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

Abstract

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.

Thermal effects on weld and unweld tensile properties of injection moulded short glass fibre reinforced ABS composites

“…These properties, however, are affected by a number of parameters, such as the fibre content, fibre length, fibre orientation and the degree of interfacial adhesion between the fibre and the matrix (e.g. : [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]). However, as most short fibre composites are fabricated by an injection moulding process, the presence of weldlines is a major design concern as weldlines could lead to a considerable reduction in mechanical properties and because of this designers often need to incorporate liberal safety factors in design analysis to compensate for this weakness.…”

“…A serious reduction in tensile strength and modulus has been reported for many polymers and their composites in the presence of cold weldlines (e.g. : [1][2][3][4][5][6][7][8][9][10][11][12]). In general, the presence of a weldline reduces tensile strength by up to 60% and tensile modulus by up to 40% depending on the polymer, the characteristic features of the reinforcing filler and the processing conditions being used.…”

“…The processing conditions such as melt temperature, injection speed and mould temperature could also play an important role in determining the integrity of the welded components and many studies have addressed this issue for both amorphous and semi-crystalline polymers (e.g. : [10,11,14])]. However, whilst the majority of weldline studies have been conducted at room temperature, very few have examined the integrity of weldlines at elevated temperatures, particularly in short fibre composite systems [3][4][5].…”

Analysis of tensile and flexural strengths of single and double gated injection moulded short glass fibre reinforced PBT/PC composites

“…In the presence of such weldlines, the strength of the composite material can be reduced by as much as 10-60% depending upon the characteristic features of the matrix, fibre concentration and the processing conditions being used. [1][2][3][4][5][6][7][8][9] In this paper, we examine the effect of weldline on tensile and flexural strength of PBT/PC composites containing up to 30% (w/w) short glass fibres (GFs) using micromechanical and a two parameter Weibull statistical strength model.…”