(4) Poor exhaust of barrel and mold.

If the barrel or mold is poorly vented, the remaining gas in the melt will cause combustion due to adiabatic compression, causing the melt to overheat and decompose, resulting in black spots and streaks.

In this regard, the injection speed can be appropriately reduced, and the structure of the barrel exhaust port can be improved under the condition that the particle size and uniformity of the raw material are suitable.

(5) Coking of stacked materials.

When the nozzle does not fit well with the main channel of the mold, the accumulation coking will occur near the gate and be injected into the cavity with the flowing material, forming black spots and stripes on the surface of the plastic part.

In this regard, the relative position of the nozzle and the main channel of the mold should be adjusted in time to make it fit well.

If the hot runner of the mold is poorly designed or made, the melt will not flow smoothly in the runner and stay in coking, which will also cause black spots and streaks on the surface of the plastic part.

In this regard, the surface luminosity of the hot runner should be increased and the heating temperature of the runner should be reduced.

(6) The raw materials do not meet the molding requirements.

If the volatile content in the raw materials is too high, the water-sensitive resin is not well dried, the amount of recycled materials is too much, there are too many fine powders, the raw materials are unevenly colored, and the lubricant varieties are not selected correctly or used excessively, which will lead to black spots and streaks on the surface of plastic parts to varying degrees.

In this regard, corresponding measures should be taken according to different situations to eliminate them separately.