Thermoplastic composite injection-overmoulding with indirectly-loaded reinforcement: Design for manufacture

Thermoplastic Overmoulding is an emerging manufacturing method combining low-cost, high-rate injection moulding with continuous fibre composite elements. The combination of materials with dissimilar stiffnesses can create stress concentrations and warpage, with further complications caused by high injection pressures distorting the continuous fibre elements (wash). This study uses a numerical model to consider a particular overmoulding approach that utilises a strategically positioned local reinforcing element (an insert) that is loaded indirectly via stresses transferred from the surrounding overmoulding material. This offers a low-cost solution by minimising use of continuous fibre material, but has a higher likelihood of the aforementioned problems. A geometrical solution (a waffle-type structure) is used to address these manufacturing challenges without compromising the primary stiffening role of the continuous fibre element. An insert efficiency factor is proposed to describe the tensile behaviour and the diminishing returns on peak load in the insert when embedding increasingly higher stiffness inserts into moderate stiffness overmoulding materials containing short fibres. According to the simulation results, the proposed Waffle structure should provide stiffness commensurate with the conventional configuration; however, the experimental coupons highlight the complexities of injection moulding short fibres and indicate the detrimental effect of anisotropic fibre distributions within the overmoulded sections.