Bio-sourced thermoplastic composites

Sirris is studying the use of natural fibre composites under different aspects: moulding, assembly, quality control. The results are very interesting.

For two years Sirris has been working in a Feder project on the use of bio-sourced composites with other research centres and Belgian university organisations.

Using bio-sourced thermoplastic composites

Based on its experience relating to the use of continuous fibre thermoplastic composites, in this Macobio project Sirris is studying the potential for substituting conventional composites with bio-sourced products. One of the specific difficulties with these materials is the maximum permissible temperature of natural fibres, clearly lower than that of synthetic fibres. This characteristic requires, for example, finely controlling tooling heat to make temperatures uniform and prevent hot spots that could degrade the product.

The choice was made to use commercially available materials, i.e. based on conventional oil-sourced matrices (like PP), and also based on bio-sourced PLA matrices, and of course on natural fibres (flax especially), materials sold either as consolidated semi-finished products, or as hybrid fabrics with co-mixed fibres.

Among the technologies tested, autoclave moulding was able to be applied and a demonstrator was produced to determine the relevance of the adopted technical choices. In this case, it is the saddle of a racing bicycle.

Apart from heat control, the difficulty of the exercise was to overcome the lack of formability of a conventional fabric to produce a good part. After optimizing the operational settings, more than satisfactory results were obtained and "sensitive" areas, previously identified by calculation using software developed by the partners as part of another project, were finally controlled.

In order to continue this work and with one of the partners to initiate an LCA study (life cycle analysis) of the bio-sourced TP composites, Sirris, thanks to the Feder funds, has acquired a special stove which completes the autoclave already available. For more detail about this new equipment, see the blog Thermoplastic composites: novelties for improved production.

The other technology examined by Sirris under this project is the overmoulding of rigid preforms (plats) previously heated to make them formable. This technology developed in a previous project was adapted to semi-finished products based on PP/flax fibres with more than satisfactory results.

A new technique for using bio-sourced TP composites starts today. It consists in creating, by compounding and then injection, bio-sourced polyethylene containing short natural fibres originating from willow bark.

Detecting defects

In order to characterise the quality of the manufactured products, a second facet of the Macobio project consisted in updating a detection tool for composite defects, a tool developed a while ago for another project. This is a system of non-destructive testing by active infrared thermography; it is efficient thanks, in part, to the uniformity of heating of the composite to be examined and in part to an optimizing technique of mathematical image processing.

The software has been updated and image processing has been automated. This update has enabled 1 hour of processing to be done in 5 minutes.

Assembly

Finally, a third section concerns assembly by welding (Joule effect) of the manufactured parts. Feasibility tests were first carried out on samples. Now it's a matter of being able to assemble the actual parts by positioning heating elements in a route related to the three-dimensional shape of the interfaces.

A co-robot, also acquired as part of Macobio, will soon be put into service to automate this process.

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