Effects of Weld Line on Strength of Fiber Reinforced Thermoplastics Injection Molding

The strength of fiber reinforced thermoplastics (FRTP) injection molded parts is reduced by weld lienes. The aim of this study is to clarify the mechanism of weld lines and the effects on strength of molding. The materials used are fiber reinforced polycarbonate (PC) with 10, 20, and 40 wt% glass fiber content. Dumbbell-shaped tensile test specimens with weld lines were molded at various injection pressures, cavity pressure, and resin temperature were noted. Measurement of tensile strength and observation of the weld region were carried out. The ratio of strength with weld line to that without weld line decreases remarkably with increasing fiber content. The strength of the molding with weld line increases with the filling pressure and/or the holding pressure. The former affects the resin temperature at the weld line and the latter affects the adhesion between fiber and resin. The weld line pressure (Pw) can be calculated from the cavity pressure. The relationship between the strength and Pw is found to be linear. A procedure for predicting the strength of a sample with a weld line is proposed.