FRACTURE-TOUGHNESS OF WELDLINES IN THERMOPLASTIC INJECTION-MOLDING

Weldlines are fatal defects in many injection moldings of thermosettin g resins and thermoplastics. Significant strength reduction by weldlin es in thermoplastics is caused by poor adhesion, molecular orientation , and a V-notch effect. These factors have been little investigated in detail, in spite of being well known. In the present article, the V-n otch effect on strength is discussed for two types of thermoplastics, polystyrene and polycarbonate. The depth of weldlines was obtained by milling on the weldline surface, and the fracture toughness was measur ed with a double edge notched specimen. Polystryrene, which was drasti cally weakened by weldlines, had relatively deeper V-notch regions and the fracture toughness was also reduced by weldlines. Although polyca rbonate had the same fracture toughness as polystyrene, it had strong weldlines since the depth of weldlines was negligibly small.