Conventional curing out-of-autoclave epoxy prepregs for in-situ manufacturing: Suitable or not?

Thermoset composites need time to cure. In additive manufacturing contexts, this prevents achieving full laminate consolidation in a single step. This work investigates the behaviour of a representative Out-Of-Autoclave (OOA) carbon fibre epoxy prepreg, in which air channels were intentionally left to facilitate air evacuation during compression, to explore new processing possibilities. Samples were consolidated through Ply-By-Ply (PBP) compaction cycles under a pressure of 1 bar, at test temperatures ranging from 30°C to 210°C. An oven heat treatment was then applied, to compare the “before and after secondary heating” states of the samples, which indicated the suitability of the material to the one-step process. It was found that the PBP processing temperature and the secondary heating step both had distinct effects on the analysed factors: thickness, voidage and microstructure. Thickness and voidage levels were evaluated via X-ray Computed Tomography (CT), and the final state microstructures were compared via optical microscopy. With preprocessing temperatures above 90°C, voidage appears to drop to a stable level (under 2%), but sample thicknesses still decrease, showing more than 0.1 mm change after the secondary heating. This ultimately demonstrates that despite an improvement in preform quality thanks to a better control of the process parameters, this category of conventional curing epoxy prepregs with air pathways in the fibre tows does not enable a single-step on-line consolidation. Future research for in-situ manufacturing with OOA thermosets should focus on faster curing material systems to allow consolidation in a single step.