Fused Filament Fabrication of PEEK: a Review of Process-Structure-Property Relationships

Review Fused Filament Fabrication of PEEK: A Review of Process-Structure-Property Relationships

Ali Reza Zanjanijam 1, Ian Major 1 , John G. Lyons 2 , Ugo Lafont 3 and Declan M. Devine 1,*

1 Materials Research Institute, Athlone Institute of Technology, N37 HD68 Athlone, Ireland; [email protected] (A.R.Z.); [email protected] (I.M.) 2 Faculty of Engineering and Informatics, Athlone Institute of Technology, N37 HD68 Athlone, Ireland; [email protected] 3 European Space Technology and Research Centre, European Space Agency, Keplerlaan 1, 2201 AZ Noordwijk, The Netherland; [email protected] * Correspondence: [email protected]

Received: 5 June 2020; Accepted: 18 July 2020; Published: 27 July 2020

Abstract: Poly (ether ether ketone) (PEEK) is a high-performance engineering thermoplastic polymer with potential for use in a variety of metal replacement applications due to its high strength to weight ratio. This combination of properties makes it an ideal material for use in the production of bespoke replacement parts for out-of-earth manufacturing purposes, in particular on the International Space Station (ISS). Additive manufacturing (AM) may be employed for the production of these parts, as it has enabled new fabrication pathways for articles with complex design considerations. However, AM of PEEK via fused filament fabrication (FFF) encounters significant challenges, mostly stemming from the semi crystalline nature of PEEK and its associated high melting temperature. This makes PEEK highly susceptible to changes in processing conditions which leads to a large reported variation in the literature on the final performance of PEEK. This has limited the adaption of FFF printing of PEEK in space applications where quality assurance and reproducibility are paramount. In recent years, several research studies have examined the effect of printing parameters on the performance of the 3D-printed PEEK parts. The aim of the current review is to provide comprehensive information in relation to the process-structure-property relationships in FFF 3D-printing of PEEK to provide a clear baseline to the research community and assesses its potential for space applications, including out-of-earth manufacturing.

Keywords: PEEK; additive manufacturing; 3D printing; fused ﬁlament fabrication (FFF); international space station (ISS)

1. Introduction Additive Manufacturing (AM) is a technology based on the principle of layer-by-layer manufacturing, which allows the production of complicated polymeric, metallic, and ceramic parts. Employing this revolutionary method decreases cycle time and cost of product development [1,2]. Fabrication of polymeric parts via AM is generally carried out using one of three different processes which are stereolithography (SLA) [3,4], selective laser sintering (SLS) [5–7], and fused filament fabrication (FFF) [8–10]. In SLA, a photopolymer resin is cured using an energy source, mostly UV laser, to form a single layer of the desired object. SLA was the first 3D printing machine developed and typically gives the best resolution [2]. However, the resin employed must be UV curable, which limits the materials that can be employed using this technique. SLS employs a laser to sinter a polymer powder as a feedstock material. It has been used as a technique to overcome

Polymers 2020, 12, 1665; doi:10.3390/polym12081665 www.mdpi.com/journal/polymers Polymers 2020, 12, 1665 2 of 29

Polymers 2020, 12, x FOR PEER REVIEW 2 of 29 drawbacks of other methods [11,12]. However, expensive instruments are required when using SLS, whichcommonly is a bigused obstacle AM technology for making for productsfabrication [13 of]. thermoplastic FFF, also known polymer as fused parts deposition based on modellingcomputer- (FDM),aided design is the (CAD). most commonly In FFF process, used a AM polymer technology filament for is fabrication continuously of thermoplastic fed to a liquefier, polymer it is melted parts basedand then on computer-aidedextruded through design a nozzle (CAD). to construct In FFF process, the layer-by-layer a polymer structure filament ison continuously a heated bed. fed This to athree-dimensional liquefier, it is melted (3D) and printing then extruded technique through offers alow nozzle manufacturing to construct cost, the layer-by-layerflexibility in structural structure ondesign, a heated and supervision-free bed. This three-dimensional operation [14–17]. (3D) Th printinge most common technique polymers offers low that manufacturing are printed via cost,FFF flexibilityare acrylonitrile-butadiene-styrene in structural design, and supervision-free(ABS) [18–20], and operation poly(lactic [14–17 acid)]. The (PLA) most common[21–23]. However, polymers thatother are polymers printed such via as FFF poly(ether are acrylonitrile-butadiene-styrene imide) (PEI) [24–26], polycarbonate (ABS) [(PC)18–20 [27–29],], and poly(lacticpolystyrene acid) (PS) (PLA)[30,31], [ 21polyamide–23]. However, (PA) [32–34], other polymers polypropylene such as (PP) poly(ether [35–37],poly(vinyl imide) (PEI) alcohol) [24–26], (PVA) polycarbonate [38–41], (PC)polyethylene [27–29], polystyrene(PE) [42,43], (PS) polycaprolactone [30,31], polyamide (PCL) (PA) [44,45], [32–34 ],polyphenylene polypropylene sulfide (PP) [35 (PPS)–37],poly(vinyl [46], and alcohol)polymer (PVA)blends [[47–51]38–41], have polyethylene been also (PE) studied. [42,43 ], polycaprolactone (PCL) [44,45], polyphenylene sulfidePoly(ether (PPS) [46 ],ether and ketone) polymer (PEEK) blends [is47 –an51 ]important have been member also studied. of poly (aryletherketone) (PAEK) familyPoly(ether with many ether excellent ketone) (PEEK) characteristics. is an important PEEK member was first of polyproduced (aryletherketone) by Imperial (PAEK) Chemicals family withIndustries many in excellent 1978, and characteristics. since then, it PEEK has attracted was first a produced lot of attention by Imperial in differen Chemicalst industries Industries [52]. Thanks in 1978, andto its since linear then, aromatic it has attracted structure a lot (Figure of attention 1), this in dihigh-performancefferent industries [thermoplastic52]. Thanks to itspolymer linear aromaticexhibits structuresuperior (Figurethermal1), resistance, this high-performance mechanical thermoplasticperformance polymer[53], low exhibits linear superiorexpansion thermal coefficient, resistance, and mechanicalchemical stability performance (soluble [53 ],only low in linear sulphuric expansion acid coeat ffi60cient, °C [54,55]. and chemical The physical stability and (soluble mechanical only in sulphuricproperties acid of PEEK at 60 are◦C[ listed54,55 ].in The Tabl physicale 1. It has and a melting mechanical temperature properties (T ofm) PEEKof 343 are°C, listedglass intransition Table1. Ittemperature has a melting (Tg) temperature of 143 °C [56], (T mand) of a 343service◦C, glasstemperature transition up temperatureto 260 °C [57]. (T Havingg) of 143 a◦ highC[56 Young’s], and a servicemodulus temperature and tensile up strength to 260 ◦ C[along57]. with Having a low a high specific Young’s gravity, modulus allows and PEEK tensile to strengthbe used along as a withreplacement a low specific of aluminium gravity, allows or steel PEEK in toa bewide used ra asnge a replacementof applications, of aluminium especially or aerospace steel in a wideand rangeautomotive of applications, applications especially with an aerospaceincreasing and intere automotivest in employing applications PEEK within the an space increasing industry interest [58], in[59]. employing Furthermore, PEEK since in the PEEK space shows industry biocompatibility [58,59]. Furthermore, and radio-transparency, since PEEK shows it biocompatibility is a promising andmaterial radio-transparency, for biomedical applications, it is a promising particularly material as for an biomedical appropriate applications, alternative particularlyto precious metal as an appropriateimplants[60–68]. alternative It can also to precious be sterilised metal repeatedly, implants [60 which–68]. is It canideal also for besurgical sterilised instruments repeatedly, and which dental is idealdevices for [64,69]. surgical instruments and dental devices [64,69].

Figure 1. Chemical structure of poly (ether ether ketone) (PEEK) [[60].60].

There areare several several methods methods used used for processingfor processing PEEK-based PEEK-based products, products, including including extrusion, extrusion, injection moulding,injection moulding, compression compression moulding, moulding, machining, machining, powder spraying, powder etc. spraying, [70–73]. etc. The [70–73]. main shortcoming The main ofshortcoming these techniques of these is techniques the lack of flexibilityis the lack for of flexibility making parts for making with complicated parts withdesigns. complicated It seems designs. that additiveIt seems manufacturingthat additive manufacturing has the potential has to the develop potential PEEK to partsdevelop without PEEK geometry parts without limitations geometry in a cost-elimitationsffective in way.a cost-effective Considering way. the Considering melt processability the melt of processability PEEK, both SLS of andPEEK, FFF both can beSLS employed and FFF forcan the be productionemployed offor 3D the printed production parts. of SLS 3D was print theed first parts. AM processSLS was used the forfirst fabrication AM process of PEEK used [ 11for]. However,fabrication it of is PEEK more complicated[11]. However, and it moreis more expensive complicated than FFFand [more74,75 ].expensive One of the than limitations FFF [74,75]. is in One the recyclingof the limitations of non-fused is in PEEK the powder,recycling which of non-fused increases processingPEEK powder, expenditure which [ 76increases]. Moreover, processing aiming toexpenditure implement [76]. such Moreover, process for aiming out-of-earth to implement manufacturing such process (in orbit for out-of-earth or on planet), manufacturing the effect of low (in gravityorbit or combinedon planet), with the effect health of and low safety gravity aspect combined makes with the use health of powder and safety less aspect suitable. makes In this the respect, use of FFFpowder process less thatsuitable. uses In filament this respect, as the FFF feedstock process material that uses has filament been successfully as the feedstock implemented material has in a been low gravitysuccessfully environment implemented on the in International a low gravity Space environm Station.ent Hence, on the there International has been a growingSpace Station. interest Hence, in 3D printingthere has of been PEEK a viagrowing FFF in interest recent years in 3D in printing situ production of PEEK of via parts FFF on in the recent ISS. However,years in situ a big production challenge regardingof parts on 3D the printed ISS. However, PEEK is a its big anisotropic challenge mechanical regarding 3D properties, printed PEEK compared is its to anisotropic PEEK parts mechanical produced usingproperties, conventional compared processing to PEEK methods parts suchproduced as injection using moulding.conventional As such,processing understanding methods the such effects as ofinjection different moulding. process parameters As such, understanding in additive manufacturing the effects of on different the final process properties parameters of the printed in additive parts, manufacturing on the final properties of the printed parts, and thereby finding optimum process parameter levels are the key to fabricate PEEK end-parts with desirable, reproducible performance. The aim of this literature review is to address challenges and provide a comprehensive and practical

Polymers 2020, 12, 1665 3 of 29 and thereby ﬁnding optimum process parameter levels are the key to fabricate PEEK end-parts with desirable, reproducible performance. The aim of this literature review is to address challenges and provide a comprehensive and practical guide to 3D printing of PEEK via FFF. This way, the focus will be on the process-structure-property relationships of the PEEK-based printed parts mostly based on tensile strength performances.

Table 1. Physical and mechanical properties of PEEK [77].

Material Properties Typical Value Density 1.3 g/cm3 Melting temperature 343 ◦C Glass transition temperature 143 ◦C Coeﬃcient of thermal expansion Average below Tg: 55 ppm/k Average above Tg: 140 ppm/k Heat deﬂection temperature 152 ◦C Thermal conductivity 0.32 W/ m.k Young’s modulus 4 GPa Tensile strength 100 MPa Elongation at break 45% Flexural modulus 3.9 GPa Flexural strength 162 MPa Compressive modulus 3.2 GPa Compressive strength 125 MPa Shore D hardness 84.5 Water absorption 0.45% Flammability V-0

2. Challenges of 3D printing of PEEK PEEK is a promising engineering polymer suitable for many high-performance applications. Recently, processing of PEEK through 3D printing has attracted a lot of attention. However, there are some barriers and difficulties in fabrication of ideal-performance parts. These barriers originate from both inherent issues of FFF such as thermal gradient, and specific physical properties of PEEK. During layer-by-layer manufacturing, residual stress is accumulated, leading to warping and interlayer delamination [78,79]. These problems are more important for additive manufacturing of semi-crystalline polymers with a high melting temperature such as PEEK and can significantly affect dimensional accuracy and mechanical properties [80–82]. In fact, the degree of crystalline perfection of PEEK is considerably influenced by the rate of cooling and thermal gradient during crystallisation from melt [83]. Processing via FFF differs from injection moulding in many aspects, so the formation of defects in the parts and a subsequent loss in the final performance of printed polymers are expected. SEM micrograph of an injection moulded-PEEK is representative of a tough fracture surface, while 3D printed PEEK shows a smooth surface due to brittle fracture. Injection moulding provides an external pressure which decreases the internal defects leading to a better toughness. This is not the case in 3D printing, so inferior properties are observed for the printed samples [84]. To achieve the desired physical and mechanical properties for PEEK parts fabricated through FFF both polymer processing characteristics must be considered. For the former, knowledge of the rheological properties, crystallisation kinetics, etc. is required. As is knowledge of the additive manufacturing process in terms of ensuring the FFF printer is a compatible high-temperature printer and optimal printing parameters are known. To illustrate variances reported in the literature using non optimised conditions, Figure2 shows the tensile properties of 3D-printed PEEK samples from various sources. A wide range of values can be seen for the mechanical properties in the reports. Most of the results are far from the properties of bulk material (processed via injection moulding). This large variation in the mechanical behaviour is due to different printing parameters such as printing orientation, nozzle temperature, environment temperature, bed temperature, printer type, PEEK type, etc. selected by the authors. Rinaldi et al. [85], Tseng et al. [86], Han et al. [87], and Yang et al. [88] Polymers 2020, 12, 1665 4 of 29 reported values which approximately equalled those of injection moulded PEEK and in some cases exceedPolymers values2020, 12, forx FOR Young’s PEER REVIEW modulus and tensile strength. Flexural and compressive data from PEEK4 of 29 parts fabricated with FFF has also being reported albeit in fewer reports than for tensile properties. properties. The compiled results for flexural and compression data of FFF 3D-printed PEEK are The compiled results for flexural and compression data of FFF 3D-printed PEEK are illustrated in illustrated in Figure 3. According to the results, the values recorded for flexural modulus and flexural Figure3. According to the results, the values recorded for flexural modulus and flexural strength strength do not match those of injection moulded PEEK. In contrast, values for compressive strength do not match those of injection moulded PEEK. In contrast, values for compressive strength of of FFF3D-printed PEEK have being reported to exceed those of injection moulded parts. For example, FFF3D-printed PEEK have being reported to exceed those of injection moulded parts. For example, Daurskikh et al. [89] and Wang et al. [90] obtained a compressive strength of 126.4 and 125 MPa, Daurskikh et al. [89] and Wang et al. [90] obtained a compressive strength of 126.4 and 125 MPa, respectively. More interestingly, the compressive strength of PEEK samples in reports by Han et al. respectively. More interestingly, the compressive strength of PEEK samples in reports by Han et al. [87] [87] and Tseng et al. [86] showed an 11% and 12% improvement in compressive strength relative to and Tseng et al. [86] showed an 11% and 12% improvement in compressive strength relative to bulk bulk PEEK, respectively. PEEK, respectively.

Figure 2. Cont.

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Figure 2. Tensile properties values for 3D-printed PEEK reported in the literature. Figure 2. Tensile properties values for 3D-printed PEEK reported in the literature. Figure 2. Tensile properties values for 3D-printed PEEK reported in the literature.

Figure 3. Flexural and compressive properties values for 3D-printed PEEK reported in theliterature. Figure 3. Flexural and compressive properties values for 3D-printed PEEK reported in the literature. Triangle and circle symbols are strength and modulus data, respectively. TriangleFigure 3. and Flexural circle andsymbols compressive are strength properties and modulus values fordata, 3D-printed respectively. PEEK reported in the literature. Triangle and circle symbols are strength and modulus data, respectively.

Polymers 2020, 12, 1665 6 of 29

A comparison of the additive manufacturing processing conditions from the literature reveals that optimal mechanical properties for FFF 3D-printed PEEK can be achieved by selecting a combination of appropriate printing parameters such as high printing temperature, bed temperature, and building direction. The parameters which have been reported in the literature for 3D printing of PEEK via FFF are listed in Table2. For nozzle and bed, a wide range of temperatures have been reported. The values are in the range of 340–520 ◦C and 100–300 ◦C, respectively. The most common nozzle diameter is 0.4 mm, although nozzles with greater diameters have also been used in some cases. The authors also employed different commercial and modified/custom-build FFF printers. Table3 summarises print conditions for commercial PEEK filaments recommended by the manufacturers. These parameters are merely for general use and may need to be altered depending on the FFF machine utilised. In fact, due to the large variety of printing machines, geometries and volumes of parts, etc., it may be necessary to employ different settings according to the filament/ FFF printer combination employed. As such, this review aims to give a comprehensive insight into the relationships between process, structure, and properties of PEEK parts produced using additive manufacturing. Additionally, the influence of printing path parameters and processing conditions on the performance of PEEK parts will be considered, in addition to the thermal stability of PEEK during 3D printing. The latter is critical when processing in enclosed environments as seen in the ISS as is the thermal stability of PEEK and its degradation in different conditions will also be considered.

Table 2. 3D printing parameters of PEEK via fused ﬁlament fabrication (FFF).

Nozzle Nozzle Printing Bed Layer Inﬁll Printing Study PEEK Temperature Diameter Speed Temperature Thickness Density Machine (◦C) (mm) (mm/s) (◦C) (mm) (%) 1st layer Indmatec Basgul, PEEK OPTIMA™ 1000–3000 height: 0.1 HPP 390–410 0.4 100 100 2018 [91] LT1 mm/min Top solid 155/Gen 2 layers: 3 Wu, 2015 PEEK Jilin custom-build - 0.4 - - 0.2–0.4 - [92] University printer Yang, 2017 custom-build VictrexPEEK 450G 360–480 0.4 40 - 0.2 - [88] printer 800 mm/min; Arif, 2018 Indmatec 410 C; 1st 0.1; 1st layer: Victrex PEEK 450G ◦ 0.4 1st layer: 300 100 100 [93] HPP 155 layer: 390 C 0.18 ◦ mm/min Rahman, Arevo Labs Arevo Labs 340 1.8 50 230 0.25 100 2015 [94] Rinaldi, Victrex PEEK 450 PF Indmatec 400 0.4 20 100 - 20–100 2018 [85] extrusion self-made Geng, 2019 speed: VICTREX 450 G printing 360 0.4–0.6 110 - - [95] 0.1–120 system mm/min Ding, 2019 custom-build 450G, Junhua, China 360–420 0.4 20 270 0.2 - [96] printer Wang, 2019 custom-build - 360–460 0.4–0.8 17–26 280 0.1–0.5 - [84] printer Deng, 2018 PEEK- custom-build 350–370 - 20–60 - 0.2–0.3 20–60 [97] ZhongshanYousheng printer Zhao, 2018 homemade Victrex PEEK450G 355–375 0.4 30 230–270 0.2 100 [69] printer a new screw Tseng, Victrex (PEEK 90G extrusion-based 370–390 - - 280 - - 2018 [86] and PEEK 450G) 3D printing system Honigmann, Apium PEEK 450 Apium P220 Up to 520 0.4 - Up to 160 0.1–0.3 - 2018 [98] Stepashkin, CF-composite custom-build 380 0.35 100 mm/min 95 0.25 100 2018 [99] Victrex 150 CA30 printer Polymers 2020, 12, 1665 7 of 29

Table 2. Cont.

Nozzle Nozzle Printing Bed Layer Inﬁll Printing Study PEEK Temperature Diameter Speed Temperature Thickness Density Machine (◦C) (mm) (mm/s) (◦C) (mm) (%) VICTREX® Berretta, MendleMax 450G/Plasticyl PEEK 350–390 - 30 300 - - 2017 [100] v2.0 101 Xiaoyon, - - - 0.4 20 25–130 0.8 100 2017 [101] Wu, 2014 PEEK Jilin custom-build 340–360 - - 150 0.3 - [102] University printer Daurskikh, Victrex PEEK 450G ------100 2018 [89] Vaezi, 2015 Victrex PEEK 450G - 350–450 - - Up to 130 0.2 - [103] Hyrel Hydra 16AS, Indmatec Wang, 2019 Apium PEEK 450 HPP 155, 400 0.4 20 100 0.1–0.2 100 [90] Natural Intamsys FUNMAT HT Cicala, Roboze one Luvocomm 420 - 20 110 0.1 75 2017 [104] 400+ Han, 2019 Jugao-AM Victrex PEEK 450G 420 0.4 40 - 0.2 - [87] Tech. Corp Han, 2019 Evonik Apium P220 480 0.4 - 130 0.2 - [105] VESTAKEEP®i4 G Gonçalve, Indmatec Victrex PEEK 450G 400 0.4 20 100 0.1 100 2018 [106] HPP 155 Li, 2019 FUNMAT ZYPEEK 550 G 400 0.4 15 160 0.1 100 [107] HT Modiﬁed Hu, 2019 Sting3d Technology Speedy 385 0.4 25 135 0.1 100 [108] Maker Wang, 2019 PEEK 450G, Junhua - 380–420 0.4 5–25 220–300 0.1–0.3 100 [109] TotalZAnyForm Sviridov, Victrex PEEK 381G 950 PRO 450 0.7 40 - 0.75 100 2019 [110] HOT+ Indmatec Basgul, PEEK OPTIMA™ 1500/2000 HPP 390–410 0.4 100 0.1 100 2019 [111] LT1 mm/min 155/Gen 2 Lafont, 2017 [106] Indmatec van Victrex 450G 390 0.4 20 100 0.1 100 HPP 155 Egmond [112]

Table 3. Parameters of 3D printing of PEEK recommended by ﬁlament manufacturers.

Nozzle Bed Chamber Bed Brand Name Temperature Temperature Printing Speed (mm/s) Temperature Preparation (◦C) (◦C) (◦C) ThermaX™ 375–410 130–145 10–50 for 0.2 mm thickness 70–140 Ultem™ Tape 3D4MAKERS 360–400 120 15–30 70–150 PEI sheet Polyﬂuor 335–390 120 - - - KetaSpire® 390–420 >200 - - - LUVOCOM 370–420 >120 - - -

3. Process-Structure-Property Relationships

3.1. Printing Path Parameters 3D printing could be performed in different building orientations and raster angles. These printing path parameters affect the microstructure and final properties of PEEK samples. Arif et al. [93] studied the effect of printing configurations and bead orientation on the mechanical behavior of PEEK processed Polymers 2020, 12, x FOR PEER REVIEW 8 of 29

3. Process-Structure-Property Relationships

3.1. Printing Path Parameters 3D printing could be performed in different building orientations and raster angles. These Polymersprinting2020 path, 12 ,parameters 1665 affect the microstructure and final properties of PEEK samples. Arif 8et of al. 29 [93] studied the effect of printing configurations and bead orientation on the mechanical behavior of PEEK processed by FFF. They evaluated the tensile and flexural properties of the samples printed byhorizontally FFF. They and evaluated vertically the with tensile a raster and angles flexural of properties 0° (H-0°) and of the90° samples(H-90°, V-90°), printed as horizontally shown in Figure and vertically4. The H-0° with and a rasterV-90° angles samples of 0exhibited◦ (H-0◦) and the 90 best◦ (H-90 and◦ ,worst V-90◦ ),mechanical as shownin properties, Figure4. The respectively. H-0 ◦ and V-90Similar◦ samples results exhibitedhave been the reported best and in worstother mechanicalworks focusing properties, on the respectively.printing path Similar parameters. results When have beenhorizontal reported building in other orientation works focusing is used onfor the printing, printing direction path parameters. of applied Whentensile horizontal or flexural building force is orientationparallel to the is used orientation for printing, of the direction polymer of filaments, applied tensile and thereby or flexural the sample force is parallelshows a to ductile the orientation fracture. ofAccording the polymer to Table filaments, 4 which and lists thereby the results the sample of mechanical shows aproperties ductile fracture. of PEEK According samples reported to Table in4 whichthe literature, lists the near results bulk of mechanical mechanical properties properties can of PEEKbe achieved samples by reportedprinting in the the horizontal literature, direction. near bulk mechanicalIn contrast, propertiesthe applied can force be achieved during bythe printing test is normal in the horizontal to interface direction. between In contrast,layers for the vertically applied forceprinted during samples the testleading is normal to a brittle to interface fracture. between In fact, layersstrength for of vertically interfacial printed bonding samples between leading beads to is a brittlethe key fracture. factor for In fact,these strength samples. of Another interfacial interesting bonding between point isbeads high standard is the key deviation factor for theseof the samples. results Anotherfor the parts interesting printed pointat vertical is high direction standard as deviationwell as the of difference the results between for the the parts reported printed values. at vertical The directionkey role of as the well building as the di orientationfference between in determining the reported final values. properties The key of PEEK role of can the also building be traced orientation using indynamic determining mechanical final properties analysis of(DMA). PEEK canFigure also 5 beshows traced the using same dynamic trend mechanicalfor the samples analysis printed (DMA). in Figuredifferent5 shows orientations the same as discussed trend for theabove. samples In encl printedosed environments in di fferent orientations with limited as space discussed as observed above. Inin the enclosed ISS the environments printing direction with limitedand raster space angle, as observed become paramount in the ISS the especial printingly when direction printing and rasterlarge angle,critical become parts. paramount especially when printing large critical parts.

Figure 4.4. ConﬁgurationsConfigurations of of samples samples fabricated fabricated (a) horizontally(a) horizontally or (b )or vertically. (b) vertically. The deposition The deposition pattern

ispattern (c) 0◦ isbeads (c) 0° orientation beads orientation or (d,e) 90or ◦(dbeads,e) 90° orientation beads orientation [93]. [93].

Polymers 2020, 12, x FOR PEER REVIEW 9 of 29

Table 4. Effect of raster layer orientation on the mechanical properties of PEEK.

BuildingPolymers 2020 Orientation, 12, 1665 /Raster Young’s Modulus Tensile FlexuralStrength9 of 29 Angle (GPa) Strength (MPa) (MPa) Horizontal/0°Table [93] 4. Eﬀ ect of raster layer orientation 3.80 on the mechanical 82.58 properties of PEEK. 142.0 Horizontal/90° [93] 3.54 72.88 124.3 Building Orientation/ Young’s Modulus (GPa) Tensile Strength (MPa) FlexuralStrength (MPa) Vertical/90°Raster Angle [93] 3.03 9.99 16.40

Horizontal/+45°/Horizontal/0◦ −[9345°[85]] 3.803.98 82.58 98.9 142.0 - Vertical/+45°/Horizontal/90−◦45°[93 ][85] 3.54 1.6 72.88 19.6 124.3 - Vertical/90◦ [93] 3.03 9.99 16.40 HorizontalHorizontal/0°/+45 / 45 [94][85 ] 3.98 2.83 98.9 73.19 - 114.16 ◦ − ◦ Horizontal/90°Vertical/+45 / 45 [94][85] 1.6 2.69 19.6 53.91 - 78.63 ◦ − ◦ Horizontal/0°/90°[94]Horizontal/0◦ [94] 2.832.73 73.19 67.75 114.16 95.22 Horizontal/90◦ [94] 2.69 53.91 78.63 Horizontal/0°Horizontal/0◦/90 ◦[106][94] 2.73 3.29 67.75 89.4 95.22 - Horizontal/+45°/Horizontal/0◦−[11045°] [106] 3.29 3.03 89.4 81.7 - - Horizontal/+45 / 45 [110] 3.03 81.7 - Horizontal/0°/90°◦ − ◦ [112] 3.38 78.1 - Horizontal/0◦/90◦ [112] 3.38 78.1 - Vertical/+45°/Vertical/+45 / −4545° 2.212.21 31.131.1 - - ◦ − ◦

Figure 5. Storage and loss moduli of PEEK filament and 3D printed samples with a 45/+45 raster Figure 5. Storage and loss moduli of PEEK filament and 3D printed samples with− a −45/+45 raster layer orientation [112]. layer orientation [112]. CT scans of PEEK based tensile test specimen before the test (Figure6) reveals that of the void fractionCT scans in theof samplesPEEK based printed tensile horizontally test specimen and vertically before is nearlythe test same (Figure (6.7% 6) and reveals 7.6%, respectively). that of the void fractionHowever, in the the sizesamples of the voidsprinted in the horizontally Z-printed samples and isvertically much larger is thannearly XY-samples. same (6.7% Additionally, and 7.6%, respectively).the orientation However, of the voidsthe size with of respect the voids to the in force the direction Z-printed can samples have a devastating is much elargerffect on than the XY- samples.properties Additionally, of the printed the partorientation [85]. Another of the example voids with by Lafont respect et al. to confirms the force that direction vertical printedcan have a devastatingsample exhibit effect on more the voids properties than horizontal of the printed printed pa samples.rt [85]. Another Moreover, example it is also by interesting Lafont et thatal. confirms the that voidvertical fraction printed candi sampleffer depending exhibit onmore the voids investigated than horizontal area (Figure printed7). For example, samples. in Moreover, ASTM Type it V is also specimen, CT scan results suggest that the narrow central part of the sample contains more voids than interesting that the void fraction can differ depending on the investigated area (Figure 7). For the grip area [106]. example, in ASTM Type V specimen, CT scan results suggest that the narrow central part of the sample contains more voids than the grip area [106].

Polymers 2020, 12, x FOR PEER REVIEW 10 of 29 Polymers 2020, 12, 1665 10 of 29 Polymers 2020, 12, x FOR PEER REVIEW 10 of 29

Figure 6. Computed Tomography scans of gauge length of PEEK-XY-100 (left) and PEEK-Z-100 (right) Figuresamples 6. 6. Computed Computedbefore tensile Tomography Tomography test [85]. scans of gauge gauge length length of of PEEK-XY-100 PEEK-XY-100 ((left)left) and and PEEK-Z-100 PEEK-Z-100 (right) (right) samples before tensile test [85]. samples before tensile test [85].

Figure 7. Void fraction calculated from CT Scan as a function of the printing orientation and scan Figure 7. Void fraction calculated from CT Scan as a function of the printing orientation and scan location [110]. Figurelocation 7. [106].Void fraction calculated from CT Scan as a function of the printing orientation and scan locationResults [106]. indicate that a raster angle of 0 (aligned in the direction of the tensile strength) can lead Results indicate that a raster angle of 0°◦ (aligned in the direction of the tensile strength) can lead to better mechanical behaviour compared to 90 (Table4). Although shorter bead deposition path, to better mechanical behaviour compared to 90°◦ (Table 4). Although shorter bead deposition path, betterResults polymer indicate diffusion, that anda raster thereby angle stronger of 0° (aligned interfacial in the bonding direction between of the te beadsnsile arestrength) conceivable can lead for tobetter better polymer mechanical diffusion, behaviour and thereby compared stronger to 90° inte (Trfacialable 4). bonding Although between shorter be beadads are deposition conceivable path, for raster angle of 90◦, however, the strength of the filaments is poorer than 0◦ [93]. It has been shown betterraster polymer angle of diffusion, 90°, however, and thereby the strength stronger of theinte firfaciallaments bonding is poorer between than 0°be ads[93]. are It hasconceivable been shown for that type of fracture of specimens prepared in different raster angles is completely different. In fact, rasterthat type angle of of fracture 90°, however, of specimens the strength prepared of thein differe filamentsnt raster is poorer angles than is completely 0° [93]. It hasdifferent. been shown In fact, there is a partial failure for the sample printed in a raster angle of 0◦, whereas 90◦and alternating 0◦/90◦ thatthere type is aof partial fracture failure of specimens for the sample prepared printed in differe in ant raster raster angle angles of is0°, completely whereas 90°and different. alternating In fact, specimens show a sudden clean fracture [94]. there0°/90° is specimensa partial failure show afor sudden the sample clean fractureprinted [94].in a raster angle of 0°, whereas 90°and alternating Printing configuration and bead orientation may also have a significant effect on the surface 0°/90°Printing specimens configuration show a sudden and cleanbead fractureorientation [94]. may also have a significant effect on the surface quality of the printed PEEK. According to Wang et al. [84], the horizontal direction provides better qualityPrinting of the configuration printed PEEK. and According bead orientation to Wang mayet al. also [84], have the horizontal a significant direction effect onprovides the surface better surface quality under the same printing conditions. This was attributed to the squeezing action of the qualitysurface of quality the printed under PEEK. the same According printing toconditions. Wang et al.This [84], was the attributed horizontal to thedirection squeezing provides action better of the nozzle. The Poisson’s ratio can be considered as a criterion for comparing surface quality. Results have surfacenozzle. quality The Poisson’s under the ratio same can printing be considered conditions. as aThis criterion was attributed for comparing to the squeezingsurface quality. action Results of the shown that printing of PEEK in the vertical direction leads to a lower Poisson’s ratio. Since applied nozzle.have shown The Poisson’s that printing ratio canof PEEK be considered in the vertical as a criteriondirection for leads comparing to a lower surface Poisson’s quality. ratio. Results Since have shown that printing of PEEK in the vertical direction leads to a lower Poisson’s ratio. Since

Polymers 2020, 12, 1665 11 of 29 force is normal to the printed layers, the number of voids increases during loading and as a result, this parameterPolymers 2020,declines 12, x FOR PEER [93]. REVIEW 11 of 29 Raster angle can change the roughness and colour of PEEK samples. 3D printing at a horizontal applied force is normal to the printed layers, the number of voids increases during loading and as a directionresult, and this a parameter raster angle declines of 90◦ [93].provides a rougher and a whiter surface than 0◦. Since the deposition path is shorter,Raster theangle thermal can change gradient the roughness between beadsand colour is lower, of PEEK and samples. thereby 3D diff printingusion is at faster a horizontal leading to a rougherdirection surface and finish a raster for theangle part of printed90° provides at a raster a rougher angle and of 90a ◦whiter[93]. surface than 0°. Since the deposition path is shorter, the thermal gradient between beads is lower, and thereby diffusion is 3.2. Printingfaster leading Layer to Thickness a rougher surface finish for the part printed at a raster angle of 90°[93]. Wang et al. [84] examined the effect of nozzle diameter and printing layer thickness on physical and 3.2. Printing Layer Thickness mechanical properties of PEEK. Results from samples printed with a 0.4 mm nozzle diameter exhibited little changeWang in theet al. density [84] examined and tensile the effect strength of nozzle of PEEK. diameter When and nozzles printing with layer larger thickness diameter on physical were used, the tensileand mechanical strength deterioratedproperties of withPEEK. an Results increase from in layersamples thickness. printed with The authorsa 0.4 mm hypothesised nozzle diameter that the exhibited little change in the density and tensile strength of PEEK. When nozzles with larger diameter interlayer bonding strength was weakened for the layers thicker than 0.35 mm. Wu et al. [92] reported were used, the tensile strength deteriorated with an increase in layer thickness. The authors that thehypothesised highest tensile, that the flexural, interlayer and bonding compressive strength strength was weakened could be for obtained the layers for PEEKthicker parts than at an optimal0.35mm.Wu layer thickness et al. [92] of reported 0.3 mm. that It isthe noteworthy highest tensile, that flexural, they used and compressive a custom-built strength 3D printer could be with a nozzleobtained diameter for ofPEEK 0.4 parts mm. at an optimal layer thickness of 0.3 mm. It is noteworthy that they used a Surfacecustom-built roughness 3D printer (R zwith) shows a nozzle an diameter increase of as 0.4 layer mm. thickness increases (Figure8) due to gaps betweenSurface layers androughness the enlargement (Rz) shows an of increase voids. as If layer the printing thickness layer increases is thicker (Figure than 8) due half to ofgaps nozzle diameter,between inappropriate layers and the quality enlargement can be observed of voids. inIf ath verticale printing direction. layer is thicker Also, there than ishalf an of apparent nozzle step effectdiameter, when thicker inappropriate layers are qual usedity can for be printing observed [ 84in ].a vertical Wang etdirect al. [ion.109 ]Also, predicted there is surface an apparent roughness step of PEEKeffect by developing when thicker a layers model. are They used found for printing that there [84]. Wang was a et good al. [109] agreement predicted between surface roughness the experimental of PEEK by developing a model. They found that there was a good agreement between the experimental and the predicted surface roughness and finally recommended a layer thickness of 0.15 mm for 3D and the predicted surface roughness and finally recommended a layer thickness of 0.15mm for 3D printingprinting of PEEK. of PEEK.

FigureFigure 8. Surface 8. Surface morphology morphology of of PEEK PEEK samples samples printed in in the the vertical vertical direction: direction: layer layer thickness thickness of (a) of (a) 0.1 mm,0.1 mm, and and (b) 0.25(b) 0.25 mm mm [84 [84].].

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3.3.3.3. ProcessingProcessing ConditionsConditions

3.3.1.3.3.1. AmbientAmbient/Printing/Printing ChamberChamber Temperature Temperature InIn 3D3D printing, printing, aa lower lower accuracy accuracy cancan bebe seen seen for for PEEK PEEK especially especially in in more more complex complex geometric geometric areasareas such such as as the the narrow narrow area area of of tensile tensile specimens specimens owing owing to to some some reasons reasons including including its its high high T Tggand and TTmm,, highhigh viscosity,viscosity, shrinkage, shrinkage, etc. etc. TheThe temperaturetemperature insideinside ofof printing printing chamber chamber can can be be a a determining determining factorfactor inin the the production production of of parts parts with with desirable desirable accuracy accuracy andand performance.performance. YangYang etet al.al. [[88]88] usedused aa temperature-controltemperature-control 3D 3D printing printing system system to to study study the the relationship relationship between between ambient ambient/printing/printing chamber chamber temperaturetemperature inin FFF FFF and and the the performance performance of of PEEK. PEEK. They They found found that that control control of of ambient ambient temperature temperature playsplays a a crucial crucial role role in thein mechanicalthe mechanical properties properties of printed of printed PEEK. AsPEEK. shown As in shown Figure9 ,in the Figure percentage 9, the ofpercentage crystalline of phase crystalline increases phase as ambientincreases temperature as ambient temperature increases which increases in turn which leads in to turn a remarkable leads to a improvementremarkable improvement in Young’s modulus in Young’s and modulus tensile strength. and tensile At strength. a temperature At a temperature of 200 ◦C, they of 200 obtained °C, they a Young’sobtained modulus a Young’s even modulus higher even than thathigher of injection-mouldedthan that of injection-moulded PEEK. PEEK.

FigureFigure 9. 9.Ambient Ambient temperaturetemperature dependency dependency of of ( (aa)) crystallinity crystallinity and and ( (bb)) mechanical mechanical properties properties of of PEEK PEEK samplessamples [ 88[88].].

WhenWhen the the ambient ambient temperature temperature is is low, low, there there is ais large a large temperature temperature difference difference between between nozzle nozzle and theand printing the printing chamber. chamber. This providesThis provides a rapid a rapid cooling cooling for PEEK for PEEK chains chains (imperfect (imperfect crystallisation) crystallisation) and causesand causes significant significant warping warping distortion. distortion. There is aTher reversee is relationshipa reverse relationship between ambient between temperature ambient andtemperature warping distortionand warping in additive distortion manufacturing. in additive ma Highernufacturing. temperatures Higher can temperatures help to slow can down help the to crystallisationslow down the phenomenon crystallisation and phenomenon reduce internal and stressesreduce internal [102]. In stresses fact, at [102]. high ambientIn fact, at temperatures, high ambient polymertemperatures, chains polymer have adequate chains have time adequate and energy time toand crystallise, energy to crystallise, so PEEK canso PEEK experience can experience a more perfecta more crystallisation perfect crystallisation process andprocess thereby and higherthereby degree higher of degree crystallinity of crystallinity is obtained. is obtained. These findings These demonstratefindings demonstrate that one can that adjust one thecan crystallinityadjust the crystallinity and stiffness and/toughness stiffness/toughness of PEEK by choosingof PEEK anby optimalchoosing ambient an optimal temperature. ambient temperature. InIn a a study study by by Wang Wang et et al. al. [ 90[90],], the the e effectffect of of ambient ambient temperature temperature on on the the physical physical and and mechanical mechanical propertiesproperties ofof PEEKPEEK waswas investigatedinvestigated throughthrough printingprinting usingusing threethree didifferentfferent printersprinters namely,namely, HyrelHyrel HydraHydra 16AS16AS (Hyrel(Hyrel 3D3D Ltd.,Ltd., Norcross,Norcross, USA),USA), IndmatecIndmatec HPPHPP 155155 (Apium(Apium AdditiveAdditive TechnologiesTechnologies GmbH,GmbH, Karlsruhe, Karlsruhe, Germany) Germany) and and Intamsys Intamsys FUNMAT FUNM HTAT (Intamsys HT (Intamsys Technology Technology Ltd., Shanghai, Ltd., Shanghai, China). TheChina). difference The difference among printed among samples printed was samples visibly wa evidents visibly based evident on their based colour, on astheir illustrated colour, inas Figureillustrated 10. Additionally,in Figure 10. X-rayAdditionally, diffraction X-ray (XRD) diffraction patterns (XRD) revealed patterns that therevealed sample that printed the sample using theprinted Hyrel, using had the a completely Hyrel, had di aff completelyerent microstructure. different microstructure. It showed the It lowest showed degree the lowest of crystallinity degree of owingcrystallinity to the owing fact that to the fact ambient that the temperature ambient temperature in this printer in this was printer 23 ◦C. was In contrast, 23 °C. In ancontrast, ambient an temperatureambient temperature of 145 ◦C inof Intamsys145 °C in printer Intamsys enabled prin theter PEEKenabled chains the toPEEK complete chains the to crystallisation complete the process.crystallisation Even though process. the Even ambient though temperature the ambient was temperature 23 ◦C in the was case 23 of °C Indmatec, in the case but of presence Indmatec, of but an adaptivepresence heating of an adaptive system provided heating asystem normal provided crystallisation. a normal These crystallisation. variations in crystallinityThese variations affected in thecrystallinity mechanical affected and dynamic-mechanical the mechanical and properties dynamic- ofmechanical PEEK. Furthermore, properties a changeof PEEK. (around Furthermore, 10 ◦C) in a glass transition temperature (T ) indicates that non-crystalline regions of PEEK can also be influenced change (around 10 °C) in glassg transition temperature (Tg) indicates that non-crystalline regions of byPEEK process can conditionsalso be influenced such as by ambient process temperature. conditions such as ambient temperature.

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Figure 10. Colour differences diﬀerences in PEEK samples (Hyrel, Intamsys,Intamsys, and Indmatec from top to bottom or from left to right, respectively)respectively) [[90].90].

To improveimprove thethe printingprinting conditions,conditions, Hu et al. [[108]108] recentlyrecently developeddeveloped a modifiedmodified FFF printer suitable forfor additiveadditive manufacturingmanufacturing of of PEEK PEEK by by adding adding a a heat heat collector collector (HC) (HC) and and a high-powera high-power heater heater to theto the nozzle nozzle module module and designingand designing a near fully-encloseda near fully-enclosed chamber. chamber. They found They that overallfound temperaturethat overall aroundtemperature the nozzle around increases the nozzle by increases adding thisby adding heat collector. this heat collector. A comparison A comparison of temperature of temperature profiles aroundprofiles thearound nozzle the shows nozzle that shows within that 50 within mm of 50 the mm nozzle, of the there nozzle, was there a large was temperature a large temperature difference (arounddifference 50 (around◦C) between 50 °C) improved between and improved traditional and nozzles. traditional This was nozzles. beneficial This for was 3D beneficial printing of for PEEK, 3D becauseprinting itof provides PEEK, because a more uniformit provides temperature a more un distribution.iform temperature According distribution. to the authors, According the sample to the printedauthors, using the sample traditional printed nozzle using shows traditional severe warpagenozzle shows (20.4%). severe An warpage increase in(20.4%). ambient An temperature increase in canambient significantly temperature solve can this significantly problem and solve reduce this theproblem warpage and rate reduce to 6.1%, the warpage but there rate was to still 6.1%, severe but delamination.there was still severe By employing delamination. the heat By collector, employing the th preparede heat collector, PEEK samples the prepared had a warpagePEEK samples rate of had 5%, highera warpage crystallinity, rate of 5%, and higher no delamination, crystallinity, which and no resulted delamination, in improved which tensile resulted and in flexural improved properties. tensile and flexural properties. 3.3.2. Nozzle Temperature 3.3.2.The Nozzle impact Temperature of nozzle temperature on characteristics of printed PEEK is a complicated one, as it can influence the melting of crystalline areas, crystallisation process, and interfacial strength between The impact of nozzle temperature on characteristics of printed PEEK is a complicated one, as it printing beads directly or indirectly. Moreover, nozzle temperature determines the performance of can influence the melting of crystalline areas, crystallisation process, and interfacial strength between the printing process for PEEK. If the printing temperature is not high enough, nozzle clogging and printing beads directly or indirectly. Moreover, nozzle temperature determines the performance of delamination of deposited layers can occur. On the other hand, high temperatures may cause thermal the printing process for PEEK. If the printing temperature is not high enough, nozzle clogging and degradation of PEEK. Dimensional inaccuracy is another issue which is due to considerable change in delamination of deposited layers can occur. On the other hand, high temperatures may cause thermal viscosity [103] which can change due to fluctuations in the nozzle temperature. degradation of PEEK. Dimensional inaccuracy is another issue which is due to considerable change The results of some studies [84,88,113,114] show that PEEK has a higher crystallinity and tensile in viscosity [103] which can change due to fluctuations in the nozzle temperature. properties when it is printed at higher nozzle temperatures. Table5 summarises the tensile strength of The results of some studies [84,88,113,114] show that PEEK has a higher crystallinity and tensile PEEK at different nozzle temperatures. A high nozzle temperature provides appropriate conditions properties when it is printed at higher nozzle temperatures. Table 5 summarises the tensile strength for chain crystallisation. Since mechanical properties are mostly governed by the crystalline phase in of PEEK at different nozzle temperatures. A high nozzle temperature provides appropriate semi-crystalline polymers, this can improve elastic modulus and tensile strength of PEEK. Analysis conditions for chain crystallisation. Since mechanical properties are mostly governed by the of fracture surface can give more information about the role of printing temperature on the tensile crystalline phase in semi-crystalline polymers, this can improve elastic modulus and tensile strength behaviour of PEEK. Spallation phenomenon occurs for PEEK parts printed at low temperatures of PEEK. Analysis of fracture surface can give more information about the role of printing (Figure 11). Presence of interlayer gaps could be the main reason for the inferior behaviour of these temperature on the tensile behaviour of PEEK. Spallation phenomenon occurs for PEEK parts printed samples in tensile testing, as illustrated in SEM micrographs (Figure 11). For the samples processed at low temperatures (Figure 11). Presence of interlayer gaps could be the main reason for the inferior at 420 C, there is no stratification on the fracture surface, and interlaminar bonding reaches its behaviour◦ of these samples in tensile testing, as illustrated in SEM micrographs (Figure 11). For the maximum [96]. It is likely that at these temperatures, the molten filament causes the previous layer to samples processed at 420°C, there is no stratification on the fracture surface, and interlaminar partially melt, thereby increasing the adhesion between layers. bonding reaches its maximum [96]. It is likely that at these temperatures, the molten filament causes the previous layer to partially melt, thereby increasing the adhesion between layers.

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TableTable 5. TensileTensile strength of PEEK at different diﬀerent nozzle temperatures.

TensileTensile Strength (MPa) (MPa)

StudyStudy 360 ◦C 360 °C 380 ◦ 380C °C 400 400◦C °C 420 ◦°CC 440 440 °C◦C 460 460°C ◦C 480 °C 480 ◦C Yang etYang al. [88 et] al. [88] 48.5 48.5 49.549.5 5454 59 55 55 57 5755 55 Wang etWang al. [84 et] al. [84] - - 59 59 67.5 67.5 70 72.5 72.5 - - - - Ding etDing al. [96 et] al. [96] 82 82 79 79 83 83 84 ------

Figure 11. Morphology of the PEEK tensile samples printed at different nozzle temperatures (left), Figure 11. Morphology of the PEEK tensile samples printed at different nozzle temperatures (left), and SEM micrographs of tensile fracture surface of printed PEEK at different printing temperatures: and SEM micrographs of tensile fracture surface of printed PEEK at different printing temperatures: (a) 370 C, (b) 380 C, (c) 390 C, and (d) 420 C[96]. (a) 370 °C,◦ (b) 380 °C,◦ (c) 390 °C,◦ and (d) 420 °C◦ [96]. Ding et al. [96] reported a different trend for mechanical properties of PEEK, as shown in Table5, Ding et al. [96] reported a different trend for mechanical properties of PEEK, as shown in Table where a decrease in tensile strength in temperatures between 360–380 C was reported. Furthermore, 5, where a decrease in tensile strength in temperatures between◦ 360–380 °C was reported. the values of tensile strength reported by Ding et al. [96] were higher than those reported by other Furthermore, the values of tensile strength reported by Ding et al. [96] were higher than those authors [84,88]. These differences appear to originate from differences in the printing parameters. reported by other authors [84,88]. These differences appear to originate from differences in the Comparison between the study conducted by Ding et al. [96] with Yang et al. [88] shows that nozzle printing parameters. Comparison between the study conducted by Ding et al. [96] with Yang et al. diameter and layer thickness are the same, but printing speed and filling direction were different. [88] shows that nozzle diameter and layer thickness are the same, but printing speed and filling It can be concluded that lower printing speed (20 mm/s as compared to 40 mm/s) leads to much higher direction were different. It can be concluded that lower printing speed (20 mm/s as compared to 40 density and tensile strength. mm/s) leads to much higher density and tensile strength. The density of PEEK also increases by applying higher nozzle temperatures. This can be attributed The density of PEEK also increases by applying higher nozzle temperatures. This can be to better fluidity of PEEK, which facilitates filling voids and pores [84]. SEM micrographs (Figure 12) attributed to better fluidity of PEEK, which facilitates filling voids and pores [84]. SEM micrographs of PEEK samples printed at 380 and 440 C clearly show the effect of processing temperature on the (Figure 12) of PEEK samples printed at◦ 380 and 440 °C clearly show the effect of processing formation of pores and internal defects. As seen, higher nozzle temperatures result in a reduction in temperature on the formation of pores and internal defects. As seen, higher nozzle temperatures the number and size of the voids. result in a reduction in the number and size of the voids.

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Figure 12. Representative SEM micrographs of the fracture surfaces of PEEK parts printed at different different printing temperatures (a) at 380 ◦°C(C (b) at 440440 ◦°C. A and B indicate voids between layers and a void between the infillinfill filamentsfilaments [[84].84].

It should be noted that there is a a limitation limitation for for increasing increasing processing processing temperature due to possible thermal degradationdegradation of of PEEK. PEEK. This This way, away, nozzle a temperaturenozzle temperature in the range in ofthe 420–440 range◦ Cof is 420–440 recommended °C is forrecommended FFF 3D printing for FFF of 3D PEEK. printing On of the PEEK. other On hand, the other fluidity hand, and fluidity melting and behaviour melting behaviour of the polymer of the inpolymer the flow in channelthe flow is channel influenced is influenced by the temperature by the temperature of the printing of the head. printing Therefore, head. Therefore, the selection the ofselection appropriate of appropriate nozzle temperature nozzle temperature is of great is of importance great importance in an ideal in an 3D ideal printing 3D printing process process for PEEK. for FigurePEEK. 13Figure illustrates 13 illustrates the viscosity the ofviscosity PEEK at of di ffPEEKerent at temperatures different temperatures and filament feedingand filament speeds. feeding A low nozzlespeeds. temperature A low nozzle (360 temperature◦C) doesn’t (360 provide °C) doesn’t enough provide heat toenough melt PEEK heat to inside melt thePEEK flow inside channel the flow and thereby,channel and a length thereby, of solid-fluid a length of (S-F) solid-fluid zone of >(S-20F) mm zone was of observed.˃ 20 mm was Applying observed. a higher Applying temperature a higher (temperature>360 ◦C) decreases (˃ 360 °C) the decreases length of the this length zone, of and this it reacheszone, and to it around reaches 10 to mm. around Furthermore, 10 mm. Furthermore, the feeding speedthe feeding of PEEK speed filament of PEEK should filament be optimised should to be prevent optimised softening to prevent of polymer softening in the coolingof polymer zone in of the flowcooling channel. zone of Based the onflow these channel. experiments, Based on the these authors experiments, [84] suggested the authors that a printing [84] suggested temperature that ofa 380–440printing ◦temperatureC and a feeding of 380–440 speed of °C 4 mmand/ sa are feeding the best speed levels of for4 mm/s FFF 3D are printing the best of levels PEEK. for FFF 3D printingTo address of PEEK. issues relating to 3D printing of highly viscous polymers such as PEEK, with a view of reaching near bulk mechanical properties, a screw extrusion-based additive manufacturing system has been developed and reported in the literature [86]. The authors achieved a Young’s modulus, and tensile strength of 3.93 GPa and 94 MPa for PEEK printed at 390 ◦C, respectively. The ratio of these values to those of bulk PEEK is 98% and 96%, respectively. Besides, the porosity of the PEEK sample is 2.6% which was much lower than the values reported in other works [103]. These results can be attributed to the role of the extrusion process of this new printing machine that can apply a shear strain on the PEEK chains, leading to high crystallinity. The degree of crystallinity of PEEK samples in this work (39–40%) was nearly doubled compared to injection moulded PEEK (21%). PEEK showed the highest impact strength at an optimal temperature of 390 ◦C[96]. Although interlaminar bonding was stronger at higher temperatures, it appeared that a specific layered structure created in the printing of PEEK at 390 ◦C could release impact energy more efficiently. Moreover, the formation of a more perfect crystalline phase at higher printing temperatures has a detrimental effect on the toughness of PEEK.

Polymers 2020, 12, 1665 16 of 29

Higher nozzle temperatures lead to a lower surface roughness for PEEK parts printed in both horizontal and vertical directions [84]. This may be due to better diﬀusion of PEEK macromolecules andPolymers surface 2020, 12 inﬁltration, x FOR PEER at REVIEW higher temperatures. 16 of 29

FigureFigure 13. 13.Viscosity Viscosity of of PEEK PEEK at variousat various heating heating temperatures temperatures and and wire wire feeding feeding speeds speeds (2, 4 (2, and 4 6and mm 6/ s) ° ° ° ° ° ° (a) atmm/s) 320 ◦C( (a)b at) at 320 380 C◦C( (b)c )at at 380 400 C◦C( (c)d at) at 400 420 C◦ C((d)e at) at 420 440 C◦C (e and) at 440 (f) atC 460 and◦C[ (f)84 at]. 460 C [84].

3.3.3.To Bed address Temperature issues relating to 3D printing of highly viscous polymers such as PEEK, with a view of reachingOne of near the printing bulk mechanical parameters properties, that controls a screw the extrusion-based quality of 3D printed additive parts manufacturing is bed or substrate system temperature.has been developed Unfortunately, and reported there in is the no literature report in [86]. the The literature authors studying achieved the a Young’s role of this modulus, parameter and intensile the performancestrength of 3.93 of PEEKGPa and parts 94 usingMPa for various PEEK physio-mechanical printed at 390 °C, respectively. tests. However, The toratio determine of these suitablevalues to bed those temperature of bulk PEEK in the is FFF98% process and 96%, of eachrespectively. polymer, Besides, Tseng et the al. porosity [86] measured of the thePEEK coe samplefficient ofis 2.6% adhesion which friction was much (µs). Therelower wasthan anthe optimal values bedreported temperature in other forworks PEEK [103]. at which Theseµ resultss showed can the be highestattributed value. to the Higher role of temperatures the extrusion weaken process the of adhesion this new process printing due machine to incomplete that can solidification apply a shear of thestrain polymer on the layer.PEEK Basedchains, on leading these criteria,to high crystallinity. Tseng et al. [ 86The] chose degree 280 of ◦crystallinityC for bed temperature of PEEK samples in the printingin this work of PEEK (39–40%) samples. was nearly doubled compared to injection moulded PEEK (21%). InPEEK terms showed of surface the quality,highest Wang impact et al.strength [109] developed at an optimal a model temperature for prediction of 390 of °C surface [96]. roughnessAlthough ofinterlaminar PEEK parts bonding prepared was via FFF-3Dstronger printing. at higher They temperatures, hypothesised it that appeared relationship that betweena specific molecular layered distructureffusion, cross-sectionalcreated in the shape,printing and of heat PEEK transfer at 390 constitutes °C could a closed-loop.release impact Initially, energy molecular more efficiently. diffusion andMoreover, neck growth the formation is fast, but of when a more the perfect temperature crystall dropsine phase down, at this higher process printing slows down,temperatures and it reaches has a itsdetrimental maximum effect at 230 on◦C the for toughness PEEK. They of also PEEK. studied the effect of some printing parameters on the surface roughnessHigher of nozzle the PEEK temperatures measured lead by laser to a scanninglower surface microscopy. roughness The for experimental PEEK parts results printed were in both in a horizontal and vertical directions [84]. This may be due to better diffusion of PEEK macromolecules and surface infiltration at higher temperatures.

3.3.3. Bed Temperature One of the printing parameters that controls the quality of 3D printed parts is bed or substrate temperature. Unfortunately, there is no report in the literature studying the role of this parameter in

Polymers 2020, 12, x FOR PEER REVIEW 17 of 29 the performance of PEEK parts using various physio-mechanical tests. However, to determine suitable bed temperature in the FFF process of each polymer, Tseng et al. [86] measured the coefficient of adhesion friction (μs). There was an optimal bed temperature for PEEK at which μs showed the highest value. Higher temperatures weaken the adhesion process due to incomplete solidification of the polymer layer. Based on these criteria, Tseng et al. [86] chose 280 °C for bed temperature in the printing of PEEK samples. In terms of surface quality, Wang et al. [109] developed a model for prediction of surface roughness of PEEK parts prepared via FFF-3D printing. They hypothesised that relationship between molecular diffusion, cross-sectional shape, and heat transfer constitutes a closed-loop. Initially, molecular diffusion and neck growth is fast, but when the temperature drops down, this process slows down, and it reaches its maximum at 230 °C for PEEK. They also studied the effect of some Polymersprinting2020 parameters, 12, 1665 on the surface roughness of the PEEK measured by laser scanning microscopy.17 of 29 The experimental results were in a good agreement with the predictions of the model. Figure 14 gooddepicts agreement the effect with of bed the temperature predictions of on the the model. surfac Figuree topography 14 depicts of PEEK the eff parts.ect of The bed temperaturesamples printed on theat higher surface temperatures topography ofshowed PEEK parts.smoother The morphology samples printed and atbetter higher interfacial temperatures bonding showed among smoother adjacent morphologyfilaments. This and originated better interfacial from the bonding fact that among molecular adjacent diffusion filaments. continues This for originated a longer time from at the higher fact thatbed moleculartemperatures diffusion leading continues to a decrease for a longer in timethe number at higher and bed temperaturesvolume of voids leading [109]. to aHowever, decrease incomparison the number between and volume bed oftemperature voids [109]. and However, nozzle comparison temperature between reveals bed that temperature the former and has nozzle a less temperaturesignificant effect reveals on thatthe surface the former roughness. has a less significant effect on the surface roughness.

FigureFigure 14. 14.Surface Surface topography topography of of the the printed printed PEEK PEEK parts parts at at bed bed temperatures temperatures of of (A ()A 220) 220°C,◦C, ( B(B)) 240 240°C,◦C, (C(C)) 260 260°C,◦C, ((DD)) 280280°C◦C( (E) 300300°C◦C and and ( (F)) experimental results andand modelmodel resultsresults ofof surfacesurface roughness roughness inin di differentfferent platform platform temperatures temperatures [ 109[109].]. 3.3.4. Printing Speed 3.3.4. Printing Speed Generally, there is a reverse relationship between printing speed and mechanical properties of printed parts. In a study by Wang et al. [84], a printing speed of 20 mm/s led to the best tensile strength and increasing this parameter caused a loss in mechanical properties of PEEK. This has been ascribed to the fact that higher speeds don’t provide sufficient time for layers and infill filaments to diffuse and crystallise. The effect of print speed on the fracture surface of the PEEK samples can be seen in Figure 15. The formation of a large number of big voids and weak bonding are the characteristics of the sample printed at higher speeds. Polymers 2020, 12, x FOR PEER REVIEW 18 of 29

Generally, there is a reverse relationship between printing speed and mechanical properties of printed parts. In a study by Wang et al. [84], a printing speed of 20 mm/s led to the best tensile strength and increasing this parameter caused a loss in mechanical properties of PEEK. This has been ascribed to the fact that higher speeds don’t provide sufficient time for layers and infill filaments to diffuse and crystallise. The effect of print speed on the fracture surface of the PEEK samples can be Polymersseen in2020 Figure, 12, 1665 15. The formation of a large number of big voids and weak bonding are18 ofthe 29 characteristics of the sample printed at higher speeds.

Figure 15. SEM micrographs of tensile-fracture surfaces of PEEK samples printed under didifferentﬀerent speeds (a) at 17 mmmm/s/s andand ((b)) atat 2626 mmmm/s/s [[84].84].

Basgul and colleagues [[91]91] printed a standardisedstandardised PEEK lumbar fusion cage and found that printing speeds lower than 25 mmmm/s/s shouldshould bebe employed.employed. The highest compressive properties were achieved whenwhen lowerlower speeds speeds were were chosen chosen for for 3D printing3D printing of PEEK. of PEEK. Porosity Porosity in printed in printed items is items created is bycreated the expansion by the expansion of voids of and voids entrapping and entrapping air bubbles air inbubbles filaments in filaments [103]. According [103]. According to the authors, to the printingauthors, speedprinting had speed a negative had a e ffnegativeect on the effect porosity on the of theporosity part. Noof the change part. in No crystallinity change in was crystallinity observed bywas increasing observed printingby increasing speed. printing As a result, speed. they As concluded a result, thatthey imperfectconcluded filament-to-filament that imperfect filament-to- bonding formedfilament at bonding high speeds formed which at high was thespeeds main which reason was for thethe deteriorationmain reason offor the the mechanical deterioration properties of the ofmechanical PEEK. properties of PEEK. In terms terms of of surface surface quality, quality, an anoptimal optimal speed speed should should be used be for used 3D forprinting 3D printing of PEEK. of Wang PEEK. et Wangal. [109] et reported al. [109] reportedthat a printing that aspeed printing of 15 speed mm/s of was 15 appropriate mm/s was appropriate for PEEK. The for surface PEEK. Thetopography surface topographydemonstrates demonstrates a large difference a large among difference samples among prin samplested at printeddifferent at speeds different (Figure speeds 16). (Figure At low16). Atprinting low printing speeds, speeds,signs of signs stacking of stacking on the surface on the surfacecan be observed. can be observed. An interaction An interaction between between the nozzle the nozzleand the and molten the molten filaments filaments (nozzle (nozzle squeezing) squeezing) occurs occurs at higher at higher speeds speeds which which adversely adversely effects eff ectsthe thesurface surface quality. quality. In another In another study study [84], [ 84the], thesame same authors authors found found that that nozzle nozzle diameter diameter can caninfluence influence the theeffect eff ectof print of print speed speed on onthe the quality quality of ofprinted printed PEEK. PEEK. When When a anozzle nozzle diameter diameter of of 0.4 0.4 mm mm was was used, used, there waswas an an optimal optimal speed speed to reachto reach the minimum the minimu roughness,m roughness, while for while nozzles for withnozzles larger with diameters, larger surfacediameters, roughness surface slightly roughness increased slightly with increased printing speed.with printing For nozzles speed. with For small nozzles diameters, with thinner small printingdiameters, layers thinner were printing used, which layerswere weremore used, sensitive which were to changes more sensitive in nozzle to speeds.changes in nozzle speeds. Geng et al. [95] addressed the effects of the extrusion speed and printing speed on the dimensions of an extruded PEEK filament in 3D printing. According to the results, high extrusion speed causes severe fluctuations in the diameter of extruded filaments. It indicates the key role of this factor in controlling dimensional accuracy and surface morphology. There is a similar observation in work by Zhao et al. [69]. They designed orthogonal experiments to optimise the mechanical strength of PEEK for medical applications. Their experiments confirmed that the deviation in diameter of PEEK filament plays an important role in 3D printing.

Polymers 2020, 12, 1665 19 of 29 Polymers 2020, 12, x FOR PEER REVIEW 19 of 29

Figure 16.16. SurfaceSurface topographytopography of of the the PEEK PEEK parts parts printed printed under under diﬀ erentdifferent printing printing speeds speeds (a) at ( 17a) mmat 17/s (mm/sb) at 20(b) mm at 20/s andmm/s (c )and at 26 (c) mm at 26/s [mm/s84]. [84].

3.3.5.Geng Heat Treatmentet al. [95] Methodsaddressed the effects of the extrusion speed and printing speed on the dimensionsVarious of heat an treatmentextruded PEEK methods filament can cause in 3D a printing. quite remarkable According di fftoerence the results, in the high crystallisation extrusion ofspeed FFF causes printed severe PEEK fluctuations parts. Findings in the diameter of a study of ex bytruded Yang filaments. et al. [88 It] indica who comparedtes the key therole role of this of furnacefactor in cooling,controlling annealing, dimensional air cooling,accuracy quenching, and surface andmorphology. tempering There methods is a similar reveals observation that a more in perfectwork by crystallisation Zhao et al. [69]. process They designed occurred orthogonal using the firstexperiments two methods. to optimise Conversely, the mechanical air cooling strength and quenchingof PEEK for couldn’t medical provide applications. an appropriate Their experiment condition fors confirmed normal crystallisation, that the devia thustion led in todiameter the lowest of crystallinity.PEEK filament The plays results an important of mechanical role in testing3D printing. indicated that annealing and tempering methods were the most efficient post-processing techniques. It seems that heat treatment methods not only control3.3.5. Heat the Treatment crystallisation Methods of PEEK, but also determine the level of residual stress in the printed items. BasgulVarious et al. [heat111] treatment printed lumbar methods spinal can cagescause viaa quite FFF re atmarkable different difference speeds (1500 in the and crystallisation 2000 mm/min) of andFFF thenprinted annealed PEEK themparts. at Findings 200 ◦C or of 300 a study◦C. They by Ya observedng et al. a 14%[88] who increase compared in compression the role strengthof furnace of thecooling, sample annealing, printed atair the cooling, slower speed.quenching, Moreover, and tempering it was found methods that although reveals annealing that a more changes perfect the structurecrystallisation of the process pores, itoccurred did not reduceusing the the first porosity two methods. formed during Conversely, printing. air cooling and quenching couldn’tRecently, provide 3D printeran appropriate manufacturers condition have triedfor no tormal find newcrystallisation, solutions forthus customising led to the degree lowest of crystallinity. 3DGenceThe results has of adopted mechanical a new testing approach indicated (rapid that cooling annealing/annealing) and tempering to control crystallinitymethods were of PEEK.the most An efficient amorphous post-processing PEEK part is techniques. 3D-printed It in seems the first that step, heat which treatment has a methods less shrinkage not only and control better distributionthe crystallisation of stress. of PEEK, Then but it is also subjected determine to annealing the level of to residual obtain astress semi-crystalline in the printed final items. part Basgul [115]. Stratasyset al. [111] have printed been lumbar granted spinal a US patent cages [via116 FFF] for at a 3D different printing speeds method (1500 that and inhibits 2000 PEEK mm/min) crystallisation. and then Inannealed this disclosure, them at the 200°C 3D part or material300°C. They includes observed a miscible a 14% blend increase of one orin more compression semi-crystalline strength polymers of the andsample one printed or more at secondary the slower materials. speed. Moreover, Polyetherimides it was (PEI) found/semi–crystalline that although PEEKannealing (60–80 changes wt.%) andthe amorphousstructure of polyaryletherketonesthe pores, it did not reduce (50–70%) the /porositysemi–crystalline formed during polyaryletherketones printing. blends are among the claimedRecently, formulations. 3D printer manufacturers It is believed thathave miscibility tried to find allows new the solutions amorphous for customising phase to impede degree the of semi-crystallinecrystallinity. 3DGence polymer has from adopted forming a new crystalline approach regions. (rapid Thecooling/annealing) additive manufacturing to control is crystallinity followed by aof post-processing PEEK. An amorphous step involving PEEK annealingpart is 3D-printed at a temperature in the first between step, glasswhich transition has a less temperature shrinkage and coldbetter crystallisation distribution of temperature stress. Then of it the is partsubjected material. to annealing Unlike layer-by-layer to obtain a semi-crystalline crystallisation during final part 3D printing,[115]. Stratasys which causeshave been curling granted effects, a post-printingUS patent [116] crystallisation for a 3D printing leads to method a uniform that shrinkage inhibits similar PEEK tocrystallisation. injection moulding In this process.disclosure, Furthermore, the 3D part annealing material includes provides a an miscible opportunity blend forof one 3D printedor more part semi- to becrystalline further crystallisedpolymers and without one or any more restriction secondary by non-shrinkable materials. Polyetherimides building plate. (PEI)/semi–crystalline PEEK (60–80 wt.%) and amorphous polyaryletherketones (50–70%)/semi–crystalline 4. Thermal and Irradiation Degradation polyaryletherketones blends are among the claimed formulations. It is believed that miscibility allowsThe the average amorphous peak heat phase release to impede rate (PHRR) the ofsemi-crystalline PEEK is 303 W polymerg 1 which from is eight forming times lowercrystalline than · − thatregions. of polyethylene The additive (PE). manufacturing Furthermore, is followed the onset by heat a post-processing release rate temperature step involving (flash annealing point), PHRR, at a temperature between glass transition temperature and cold crystallisation temperature of the part material. Unlike layer-by-layer crystallisation during 3D printing, which causes curling effects, post-

Polymers 2020, 12, x FOR PEER REVIEW 20 of 29

printing crystallisation leads to a uniform shrinkage similar to injection moulding process. Furthermore, annealing provides an opportunity for 3D printed part to be further crystallised without any restriction by non-shrinkable building plate.

4. Thermal and Irradiation Degradation Polymers 2020, 12, 1665 20 of 29 The average peak heat release rate (PHRR) of PEEK is 303 W·g−1 which is eight times lower than that of polyethylene (PE). Furthermore, the onset heat release rate temperature (flash point), PHRR, andand limitinglimiting oxygen index (LOI) of of PEEK PEEK are are 601, 601, 619, 619, and and 37.3%, 37.3%, respectively respectively [117]. [117 ].Having Having such such as assuperior superior heat heat stability stability allows allows PEEK PEEK to to be be used used in in specific specific applications applications and has great potentialpotential toto replacereplace metallicmetallic partsparts onon thethe ISS.ISS. PEEKPEEK exhibitsexhibits aa two-steptwo-step weightweight lossloss inin anan inertinert environmentenvironment (Figure(Figure 17 17).). TheThe firstfirst stagestage occursoccurs atat aa temperaturetemperature rangerange ofof 500–600500–600 ◦°CC andand itit isis attributedattributed toto hemolytichemolytic polymerpolymer chainchain scissionscission whichwhich leadsleads toto formationformation ofof phenols,phenols, carboncarbon monoxide,monoxide, andand carboncarbon dioxide.dioxide. TheThe formerformer ifif producedproduced cancan causecause chemicalchemical burnsburns whichwhich isis aa concernconcern forfor spacespace flightflightapplications. applications. InIn thethe secondsecond stepstep (above(above 600600°C),◦C), polymer polymer residue undergoes cracking andand dehydrogenation.dehydrogenation. Figure Figure 18 shows thethe FT-IRFT-IR spectraspectra ofof PEEKPEEK beforebefore thethe startstart ofof degradationdegradation andand atat maximummaximum raterate ofof weightweight loss.loss. TheThe massmass lossloss duringduring firstfirst andand secondsecond stepssteps ofof thermalthermal degradationdegradation ofof PEEKPEEK inin nitrogennitrogen environment,environment, andand charchar yieldyield at 1000 °C◦C are are around around 40–45%, 40–45%, 10%, 10%, and and 45–50%, 45–50%, respectively. respectively. Thermal Thermal decomposition decomposition of PEEK of PEEKin oxidative in oxidative atmosphere atmosphere includes includes two fast, two consecutive fast, consecutive stages. stages. Cleavage Cleavage of ether of and ether ketone and ketone bonds bondsbegins begins at temperatures at temperatures below below500 °C. 500 Ether◦C. bonds Ether bondsundergo undergo degradation degradation at lower at lowertemperatures, temperatures, while whiledecomposition decomposition of carbonyl of carbonyl bonds bonds mostly mostly occurs occurs at athigher higher temperatures. temperatures. Thermal Thermal oxidation ofof carbonaceouscarbonaceous materialmaterial takestakes placeplace inin thethe nextnext step,step, leavingleaving nono charchar at at 650–700 650–700◦ °CC[ [118–120].118–120].

Figure 17. Thermal decomposition of PEEK and its glass (GL30) and carbon (CA30) ﬁbre composites under nitrogen (top) and in air (bottom)[119].

Polymers 2020, 12, x FOR PEER REVIEW 21 of 29

PolymersFigure2020, 17.12, 1665Thermal decomposition of PEEK and its glass (GL30) and carbon (CA30) fibre composites21 of 29 under nitrogen (top) and in air (bottom) [119].

Figure 18. FourierFourier Transform Transform Infrared spectra of PEEK: ( a) before start of degradation, and (b) at maximum rate of weight loss.

A summary of the thermal degradation parameters for PEEK (obtained from TGA testing) are presented inin TableTable6 .T6. Tonsetonset isis onsetonset degradationdegradation temperature.temperature. TT10%10% andand TTmaxmax areare the temperaturestemperatures corresponding toto 10%10% weight weight loss loss and and maximum maximum degradation degradation rate, rate, respectively. respectively. According According to the to data, the oxidativedata, oxidative degradation degradation starts starts at 460–480 at 460–480◦C which °C which is 40–50 is 40–50◦C lower °C lower than than that that of degradation of degradation in anin inertan inert atmosphere. atmosphere. Furthermore, Furthermore, Tsai Tsai et al. et [ 121al. ][121] studied studied thermal thermal decomposition decomposition of PEEK of PEEK using using flash pyrolysisflash pyrolysis gas chromatography gas chromatography/mass/mass spectrometry spectrometry (pyGC/MS) and(pyGC/MS) found that and 1,4-diphenoxybenzene found that 1,4- anddiphenoxybenzene 4-phenoxyphenol and are 4-phenoxyphenol the decomposition are productsthe decomposition at 450 ◦C. products This indicates at 450 that °C. 3DThis printing indicates of PEEKthat 3D in printing the space of environment PEEK in the shouldspace environmen be performedt should at temperature be performed lower at than temperature this. Based lower on thesethan findings,this. Based it on is reasonablethese findings, to set it ais maximum reasonable temperature to set a maximum of 440 ◦temperatureC to not only of achieve 440 °C anto not effi cientonly printingachieve an process, efficient but printing also avoid process, any probable but also thermalavoid any degradation. probable thermal degradation.

Table 6. Summary of the thermal degradation parametersparameters (obtained from TGA test)test) forfor PEEK.PEEK.

HeatingHeating Rate Rate StudyStudy Tonset (°C) Tonset (◦C) T10% (°C) T10% (◦C) TMaxMax (°C)(◦C) (◦C(°C/min)/min) inert: 514 inert: 554 oxidative: [122inert:] 514 inert: 541 oxidative: 512 inert: 554 oxidative: Tmax-І: 10 [122] oxidative: 466inert: 541 oxidative: 512 T : 517 T : 575 10 oxidative: 466 max-I517 Tmax-max-IIП: 575 [118] [118 inert:] 521 inert: 521 inert: inert: 544 544 inert: inert: 558 10 10 inert: 520inert: 520 inert:inert: 558 558oxidative: oxidative: Tmax-І: [123] [123] inert: inert:544 oxidative: 544 oxidative: 520 520 10 10 oxidative:oxidative: 478 478 Tmax-I530: 530Tmax- Tmax-IIП: 584: 584 inert: 588 (MFI: 15 inert:g/10min) 592 588 (MFI: 15 g/10 min) [124] [124]-- (MFI: 27 g/10min) 599 (MFI: 85 inert:590–600inert: 590–600 10 10 592 (MFI: 27 g/10 min) g/10min) 599 (MFI: 85 g/10 min)

Vaezi et al. [103] tested different extrusion temperatures (350–450 °C) and observed colour changeVaezi or etentrapped al. [103] testedvoids diinsidefferent PEEK extrusion samples. temperatures According (350–450 to the ◦results,C) and a observed temperature colour range change of or400–430 entrapped °C was voids suggested inside PEEK by samples.the authors According for additive to the manufacturing results, a temperature of PEEK. range To of address 400–430 the◦C waspossibility suggested of degradation by the authors of PEEK for additiveduring FFF manufacturing 3D printing, ofDing PEEK. et al. To [96] address printed the PEEK possibility at 420 °C of degradationand then compared of PEEK the duringFT-IR spectra FFF 3D of printing,PEEK filame Dingnt and et al. the [96 final] printed part. The PEEK printed at 420 sample◦C and showed then comparedthe PEEK’s thecharacteristic FT-IR spectra absorption of PEEK peaks filament (C = O and at 1651 the final cm−1, part. -O- at The 1186 printed cm−1, and sample C-O at showed 1224 cm the−1) 1 1 1 PEEK’s characteristic absorption peaks (C = O at 1651 cm− , -O- at 1186 cm− , and C-O at 1224 cm− )

Polymers 2020, 12, 1665 22 of 29 with same intensity, so they concluded that there was no thermal decomposition in printing of PEEK at this temperature. The same results were reported by Zhao [69] where the authors comparing FT-IR spectra of the PEEK granule, filament, and samples printed at 375 ◦C for medical applications. Aiming to use PEEK in an out-off-earth environment and on ISS, irradiation effect on this high-performance polymer must be considered. It has been shown by Mylläri et al. [125] that UV irradiation for 1056 h had a little effect on strength and elastic modulus of PEEK fibres, while it significantly deteriorated elongation at break due to random scission of the chains. There are also other works [126,127] that reported even an increase in yield strength of PEEK sheets caused by crosslinking. Although an increase in Tg and zero shear viscosity of PEEK could be seen by increasing irradiation time, but crystallinity and melting temperature are not influenced by this parameter [125]. Nakamura et al. [128] examined the effect of low Earth orbit (LEO) environment on the PEEK films and concluded that UV constituent in LEO is the main reason for surface browning and crosslinking phenomenon. They also observed that atomic oxygen (AO) in ISS orbit may cause surface erosion and a decrease in thickness. An interesting finding was that the degree of degradation was much smaller than the predicted values. The authors attributed it to the attitude change of the ISS during flight and formation of SiOx layer on surface of the samples. This layer has a low light transmission and can prevent detrimental effect of AO. These results show that stabilisation is a critical part of development of PEEK-based devices. This way, it seems incorporating appropriate anti-aging additives into PEEK bulk is as one of the best solutions.

5. Conclusions This review has examined FFF 3D-printing of PEEK with an aim to serve as a foundation for the development of 3D printed PEEK parts in the international Space Station. Here, the focus was on understanding process-structure-property relationships in printing of this high-performance plastic. The effects of various printing parameters including building orientation, nozzle temperature, ambient temperature, bed temperature, layer thickness, print speed, and heat treatment on the physical/mechanical properties and print quality were clarified. Based on the findings of published works, the recommended levels of the parameters in order to approximate the performance of conventionally moulded samples are presented in Table7. However, it is important to note that these are based on tensile mechanical performance mostly.

Table 7. Recommended values of the parameters for FFF 3D-printing of PEEK.

Parameter Value/Type

Raster angle 0◦ [93] Layer thickness 0.1–0.3 mm [92] Ambient temperature 150–200 ◦C[88] Nozzle temperature 420–440 ◦C[84] Print speed 20 mm/s [91] Inﬁll 100% Heat treatment method Annealing [88]

The review of the reported literature revealed some promising results for mechanical properties of PEEK samples printed via FFF process. However, there are still many challenges regarding 3D printing of PEEK, which needs to be taken into account. From technical point of view, optimising the temperature diﬀerence between nozzle and bed/chamber in 3D-printers is the most critical aspect. In fact, thermal gradients across beads should be minimised to fabricate parts with desirable mechanical performance and dimensional stability. Therefore, development of a customised printer considering this point is necessary for 3D printing of PEEK. On the other hand, it appears that there is a strong need for more experimental and modelling research on the microstructure, fracture phenomenon, Polymers 2020, 12, 1665 23 of 29 rheological behaviour, and crystallisation of PEEK. Moreover, the interactive eﬀects of the printing parameters should also be studied by the wider research community.

Author Contributions: Writing—original draft preparation, A.R.Z.; review and editing, D.M.D., U.L., I.M., J.G.L.; funding acquisition & project administration, J.G.L., D.M.D., I.M.; Providing some experimental data, U.L.; All authors have read and agreed to the published version of the manuscript. Funding: This work was supported under the frame of IMPERIAL project/ESA contract (4000126158/18/NL/FE). Acknowledgments: The authors would like to thank Anna Daurskikh, Antonella Sgambati, and Advenit Makaya for their expert advice in the development of this review. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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