Fix Flow Marks On Plastic Auto Part Mold

More and more large size plastic injection molding parts are used in the automotive industry. For consideration of cost, the application of spray-free injection parts is more and more extensive. However, this also requires higher cosmetic quality of injection molding parts. On some large parts such as bumpers, dashboards, door panels, and long interior plastic accessories, it is very common to see an appearance defect characterized by a streak or stripes perpendicular to the plastic flow direction, which looks like a pattern on the skin of a tiger, we call this “Tiger Flow Line".

1. Possible causes

Molding process: In this case, the injection speed was very fast at the start. Then the material temperature was increased up to 255 ℃ while the actual mold temperature was about 49 ℃.  With this adjustment, we slowed down the injection speed but the flow line was still there. Then we noticed that the interval between the first valve gate and the other valve gate is only 3 seconds.  It tells that the molding pressure of the first valve gate may be insufficient due to the opening of other gates when the injection speed slows down, resulting in unstable pressure at the first stage of filling and unstable flow of molten plastic, thus causing tiger flow lines.

2. Remedy

In order to provide sufficient pressure to maintain the stable flow of the molten plastic at the first gate, close all other gates pressure and use slow injection speed as 15cm3/s. The tiger flow line disappears.

3. Important note

The result shows that the occurrence of the tiger flow line is not related directly to the viscosity of the melt. More and more studies have shown that instability in the flow process is the main cause of tiger flow lines.