3D-Printed Fiber-Reinforced Polymer Composites by Fused Deposition Modelling: History
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Khairul Izwan Ismail
,
Tze Chuen Yap
,
Rehan Ahmed

The mechanical properties of FDM printed parts are still weaker compared to conventionally manufactured products. Numerous studies and research have already been carried out to improve the mechanical properties of FDM printed parts. Reinforce polymer matrix with fiber is one of the possible solutions. Furthermore, reinforcement can enhance the thermal and electrical properties of FDM printed parts. Various types of fibers and manufacturing methods can be adopted to reinforce the polymer matrix for different desired outcomes.

  • additive manufacturing
  • fused filament fabrication
  • 3D printing

1. Introduction

Additive manufacturing (AM) or 3-dimensional printing (3DP) technology is one of the most promising areas in component manufacturing. AM has paved its way into application areas ranging from automotive [1], construction [2], aerospace [3] and consumer products to biomedical products such as prosthetics [4]. AM refers to a group of fabrication techniques where parts are fabricated layer-by-layer directly from a computer-aided design (CAD) file. AM technology is a very broad term that encompasses many methods such as Stereolithography (SLA) of a photopolymer liquid [5], Laminated Object Manufacturing (LOM) from plastic laminations [6], Selective Laser Sintering (SLS) from plastic or metal powder [7] and Fused Deposition Modelling (FDM) from plastic filaments [8]. Since 1980, many studies have been conducted to maximize the potential applications of these technologies, as AM is well-known and still a far more cost-effective alternative to subtractive manufacturing technologies such as milling, drilling and turning [9]. FDM, also called Fused Filament Fabrication (FFF), is one of the most popular techniques due to its relatively low cost, low material wastage and ease of use. Nowadays, most people can even purchase and use this technique at home. However, FDM 3D print is yet to replace conventional manufacturing in producing functional parts. FDM 3D print parts are weaker than conventionally manufactured counterparts due to their layer-by-layer fabrication method. Research has been carried out to improve the mechanical properties of FDM printed parts by using various methods, such as optimizing printing parameters, annealing, snap-fitting [10], printing in an oxygen free environment [11], mechanical pressing [12] and fiber reinforced thermoplastics. Of all the methods, fiber reinforced polymer composites (FRPC) are known to have high stiffness, strength, damage tolerance, fatigue resistance and corrosion resistance. FRPCs are produced by adding fibers or particles into the thermoplastic matrix to improve the mechanical strength of the printed components [13]. This method reduces voids and increases interlaminar bonding between the deposited filaments. There are two types of fiber reinforcement: continuous and discontinuous, depending on fiber length. Fiber reinforced composites have a long history and are traditionally produced by techniques like hand lay-up, molding, etc. FDM is a relatively new technique for manufacturing fiber reinforced polymer composites. Research in FDM 3D printed fiber reinforced polymer composites has flourished recently. Previous works in FDM 3D printed fiber reinforced polymer composites have been reviewed by several state-of-the-art review papers with a different emphasis [14][15][16][17][18][19][20][21].

2. Polymer Sintering and Voids Formation in Fused Deposition Modelling

2.1. Fused Deposition Modelling Process

The process of creating an object with an FDM printer begins with the product design using CAD software such as CATIA and SOLIDWORKS, which is saved in a Surface Tessellation Language (STL) file. Before such a file can be printed, it must be converted into a format that the 3D printer can understand, namely a G-code file. Slicer software such as Cura, Ideamaker and Simpliy 3D are used to convert the STL file into a G-code file. The G-code contains commands for moving parts within the printer. It consists of G- and M-commands that have assigned actions and movements in x-, y- and z-directions of the nozzle and bed of the FDM printer. 
 
A 3D geometry is produced in the FDM process by building up an extruded thermoplastic filament layer-by-layer. The filament is fed through the extrusion head (nozzle), which is heated to a semi-liquid state and applied to the build platform through a nozzle in layers. Each layer is bonded to the adjacent layers in the semi-liquid state. Thus, it is crucial to control the feed rate of the printer to ensure that the previous layer does not solidify too early. Feed rate can be easily adjusted in the slicer software. Figure 1b shows a schematic diagram of the FDM extrusion head and filament deposition process. The filament is first driven into the print head by rollers. As it passes through a liquefier, the feedstock is heated by a heater to a viscous melt and pushed out of the print nozzle by the incoming still-solid filament.

2.2. Polymer Sintering of Deposited Thermoplastics

The FDM process uses a heated nozzle to melt and extrude thermoplastic filaments such as Acrylonitrile-butadiene-styrene (ABS), poly-lactic acid (PLA), nylon, polypropylene (PP), polyethylene (PE), and so on. These materials are common thermoplastics used in 3D printing. Each material has a different melting point, and the printer must be set accordingly. An error in setting up the temperature of the feedstock material will affect the cosmetic and strength of 3D printed products. During the FDM process, each filament extruded through the heated nozzle solidifies and forms a cross-bond with the adjacent filaments extruded previously. These filaments form a bridge between them, known as the “neck” by the process of polymer sintering [22]. This bond, which is responsible for growing the necks within a layer, may be termed as the “intra-layer bonding”. Since the temperature of the previously solidified layer is still high, there is a good tendency for similar bonds to form between the filaments of the two successive layers, which can be termed as “inter-layer bonding”.
Gurrala and Regalla et al. [22] also investigated the effects of inter-layer bonding, intra-layer bonding and neck formation between adjacent filaments on the tensile strength of FDM products, both experimentally and theoretically. They found that in the FDM sample with 0° raster angle, the failure of a specimen was due to inter-layer fracture, whereas at 45° raster angle, the specimens failed due to both inter-layer and intra-layer fracture. This research has shown that inter-layer and intra-layer bonds play an important role in the mechanical properties of FDM products. Figure 3 and Figure 4 are schematic diagrams of multi-layer extruded filaments. The strength of printed parts depends on these two interlaminar bonds. To improve these interlaminar bonds, much research has been done focusing on the printing parameters. They believe that an optimal setup results in high strength FDM products with great interlaminar bonding. An overview of these research will be presented in the next section (Section 3).
 
Multiple attempts have been made to numerically model the sintering process of polymers based on heat transfer calculations. Early work by Yardimci et al. [23][24] presented different modelling approaches to capture the heat transfer between printed beads, but did not consider the polymer flow dynamics. Bellehumeur et al. [25] used a model based on a polymer sintering model described by Pokluda et al. [26]. Pokluda et al. performed an energy balance between surface tension and viscous dissipation [26], and Bellehumeur et al. incorporated temperature-dependent surface tension and viscosity into Pokluda et al.’s model. Although they did not model molecular diffusion, they found that the extruded material cooled too quickly for complete bonding. They also reported that the convective heat transfer coefficient greatly affects bond formation and neck growth, with less heat transfer leading to better neck formation. However, they modelled isothermal polymer sintering and did not consider heat transfer from the hot extruded material to the surrounding material. Bellini [27] performed extensive modelling of the entire FDM process with ceramic-filled filament using four different numerical simulations focusing on: the liquefier, the nozzle contraction, deposition on the printing bed and on stacked layers. This enabled the tracking of material temperature, swelling and filling as a function of the various printing parameters. It was found that the higher thermal conductivity of the filled material increases heat transfer from the liquefier to the printed material and improve the flow behavior.

2.3. Voids in FDM Printed Components

The strength of components produced by the FDM process differs from that of parts fabricated by conventional injection molding. The presence of voids and gaps between the individual layers reduces the layer-to-layer bond strength. The strength of fabricated components by FDM is compromised by significant voids and weak interlaminar bonding between layers. The percentage of void is depends on printing parameters and typically ranged from 4% to 18.5% [23][25][28]. Although the deposition filaments can be integrated into the adjacent deposition filaments by their gravity and the force of the printer’s stepper motor, the presence of significant voids between them greatly affects the mechanical properties of fabricated components. In addition, the extruded filament cools rapidly from the melting temperature to the chamber temperature, developing inner stresses responsible for a weak bond between two deposition filaments. This leads to a deformation between layer (inter-layer) and within layer (intra-layer) in the form of cracks, delamination or even part fabrication failure [24].
Voids in FDM printed part can be classified into five categories according to their formation mechanism: raster gap voids, partial neck- growth voids, sub-perimeter voids, intra-bead voids and infill voids [28]. Raster gap voids are formed by gaps between the raster surfaces and are visible. Partial neck growth voids are internal voids formed by incomplete neck growth between adjacent intra- and inter-layer rasters. This occurs when rasters solidify before coalescence is complete. Partial neck growth voids are a major contributor to voids in FDM prints. Due to physical limitations, sub-perimeter voids form in between turning rasters along the perimeter of the FDM layer. Even when the printer is set into 100% infill density, voids form between the blue wall lines and the infill zones. Intra-bead voids are specific to composites due to fiber loading. Finally, infill voids are voids in the infill depending on the infill pattern selected for printing the parts and can be controlled/adjusted.
 
2.4. Quantification of Voids
Density measurement, imaging technique, optical microscope (OM), scanning electron microscope (SEM) and CT scan are commonly used to study voids [28]. OM is widely used to study the meso-structures of printed parts, while SEM is often used to analyze microstructures. OM and SEM can both capture 2D images, with the right angle proportional to the FDM layers, to capture valuable data for the analysis, such as raster gap voids, partial neck growth voids, sub-perimeter and intra-bead voids. In contrast, a CT scan is a valuable tool for observing and investigating the effects of FDM voids in 3D. Moreover, CT scans can also be used to reconstruct 3D models of scanned specimens in great detail [28].

3. Fiber Reinforced Polymer Composite (FRPC)

The development of composites that are compatible with FDM printers has attracted a lot of attention. This is because composites promise better mechanical properties and performances compared to neat polymers. Many results in the development of new printable composites reinforced with particles, fibers or nanomaterials have already been demonstrated. Carbon black, platelets, chopped fibers and polymer fibrils are mixed with the polymer matrix and then extruded together during printing. However, the performance of these composites depends largely on the fiber orientation in the plastic and the fiber-volume-fraction (FVF). Parts manufactured with FDM from neat polymer have shown insufficient strength in load tests. This limits the range of applications in which FDM technology can be used for functional parts and not for prototypes.
Researchers have used fiber reinforced polymer composites (FRPCs) to overcome the aforementioned limitations in their work. In FRPCs, the material properties of a component are enhanced by combining reinforcing fibers and polymer matrix. Various fibers have been used for reinforcement, including chopped carbon fibers, carbon nanotubes, glass fibers, natural fibers etc. [29][30][31][32]. There are certain requirements that FRPC materials must meet in order to be processed by AM, namely:
  • Types of reinforcement and matrices;
  • Good fiber-to-matrix bonding;
  • Fiber homogeneity;
  • Fiber alignment;
  • Good interlayer bonding;
  • Minimal porosity.
The fiber reinforcement must be matched in size, shape and length to the part’s intended use. Both the matrix material, which holds the fibers in place, and the reinforcement must be compatible with the selected AM technique. A good bond between fibers and matrix is required at the fiber-matrix interface to transfer loads efficiently from the matrix, resulting in composites that follow the “rule of mixtures”. Fiber loading is also crucial to obtain AM composites with good mechanical properties. Mechanical properties such as elastic modulus increase with fiber loading at a low loading ratio but degrade after reaching an optimum value [33]. This phenomenon generally occurs due to poor wettability of the fiber with the thermoplastic, which results in a poor fiber-matrix interface.
Higher loading leads to an increase in viscosity and a decrease in flowability, leading to processability problems such as clogging of the nozzle. Furthermore, fiber reinforcement may cause negative effects on interlaminar bonding and the properties of printed parts. Based on previous research, interlamellar matrix regions between the reinforced fiber layers are critical regions that are highly prone to delamination when subjected to mechanical stress. Delamination can result from weak fiber-matrix bonding, which often leads to internal damage in composites, potentially leading to global failure of the component with reduced strength and stiffness [34]. Furthermore, porosity and weak interface bonding between fibers and matrix have been cited as a major problem for 3D printed fiber reinforced polymer composites [35]. Understanding the mechanism of filament bonding is important to further investigate how FRPC works to reduce voids and increase the strength of the interlaminar bond between the deposited filaments.

3.1. Synthetic Fibers vs. Natural Fibers

Various fibers were used as reinforcement for polymer composites and can be grouped under two categories: synthetic fibers and natural fibers [13]. Natural fibers were first used as reinforcement for polymers since 1936 [36] and were slowly replaced by synthetic fibers because synthetic fibers are usually much stronger than natural fibers. However, natural fibers re-emerged as reinforcing materials for polymer composites when environmental issues became more important in engineering applications. In FDM 3D printed polymer composites, both synthetic and natural fibers were used to reinforce polymers, although synthetic fibers are a more popular choice.
Synthetic fibers are commonly used as reinforcement for FDM printed composites, and the popular fibers are carbon fiber [37][38][39][40][41][42][43][44][45][46][47], glass fiber [39][48][49] and Kevlar fibers [39]. Other possible synthetic fibers are Graphene, CNTs [50], powder [51], copper powder [51] etc. Generally, synthetic fibers are added to polymer matrix during FDM 3D printing to enhance the mechanical properties of polymer composites, and plenty of works were reported previously [39][40][44][46][52][53]. In addition, synthetic fibers were also used to improve or alter thermal properties/thermal conductivities of FDM 3D printed polymer composites [54][55] and electrical properties [56]. A systematic review on synthetic fibers as reinforcement for polymer matrix was presented recently [57], although their review does not focus on FDM 3D printed polymer composites specifically.
Natural fibers are used as reinforcement to reduce the inorganic content in thermoplastic composites without compromising mechanical strength, ultimately improving biodegradability and reducing costs [18]. Common natural fibers used in FDM 3D printed polymer composites are jute [45], wood [58], harakeke/flax [59][60], bamboo [60], sugarcane and many more. Recent and systematic reviews on natural fiber reinforced polymer composites as feeders in FDM-Based 3D Printing were reported by researchers [18][61][62]. Natural fibers are a cheaper and greener alternative to reinforce polymer matrix during the FDM 3D printing process, but challenges such as fiber agglomeration, clogging in the nozzle, poor fiber-matrix interface, non -homogenous mixing etc., have to be investigated further. Furthermore, various treatments such as chemical and thermal are required to be applied to natural fibers to enhance the performance of natural fibers reinforced polymers. In addition, a different combination of polymer matrix and natural fibers requires different treatments and processes. As such, more research works are required to improve the performance of natural fibers reinforced polymers. Environmentally friendly engineering materials are getting more attention recently. Therefore, polymer composites produced by bio-based polymers such as PLA [58], soy-based resin [63][64] etc., and reinforced with natural fibers have great potential because they are biodegradable and environmentally friendly.
The advantages and limitations of synthetic and natural fibers as reinforcing materials for FDM printed polymer are summarized in Table 1.
Table 1. The advantages and limitations for synthetic fibers and natural fibers as reinforced material for FDM printed polymer [18][19][65].

 

Type of Fibers

Advantages

Limitation

Synthetic fibers

Higher strength

Higher stiffness

Corrosion resistance

Flame retardancy

Chemical resistance

Commercially available

Higher cost

Not ‘green’

Natural fibers

Biocompatible, Biodegradable, renewable

Recyclable

Relatively cheap

Lower strength compared to synthetic fibers

Requires treatment of fibers (in general)

Fibers are discontinuous (in general)

Not all fibers are commercially available

 

3.2. Continuous vs. Discontinuous Fiber

Fiber reinforced polymer composite is a subcategory of fiber reinforced composites. Generally, fiber reinforcement can be categorized into discontinuous and continuous fibers according to critical fiber length [66]. Critical length lc is the fiber length that allows applied load transfer to the reinforced fibers by the matrix, and depends on fiber’s ultimate strength σf, fiber diameter d, and fiber-matrix bond strength or shear yield strength of the matrix τc. Continuous fibers are referred to fiber with length more than 15 lc, and discontinuous fibers are fibers with length less than 15 lc [66]. Nevertheless, some other researchers have slightly different definition. Krajangsawasdi et al. further classified short and discontinuous fiber, where short fibers are fibers shorter than critical length lc, and discontinuous fiber are those with length above critical length lc [16]. Pruß and Vietor defined discontinuous fibers as fibers with fiber length less than 1 mm (0.04 in.), while continuous fibers are fibers with a length above 50 mm (2 in.) [67].
Besides the obvious motivation of improving mechanical properties, reinforcement can also be used to provide the material with additional functions such as electro-conductivity, thermal conductivity or biocompatibility. Kalsoom et al. [68] and Wang et al. [69] have provided a general overview of 3D printable composites; this paper instead focuses in more detail on the engineering aspects of FDM as a composite manufacturing method.
Conventionally, fiber-reinforced composites can be classified into: (a) continuous and aligned fiber composites, (b) discontinuous and aligned-fiber composites, and (c) discontinuous and randomly oriented-fiber composites, depending on the length and alignment of the fibers [66]. The major advantages and disadvantages are listed in Table 2.
Table 2. Brief comparison of fiber reinforced composites, according to length and orientation of fiber [66][70].

 

Continuous and Aligned Fiber Composites

Discontinuous and Aligned-Fiber Composites

Discontinuous and Randomly Oriented-Fiber Composites

Properties of the composite are highly anisotropic

Properties of the composite are highly anisotropic

Composites are isotropic

Most effective strengthening but only along the designed direction; weaker along other directions

Less effective in strengthening than continuous and aligned fiber composites and only along the designed direction

Least effective in strengthening mechanical but all directions are strengthened

Limited manufacturing methods, hard to be manufactured, the highest cost

Difficult to maintain good alignment of discontinuous fiber during manufacturing; higher cost than discontinuous and randomly oriented-fiber composites

Easier to be manufactured, lowest cost

3.2.1. Continuous and Aligned Fiber Composites

The continuous and aligned fibers can reinforce composites in the intended direction but have no significant effect in the transverse direction. Conventional methods for producing continuous and aligned fiber composites are pultrusion, prepreg, and filament winding [66]. In terms of additive manufacturing, the FDM 3D printed ‘continuous and aligned fiber composites’ are being investigated by various researchers [38][39][40][44][46][53][56][71]. Previously, 3D printed continuous and aligned fiber composites were mostly printed using in-house developed or modified 3D printers [38][44]. The first commercial 3D printer capable of printing continuous and aligned fiber composites was developed by MarkForged. With the availability of commercial machines such as Markforged’s Markone, Marktwo 3D printers, research on FDM printing of continuous fiber reinforced thermoplastics (CFRT) composites is booming. Most of the recent research on FDM 3D printed continuous and aligned fiber composites uses Markforged’s Markone, Marktwo 3D printers [39][40][46][53][56][71]. Various types of continuous fibers, such as carbon fibers [38][39][40][44][46], glass fibers [39][53], and Kevlar fibers [39], have been used as reinforcement. In general, the FDM printed continuous and aligned fibers can have better electrical properties [56] and mechanical properties, such as tensile strength [39][40][44][46][53], flexural strength [44] if the printing parameters are properly selected. A systematic review of 3D printed continuous fiber polymer composites is presented by [72]. However, 3D printed continuous and aligned fiber composites are limited in terms of design freedom, as fiber placement is challenging and more voids are created, especially when printing complex shapes [19][46]. Design freedom is one of the main advantages of additive manufacturing over conventional manufacturing, and incorporating continuous fibers into FDM 3D printing, negates this advantage.

3.2.2. Discontinuous and Randomly Oriented-Fiber Composites

Discontinuous fiber composites have a long history, and the first scientific publication dates back to 1936 [36]. Due to the nature of reinforced fibers and conventional fabrication methods, such as hand lay-up, resin transfer molding, etc., early fiber reinforced composites are mainly discontinuous and randomly oriented. FDM 3D printed discontinuous fiber composites are manufactured using composite filaments by commercial FDM 3D printers. Generally, discontinuous fibers were premixed with the polymer matrix as composite filament, and the composite filaments were then used in FDM 3D printing to produce discontinuous fiber composites. To date, more than 10,000 published papers have been found in Scopus using the keywords “additive manufacture” and “short fiber reinforced polymers”, and it is not possible to discuss them all here. However, most of these research papers focused on the mechanical or thermal properties of the composites. They did not report on the orientation of the fibers in FDM 3D printed discontinuous fiber composites. Nevertheless, research with FDM 3D printed discontinuous and randomly oriented-fiber composites have been reported by several researchers [49], although not all of them emphasized the orientation of the fibers.
One of the recent works with FDM 3D printed discontinuous and randomly oriented-fiber composites was reported by Zhao et al. [49]. They compared the tensile properties of 3D printed CNT-short glass fiber (SGF) reinforced PLA composite with the tensile properties of 3D printed PLA, SGF/PLA, and found that both composites are better than neat PLA in terms of tensile strength and tensile modulus. In addition, CNT-SGF /PLA composite has a higher tensile strength than SGF/PLA composite. From the SEM images of the fracture surfaces of the composite specimens, they found that the fibers in the composites are randomly oriented. Su et al. reinforced polyamide with reclaimed carbon fiber in four different weight percentages (10%, 20%, 30%, 40%). They found that the fibers were better aligned at low fiber contents (10–20%) and had no significant alignment at 40%. They concluded that the tensile performance of the reclaimed carbon fiber reinforced polyamide composites (rCF/PA) largely depended on the fiber content and orientation, with higher fiber content and aligned fiber being able to improve tensile strength. All composites, including rCF/PA with 40 wt% and non-aligned fiber performed better than neat PLA [73].

3.2.3. Discontinuous and Aligned-Fiber Composites

Discontinuous and aligned fibers are an alternative to continuous fibers in 3D printing of polymer composites, with the advantage of better design freedom. Early research on discontinuous and aligned-fiber composites (also named as aligned discontinuous fiber thermoplastic) produced by non-additive manufacturing processes was summarized by Such et al. [74]. Although the manufacturing methods for 3D printed FDM 3D printed discontinuous and aligned-fiber composites are different from the conventional make discontinuous and aligned-fiber composites, the motivations for reinforcing polymers with discontinuous and aligned-fiber are similar. In general discontinuous and aligned-fiber are added to polymers for three main reasons: (1) to improve mechanical, thermal, or electrical properties in the desired direction, (2) to reduce the cost and complexity of manufacturing compared to composites with continuous fibers, and (3) enabling design freedom or complex geometries [20][36][74][75]. FDM 3D printed discontinuous and aligned-fiber composites are mainly manufactured using composite filaments by commercial FDM 3D printers. The discontinuous fibers were aligned by shear (referred to as shear-induced alignment or flow-induced alignment), where the shear force between a nozzle and the molten material forces the fibers to align in the direction of extrusion or flow [20][76]. Furthermore, the orientation of fibers is affected by experimental extrusion width, where experimental extrusion width depends on extrusion temperature, speed and width. Fibers were more aligned in a narrow extruder than in a wider extruder [77]. One of the first published papers on FDM 3D printed discontinuous, and aligned-fiber composites was by Tekinalp et al. [33]. They fabricated the carbon fiber reinforced ABS filament and used the filament with a commercial FDM 3D printer. They applied the method of Bay and Tucker [78] to characterize the fiber orientation in the printed part and found that the carbon fibers in the printed parts are mainly oriented in the load-bearing direction. They concluded that the carbon fibers could increase the strength and modulus of both the FDM printed and compression molded samples, but the FDM samples have significant voids [33].
 
 
Jia et al. fabricated graphite flakes reinforced PA6/POE-g-MAH/PS composite with an FDM 3D printer and verified by microscopy that the graphite flakes were aligned along the through-plane direction (parallel to the x-y plane) via microscopy. With this designed composite, they were able to improve the thermal conductivity of the polymer [79]. However, they also pointed out that the presence of voids in FDM- printed composites affects the through-plane thermal conductivity of the composites. Papon and Haque investigated fracture toughness of 3D printed carbon fiber reinforced PLA composites with different fiber content (3 wt.%, 5 wt.%, 7 wt.% and 10 wt.%), manufactured by two different nozzle shapes (circular and square) [80]. The square shape nozzle was custom-made to improve the contact area and inter-bead void. The fibers are mostly aligned in the extrusion direction, but they did not report how nozzle shape affects the fiber orientation. Their experimental results show that the fracture toughness increased with fiber content from 0% to 5%, at both layer orientation of 45°/−45° and 0°/90°. The print layer orientation of 45°/−45° and 0°/90° has no major different in fracture properties. Furthermore, parts printed by a square nozzle have better fracture toughness than parts printed by a circular nozzle because less void is produced in parts produced by the square nozzle.
Researchers at the University of Bristol developed a method named High Performance Discontinues Fiber (HiPerDIF) to manufacture discontinuous and aligned-fiber composites [81] and investigated the performance of composites produced with this method [75]. Generally, the fibers were suspended in a liquid medium (water), and the orientation of fiber was controlled by the orientation head [81]. With this method, they fabricated discontinuous and aligned-fiber epoxy composites using carbon fiber [81] and recycled carbon fibers [82]. They reported that the mechanical properties of composites are proportional to the fiber lengths [82]. To expand the HiPerDIF technology to additive manufacturing/FDM, Blok et al. have identified 4 different polymers (ABS, PLA, Nylon, PETG) as the potential polymer matrix materials to be reinforced with high performance discontinues and formed the feedstock materials for FDM. The four polymers were selected based on 14 factors. They fabricated the composite tapes using an in-house consolidation method, where the HiPerDiF fiber was sandwiched between two layers of polymer matrix films of 0.125 mm.
They proofed that aligned discontinuous fiber composites produced using HiPerDIF technology are better than currently available short fiber thermoplastic. Furthermore, the composite fabricated with HiPErDIF technology has comparable mechanical behavior compared with continuous fiber composite but with better manufacturing flexibility [47]. Krajangsawasdi et al. recently extended their work by fabricating 3D printer filament using ADFRC fiber to reinforce PLA thermoplastic. They managed to produce HiPerDiF-PLA filament and also identified the optimal printing parameters of their newly developed filament. They compared the mechanical properties of the HiPerDiF-PLA printed parts with PLA, PLA-short carbon fiber, PLA-continuous carbon fiber, and Markforged continuous carbon fiber [75], and they concluded that HiPerDiF-PLA outperformed other PLA composites in terms of mechanical performance.

This entry is adapted from the peer-reviewed paper 10.3390/polym14214659

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