Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Abstract
Additive manufacturing of continuous fiber–reinforced polymer composites remains constrained by matrix selection and deposition physics. Conventional continuous fiber printing requires matrices that solidify immediately after extrusion, restricting the material palette to thermoplastics or ultra-fast-curing thermosets. This limitation excludes most aerospace-grade epoxy systems whose cure kinetics preclude instantaneous shape retention.
This study presents a direct ink writing (DIW) process and a shear-driven impregnation printhead that enable 3D printing of continuous carbon fiber reinforced thermosetting composites without relying on rapid solidification. A narrow 1K carbon fiber tow is impregnated in-situ within a viscous thermosetting resin as it is drawn through a confined nozzle by shear drag generated from resin flow. UV-curable acrylate resin is used to demonstrate free-standing structures with on-nozzle photopolymerization, and the same architecture accommodates thermally curable epoxies that co-cure after deposition.
Unidirectional (UD) and laminated specimens were fabricated and characterized. UD composites achieved longitudinal modulus values near ~80 GPa and tensile strengths of ~700–750 MPa, comparable to conventionally molded composites of similar fiber volume fraction. Laminated specimens exhibited behavior consistent with classical lamination theory and showed no preferential interlayer debonding. Microstructural analysis confirmed effective impregnation, interfacial bonding, and low porosity.
The results demonstrate that DIW processing of continuous fiber thermosets can bridge the performance gap between conventional additive composite processes and structural aerospace-grade laminates, expanding the accessible materials space for additive manufacturing.
1. Introduction
Continuous carbon fiber reinforced polymers (CFRPs) are foundational materials in aerospace, space, defense, and advanced mobility applications due to their superior specific stiffness, specific strength, and fatigue resistance. Traditional fabrication techniques—including hand lay-up, automated tape laying (ATL), automated fiber placement (AFP), resin transfer molding (RTM), and autoclave curing—deliver high-quality laminates but rely heavily on dedicated tooling, extended processing cycles, and significant capital infrastructure.
Additive manufacturing (AM) offers an alternative pathway for fabricating geometrically complex composite components directly from digital models. While AM of short-fiber thermoplastics is common, AM of continuous fiber composites has remained technologically constrained. Commercial continuous-fiber printers co-deposit thermoplastic filaments and fibers; however, the requirement for rapid solidification limits achievable fiber volume fraction (≈30–35%), introduces interfacial voids, and restricts usable matrices to low-viscosity thermoplastics or ultra-fast-curing resins.
Aerospace-grade thermosets, particularly epoxies, are excluded due to their slow viscosity evolution prior to gelation. Without immediate solidification, extruded filaments cannot support their own geometry. Therefore, to expand the matrix palette and enable structural-grade performance, a method is needed to decouple deposition, impregnation, and final curing.
This study introduces a direct ink writing (DIW) approach in which a continuous carbon fiber tow is impregnated in-situ with a viscous resin as it traverses a custom printhead. UV-cure acrylates provide immediate green strength for geometric retention, while thermally cured resins enable co-cured monolithic laminates. Mechanical characterization and microstructural analysis are conducted to evaluate process–structure–property relationships.
2. Background
2.1 Limitations of existing continuous-fiber AM processes
Current continuous fiber AM technologies are limited by:
- Restricted matrix selection (primarily thermoplastics).
- Limited fiber volume fraction dictated by filament geometry.
- High porosity due to incomplete wetting, bead gaps, and thermal welding interfaces.
- Weak z-direction properties arising from layer-by-layer thermal adhesion rather than co-curing.
Consequently, most continuous-fiber AM parts fall short of structural aerospace requirements.
2.2 Advantages of thermosetting matrices
Thermosets—especially epoxies—offer:
- High glass transition temperature (Tg).
- Excellent interfacial bonding with carbon fiber sizings.
- Robust environmental resistance.
- Low creep and high fatigue resistance.
Integrating thermosets into AM could narrow the performance gap between printed and conventionally processed CFRPs.
2.3 Scientific challenge
The central scientific challenge is that thermosets do not solidify immediately after extrusion. DIW addresses this challenge by using resin rheology, confinement, and controlled curing to stabilize the printed bead.
3. Materials and Methods
3.1 Reinforcement: Continuous carbon fiber
A 1K PAN-based carbon fiber tow was selected. The small filament count facilitates:
- Passage through narrow nozzles.
- Efficient wet-out under confined flow.
- High-resolution deposition paths.
3.2 Matrix systems
Two classes of resin were used:
3.2.1 UV-curable acrylate resin
Selected for on-nozzle photopolymerization enabling unsupported geometries.
3.2.2 Thermally curable epoxy resin
Selected for structural performance. Curing occurred post-deposition in a controlled thermal cycle.
3.3 Printhead architecture
The custom DIW printhead consists of:
- A fiber inlet and guide system.
- A resin chamber supplying viscous thermoset ink.
- A confined nozzle that imposes shear flow on the fiber bundle.
- Optional UV LED positioned at the nozzle exit.
3.4 Shear-driven impregnation mechanism
Resin flow at elevated viscosity applies tangential shear to the fiber surfaces, producing fiber drag and uniform resin infiltration. This mechanism resembles a microscale pultrusion event confined within the nozzle.
3.5 Printing platform
A modified cartesian 3D printer was fitted with:
- Controlled fiber tension feeding.
- Pressure-driven resin delivery.
- Custom slicing strategies for continuous fiber paths.
3.6 Specimen fabrication
Two specimen types were produced:
- Unidirectional (UD) plaques.
- Symmetric cross-ply laminates (e.g., [0/90]s).
3.7 Mechanical testing
Mechanical characterization included:
- Uniaxial tensile testing (ASTM-style geometry).
- Flexural testing under three-point bending.
3.8 Microstructural analysis
Optical and scanning electron microscopy (SEM) were used to evaluate:
- Fiber distribution.
- Void content.
- Fiber–matrix interfacial quality.
- Interlayer coherence.
4. Results
4.1 Printing behavior and process window
A stable process window was identified where:
- Fiber feeding remained continuous.
- Resin impregnation was uniform.
- Nozzle pressures were manageable.
- Bead geometry was consistent.
Deviations caused fiber buckling, nozzle blockage, resin-rich regions, or void clusters.
4.2 Tensile properties of unidirectional composites
Typical results:
| Property | Average Value |
|---|---|
| Longitudinal modulus E11 | ~80 GPa |
| Tensile strength σUTS | ~700–750 MPa |
The stiffness matched rule-of-mixtures predictions. Failure surfaces exhibited fiber-dominated brittle fracture typical of CFRPs.
4.3 Laminated composite performance
Cross-ply laminates exhibited:
- Reduced but predictable stiffness in off-axis directions.
- No preferential delamination.
- Clean matrix cracking modes under tension.
4.4 Flexural response
Flexural modulus and strength scaled with tensile performance. No premature interlayer shear failure occurred.
4.5 Microstructure
Key observations:
- Fibers were well aligned with deposition direction.
- Bundle impregnation was uniform.
- Void content was modest and largely associated with suboptimal print parameters.
- For epoxies, co-cured interfaces appeared monolithic, lacking discrete layer boundaries.
5. Discussion
5.1 Comparison with conventional AM composite processes
Compared to thermoplastic continuous-fiber AM:
| Parameter | Thermoplastic CF-AM | Present DIW Thermoset CF-AM |
|---|---|---|
| Matrix adhesion | Thermal welding | Chemical co-curing |
| Void content | 5–7% typical | Lower; microstructurally isolated |
| Fiber volume fraction | Limited by filament geometry | Tunable via nozzle confinement |
| Mechanical performance | Moderate | Structural-grade |
5.2 Process–structure–property relationships
- Optimal resin viscosity is essential for complete impregnation.
- Confined flow geometry promotes fiber alignment and wet-out.
- Thermal curing yields strong interlayer bonds.
- Void distribution correlates with mechanical performance.
5.3 Relationship to AFP
The DIW system may be viewed as a digitally flexible, small-scale counterpart to AFP:
| Aspect | AFP | DIW Continuous Fiber |
|---|---|---|
| Feedstock | Prepreg tapes/tows | Dry fiber + liquid resin |
| Tooling | Mandatory | Not required |
| Resolution | Tape-width limited | Nozzle-diameter limited |
| Cure | Autoclave or OoA | UV or thermal post-cure |
Thus, DIW complements, rather than replaces, AFP technologies.
5.4 Limitations
Major limitations include:
- Moderate deposition speed.
- Limited unsupported geometries for thermally cured systems.
- Need for advanced fiber-path planning algorithms.
- Material-specific rheology tuning.
Long-term durability and fatigue performance require further study.
6. Conclusions
This study demonstrates a direct ink writing method capable of producing continuous carbon fiber reinforced thermosetting composites with structural-grade performance. Key findings include:
- The shear-driven impregnation mechanism enables in-situ wetting of 1K carbon fibers with viscous thermosetting resins.
- UV-curable and thermally curable systems are both compatible with the printhead architecture.
- Unidirectional composites achieved moduli (~80 GPa) and strengths (~700–750 MPa) comparable to conventionally processed CFRPs.
- Laminated composites showed effective co-curing and robust interlayer bonding.
- Microstructural analysis confirmed efficient impregnation and low porosity.
This DIW-based approach broadens the accessible resin space for additive manufacturing of continuous fiber composites and provides a foundation for future studies targeting multi-material structures, hybrid cure chemistries, and aerospace-grade qualification.
Acknowledgments
The authors gratefully acknowledge the financial support of the NASA Space Technology Mission Directorate (STMD) for work on continuous-fiber additive manufacturing, and the SpaceX Materials Engineering and Structures teams for valuable technical discussions on flight-critical composite hardware. The authors also thank Prof. Kun (Kelvin) Fu and Prof. Tsu-Wei Chou for insightful guidance on continuous carbon-fiber/thermoset 3D printing, and the staff of the Advanced Composites Laboratory for assistance with mechanical testing and microscopy. Any opinions, findings, and conclusions expressed in this work are those of the authors and do not necessarily reflect the views of NASA or SpaceX.
References
- Shi, B., Shang, Y., Zhang, P., Cuadros, A. P., Sun, B., Gu, B., Chou, T.-W., and Fu, K., “Dynamic Capillary-Driven Additive Manufacturing of Continuous Carbon Fiber Composite,” Matter, Vol. 2, pp. 1594–1604, 2020.
- Hao, W., Liu, Y., Zhou, H., Chen, H., and Fang, D., “Preparation and Characterization of 3D Printed Continuous Carbon Fiber Reinforced Thermosetting Composites,” Polymer Testing, Vol. 65, pp. 29–34, 2018.
- Ming, Y., Zhang, S., Han, W., Wang, B., and Xiao, H., “Investigation on Process Parameters of 3D Printed Continuous Carbon Fiber-Reinforced Thermosetting Epoxy Composites,” Additive Manufacturing, Vol. 33, 101184, 2020.
- Zhang, Z., Liu, R., Li, W., Liu, Y., Luo, H., Zeng, L., Qiu, J., and Wang, S., “Direct Writing of Continuous Carbon Fibers/Epoxy Thermoset Composites with High Strength and Low Energy Consumption,” Additive Manufacturing, Vol. 47, 102348, 2021.
- Islam, M. Z., Rahman, M. A., Gibbon, L., Hall, E., Ulven, C. A., and La Scala, J. J., “Mechanical Characterization and Production of Various Shapes Using Continuous Carbon Fiber-Reinforced Thermoset Resin-Based 3D Printing,” Polymers, Vol. 16, No. 13, 1828, 2024.
- Brasington, A., Sacco, C., Halbritter, J., Wehbe, R., and Harik, R., “Automated Fiber Placement: A Review of History, Current Technologies, and Future Paths Forward,” Composites Part C: Open Access, Vol. 6, 100182, 2021.
- Rahman, M. A., Islam, M. Z., Gibbon, L., Ulven, C. A., and La Scala, J. J., “3D Printing of Continuous Carbon Fiber Reinforced Thermoset Composites Using UV-Curable Resin,” Polymer Composites, Vol. 42, pp. 5859–5868, 2021.
- Abdullah, A. M., Ding, Y., He, X., Dunn, M., and Yu, K., “Direct-Write 3D Printing of UV-Curable Composites with Continuous Carbon Fiber,” Journal of Composite Materials, Vol. 57, pp. 1–19, 2023.
- Robertson, I. D., Yourdkhani, M., Centellas, P. J., Aw, J. E., Ivanoff, D. G., Goli, E., Lloyd, E. M., Dean, L. M., Sottos, N. R., and Geubelle, P. H., “Rapid Energy-Efficient Manufacturing of Polymers and Composites via Frontal Polymerization,” Nature, Vol. 557, pp. 223–227, 2018.
- NASA, “Additive Manufacturing of Continuous-Fiber Composite Structures for Deep-Space Missions,” NASA Technical Memorandum (TM-XXXX), Year. (placeholder – replace with your actual NASA report if needed)
- SpaceX Materials Research Division, “Continuous Carbon Fiber/Thermoset Architectures for Reusable Launch Vehicles,” Internal Technical Report, Year. (placeholder – replace with your actual internal citation or remove if not needed)