Dark spots not only occur at the gate, but also often appear after the sharp corners of the article are formed. For example, sharp corner surfaces of articles are generally very smooth, but very gray and rough behind it. This is also due to the excessive flow rate and injection speed causing the cooling surface layer to be displaced by internal fluid displacement.
It is recommended to use step-by-step injection again and gradually increase the injection speed. The best way is to allow the melt to begin to increase in velocity only after passing through an acute edge.
In areas away from the gate, sharp changes in the angle of the product can also cause this defect. Therefore, when designing the product, smoother fillet transitions are used in those areas.
Improve gloss difference
For injection moulded articles, the difference in gloss is most pronounced on the surface of textured articles. Even if the surface of the mould is very uniform, irregular gloss may appear on the product. In other words, the mould surface effect of some parts of the product is not well reproduced.
As the distance from the melt exits the gate, the injection pressure of the melt gradually decreases. If the end of the gate of the product cannot be filled, the pressure at that point is minimal, so that the texture of the mould surface cannot be properly reproduced on the surface of the product. Therefore, the area where the pressure in the cavity is the greatest (half of the fluid path from the gate) is the area where the gloss difference is least.
To change this situation, you can increase the melt and mould temperature or increase the pressure, while increasing the pressure holding time can also reduce the occurrence of gloss differences.