A critical review on the fused deposition modeling of thermoplastic polymer composites
Introduction
Fused Deposition Modeling (FDM), a layer-wise 3D printing technology, has been developed by Stratasys © for fabricating complex geometrical parts [1]. Among different additive manufacturing processes, FDM became more popular due to low cost and flexibility to use different materials. Over the years, this process has been used for making products from a wide range of materials such as plastics, metal powder, ceramics and composites [2,3] for aerospace [4], medical [5], mold design [6] and automobile applications [7]. In this process, filament material is extruded through a heated nozzle and deposited layer-by-layer in the semi-solid state on a partially constructed part. As shown in Fig. 1, the process consists of a build platform, print bed, liquefier head and build material spool. First, part model's STL file format that has to be fabricated is created in the geometry creation software. Then, it is imported to the software in which it is sliced into thin two-dimensional layers. The tool path motion is generated using this two-dimensional contour information. A 3-axis system controls the movement of the liquefier head. It moves in the X–Y plane as per the tool path made by the software and deposits the first layer. After completing one layer, the head moves downward in the z-direction by an amount of set layer thickness. The newly added layer fuses with the already deposited layer and forms a bond. Successive layers of the material deposition will occur until the entire part finishes. After the completion, the structure can be taken out from the print bed manually or chemically removing the support structure [8]. According to the ASTM F42 (Additive Manufacturing Technologies) terminology, this process could also be termed as Fused Filament Fabrication (FFF) or material extrusion additive manufacturing (AM).
Initially, FDM printed parts have been used for low load prototypes such as domestic toy applications. With technological advancements, the process is extended nowadays to print parts for real time industrial applications. Large scale geometries of several meters of length and width are recently printed at very high material output rates of more than 200 kg/h [9]. The Big Area Additive Manufacturing (BAAM) system developed at Oak Ridge National Laboratory (ORNL) in collaboration with Cincinnati Incorporated© can print large scale geometries with dimensions of several meters. The design principle of BAAM is similar to FFF. The nozzle size and diameter are much bigger than lab scale printers. Also, polymer pellets are fed to the extruder feeding system, instead of filaments. The extruder is controlled by gantry to print different sizes. Similarly, Thermwood© developed Large Scale Additive Manufacturing System (LSAM). This system contains CNC machine in addition to FFF printer, to machine the printed parts. Readers are suggested to refer company websites to see illustrations of these big scale machines and printed parts [10,11]. The research group in Spain also developed large format Polymeric Pellet-Based Additive Manufacturing (PPBAM) system for printing different polymeric parts for Naval applications [13]. Continuous research is going on to print parts by FFF for more industrial applications. Composite Additive Manufacturing Research Institute (CAMRI) was developed at Purdue university to print different high performance polymer composites for tooling applications [12].
The quality of the printed parts depends on various underlying physical phenomena during printing, which are highlighted in Fig. 1. The part integrity and properties depend highly on bonding phenomena and bond quality. The bond formation between two layers includes surface contacting, neck growth and molecular diffusion [[14], [15], [16]]. This bonding phenomena occur between the adjacent filaments in a layer (intra-layer) and among the successive layers (inter-layer). Gurrala et al. demonstrated that during the bonding process, the total time available for solidification is less and partial neck growth and coalescence occurs resulting in the void formation between layers [17]. Due to the formation of voids, the strength of the FDM parts is quite low when compared to the parts made by other processes such as injection molding. Also, the difference in temperature profiles between adjacent layers during the solidification cause shrinkage, residual stress and distortion of the printed part. The built chamber temperature is a critical factor in FDM process. The difference between the filament temperature and the built chamber temperature causes additional thermal stresses and warpage. Often, the built chamber is also heated to a high temperature to minimize thermal stress.
To improve the properties of the FDM printed parts, literature works focused on developing composite material systems by reinforcing different fillers to the base polymer and printing of these composites [18,19]. Parts made using these composites found to exhibit higher mechanical, thermal and electrical properties compared to the unreinforced polymer printed parts. Driven by new applications and challenges, the FDM technology keeps growing in producing components made of new composite material systems [20]. The properties of composite materials are highly sensitive to the reinforcement type, i.e., particle, short fiber or continuous fiber. Also, the reinforcement size, shape and distribution affect the properties. Generally, in the case of nano particles, more than 8–10 wt% may decrease the properties due to agglomerates formation. Short fiber composites show a decrement in properties at loadings higher than 30 wt%. During the FDM printing of short fiber or particle reinforced composites, the orientation of fibers in the extruding nozzle and in the print layer depends on the flow field in the liquefier and in the nozzle [21]. The reinforcement size alters the melt viscosity, which effects the flow field. The nozzle may act as a filter for higher particle sizes, which causes the nozzle blockage after few prints. Few thermal and electrical properties of 3D printed parts using short-fiber composites have been improved [22]. However, the mechanical performance of printed parts with particle or short fiber composites are still inferior to the parts made using other short fiber composite processing techniques such as compression molding [23].
The mechanical performance of continuous fiber reinforced composites is higher compared to short fiber composites. Few works focused on the 3D printing of continuous fiber reinforced composites. Matsuzaki et al. proposed a new method for the first time for printing continuous fiber composites using an in-nozzle impregnation technique. In this, the thermoplastic resin and preheated reinforcing fibers are supplied from two separate rollers and are impregnated in the heated nozzle [24]. The heated resin inside the nozzle consolidates the fibers. These impregnated fibers are extruded from the nozzle, as shown in Fig. 2a. These modifications to the nozzle system offers the advantage of printing different fiber and resin combinations. In this work, continuous fiber reinforced composites have been printed by reinforcing carbon fibers in polylactic acid (PLA). The modulus and strength of the 3D printed continuous carbon fiber reinforced composites is higher compared to the conventional 3D printed composites. Few other works have been presented to print continuous fiber reinforced composites by modifying traditional FDM printers [25,26]. Printing through this in-nozzle impregnation technique has less control over the fiber volume fraction in the printed materials. The properties of continuous fiber reinforced composites can be controlled by volume fraction of fibers.
Few works reported about the printing of continuous fiber reinforced composites using two separate print heads. The matrix material (in the filament form) has been printed in one stage and reinforcement material in another stage [19]. This process schematic is shown in Fig. 2b. Initially, the fiber bundle will be printed with concentric or isotropic pattern, and then the matrix will be printed in the gaps before the next layer of fiber printing. As can be seen from Fig. 2c, in the concentric pattern, fiber printing starts at the outer edge and wraps towards the center and the isotropic pattern consists of parallel lines. It should be noted that the fiber should be stiff enough to print in concentric and isotropic patterns. Markforged© has further developed and commercialized these printers with separate supply spools for both continuous fiber and thermoplastic filaments [27]. However, these printers are mostly limited to print only nylon as matrix material and printing using other thermoplastic material is difficult. To improve the adhesion of the reinforcement to the nylon matrix, the reinforcing fiber filaments are also coated with a nylon material. In general, the design of printing using two separate heads offers advantages of selecting and customizing different combinations of fibers and polymers. In addition to that, the fiber volume fraction in the individual deposited layer can be controlled.
There are still some challenges that need to be resolved in FDM of composite materials. The critical requirement for any material to be used in FDM is that it can be processed into a feedstock filament. Most of the commercial printers require a filament of 1.75 mm diameter as the feedstock material. Making this filament of constant cross sectional diameter is quite challenging, especially for the particle or short fiber reinforcements. The composite material that needs to be printed should possess sufficient melt viscosity, strength, modulus and ductility. In the liquefier head, the filament forces the upcoming material out of the nozzle. If the filament has high viscosity and low stiffness, filament buckling will occur. It is desired to have a less viscosity and high strength for the filament. The flow of reinforcements in the nozzle and the pressure drop required for extruding the material through the nozzle highly depend on the melt viscosity. Venataraman et al. proposed an index for printability as the ratio of the elastic modulus to the melt viscosity. It is reported that if this ratio is higher than a critical value (3-5 × 105) will not cause buckling [28]. The addition of fillers tends to increase the viscosity by several orders. Often, to reduce the viscosity with the addition of reinforcments, other additives such as surfactants and plasticizers need to be added, making the process even more challenging. The addition of fillers cause the agglomerate formation, blockage of printer heads and improper adhesion during printing.
The final properties of the printed parts are difficult to control because several process parameters effects the process [29,30]. As shown in Fig. 3, these parameters can be categorized as.
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Polymer type: Amorphous or semi-crystalline nature of the polymer and their crystallization temperatures. (The FDM process is highly suitable for printing amorphous polymers because they quickly solidify with less degree of shrinkage. This behavior of the printed layer is essential to stick to the upcoming layer. The solidification of semi-crystalline polymers may take longer time depending the degree of crystallinity and the cooling rate. The crystalline nature also causes a high degree of shrinkage and part distortion.)
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Filler type: Discontinuous (Particles, short fibers) and continuous fibers
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Filler morphology: Size and shape of discontinuous reinforcement
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Build Orientation: It refers to the inclination of the part in a build platform with respect to X, Y, Z axis. X and Y-axis are considered parallel to build platform and Z-axis is along the direction of part build.
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Raster angle: Direction of the raster relative to the X-axis of build table.
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Layer thickness: It refers to the thickness of the deposited layer.
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Nozzle diameter: It depends upon the type of nozzle used. Commercial printers mostly use 0.4 mm nozzle diameter. During the printing of short fiber or particle reinforced composites, the agglomeration of reinforcements yields nozzle blockage. For composite material printing, generally high nozzle diameters are recommended.
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Raster width: Width of raster pattern used to fill interior regions of the part.
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Number of contours: The number of contours of the part outside
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Raster to raster gap (air gap): It is the gap between two adjacent rasters in a same layer. Negative air gap refers to the overlap of rasters. Positive air gap allows space between rasters. Printing with zero air gap is highly recommended.
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Infill density: The amount of material that is used to build the part inside. For example; the inner layers of the part can be printed in hexagonal or rectangular pattern.
Few review papers are available in the literature, which gives an overview of 3D printing of composite materials. These reviews briefly explain about different printing techniques such as Selective Laser Sintering (SLS), Stereolithography (SLA), Ink jet printing etc. [20,[31], [32], [33], [34], [35], [36], [37]]. Among these techniques, FDM is a simple and cost effective process. Therefore, most research works focused on expanding the capability of FDM to print different composite material systems [[38], [39], [40], [41]]. The available reviews on FDM of composites concentrated only on few aspects such as FDM of carbon fiber reinforcements [39], natural fiber reinforcements [40], PLA composites [41] and composite printing for tooling applications [9]. Given the importance of FDM, a thorough review of the past work is needed for different composite materials ranging from amorphous to crystalline and intermediate to high temperature polymers. The present paper gives an in depth analysis of the FDM of acrylonitrile-butadiene-styrene (ABS), polylactic acid (PLA), polyamide, polypropylene (PP), polyethylene (PE), polyethylene terephthalate glycol (PETG), polyether-ether-ketone (PEEK), polyetherimide (PEI), polyphenylenesulfide (PPS), polycarbonate (PC), polycaprolactone (PCL) and thermoplastic elastomers (TPE). The effect of all types of reinforcements ranging from powders to continuous fibers is discussed. Finally, this review explains the application of FDM printed composite parts in biomedical, electrical, functional and aerospace applications. The use of mathematical modeling for simulating the printing process and models for estimating the mechanical properties of the composite printed parts is also highlighted. The present paper may serve as a primary reference for young researchers.
Section snippets
FDM of thermoplastic composites
Major issues during the FDM printing are the presence of void content in the microstructure, dimensional inaccuracy and the anisotropy of the printed parts. They arise due to the weak intra layer bonding between beads in a single layer and weak inter layer bonding between different layers in the thickness direction. Therefore, the strength of printed parts is limited. All research work in the area of layer wise additive manufacturing mainly focus on overcoming one or two of these problems. Even
Biomedical applications
The FDM technology is rapidly used in tissue engineering and in developing patient-specific implants for prosthetics and bones. The complete 3D information and microarchitecture of tissues and organs obtained from Computed Tomography (CT) techniques in the form of image data can be used to process tissues by 3D printing (refer Fig. 17) [122]. The materials that are to be used to process FDM feedstock should possess biocompatibility, along with required mechanical properties [123]. The internal
Mathematical modeling
Mathematical models provide useful information to understand the effect of different physical phenomena on the properties of printed parts [159]. Several key inputs can be obtained through these models to optimize the printing process to get quality products [160]. These mathematical models are categorized in this work as the models for simulating the printing process itself and models for estimating the mechanical behavior of printed parts. Numerical techniques such as Finite Difference (FD)
Summary, challenges and future scope
This paper presents an overview of the fused deposition modeling of thermoplastic composite materials. Various physical phenomena during printing that contribute to the strength of the printed parts have been explained clearly. The processing of composite filaments and the microstructural characteristics of the printed parts has been explained for a range of polymers; low strength amorphous polymers to high strength semi-crystalline polymers. Applications of the composite printed parts in
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the funding received from Science and Engineering Research Board (SERB), Department of Science and Technology (DST), India through the project: ECR/2018/002281.
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