A B S T R A C TRecycling of thermoplastic composites has attracted considerable attention in the recent years. Several recycling solutions include shredding scrap to centimetre-sized flakes to retain long fibres, followed by a remanufacturing step that prevents fibre breakage. Determining the exact fibre length distribution (FLD) for these routes is crucial, as it is of importance for the processibility of the material as well as the mechanical performance of the recycled parts. In this paper, novel analysis methods are introduced to calculate FLDs based on photographs of flakes. The reliability of the method and of the sampling was found to be high. The relation between flake size and FLD was studied, showing that offcut layup barely influences the FLD in comparison to flake size. The effects of shredding settings and sieving were studied, showing a strong correlation between machine parameters and FLD, whereas the offcut size was found to have no effect on FLD.

The transportation sector, aerospace and automotive in particular, is striving to reduce vehicle weight to improve fuel economy. The high specific mechanical properties of fibre-reinforced composites make them an attractive option for reaching this goal. Among this class of materials, thermoplastic composites are becoming increasingly popular due to several benefits compared to their thermoset counterparts: superior toughness, short processing times, possibility to weld components and their easier recyclability. Recent years have shown an increase in applications and a rising demand for these thermoplastic composites. This comes with the drawback of larger volumes of industrial scrap. The combination of the high value of the scrap material and legislative incentives is pushing the industry to develop recycling solutions for this material.

A novel recycling solution for thermoplastic composites (TPCs) was recently implemented. The processing steps comprise shredding of TPC offcuts to flakes of a few centimetres, melting and blending of the flakes in a low-shear mixer, extrusion of a molten mixed dough and subsequent compression moulding in a press. This material and process are similar to the compression moulding of long-fibre thermoplastics (LFTs) that have been in the market for decades, such as glass mat thermoplastics (GMT) or direct-LFT. However, the input material in this recycling route consists of multi-layered woven flakes, which is very different from the pellets or chopped rovings of other LFTs. Process- and material-induced heterogeneities such as fibre orientation, percolation, variation of fibre fraction, or fibre attrition may be different for this new material. The development of this recycling technology and future industrial applications require more confidence in the material and process. The objective of this study is to characterise these heterogeneities for this recycling solution, and compare them to those generated in regular LFTs. It was found that the process- and material-induced heterogeneities of the recycled TPCs are similar to other LFTs, for the aspects listed here: fibre orientation, percolation, variation of fibre fraction and fibre attrition. In comparison to GMT, the effect of the mixing step is particularly noticeable on the local variation of fibre fraction within the panels. Industrial applications of this recycling route will benefit from this similarity, as it improves the confidence in the material and process combination.

An integrally-stiffened access panel for a rotorcraft is selected for detail design, testing and actual flight to demonstrate a novel recycling route for thermoplastic composites. The design, development and validation followed the 'Building Block approach'. The used material is postindustrial carbon fiber reinforced polyphenylene sulfide waste. This material originates from thermoplastic components of the very same rotorcraft as the panel will be mounted on, improving traceability, logistics and fixing supply and demand. Material data have been gathered from mechanical tests and used to predict the panels strength and stiffness. A critical design detail was selected and tested for validation. This section was included in a manufacturing demo, along with other integrated design features, enabling testing the processability. The final panel design was successfully produced and tested on component level. The re-manufacturing process includes simultaneously applied heat and low-shear mixing, followed by compression molding in an isothermal mold. This offers the possibility to retain long fibers and therefore high mechanical properties at short cycle times. In comparison to the current carbon/epoxy solution, the resulting product is lighter, significantly more cost-effective and made of recycled material (fiber and matrix). The prototype panel is targeted for flight testing on the rotorcraft in 2019.

A low-shear mixer was used to blend thermoplastic composite scrap material into a molten mixed dough, which was then compression moulded. This process is a key step in a novel recycling solution for thermoplastic composites. A study was carried out to characterise the quality of mixing (QoM) of the blended doughs to understand how to improve the QoM of mixed doughs towards further improvement and implementation of the recycling solution. In order to achieve this, the effect of mixing parameters and fibre length on the QoM were studied. This study used shredded C/PPS flakes, originating from consolidated laminate scrap. These flakes are about 20 mm in size, and contain woven fabric reinforcement, making them far different from regular pellets, and therefore more difficult to mix. The QoM was characterised by means of image analysis of a large set of cross-sectional microscopy images, based on which the scale and intensity of segregation of the fibre clusters were evaluated. Bundle size distribution was determined by applying Delaunay triangulations to cluster the fibre centres. These methods were found to be suitable for characterising the QoM of such doughs. Increasing the mixing time and mixing speed were identified as key ways to improve the mixing process. With the current mixing machine, it is also suggested not to use fibres longer than 15 mm on average in order to limit intra-dough variability. For doughs made of fibres longer than 15 mm, improvements on the mixing device could sufficiently increase the QoM.

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