Influence of Layer Thickness and Raster Angle on the Mechanical

Materials 2015, 8, 5834-5846; doi:10.3390/ma8095271 OPEN ACCESS materials ISSN 1996-1944 www.mdpi.com/journal/materials

Article Inﬂuence of Layer Thickness and Raster Angle on the Mechanical Properties of 3D-Printed PEEK and a Comparative Mechanical Study between PEEK and ABS

Wenzheng Wu 1, Peng Geng 1, Guiwei Li 1, Di Zhao 1, Haibo Zhang 2 and Ji Zhao 1,*

1 School of Mechanical Science and Engineering, Jilin University, Renmin Street 5988, Changchun 130025, China; E-Mails: [email protected] (W.W.); [email protected] (P.G.); [email protected] (G.L.); [email protected] (D.Z.) 2 Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, China; E-Mail: [email protected]

* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +86-150-4306-6406.

Academic Editor: Reza Montazami

Received: 20 May 2015 / Accepted: 19 June 2015 / Published: 1 September 2015

Abstract: Fused deposition modeling (FDM) is a rapidly growing 3D printing technology. However, printing materials are restricted to acrylonitrile butadiene styrene (ABS) or poly (lactic acid) (PLA) in most Fused deposition modeling (FDM) equipment. Here, we report on a new high-performance printing material, polyether-ether-ketone (PEEK), which could surmount these shortcomings. This paper is devoted to studying the inﬂuence of layer thickness and raster angle on the mechanical properties of 3D-printed PEEK. Samples with three different layer thicknesses (200, 300 and 400 µm) and raster angles (0˝, 30˝ and 45˝) were built using a polyether-ether-ketone (PEEK) 3D printing system and their tensile, compressive and bending strengths were tested. The optimal mechanical properties of polyether-ether-ketone (PEEK) samples were found at a layer thickness of 300 µm and a raster angle of 0˝. To evaluate the printing performance of polyether-ether-ketone (PEEK) samples, a comparison was made between the mechanical properties of 3D-printed polyether-ether-ketone (PEEK) and acrylonitrile butadiene styrene (ABS) parts. The results suggest that the average tensile strengths of polyether-ether-ketone (PEEK) parts were 108% higher than those for acrylonitrile butadiene styrene (ABS), and compressive strengths were 114% and bending strengths were 115%. However, the modulus of elasticity for both Materials 2015, 8 5835

materials was similar. These results indicate that the mechanical properties of 3D-printed polyether-ether-ketone (PEEK) are superior to 3D-printed ABS.

Keywords: 3D printing; polyether-ether-ketone (PEEK); raster angle; layer thickness

1. Introduction

3D printing is a new integrated manufacturing technology that involves a variety of disciplines. 3D printing has shown excellent potential to reduce both the cycle time and cost of product development [1]. With the development of 3D printing, a large number of processes have been developed that allow the use of a variety of materials and methods [2,3]. Amongst these technologies, one of the most commonly used is fused deposition modeling (FDM) [4,5], a layer-by-layer additive manufacturing technique, based on computer-aided design (CAD) and computer-aided manufacturing (CAM) [6]. The advantages of this technology are as follows [7]: easy material change, low maintenance costs, supervision-free operation, compact size and low working temperature [8]. However, the main disadvantage of FDM is the narrow range of available materials [9]. Many commercial 3D printers can print only acrylonitrile butadiene styrene (ABS) or poly (lactic acid) (PLA). Most parts fabricated by commercial 3D printers are used as demonstration parts, not as working parts. This limits the use of FDM in industrial applications [10]. Polyether-ether-ketone (PEEK), a semi-crystalline thermoplastic material, is an engineering plastic developed by Imperial Chemical Industries (ICI) in 1977 that has high temperature resistance, superior mechanical strength and outstanding chemical stability. In addition, PEEK is biocompatible and ideally suited for use in biomedical applications. As a consequence of these advantages, PEEK can be used in a wide variety of fields, such as the aerospace, automotive, electronics and medicine industries. The mechanical properties of 3D-printed parts are important indices for evaluating printing quality [11–14]. Regarding this issue, some work has been carried out to determine the effects of production parameters on the mechanical properties of 3D printed parts. Ismail et al. [15] studied the influence of raster angle and orientation on the mechanical properties of ABS printed parts. Tensile and three-point bending tests have been studied. Sood et al. [16] focused on the influence of orientation, layer thickness, raster angle, part raster width and raster-to-raster gap. Tymrak et al. [17,18] described the influence of orientation, layer thickness and raster-to-raster gap on parts that were printed using several commercial 3D printers. The above research has mainly been concerned with ABS materials. This paper reports on the mechanical properties of PEEK samples built by a custom-built 3D printing system. PEEK samples were fabricated for two experiments. The first experiment was used to investigate the influence of printing parameters on mechanical properties. The second experiment was used to compare the mechanical properties of PEEK and ABS FDM parts.

2. Experimental Section

Mechanical properties of 3D-printed samples can be influenced by many factors, such as layer thickness, raster angle, build orientations, fill pattern, air gap and model build temperature. Layer thickness is the thickness of the layer deposited by the nozzle. Raster angle is the direction of raster Materials 2015, 8 3885 Materials 2015, 8 5836 thickness is the thickness of the layer deposited by the nozzle. Raster angle is the direction of raster with respectrespect toto thethe loadingloading direction direction of of stress, stress, as as shown shown in Figurein Figure1. Air 1. Air gap gap is the is distancethe distance between between two adjacenttwo adjacent deposited deposited filaments filaments in the in the same same layer. layer. The The number number of of contours contours is is thethe numbernumber ofof filaments filaments initially deposited deposited along along the the outer outer edge. edge. Bead Bead width width is isthe the width width of the of thefilament filament deposited deposited by the by the3D 3Dprinter printer nozzle. nozzle.

Figure 1. The raster angle of 3D printing. Figure 1. The raster angle of 3D printing.

The workwork describeddescribed hereinherein investigatedinvestigated the the influence influence of of layer layer thickness thickness (LT) (LT) and and raster raster angle angle (RA) (RA) on theon the mechanical mechanical properties properties of PEEK of PEEK samples samples built bu byilt a by custom-built a custom-built 3D printing 3D printing system. system. Samples Samples with threewith three raster raster angles angles (0˝, 30 (0°,˝ and 30° 45 and˝) and 45° three) and layerthree thicknesseslayer thicknesses (200, 300(200, and 300 400 andµm) 400 were μm) built were by built 3D printingby 3D printing of PEEK of and PEEK tested and for tensile,tested for compressive tensile, compressive and bending strengths.and bending The strengths. printing parameters The printing are shownparameters in Table are 1shown. Build in orientation Table 1. Build refers orientation to the inclination refers ofto partthe inclination in a build platform of part in with a build respect platform to X, Y,with Z axisrespect (side, to X, flat Y, and Z up)axis [ 18(side,]. In flat our and study, up)samples [18]. In grownour study, up along samples Z axis grown and up sat along down Z flat axis during and thesat down building flat process. during the Fill building pattern process. is the motion Fill pattern path of is thethe 3Dmotion printing path machine’s of the 3D liquefierprinting headmachine’s [19]. Duringliquefier the head printing [19]. process,During the liquefierprinting headprocess, move the back liquefier andforth head forming move back a rectangular and forth structure forming toa fillrectangular the entire structure internal to portion fill the of enti layer.re internal Air gap portion is the gap of betweenlayer. Air two gap adjacent is the gap rasters between on same two layer.adjacent rasters on same layer. Table 1. The printing parameters of polyether-ether-ketone (PEEK) 3D printing. Table 1. The printing parameters of polyether-ether-ketone (PEEK) 3D printing. Control Factors Fixed Factors FactorControl Factors Level Unit FactorFixed Factors Value Unit Factor Level Unit Factor Value Unit 200200 mm Build Build orientation orientation Y-direction Y-direction (Flat) (Flat) - - LayerLayer thickness thickness 300300 mm Fill Fill pattern pattern Line Line - - 400400 mm Air Air gap gap 0 0 mm mm 0 0 ˝ ° NumberNumber of of contours contours 2 2 - - RasterRaster angle angle 30 ° NozzleNozzle inner innerdiameter 0.4 mm 30 ˝ 0.4 mm 45 ° diameter 45 ˝

Materials 2015, 8 5837

Materials 2015, 8 3886 To investigate the relationships between printing factors and mechanical properties, five samples of each geometryTo investigate were created,the relationships with different between layerprinting thicknesses factors andand mechani rastercal angles, properties, using five PEEK samples material. of Identicaleach samplesgeometry werewere builtcreated, using with ABS different material layer tothicknesses compare and the raster mechanical angles, using properties PEEK of material. 3D-printed PEEKIdentical and ABS samples parts. were Thebuilt geometricusing ABS material models to of compare the tensile, the mechanical bending properties samples of and 3D-printed compressive samplesPEEK were and similar ABS parts. to the The specifications geometric models in GB/T of the 16421-1996, tensile, bending GB/T samples 9341-2008 and compressive and GB/T1041-2008, samples were similar to the specifications in GB/T 16421-1996, GB/T 9341-2008 and GB/T1041-2008, respectively. Mechanical test sample models conforming to the relevant mechanical test standards were respectively. Mechanical test sample models conforming to the relevant mechanical test standards designed in CATIA V5, and the geometric models were exported as files in stereolithography (STL) were designed in CATIA V5, and the geometric models were exported as files in stereolithography format(STL) for importformat for by import the FDM by the software. FDM software. The geometric The geometric models models are shown are shown in Figure in Figure2. 2.

Figure 2. Standard samples for mechanical tests. (a) Tensile samples; (b) Bending samples; Figure 2. Standard samples for mechanical tests. (a) Tensile samples; (b) Bending samples; (c) Compressive(c) Compressive samples. samples.

The experimentalThe experimental samples samples were were made made from from PEEK PEEK and ABSand ABS plusTM plus-P430.TM-P430. High-performance High-performance PEEK PEEK material was obtainedmaterial fromwas theobtained Jilin University from the Special Jilin University Plastic Engineering Special Plastic Research Engin Co.eering Ltd. (Changchun, Research Co. China), Ltd. with (Changchun, China), with a melting point of 334 °C, a glass transition temperature of 143 °C and a melting point of 334 ˝C, a glass transition temperature of 143 ˝C and a melt index of 44 g/10 min. a melt index of 44 g/10 min. PEEK samples were then built with a custom-built 3D printing system. In PEEK samples were then built with a custom-built 3D printing system. In this printing machine, a stage this printing machine, a stage platform moves in the X- and Y- axes, and a head including a nozzle tip platformand nozzle moves heater in the moves X- and along Y- axes,the Z-axis, and awhile head fusing including and depositing a nozzle mate tip andrial. ABS nozzle is inexpensive, heater moves has along the Z-axis,high mechanical while fusing strength, and depositingand is widely material. used in the ABS 3D isprinting inexpensive, industry has[20]. high ABS mechanical plusTM-P430 strength,material and is widely(Stratasys used Inc., in the Eden 3D Prairie, printing MN, industry USA) [was20]. used ABS to plusmakeTM ABS-P430 samples. material ABS (Stratasys is ideal for Inc., building Eden 3D Prairie, MN,parts USA) when was combined used to makewith uPrint ABS SE samples. 3D Printers ABS (Stratasys is ideal Inc., for Eden building Prairi 3De, MN, parts USA). when ABS combined samples with uPrintwere SE built 3D Printerswith uPrint (Stratasys SE 3D Printers Inc., Eden using Prairie,default building MN, USA). parameters ABS to samples obtain optimal were built mechanical with uPrint SE 3Dproperties. Printers The using SR-30 default Soluble building Support parameters (synthetic therm to obtainoplastic optimal polymer) mechanical was used as the properties. support material. The SR-30 The mechanical properties of PEEK and ABS materials are shown in Table 2. Testing of mechanical Soluble Support (synthetic thermoplastic polymer) was used as the support material. The mechanical properties of the raw materials were performed on samples manufactured by injection molding. properties of PEEK and ABS materials are shown in Table2. Testing of mechanical properties of the raw materialsTable were performed2. Mechanical on properties samples manufacturedof PEEK and acrylonitrile by injection butadiene molding. styrene (ABS). Factor PEEK ABS Table 2. Mechanical properties of PEEK and acrylonitrile butadiene styrene (ABS). Tensile Strength 100.0 MPa 37.0 MPa Elastic Limit 72.0 MPa 31.0 MPa Factor PEEK ABS Compressive Strength 118.0 MPa 37.0 MPa CompressiveTensile Strength Modulus 100.0 3.8 GPa MPa 2.3 37.0 GPa MPa BendingElastic Limit Strength 163.0 72.0 MPa 53.0 31.0 MPa MPa CompressiveBending Modulus Strength 118.0 4.0 GPa MPa 2.2 37.0 GPa MPa Compressive Modulus 3.8 GPa 2.3 GPa Bending Strength 163.0 MPa 53.0 MPa Bending Modulus 4.0 GPa 2.2 GPa Materials 2015, 8 5838

The mechanical tests were carried out on an autograph universal materials testing machine, equipped with a 50-kN load cell. To minimize experimental error, ﬁve samples were built and tested under identical conditions for each mechanical test and the mean values were taken as the results. The samples were performed according to the GB/T 16421-1996 Standard and the samples stretched at a strain rate of 1 mm/min. The three-point bending tests were performed according to the GB/T 9341-2008 Standard with the loading rate of 2 mm/min, the span was 64 mm and the head of loading device was composed of steel cylinders 10 mm in diameter. The compressive test was performed according to the GB/T1041-2008 Standard at a loading rate of 2 mm/min.

3. Results and Discussion

3.1. Inﬂuence of Layer Thickness and Raster Angle on Mechanical Properties of PEEK

Table3 shows the practical mean values of mechanical tests for the 3D-printed PEEK samples. From Table3, it can be noted that the samples built with a 300- µm layer thickness had the greatest strengths in all mechanical tests. The strength of samples with a 400-µm layer thickness decreased significantly. We can also see that samples built with raster angles of 0˝/90˝ had the greatest mechanical strengths. Although the layer thickness had a great influence on tensile strength, it had little influence on bending and compressive strengths. Because the compressive sample was cylindrical, there was no effect of the raster angle.

Table 3. Mechanical properties in different layer thickness and different raster angles.

Tensile strength Bending strength Compressive strength Factors (MPa) (MPa) (MPa)

200 40.1 52.1 53.6 Layer Thickness 300 56.6 56.1 60.9 (µm) 400 32.4 48.7 54.1 0˝/90˝ 56.6 56.1 - Raster Angle (˝) 30˝/´60˝ 41.8 48.5 - 45˝/´45˝ 43.3 43.2 -

As indicated in Table3, the tensile strengths of the PEEK samples were significantly affected by the layer thickness and raster angle. In samples printed at raster angles of 0˝/90˝, the filaments were oriented parallel to the load direction, producing the strongest samples. Similarly, in samples printed in other orientations, there was a finite angle between the printed microstructural elements and the load direction. The filaments were thus subjected to both tensile and shear stresses, leading to failure at low tensile strength. During compressive tests, the samples undergo pressure aligned in the axial direction and the adjacent layers bear the shear stress because of the additive build up, which can cause the single layers to slide along one another until the sample finally breaks. The compressive strength of printed samples is significantly lower than that of injected materials because the printed samples are subject to flaws, comprising both extensive pores that are initially squeezed out and weak interlayer bonding. Materials 2015, 8 5839 Materials 2015,, 8 3888

Weak interlayerinterlayer bondingbonding isis responsibleresponsible forfor aa decreasedecrease inin bendingbending strength,strength, whichwhich cancan causecause weldedwelded layerslayers to to delaminate. delaminate. DuringDuring During printing,printing, eacheach newnew layerlayer will will overlay overlay the top of th thee previous layer beforebefore material solidification solidiﬁcation from from the the melt occurs, which lead leadsss to to shrinkage shrinkage inin the the previous previous layer. layer. The The residualresidual stressesstresses in in the the printed printed samples samples resulting resulting from from the the volume volume shrinkage shrinkage werewere examinedexamined toto determinedetermine thethe weak interlayer bonding.bonding. As As the the layer layer thickness thickness incr increases,eases, the the accuracy accuracy of of the the overall overall finished ﬁnished samplesample geometry decreases andand thethe outlineoutline moremore readilyreadily displaysdisplays thethe staircasestaircase phenomenon.phenomenon.phenomenon.

3.2. Comparison of ABS and PEEK Tensile StrengthsStrengths

To evaluateevaluate thethe performanceperformance ofof printedprinted PEEKPEEK samples,samples, wewe builtbuilt samplessamples fromfrom PEEKPEEK andand ABSABS using optimaloptimal parameters.parameters. Figure Figure 33 showsshowsshows ABSABSABS andandand PEEK PEEKPEEK samples samplessamples after afteafterr the thethe tensile tensiletensile test. test.test. Engineering EngineeringEngineering stress-strainstress-strain curves curves forfor PEEKPEEK andand ABSABS samplessamples areare shownshown inin FigureFigure4 4..

Figure 3.3. FracturedFractured tensiletensiletensile samples.samples.samples. (a((a))) acrylonitrile acrylonitrileacrylonitrile butadiene butadienebutadiene styrene styrenestyrene (ABS); (ABS);(ABS); (b) (polyether-ether-ketone(b)) polyether-ether-ketonepolyether-ether-ketone (PEEK). (PEEK).(PEEK).

Figure 4. Tensile stress-strain curves of acrylonitrile butadiene styrene (ABS) and Figure 4. TensileTensile stress-strainstress-strain curvescurves ofof acracrylonitrile butadiene styrene (ABS) and polyether-ether-ketone (PEEK). polyether-ether-ketone (PEEK).

The ABS curve includesincludes regionsregions ofof elasticelastic andand plasticplastic deformation,deformation, accompanied byby neckingnecking deformation. The The stress-strain stress-strain be behaviorhavior under tensile stress waswas initiallyinitiallyially nonlinear.nonlinear.nonlinear. After After the the samplesample

Materials 2015, 8 5840 Materials 2015, 8 3889 reached peak stress, the resisting stress was almost constant with the increase in strain through shape. It reached peak stress, the resisting stress was almost constant with the increase in strain through shape. can be concluded that the fracture of ABS occurs mainly via damage to the raster. As the tensile stress It can be concluded that the fracture of ABS occurs mainly via damage to the raster. As the tensile increases, the failure will begin at the weakest raster and next weakest raster will break, in sequence, stress increases, the failure will begin at the weakest raster and next weakest raster will break, untilin sequence, total failure untilof total the failure sample. of Whenthe sample. the stress When reaches the stress a certain reaches constant a certain value, constant a long value, propagation a long processpropagation occurs process in the occurs neck. in Craze the neck. is the Craze main is plasticthe main deformation plastic deformation mechanism mechanism of ABS, of with ABS, a greatwith numbera great number of crazes of appearingcrazes appearing perpendicular perpendicular to the direction to the direction of the tensile of the loading.tensile loading. Crazes initiate,Crazes initiate, widen, thenwiden, suffer then breakdown suffer breakdown of the raster of the as tensionraster as increases. tension increases. If crazes extend If crazes to bothextend ends to of both the sample,ends of thethe samplesample, failsthe sample with insignificant fails with neckinginsignificant deformation, necking becausedeformation, molecular because chains molecular initially chains in an unorientedinitially in statean unoriented transform state to a transform more highly to a oriented more highly state oforiented necking. state The of n transformationecking. The transformation process causes process strain hardening,causes strain and hardening, ensures theand uniformensures expansionthe uniform of expansion crazes extending of crazes to extending both ends, to whichboth ends, is similar which tois metalsimilar deformation to metal deformation hardening hardening caused by caused uniform by deformation. uniform deformation. The PEEK curve clearly differs from the the ABS ABS curve. curve. As As the the load load increased, increased, PEEK PEEK first first yielded yielded at at a amaximum maximum stress, stress, then then necking necking deformation deformation appeared appeared at at the the tensile tensile fracture fracture surface surface and the samplessamples broke afterafter reachingreaching thethe maximum maximum stress, stress, when when the the strain strain reached reached 87%. 87%. In In PEEK PEEK samples, samples, there there was was no obviousno obvious necking necking deformation deformation at the at th tensilee tensile fracture fracture surface, surface, and and no obviousno obvious neck neck propagation propagation until until the fracturethe fracture of samples of samples after after yielding. yielding. Figure5 5 shows shows the the average average tensile tensile property property values values for for both both ABS ABS and and PEEK. PEEK. The The elastic elastic limit limit from from fivefive tensiletensile experimentsexperiments withwith PEEKPEEK waswas 50.850.8 MPa;MPa; thethe tensiletensile strengthstrength ofof PEEKPEEK waswas 56.656.6 MPa,MPa, whilewhile thethe elasticelastic limitlimit ofof ABSABS waswas 22.922.9 MPa,MPa, andand thethe tensiletensile strengthstrength ofof ABSABS waswas 27.127.1 MPa.MPa. The values ofof thesethese propertiesproperties forfor PEEK PEEK were were 122% 122% and and 108% 108% higher highe thanr than for ABS.for ABS. As shown As shown in the in figure, the figure, the tensile the propertiestensile properties of 3D-printed of 3D-printed ABS samples ABS samples were lower were than lower injection-molded than injection-mo ABSlded by ABS 26.2% by for26.2% the for elastic the limitelastic and limit 26.8% and for26.8% the tensilefor the strength.tensile strength. Likewise, Likewise, the tensile the properties tensile properties of 3D-printed of 3D-printed PEEK samples PEEK weresamples lower were than lower injection-molded than injection-molded PEEK by 29.4% PEEK for by the 29.4% elastic for limit the and elastic 43.4% limit for theand tensile 43.4% strength. for the However,tensile strength. the lower However, strength the of lowe 3D-printedr strength samples of 3D-printed compared samples with injection-moldedcompared with injection-molded samples can be mainlysamples attributed can be tomainly the gaps attributed between to filaments the gaps and between the extensive filaments pores and within the filaments,extensive especiallypores within for PEEKfilaments, samples especially because for of PEEK the poor samples printing because quality of obtained.the poor printing quality obtained.

Figure 5. Comparison on tensile property between acrylonitrile butadiene styrene (ABS) Figure 5. Comparison on tensile property between acrylonitrile butadiene styrene (ABS) and polyether-ether-ketone (PEEK). and polyether-ether-ketone (PEEK).

Materials 2015, 8 5841 Materials 2015, 8 3890

Figure6 6 shows shows scanningscanning electronelectron microscopy microscopy (SEM) (SEM) images images ofof fracturefracture cross-sectionscross-sections of of PEEK PEEK and and ABS alongalong thethe longitudinal longitudinal direction. direction. These These images images of theof fracturethe fracture surface surface show thatshow failure that failure was caused was bycaused differing by differing reasons. Althoughreasons. ABSAlthough individual ABS rastersindividual had melted rasters together, had me welted can together, still distinguish we can every still rasterdistinguish in the every images, raster and in the the fracture images, of and ABS the was fracture mainly of caused ABS was by damagemainly caused to the rasters by damage pulling to andthe rupturing.rasters pulling As theand load rupturing. force increased, As the load the forceforce perincreased, unit area the would force reach per unit the filamentsarea would tensile reach limit. the Infilaments the printed tensile samples limit. theIn fracturethe printed would samples begin th approximatelye fracture would at the begin weakest approximately filament, and at the the weakest fracture wouldfilament, propagate and the until fracture the samples would failed.propagate The resultuntil th ise that samples the stress failed. continues The result to increase is that and the the stress next weakestcontinues raster to increase will fail. and By the observing next weakest the failure raster surfaces will fail. of By PEEK observing samples, the we failure can seesurfaces that there of PEEK were nosamples, obvious we rasters can see and that the there samples wereappeared no obvious to haveraster melteds and the into samples a block. appeared This was to mosthave likelymelted caused into a byblock. a combination This was most of the likely high extrudercaused by temperature a combinati andon filaments of the high that extruder created significanttemperature thermal and filaments bonding betweenthat created both significant raster and therma layers,l causingbondinggreater between fusion. both raster and layers, causing greater fusion.

Figure 6. SEM images of fracture cross sections of polyether-ether-ketone (PEEK) Figure 6. SEM images of fracture cross sections of polyether-ether-ketone (PEEK) and acrylonitrile butadiene styrene (ABS). (a) ABS, 25ˆ;(b) ABS, 70ˆ;(c) PEEK, 25ˆ; and acrylonitrile butadiene styrene (ABS). (a) ABS, 25×; (b) ABS, 70×; (c) PEEK, 25×; (d) PEEK, 70ˆ. (d) PEEK, 70×.

3.3. Comparison of ABS and PEEK Compressive PropertiesProperties

Figure7 7shows shows ABS ABS and and PEEK PEEK samples samples after after compressive compressive tests. tests. FigureFigure8 8shows shows compressive compressive engineering stress-strain stress-strain curves curves for for ABS ABS and and PEEK. PEEK. As As the the figure figure illustra illustrates,tes, the the compressive compressive strength strength of ofABS ABS is obvious, is obvious, while while it is difficult it is difficult to confirm to confirma unique acompressive unique compressive strength for strength PEEK. The for PEEK.stress-strain The stress-straincurves for ABS curves are for very ABS similar are very to the similar tensile to thecurv tensilees for curvesABS. The for ABS.stress-stain The stress-stain curve of PEEK curve ofis PEEKinitially is linear initially and linear the andstress the increases stress increases with the with development the development of significant of significant deformation. deformation. With With an anincrease increase in incompressive compressive stress, stress, the the curve curve become becomess nonlinear nonlinear and and inelastic. The The significant significant initial

Materials 2015,, 8 38915842 deformation appearsappears toto result result from from extensive extensive pores pores becoming becoming squeezed squeezed out. out. If we If could we could reduce reduce the pores the poreswithin within the printed the printed part, the part, compressive the compressive strength stre andngth printing and printing quality ofquality 3D-printed of 3D-printed PEEK parts PEEK could parts be couldsubstantially be substantially improved. improved.

Figure 7. Fractured compressive samples. (a) acrylonitrile butadiene styrene (ABS); Figure 7. FracturedFractured compressivecompressive samples.samples. ((a) acrylonitrile butadiene styrene (ABS); (b) polyether-ether-ketone (PEEK). (b) polyether-ether-ketone (PEEK).

Figure 8. Compressive stress-strain curves of acrylonitrile butadiene styrene (ABS) and Figure 8. CompressiveCompressive stress-strainstress-strain curvescurves ofof acacrylonitrile butadiene styrene (ABS) and polyether-ether-ketone (PEEK). polyether-ether-ketone (PEEK).

The compressive strength and compressive modulus of of five ﬁvefive experiments areare shown in Figure Figure9 9.9.. According to Figure 99,, thethe compressivecompressive strengthstrength ofof PEEK PEEKPEEK was waswas 60.9 60.960.9 MPa MPaMPa and andand the ththe compressivecompressive strengthstrength of ABS was 28.428.4 MPa.MPa. Thus, Thus, the the value value for for PEEK PEEK wa wass 114% 114% higher higher than than that that forfor ABS,ABS, while while the the compressive modulusmodulus waswas similar similar for for both. both. As As the the ﬁgure figure shows, shows, the 3D-printed the 3D-printed samples samples failed atfailed 76.7% at 76.7%of the valueof the of value the injection-molded of the injection-molded ABS. Likewise, ABS. theLikewise, injection-molded the injection-molded PEEK failed atPEEK 118 MPa,failed and at 118the 3D-printedMPa, and the samples 3D-printed failed atsamples 76.7% failed of the at injection-molded 76.7% of the injection-molded PEEK. The test resultsPEEK. indicateThe test that results the indicatecompressive that strengththe compressive and modulus strength of injection-molded and modulus of ABSinjection-mold samples wereed ABS higher samples by 76.9% were and higher 35.3% by 76.9%than those and of35.3% the 3D-printedthan those of ABS the samples.3D-printed Similarly, ABS samples. the 3D-printed Similarly, PEEK the 3D-printed samples failedPEEK at samples 51.6% failedof the at injection-molded 51.6% of the injection-molded PEEK’s compressive PEEK’s strength, compressive and 3D-printed strength, PEEK and 3D-printed samples had PEEK 79.1% samples lower hadcompressive 79.1% lower modulus compressive than that modulus of injection-molded than thatthat ofof PEEK.injection-moldedinjection-molded PEEK.PEEK.

Materials 2015, 8 5843 Materials 2015,, 8 3892

Figure 9. Comparison on compressive property between acrylonitrile butadiene styrene Figure 9. ComparisonComparison onon compressivecompressive propertyproperty bebetween acrylonitrile butadiene styrene (ABS) and polyether-ether-ketone (PEEK). (ABS) and polyether-ether-ketone (PEEK).

3.4. Comparison of ABS and PEEK Bending Properties

Figure 10 shows ABSABS andand PEEKPEEK samplessamples afterafter three-pointthree-point bendingbending tests.tests. Figure 11 shows bending engineering stress-strainstress-strain curves curves for for ABS ABS and and PEEK PEEK samples. samples. According According to the GB/T9341-2008to the GB/T9341-2008 bending bendingstandard, standard, the bending the strengthbending isstrength set as theis set value as th thate value causes that 3.5% causes deformation. 3.5% deformation. The bending The strengthbending strengthand bending and modulusbending ofmodulus five experiments of five experiment are showns are in Figureshown 12 in. Figure The bending 12. The strength bending of strength PEEK was of PEEK56.2 MPa, was 15%56.2 higherMPa, 15% than higher that of than ABS that (48.6 of AB MPa).S (48.6 The MPa). bending The modulus bending ofmodulus PEEK of was PEEK 1.6 GPa, was 1.6which GPa, was which very was close very to close that of to ABS.that of The ABS. main The reason main forreason the for poor the flexural poor flexural property property is the is weak the weakinterlayer interlayer bonding. bonding. The 3D-printed The 3D-printed ABS samples ABS samples had bending had bending strength strength and bending and bending modulus modulus reduced reducedby up to by 8.2% up andto 8.2% 20.8%, and respectively, 20.8%, respectively, compared comp withared those with of injection-moldedthose of injection-molded ABS. The ABS. weak Theinterlayer weak bonding interlayer greatly bonding influenced greatly 3D-printed influenced PEEK 3D-printed sample properties,PEEK sample because properties, the bending because strength the bendingand bending strength modulus and bending were reduced modulus by upwere to 65.5%reduced and by 58.9%,up to 65.5% respectively. and 58.9%, respectively.

Figure 10.10. FracturedFracturedFractured bendingbending bending samples.samples. ((a ()a )acrylonitrile acrylonitrile butadie butadienene styrene styrene (ABS); (b) polyether-ether-ketone (PEEK).(PEEK).

Materials 2015, 8 5844 Materials 2015, 8 3893

Materials 2015, 8 3893

FigureFigure 11. 11.Bending Bending stress-strain stress-strain curvescurves of acryl acrylonitrileonitrile butadiene butadiene styrene styrene (ABS) (ABS) and and polyether-ether-ketoneFigure 11. Bending (PEEK). stress-strain curves of acrylonitrile butadiene styrene (ABS) and polyether-ether-ketonepolyether-ether-ketone (PEEK). (PEEK).

Figure 12.FigureComparison 12. Comparison on bending on bending property property betweenbetween acrylonitrile acrylonitrile butadiene butadiene styrene styrene(ABS) (ABS) and polyether-ether-ketoneFigureand 12. polyether-ether-ketone Comparison on (PEEK). bending (PEEK). property between acrylonitrile butadiene styrene (ABS) 4.and Conclusions polyether-ether-ketone (PEEK). 4. Conclusions 4. ConclusionsThe aim of this paper was to investigate the effects of raster angle and layer thickness on The aimmechanical of this paper properties was of to 3D-printed investigate samples. the effects The experiments of raster confirmed angle and that layer raster thickness angle and onlayer mechanical Thethickness aim of both this have paper a markedwas to effect investigate on tensile, the compressive effects of andraster three-poi anglent andbending layer properties. thickness on properties of 3D-printed samples. The experiments confirmed that raster angle and layer thickness both mechanicalThe optimal properties mechanical of 3D-printed properties ofsamples. PEEK were The found experiments in samples confirmed with a 300- thatμm layerraster thickness angle andand layer have athickness markeda raster effectboth angle have on of tensile,0°/90°. a marked compressive effect on andtensile, three-point compressive bending and properties.three-point Thebending optimal properties. mechanical ˝ ˝ propertiesThe optimal ofOur PEEK studymechanical were also compared found properties in the samples mechanical of PEEK with propertieswere a 300- found ofµm PEEKin layersamples and thickness ABS with sample a 300- and parts.μ am raster layerComparing thickness angle the of 0and/90 . mechanical properties of ABS and PEEK samples made by 3D printing, it can be concluded that the Oura raster study angle also of compared 0°/90°. the mechanical properties of PEEK and ABS sample parts. Comparing the properties of each part are decreased through 3D-printing compared with those of the raw materials. Our study also compared the mechanical properties of PEEK and ABS sample parts. Comparing the mechanicalIn propertiesthis study, the of tensile ABS strength and PEEK of 3D-printed samples PEEK made was about by 3D 56 MPa, printing, which itis equivalent can be concluded to that of that the propertiesmechanicalnylon of each injectionproperties part parts. are of decreasedInABS future and st udies,PEEK through the samples mechanical 3D-printing made properties by compared3D printing,of 3D-printed with it can thosePEEK be concluded ofparts the may raw bethat materials. the properties of each part are decreased through 3D-printing compared with those of the raw materials. In this study, the tensile strength of 3D-printed PEEK was about 56 MPa, which is equivalent to that In this study, the tensile strength of 3D-printed PEEK was about 56 MPa, which is equivalent to that of of nylon injection parts. In future studies, the mechanical properties of 3D-printed PEEK parts may nylon injection parts. In future studies, the mechanical properties of 3D-printed PEEK parts may be be improved by increasing the control accuracy and hardware precision of the 3D-printing system. In this study, the mechanical properties of 3D-printed PEEK samples (tensile, compressive and three-point bending) were higher than those of ABS samples printed by commercial 3D printers. Specifically, the tensile, compression and bending strengths of PEEK samples were higher than those of ABS samples Materials 2015, 8 5845 by 108%, 114% and 115%, respectively, while little significant difference was found between the compressive and flexural modulus of PEEK and ABS. Experimental studies and comparative analyses were carried out to study the factors affecting PEEK 3D print forming quality, in the hope of providing reference conditions under which to print PEEK. However, we recognize that there is much work left to do in this area. Further research is needed to reduce pore formation during the printing process and to improve interlayer bonding. PEEK has favorable properties, including excellent mechanical properties in both static and dynamic tests and high chemical resistance [21,22]. It is believed that PEEK may be a significant and promising material for industrial applications of 3D-printed components.

Acknowledgments

This research is supported by National Natural Science Foundation of China (No. 51205163), Specialized Research Fund for the Doctoral Program of Higher Education of China “SRFDP” (No. 20120061120030), Young Research Fund of Jilin Province Science and Technology Development Plan (No. 20140520124JH) and Young Teachers and Students to Interdisciplinary Cultivating Project of Jilin University (No. JCKY-QKJC28).

Author Contributions

Wenzheng Wu designed the experiment and wrote the ﬁnal manuscript. Peng Geng Guiwei Li and Di Zhao wrote the ﬁrst draft and prepared the experiment. Haibo Zhang carried out the experimental study. Ji Zhao supervised the research, design of the study and carried out the conception. All authors contributed to the analysis for results and conclusions and revised the paper.

Conﬂicts of Interest

The authors declare no conﬂict of interest.

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