3D Printing of Fibre-Reinforced Thermoplastic Composites Using Fused Filament Fabrication—A Review

polymers

Review 3D Printing of Fibre-Reinforced Thermoplastic Composites Using Fused Filament Fabrication—A Review

Andrew N. Dickson *, Hisham M. Abourayana and Denis P. Dowling

School of Mechanical and Materials Engineering, University College Dublin, Belﬁeld, D04 V1W8 Dublin, Ireland; [email protected] (H.M.A.); [email protected] (D.P.D.) * Correspondence: [email protected]

Received: 26 August 2020; Accepted: 20 September 2020; Published: 24 September 2020

Abstract: Three-dimensional (3D) printing has been successfully applied for the fabrication of polymer components ranging from prototypes to final products. An issue, however, is that the resulting 3D printed parts exhibit inferior mechanical performance to parts fabricated using conventional polymer processing technologies, such as compression moulding. The addition of fibres and other materials into the polymer matrix to form a composite can yield a significant enhancement in the structural strength of printed polymer parts. This review focuses on the enhanced mechanical performance obtained through the printing of fibre-reinforced polymer composites, using the fused filament fabrication (FFF) 3D printing technique. The uses of both short and continuous fibre-reinforced polymer composites are reviewed. Finally, examples of some applications of FFF printed polymer composites using robotic processes are highlighted.

Keywords: fused ﬁlament fabrication; polymers; ﬁbre reinforcement; mechanical properties

1. Introduction Three-dimensional (3D) printing, also known as additive manufacturing (AM), can be used to print a range of metallic, polymer and composite parts with complex geometries and great design flexibility, while minimising processing waste [1,2]. Applications of this processing technology have included parts fabricated for use in the biomedical, automotive and aerospace sectors [3]. Three-dimensional printing was first introduced during the early 1980s, using the process of stereolithography, in which UV lasers are used to cure layers of polymer into 3D shapes [4]. These methods can be used to process materials such as epoxies. For example, a hydrogel combined with a UV curable adhesive to form a composite exhibiting properties similar to organic tissues, such as cartilage, was demonstrated [5]. A range of other polymer 3D printing technologies are also available, including Selective Laser Sintering (SLS), Laminated Object Modelling (LOM), Multi Jet Fusion Printing and Fused Filament Fabrication (FFF) processes [6–8]. The latter technique, which is also known by the trade name Fused Deposition Modelling, is one of the most widely used amongst all the 3D printing techniques, showing great potential for fabricating 3D geometry parts with the capacity to compete with conventional processing techniques [9–11]. In this technique, the polymer filament is extruded through the nozzle that traces the part’s cross sectional geometry layer by layer, as shown in Figure1[ 12]. The nozzle contains resistive heaters that keep the polymer at a temperature just above its melting point, so that it flows easily through the nozzle and forms the layer [13]. The extruding apparatus is typically mounted onto an X–Y computer numerical control (CNC) gantry, allowing the printing of complex geometric patterns. Once a pattern is deposited, the build platform is lowered, or the extruding orifice is raised-up, to deposit the next layer [14,15].

Polymers 2020, 12, 2188; doi:10.3390/polym12102188 www.mdpi.com/journal/polymers Polymers 2020, 12, 2188 2 of 18 Polymers 2020, 12, x FOR PEER REVIEW 2 of 18

FigureFigure 1. SchematicSchematic of of FFF FFF process process for for the the printing printing of of parts using the melted polymer filament. ﬁlament.

At present, thermoplasticthermoplastic polymers polymers are are the the most most frequently frequently utilised utilised feed-stock feed-stock materials materials for thefor FFFthe FFFprocess, process, due todue their to relativelytheir relative lowly cost low as cost well as theirwell lowas their melting low temperaturesmelting temperatures [16]. These [16]. polymers These polymersinclude acrylonitrile include acrylonitrile butadiene butadiene styrene (ABS), styrene polylactic (ABS), polylactic acid (PLA), acid polycarbonate (PLA), polycarbonate (PC), polyether (PC), polyetherether ketone ether (PEEK) ketone and (PEEK) nylon [and17]. nylon The resulting [17]. The pure resulting polymer pure products, polymer however, products,can however, often lack can the oftenstrength lack to the produce strength fully to produce functioning fully engineering functioning parts,engineering which parts, has restricted which has the restricted wider adoption the wider of adoptionthis technology of this [ 18technology]. In order [18]. to address In order this to issue,address reinforcing this issue, materials, reinforcing such materials, as fibres, such are added as fibres, into arethe added polymer into matrix the polymer during printing, matrix during in order printing, to produce in order a composite to produce structure a composite which typicallystructure exhibits which typicallyimproved exhibits mechanical improved properties mechanical [19]. properties [19]. Reinforcing fibresfibres in in composite composite materials materials can becan in thebe formin the of continuous form of fibrescontinuous or discontinuous fibres or discontinuous(short) fibres [(short)20]. Continuous fibres [20]. fibresContinuous have longfibres aspect have ratioslong aspect and have ratios a and preferred have a orientation, preferred orientation,while short fibreswhile haveshort shortfibres aspect have short ratios aspect and generally ratios and have generally a random have orientation a random orientation [21]. Due to [21]. the Duefibre to orientation, the fibre orientation, continuous continuous fibre composites fibre ocompositesffer higher offer strength higher and strength stiffness and qualities stiffness than qualities those of thandiscontinuous those of discontinuous fibre composite fibre [22 ].composite This paper [22]. will This initially paper introduce will initially and discussintroduce short and fibre-reinforced discuss short fibre-reinforcedpolymer composites, polymer before composites, reviewing before the rapidly reviewing expanding the rapidly field ofexpanding 3D printed field continuous of 3D printed fibre continuouscomposites. fibre Commercial composites. developments Commercial of developments FFF, including of theFFF, use including of robotic the printing use of robotic techniques printing for techniqueslarger scale for printing, larger arescale also printing, reviewed. are also reviewed.

2. 3D 3D Printing Printing of of Short Short Fibre-Reinforced Fibre-Reinforced Composite Composite Composites fabricated fabricated reinforced reinforced using using short short fibre fibress are attractive because of their ease of fabrication, economy economy and and superior mechanical properties [[23].23]. They They are are typically typically produced produced by by extrusion compounding, compounding, injection, injection, or or compression compression moulding moulding processes processes [22]. [22 For]. ForFFF FFFprocessing, processing, the filamentsthe filaments are arefabricated fabricated in a in two-step a two-step process; process; this this firstly firstly involves involves mixing mixing the the polymer polymer pellets pellets and fibrefibre in a blender and secondly extruding the compound to create the filamentfilament [[24].24]. Typical Typical short fibresfibres used as reinforcements include carbon and gl glassass fibres. fibres. Recently, Recently, basalt fibre fibre has also received attention [25]. [25]. As detailed detailed in in Table Table 11,, severalseveral authors have investigated investigated the the addition addition of of short short fibres fibres into into a thermoplastica thermoplastic polymerpolymer to toprovide provide composite composite fila filamentsments used used as a as feedstock a feedstock for FDM for FDM process. process. The reportedThe reported studiesstudies included included invest investigationsigations of the of the effect effect of offibre fibre content, content, as as well well as as its its orientation orientation and and length,length, onon the the processability processability and and performance performance of the of resultant the resultant fibre-reinforced fibre-reinforced composites. composites.Some studies Some studiesinvolved involved a comparison a comparison between between the properties the properties of 3D of printed 3D printed composites composites and and those those fabricated fabricated by bytraditional traditional compression compression moulding moulding techniques. techniques.

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Table 1. A summary of materials used for 3D printing of short ﬁbre-reinforced polymer composites.

Matrix Reinforcement wt % Studying Ref. Experimental characterisation and PLA Carbon 15 micrography of 3D printed PLA and PLA [26] reinforced with short carbon fibres. Effects of process parameters on the tensile ABS Carbon 5 properties of FDM fabricated carbon fibre [27] composite parts. Investigate the effects of reinforcements on Carbon 5 ABS porosities and tensile properties of [28] Graphite 5 FFF-fabricated parts built at two raster angles. Explore the potential engineering application Carbon 5–15 PEEK of FDM-3D printing short fibre-reinforced [29] Glass 5–15 PEEK composites Analytical study of the 3D printed structure and mechanical properties of basalt PLA Basalt [30] fibre-reinforced PLA composites using X-ray microscopy. Development and mechanical properties of ABS Basalt 0–60 fibre-reinforced ABS for in-space [31] manufacturing applications. Study of short fibre-reinforced ABS polymers ABS Glass 10–18 [32] for use as FFF feedstock material. Investigate the effects of fibre reinforcements PP Glass 30 on the physical and mechanical properties of [33] FFF-fabricated parts.

Fibre content plays an important role in determining the properties of FFF composite filaments. Generally, tensile strength increases with increasing fibre content. Composite filaments, however, with high fibre content, can be very difficult to print, due to issues with nozzle clogging, in addition to the excessive viscosity of the melted composite filament [34,35]. Therefore, the determination of an appropriate fibre content in the composite used as a filament for FFF is often a compromise between processing difficulty and the performance characteristics of the resulting composites [32]. Ning et al. [36] investigated the effect of carbon fibre content and length on the mechanical properties and porosity of FFF printed ABS/carbon fibre composite parts. The composite filaments were fabricated with different fibre contents (3 to 15 wt %) and different fibre lengths (100 and 150 µm). This study demonstrated that the best performing FFF printed parts were obtained for samples reinforced with 5 wt % carbon fibre, which achieved 22.5% and 30.5% increases in tensile strength and Young’s modulus, respectively, compared with the ABS only specimens. A further increase in the fibre content to 10% or higher resulted in a decrease in tensile strength due to the higher porosity. Moreover, the composite specimens reinforced with longer carbon fibres (150 µm) exhibited higher tensile strength and Young’s modulus values, and lower toughness as well as ductility, compared with those reinforced with shorter carbon fibres (100 µm). A study by Tekinalp et al. [34] investigated short fibre (10–40 wt %)-reinforced ABS composites as a feedstock for FFF printing, in order to report on their processability, microstructure and mechanical performance. The additive components were also compared with traditional compression moulded (CM) composites. The results showed that FFF 3D printing yielded samples with very high fibre orientation, lower average fibre length and high porosity levels (16–27%) compared with those obtained using the CM process. Tensile test results demonstrated that tensile strength and modulus were increased, with increasing fibre content for both the FFF and compression moulded samples (Figure2). However, the improvement in the mechanical properties of FFF 3D printed samples is close to that Polymers 2020, 12, 2188 4 of 18 of those fabricated with CM process, attributed to the high degree of fibre alignment in FFF 3D printed samples compared to the random orientation of the fibres in moulded samples. The authors compensated for some of the loss of strength due to high porosity and decreased fibre length. For load bearing applications, the composite filaments used in the FFF process must exhibit adequate mechanical properties, such as strength, stiffness, ductility and flexibility [19,32]. The addition of short fibres into pure thermoplastics polymers, however, while improving the resulting printed parts’ tensile and flexural strengths, can be at the cost of the reduced flexibility and handleability of the resulted composite filaments. Authors have addressed this issue through the addition of a small amount of plasticiser and compatibility agents. For example, the processability values of short glass fibre-reinforced ABS composites with three different glass fibre contents (10.2, 13.2 and 18 wt %) were investigated in relation to their use as a feedstock filament for FFF [32]. The ABS was mixed with glass fibre in a twin-screw extruder, and then granulated into small pellets. These pellets were then fed into a single screw extruder and extruded into a filament. The glass fibres were found to reduce the flexibility of the resulting filament and make it impossible to feed into the FFF printer, therefore plasticiser (linear low-density polyethylene) and compatibiliser (hydrogenated Buna-N) were added to improve the ability to process the filaments through the printer nozzle and the properties of the FFF parts. The properties of the resulting composite filaments showed they would work well as a feedstock for FFF processes. This study also demonstrated that adding short glass fibre to ABS polymer resulted in a reduction in adhesive strength between the layers in the resulting FFF 3D printed samples, while the tensile strength was increased with increasing fibre contents. The authors reported that this may be due to enhanced fibre bridging across layers during printing, as fibre content increased. A study by Sodeifian et al. [33] reported on how the flexibility of glass fibre-reinforced polypropylene composite filaments was enhanced by adding maleic anhydride polyolefin (POE-g-MA) as a modifier. POE-g-MA was added with three different weight percentages, namely 10, 20 and 30 wt %. The filaments were used to produce test specimens using FFF printing. The test specimens were also provided using the CM method, to compare the results with those of the FFF method. This study demonstrated that the tensile strengths of the specimens with 10 wt % POE-g-MA were the same irrespective of the manufacturing method used, but the FFF 3D printed specimens exhibited higher flexibility. The FFF 3D printed specimen with 20 wt % POE-g-MA showed superior mechanical properties, compared with those prepared by using the CM method. Increasing POE-g-MA to 30 wt % yielded an increase in the strength and a decrease in flexibility. X-ray diffraction analysis indicated the higher crystallinity of the specimens prepared by compression moulding, compared with that obtained using 3D printing. Figure3 helps to demonstrate the interlayer and intralayer adhesion of the 3D printedPolymers 2020 specimens,, 12, x FORas PEER compared REVIEW to that prepared using the CM method. 4 of 18

Figure 2. EffectEffect of fibre fibre content and preparation process on (a) tensile strength and (b) modulus of ABSABS/carbon/carbon fibrefibre compositescomposites [[34]34]..

For load bearing applications, the composite filaments used in the FFF process must exhibit adequate mechanical properties, such as strength, stiffness, ductility and flexibility [19,32]. The addition of short fibres into pure thermoplastics polymers, however, while improving the resulting printed parts’ tensile and flexural strengths, can be at the cost of the reduced flexibility and handleability of the resulted composite filaments. Authors have addressed this issue through the addition of a small amount of plasticiser and compatibility agents. For example, the processability values of short glass fibre-reinforced ABS composites with three different glass fibre contents (10.2, 13.2 and 18 wt %) were investigated in relation to their use as a feedstock filament for FFF [32]. The ABS was mixed with glass fibre in a twin-screw extruder, and then granulated into small pellets. These pellets were then fed into a single screw extruder and extruded into a filament. The glass fibres were found to reduce the flexibility of the resulting filament and make it impossible to feed into the FFF printer, therefore plasticiser (linear low-density polyethylene) and compatibiliser (hydrogenated Buna-N) were added to improve the ability to process the filaments through the printer nozzle and the properties of the FFF parts. The properties of the resulting composite filaments showed they would work well as a feedstock for FFF processes. This study also demonstrated that adding short glass fibre to ABS polymer resulted in a reduction in adhesive strength between the layers in the resulting FFF 3D printed samples, while the tensile strength was increased with increasing fibre contents. The authors reported that this may be due to enhanced fibre bridging across layers during printing, as fibre content increased. A study by Sodeifian et al. [33] reported on how the flexibility of glass fibre-reinforced polypropylene composite filaments was enhanced by adding maleic anhydride polyolefin (POE-g- MA) as a modifier. POE-g-MA was added with three different weight percentages, namely 10, 20 and 30 wt %. The filaments were used to produce test specimens using FFF printing. The test specimens were also provided using the CM method, to compare the results with those of the FFF method. This study demonstrated that the tensile strengths of the specimens with 10 wt % POE-g-MA were the same irrespective of the manufacturing method used, but the FFF 3D printed specimens exhibited higher flexibility. The FFF 3D printed specimen with 20 wt % POE-g-MA showed superior mechanical properties, compared with those prepared by using the CM method. Increasing POE-g- MA to 30 wt % yielded an increase in the strength and a decrease in flexibility. X-ray diffraction analysis indicated the higher crystallinity of the specimens prepared by compression moulding, compared with that obtained using 3D printing. Figure 3 helps to demonstrate the interlayer and intralayer adhesion of the 3D printed specimens, as compared to that prepared using the CM method.

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Figure 3. Scanning electron microscopy (SEM) images of the specimens manufactured using (a) 3D Figure 3. Scanning electron microscopy (SEM) images of the specimens manufactured using (a) 3D printing and (b) CM methods [33]. These images help to illustrate the differences in interlayer adhesion printing and (b) CM methods [33]. These images help to illustrate the differences in interlayer of 3D printed samples compared to that prepared using CM process. adhesion of 3D printed samples compared to that prepared using CM process. A study by Sang et al. [35] investigated PLA composites reinforced with silane treated basalt A study by Sang et al. [35] investigated PLA composites reinforced with silane treated basalt fibre at three different fibre weight fractions (5, 10 and 20 wt %). The effects of fibre type, as well as fibre at three different fibre weight fractions (5, 10 and 20 wt %). The effects of fibre type, as well as its weight fraction, on the thermal properties, mechanical performance and rheological behaviour of its weight fraction, on the thermal properties, mechanical performance and rheological behaviour of PLA/BF composite filaments were investigated. The study included a comparison with composites PLA/BF composite filaments were investigated. The study included a comparison with composites fabricated using both compression-moulded and carbon fibre-reinforced counterparts with the same fabricated using both compression-moulded and carbon fibre-reinforced counterparts with the same fibre weight fraction. The results of tensile tests indicated that 3D printed specimens exhibit similar fibre weight fraction. The results of tensile tests indicated that 3D printed specimens exhibit similar tensile strength values to the compression moulded specimens, and this was attributed to the high tensile strength values to the compression moulded specimens, and this was attributed to the high alignment of fibre orientation in FFF 3D printed samples versus the random orientation of fibres in alignment of fibre orientation in FFF 3D printed samples versus the random orientation of fibres in the moulded samples. A 33% increase in tensile strength was obtained for FFF samples reinforced the moulded samples. A 33% increase in tensile strength was obtained for FFF samples reinforced with 20 wt % basalt fibre. Rheological results, as anticipated, demonstrated that the viscosity of the with 20 wt % basalt fibre. Rheological results, as anticipated, demonstrated that the viscosity of the carbon fibre composites is higher than that of the PLA matrix, while that of the basalt fibre composite carbon fibre composites is higher than that of the PLA matrix, while that of the basalt fibre composite counterparts is close to the PLA only material. The high viscosity of the carbon fibre composite results counterparts is close to the PLA only material. The high viscosity of the carbon fibre composite results in poor printing flowability and interlayer bonding defects, which causes stress concentration and in poor printing flowability and interlayer bonding defects, which causes stress concentration and failures in test samples. This study indicates the ease of processability of basalt compared to carbon failures in test samples. This study indicates the ease of processability of basalt compared to carbon fibre for the fabrication of FFF composites. fibre for the fabrication of FFF composites. Applications of FFF Printed Short Fibre-Reinforced Composites Applications of FFF Printed Short Fibre-Reinforced Composites Some examples of the applications of fibre-reinforced polymer composites include high-temperatureSome examples inlet of guide the vanesapplications (IGV) forof aerospacefibre-reinforced applications, polymer printed composites using polyetherimides- include high- Ultemtemperature 1000 mixedinlet guide with 10%vanes chopped (IGV) for carbon aerospace fibre [19applications,,37]. Sang etprinted al. [38 using] developed polyetherimides- promising PLA-PCLUltem 1000/basalt mixed fibre with composite 10% chopped filaments, carbon to befibre used [19,37]. as FFF Sang feedstock et al. [38] for developed manufacturing promising honeycomb PLA- structuresPCL/basalt that fibre exhibit composite elastic deformationsfilaments, to withbe us superiored as FFF energy feedstock absorption for manufacturing under compressive honeycomb loading, andstructures which that provide exhibit valuable elastic means deformations to obtain with an excellent superior compressive energy absorption mechanical under performance compressive in honeycombloading, and structures which provide (Figure4 ).valuable means to obtain an excellent compressive mechanical performance in honeycomb structures (Figure 4).

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FigureFigure 4.4.3D 3D printed printed circular circular honeycombs honeycombs of PLA-PCL of PLA-PCL/KBF/KBF with varyingwith varying ratios [ 38ratios]. The [38]. PLA-PCL The PLA-/KBF compositePCL/KBF composite consists of consists polylactic of acidpolylactic (PLA) acid as the (PLA) stiff asmatrix, the stiff polycaprolactone matrix, polycaprolactone (PCL) as an (PCL) elastomer as an phaseelastomer and silane-treatedphase and silane-treated basalt fibres basalt (KBF) fibres as the (KBF) reinforcing as the reinforcing filler. filler. 3. 3D Printing of Continuous Fibre-Reinforced Composite 3. 3D Printing of Continuous Fibre-Reinforced Composite As discussed in Section2, the incorporation of short fibre reinforcement can usually increase the As discussed in Section 2, the incorporation of short fibre reinforcement can usually increase the stiffness of the resulting composite; however, the part strength is often only marginally increased. stiffness of the resulting composite; however, the part strength is often only marginally increased. This is due to reliance on the matrix material to transfer loads between fibres. In contrast, a continuous This is due to reliance on the matrix material to transfer loads between fibres. In contrast, a fibre reinforcement transfers and retains primary loads within unbroken strands of fibre, and this continuous fibre reinforcement transfers and retains primary loads within unbroken strands of fibre, results in a significantly lower load transfer through the polymer and allows for a load-bearing capacity and this results in a significantly lower load transfer through the polymer and allows for a load- orders of magnitude higher than that which short reinforcement is capable of achieving. In the case bearing capacity orders of magnitude higher than that which short reinforcement is capable of of continuous fibre composites, the polymer serves to transfer off-axis loads between fibres, such as achieving. In the case of continuous fibre composites, the polymer serves to transfer off-axis loads shear forces. This protects the fibres, as high modulus reinforcement such as carbon and glass fibre between fibres, such as shear forces. This protects the fibres, as high modulus reinforcement such as exhibit poor mechanical properties under shear loading [39]. Continuous fibre-reinforced polymers carbon and glass fibre exhibit poor mechanical properties under shear loading [39]. Continuous fibre- are currently one of the largest areas of focus in 3D printing research [40]; this is due to their potential reinforced polymers are currently one of the largest areas of focus in 3D printing research [40]; this is to match or exceed the mechanical performance of conventional composites. A number of authors due to their potential to match or exceed the mechanical performance of conventional composites. A have reported on modifications of the standard FFF process for the printing of continuous fibres; these number of authors have reported on modifications of the standard FFF process for the printing of include hardware changes, such as more wear-resistant nozzle materials, fibre cutting mechanisms, continuous fibres; these include hardware changes, such as more wear-resistant nozzle materials, dual inlet hot-ends and fibre preheaters [41,42]. fibre cutting mechanisms, dual inlet hot-ends and fibre preheaters [41,42]. Two main categories of continuous fibre printing have been described in the literature, these being Two main categories of continuous fibre printing have been described in the literature, these “in-situ fusion”, and “ex-situ prepreg” [41,42]. The in-situ systems utilise two input materials, typically being “in-situ fusion”, and “ex-situ prepreg” [41,42]. The in-situ systems utilise two input materials, a dry fibre feedstock (the reinforcing fibre) and a neat polymer (the matrix polymer), which are typically a dry fibre feedstock (the reinforcing fibre) and a neat polymer (the matrix polymer), which combined together during the printing process. One of the most widely used techniques is known are combined together during the printing process. One of the most widely used techniques is known as “in-nozzle impregnation” [43]. In this process, the dry fibre is typically pre-threaded through the as “in-nozzle impregnation” [43]. In this process, the dry fibre is typically pre-threaded through the printer nozzle prior to printing, and the fibre is also preheated using a coil heater or IR lamps, so as not printer nozzle prior to printing, and the fibre is also preheated using a coil heater or IR lamps, so as to excessively cool the molten polymer during printing. The polymer is fed by a motor-driven hobbed not to excessively cool the molten polymer during printing. The polymer is fed by a motor-driven gear into the melt zone of the hot-end, and the preheated fibre and melted polymer converge in this hobbed gear into the melt zone of the hot-end, and the preheated fibre and melted polymer converge melt zone where they are pushed together by the feeding polymer filament. The polymer continues to in this melt zone where they are pushed together by the feeding polymer filament. The polymer flow as long as the motor drives the filament, and the continuous fibre bundle is pulled through the continues to flow as long as the motor drives the filament, and the continuous fibre bundle is pulled nozzle by traction force as it is anchored to the build plate, as shown in Figure5a. This method has through the nozzle by traction force as it is anchored to the build plate, as shown in Figure 5a. This the advantage of a single manufacturing step, which uses low-cost commercially available feedstocks, method has the advantage of a single manufacturing step, which uses low-cost commercially such as carbon fibre tow and FFF filaments. It also allows for real-time control over the local volume available feedstocks, such as carbon fibre tow and FFF filaments. It also allows for real-time control fraction of the part by altering the flowrate of polymer. This single-step printing approach, while rapid, over the local volume fraction of the part by altering the flowrate of polymer. This single-step printing has been reported to yield relatively poor-quality composite parts [44,45]. The short dwell time within approach, while rapid, has been reported to yield relatively poor-quality composite parts [44,45]. The the heated nozzle results in poor polymer infusion into the fibre bundles, and ultimately increases the short dwell time within the heated nozzle results in poor polymer infusion into the fibre bundles, porosity of the composite [43–45]. Tian et al. [45] reported that increasing the printing temperature and ultimately increases the porosity of the composite [43–45]. Tian et al. [45] reported that increasing led to a marked increase in polymer impregnation and decreased porosity, however a quantitative the printing temperature led to a marked increase in polymer impregnation and decreased porosity, however a quantitative analysis was not performed. Despite these issues, these studies have

Polymers 2020, 12, 2188 7 of 18 Polymers 2020, 12, x FOR PEER REVIEW 7 of 18 analysisdemonstrated was not that performed. significant Despite strength these increases issues, can these be achieved studies have by the demonstrated addition of thatthe continuous significant strengthfibre to polymer increases materials. can beachieved Matsuzaki by et the al. addition[46] reported of the on continuousa 3.4-fold improvement fibre to polymer in the materials. strength Matsuzakiof carbon fibre-reinforced et al. [46] reported PLA, on versus a 3.4-fold unreinforc improvemented polymer, in the when strength only of 6.6 carbon volume fibre-reinforced fibre percent PLA,(VF%)versus was used. unreinforced Similarly,polymer, Bettini et when al. [47] only observed 6.6 volume up to fibre a six-fo percentld improvement (VF%) was in used. strength Similarly, with Bettinithe incorporation et al. [47] observed of 8.6 VF% up toofa aramid six-fold fibre improvement in PLA. This in strength aramid with composite the incorporation was shown of to 8.6 exhibit VF% of a aramidlower porosity fibre in PLA.compared This aramid with that composite obtained was for shown carbon to exhibitfibre composites. a lower porosity Another compared “in-situ with fusion” that obtainedmethod involves for carbon first fibre printing composites. polymer Another parts “in-situusing standard fusion” methodFFF 3D involvesprinting firsttechniques; printing these polymer are partsthen sandwiched using standard together FFF 3D with printing a composite techniques; reinforcement these are then and sandwiched bonded with together the application with a composite of heat reinforcementand/or pressure and [48]. bonded The fibres with can the applicationbe added duri of heatng the and printing/or pressure process [48 and]. The overprinted fibres can beor added duringafter printing the printing between process layers and of overprintedthe printed orpolymer, added afterand printingthen placed between into layersan oven of to the facilitate printed polymer,bonding (Figure and then 5b). placed Mori intoet al. an [48] oven developed to facilitate a method bonding for (Figureoverprinting5b). Mori and etcompared al. [ 48] developedit with the ain-nozzle method forimpregnation overprinting method and compared discussed it earlier. with the Whilst in-nozzle overprinted impregnation carbon method fibre/PLA discussed samples earlier. led Whilstto a 180% overprinted increase in carbon load before fibre/PLA failure samples versus led the to unreinforced a 180% increase PLA, in the load in-nozzle before failure impregnated versus thecomposite unreinforced withstood PLA, a force the in-nozzle of 500%, impregnatedwhich was obtained composite using withstood PLA only. a This force superior of 500%, performance which was obtainedwas attributed using to PLA the only.better This contact superior between performance the matrix was and attributed fibre, and to as the a result better significantly contact between reduced the matrixporosity. and A fibre, case andstudy as aby result Brooks significantly et al. [49] reduced used porosity.a topologically A case studyoptimised by Brooks FFF etpolymer al. [49] usedbase astructure, topologically onto optimisedwhich large FFF fibre polymer tows were base adhesi structure,vely onto bonded, which and large the fibreresults tows were were a lighter adhesively part bonded,that exhibited and the a results4000% werehigher a lighter strength, part compared that exhibited with a 4000%the unreinforced higher strength, equivalent. compared A potential with the unreinforcedweakness of these equivalent. processes A potential is the short weakness amount of theseof time processes the fibre is spends the short in amountthe molten of time polymer, the fibre as spendswell as inthe the low molten pressure polymer, applied as well to the as thepolymer, low pressure which appliedtypically to leads the polymer, to poor whichinfiltration typically and leadshigh toporosity. poor infiltration The approach and highalso porosity.necessitates The that approach multiple also manufacturing necessitates thatsteps multiple occur simultaneously, manufacturing stepswhich occur makes simultaneously, optimisation difficult. which makes optimisation difficult.

Figure 5. Schematics of in-situ fusion techniques: (a) in-nozzle impregnation with polymer and coaxial Figure 5. Schematics of in-situ fusion techniques: (a) in-nozzle impregnation with polymer and fibre extrusion, and (b) embedding of continuous carbon fibre (CCF) after 3D printing in a thermal coaxial fibre extrusion, and (b) embedding of continuous carbon fibre (CCF) after 3D printing in a bonding process. Images from [42]. thermal bonding process. Images from [42]. In contrast with “in-situ fusion”, the “ex-situ prepreg” systems separate the manufacturing of the filamentIn contrast and with the printing “in-situ of fusion”, the composite the “ex-situ into twoprepreg” separate systems steps. separate This allows the formanufacturing greater control of overthe filament the individual and the processes. printing of As the with composite in-situ systems, into two the separate method steps. utilises This two allows input for materials greater control (a fibre towover andthe polymer);individual however,processes. these As with are combined in-situ systems, together the prior method to printing utilisesinto twoa input pre-impregnated materials (a filamentfibre tow (prepreg), and polymer); via a separate however, extrusion these processare comb (Figureined 6together). The prepreg prior filamentto printing is then into spooled a pre- andimpregnated transferred filament to the printing(prepreg), system via a forseparate deposition. extrusion Where process in-nozzle (Figure impregnation 6). The prepreg requires filamenta drive is motorthen spooled for the constantand transferred extrusion to ofthe the printing polymer, syst thisem methodfor deposition. only requires Where a motorin-nozzle to feedimpregnation the initial fewrequires millimetres a drive ofmotor filament for the through constant the nozzle.extrusion After of the the polymer, filament isthis anchored method to only the requires plate, this a motor disengagesto feed the initial and is few pulled millimetres through theof filament nozzle bythrough the continuous the nozzle. fibre After reinforcement, the filament whichis anchored remains to solidthe plate, throughout this motor the process. disengages This simplifies and is thepulled printing through process the significantly, nozzle by andthe ascontinuous well as allowing fibre forreinforcement, superior fibre which impregnation remains solid during throughout the filament the process. manufacture This simplifies stage, the the dedicated printing extrusion process apparatussignificantly, can and exert as more well pressure as allowing on the polymerfor superior to fully fibre infuse impregnation the fibre tow, andduring allows the forfilament higher manufacturingmanufacture stage, speeds. the dedicated extrusion apparatus can exert more pressure on the polymer to fully infuse the fibre tow, and allows for higher manufacturing speeds.

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FigureFigure 6. 6.Schematic Schematic of of ex-situ ex-situ prepregprepreg process:process: ( (aa)) The The extrusion extrusion and and cooling cooling apparatus apparatus for for production production ofof the the prepreg prepreg filament. filament. ( b(b)) The The printingprinting processprocess utilising utilising the the prepreg prepreg filament filament requires requires no no drive drive gear gear asas the the fibre fibre is is pulled pulled through through the the nozzle, nozzle, extrudingextruding the polymer as as it it moves moves [42]. [42].

InIn 2014, 2014, the the MIT MIT spin-out spin-out company company MarkforgedMarkforged was was the the first first to to commercially commercially offer off erthe the ex-situ ex-situ prepregprepreg composite composite printing printing system system [[50].50]. Their system system could could print print using using carbon, carbon, glass glass and and Kevlar Kevlar fibre,fibre, and and used used aa cutting apparatus apparatus to todeposit deposit the thecorrect correct amounts amounts of fibre of in fibre specific in specificlocations locations in the inpart. the part.The printer The printer included included a second a secondFFF printhead FFF printhead for the printing for the of printing unreinforced of unreinforced nylon. Selected nylon. Selectedregions regions of a polymerof a polymer part can be part reinforced, can be reinforced, rather than ratherstrengthening than strengthening the entirety of the the entiretycomponent. of the component.In doing so In the doing components’ so the components’ fibre fractions fibre are fractions limited to are approximately limited to approximately 34 VF% (slightly 34 VF%lower (slightly than lowerthe thanVF% theof VF%the prepreg of the prepreg filament filament used), used),and larg ande largeareas areasof the of parts the parts remain remain unreinforced unreinforced [41]. [41 ]. ComparedCompared with with printed printed polymer polymer only only parts, parts, the FFF the of FFF the compositesof the composites exhibited exhibited significantly significantly improved mechanicalimproved performances.mechanical performances. For example, For Blok example, et al. [Blok51] reported et al. [51] tensile reported strengths tensile asstrengths high as as 725 high MPa as 725 MPa for carbon fibre/PA, compared with 84 MPa for the printed PA polymer. It is important for carbon fibre/PA, compared with 84 MPa for the printed PA polymer. It is important to highlight, to highlight, however, that the results from this study are based upon samples that were modified however, that the results from this study are based upon samples that were modified after printing. after printing. Several studies utilise an in-house designed filament or printer with a similar Several studies utilise an in-house designed filament or printer with a similar mechanism to that used mechanism to that used by Markforged. Hu et al. [42], for example, produced a custom PLA/carbon by Markforged. Hu et al. [42], for example, produced a custom PLA/carbon fibre prepreg filament fibre prepreg filament printed using a modified open source 3D printer, and these composites printedachieved using flexurala modified strengthsopen five source times 3D higher printer, than and unreinforced these composites PLA. achievedHowever, flexural the VF% strengths was not five timesprovided higher for than cross unreinforced comparisons PLA. to be However, made. The the air VF%void content was not of provided these samples for cross is typically comparisons lower to bethan made. in Thethe in-nozzle air void contentimpregnated of these equivalents, samplesis pr typicallyimarily due lower to the than initia in thel impregnation in-nozzle impregnated step. As equivalents,highlighted primarily by Matsuzaki due toet theal. [52], initial increasing impregnation the fibre step. count As in highlighted a tow results by Matsuzakiin an increased et al. air [52 ], increasingcontent, however the fibre countafter deposition in a tow results this is intypically an increased reduced. air This content, study however also demonstrated after deposition that after this is typicallyprinting reduced. under pressure This study (from also the demonstratedprinting head) thatthe nozzle after printingserving to under push pressurethe air out (from of the the filament, printing head)a reduction the nozzle in porosity serving tofrom push 33% the to air 4% out was of theobserv filament,ed for asome reduction samples in porosity[52]. Whilst from this 33% result to 4% was was observedreported for for some a single samples line of [ 52printed]. Whilst filament, this result the overall was reported composite’s for avoid single content line of increased printed due filament, to thethe overall formation composite’s of air pockets void contentbetween increased filaments dueand tosubsequent the formation layers, of as air they pockets are placed between adjacent filaments to andand subsequent on top of one layers, another. as they are placed adjacent to and on top of one another. BothBoth “in-situ “in-situ fusion”fusion” and “ex-situ “ex-situ prepreg” prepreg” sy systemsstems demonstrate demonstrate that thatentrained entrained air within air within the thecomposite composite matrix matrix is the is primary the primary challenge challenge when 3D when printing 3D printingcontinuous continuous fibre composites. fibre composites. Prepreg Prepregsystems systems have thehave advantage the advantage of a dedicated of a dedicated manufacturing manufacturing step, which step,can reduce which filament can reduce air content filament airto content nearly 0%. to nearly However, 0%. some However, level of some porosity level rema of porosityins after remains printing, after and printing,it is therefore and evident it is therefore that the printing process itself induces porosity and still requires optimisation. Goh et al. [41] observed evident that the printing process itself induces porosity and still requires optimisation. Goh et al. [41] that the overlapping of fibre bundles can reduce this porosity, however it could not be eliminated observed that the overlapping of fibre bundles can reduce this porosity, however it could not be completely. The reduction in printed part porosity associated with the use of low pressure processing eliminated completely. The reduction in printed part porosity associated with the use of low pressure conditions during printing (1 Pa) has been successfully shown to increase the interlaminar shear processing conditions during printing (1 Pa) has been successfully shown to increase the interlaminar strength (ILSS) of carbon, glass and Kevlar, by 33%, 22% and 12% respectively, compared to those shearmaterials strength printed (ILSS) under of carbon, atmospheric glass and pressure Kevlar, [53]. by 33%,Another 22% method and 12% of respectively, both decreasing compared the porosity to those materialsof the FFF printed printed under parts, atmospheric as well as enhancing pressure inte [53].rlayer Another adhesion, method is the of bothuse of decreasing atmospheric the plasma porosity ofsurface the FFF activation printed parts, treatments. as well This as enhancingwas demonstrat interlayered through adhesion, the use is the of an use in-line of atmospheric air atmospheric plasma surfaceplasma activation jet treatment treatments. for the activation This was of demonstrated sized basalt fibres, through immediately the use of prior an in-line to the airapplication atmospheric of plasmapolypropylene jet treatment by extrusion for the activation coating, to ofform sized the basaltpolymer-coated fibres, immediately filaments [2 prior5]. The to flexural the application modulus of polypropyleneand the maximum by extrusion shear stress coating, values to of form the theresultin polymer-coatedg FFF composites filaments were [found25]. The to increase flexural by modulus 12%

Polymers 2020, 12, 2188 9 of 18

and thePolymers maximum 2020, 12, xshear FOR PEER stress REVIEW values of the resulting FFF composites were found to increase9 of by18 12% and 13%, respectively, compared with those obtained for composites fabricated using untreated fibres. It wasand concluded 13%, respectively, that this increasedcompared mechanicalwith those obta performanceined for composites is likely due fabricated to the enhancedusing untreated interfacial bondfibres. strength It was between concluded thefibres that this and increased the polypropylene mechanical polymer,performance with is anlikely associated due to the reduction enhanced in the levelinterfacial of air incorporation bond strength around between the basaltthe fibres filaments. and the polypropylene polymer, with an associated reduction in the level of air incorporation around the basalt filaments. 3.1. 3D Printed Composites—Mechanical Performance Comparison 3.1. 3D printed Composites—Mechanical Performance Comparison Agarwal et al. [54] demonstrated how the 3D printed composites can outperform those produced Agarwal et al. [54] demonstrated how the 3D printed composites can outperform those usingproduced conventional using approaches,conventional particularlyapproaches, whenparticularly the printing when the is carriedprinting out is usingcarried optimised out using fibre orientations.optimised Figure fibre orientations.7 provides Figure an overview 7 provides of thean overview tensile properties of the tensile of properties continuous of continuous reinforcement compositesreinforcement versus short,composites particle versus and short, unreinforced particle polymers,and unreinforced based onpolymers, values reportedbased on by values a number of authorsreported in by the a number literature. of authors The wide in the rangeliterature. of tensileThe wide properties range of tensile obtained properties for a obtained given composite for a type isgiven likely composite to be influenced type is likely by parametersto be influenced such by as parameters fibre content, such as as well fibre as content, composite as well processing as techniquecomposite used. processing technique used.

Figure 7. Literature values for tensile strength and modulus for short and continuous ﬁbre-reinforced Figure 7. Literature values for tensile strength and modulus for short and continuous fibre-reinforced composites,composites, as as well well as as unreinforcedunreinforced polymers polymers for comparison. for comparison. A comparison A comparison between similar between additive similar additivemanufactured manufactured (AM) and (AM) compression and compression moulded (CM) moulded woven (CM)PA66/CF woven composites PA66/ CFis highlighted, composites is highlighted,with tensile with performance tensile performance being comparable. being comparable. Key: Author, Key: matrix, Author, reinforcement, matrix, reinforcement, fibre % ﬁbre %[18,27,29,34,43,46,51,55–58]. [18,27,29,34,43,46,51,55 –58].

Polymers 2020, 12, x FOR PEER REVIEW 10 of 18 Polymers 2020, 12, 2188 10 of 18 3.2. Continuous Fibre Printing—Pathing Behaviour 3.2. Continuous Fibre Printing—Pathing Behaviour 3.2.1. Open Source Programme 3.2.1.Printing Open Source parameters Programme such as temperature, speed, nozzle height, cornering radii and filament overlapPrinting can have parameters an impact such on the as temperature,printed composite’s speed, nozzlemechanical height, performance. cornering radiiAs the and majority filament of compositeoverlap can printing have an systems impact under on the development printed composite’s are based mechanical upon a 3-Axis performance. CNC (Computer As the majority numerical of control)composite platform, printing they systems utilise under Gcode development to control movements. are based upon Toolpath a 3-Axis logic CNC and (Computer printing behaviour numerical mustcontrol) accommodate platform, they the utilise unique Gcode behaviour to control movements.of a filament, Toolpath containing logic and a printingcontinuous behaviour unbroken must reinforcement.accommodate theAs the unique fibre behaviour reinforcement of a filament,remains in containing a solid state a continuous throughout unbroken the printing reinforcement. process, its mechanicalAs the fibre properties reinforcement should remains be unaffected in a solid by state the throughoutprinting process, the printing however process, this assumes its mechanical that no destructiveproperties shouldprinting be behaviour unaffected has by theoccurred printing during process, deposition. however “Destructive this assumes printing that no destructive behavior” refersprinting to any behaviour movement has or occurred printing during parameter deposition. that results “Destructive in a reduction printing of the behavior” final properties refers toof anythe composite,movement orcompared printing parameterwith those that ofresults the pre-depo in a reductionsition ofmaterial the final (such properties as porosity of the composite, or fibre discontinuity).compared with thoseFFF slicing of the pre-deposition software (GrabCAD, material (suchUltimakers as porosity CURA, or fibre Slic3r, discontinuity). simplify3DFFF etc.), slicing has developedsoftware (GrabCAD, significantly Ultimakers over the last CURA, 30 years, Slic3r, since simplify3D its invention etc.), hasin 1989 developed by Scott significantly Crump, founder over the of Stratasyslast 30 years, Ltd. since[59]. The its invention majority of in studies 1989 by perfor Scottmed Crump, on custom-built founder of Stratasys continuous Ltd. composite [59]. The majoritysystems useof studies a simple performed raster deposition on custom-built pattern, continuouswhich is a ba compositeck and forth systems movement use a simpleseparated raster by raster deposition gaps [42,44–46],pattern, which as shown is a back inand Figure forth 8a. movement As raster separated patterns can by raster be easily gaps programmed [42,44–46], as shownusing existingin Figure FFF8a. software,As raster or patterns with milling can be CNC easily software, programmed they allow using for existing the creation FFF software, of rectangularor with specimensmilling CNC in a shortsoftware, period they of allowtime. This for the is likely creation the ofreason rectangular for many specimens studies focusing in a short on period tensile of and time. flexural This istesting, likely asthe samples reason for can many be manufactured studies focusing without on tensile comple and flexuralx pathing testing, software. as samples The tight can becorners manufactured of 180° introduce major fibre damage and dislocation during printing, however the attachment of tabs without complex pathing software. The tight corners of 180◦ introduce major fibre damage and duringdislocation testing during usually printing, obscures however this fact the[52]. attachment Another popular of tabs printing during method testing usuallyis to use obscuresFFF software this perimeter-followingfact [52]. Another popular logic toprinting form rings method of material is to use in FFF a spiral software like perimeter-following motion. This mode logiccan mitigate to form mostrings of materialthe problems in a spiral of a likeraster motion. pattern This by modetaking can corners mitigate at mostreduced of the angles problems [47]. ofIt can a raster be used pattern to makeby taking simple corners geometries, at reduced such angles as cylinders, [47]. It canwing be cross-sections used to make or simple dog bones, geometries, or any such solid as shape cylinders, that containswing cross-sections no internal orfeatures dog bones, (i.e., no or anyhollow solid spaces) shape (see that Figure contains 8b,c). no internalThe primary features reason (i.e., behind no hollow the usespaces) of these (see patterns Figure8b,c). is that The the primary toolpath reason generat behinded is thecontinuous, use of these as most patterns custom-built is that the systems toolpath do notgenerated include is a continuous, cutting apparatus, as most wh custom-builtich is necessary systems for dothenot printing include of amore cutting geometrically apparatus, complex which is shapes.necessary for the printing of more geometrically complex shapes.

Figure 8 8.. ((aa)) Example Example of of a a raster raster pattern pattern generated generated fo forr printing printing tensile testing samples [[41].41]. ( (b)) Objects Objects printed using “spiral” generated by an FFF slicer software [47]. (c) Printer producing a bowl-shaped printed using “spiral” generated by an FFF slicer software [47]. (c) Printer producing a bowl-shaped component from PLA/CF [45]. component from PLA/CF [45]. 3.2.2. Proprietary/Commercial Programs 3.2.2. Proprietary/Commercial Programs In order to facilitate the commercialisation of the 3D printing technology for composite production, severalIn companiesorder to facilitate have simpliﬁed the commercialisation the tool pathing of software the 3D andprinting aligned technology the method for of composite operation production, several companies have simplified the tool pathing software and aligned the method of

Polymers 2020, 12, 2188 11 of 18

Polymers 2020, 12, x FOR PEER REVIEW 11 of 18 more closely with that of FFF systems. Between 2014 and 2019, only one such software was available commercially;operation more this closely was the with Markforged that of FFF slicer systems. software Between “Eiger”. 2014 Thisand 2019, software only allows one such for automatedsoftware was fibre placementavailable basedcommercially; upon existing this was slicer the logic, Markforged such as slicer perimeter software placement, “Eiger”. but This can software only be allows utilised for with Markforgedautomated Markfibre seriesplacement composite based printersupon existing [60]. Asslic ofer 2019,logic, additional such as peri companiesmeter placement, have released but can open sourcedonly be equivalents utilised with of slicing Markforged software, Mark with series a specific composite focus printers on turning [60]. FFF As printers of 2019, into additional composite printers.companies For have example, released 9T labs open of sourced Zurich, Switzerlandequivalents of and slicing Anisoprint software, of Moscow,with a specific Russia focus have on both releasedturning openFFF printers sourced into slicer composite software printers. for use onFor aexample, wide range 9T labs of 3-axis of Zurich, printing Switzerland systems [and61,62 ]. 9TAnisoprint labs is in earlyof Moscow, beta testing Russia ofhave its “CarbonKit”both released andopen accompanying sourced slicer software slicing software; for use on this a wide system servesrange as of a drop-in3-axis printing kit for existingsystems FFF[61,62]. printers, 9T labs expanding is in early their beta capabilities testing of intoits “CarbonKit” composite printing. and Theaccompanying accompanying slicingsoftware software; “Fibrify” this appears system similar serves in as function a drop-in to Markforged kit for existing “Eiger”, FFF with printers, a greater emphasisexpanding on their fibre capabilities optimisation. into Anisoprint composite printing. has followed The accompanying a similar route software to that of“Fibrify” Markforged appears with similar in function to Markforged “Eiger”, with a greater emphasis on fibre optimisation. Anisoprint its “Aura” slicer, which enables fibre printing with their desktop “Composer” series printers [61,62]. has followed a similar route to that of Markforged with its “Aura” slicer, which enables fibre printing As with “Eiger” and “Fibrify”, this slicer is fundamentally an FFF slicer, with the added functionality with their desktop “Composer” series printers [61,62]. As with “Eiger” and “Fibrify”, this slicer is of fibre inclusion. These commercial systems cater to a model of “improving printed part performance” fundamentally an FFF slicer, with the added functionality of fibre inclusion. These commercial rather than “improving composite part performance”. Figure9 compares the Markforged “Eiger” and systems cater to a model of “improving printed part performance” rather than “improving composite Anisoprintpart performance”. “Aura” slicer Figure software 9 compares for the fibre Markforged composite “Eiger” printing. and Anisoprint As demonstrated “Aura” slicer in this software literature review,for fibre the composite inclusion printing. of fibres As will demonstrated ultimately lead in this to an literature increase review, in the strengththe inclusion and of sti fffibresness will of FFF parts,ultimately however lead the to an performance increase in the of thestrength resulting and stiffness composite of FFF part parts, can however be significantly the performance enhanced of by optimisingthe resulting the composite fibre placement. part can be significantly enhanced by optimising the fibre placement.

FigureFigure 9. 9Comparison. Comparison ofof (a(a)) MarkforgedMarkforged “Eiger” and and ( (bb) )Anisoprint Anisoprint “Aura” “Aura” slicer slicer software software forfor fibre fibre compositecomposite printing. printing. TheThe EigerEiger softwaresoftware facilitates fi fibrebre placement placement in in tighter tighter spaces, spaces, with with blue blue lines lines indicatingindicating fibre fibre paths. paths.

3.3.3.3. Geometrically Geometrically Complex Complex CompositeComposite FabricationFabrication through through 3D 3D Printing Printing TheThe majority majority of of the the studies studies toto datedate onon composites have have focused focused on on improving improving their their mechanical mechanical properties.properties. 3D 3D printing printing systems, systems, however,however, also fa facilitatecilitate greater greater design design freedom freedom than than automated automated fibre fibre placementplacement (AFP), (AFP), automated automated tape tape laying laying (ATL) (ATL) and an mouldingd moulding techniques, techniques, particularly particularly regarding regarding fibre placement,fibre placement, localising localising volume volume fractions fractions and part and geometry.part geometry. SeveralSeveral studies studies have have produced produced novel novel continuous continuous fibre fibre pathing pathing programs programs to print to unique print structures.unique Houstructures. et al., forHou example, et al., for printed exampl ae, corrugatedprinted a corrugated structure forstructure use as for a coreuse as material a core material in composite in sandwichcomposite paneling, sandwich by paneling, printing by the pr panelinting sideways the panel in sideways the Z-direction in the Z-direction of the printer of the [63 printer]. These [63]. were reportedThese were to exhibit reported superior to exhibit compression superior compressio strengthsn and strengths lower and core lower densities core todensities those ofto aluminumthose of corrugatedaluminum structures.corrugated structures. Similarly, Simi Sugiyamalarly, Sugiyama et al. printed et al. printed sandwich sandwich structures structures from from carbon carbon fibre; fibre; however, these were printed in the XY plane utilising fibre tension to print over the gaps in the

Polymers 2020, 12, 2188 12 of 18 however, these were printed in the XY plane utilising fibre tension to print over the gaps in the core structure [64]. Utilising the continuous fibre filament under tension to form a bridge allows for printing with very little support material. A multitude of core patterns and shapes were tested in 3-point bending, with supporting abutment material being removed using a saw prior to testing. The printing was performed in a single continuous movement with no cutting apparatus present. These studies utilised a layer-by-layer approach for deposition, however a number of studies have taken this a step further to produce 3D curvilinear composites [65,66]. Liu et al. [65] produced a core structure for sandwich paneling, utilising a novel method of depositing small sections of composite in mid-air, without the need for a mould. In this study, the latticed core was printed and then adhesively bonded to epoxy composite face sheets. Compressive strengths were low, however the repeatability and filament placement error were significantly improved during the course of the study. To make a similar curvilinear component, Tse et al. [66] utilised a spring-loaded mechanism to reciprocate a heated nozzle over a 3D printed dissolvable mould. A number of contoured composite parts were produced, and in this case a 2D coordinate system was used to guide the head, with the spring mechanism following the mould in the Z-direction [66]. The exact method of achieving these printing paths is not disclosed in these studies, presumably to prevent replication, however it is likely that each study would have required bespoke programs/scripts to be written. The use of 3D printing to minimise stress concentrators such as holes and notches has been explored in a number of modelling studies [67,68]. Stress concentrators are a major cause of the early/catastrophic failure of fibre composites. These can be associated, for example, with the machining of holes and notches in conventional composites. 3D printing has the potential to create holes within a part by reorienting fibres around the opening of a hole, rather than breaking the fibres through machining. Yamanaka et al. [67] produced a preliminary model of such a structure, indicating that by 3D printing the composite structure and preventing fibre breakage a significantly increased tensile strength could be achieved, compared with that obtained by the cutting of unidirectional reinforcements (Figure 10c) [67]. This study did not consider the width of a 3D printed fibre filament, and filaments are still cut at the point they reach the perimeter of the hole, meaning that fibre damage was also not accounted for at the point of cutting. Zhang et al. [68] performed a similar study; however, material properties were taken from the 3D printed composite literature, rather than the conventional composite values used by Yamanaka. Laminates were tested in single ply and cross ply configurations. In contrast to the procedure of Yamanaka, the fibres were not cut when the hole perimeter was reached, and instead formed a densified region on either side of the hole perimeter. This densified region prevented a major strain riser formation during testing, increasing the laminates strength and stiffness. Figure 10a,b represent the optimised and cut samples respectively. Both studies are based upon the simulation of non-woven materials, with precise fibre placement, which are needed in order to reduce the effects of the induced off-axis forces generated by the stress concentration around the hole. Woven materials, however, typically exhibit superior through-thickness and off-axis properties [69]. It is therefore reasonable to conclude that woven laminates would be potentially favourable for resisting the forces developed around a notch. Polymers 2020, 12, 2188 13 of 18 Polymers 2020, 12, x FOR PEER REVIEW 13 of 18

FigureFigure 10. 10Modelled. Modelled fibre fibre layouts layouts (left) (left) and and resulting resulting stressstress heat maps (right). (right). (a (a) )represents represents an an ideal ideal fibrefibre placement placement scenario scenario with with reduced reduced strain strainconcentration concentration [[68].68]. ( (bb)) represents represents a adrilled/cut drilled/cut sample sample withwith discontinuous discontinuous fibres fibres resulting resulting in large in strainlarge concentrators.strain concentrators. (c) represents (c) represents a cut sample a cut containing sample fibrecontaining vorteces fibre to reduce vorteces the to strain reduce concentration the strain concentration at the centre at hole the centre [67]. hole [67].

3.4. Six-AxisMany Robotics companies in the have 3D incorporated Printing of Composites robotics as part of their composite printing activities, with a wider range of designs made possible by multi-axis printing. Arevo Labs, for example, reported on In order to facilitate the wider adoption of 3D composite printing as a manufacturing process, the printing of short fibre composites as early as 2016, and has since unveiled a new robotic printing automation, including robotics, is vital. The use of robotic techniques has been utilised for the layup system for the printing of PEEK/CF composites [71]. Arevo have also produced a demonstrator of process of composites for a number of years, and most are utilised in combination with ATL systems their technology in the form of a 3D printed CF bicycle frame [71]; this frame is made of layered CF which,filaments for example, that are deposited are widely using used a incustom-built, the aerospace laser-heated industry deposition [40]. A multi-axis head, the articulatingdetails of which robot canare be not used disclosed to hold (Figure the ATL 11). head,9T labs which have demonst is in turnrated connected a robotic to system a gantry for the or placing rail. This of CF/PA12 can allow 5 tocomposite 10 degrees ontoof freedom.curved surfaces, This type however of system the details is efficient of this for design the depositing have also of not large been non-complex disclosed (flatpublicly or single [62]. curvature) Atropos parts;is a robotic however, technology for non-flat demon structuresstrator that with originated higher curvature, at the Politechnico a second Milano forming process+Lab, is and required features after a unique placement. thermosetting This second resin forming print stepheadhas [72]. a tendencyThis system to causeis reported the warpage to utilise and bucklingcarbon, of glass, the laminates, or basalt leadingfibres, and to fibre can damageuse a nu [70mber]. The of tapeUV curable width also resins precludes to produce tight complex cornering, andgeometric results in parts machining (Figure being11). The required processing for small efficien featurecies obtained inclusion. through the further automation of FFFMany composite companies printing have are clearly incorporated key for the robotics wider asadoption part of of theirthis 3D composite printing technology printing activities, for the withmanufacture a wider range of individualised of designs made consumer possible products, by multi-axis such as sports printing. bicycles, Arevo tennis Labs, rackets for and example, golf reportedclubs, as on wellthe as printing medical of devices short fibresuch as composites prostheses. as early as 2016, and has since unveiled a new robotic printing system for the printing of PEEK/CF composites [71]. Arevo have also produced a demonstrator of their technology in the form of a 3D printed CF bicycle frame [71]; this frame is made of layered CF filaments that are deposited using a custom-built, laser-heated deposition head, the details of which are not disclosed (Figure 11). 9T labs have demonstrated a robotic system for the placing of CF/PA12 composite onto curved surfaces, however the details of this design have also not been disclosed publicly [62]. Atropos is a robotic technology demonstrator that originated at the Politechnico Milano +Lab, and features a unique thermosetting resin print head [72]. This system is reported to utilise carbon, glass, or basalt fibres, and can use a number of UV curable resins to produce complex geometric parts (Figure 11). The processing efficiencies obtained through the further automation of FFF composite printing are clearly key for the wider adoption of this 3D printing

Polymers 2020, 12, 2188 14 of 18 technology for the manufacture of individualised consumer products, such as sports bicycles, tennis Polymers 2020, 12, x FOR PEER REVIEW 14 of 18 rackets and golf clubs, as well as medical devices such as prostheses.

Figure 11. The Arevo labs robotic AM system printing a portion of a bicycle frame (left)[71]. TheFigure Atropos 11. TheRobot Arevo system labs printing robotic aAM glass system/epoxy printing turbine blade a portion without of a mouldbicycle for frame support (left) (right [71].)[ The72]. Atropos Robot system printing a glass/epoxy turbine blade without a mould for support (right) [72]. 4. Summary and Conclusions 4. SummaryThis paper and provided Conclusions an overview of the use of the fused filament fabrication (FFF) technique for theThis manufacture paper provided of both an overview short and of continuous the use of fibre-reinforcedthe fused filament polymeric fabrication materials, (FFF) technique along with for thedetails manufacture of the mechanical of both properties short and of continuous the resulting fib composites.re-reinforced The polymeric most widely materials, reported along short fibreswith detailsin the literature of the mechanical are those ofproperties carbon and of glassthe resu fibres,lting which composites. is primarily The most due to widely the application reported focusshort withinfibres in aerospace the literature and automotive.are those of carbon Several and other glass reinforcement fibres, which fibres, is primarily including due basalt, to the aramidapplication and focusjute (and within other aerospacenatural fibres),and automotive. are also reported, Severaland othe haver reinforcement also been shown fibres, to including improve thebasalt, mechanical aramid propertiesand jute (and of polymerother natural composites. fibres), Theare additionalso reported, of short and fibres have into also neat been thermoplastics shown to improve polymers the mechanicalcan significantly properties improve of polymer their stiff composites.ness and strength. The ad However,dition of short the maximum fibres into achievable neat thermoplastics properties polymersof these composites can significantly are limited improve due their to the stiffness presence and ofstrength. porosity However, in the printed the maximum parts. Despite achievable the propertiesidentified mechanicalof these composites limitations, are thelimited FFF due printing to the of presence short fibre-reinforced of porosity in thermoplasticthe printed parts. composites Despite theshows identified potential mechanical in 3D printing limitations, to produce the some FFF “end-use”printing of components, short fibre-reinforced such as moulds thermoplastic for tooling. Whilstcomposites short shows fibre compositespotential in have3D printing excellent to produce utility, and some can “end-use” be processed components, through such a standard as moulds FFF process,for tooling. continuous Whilst fibreshort composites fibre composites offer orders have of excellent magnitude utility, higher and strength can be and processed stiffness, versus through neat a polymersstandard orFFF short process, fibre composites.continuous Thesefibre composites materials contain offer continuousorders of magnitude unbroken strandshigher ofstrength reinforcing and stiffness,fibre, which versus allows neat for polymers greater load or capacity,short fibre but composites. also requires These specialist materials hardware contain for continuous processing, unbrokensuch as cutting strands devices of reinforcing and multi-input fibre, printheadswhich allows for for proper greater polymer–fibre load capaci infusion.ty, but also Two requires primary specialistmethods of hardware continuous for composite processing, printing such haveas cutti beenng highlighted.devices and “In-situmulti-input fusion” printheads shows great for promise proper polymer–fibrefor the rapid manufacturing infusion. Two of compositeprimary methods parts, with of potentialcontinuous to producecomposite variable printing volume have fraction been partshighlighted. with a single“In-situ manufacturing fusion” shows process. great Thispromise technique, for the however, rapid manufacturing typically produces of composite inferior quality parts, partswith potential due to entrained to produce air contents variable and volume poor polymerfraction parts permeation. with a single “Ex-situ manufacturing prepreg” provides process. superior This qualitytechnique, parts however, with lower typically air contents produces and inferior excellent qu polymerality parts infusion, due to at entrain the expenseed air contents of a more and complex poor polymermulti-stage permeation. manufacturing “Ex-situ process. prepre Ag” di providesfficulty with superior the resulting quality parts 3D printed with lower composites, air contents however, and excellentis the presence polymer of porosity,infusion, which at the canexpense significantly of a more impact complex on mechanical multi-stage performance. manufacturing Amongst process. the A difficultymethods ofwith addressing the resulting this are3D theprinted application composites of pressure, however, during is the printing, presence fibre of bundleporosity, overlapping, which can significantlythe use of low-pressure impact on printingmechanical conditions, performance. as well Amongst as the use the of methods atmospheric of addressing plasma pre-treatments. this are the applicationThe wider of pressure adoption during of 3D printingprinting, for fibre consumer bundle products overlapping, is facilitated the use throughof low-pressure the use of printing robotic printingconditions, techniques. as well as Developments the use of atmospheric in this area plasma were reviewedpre-treatments. and, when combined with the superior materialThe propertieswider adoption of the of high 3D strength printing and for lowconsumer weight products of the 3D is printed facilitated composites, through havethe use the of potential robotic printing techniques. Developments in this area were reviewed and, when combined with the superior material properties of the high strength and low weight of the 3D printed composites, have the potential to produce a wide range of individualised parts, particularly for sectors such as sports goods and medical devices.

Polymers 2020, 12, 2188 15 of 18 to produce a wide range of individualised parts, particularly for sectors such as sports goods and medical devices.

Funding: This work was funded by Science Foundation Ireland, through the I-Form Advanced Manufacturing Research Centre, grant number [16/RC/3872]. And the APC was funded by I-Form. Acknowledgments: This work is partially supported by the Irish Centre for Composite Research (IComp) and the I-Form Advanced Manufacturing Research Centre (Grant Number 16/RC/3872). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study, in the collection or interpretation of data, in the writing of the manuscript, or in the decision to publish.

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