Overview
Different FDM mechanisms have been developed to fabricate continuous fiber-reinforced composites. The two main mechanisms are in-situ fusion (single nozzle) and dual extruder/ex-situ prepreg (two nozzles). Modified mechanisms such as 3D compaction printing have also been introduced to address limitations.
1. In-Situ Fusion Mechanism
This mechanism uses two input materials—the reinforcement (dry fiber feedstock) and the neat polymer matrix—combined during printing through a single nozzle.
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.
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
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
Mechanism Comparison
| 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 |
Common Failure Modes
| 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] |