Welding angle is the meeting angle between two melt fronts, ranging from 0 to 135 degree, between two converging melt fronts. If the welding angle is 180 °, then the two melt fronts can be considered as one. If the welding angle is 0°, then two melt fronts converge head-on. Weld line are more likely to form when the welding angle is smaller than 135°.