Vacuum-bag-only consolidation of fiber-placed thermoplastic composite structures - A study on the void reduction mechanisms for unidirectional C/PEKK materials
Jagadeesh Narayana Swamy is a PhD student in the Department Production Technology. (Co)Promotors are prof.dr.ir. R. Akkerman and dr.ir. W.J.B. Grouve from the Faculty of Engineering Technology.
Over the past decade, the aerospace industry has increasingly adopted thermoplastic composites because of their melt-processable nature, which enables higher production rates, assembly through welding, and recycling by means of remolding. Thermoplastic composites also offer superior toughness compared to thermoset composites, potentially allowing additional weight savings. Despite these advantages, their use has mostly been limited to small components like press-formed ribs, clips and cleats, due to the lack of cost-effective manufacturing technologies for larger structures. The combination of automated fiber placement (AFP) and vacuum-bag-only (VBO) consolidation provides promising solution for the manufacture of larger parts from unidirectional thermoplastic composite tapes. AFP is used to layup complex preforms at high deposition rates, while the VBO process, either in an oven or on heated tooling, ensures high-quality consolidation.
The key mechanisms governing void-free VBO consolidation are gas removal, which predominantly concerns entrapped air and volatiles that evaporate during heating, and void filling. Gas removal occurs via gas dissolution, diffusion, and in-plane evacuation through porous inter-layer pathways in the layup. Studies on hand-stacked plies showed that tapes with rough surfaces provide interlaminar air channels that aid air evacuation. In the proposed manufacturing route, however, the tape surfaces are melted and bonded during the APF process, closing off any interlaminar channels and limiting void removal to gas diffusion through the thickness of the layup.
This study introduced engineered channels in the preform to enhance air evacuation by deliberately leaving gaps between the placed thermoplastic composite tapes. A predictive simulation tool was developed to optimize the channel dimensions and to estimate the time required for air evacuation. The study also addressed challenges in filling gaps and ply-drop regions in AFP preforms, showing that 1 bar of pressure during VBO is sufficient to fill these voids, with transverse squeeze flow as the main mechanism. It was demonstrated that the channels reduced void content to less than 1%, which would be acceptable for aerospace applications.
In conclusion, the AFP and VBO process offers a viable method for producing large thermoplastic composite structures, paving the way for large scale application of these materials in aircraft applications.
