Induction Melt Incremental Thermoforming of Advanced Thermoplastic Composites

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Transport is responsible for around a quarter of the UK's total greenhouse gas emissions. Light-weight recyclable materials, such as thermoplastic composites, are recognised as a sustainable solution to reducing transport emissions via "light-weighting". Thermoplastic composites have several advantages over their thermosetting polymer composite cousins, including faster processing, improved recyclability and increased toughness, though they are more challenging to process which partly explains their smaller market share (~1/3rd that of thermosets). Consequently, there is an urgent need to address the manufacturing issues to realise their full weight and fuel saving potential. However, forming advanced thermoplastic composites into complex geometries is not always straightforward and production defects such as wrinkling, bridging and tearing of the forming sheet are commonplace.The main idea of the project is to develop a novel sheet forming process for advanced thermoplastic composites designed to mitigate manufacturing defects when thermoforming multi-axial laminates into complex geometries. This will be achieved through the creation of wrinkle-resistant 'lubricated-blanks' using a combination of induction heating and a novel incremental forming process. Typically, to achieve optimum mechanical properties in composites structures, placement of fibres in multiple directions is required to accommodate complex loading conditions. However, despite the development of sheet forming processes for advanced composites dating back over 30 years, questions regarding production defects have yet to be satisfactorily answered, namely: 'How to form complex components from pre-consolidated multi-axial thermoplastic laminates without inducing wrinkles?' and 'How to form highly complex geometries involving multiple recesses, without bridging or tearing of the forming laminate?' The proposed research combines two distinct new ideas. The first is to create a 'lubricated blank', using induction heating and melting of metallic inter-layers placed within the forming composite sheet to facilitate ultra-low inter-ply sliding friction and consequently, wrinkle-free forming of multi-axial pre-consolidated advanced thermoplastic composites. The electromagnetic properties of certain metals coupled with their high surface tension and low viscosity when molten mean they can be used as a medium with which to both inductively heat and lubricate the forming composite blank, thereby preventing wrinkles. The second idea is to use a multi-step forming tool designed to create an automatic sequential and incremental forming process in a single press-forming down stroke, beginning at the centre and subsequently moving outwards towards the perimeter of the sheet. The multi-step forming tool serves to both mitigate bridging and tearing and crucially, squeezes the molten metal out of the composite layup (like squeezing toothpaste from a tube!) and into the surrounding rubber diaphragm during the thermoforming process, leaving the final consolidated composite part almost completely free of embedded metallic inclusions.