Mechanical Properties Optimization of Poly-Ether-Ether-Ketone Via Fused Deposition Modeling

materials

Article Mechanical Properties Optimization of Poly-Ether-Ether-Ketone via Fused Deposition Modeling

Xiaohu Deng 1,2 ID , Zhi Zeng 3,4,*, Bei Peng 3, Shuo Yan 3 and Wenchao Ke 3

1 National Local Joint Engineering Laboratory of Intelligent Manufacturing Oriented Automobile Die & Mold, Tianjin University of Technology and Education, Tianjin 300222, China; [email protected] 2 Tianjin Key Laboratory of High Speed Cutting and Precision Machining, Tianjin University of Technology and Education, Tianjin 300222, China 3 School of Mechatronics Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China; [email protected] (B.P.); [email protected] (S.Y.); [email protected] (W.K.) 4 Institute of Electronic and Information Engineering of UESTC in Guangdong, Dongguan 523808, China * Correspondence: [email protected] (Z.Z.); Tel.: +86-028-6183-229

Received: 2 January 2018; Accepted: 28 January 2018; Published: 30 January 2018

Abstract: Compared to the common selective laser sintering (SLS) manufacturing method, fused deposition modeling (FDM) seems to be an economical and efficient three-dimensional (3D) printing method for high temperature polymer materials in medical applications. In this work, a customized FDM system was developed for polyether-ether-ketone (PEEK) materials printing. The effects of printing speed, layer thickness, printing temperature and filling ratio on tensile properties were analyzed by the orthogonal test of four factors and three levels. Optimal tensile properties of the PEEK specimens were observed at a printing speed of 60 mm/s, layer thickness of 0.2 mm, temperature of 370 ◦C and filling ratio of 40%. Furthermore, the impact and bending tests were conducted under optimized conditions and the results demonstrated that the printed PEEK specimens have appropriate mechanical properties.

Keywords: fused deposition modeling; polyether-ether-ketone; tensile properties; ﬂexural properties; impact properties

1. Introduction Poly-ether-ether-ketone (PEEK) has received widespread attention due to the good mechanical properties, biocompatibility and elastic modulus closing to human bones, and is considered one of the most promising bone repair materials [1,2]. As demands of PEEK increase, additive manufacturing (AM) is starting to be used in the forming process of PEEK components. In the past decades, selective laser sintering (SLS) is the most popular additive manufacturing technology for PEEK [3,4]. However, the high cost, poor penetrability and concentrated beam of laser restrict it from sintering large areas or laminates. So far, fused deposition modeling (FDM) is one of the most mature, popular and fastest growing three-dimensional (3D) printing methods, which are employed to produce large size medical implants. Valentan et al. [5] reported that FDM has been an alternative method to process PEEK parts. The mechanical properties of FDM parts are mainly dependent on production parameters such as printing temperature, printing speed, scanning path, nozzle diameter, layer thickness, chamber temperature, and ﬁlling ratio [6]. Several studies have been carried out to evaluate various mechanical parameters of polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS), including tensile,

Materials 2018, 11, 216; doi:10.3390/ma11020216 www.mdpi.com/journal/materials Materials 2018, 11, 216 2 of 11 compressive, flexural and impact strength. Gordon et al. [7] presented the effects of several production parameters on the mechanical properties of FDM specimens with PLA. Luzanin et al. [8] discussed the influence of the layer thickness, deposition angle and infill on the maximum flexural force in FDM specimens made of PLA. Ismail et al. [9] developed the functional relationship between the FDM parameters (raster angles and part orientations) and strength, surface roughness and production cost of specimens. Pfeifer et al. [10] developed a theoretical relationship among four FDM printing parameters (extrusion rate, nozzle travel speed, layer height and path width). Chacon et al. [11] investigated the influences of build orientation, layer thickness and feed rate on the mechanical performance of PLA specimens. Compared to the PLA and ABS wires, PEEK has a higher melting point, it is liable to cause excessive thermal stress (unevenly distributed between layers) and thermal cracks. Therefore, PEEK material needs specific parameter setting to adapt its own characteristics [12]. Wu et al. [13,14] investigated the mechanical strengths and the thermal deformation performance of PEEK material with different printing parameter settings in FDM. PEEK can be processed into a filament, and specimens can be manufactured by using several build orientations and extrusion paths. Kazi et al. [15] investigated the relationship between various thermal processing conditions (the ambient temperature, the nozzle temperature and heat treatment methods) in the FDM process, and crystallinity and mechanical properties. Vaezi et al. [16] investigated the mechanical properties of extrusion freeformed porous PEEK parts. Sun et al. [17] analyzed the effect of temperature on the mechanical properties and forming precision. Yang et al. [18] investigated the relationship among various thermal processing conditions in the FDM process, and mechanical properties. Gianluca et al. [19] investigated the tensile properties of the PEEK printed specimens for FDM printing technology. It was verified that PEEK has excellent mechanical properties compared with other filament materials. The mechanical properties of the 3D-printed parts are important indices for evaluating printing quality. However, it has not been sufficiently studied. Therefore, it is important to evaluate the mechanical properties of a new FDM technique for PEEK. The main purpose of the present paper is to optimize the process parameters of FDM for PEEK components. A quadrature design of four factors and three levels was taken to study the processing parameters effect on the tensile strength and microstructure, as applying the printing temperature, printing speed, layer thickness and filling ratio. Furthermore, the bending test and the impact test were also implemented to verify the optimum process parameters.

2. Materials and Experiments It is generally accepted that higher or lower printing temperature, layer thickness or printing velocity may result in warpage of the specimen, which are directly related to properties of the FDM components [6]. The present research investigated the influence of the printing temperature, printing speed, layer thickness and filling ratio on the mechanical properties of the PEEK specimens. Specimens were made from 1.75 mm PEEK-1000 bar (Zhongshan Yousheng Plastic materials Co., Ltd., Zhongshan, Guangdong) and the custom-built FDM equipment, as shown in Figure1. Optimum FDM process parameters of PEEK tensile specimens were selected by the orthogonal test of four factors and three levels, with the printing temperature, printing speed, layer thickness and filling ratio, represented as A, B, C and D respectively. In the present study, the interactions among four parameters were neglected in order to simplify the test. Simplification has been proved actually feasible by the previous literature [10,19,20]. The orthogonal factor level table is shown in Table1. L9 orthogonal array design was selected to improve experimental efficiency, as shown in Table2. To enhance the experimental precision and the reliability of statistical analysis, as well as to eliminate the interference of experimental error, each test was conducted three times. Tensile properties of the PEEK specimens were measured with the universal material experiment machine (KL-WS-30S, Dongguan Kunlun Instrument Co., Ltd., Dongguan, Guangdong) by ISO 178:2001 Standard Test Materials 2018, 11, 216 3 of 11

Method. The micro-structure and morphology of tensile specimens were examined using scanning electronMaterials 2018 microscopy, 11, x FOR (SEM)PEER REVIEW (TESCAN VEGA3, Brno, Czech Republic). 3 of 11

FigureFigure 1.1.Schematic Schematic of of customized customized polyether-ether-ketone polyether-ether-ketone (PEEK) (PEEK) fused fused deposition deposition modeling modeling (FDM) system. (FDM) system. Table 1. Orthogonal factor level table for PEEK FDM. Table 1. Orthogonal factor level table for PEEK FDM. Factor Level Factor ◦ Level PrintingPrinting Speed Speed (mm/s) (mm/s) LayerLayer Thickness Thickness (mm) Printing Temperature Temperature ( C) (°C) FillingFilling Ratio Ratio (%) (%) A B C D A B C D 1 120 20 0.20 0.20 350 350 20 20 2 40 0.25 360 40 2 340 60 0.25 0.30 370 360 60 40 3 60 0.30 370 60

Table 2. L9 orthogonal array design for PEEK FDM. Table 2. L9 orthogonal array design for PEEK FDM.

Factor No.No. ABCDAB CD 11 20 20 0.200.20 350 350 20 20 22 20 20 0.250.25 360 360 40 40 33 20 20 0.300.30 370 370 60 60 4 40 0.20 360 60 54 40 40 0.250.20 360 370 60 20 65 40 40 0.300.25 370 350 20 40 76 60 40 0.200.30 350 370 40 40 87 60 60 0.250.20 370 350 40 60 9 60 0.30 360 20 8 60 0.25 350 60 9 60 0.30 360 20 The impact test and bending test were conducted on the printed specimens by using the optimized processingThe impact parameters. test and The impactbending test test was were carried cond outucted with on an the impact printed velocity specimens of 3.5 m/s, by impactingusing the angleoptimized of 150 processing◦ and impact parameters. energy ofTh 2.5e impact J. Impact test testingwas carried specimens out with were an designed impact velocity according of 3.5 to ISOm/s, 179-1:2000 impacting. Theangle three-point of 150° and bending impact tests energy were of performed 2.5 J. Impact according testing tospecimens the ISO 527-2:1993 were designedwith theaccording loading to rate ISO of 2179-1:2000. mm/min. The Both three-point the impact testbending and three-pointtests were ﬂexuralperformed test according were also conductedto the ISO using527-2:1993 KL-WS-30S with the universal loading material rate of experiment 2 mm/min. machine Both the (KL-WS-30S, impact test Dongguan and three-point Kunlun flexural Instrument test Co.,were Ltd., also Dongguan, conducted Guangdong). using KL-WS-30S The geometric universal models material are shown experiment in Figure machine2. (KL-WS-30S, Dongguan Kunlun Instrument Co., Ltd., Dongguan, Guangdong). The geometric models are shown in Figure 2.

Materials 2018, 11, 216 4 of 11 Materials 2018, 11, x FOR PEER REVIEW 4 of 11 Materials 2018, 11, x FOR PEER REVIEW 4 of 11

Figure 2. Geometric models of the mechanical test specimens. (a) Tensile specimen; (b) Bending Figure 2. Geometric models of the mechanical test specimens. (a) Tensile specimen; (b) Bending specimen; specimen;Figure 2. (Geometricc) Impact specimen.models of the mechanical test specimens. (a) Tensile specimen; (b) Bending (c) Impact specimen. specimen; (c) Impact specimen. 3. Results and Discussion 3. Results3. Results and and Discussion Discussion The stress-strain relationship was calculated, as shown in Figure 3. The elastic modulus, tensile strength,The stress-strainThe elongationstress-strain relationship for relationship different was processwas calculated, calculated, paramete asrs shown are listed in in Figure Figurein Table 3.3 The. The3. elasticThe elastic maximum modulus, modulus, tensile tensile tensile strength,strengthstrength, elongation is elongation 40.0 ± 4.4 for MPa, for different different which processis processlower than parametersparamete that reportedrs are are listed listedfor printed in in Table Table PEEK 3.3 .The with The maximum an maximum infill density tensile tensile strengthofstrength 100% is 40.0 [14]. is 40.0± N4.4ormally, ± 4.4 MPa, MPa, the which whichmechanical is is lower lower properties thanthan that are reported reported proportional for for printed printedto the PEEK infill PEEK densitywith with an duringinfill an infill density FDM density of 100%process.of 100% [14]. The[14]. Normally, tensile Normally, strength the the mechanical mechanical dropped to properties properties 49.2 MPa are when prop proportional theortional infill density to tothe the infill decreased infill density density to during 80% during in FDM ref. FDM [16],process. which The wouldtensile strengthstill maintain dropped an toappropriate 49.2 MPa whenstrength the infilland densitycause lower decreased weight to 80%of PEEKin ref. process. The tensile strength dropped to 49.2 MPa when the infill density decreased to 80% in ref. [16], components.[16], which would still maintain an appropriate strength and cause lower weight of PEEK which would still maintain an appropriate strength and cause lower weight of PEEK components. components. Table 3. Tensile properties of orthogonal array designed specimens. TableTable 3. Tensile3. Tensile properties properties of oforthogonal orthogonal array array designed designed specimens. specimens. Factor No. Tensile Strength (MPa) Elongation (%) Young’s Modulus (MPa) A BFactor C D No. TensileTensile Strength Strength (MPa) Elongation Elongation (%) (%) Young’s Young’s Modulus Modulus (MPa) (MPa) No.1 1A 1B 1C 1D 27.2 ± 1.5 11.6 ± 1.1 379.9 ± 20.0 ABCD 21 1 1 2 1 2 1 2 1 33.2 27.2 ± ± 7.3 1.5 11.8 11.6 ± ± 1.4 1.1 416.3 379.9 ± ± 20.6 20.0 ± ± ± 132 1 1 3 1 2 3 2 1 3 2 1 35.5 33.227.2 ± ± 5.0 7.31.5 11.1 11.8 11.6 ± ± 1.9 1.41.1 453.8 416.3 379.9 ± ± 91.3 20.620.0 2 1 2 2 2 33.2 ± 7.3 11.8 ± 1.4 416.3 ± 20.6 4 2 1 2 3 29.9 ± 4.2 8.8 ± 1.3 576.5 ± 49.9 33 1 1 3 3 3 3 3 3 35.5 ± 5.05.0 11.1 11.1 ±± 1.91.9 453.8 453.8 ± ±91.391.3 454 2 2 2 1 1 3 2 2 1 3 3 33.7 29.9 ± ± 3.2 4.24.2 13.5 8.8 8.8 ±± 1.31.3 423.6 576.5 576.5 ± ± 15.2 ±49.949.9 565 2 2 3 2 2 1 3 3 2 1 1 25.6 33.7 ± ± 1.5 3.23.2 11.3 13.5 13.5 ± ±± 0.7 1.31.3 355.2 423.6 423.6 ± ± 25.0 ±15.215.2 676 23 2 1 3 3 3 1 1 2 2 2 40.0 25.6 ± ± 4.4 1.51.5 14.3 11.3 11.3 ± ±± 1.1 0.70.7 522.9 355.2 355.2 ± ± 32.1 ±25.025.0 787 3 3 2 1 1 1 3 3 3 2 2 39.1 40.0 ± ± 5.2 4.44.4 10.4 14.3 14.3 ± ±± 0.9 1.11.1 533.1 522.9 522.9 ± ± 51.4 ±32.132.1 ± ± ± 898 3 3 3 2 2 2 1 1 1 3 3 27.9 39.1 ± ± 1.5 5.25.2 8.510.4 10.4 ± ±0.8 0.90.9 475.7 533.1 533.1 ± ± 43.2 51.451.4 9 3 3 2 1 27.9 ± 1.5 8.5 ± 0.8 475.7 ± 43.2 9 3 3 2 1 27.9 ± 1.5 8.5 ± 0.8 475.7 ± 43.2

Figure 3. Stress-strain curves of PEEK FDM tensile specimens designed in Table 2. Figure 3. Stress-strain curves of PEEK FDM tensile specimens designed in Table 2. Figure 3. Stress-strain curves of PEEK FDM tensile specimens designed in Table2.

Materials 2018, 11, 216 5 of 11

In order to obtain the optimized process parameters, range analysis was used with the elastic modulus, tensile strength and elongation. The range analysis of orthogonal experiments is listed in Tables4–6, respectively. In these tables, K iij (i = 1, 2, 3) is the sum of the experimental results in terms of level i, kij is the mean of Kij, R is range, which can be calculated by

R = max (kij) − min(kij) (1)

As shown in the Table4, four level ranges of tensile strength are largely consistent, which have almost the same influence on the tensile strength. It can be seen that the optimal combination of tensile strength is A3B2C3D3, which is corresponding to the printing velocity of 60 mm/s, layer thickness of 0.25 mm, printing temperature of 370 ◦C and filling rate of 60%. In contrast, four ranges of elongation have a significant difference, as shown in Table5. The sequence according to the effect of the elongation is as follows: printing temperature, filling rate, layer thickness and printing velocity. It can be concluded that the optimal combination of elongation is A1B2C3D2, which is corresponding to the printing velocity of 20 mm/s, layer thickness of 0.25 mm, printing temperature of 370 ◦C and filling rate of 40%.

Table 4. Range analysis of tensile strength.

Range A B C D

K1j 95.8 97.1 91.9 88.8 K2j 89.1 105.9 98.8 98.8 K3j 107.1 89.0 109.2 104.4 k1j 31.9 32.4 30.6 29.6 k2j 29.7 35.3 32.9 32.9 k3j 35.7 29.7 36.4 34.8 R 6.0 5.6 5.8 5.0 optimum levels A3 B2 C3 D3 optimum assembly A3B2C3D3 order of priority A C B D

Table 5. Range analysis of elongation.

Range A B C D

K1j 34.5 34.6 33.4 33.6 K2j 33.6 35.8 29.0 37.4 K3j 33.2 30.9 38.9 30.3 k1j 11.5 11.5 11.1 11.2 k2j 11.2 11.9 9.7 12.5 k3j 11.1 10.3 13.0 10.1 R 0.4 1.6 3.3 2.4 optimum levels A1 B2 C3 D2 optimum assembly A1B2C3D2 order of priority D C B A

Table6 shows the range analysis of elastic modulus. The optimal combination of the elastic modulus is A3B1C2D3, which is corresponding to the printing velocity of 60 mm/s, layer thickness of 0.2 mm, printing temperature of 360 ◦C and ﬁlling rate of 60%. The sequence according to the magnitude of the elastic modulus’ range is as follows: ﬁlling rate, printing velocity, printing temperature and layer thickness. Materials 2018, 11, x FOR PEER REVIEW 6 of 11 Materials 2018, 11, 216 6 of 11 Table 6. Range analysis of elastic modulus.

RangeTable 6. Range analysisA of elastic B modulus. C D K1j 1250.1 1479.3 1268.2 1279.2 RangeK2j 1355.3 A 1373.0 B 1468.5 C 1294.4 D

K1j3j 1250.1 1531.6 1479.31284.7 1268.21400.3 1279.21563.5 Kk1j2j 1355.3416.7 1373.0493.1 1468.5422.7 1294.4426.4 K 1531.6 1284.7 1400.3 1563.5 k2j3j 451.8 457.7 489.5 431.5 k1j 416.7 493.1 422.7 426.4 k3j 510.5 428.2 466.8 521.2 k2j 451.8 457.7 489.5 431.5 R 93.9 64.9 66.8 94.8 k3j 510.5 428.2 466.8 521.2 optimumR levels 93.9 A3 64.9 B1 66.8 C2 94.8 D3 optimumoptimum assembly levels A3 B1 A3B1C2D3 C2 D3 optimumorder of priority assembly A3B1C2D3 D A C B order of priority D A C B It can be concluded that the optimal tensile properties vary with different process parameters. Therefore,It can bethe concludedinfluence thatof these the optimal factors tensileon the properties tensile properties vary with should different be process comprehensively parameters. consideredTherefore, the in influencedetermining of these the optimal factors on printing the tensile parameters. properties Factor should A beexerted comprehensively the greatest considered influence onin determiningthe tensile strength. the optimal Moreover, printing it parameters. is of second Factorary importance A exerted theto the greatest elastic influence modulus. on Therefore, the tensile thestrength. optimal Moreover, choice of it isfactor of secondary A is A3. importanceIn the same to analysis, the elastic the modulus. optimum Therefore, of factor theB, C optimal and D choiceis B1, C3of factorand AD2, is A3.respectively. In the same From analysis, the theabove, optimum the op oftimal factor combination B, C and D is of B1, printing C3 andD2, parameters respectively. is A3B1C3D2,From the above, which the corresponds optimal combination to the printing of printing velo parameterscity 60 mm/s, is A3B1C3D2, layer thickness which corresponds0.2 mm, printing to the ◦ temperatureprinting velocity 370 60°C mm/s,and filling layer rate thickness 40%. 0.2 mm, printing temperature 370 C and filling rate 40%. The mechanical tests show that the tensile strength of No. 7 7 and and 8 is higher, and elongation of No. 5 and No. 7 7 is is greater. greater. It can be concluded that the mechanical properties of No. 7 are the best in this investigation.investigation. ItIt isis consistent consistent with with the the range rang analysise analysis results results of orthogonal of orthogonal test. Intest. order In order to verify to verifythe results, the theresults, tensile the specimens tensile specimens were prepared were by prepared using optimized by using parameters optimized of A3B2C3D3parameters and of A1B2C3D2.A3B2C3D3 and Figure A1B2C3D2.4 shows the Figure comparison 4 shows on the tensile compar strengthison on and tensile elongation strength for and different elongation printing for differentparameters. printing The tensileparameters. strength The increased tensile tostre 41.2ngth MPa increased using A3B2C3D3 to 41.2 MPa processing using parametersA3B2C3D3 whichprocessing is only parameters 2.8% higher which than is only that 2.8% of No. higher 7 specimen, than that while,of No. the7 specimen, elongation while, is lower the elongation by 21.1%. isLikewise, lower by when 21.1%. the Like A1B2C3D2wise, when parameters the A1B2C3D2 was applied, parameters the elongation was applied, could the be elongation improved could to 14.8%, be improvedbut the tensile to 14.8%, strength but the is lower tensile by strength 13.3%comparing is lower by to 13.3% that comparing of No. 7 specimen. to that of Therefore,No. 7 specimen. it can Therefore,be concluded it can that be the concluded excellent that integrated the excellent tensile integrated properties tensile were obtainedproperties using were theobtained No. 7 using FDM theprocessing No. 7 FDM parameters processing (A3B1C3D2). parameters (A3B1C3D2).

Figure 4. ComparisonComparison on on tensile tensile properties for different printing parameters.

The reason is that too high or too low process parameters may exert poor mechanical The reason is that too high or too low process parameters may exert poor mechanical performance. performance. Although, the rapid printing speed could improve the printing efficiency, the Although, the rapid printing speed could improve the printing efficiency, the extrusion materials do extrusion materials do not display enough plasticizing effect. If the printing speed is too slow, the not display enough plasticizing effect. If the printing speed is too slow, the fuse filament diameter fuse filament diameter will be uneven, and the fusion bonding would be much more difficult. The will be uneven, and the fusion bonding would be much more difficult. The printing temperature also printing temperature also has a significant effect on the level of crystallinity, and thus the has a significant effect on the level of crystallinity, and thus the mechanical properties [17]. Too high mechanical properties [17]. Too high extrusion temperature may cause the material degradation or

Materials 2018, 11, 216 7 of 11

Materials 2018, 11, x FOR PEER REVIEW 7 of 11 extrusion temperature may cause the material degradation or extrudate, which cannot retain its shape uponextrudate, deposition, which cannot resulting retain in filament its shape deformation upon deposition, and dimensional resulting in inaccuracy. filament deformation However, if and the extrusiondimensional temperature inaccuracy. is notHowever, high enough, if the theextrusion material temperature does not have is enoughnot high time enough, to get the fully material melted whichdoes not could have result enough in the time nozzle to get clogging. fully melted Layer which thickness could affects result the in width the nozzle and speed clogging. of filament, Layer andthickness thusbonding affects the and width mechanical and speed properties. of filament, Thefilling and thus rate bonding has a direct and effect mechanical on the realproperties. section areaThe offilling FDM rate components has a direct and effect the on tensile the real stress. section area of FDM components and the tensile stress. In this investigation,investigation, No. 4 and No. 7 7 specimens were selected to analyze the effect of process parameters on tensile properties. Figure 55 showsshows thethe fracturedfractured PEEKPEEK specimensspecimens afterafter thethe tensiletensile test.test. The elongation rate is 8.8%8.8% andand 14.3%14.3% respectively.respectively. InIn addition,addition, No.No. 7 specimen has higher tensile strength than No. 4 specimen.

Figure 5. Fractured tensile specimens. ( a) No. 4 4 specimen specimen in in Table Table3 3;;( (b) No. 7 specimen in Table 3 3..

Figure 6 shows the microstructures of two tensile PEEK specimens. It is evident that the lower strengthFigure specimen6 shows has the more microstructures defects, such of as two air tensile gaps and PEEK fusion specimens. line between It is evident infill filaments. that the lower It can strengthbe seen specimenfrom Figure has 6a more that defects, the interlamellar such as air gaps connectivity and fusion shows line between good fusion infill filaments. effects for It canNo. be7 seenspecimen, from Figurewhich6 isa thatevidence the interlamellar of uniform connectivityfusion PEEK shows wire deposition good fusion during effects the for FDM No. 7 process. specimen, In whichFDM, isthe evidence first deposited of uniform layer fusion was PEEKbetween wire the deposition aluminum during build the plate FDM an process.d fused InPEEK FDM, wire, the firstthe depositedconnectivity layer basically was between depends the on aluminum the subcooled build plate temperature and fused among PEEK wire, the fused the connectivity wire temperature, basically dependsbuild plate on temperature the subcooled and temperature working chamber among the temperature. fused wire temperature,In this investigation, build plate the temperature build plate and working chamber temperature. In this investigation, the build plate temperature and working temperature and working chamber temperature◦ ◦ were set at 95 °C and 150 °C respectively and kept chamberstable considering temperature the were high setconductivity at 95 C and of 150aluminumC respectively alloy, which and kept could stable develop considering a dense the joining high conductivityon the base plate. of aluminum For the alloy,above which layers, could the prin developting coherence a dense joining and connectivity on the base plate. primarily For the resulted above layers,from the the fused printing temperature coherence and and deposited connectivity speed primarily listed in resulted Table 4. from By thecomparing fused temperature with the No. and 4 depositedspecimen speedmicrostructure listed in Tablein Figure4. By comparing6c, it can be with concluded the No. 4that specimen the lower microstructure fused temperature in Figure and6c, itdeposited can be concluded speed could that result the lower in more fused slag temperature inclusion, micro-pores and deposited and speed un-uniform could resultdeposition. in more slag inclusion,Figure micro-pores 6b also shows and un-uniformthat this region deposition. possesses excellent filament-to-filament and interlayer bonding.Figure Furthermore,6b also shows the that fracture this regioncross-sections possesses show excellent the failure filament-to-filament mechanism. The and center interlayer of the bonding.specimen is Furthermore, the initial failing the fracture zone for cross-sections air gaps. This show is contrast the failure to the mechanism.SLS on PEEK The powders, center which of the specimenhad a high is theporosity initial and failing many zone connected for air gaps. pores. This In is contrastthe SLS toprocessing, the SLS on the PEEK feedstock powders, powders which hadare aalways high porosity not totally and molten many connected or flatten pores. in the In as-sprayed the SLS processing, coatings. theThe feedstock powders powders are only are partially always notmolten totally in the molten flame or spraying flatten in process the as-sprayed and incompact coatings. connected The powders with each are onlyother partially when sprayed molten on in the flamesubstrate spraying or on process the pre-deposited and incompact coating connected [21]. SLS with process each other boundaries when sprayed are shown on the concerning substrate orlaser on sintering the pre-deposited temperature coating and [21energy]. SLS input. process The boundaries powder morphology are shown concerning and powder laser preparation sintering temperaturealso have great and influence energy input.on the Themechanical powder performance morphology of and the powder composite. preparation In this investigation, also have great the influenceas-received on PEEK the mechanical wires were performance molten and extruded of the composite. in the nozzle In this during investigation, the FDM theprocess, as-received which PEEKcould wiresresult werein a moltenrelatively and tight extruded connection in the nozzlebetween during the layers the FDM by process,extrusion which force. could Meanwhile, result in aPEEK relatively powders tight were connection only betweenmelted by the laser layers in by the extrusion SLS, which force. tends Meanwhile, to higher PEEK porosity powders among were onlyparticles melted [4]. by laser in the SLS, which tends to higher porosity among particles [4].

Materials 2018, 11, 216 8 of 11 Materials 2018, 11, x FOR PEER REVIEW 8 of 11

Figure 6. Microstructure and fracture surface. ( a) side face microstructure of tensile specimen No. 7; ((b)) No.No. 77 fracturedfractured surfacesurface ofof tensile tensile sample sample No. No. 7; 7; ( c()c) side side face face microstructure microstructure of of tensile tensile sample sample No. No. 4; (4;d )(d fractured) fractured surface surface of tensileof tensile sample sample No. No. 4. 4.

According to the analysis above, the optimal process parameters are set as printing speed of 60 According to the analysis above, the optimal process parameters are set as printing speed of mm/s, layer thickness of 0.2 mm, temperature of 370 °C and filling ratio of 40%. Both the three-point 60 mm/s, layer thickness of 0.2 mm, temperature of 370 ◦C and filling ratio of 40%. Both the three-point bending test and impact test were also performed to verify the mechanical properties. bending test and impact test were also performed to verify the mechanical properties. Figure 7a shows that the fractured bending specimens. The corresponding bending Figure7a shows that the fractured bending specimens. The corresponding bending stress-strain stress-strain curves are shown in Figure 7b. The maximum bending strength and bending modulus curves are shown in Figure7b. The maximum bending strength and bending modulus are 68.2 MPa are 68.2 MPa and 1658.6 MPa, respectively. It can be concluded from Figure 6 that bending strength and 1658.6 MPa, respectively. It can be concluded from Figure6 that bending strength is greater than is greater than the tensile strength for the same process parameters. The reason for this is that the the tensile strength for the same process parameters. The reason for this is that the specimens are specimens are subjected to both tensile and compressive stresses during the bending tests. subjected to both tensile and compressive stresses during the bending tests. Moreover, the quality of the Moreover, the quality of the interlayer bonding has a great influence on flexural properties, which interlayer bonding has a great influence on flexural properties, which mainly correspond to the layer mainly correspond to the layer thickness [10]. It also can be seen from Figure 6 that the failure thickness [10]. It also can be seen from Figure6 that the failure strength is 60.1 Mpa. The difference strength is 60.1 Mpa. The difference between the bending strength and the failure strength show the between the bending strength and the failure strength show the plastic deformation ability of the plastic deformation ability of the specimen which indicates the present specimen can sustain certain specimen which indicates the present specimen can sustain certain plastic deformation. plastic deformation. Figure8 shows the impact test results. The mean impact strength and the absorbed energy of Figure 8 shows the impact test results. The mean impact strength and the absorbed energy of three specimens are 101.2 KJ/m2 and 3.25 J, respectively. In general, the impact strength of ABS three specimens are 101.2 KJ/m2 and 3.25 J, respectively. In general, the impact strength of ABS approximates 15~40 KJ/m2. It is evident that the impact strength of PEEK is better than the common approximates 15~40 KJ/m2. It is evident that the impact strength of PEEK is better than the common thermoplastic [22]. Moreover, the raster orientations have a significant effect on the absorbed energy, thermoplastic [22]. Moreover, the raster orientations have a significant effect on the absorbed and the present specimens absorbed more energy comparing to the previous literature results for energy, and the present specimens absorbed more energy comparing to the previous literature results for the same raster orientation [15]. Two main factors of fracture properties are the layer thickness and infilling rate. The high-filling and low layer thickness could strengthen the structure

Materials 2018, 11, 216 9 of 11

Materialsthe same 2018 raster, 11, x FOR orientation PEER REVIEW [15]. Two main factors of fracture properties are the layer thickness9 of and 11 infilling rate. The high-filling and low layer thickness could strengthen the structure by providing Materials 2018, 11, x FOR PEER REVIEW 9 of 11 byadditional providing bulk additional and crack bulk resistance and crack to the specimensresistance [to7]. the Therefore, specimens low layer[7]. Therefore, thickness islow necessary layer to improve the fracture properties under the low infilling density. thicknessby providing is necessary additional to improve bulk andthe fracturecrack resistance properties to under the thespecimens low infilling [7]. Therefore,density. low layer thickness is necessary to improve the fracture properties under the low infilling density.

FigureFigure 7. 7. BendingBending test test tests tests of of PEEK PEEK FDM FDM specimens. specimens. (a (a) )Fractured Fractured bending bending specimens; specimens; (b (b) )Bending Bending stress-strain curves. stress-strainFigure 7. Bending curves. test tests of PEEK FDM specimens. (a) Fractured bending specimens; (b) Bending stress-strain curves.

Figure 8. Impact test of PEEK FDM specimens. (a) Fractured impact specimens; (b) Impact strength and absorbed energy. Figure 8. Impact test of PEEK FDM specimens. (a) Fractured impact specimens; (b) Impact strength Figure 8. Impact test of PEEK FDM specimens. (a) Fractured impact specimens; (b) Impact strength and absorbed energy. 4. Conclusionsand absorbed energy. 4. Conclusions 4. ConclusionsThe effect of FDM processing parameters on the mechanical properties of high temperature PEEK Thematerials effect wasof FDM first processingsystematically parameters investigated on th ine mechanicalthis study, propertiesincluding theof hightensile, temperature bending The effect of FDM processing parameters on the mechanical properties of high temperature andPEEK impact materials tests. wasThe firstinfluences systematically of printing investigated temperature, in thisfilling study, rate, including layer thickness the tensile, and printing bending PEEK materials was first systematically investigated in this study, including the tensile, bending and velocityand impact on tensile tests. propertiesThe influences were ofanalyzed printing by temp orthogonalerature, testfilling of fourrate, factorslayer thickness and three and levels. printing The impact tests. The influences of printing temperature, filling rate, layer thickness and printing velocity rangevelocity analysis on tensile indicated properties that thewere optimal analyzed combinat by orthogonalion of tensile test of strength four factors is printing and three speed levels. of The60 on tensile properties were analyzed by orthogonal test of four factors and three levels. The range mm/s,range layeranalysis thickness indicated of 0.25that themm, optimal printing combinat temperatureion of oftensile 370 °Cstrength and filling is printing rate of speed 60%, ofthe 60 analysis indicated that the optimal combination of tensile strength is printing speed of 60 mm/s, optimalmm/s, layercombination thickness of elongationof 0.25 mm, rate printing is set as temperature printing speed of 370of 20 °C mm/s, and fillinglayer thicknessrate of 60%, of 0.25 the layer thickness of 0.25 mm, printing temperature of 370 ◦C and filling rate of 60%, the optimal mm,optimal printing combination temperature of elongation of 370 °C rateand isfilling set as rate printing of 40%, speed and theof 20 optimal mm/s, combinationlayer thickness of elasticof 0.25 combination of elongation rate is set as printing speed of 20 mm/s, layer thickness of 0.25 mm, printing modulusmm, printing is printing temperature velocity of of 370 60 mm/s,°C and layer filling thickness rate of 40%, of 0.2 and mm, the printing optimal temperature combination of of 360 elastic °C temperature of 370 ◦C and filling rate of 40%, and the optimal combination of elastic modulus is andmodulus filling israte printing of 60%. velocity The influence of 60 mm/s, sequence layer thicknesson tensile ofproperties 0.2 mm, printingwas varied. temperature According of to360 the °C printing velocity of 60 mm/s, layer thickness of 0.2 mm, printing temperature of 360 ◦C and filling comprehensiveand filling rate analysis, of 60%. Thethe influenceoptimal combination sequence on of tensile printing properties parameters was varied.is printing According speed ofto 60the rate of 60%. The influence sequence on tensile properties was varied. According to the comprehensive mm/s,comprehensive layer thickness analysis, of 0.2 the mm, optimal temperatur combinatione of 370 °Cof andprinting filling parameters ratio of 40%. is printing speed of 60 analysis, the optimal combination of printing parameters is printing speed of 60 mm/s, layer thickness mm/s,Furthermore, layer thickness the tensileof 0.2 mm, test temperaturindicated thate of 370the °Coptimum and filling tensile ratio properti of 40%. es (tensile strength of 0.2 mm, temperature of 370 ◦C and filling ratio of 40%. 40.0 Mpa,Furthermore, elongation the 14.3%) tensile could test indicatedbe obtained that with the optimumthe same parameterstensile properti as thees (tensilerange analysisstrength Furthermore, the tensile test indicated that the optimum tensile properties (tensile strength results.40.0 Mpa, Good elongation fusion effects 14.3%) and could filament-to-filament be obtained with and the interlayer same parameters bonding asat thethese range parameters analysis 40.0 Mpa, elongation 14.3%) could be obtained with the same parameters as the range analysis results. couldresults. be Goodseen fromfusion SEM effects images. and filament-to-filamentIn addition, the printed and interlayerbending andbonding impact at thesespecimens parameters with Good fusion effects and filament-to-filament and interlayer bonding at these parameters could be seen optimizedcould be parametersseen from SEMwere performed.images. In addition,The flexural the strength printed an bendingd impact and strength impact are specimens 68.2 Mpa andwith from SEM2 images. In addition, the printed bending and impact specimens with optimized parameters 101.2optimized KJ/m ,parameters respectively, were which performed. is satisfactory The flexural for hi ghstrength performance and impact engineering strength plastics.are 68.2 MpaFurther and were performed. The flexural strength and impact strength are 68.2 Mpa and 101.2 KJ/m2, respectively, research101.2 KJ/m is 2necessary, respectively, to discusswhich isthe satisfactory effects of for other high parameters,performance includingengineering the plastics. raster Furtherangle, orientationresearch is and necessary diameter to of discuss the nozzle. the effectsMoreover of , otherthe interactions parameters, between including different the rasterparameters angle, shouldorientation be considered and diameter in the offuture the nozzle.experimental Moreover design., the interactions between different parameters should be considered in the future experimental design.

Materials 2018, 11, 216 10 of 11 which is satisfactory for high performance engineering plastics. Further research is necessary to discuss the effects of other parameters, including the raster angle, orientation and diameter of the nozzle. Moreover, the interactions between different parameters should be considered in the future experimental design.

Acknowledgments: Zhi Zeng acknowledges Science and Technology Planning Project of Guangdong Province (Grant No. 2016A010102002) and the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2015J149). Moreover, this research receives support from Innovation Team Training Plan of Tianjin Universities and colleges (Grant No. TD12-5043). Finally, we would like to thank Jiangang Wang (HEBUST) and Zhenghuan Wu (TUTE) for valuable assistance with the SEM and mechanical tests. Author Contributions: Zhi Zeng and Bei Peng conceived and designed the experiments; Xiaohu Deng and Shuo Yan performed the experiments; Zhi Zeng and Wenchao Ke analyzed the data; Xiaohu Deng wrote the paper. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

1. Kurtz, S.M. PEEK Biomaterials Handbook, 1st ed.; William Andrew Publishing: Norwich, CT, USA, 2012; pp. 1–7. ISBN 9781437744637. 2. Garcia-Gonzalez, D.; Jayamohan, J.; Sotiropoulos, S.N. On the mechanical behaviour of PEEK and HA cranial implants under impact loading. J. Mech. Behav. Biomed. Mater. 2017, 69, 342–345. [CrossRef][PubMed] 3. Tan, K.; Chua, C.; Leong, K. Selective laser sintering of biocompatible polymers for applications in tissue engineering. Biomed. Mater. Eng. 2005, 15, 113–124. [PubMed] 4. Schmidt, M.; Pohle, D.; Rechtenwald, T. Selective laser sintering of PEEK. CIRP Ann. Manuf. Technol. 2007, 56, 205–208. [CrossRef] 5. Valentan, B.; Kadivnik, Z.; Brajlih, T.; Anderson, A. Processing poly(ether etherKetone) on a 3d printer for thermoplastic modeling. Mater. Technol. 2013, 47, 715–721. 6. Chavan, S.A.; Anwar, M.A.; Shaha, B.R. Review of effect of process parameters on mechanical properties of 3D printed components. In Proceedings of the 6th International Conference on Recent Development in Engineering Science, Humanities and Management, Chandigarh, India, 14 May 2017; pp. 461–466. 7. Gordon, A.P.; Torres, F.; Cole, M.; Owji, A.; DeMastry, Z. An approach for mechanical property optimization of fused deposition mdeling with polylactic acid via design of experiments. Rapid Prototyp. J. 2016, 22, 1–18. 8. Pfeifer, T.; Koch, C.; Van Hulle, L.; Capote, G.A.M.; Rudolph, N. Optimization of the FDM additive manufacturing process. In Proceedings of the SPE ANTEC™ Indianapolis, Indianapolis, IN, USA, 22–25 May 2016; pp. 22–29. 9. Durgun, I.; Ertan, R. Experimental investigation of FDM process for improvement of mechanical properties and production cost. Rapid Prototyp. J. 2014, 20, 228–235. [CrossRef] 10. Luzanin, O.; Movrin, D.; Plancak, M. Effect of layer thickness, deposition angle and infill on manimum flexural force in FDM-built specimens. J. Technol. Plast. 2014, 39, 49–57. 11. Chacóna, J.M.; Caminerob, M.A.; García-Plazab, E.; Núñezb, P.J. Additive manufacturing of PLA structures using fused deposition modelling: Effect of process parameters on mechanical properties and their optimal selection. Mater. Des. 2017, 124, 143–157. [CrossRef] 12. Bala, A.S.; Wahab, S.B.; Ahmad, M.B. Elements and materials improve the FDM products: A review. Adv. Eng. Forum 2016, 16, 33–51. [CrossRef] 13. Wu, W.; Geng, P.; Zhao, J.; Zhang, Y.; Rosen, D.; Zhang, H. Manufacture and thermal deformation analysis of semicrystalline polymer polyether ether ketone by 3D printing. Mater. Res. Innov. 2014, 18, 5–12. [CrossRef] 14. Wu, W.; Geng, P.; Li, G.; Zhao, D.; Zhang, H.; Zhao, J. Influence of Layer Thickness and Raster Angle on the Mechanical Properties of 3D-Printed PEEK and a Comparative Mechanical Study between PEEK and ABS. Materials 2015, 8, 5834–5846. [CrossRef][PubMed] 15. Kazi, M.R.; Todd, L. Mechanical properties of additively manufactured PEEK components using fused filament fabrication. In Proceedings of the ASME 2015 International Mechanical Engineering Congress and Exposition, Houston, TX, USA, 13–19 November 2015. 16. Mohammad, V.; Yang, S. Extrusion-based additive manufacturing of PEEK for biomedical applications. Virtual Phys. Prototyp. 2015, 10, 1–13. Materials 2018, 11, 216 11 of 11

17. Sun, X.; Cao, L.; Ma, H.; Gao, P.; Bai, Z.; Li, C. Experimental Analysis of High Temperature PEEK Materials on 3D Printing. In Proceedings of the 2017 9th International Conference on Measuring Technology and Mechatronics Automation, Changsha, China, 14–15 January 2017; pp. 13–16. 18. Yang, C.; Tian, X.; Li, D.; Cao, Y.; Zhao, F.; Shi, C. Inﬂuence of thermal processing conditions in 3D printing on the crystallinity and mechanical properties of PEEK material. J. Mater. Process. Technol. 2017, 248, 1–7. [CrossRef] 19. Gianluca, C.; Alberta, L.; Barbara, D.; Alessio, L.; Giuseppe, R.; Silvia, F. Engineering thermoplastics for additive manufacturing: A critical perspective with experimental evidence to support functional applications. J. Appl. Biomater. Funct. Mater. 2017, 15, 10–18. 20. Wu, W.; Ye, W.; Wu, Z.; Geng, P.; Wang, Y.; Zhao, J. Inﬂuence of layer thickness, raster angle, deformation temperature and recovery temperature on the shape-memory effect of 3D-printed polylactic acid samples. Materials 2017, 10, 970. [CrossRef][PubMed] 21. Wang, Y.; Rouholamin, D.; Davies, R.; Ghita, O.R. Powder characteristics, microstructure and properties of graphite platelet reinforced Poly Ether Ether Ketone composites in High Temperature Laser Sintering (HT-LS). Mater. Des. 2015, 88, 1310–1320. [CrossRef] 22. Ziemian, C.; Sharma, M.; Ziemian, S. Anisotropic Mechanical Properties of ABS Parts Fabricated by Fused Deposition Modelling. In Mechanical Engineering; Gokcek, M., Ed.; InTech: Rijeka, Croatia, 2012; pp. 160–178. ISBN 978-953-51-0505-3.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).