OVERCOMP: Interface Formation and Bond Strength Prediction in Composite Injection Overmoulding

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The rapid growth of the global composite market is primarily driven by the ever-more critical need for lightweighting, especially in the automotive and aerospace sectors constituting over 70% of the UK's demand for composites. The increasing need for high-volume manufacture of composite components cannot be addressed by solely relying on time-consuming traditional composite processing technologies. This is one of the main factors contributing to the UK's reliance on non-domestic production, especially in the automotive industry, where over 50% of the required composite parts are currently imported. High-speed manufacture of near-net-shape hybrid thermoplastic composite components via the highly automated injection over-moulding process (or hybrid injection moulding) is arguably one of the only available solutions to address such a significant demand. Composite injection overmoulding is characterised by its capability to manufacture selectively reinforced, highly complex multi-material components within a few minutes, thereby eliminating several hours-long manufacturing steps that would otherwise be required to produce a part at a similar level of complexity. This will, in turn, largely reduce the waste formation, carbon footprint, and by-to-fly ratio. However, despite these advantages, the adoption of this technology has been hampered by the inconsistent and unpredictable performance of the overmoulded components under loading, predominantly caused by a premature failure at the interface. The lack of a fundamental understanding of the interface formation between the injected polymer and the thermoplastic composite insert is the underlying reason for the current inability to control the bond strength and hence the performance of the overmoulded composite. The complexity of the problem mainly arises from the multitude of factors that affect the interpenetration of the polymer chains and at the interface. Even the slightest changes in processing conditions or the composition of the injected polymer or the thermoplastic insert can significantly affect the bonding quality and hence the service life of the components. The absence of a reliable method to support a high-confidence prediction of the structural performance of overmoulded components has left the manufacturers with no other option but to consider costly trials or resort to other, often highly time-consuming labour-intensive multistep alternative processes. To address this gap in the knowledge base, the OVERCOMP project aims to deliver a reliable multi-scale model to predict the interfacial strength between the two thermoplastic phases involved in an overmoulded component. To this end, the project will focus on the three main aspects that contribute to interface formation during overmoulding. These include (i) heat transfer and rheology, (ii) material compatibility; and (iii) time and temperature-dependent interdiffusion of the polymer chain at the interface (healing). This way, the model will ensure a complete picture of the interface formation during overmoulding and reduce the risk of transitioning to this processing method. This model will enable the manufacturers and part designers to make informed decisions during the material selection step and have a clear picture of the part performance before undertaking to manufacture.