Journal Club for January 2023: Design and 3D Printing of Continuous Fiber Composites: Status, Challenges, and Opportunities

Design and 3D Printing of Continuous Fiber Composites: Status, Challenges, and Opportunities

Kai Yu

Associate Professor, Mechanical Engineering, University of Colorado Denver

 

1. Introduction

Continuous fiber-reinforced polymer composites, with their superior combination of stiffness, strength, and lightweight, have been leading contenders in various applications ranging from aerospace to ground transportation. 

Conventional composite manufacturing methods usually have mutually exclusive degree of automation and design freedom (Figure 1a). They use expensive molding tools to shape the resin and fibers before and during curing. Mass production is required to even out the overhead cost of tooling, labor, and production. In addition, the geometry of the final products is usually limited to simple profiles. Due to the high cost of tooling, the barrier to creating complex composite parts or changing the design of existing ones is significant.

In contrast, 3D printing empowers exceptional design freedom in composite manufacturing and is ideal for rapid prototyping and product development. The cost-per-part remains the same even for small-quantity production (Figure 1b). It allows the user to design composites with tailored structural topology, fiber distribution, and orientation. Various functions can be readily incorporated into the composites. For example, metallic wires or fiber optics can be printed into the composites to introduce the capabilities of sensing, collecting data, powering electronics, and real-time health monitoring.

3D printing of continuous fiber composites is an emerging field that attracts more and more researchers in materials science, mechanics, computational design, system control, etc. According to Web of Science [1, 2], only three articles published in 2015 were relevant to the printing of continuous fiber composites. This number had grown substantially over the next eight years. In 2021, the number of articles published was 112, showing a 37-time increase.

This review briefly summarizes recent studies on the 3D printing methods of continuous fiber thermoplastics and thermoset composites, as well as their exciting design opportunities and potential applications.  

Figure 1. (a) Comparison of conventional composite manufacturing techniques and composite 3D printing. (b) An illustration on the average cost per part of composites fabricated by injection modeling and 3D printing. 

 

2. 3D Printing of Thermoplastic Composites with Continuous Fiber

Among a variety of polymer 3D printing techniques, extrusion-based printing methods show great promise in the fabrication of continuous-fiber reinforced composites. In 2016, Matsuzaki et al. [3] customized the a fused deposition modeling (FDM) printer head to fabricate continuous fiber composites based on the in-nozzle impregnation (Figure 2a). The polylactic acid (PLA) filament and the continuous carbon fiber were supplied separately to the printer head. When the nozzle was heated, the PLA was melted and fused to the fiber bundle. After the extrusion of the filament, the PLA matrix quickly solidified and adhered to the previous layer. Subsequent studies in this field used similar designs of the printer head to investigate composite printability and mechanical properties with different thermoplastic matrix and reinforcement fibers. 

For commercial printers, in 2016, Markforged Inc. (Watertown, MA) released the first 3D composite FDM desktop printer (Mark Two™, ~$20k) to print continuous fiber thermoplastic composites. The printer features two separate extrusion nozzles for plastic filaments and continuous fiber supply (Figure 2b). The continuous fiber is pre-impregnated with plastic so that it can stick to the previous layer during the extrusion. Unlike the single-nozzle system, the dual-nozzle design enables the individual printing layer to be selectively reinforced at different locations. The printer is able to print composites with different thermoplastic matrix (nylon or nylon with chopped carbon fiber) and reinforcement fibers (carbon fiber, kevlar, fiberglass). Other manufacturers of thermoplastic composite printers include Orbital Composites Inc. (San Jose, CA) and Arevo Inc. (Silicon Valley, CA).

In addition to the FDM printing method, directed energy deposition [4] and laminated object manufacturing [5] are also investigated to print continuous fiber thermoplastic composites. However, in terms of printing flexibility and accessibility, FDM printing methods are the most popular ones and are widely adopted by engineers and researchers for the rapid prototyping or examining the material-process-property relationships. The quality and mechanical performances of printed composites are shown to be affected by various material and machine factors (fiber and matrix, fiber content, nozzle size, etc.), process parameters (printing speed, nozzle temperature, infill pattern, etc.), and post-processing conditions (temperature, humidity, etc.). Interested readers are suggested to check the recent review articles [6-8].

Figure 2. Schematic representation of the FMD 3D printing process for continuous fiber composites with (a) a single nozzle [9] and (b) dual nozzles [10].

 

Despite the exciting developments and high accessibility of FDM printing methods, a major limitation is that they can only print thermoplastic composites, which melt and lose their load-bearing capabilities at high temperatures. The printed composites also exhibit notable void formation [11-14] and weak bonding strength [15-18] because the interfaces of the composite filaments are bonded primarily by non-covalent bonds. As shown in Figure 3a, the incorporation of continuous fiber dramatically improves the mechanical strength of printed composites compared to those with short fibers. However, the tensile strength is notably lower than that of conventional composites [19]. In the case of the single-nozzle FDM process, the fiber bundle is fed directly into the printer head with a thermoplastic matrix. Fiber impregnation is usually insufficient and requires a high nozzle temperature to reduce the viscosity of the molten polymer matrix (Figure 3b) [20].

Figure 3 (a) Tensile strength versus fiber volume ratio of composites manufactured using various conventional and AM techniques [19]. (b) Microstructures of fractured cross section of 3D printed carbon fiber/PLA composites. Top: nozzle temperature 180 °C. Bottom: nozzle temperature 240 °C. A higher printing temperature is required to improve fiber impregnation and fiber-matrix bonding strength [20].

 

3. 3D Printing of Thermosetting Composites with Continuous Fiber

The 3D printing of continuous fiber composites with thermosetting matrix is in the early stages of development. In 2015, Continuous Composite Inc. (Coeur d’Alene, ID) patented the design of a direct-ink-write (DIW) printer head for UV-curable thermoset composites with continuous fiber [21]. The printer head comprises a syringe for resin storage, a deposition nozzle, a feeder for continuous fiber, and a UV lamp for photopolymerization (Figure 4a). The fiber feeder connects to the syringe through a one-way check valve, which prevents the backflow of liquid resin into the feeder. The deposited composite filaments quickly solidify and stick to the stage upon UV irradiation, which provides a persistence force to hold the filament and pull the fiber when the printer head moves forward.

Most high-performance engineering composites use thermally curable thermosets as the matrix. It is reported that the epoxy, polyester, vinyl ester, and polyurethane share 94% of the current global composite resin market [22]. Printing with these thermosets with conitnuous fiber is challenging because the viscous resins cannot solidify after the filament extrusion to provide a persistence force. Recently, Fang et al. [23] and Ming et al. [24] demonstrated the 3D printing of epoxy composites with continuous carbon fiber. As shown in Figure 4b, the fiber bundle was first impregnated with the resin in a tank and then extruded through a squeezing nozzle at 130 ° C. Due to the high molecular weight of the epoxy resin, it remained in a nearly solid state at room temperature and thus can be printed in a process similar to that of FDM. Finally, the printed composite was subjected to vacuum heating to complete the curing of the epoxy matrix.

Figure 4. (a) Design of the DIW printer head for UV-curable thermoset composites with continuous fiber. Continuous Composite Inc. (Coeur d’Alene, ID). (b) Schematic illustration of the 3D printing process for continuous fiber composites with epoxy matrix.

 

Our research group at CU Denver has been working on the 3D printing of thermosetting composites since 2018. Our initial objective was to design a printing method that could be applied to a wide range of thermoset resins and commercial fibers. It was also expected to be very accessible to other researchers to design their applications or test the process-property relationships. After a few design iterations, we developed a DIW printer head as shown in Figure 5a [25]. To print continuous fibers with thermally curable resins (e.g., epoxy, polyimine), the monomers and cross-linkers are first partially cured as printable ink. The fiber bundles are fed into the syringe through a tube. Two deposition pressures are supplied to the syringe, one for ink deposition and the other one to prevent the ink backflow to the feeding tube. When deposition pressures are applied, the viscous flow applies shear stress to the fiber, which drives its flow through the nozzle. During printing, the nozzle moving speed is set to equal to the filament extrusion speed. After printing, the thermoset composites are transferred to an oven for post-curing. The designed printer head allows for a controllable fiber content (5-36%) by using a different needle (Figure 5b). Composite structures with different fiber content are printed without supporting materials (Figure 5c).

During the post-curing of printed composites, the interfacial polymerization among filaments leads to strong interfaces connected by covalent bonds, which substantially promote the mechanical strength of printed composites. The longitudinal modulus and transverse modulus of the printed composite lamina are shown to be notably higher than the values reported in some existing studies (Figure 5d) and close to the Rule of Mixture predictions. However, composites printed with a high fiber content (36% and above) exhibit weak mechanical performance (especially the transverse strength) compared to directly molded samples because there is not enough resin on the surface of the filaments to enable their tight and covalent bonding. How to improve the mechanical performances of printed composites at high fiber contents is still a fundamental challenge in the field and needs future research efforts.

A unique feature of the developed printing method is that the composite filaments are not subject to mechanical interferences after extrusion, which enables its free-standing 3D printing of UV-curable components. As shown in Figure 5e, an acrylate spring without and with a carbon fiber bundle can be printed without supporting materials. A lattice structure with 9% carbon fiber can be created, wherein member is printed at the designed positions and then manually cut after being welded at the joint.

Figure 5. (a) Design of the DIW deposition syringe for the printing of thermally curable composites. (b) Cross-sectional views of filaments with different fiber content. (c) 3D printed composite structures with different fiber content. (d) Longitudinal and transverse modulus of printed composites. Reference data points are collected in existing studies on composite 3D printing, including continuous carbon fiber thermoplastic composites printed with FDM (blue squared dots), short carbon fiber thermoplastic composites printed with FDM (red circular dots), and short carbon fiber thermoset composites printed with DIW (green triangular dots). (e) Free-standing 3D printed UV curable composites [25].

 

In our recent work, the designed DIW printer head is integrated with a six-axis robotic arm to dramatically promote design and manufacturing freedom. A UV-curable thermoset resin is used as a composite matrix. Composites with more than 50% continuous carbon fiber are successfully printed with a low void density and an excellent degree of fiber impregnation. The influences of different material and processing parameters are studied. Our major finding is that the nozzle diameter, ink viscosity, and nozzle moving speed are the major parameters that determine the fiber volume fraction in the printed composites. The maximum printable curvature of the composite filament depends on the fiber content and UV intensity.

In addition, we demonstrated the printing of composites on 3D curved surfaces. In our workflow, the first step is to mathematically calculate the coordinate of intended printing pathway (Figure 6a), either in directional parallel pattern or contour parallel pattern. A 3D scanner was incorporated into the system for the composite printing on a substrate with unknown profile. The second step is to translate the coordinate of the printing pathway into the coordinate of the robotic arm and generate the G-code for the robotic arm (Figure 6b). The major printing parameters, such as the direction of the DIW needle axis, the printing speed, and the UV intensity, are specified in this step. Figure 6c shows composites printed on 3D curved surfaces, in which the filament pattern and spacing of the filaments are precisely controlled. Because the motion of DIW printing is driven by the robotic arm, composite structures can be easily printed in large scale (Figure 6d). Our printing method was also shown to be applicable to other types of continuous fibers, such as the Kevlar fiber and conductive metallic wires.

 

Figure 6. (a) Determination of the XYZ coordinate of the intended printing pathway. If the substrate profile is unknown, a 3D scanner will first be used to obtain the substrate model. (b) Composite printing setup with a six-axis robotic arm. (c) Demonstrations of composite 3D printing on curved surfaces. (d) Large-scale composite 3D printing (unpublished results, non-provisional patent filled).

 

4. Design for Composite 3D Printing: Put the Right Materials at the Right Place

The computational design of composites optimally determines where to place materials in a 3D domain to achieve the desired mechanical performance and function. The integration between design and 3D printing would disrupt the conventional paradigm of composite product development, better integrate skill sets and trades in the value chain, and simultaneously increase efficiency and productivity while unlocking new capabilities and functions of contemporary composites.

4.1 Design with Innovative Materials

Materials play an important role during the entire process of engineering design. Material selection or material identification should be based on the objective of the design activities and consider the various manufacturing constraints. In recent years, with advances in manufacturing techniques, material metrics for composite 3D printing have been dramatically expanded, but fundamental challenges remain. The mechanical performances of printed composites are notably lower than those of convention composites. On the one hand, printing with the thermally curable resin can promote the bonding strength among filaments and printing layers, but it is challenging for current printing methods to fabricate composites with a high content of continuous fiber. UV-curable resins allow 3D printing of composites with a high fiber content, but the filament bonding strength is limited by the weak van der Waals forces. In addition, the printing may also suffer from the non-uniform curing of matrix materials as the carbon fiber significantly blocks the light penetration.

There is very little existing study on the design with materials for composite 3D printing. In this review, I would like to highlight two innovative thermosetting resins that can enrich design metrics and potentially tackle the ground challenges of composite 3D printing: two-stage UV-curable resin [26] and frontal polymerization resin [27]. A two-stage UV-curable resin can be cured rapidly upon UV irradiation (the first-stage polymerization). Subsequently, the materials will be subject to post-heating, wherein the covalent reactions on the chain backbone form a second interpenetrating network to dramatically increase the density and mechanical strength of the cross-linking of the material (second stage polymerization) (Figure 7a). This can be realized by thermal curing of unreacted chemical bonds or by leveraging recently emerged dynamic covalent reactions. The two-stage curable resin would also promote the bonding strength among printing filaments and printing layers, as the covalent reactions during the post heating leads to chain connection on the interface. For the frontal polymerization resin, the ink is initially in a high-viscosity fluid stage. After extrusion of a filament from a print head, the resin is immediately cured via frontal ring-opening metathesis polymerization of dicyclopentadiene (Figure 7b), allowing for simultaneous free-form printing and curing of thermoset polymers (Figure 7c). Both two innovative resins would enable rapid and homogeneous curing of matrix resin during composite 3D printing.

Figure 7. (a) Representation working mechanism for the two-stage UV curable resin. After thermal processing, the network has increased dramatically to increase the material stiffness and strength. (b) Scheme for the frontal polymerization using a ruthenium catalyst and an alkyl phosphite inhibitor. (c) 3D printing of frontal polymerization resin that solidifies immediately after extrusion [27].

 

4.2 Optimization Design of Printing Pathway

The composite filaments with continuous fiber exhibit transversely isotropic behaviors. Their laying pattern within the printed composites strongly affects the mechanical properties and other functions. Early studies examined the influences of simple pathway patterns from material extrusion, e.g., straight line [28], zigzag [29], contour [30], and honeycomb [31]. On the basis of parametric studies, the printing pathway was improved to minimize the frequency of fiber cutting or avoid sharp corners, which serve as defects and compromise the mechanical strength of printed composites.

Recent studies have integrated computational optimization design to determine the complex printing pathway that can maximize the mechanical performance of fabricated components. The main idea is to improve the local properties of composites by controlling the local fiber orientation and volume fraction. Optimization design can be performed as a one-time computation or based on iterative methods. Both methods begin by assigning all fibers within composites with a single orientation and identical fiber volume fraction. Then, the initial design is analyzed using numerical methods under given boundary and load conditions. Based on internal load transfer, a series of concepts have been developed to determine the optimized fiber trajectory, including fluid streamlines (Figure 8a) [32], maximum principal stress (Figure 8b) [33], pointing stress vector (Figure 8c) [34], and stiffness decay vector [35]. The one-shot method updates the fiber direction without considering the discrepancy between the updated and initial designs. The iterative method continues the process by repeatedly updating the analysis results and comparing the actual and expected performance until the optimization is converged. Interested readers are suggested to check the recent review article by Liu et al. [36]

Figure 8 Curvilinear fiber optimization using different methods. (a) Fluid streamlines [32], (b) principal stress [33], and (c) point stress vector [34].

 

4.3 Topology Optimization Design of Composites

The combination of topology optimization design and composite 3D printing would enable a tightly integrated digital design-manufacturing workflow to generate creative solutions of composite products with optimized performances and functions. The main challenge of the design activity is that the properties or functions depend on both macroscale structural topology and local material properties (e.g., fiber orientation, volume content), which should be fully considered and optimized simultaneously.

Early studies on the topology optimization of 3D printed composites separated the material and structure and created optimal structures from specified materials. For example, Li et al. [37] propose a two-step approach, in which the structure is optimized first using the solid isotropic material with the material penalization method and then filled with reinforced fibers that take into account the stress states.

To simultaneously design the composite structure and material distributions, numerical homogenization is first performed to bridge the micro- and macroscale material properties [38, 39]. Both the composite topology and the material microstructure are optimized within one formulation using iterative design algorisms. To improve computational efficiency, possible microstructure designs or material distributions can be pre-computed and used as material libraries [40, 41]. As a notable example, Boddeti et al. [42, 43] develop an integrated digital design-manufacturing workflow that allows the simultaneous design of macroscopic structural topology and microscopic fiber orientation of continuous fiber composites. The digital workflow consists of three steps (Figure 9a): (i) a design automation process that involves solving a multiscale optimization problem using finite element simulations to determine the optimal composite topology and its fiber orientation. (ii) a material compilation process to translate the mathematical description of the optimal macro and microstructure into a physically realizable 3D material layout and generate the machine code for printing, and (iii) a digital fabrication step where the composites are printed using a multimaterial photopolymer 3D printer. The workflow was validated by designing, fabricating, and testing a series of 2D cantilever beam and 3D components that illustrate capabilities.

The optimized fiber orientation can be determined using other approaches. For example, Safonov et al. [44] utilize a dynamic system method to find the density distribution together with a method to align the fiber direction with the direction of principal stress. Wang et al. [45] propose a load-dependent path planning method, in which the printing path follows the load transmission and the variable printing speed fits the geometrical characteristics (Figure 9b). Papapetrou et al. [46] introduce three new approaches for the design of fiber infill patterns, including the equally spaced method, the streamline method, and the offset method, to generate a continuous fiber path across the domain.

Figure 9 (a) Simultaneous multiscale design optimization to manufacture workflow adapted for continuous fiber composites [43]. (b) A load-dependent path planning method, in which the printing path follows the load transmission and the variable printing speed fits the geometrical characteristics [45].

 

5. Applications of Composite 3D Printing beyond Mechanical Properties Enhancement

In addition to the enhancement of mechanical properties, the 3D printing of continuous fiber composites endows other functions to the printed material structures. Existing proof-of-concept demonstrations primarily utilize the unique thermal and electrical properties of embedded continuous fibers.

Shape changing and 4D printing: Shape memory polymers (SMPs) are an intensively studied smart material that can recover their programmed shapes when temperatures are above their phase transition temperatures. When composites are printed with an SMP matrix, the conductive Joule heating effect of continuous carbon fiber (or other conductive wires) enables the phase transition and shape changes of SMP composites. This a 4D printing technique was recently shown to enable the shape morphing of printed composite honeycomb structures [47]. Another shape-changing mechanism is based on the mismatching coefficients of thermal expansion between the fibers and the matrix. As shown in Figure 10a, continuous fiber filaments were printed on a flexible substrate. After changing the temperature, the structure morphed into different shapes depending on the pattern of the fiber. The angle between the intersecting fibers determines the size of the principal curvature, while the bisector of the angle of the fibers determines the direction of the principal curvature [48]. Shape-changing patterns strongly depend on the fiber printing pathway and can be incorporated with the optimization design to achieve deformations of arbitrary surfaces.

Sensing and Self-monitoring: The electromechanical properties of continuous fibers enable the sensing or self-monitoring capabilities of 3D printing composites. This function is primarily realized by measuring and analyzing the changes in electrical conductivity of printed composites under external loading. Recent studies include strain sensors with continuous copper and nichrome wires [49], smart honeycomb structure [50] and truss lattice structures [51] with continuous carbon fiber for self-sensing of strain, stress, and damage (Figure 10b), continuous carbon fiber composites applied to an artificial self-monitoring hand (Figure 10c) [52].

Energy Storage: Continuous fiber composites can be 3D printed as a structural battery and used for energy storage. A representative recent study is shown in Figure 10d [53], Within the structural battery composites, the carbon fiber reinforcement acts as anode and current collector, the doped polymer matrix with high electrical and ionic conductivity acts as cathode, and the solid polymer electrolyte coated with the carbon fiber is used as an electrolyte and separator. Both photopolymer and PLA were reported as polymer matrix materials. The studies demonstrated that the introduced polymer-coated carbon fibers not only enable energy storage, but also improve mechanical performance.

Figure 10. (a) Shape-morphing surfaces with continuous carbon fiber composites. Different configurations are realized by controlling the temperature [48]. (b) 3D printed self-monitoring lattice truss cells with continuous carbon fiber [51]. (c) Conception of 3D printed continuous carbon fiber composites applied to an artificial self-monitoring hand [52]. (d) Schematic of the 3D structural battery composite architecture fabricated by UV-assisted coextrusion [53].

 

6. Conclusions, Challenges, and Opportunities

As an emerging advanced manufacturing technique, 3D printing of continuous fiber composites allows composite products to be customized quickly and inexpensively to meet unique specifications and provide new functionalities. In the past eight years, exciting progress has been made in materials innovation, manufacturing technique, computation, and optimization design. These advances have substantially promoted the material choices, quality, and mechanical performances of printed composite parts. There is no doubt that the composite 3D printing market will continue to grow, and its potential applications would affect key industries such as aerospace, electronics, biomedical, healthcare, and defense.

Despite exciting progress, fundamental challenges remain in the field. Currently, the mechanical properties of 3D printed composites cannot match those of conventional composites. Major limiting factors include the numerous interfaces between printed filaments and layers, the notable internal void identity, and the unsatisfied fiber impregnation and fiber straightness. Additionally, the highest printable fiber content is much lower than those of conventional manufacturing methods. To date, printing composites with more than 50% continuous fiber is challenging. Tackling these grand challenges requires interdisciplinary collaborations in materials, manufacturing processes, and computational designs. For example, surface treatment of continuous fibers is one of the suggested methods to improve the adhesion between the fiber and the polymer matrix. Using the resins with fast and homogeneous curing mechanisms, or the ones with intrinsic healing mechanisms, seems to be good choices to print high-quality composite samples with uniform matrix curing and strong inter-filament bonding. Hybrid additive manufacturing that incorporates composite 3D printing with other advanced manufacturing techniques is a promising approach to further improve the quality, surface finishing, and reduce the void density of the fabricated composites.

Like most other extrusion-based 3D printing techniques, composite 3D printing is still limited by its slow manufacturing speed because the printing is essentially a line-by-line writing process. Low productivity is one of the key technological barriers to adopting them for commercialization. Developing techniques that can synchronize multiple robots for faster processing using scalable materials can expand manufacturing scale and enhance productivity. The incorporation of robots would also provide smart assistance for the 3D printing of continuous fiber composites. For example, the research team led by Dr. Satyandra Gupta at the University of Southern California is making advances in physics-informed artificial intelligence to enable robots to exhibit smart behaviors during additive manufacturing, such as automatic generation of optimal trajectories in real-time and the self-supervised learning from observing the performance of previously executed tasks.

Computational design plays a critical role in the development of the composite printing process. For the 3D printing of composites, their mechanical performances are determined by various material, process parameters, and manufacturing constraints, such as the matrix stiffness, printing speed, fiber content, temperature or UV intensity etc. These parameters affect the interfacial defects between fiber and the matrix, the internal voids, and the straightness of the fiber, thus determining the final stiffness and strength. Existing studies focused on the optimization of fiber layout and composite topologies with the assumptions of no interfacial defects and perfect interfacial bonding. There is no design framework that incorporates the various material and manufacturing influences. In future studies, a product-process codesign methodology is in high demand for 3D composite printing that simultaneously optimizes the composite topology, fiber placement, and printing parameters to maximize the mechanical performances of continuous fiber composites or design their various functions.

 

Key References

1.           Cheng, P., et al., 3D printed continuous fiber reinforced composite lightweight structures: A review and outlook. Composites Part B: Engineering, 2023. 250: p. 110450.

2.           Kabir, S.M.F., K. Mathur, and A.-F.M. Seyam, A critical review on 3D printed continuous fiber-reinforced composites: History, mechanism, materials and properties. Composite Structures, 2020. 232: p. 111476.

3.           Matsuzaki, R., et al., Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation. Scientific Reports, 2016. 6: p. 23058.

4.           Janssen, H., T. Peters, and C. Brecher, Efficient Production of Tailored Structural Thermoplastic Composite Parts by Combining Tape Placement and 3d Printing. Procedia CIRP, 2017. 66: p. 91-95.

5.           Parandoush, P., C. Zhou, and D. Lin, 3D Printing of Ultrahigh Strength Continuous Carbon Fiber Composites. Advanced Engineering Materials, 2019. 21(2): p. 1800622.

6.           Zhang, H., et al., Recent progress of 3D printed continuous fiber reinforced polymer composites based on fused deposition modeling: a review. Journal of Materials Science, 2021. 56(23): p. 12999-13022.

7.           Safari, F., A. Kami, and V. Abedini, 3D printing of continuous fiber reinforced composites: A review of the processing, pre-and post-processing effects on mechanical properties. Polymers and Polymer Composites, 2022. 30: p. 09673911221098734.

8.           Zhao, H., et al. An Overview of Research on FDM 3D Printing Process of Continuous Fiber Reinforced Composites. in Journal of Physics: Conference Series. 2019. IOP Publishing.

9.           Yang, C., et al., 3D printing for continuous fiber reinforced thermoplastic composites: mechanism and performance. Rapid Prototyping Journal, 2017. 23(1): p. 209-215.

10.         Mei, H., et al., Influence of mixed isotropic fiber angles and hot press on the mechanical properties of 3D printed composites. Additive Manufacturing, 2019. 27: p. 150-158.

11.         Wang, X., et al., 3D printing of polymer matrix composites: A review and prospective. Composites Part B-Engineering, 2017. 110: p. 442-458.

12.         M, N., et al., 3D printing of continuous fiber reinforced plastic. Proc Soc Adv Mater Process Eng, 2014.

13.         J, W., et al., A novel approach to improve mechanical properties of parts fabricated by fused deposition modeling. Materials and Design, 2016. 105: p. 152-159.

14.         HL, T., et al., Highly oriented carbon fiber–polymer composites via additive manufacturing. Composite Science and Technology, 2014. 105: p. 144-150.

15.         Zou, R., et al., Isotropic and anisotropic elasticity and yielding of 3D printed material. Composites Part B-Engineering, 2016. 99: p. 506-513.

16.         Ahn, S.H., et al., Anisotropic material properties of fused deposition modeling ABS. Rapid Prototyping Journal, 2002. 8(4): p. 248-257.

17.         Sun, Q., et al., Effect of processing conditions on the bonding quality of FDM polymer filaments. Rapid Prototyping Journal, 2008. 14(2): p. 72-80.

18.         Gibson, I., D. Rosen, and B. Stucker, Additive manufacturing technologies: 3D printing, rapid prototyping, and direct digital manufacturing. 2015: Springer; 2nd ed. . ISBN 978-1-4939-2113-3.

19.         Goh, G.D., et al., Recent Progress in Additive Manufacturing of Fiber Reinforced Polymer Composite. Advanced Materials Technologies, 2019. 4(1): p. 1800271.

20.         Tian, X.Y., et al., Interface and performance of 3D printed continuous carbon fiber reinforced PLA composites. Composites Part a-Applied Science and Manufacturing, 2016. 88: p. 198-205.

21.         Tyler, K., Method and apparatus for continuous composite three-dimensional printing. 2012: USA.

22.         Mazumdar, S., Opportunities for Thermoset Resins in the Composites Industry, in Lucintel Report. 2008: Dallas, USA.

23.         Hao, W.F., et al., Preparation and characterization of 3D printed continuous carbon fiber reinforced thermosetting composites. Polymer Testing, 2018. 65: p. 29-34.

24.         Ming, Y.K., et al., Investigation on process parameters of 3D printed continuous carbon fiber-reinforced thermosetting epoxy composites. Additive Manufacturing, 2020. 33.

25.         He, X., et al., 3D printing of continuous fiber-reinforced thermoset composites. Additive Manufacturing, 2021. 40: p. 101921.

26.         Kuang, X., et al., High-Speed 3D Printing of High-Performance Thermosetting Polymers via Two-Stage Curing. Macromolecular Rapid Communications, 2018. 39(7): p. 1700809.

27.         Robertson, I.D., et al., Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization. Nature, 2018. 557(7704): p. 223-+.

28.         Heidari-Rarani, M., M. Rafiee-Afarani, and A.M. Zahedi, Mechanical characterization of FDM 3D printing of continuous carbon fiber reinforced PLA composites. Composites Part B: Engineering, 2019. 175: p. 107147.

29.         Dutra, T.A., et al., Mechanical characterization and asymptotic homogenization of 3D-printed continuous carbon fiber-reinforced thermoplastic. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 2019. 41(3): p. 133.

30.         Li, N., Y. Li, and S. Liu, Rapid prototyping of continuous carbon fiber reinforced polylactic acid composites by 3D printing. Journal of Materials Processing Technology, 2016. 238: p. 218-225.

31.         Quan, C., et al., 3d printed continuous fiber reinforced composite auxetic honeycomb structures. Composites Part B: Engineering, 2020. 187: p. 107858.

32.         Yamanaka, Y., et al., Fiber Line Optimization in Single Ply for 3D Printed Composites. . Open Journal of Composite Materials, 2016. 6: p. 121-131.

33.         Sugiyama, K., et al., 3D printing of optimized composites with variable fiber volume fraction and stiffness using continuous fiber. Composites Science and Technology, 2020. 186: p. 107905.

34.         Suzuki, T., S. Fukushige, and M. Tsunori, Load path visualization and fiber trajectory optimization for additive manufacturing of composites. Additive Manufacturing, 2020. 31: p. 100942.

35.         Zhao, S., N. Wu, and Q. Wang, Load path-guided fiber trajectory in composite panels: A comparative study and a novel combined method. Composite Structures, 2021. 263: p. 113689.

36.         Liu, G., Y. Xiong, and L. Zhou, Additive manufacturing of continuous fiber reinforced polymer composites: Design opportunities and novel applications. Composites Communications, 2021. 27: p. 100907.

37.         Li, N., et al., Path-designed 3D printing for topological optimized continuous carbon fibre reinforced composite structures. Composites Part B: Engineering, 2020. 182: p. 107612.

38.         Xia, L. and P. Breitkopf, Concurrent topology optimization design of material and structure within FE2 nonlinear multiscale analysis framework. Computer Methods in Applied Mechanics and Engineering, 2014. 278: p. 524-542.

39.         Sivapuram, R., P.D. Dunning, and H.A. Kim, Simultaneous material and structural optimization by multiscale topology optimization. Structural and Multidisciplinary Optimization, 2016. 54(5): p. 1267-1281.

40.         Xia, L. and P. Breitkopf, Multiscale structural topology optimization with an approximate constitutive model for local material microstructure. Computer Methods in Applied Mechanics and Engineering, 2015. 286: p. 147-167.

41.         Zhu, B., et al., Two-Scale Topology Optimization with Microstructures. Acm Transactions on Graphics, 2017. 36(5).

42.         Boddeti, N., et al., Simultaneous Digital Design and Additive Manufacture of Structures and Materials. Scientific Reports, 2018. 8.

43.         Boddeti, N., et al., Optimal design and manufacture of variable stiffness laminated continuous fiber reinforced composites. Scientific Reports, 2020. 10(1): p. 16507.

44.         Safonov, A.A., 3D topology optimization of continuous fiber-reinforced structures via natural evolution method. Composite Structures, 2019. 215: p. 289-297.

45.         Wang, T., et al., Load-dependent path planning method for 3D printing of continuous fiber reinforced plastics. Composites Part A: Applied Science and Manufacturing, 2021. 140: p. 106181.

46.         Papapetrou, V.S., C. Patel, and A.Y. Tamijani, Stiffness-based optimization framework for the topology and fiber paths of continuous fiber composites. Composites Part B: Engineering, 2020. 183: p. 107681.

47.         Cheng, Y., et al., 3D printed recoverable honeycomb composites reinforced by continuous carbon fibers. Composite Structures, 2021. 268: p. 113974.

48.         Wang, Q., et al., Programmable morphing composites with embedded continuous fibers by 4D printing. Materials & Design, 2018. 155: p. 404-413.

49.         Saleh, M.A., R. Kempers, and G.W. Melenka, 3D printed continuous wire polymer composites strain sensors for structural health monitoring. Smart Materials and Structures, 2019. 28(10): p. 105041.

50.         Ye, W., et al., Self-sensing properties of 3D printed continuous carbon fiber-reinforced PLA/TPU honeycomb structures during cyclic compression. Materials Letters, 2022. 317: p. 132077.

51.         Wang, Z., et al., Mechanical and self-monitoring behaviors of 3D printing smart continuous carbon fiber-thermoplastic lattice truss sandwich structure. Composites Part B: Engineering, 2019. 176: p. 107215.

52.         Yao, X., et al., Evaluation of carbon fiber-embedded 3D printed structures for strengthening and structural-health monitoring. Materials & Design, 2017. 114: p. 424-432.

53.         Thakur, A. and X. Dong, Printing with 3D continuous carbon fiber multifunctional composites via UV-assisted coextrusion deposition. Manufacturing Letters, 2020. 24: p. 1-5.