When the two streams meet, a weld line is formed on the injection molded part. At this time, the tangent of the melt front has an angle, which is the junction angle of the weld line, as shown in the figure below. The size of the junction angle has an important influence on the strength and clarity of the weld line. According to Moldflow's analysis criteria, when the junction angle is greater than 135°, the weld line is invisible to the naked eye, and the strength is very high; when 75° <mergence angle <135°, the weld line is clearer, but the strength is still very high. , The spray paint can cover the weld line; when the confluence angle is less than 75°, even if the weld line is processed, the spray paint cannot cover it, and the strength is poor.
solution:
For larger injection parts such as car bumpers, dashboards, door panels, skirts, etc., hot runners with sequence valves are usually used to eliminate weld lines. Generally, the needle valve in the middle is opened first, when the melt front flows through the next When the needle valve is opened, the needle valve is opened, so as to avoid forming more weld lines on the injection molded parts during filling. Even if the needle valve hot runner is used, the weld line around the hole is difficult to eliminate, but it can also be weakened in structural design. Generally, a structure such as a groove is added around the hole to disturb the formation of the weld line, thereby playing the role of weakening the weld line.
In the design of the mold, the gate should be avoided facing the hole, otherwise the weld line formed is difficult to adjust. In the injection molding process, using a lower injection speed in this section can weaken the weld line, but when the processing temperature or material melt flow rate fluctuates slightly, the injection molding process needs to be adjusted, which causes certain difficulties in production.