans over this layer, heats the PA12 powders to molten state. After one layer of scanning, the building platform drops a small distance that is called layer thickness and a new powder layer is tiled by the roller. This process is then repeated and the parts is fabricated layer-by-layer (Fig. 1). The bulit-in factory processing parameters of SLS are shown in Table 1. Energy density (ED) is the most important parameter in the SLS process, which is determined by laser power (P), scan speed (v), layer thickness (t) and scan space (s) [19]:
2.3. Characterisations The particle size distributions of PA12 powders were analysed by a laser particle size analyser (Mastersizer 3000E, Malvern Instruments). The dimensions of the printed specimens were measured by calliper and spiral micrometre. The surface roughness was measured by a surface roughness measuring instrument (SV-400, Mitutoyo). The micro morphologies of the raw PA12 powder and the specimen surfaces were observed using scanning electron microscope (SEM, Zeiss Merlin). The
Fig. 1. Schematic profile of SLS.
Fig. 2. Schematic illustration of MJF. 420
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Table 2 The MJF parameters. Parameters Effective building area Building speed Layer thickness
380 mm × 284 mm 10 s/layer 0.08 mm
specimens and powders were sputter coated with platinum under vacuum condition for 240 s to conducting. The differential scanning calorimeter (DSC, TA Instruments) and the corresponding software were used to measure the thermal properties of powders and printed specimens. The samples were heated from 25 to 200℃ with a constant rate of 5℃/min. Samples were held at 200℃ for one minute, then cooled to 25℃ with the same rate [20]. The tensile properties of the SLS and MJF specimens were tested under ambient conditions through universal testing machine (CMT4304, MTS-SANS) at a constant tensile speed of 5 mm/min. Standard tensile test specimens were designed with reference to ISO 527-2. For each data, the average value was taken from at least three tests. The impact properties were tested through Charpy impact test according to ISO 179.
Fig. 4. Particle size distribution of MJF and SLS PA12 powder. Table 3 The volume median diameter of the MJF and SLS powders (unit: μm).
3. Results and discussion 3.1. Powders In the polymer powder 3D printing process, the spherical shapes and smooth surfaces of the powder materials will ensure a good flowability in powder feeding, and the accuracy of the printed parts. The SEM shown in Fig. 3 revealed the particle geometry and surface morphologies of the PA12 powders used by SLS and MJF. It can be seen that both MJF and SLS powders had approximately elliptical shapes and rough surfaces, which could be further developed by improving the sphericity and surface qualities. In the powder bed fusion process, suitable particle size and distribution are very important for the powder processability and the mechanical properties of the printed parts. Goodridge et al. revealed that the most promising size range for polymer powder bed additive manufacturing was from around 45 to 90 μm [21]. The laser diffraction analysis shown in Fig. 4 and Table 3 revealed the particle size distributions, which were extremely close for MJF and SLS powders. The average particle sizes were 54.76 μm for MJF and 57.24 μm for SLS, which were in the suggested particle size distribution range for polymer powder bed additive manufacturing.
Type
MJF
SLS
Dv(10) Dv(50) Dv(90)
34.4 55.4 87.0
36.6 58.6 90.8
designed and the printed parts by SLS and MJF. It can be seen that the sample width were well controlled for both SLS and MJF. However the dimensional accuracy of the SLS parts on the vertical direction was not as good as MJF parts, which might be due to the exceeded layer-overlapping during the SLS process as the result of the strong laser energy input. Embossment on edges, as shown in Fig. 5b, obviously existed for both SLS and MJF parts in this study. The main cause of embossment in both SLS and MJF can be speculated as the residual stress caused by temperature gradient mechanism (TGM) [22]. As shown in Fig. 6, when powders were heated, the temperature difference between the high temperature as-heated layer and the underlying of the part caused a temperature gradient. Because of detailing agent, the temperature gradient on edges would be distinct. The expansion of the heated layer is limited by the previous layers, so elastic and plastic compression successively occurred, and the part bended. When the part was cooled, the last layer shrank and the plastic compression section was tensed. Afterwards, the part bended in the opposite direction and the bottom surface restored to nearly flat. Another potential cause is possibly due to the shrinkages difference of the block's inside and the outer wall connected to the powders, as shown in Fig. 7. Voids always exist in powder bed. During the crystallization, as the void had been occupied by melted PA12, the vertical
3.2. Dimensional accuracy To meet the requirement of various industry applications, the dimensional accuracy is very important for the printed components.Table 4 shows the dimension comparison between the
Fig. 3. SEM morphology of (a) MJF and (b) SLS PA12 powder. 421
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Table 4 The average dimensions of printed specimens. Type
width(w) (mm)
thickness(t) (mm)
side width(sw) (mm)
section area (mm2)
embossment(e) (mm)
Design MJF SLS
10 9.93 ± 0.02 10.00 ± 0.02
4 3.93 ± 0.01 4.24 ± 0.01
20 19.95 ± 0.02 20.05 ± 0.03
40 39.04 ± 0.12 42.34 ± 0.10
– ∼0.17 ∼0.07
shrinkage would occur more clearly at the internal of the printed parts. At the outer wall of the printed parts, due to the obstruction of unmelted powders which were adhered by outer wall of parts, the vertical shrinkage would occur less clearly. Apart from the reason above mentioned, the higher embossment of MJF was possibly due to the effect of the detailing agent, but the specific mechanism should be studied more deeply.
printed parts are shown in Fig. 9. During the melting processes, the MJF parts showed two obvious melting peaks, which illustrates that there were two phases in the parts. The two phases phenomenon could also been seen in the melting curve of the SLS parts, though the second phase had a tiny dent in the DSC curve of SLS parts. Zarringhalam et al. [23] revealed that the two distinct melt peaks correspond to the melted outer section and un-melted core of the PA12 powder respectively. In the SLS processing, due to powerful instant heating capability of laser, most portion of the PA12 powder was melted except a little un-melt core, so the existence of SLS un-melt core is extremely inconspicuous in the DSC curve of the SLS parts. Table 6 lists melting enthalpy ( Hm ), crystallization enthalpy ( Hc ), initial and peaking melting, core peaking melting temperature (Tim, Tpm, Tcpm ), initial and peaking crystallization temperature (Tic , Tpc ) from the DSC curves. Sintering window (Ws ) is the difference between Tim and Tic , which is the reflection of sintering processability. The crystallinity is the ratio between Hm and the melting enthalpy of an 0 entirely crystalline specimen ( Hm ). The calculation formula is the following Eq. (2) [24]:
3.3. Surface quality The surfaces quality of printed parts were examined by SEM, which are shown in Fig. 8. The morphologies of top (Fig. 8c) and bottom (Fig. 8d) surface of the SLS parts were similar, whose PA12 particles and sintering necks were apparent meanwhile some voids could be seen on the surface. The surface particles which almost did not melt were adhered by the melt PA12 layer. The top and bottom surface roughness of the SLS were very similar around 14 μm, shown in Table 5. For another, the MJF specimen exhibited a denser surface (Fig. 8a and b) without obvious pores and particles compared with SLS parts. Both of MJF surfaces were tight, but there was a large amount of upwarp features on the top surface possibly due to the lack of detailing agent, which could be the reason of roughness difference (Table 5). Although the specification of HP only shows that the detailing agent was used on edge of cross-section, the fact that detailing agent was also possibly used on bottom surface can be speculated by the roughness difference.
Xc =
Hm 0
Hm
× 100% 0
(2)
Where Hm is taken to be 209.3 J/g [25]. According to the datasheet of VESTAMID [18], both extrusion and injection moulding PA12 had a melting temperature of 178 ℃, which was in the melting temperature intervals of the MJF and SLS PA12 materials. Ws is the range of the powder bed temperature (Tb ). In general, a higher Ws value means that the controllability and precessability
3.4. Thermal properties The DSC of heating/cooling cycle of the raw PA12 powders and
Fig. 5. (a) Model and (b) cross section sketch of printed tensile test specimens; (c) MJF and (d) SLS printed tensile test parts. 422
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Fig. 6. TGM inducing residual stress in MJF.
Fig. 7. The microscopic process of MJF at part edge.
Fig. 8. The SEM images of MJF specimens: (a) top; (b) bottom surface, and the SLS specimens: (c) top; (d) bottom surface.
caused by lower crystallinity, which means that the parts' dimensions are close to designed values [14]. Moreover, for the same polymer, higher crystallinity could leads brittleness of the parts [27], which match the lower impact properties of the MJF parts.
Table 5 Surface roughness of printed tensile test specimens. Type
Ra(top)(μm)
Ra(botom)(μm)
MJF SLS
15.58 ± 0.59 14.62 ± 0.45
6.31 ± 0.43 14.40 ± 1.06
3.5. Mechanical properties The mechanical properties of the MJF, SLS printed and injection moulding, extrusion parts are compared in Table 7. The stress-strain curves of the special data are shown in Fig. 10, where the strain hardening phenomena did not occur. The properties of the SLS parts were notably more excellent than the MJF. The SLS specimens exhibited an outstanding elongation at break of 31.55%, high Young's
of the sintering process is better [26]. In Table 6, the Ws of MJF raw PA12 powder was slightly greater than the Ws of SLS raw PA12 powder, so MJF raw PA12 powders would have a better controllability and processability. Crystallinity is another important parameter to evaluate the sintering processability. Generally, lower volume shrinkage is 423
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Fig. 10. Stress-strain curves of the SLS and MJF specimens.
Modulus of 1.76 GPa and impact toughness of 0.34 J/cm2, which were nearly 2 times, 1.24 times and 1.3 times of the MJF specimens respectively. The tensile strength of the MJF parts (40.10 MPa) was close but still slightly lower than the SLS parts of 43.61 MPa. However, in addition to the Young’s Modulus of SLS parts, the mechanical properties of MJF and SLS parts were lower than extrusion and injection moulding parts. The SEM images of fracture surfaces of the MJF and SLS components are shown in Fig. 11. It can be found that the spherical voids existed on the fracture surface of both SLS and MJF samples without distinct PA12 particle. The PA12 parts densities tested by Archimedes principle was 0.93 g/cm3 for MJF and 0.99 g/cm3 for SLS severally, which means that the porosity of MJF parts was higher than that of SLS parts. The ductility and modulus are especially sensitive to the internal voids of the solid components. The speed of crack and failure would rise because of the existence of inherent pores where stress concentration occurs. Higher density normally results in good mechanical property, which should be the main reason for the SLS part’s better mechanical properties.
Fig. 9. The DSC thermograms during the heating/cooling cycle of the (a) MJF part, powder and (b) SLS part, powder. The peak temperature of melted outer section is called peaking melting temperature, and the peak temperature of unmelted core is called core peaking melting temperature.
3.6. Technique comparison SLS and MJF have many similarities and distinctions. The most similarity is they are both powder-based processes, which means that the techniques of SLS's powder system, such as the powder roller, powder recycle and build platform, are similar to MJF's. Second, both printing processes belong to sintering, in which the powder is heated to fully or partially melt and then cooled to semi-crystallize solids. Whereas, the detailed processing methods have huge differences. In SLS, the heating power source is the laser, which can increase the temperature rapidly without the assistance of any reagents. In MJF, the heating source is the infrared light and the powder deposited with fusing agent ink is heated by the infrared lamps, which are fixed on the top of the building chamber and scanner in the printer respectively. Instant heating effect of infrared lamp is not as good as laser, so the processing temperature of MJF is presumed to be lower than SLS’s. During the SLS process, the laser scans line-by-line and the printed area
Table 6 Heating/cooling cycle characteristics of the raw PA12 and printed parts. Sample
Hm (J/g)
Hc (J/g) Tim (℃) Tpm (℃)
MJF raw PA12 82.587
39.989
173.577 185.387
Tcpm (℃)
–
Ws (℃) X c (%)
17.505 39.46
Tic (℃) Tpc (℃)
156.072 151.177
MJF parts
SLS raw PA12
SLS parts
62.8
74.477
51.014
171.823 179.417
170.992 183.895
171.205 179.737
155.81 150.893
156.564 151.435
53.937
37.951
186.118
–
– 30
15.182 35.58
159.301 155.288
41.954
187.173
– 24.37
Table 7 The average mechanical properties of MJF, SLS, injection moulding (IM) and extrusion (E) specimens. Type
Tensile Strength (MPa)
Elongation at Break (%)
Young's Modulus (GPa)
Impact Toughness (J/cm2)
Density (g/cm3)
MJF SLS IM [18] E [18]
40.10 ± 1.49 43.61 ± 0.46 53 46
17.45 ± 3.87 31.55 ± 2.93 > 50 > 50
1.42 ± 0.04 1.76 ± 0.02 1.7 1.6
0.26 ± 0.01 0.34 ± 0.03 – –
0.93 ± 0.08 0.99 ± 0.04 1.03 1.02
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Fig. 11. The SEM fracture surface images of (a) MJF, (b) SLS.
is the laser scanning area. In MJF, infrared lamps scans the whole building platform, and the sintered area is the region where the inks are applied, which sharply and fundamentally raised the printing speed. According to the data sheet, the printing speed of the MJF under standard mode is about 10 s per layer which equals to 10,792 mm2/s; however, the maximum print speed for the SLS is just 5 m/ s × 0.25 mm = 1250 mm2/s, whose maximum laser scan speed is 5 m/ s. MJF shows great promotion of the printing speed compared to SLS, however as discussed in the previous section, the mechanical properties of the MJF parts was not as good as SLS. In terms of the mechanical properties, the MJF printed parts showed some inferior compared to the SLS parts. SLS has been well developed for several decades, and the processing parameters for the PA12 is mature and well optimised. As an emerging technology, the MJF process and the processing parameters for different materials are still under developing. Furthermore, based on the authors’ knowledge, the PA12 powders, which are almost the same for SLS and MJF, have been processed by SLS for a long time. As a new 3D printing process, a specific or exclusive powder materials for the MJF process should be developed, which could further unlock the potential of the MJF process. The excellent printing speed reveals the potential of MJF in the fast growing AM field, and the following aspects could be focused to further improve the industry applications of this technology. Firstly, the printing materials for the MJF should be developed and expanded. Currently, similar to SLS, the PA12 and its composites are the main materials used for MJF. As a unique technology, the powder materials should be investigated specifically for MJF. Secondly the processing parameters should be continuously optimized during the material development. Thirdly, functional ink, such as nanofiller ink, can be developed and applied to facilitate the printing process and improve the properties of the MJF parts.
4) The mechanical properties of the SLS specimens were higher than MJF specimens, especially Young's modulus and elongation at break, which could be due to the higher density of the SLS specimens. 5) Compared with SLS, the printing speed of MJF sharply and fundamentally raised due to the innovative face-sintering processing method. It is believed that materials and process development will further push the industry applications of MJF. Acknowledgements The study was supported National Natural Science Foundation of China (Grant No. 51805239) and the Shenzhen Key Laboratory for Additive Manufacturing of High-performance Materials (Grant No: ZDSYS201703031748354). References [1] Holmes Larry Jr. R, Riddick Jaret C. Research summary of an additive manufacturing technology for the fabrication of 3D composites with tailored internal structure. JOM 2014;66:270–4. https://doi.org/10.1007/s11837-013-0828-4. [2] Jacobs Paul F. Stereolithography and other RP&M technologies-from rapid prototyping to rapid tooling. Dearborn: Society of Manufacturing Engineers; 1996. [3] Melchels Ferry PW, Domingos Marco AN, Klein Travis J, Malda Jos, Bartolo Paulo J, Hutmacherad Dietmar W. Additive manufacturing of tissues and organs. Prog Polym Sci 2012;37(8):1079–104. https://doi.org/10.1016/j.progpolymsci.2011.11.007. [4] Ngo Tuan D, Kashani Alireza, Imbalzano Gabriele, Nguyen Kate TQ, Hui David. Additive manufacturing (3D printing): a review of materials, methods, applications and challenges. Compos Part B Eng 2018;143:172–96. https://doi.org/10.1016/j. compositesb.2018.02.012. [5] Melchels FP, Feijen J, Grijpma DW. A review on stereolithography and its applications in biomedical engineering. Biomaterials 2010;31:6121–30. https://doi.org/ 10.1016/j.biomaterials.2010.04.050. [6] Zein Iwan, Hutmacher Dietmar W, Tan Kim Cheng, Teoh Swee Hin. Fused deposition modeling of novel scaffold architectures for tissue engineering applications. Biomaterials 2002;23:1169–85. https://doi.org/10.1016/S0142-9612(01)00232-0. [7] Kruth J-P, Mercelis P, Vaerenbergh J Van, Froyen L, Rombouts M. Binding mechanisms in selective laser sintering and selective laser melting. Rapid Prototyp J 2005;11:26–36. https://doi.org/10.1108/13552540510573365. [8] Carl R. Deckard, inventor; University of Texas System, assignee. Method and apparatus for producing parts by selective sintering. United States patent US 4863538A. 1989 Sep 5. [9] Olakanmi EO, Cochrane RF, Dalgarno KW. A review on selective laser sintering/ melting (SLS/SLM) of aluminium alloy powders: processing, microstructure, and properties. Prog Mater Sci 2015;74:401–77. https://doi.org/10.1016/j.pmatsci. 2015.03.002. [10] Shirazi Seyed Farid Seyed, Gharehkhani Samira, Mehrali Mehdi, Yarmand Hooman, Metselaar Hendrik Simon Cornelis, Kadri Nahrizul Adib, et al. A review on powderbased additive manufacturing for tissue engineering: selective laser sintering and inkjet 3D printing. Sci Technol Adv Mater 2015;16:033502. https://doi.org/10. 1088/1468-6996/16/3/033502. [11] Yuan Shangqin, Shen Fei, Bai Jiaming, Chua Chee Kai, Wei Jun, Zhou Kun. 3D soft auxetic lattice structures fabricated by selective laser sintering: TPU powder evaluation and process optimization. Mater Des 2017;120:317–27. https://doi.org/10. 1016/j.matdes.2017.01.098. [12] Yuan Shangqin, Bai Jiaming, Chua Chee Kai, Wei Jun, Zhou Kun. Material evaluation and process optimization of CNT-Coated polymer powders for selective laser sintering. Polymers 2016;8:370. https://doi.org/10.3390/polym8100370. [13] Bai Jiaming, Goodridge Ruth D, Yuan Shangqin, Zhou Kun, Chua Chee Kai, Wei Jun. Thermal influence of CNT on the polyamide 12 nanocomposite for selective laser sintering. Molecules 2015;20:19041–50. https://doi.org/10.3390/
4. Conclusions This study investigated the process and parts performance of PA12 parts fabricated by SLS and MJF. Specimens were printed to evaluate the morphology, mechanical properties and thermal characteristic of the PA12 parts. The major findings include: 1) Both PA12 powder used by SLS and MJF were near ellipsoid geometry. The particle size and distribution of the two types were close. But the MJF powder had higher crystallinity and wider sintering window. 2) The top surface roughness of SLS and MJF parts were extremely close, but the bottom surface roughness of MJF part, was distinctly better than SLS part bottom’s, which might be due to the applying of detailing agent. 3) Due to the powerful instant heating capability of laser, the unmelted core of SLS was distinctly smaller than MJF. The crystallinity of SLS parts was higher than MJF, which lead better impact property and dimension accuracy. 425
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