| Résumé | Direct long fibre thermoplastics (D-LFT) composites based on polypropylene and glass fibre (PP/GF) are widely used for the manufacturing of semi-structural and structural automotive applications that include front-end carriers, running boards and door carriers, among other parts. The D-LFT process has a number of advantages when compared to the two step pre-compounded LFT pellet process, of which the most relevant are the cost reduction associated to the elimination of an intermediate step of producing pellets, the flexibility of modifying material formulations in-house at convenience and the reduction of heat and shear history of the material. Among the various configurations commercially available for D-LFT systems, two processes were considered for the present project. In the first process reinforcing fibres are fed and mixed with a molten polymer in a twin screw compounder and then extruded into a rectangular cross-section profile. This extrudate (sometimes referred to as a "log") is automatically cut to a specific length and transferred by a robot arm to a vertical compression moulding press where the polymer/fibre compound is moulded to the desired shape. The moulding can be done by means of direct compression of the log or by its placement into a mould shot-pot device that actuates after the mould is completely closed. A second approach involves the use of an injection moulding compounding unit, comprised of a twin screw compounder and a shot-pot (incorporated this time into the compounding-injection unit) that discharges material through a long nozzle into the sprue of the mould in similar fashion as the conventional injection moulding process. This paper deals with the hypothesis that the differences in the geometry of the flow channels for these two approaches affect the length, dispersion and distribution of fibres in the polymer matrix. It has been well documented in literature that fibre length affects the mechanical properties of the composite material while dispersion and distribution affect the uniformity of these properties ['"]. A systematic analysis of composite material produced with the two processing methods described above was carried out. A Coperion-Dieffenbacher D-LFT system, using a shot-pot mould, and a Coperion-Husky, using the compounding-injection moulding process, were employed to mould identical parts under similar compounding conditions. Samples were taken from the parts for a comparison of the respective fibre lengths and distribution patterns of the two moulding technologies using the micro computed tomography (CT) scanning technique. A characterization of fibre length distribution was performed on the samples using image analysis software after pyrolysis. Resulting mechanical properties were compared to provide a comprehensive picture of the two processing technologies. |
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