Development of optimised thermoplastic composites by tailored fibre placement

Sirris is involved in a European project whose objective is to develop a new composite concept with complex optimised geometry. It is based on the combination of several technologies, from tailored fibre placement to produce the preform up to manufacture of the mould by incremental sheet forming.

The development of fibre reinforced thermoplastics has been strongly accelerated in recent years because these materials offer high impact resistance and toughness, short implementation cycles and good recyclability.

Over time Sirris has acquired considerable expertise with these materials.

To achieve resource optimisation and the profitability of a complete production line, in a holistic approach, it is essential to look at optimising fibre distribution, tool production and the forming process. The repercussions of each individual step can then be identified and taken into account.

Recently, a Cornet project (Tailcomp) was carried out at two German centres, IPF in Dresden and Fraunhofer IWU in Chemnitz in order to combine the advantages of thermoplastic composites with those of innovative manufacturing technologies.

A first demonstrator was selected to validate the concepts: a racing bike saddle. Through its involvement in the bike tests for UCI, Sirris was able to supply information on this product's stress modes.

Fibre impregnation with thermoplastics is difficult because of the viscosity in the molten state of the mould, such that one of the ways of proceeding is to mix the TP fibres and the reinforcement. As part of this project IPF produced threads combining polybutylene terephthalate (PBT) and glass fibres to obtain high mechanical performance. With this material, the centre produced textile preforms by TFP (Tailored Fibre Placement), a technique inspired by embroidery which consists in placing fibres with the orientation required by the application, according to the forces to be exerted on the product in service. 

To consolidate these preforms, Fraunhofer IWU produced light and inexpensive moulds by an incremental forming technique where the principle is to deform a sheet locally to shape it gradually with a spinning operation.

In its autoclave Sirris used different preforms on the moulds under pressure and temperature. It was able to see that the material formed much more easily to the required geometry than when the reinforcement was a conventionally woven textile, the much poorer formability of which required many tests before achieving a result.

An additional difficulty to be controlled is the relatively high temperature of using PBT which poses various technical problems, especially regarding the consumables which also have to withstand the same conditions.

Sirris also used a 3D scanner to measure the deviations between the moulded products and their CAD and proved that 80% came from the mould and 20% from moulding (shrinkage, etc.).

This research thus clearly shows the interactions between all the steps of the production line of a thermoplastic composite. Some of them are already well known in the thermosetting industry, but when the technologies are more "physical" (temperature, etc.) than "chemical" (cross-linking), other effects have to be mastered.

Autoclave treatment is known to enable the production of complex 3D parts, but this solution is expensive. A second phase of the project now consists in using the same preforms in a vacuum bag process, which will increase economic performance while keeping the mechanical properties of the final product.

A second demonstrator, with other levels of complexity, is also planned. It will be a medical orthosis produced from a "customised" preform configured on a single use plaster mould.