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Weldlines in Injection-Moulded Reinforced Thermoplastics: Mechanical Performance Improvement through Intrusion Induced Fibre Reorientation

The effect of processing conditions on the mechanical properties of a weldline in short glass fibre reinforced polypropylene was investigated. It appears that the presence of a weldline has a deleterious effect on the tensile properties, with a serious loss of strength and rigidity. The optimisation of the process parameters through a Taguchi methodology showed some improvement, the properties remaining however quite distant from those of specimens without weldline. The analysis of the fibre arrangement around the weldline gave clues to explain this situation. In the vicinity of the weldline, the typical skin-core structure was deeply disrupted with a skin layer almost non-existent. Therefore, most of the fibres were oriented perpendicularly to the direction of loading, resulting in poor load transfer. Calculations showed that the strength was almost reduced to the transverse resistance of a unidirectional composite because of the very specific orientation of the fibres around the weldline. The generation of a back-flow in the melt, thanks to a differential gate solidification, brought some reorientation of the fibres and some increase in the properties.