Gate blush Troubleshooting

What is gate blush?

Gate blush is a molding defects commonly caused at the gate location of injection molded part, It shows as rings or ripples in the mold cavity surrounding the gate area. This defect occurs when plastics material flow across the mold cavity rather than forming a fountain flow and freezing to the mold cavity. As more material enters the mold cavity, it erodes the material off of the mold surface, causing the blushed appearance, gate blush sometimes we call gate shear defects.

Cause

Injection pressure and speed is too high.

Gate blush defects caused by high pressures and speed going through a mold gate at the 1^st stage injection, the melt plastics pass through the gate too quickly so that create excessive shear.

Mold temperature too high.

The mold temperature too low or too high that could cause the gate blush problems, so it can be important to understand the correct mold temperature for different plastics materials. Gate blush is a strange phenomenon that can be magnified by the steel temperature and both too low and too high steel temperatures may lead to a poor formation of the initial plastic through the gate.

Some of materials like ABS, PC, PA66, increase the mold temperature will improve the mold surface, so the gate blush will be improved as well.

Mold gate size is too small

Gate size will mostly cause this gate blush issue as well, small size that requires the high injection pressure and speed.

Solution

There are some solutions can be applied to solve gate blush. The first is to reduce the injection speed so that the plastic resin do not extend as much as with higher speeds. This will lay down a clean skin of material and improve the gate blush issue. Reduce the injection speed can also be used, at the beginning of injection to start with a slow injection speed, switch to a higher injection speed once the cavity is filled.

Increase the mold temperature to improve this gate blush issue, low temperature will occur the material hard to fill and this will require the high injection speed and pressure as well.

Increase the gate size and add a cold slug well, big gate size will reduce the gate shear force, add a cold slug well is to receive the first slug of plastics material from the first step of nozzle and keep same material temperature with the flow path of the hotter material behind. Lack of a cold slug well can force this colder material into the gate and into the part showing up as gate blush. It is very important to add big cold slug well at each end of runner.

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