A process combining the thermoforming of thermoplastic composites with injection overmoulding

Sirris along with Optim Test Center has developed a manufacturing process combining the thermoforming of continuous fibre thermoplastic composites with injection overmoulding in a single mould.

The Surcomp two-year project financed by the Walloon region (CWality) completed recently. This industrial research project aimed to develop new implementation technology for manufacturing complex structural thermoplastic composites with continuous fibres (glass, carbon, natural fibres, etc.). It benefited from the united skills of the Optim Test Center SME (Simonis Group) and Sirris in terms of knowledge of materials, product development, implementation processes and characterisation.

The process consists in starting with a sheet of laminated thermoplastic composite which provides the part with its general stiffness, heating it to make it malleable and placing it in the mould of an injection machine. This mould is specially designed to hold the softened sheet in position. Closing it causes the composite to be deformed. Conventional injection then enables complex 3D elements (e.g. stiffeners or clips), in the same thermoplastic material, to be added to the resulting shape. 

Sirris studied the conditions of implementation of this single-step process, in particular the heating techniques of the composite, the transfer system from the heating zone to the mould, holding in the mould and the injection overmoulding. It also paid attention to the simulation of the process and to the characterisation of the parts. 

The advantage of this hybrid system is to obtain a high-performance material (continuous fibre composite) in a geometrically complex product in a single step, which, compared with other two-step projects carried out by Sirris, ensures better material distribution and better performance of the finished product. 

As the composite and the injection overmoulded elements have the same nature, the welding is of good quality. The final hybrid part is better in stiffness than an injected part, even reinforced with short fibres. As regards the strength, tests indicated that suitable design was required so as not to create weak points at the welding between the two components. 

The homogeneous chemical nature of the parts also ensures their recyclability. This was demonstrated by grinding the waste from production rejects, compounding it and re-injecting the granules in the form of real parts (see photo, rack angle).

The performance of the injected parts made with the recycled material is equivalent to a short fibre filled polymer. 

The hybrid overmoulded composite is intended as a metal replacement for manufacturing lighter structures, ranging from structural reinforcements for car bodies to seat back structures and to seat supports for earthmoving machinery. 

This type of material is achieving growing success, firstly with carmakers and in other industrial sectors; Sirris and its partner Optim Test Center are ready to help companies respond to new challenges in this field.