Overview
Fused Deposition Modeling (FDM) of continuous fiber-reinforced composites represents a significant advancement in additive manufacturing, enabling the production of lightweight, high-strength components without the tooling requirements of traditional composite manufacturing. Different FDM mechanisms have been developed to fabricate these composites, each offering distinct advantages for specific applications. The two primary mechanisms are in-situ fusion (single nozzle impregnation) and dual extruder/ex-situ prepreg (two-nozzle systems using pre-impregnated fibers). Modified mechanisms such as 3D compaction printing have also been introduced to address the porosity and bonding limitations inherent in earlier approaches.
The choice of fabrication mechanism fundamentally affects the quality, cost, and complexity of the final composite part. This means that selecting the right mechanism requires balancing multiple factors. For example, in-situ fusion offers simplicity and lower equipment cost but typically produces parts with higher void content (5-15%) and weaker interlayer bonding, because the fiber has limited time to be wetted by the polymer in the nozzle. In contrast, dual-extruder systems using prepreg filaments achieve better mechanical properties through optimized fiber-matrix interfaces, specifically because the impregnation occurs before printing in controlled conditions. Each mechanism represents a different trade-off between process simplicity and part quality, with ongoing research aimed at combining the best features of multiple approaches.
Research on FDM mechanisms is primarily published in journals including Composites Part A, Composites Part B, Additive Manufacturing, and Polymers. Key contributions have come from institutions including the University of Tehran, Tokyo University of Science, Xi'an Jiaotong University, and commercial entities like MarkForged and Anisoprint. Understanding these mechanisms is essential for selecting the appropriate approach for a given application, whether prioritizing cost-effectiveness for prototyping or maximum mechanical performance for structural components.
See also: Fiber Types | Matrix Materials | Top Journals | Research Teams
Historical Evolution
The development of continuous fiber FDM has progressed through several generations, each addressing limitations of earlier approaches:
- 2014-2016: Pioneering work by Matsuzaki et al. demonstrated in-nozzle impregnation, proving the concept of combining dry fibers with thermoplastic matrix during printing. This foundational research opened the field but revealed challenges with fiber-matrix adhesion and void content.
- 2016-2018: Commercial systems from MarkForged introduced dual-extruder approaches using pre-impregnated fiber filaments, offering improved quality at higher cost. Academic researchers extensively characterized these commercial systems.
- 2019-present: Modified mechanisms like 3D compaction printing emerged to address porosity and bonding issues, while researchers at multiple institutions developed enhanced in-situ approaches with improved fiber control.
Today, the field continues to evolve with hybrid approaches combining elements of multiple mechanisms, as well as integration with other manufacturing technologies like thermoforming and post-processing consolidation.
1. In-Situ Fusion Mechanism
The in-situ fusion mechanism, also known as "nozzle impregnation" or "co-extrusion," combines dry reinforcing fibers with molten thermoplastic matrix within the print nozzle during the deposition process. This approach was pioneered by Matsuzaki et al. (2016) at Tokyo University of Science, who demonstrated that continuous carbon and jute fibers could be impregnated with PLA during FDM printing. The mechanism uses two input materials—the reinforcement (dry fiber feedstock) and the neat polymer matrix—combined during printing through a single nozzle.
The primary advantage of in-situ fusion is its relative simplicity: it requires only a modified single-nozzle extruder rather than specialized equipment. This makes it accessible for research laboratories and allows flexibility in material combinations. However, the short dwell time in the nozzle limits polymer infiltration into the fiber bundle, resulting in higher void content (typically 5-15%) compared to prepreg approaches. Research published in Scientific Reports, Rapid Prototyping Journal, and Composites Part A has extensively characterized these limitations and proposed various solutions.
Process Description
- Reinforcing fiber is drawn into the nozzle and preheated
- Matrix polymer is fed into the melt zone via a motor-driven hobbed gear
- Melted polymer and preheated fiber combine under pressure in the melt zone
- Combined material is deposited layer-by-layer
Advantages and Disadvantages
Advantages
- User control over thermoplastic flow rate
- Single-step manufacturing
- Lower equipment cost
- Adjustable fiber content
Disadvantages
- Poor bonding between layers due to short dwell time
- Inadequate polymer infusion into fiber bundles
- Increased porosity and weaker mechanical properties
- Subpar fiber-matrix interface
Published Results
| Matrix | Fiber | Volume % | Results | Reference |
|---|---|---|---|---|
| ABS | Carbon | 1.6% | Enhanced tensile and fatigue strength with thermal bonding | Nakagawa et al., 2017 |
| PLA | Carbon | 6.6% | Developed in-nozzle impregnation method | Matsuzaki et al., 2016 |
| PLA | Jute | 6.1% | Tensile strength slightly higher than unreinforced PLA | Matsuzaki et al., 2016 |
| ABS | Carbon | 10 wt.% | Flexural strength 127 MPa (vs 80 MPa unreinforced) | Yang et al., 2017 |
Key Studies
Nakagawa et al. (2017) used bundled carbon fibers (6 μm diameter, 5.3 GPa tensile strength) with ABS filament (1.75 mm diameter, 30 MPa tensile strength) through nozzles with 0.4 mm and 0.9 mm exit diameters. Thermal bonding with a heating pin significantly improved tensile strength. Samples with 0.9 mm nozzle showed cavities resulting in lower strength.
Matsuzaki et al. (2016) produced Filled and Reinforced Thermoplastics (FRTP) using PLA matrix with carbon fiber tow (CFRTP) and jute fiber yarn (JFRTP). Feeding rates: 100 mm/s for CFRTP, 60 mm/s for JFRTP. Both samples exhibited fiber pull-out indicating poor fiber-matrix adhesion.
Yang et al. (2017) created CFRTPCs using a modified extrusion head where carbon fibers (1000 fibers/bundle, 10 wt.%) passed through the extruder's inner core for infiltration with molten ABS. Flexural strength increased from 80 MPa to 127 MPa, approaching injection molded CCF/ABS (140 MPa).
2. Dual Extruder / Ex-Situ Prepreg Mechanism
Uses two extruders: one deposits pure polymer filament, the other deposits pre-impregnated (prepreg) reinforcing filament manufactured before printing. This approach separates the impregnation step from the printing step, allowing optimization of each process independently.
The prepreg approach has been commercialized most successfully by MarkForged and Anisoprint, enabling industrial adoption of continuous fiber 3D printing. Pre-impregnated filaments ensure consistent fiber-matrix ratios and eliminate the in-nozzle wetting challenges that limit in-situ approaches.
Commercial Systems
| Company | Products | Features |
|---|---|---|
| MarkForged | CFRPF/PA prepregs | Continuous carbon, glass, aramid fiber with polyamide resin |
| Anisoprint | CCFRC, CBFRC | Continuous carbon and basalt fiber composites with thermosetting resin |
Advantages and Disadvantages
Advantages
- Greater flexibility and precision
- Control over fiber content and position
- Different material combinations possible
- Improved mechanical properties
- Simultaneous dual-part printing capability
Disadvantages
- Higher equipment cost and maintenance
- More filament consumption
- Time-consuming setup and parameter balancing
- Requires prepreg filament manufacturing
Published Results
| Matrix | Fiber | Volume % | Results | Reference |
|---|---|---|---|---|
| Nylon | Kevlar | 4.04-10.1% | Elastic modulus 1767-9001 MPa (increasing with fiber content) | Melenka et al., 2016 |
| Nylon | Carbon | 6CF layers | Tensile strength 370-520 MPa | Van Der Klift et al., 2016 |
| PA6 | Carbon/Glass | 26.8-73.4% | Highest shear strength for carbon fiber | Caminero et al., 2018 |
| PA | Carbon | Various layups | Linear elastic behavior until failure at 1-1.2% strain | Lupone et al., 2022 |
Key Studies
Melenka et al. (2016) studied nylon filament with continuous Kevlar fiber rings (2, 4, 5 rings = 4.04%, 8.08%, 10.1% volume fraction). All sample fractures occurred at the fiber deposition start location, indicating this as a weak point.
Van Der Klift et al. (2016) created CFRTP using Nylon with carbon fiber layers. For 6CF samples, tensile strength reached 370-520 MPa vs 17 MPa for pure nylon. Failure occurred near tabs (clamping locations) rather than the smallest cross-section.
Lupone et al. (2022) fabricated CCF/PA composites using MarkForged Mark Two printer with four layups: longitudinal (0), cross-ply (0,90)s, quasi-isotropic (0/±60)s, and (0/+45/90/−45)s. All showed linear elastic behavior with strain at break of 1-1.2%.
Chabaud et al. (2019) studied moisture effects on PA6 with continuous carbon and glass fibers. At 95% relative humidity: carbon fiber composites showed 25% decrease in longitudinal tensile modulus and 18% decrease in tensile strength; glass fiber composites showed stable modulus but 25% decrease in strength. Carbon fiber samples exhibited 40% more internal porosity than glass.
3. Modified Mechanisms
While in-situ fusion and dual-extruder approaches represent the two primary paradigms, researchers have developed several modified mechanisms to address their respective limitations. These modifications typically focus on improving fiber-matrix adhesion, reducing porosity, or enhancing process control—the three factors that most significantly impact final part quality.
3D Compaction Printing (3DCP)
Developed by: Ueda et al. (2020)
A hot compaction roller (10 mm diameter, aluminum) is attached to press deposited layers immediately after extrusion, reducing voids and promoting interlayer adhesion.
| Property | Conventional 3D Printing | 3D Compaction Printing | Improvement |
|---|---|---|---|
| Tensile strength | Baseline | +33% | Significant |
| Tensile modulus | Baseline | No significant change | — |
| Flexural modulus | Baseline | +26% | Moderate |
| Flexural strength | Baseline | +62% | Significant |
| Void distribution | Large voids | Dispersed small voids | Improved |
Reference: Ueda et al., 2020
Modified In-Situ Fusion
Developed by: Akhoundi et al. (2020)
Modified nozzle with an orifice plate guides continuous glass fiber directly to the melt zone for impregnation with molten PLA matrix. Uses fixed and idle pulleys for fiber feeding.
Key Features
- Online changing of fiber fraction volume capability
- Good agreement between experimental and theoretical (mixture rule) tensile results
- Volume fraction range: 35.1% to 49.3%
Reference: Akhoundi et al., 2020
Other Emerging Approaches
Additional modifications under active research include:
- Laser-assisted deposition: Using localized laser heating to improve fiber-matrix wetting and reduce thermal gradients
- Ultrasonic consolidation: Applying ultrasonic vibration during deposition to enhance interlayer bonding
- Hybrid thermoset-thermoplastic: Combining thermoplastic matrices with thermoset sizing for improved fiber adhesion
- Multi-axis deposition: Non-planar printing to align fibers with load paths in complex geometries
Mechanism Comparison
The following comparison summarizes key characteristics across all mechanism types, enabling informed selection based on application requirements:
| Feature | In-Situ Fusion | Dual Extruder | 3DCP | Modified In-Situ |
|---|---|---|---|---|
| Number of nozzles | 1 | 2 | 1 | 1 |
| Equipment cost | Low | High | Moderate | Low |
| Setup complexity | Low | High | Moderate | Low |
| Fiber-matrix bonding | Poor | Good | Excellent | Good |
| Void content | High | Moderate | Low | Moderate |
| Online fiber control | Yes | Yes | Yes | Yes (enhanced) |
| Commercial availability | Limited | Yes (MarkForged, Anisoprint) | Research | Research |
Selection Guidelines
Choosing the appropriate FDM mechanism for continuous fiber composites depends on several interrelated factors. The following guidelines can help navigate this decision:
By Application Type
- Prototyping and proof-of-concept: In-situ fusion offers the lowest barrier to entry with acceptable quality for non-structural parts. Equipment modifications to standard FDM printers are straightforward.
- Structural components: Dual-extruder systems (MarkForged, Anisoprint) provide the consistency required for load-bearing applications. The higher cost is justified by reliability.
- Research and development: Modified mechanisms like 3DCP offer the best properties but require custom equipment. Ideal for pushing performance boundaries.
- Production environments: Commercial dual-extruder systems dominate due to process stability, repeatability, and vendor support.
By Resource Constraints
Decision Matrix
- Limited budget, moderate quality needs: In-situ fusion with process optimization
- Higher budget, consistent quality: Commercial dual-extruder system
- Maximum performance, flexible budget: Modified mechanisms or post-processing consolidation
- High volume, aerospace/automotive: Consider hybrid approaches combining AM with traditional consolidation
Key Trade-offs
The fundamental trade-off in mechanism selection is between process complexity and part quality. In-situ fusion maximizes simplicity but sacrifices fiber-matrix interface quality. Dual-extruder systems improve quality through prepreg materials but add complexity and cost. Modified mechanisms like 3DCP achieve the best properties but require specialized equipment not yet commercially available.
For most applications, the recommendation is to start with commercial dual-extruder systems and only move to modified mechanisms when the application demands it and resources permit custom development.
Common Failure Modes
Understanding failure modes is essential for both mechanism selection and part design. These failures are inherent to FDM composite fabrication and must be accounted for in design margins:
| Failure Mode | Description | Observed In |
|---|---|---|
| Fiber pull-out | Fibers pull from matrix before breaking | CFRTP, JFRTP [Matsuzaki et al., 2016] |
| Surface tension fracture | Matrix fractures, then fibers pull out and break | CCF/ABS [Yang et al., 2017] |
| Fracture at fiber start | Failure at fiber deposition initiation point | Kevlar/Nylon [Melenka et al., 2016] |
| Tab vicinity failure | Failure near clamping points | Carbon/Nylon [Van Der Klift et al., 2016] |
| Delamination | Layer separation, more significant at high humidity | Carbon/PA [Chabaud et al., 2019] |
Mitigating these failure modes typically requires a combination of process optimization (temperature, speed, layer height), post-processing (heat treatment, consolidation), and design considerations (avoiding sharp corners, minimizing fiber discontinuities).
Recent Developments (2024-2025)
Significant advances in FDM mechanisms for continuous fiber composites have emerged in recent research:
High-Throughput Multifilament Printing
Researchers have developed multifilament approaches to increase production rates. Tu et al. (2024) demonstrated simultaneous deposition of three filaments using robotic manufacturing systems, coupled with thermal-mechanical modeling to optimize filament compaction and improve mechanical properties [DOI].
Dual-Nozzle In-Situ Impregnation
A novel two-stage in-situ impregnation method using commercial dual-nozzle 3D printers enables simultaneous manufacturing of both prepreg filaments and final composites. This approach significantly improves printing geometrical accuracy for rigid continuous carbon fiber reinforcement [Composites Part A, 2024].
Thermoset CFRP Direct Ink Writing
While FDM dominates thermoplastic CFRP printing, recent innovations have enabled thermoset CFRP printing using direct ink writing (DIW). Yu and Dunn (2024) reviewed these emerging techniques, including integration with robotic arms for enhanced manufacturing capabilities [Langmuir].
Process Parameter Optimization
Temperature field modeling has enabled systematic optimization of in-situ impregnation parameters. For CF/PLA composites, optimal parameters of 1.25 mm/s printing speed and 235°C nozzle temperature were identified through coupled simulation of heat transfer mechanisms during FDM deposition [Composites Part A, 2025].
| Development | Key Finding | Reference |
|---|---|---|
| Hybrid fiber printing | Carbon/glass hybrid PLA composites show synergistic mechanical properties | Chen et al., 2024 |
| Comprehensive ME review | State-of-the-art CCF thermoplastic processing and pre-impregnation effects | Maqsood et al., 2024 |
| Non-planar printing | Curved-layer deposition for improved fiber continuity on complex surfaces | Composites Part A, 2025 |
Leading Research Teams
Key research groups advancing FDM mechanism development:
| Institution | Key Researchers | Mechanism Focus |
|---|---|---|
| Tokyo University of Science | Matsuzaki, R. [Scholar], Ueda, M. [Scholar], Todoroki, A. [Scholar] | In-nozzle impregnation, 3D compaction printing |
| Xi'an Jiaotong University | Tian, X. [RG], Yang, C., Li, D. | In-situ fusion, modified extrusion heads |
| University of Tehran | Baghani, M. [Scholar], Rahmatabadi, D. [Scholar] | Mechanism review, 4D printing mechanisms |
See Research Teams for complete listing.
Key Journals
Research on FDM mechanisms for continuous fiber composites is primarily published in:
- Composites Part A - Applied science and manufacturing
- Composites Part B - Engineering applications
- Additive Manufacturing - AM-focused research
- Polymers (MDPI) - Open access polymer science
See Top Journals for complete journal list with impact metrics.