Despite the increasing use of fiber-reinforced polymer composites (FRPCs) across multiple industries for the development of lightweight structures, the conventional manufacturing methods for composites still remain lengthy, inflexible, labor-intensive, energy-inefficient, and cost-prohibitive1–3. A major drawback of present composite manufacturing methods is the need for complex tooling for every new part design, which can take up to several months to produce based on the traditional manufacturing and account for more than 30% of the final product cost4,5. In addition, it might be quite challenging, or even impossible, to develop tooling for the manufacture of composite structures with complex geometries6,7. Production of a composite part using the developed tooling will then involve sustained, bulk heating of the material (and tooling) for several hours until the matrix thermoset resin of the composite is cured, leading to slow and energy-intensive manufacturing processes. Additive manufacturing (AM), also known as 3D printing, is promising for addressing the inefficiencies associated with the conventional composite manufacturing and enabling the rapid, flexible, and cost-effective manufacture of FRPC structures6. Additive manufacturing of FRPCs requires automated deposition or placement of fully integrated constituents along designated paths, accompanied by rapid phase transformations of the matrix polymer for the high-fidelity fabrication of high-quality composite structures in the absence of tooling surfaces and at ambient conditions. Current methods for the AM of FRPCs, using discontinuous or continuous fibers, primarily rely on the melt processing of thermoplastic polymers or photopolymerization of thermoset resins as the matrix polymer of composites8–15. While the AM of thermoplastic-matrix composites has garnered attention for its ease of processing and potential recyclability, most of the thermoplastic polymers employed in AM are unsuitable for structural applications, mainly due to their limited mechanical properties, thermal stability, and creep resistance16,17. In addition, the high melt-viscosity of thermoplastics presents a challenge in creating composites with a high concentration of fiber reinforcements while maintaining a low void content, restricting the performance of produced composites18–21. In contrast, photocurable thermoset resins with substantially lower viscosities facilitate the AM of composites containing a high volume-fraction of fiber reinforcements; however, the low penetration depth of the incident light (i.e., ultraviolet or visible light) caused by the screening effect of nontransparent fillers or reinforcements leads to partial and nonuniform curing of the matrix resin, limiting the print fidelity and printing speed and requiring post-curing in size-limiting ovens22–27. While thermally curable thermosets, like those used in traditional composites, are ideal polymers for the AM of FRPCs, adoption of such resins in AM technologies has been hindered by their long cure cycles and flow-induced instabilities during high-temperature curing processes28–30. Direct ink writing (DIW) of highly thixotropic inks or reactive extrusion of thermoset resins reinforced by discontinuous fibers have been previously used to mitigate the processing issues of thermoset resins8,31,32. These approaches, however, are mostly suitable for the manufacture of simple 2D geometries and cannot easily be used with continuous fibers. Additive manufacturing of continuous fiber-reinforced thermoset composites has been demonstrated by capillary-driven impregnation of fibers and in-situ thermal curing via Joule heating33. This approach similarly produces low-fidelity composite parts with a simple geometry, cannot be applied to the AM of discontinuous FRPCs, and has limited processability and printing speeds due to the requisite wicking effect. Frontal polymerization (FP) has been recently introduced as a promising curing strategy for 3D printing of discontinuous- and continuous fiber-reinforced composites by in-situ curing of the matrix thermoset resin through a self-propagating chemical reaction34–37. While the self-sustaining, FP-assisted 3D printing is promising for the freeform manufacturing of composite structures, its practical application is limited by the low front velocities of the FP chemistries (< 10 cm min− 1), reaction quenching caused by heat losses through boundaries, and difficulties in maintaining a high frontal reactivity in the presence of a high concentration of fillers or reinforcements34,38–40. It is thus desirable to develop robust AM techniques for the high-quality and high-fidelity manufacture of thermoset-matrix composites for use in aerospace, space, marine, energy, automotive, and biomedical applications.
Here, we present a new AM method for the rapid, tool-free production of high-quality thermoset-matrix composites by in-situ, instantaneous thermal curing of composite materials. Use of a rapid-curable, thermoresponsive thermoset resin along with a local photothermal stimulus mounted on the printhead of a robotic platform allows for on-demand, instantaneous curing and rigidization of carbon fiber composites on existing surfaces or in midair. By tuning the cure kinetics and rheological profiles of the resin system, we demonstrate the AM of FRPCs using both discontinuous and continuous carbon fibers. Systematic experimental studies are performed to understand the effect of various processing parameters on the printability of the composite materials and guide the design of reliable AM processes. This method offers the rapid, scalable, and resource-efficient manufacture of FRPC structures while eliminating the need for tooling and post-curing steps and can be applied to a variety of rapid-curable polymer resins, reinforcement types, and thermal stimuli.