The influence of knit-lines on the tensile properties of fiberglass reinforced thermoplastics

Abstract

The adverse effect of knit-line formation on the mechanical properties of thermoplastics has been well documented for unfilled materials. A majority of these investigations concern amorphous polymers, whereas a number of engineering polymers are semicrystalline and also contain reinforcing fillers and fibers. The present investigation deals with the effect of knit-lines on the mechanical behavior of unfilled and glass fiber-reinforced semicrystalline polymers, such as polypropylene, poly(butylene terephthalate), poly(ethylene terephthalate), and poly(phenylene sulfide). Unfilled and fiber-reinforced polystyrene was also investigated for reference. Tensile specimens with knit-lines were produced by impinging melt fronts from two gates located at opposite ends of the dumbbell-shaped mold. The effect of process conditions on the knit-line strength was studied by varying primarily the melt and mold temperatures. The presence of glass fibers significantly reduced the knit-line factor based on strength and strain to failure for the reinforced grades of all polymers relative to their respective unfilled grades. This is attributed to the lack of fiber flow across the knit-line, which makes the material in the knit-line region act as if it is not reinforced. The knit-line strength could be changed through process modifications. For the unfilled grades of semicrystalline polymers, the presence of knit-lines did not affect the yield strength but reduced the elongation at onset of necking, indicating that little spherulitic growth takes place across the knit-line.