1. Causes of dents

1-1 The thickness of each part of the product is different
1-2 The internal pressure of the mold is insufficient
1-3 Insufficient mold cooling
1-4 Deformation caused by insufficient cooling time

2. Associated knowledge

2-1 In the process of producing products, dents are the most frequent occurrence of undesirable phenomena, the plastic injected into the mold shrinks in volume when cooling, and the surface of the early cooling part hardens first, and bubbles will be produced inside, the so-called dents, which are the parts that cool slowly in the direction of bubble contraction.
2-2 Materials with high shrinkage are also prone to dents. If you want to change the molding conditions to eliminate dents, you should set the setting conditions in the direction of small shrinkage. That is, the mold temperature and barrel temperature decrease, and the injection pressure increases, but it should be noted that this may cause residual internal stress.
2-3 Because it is better to have inconspicuous dents, if they do not affect the appearance, they are deliberately processed on the mold to form a corroded pattern, such as grained or granular. In addition, if the molding material is HIPS, it is also effective to reduce the finish by lowering the mold temperature. However, once a dent occurs in these methods, it is difficult to repair the polished product.

3. Solution

3-1 Immediate: Increase the injection pressure, prolong the injection pressure holding time, reduce the barrel temperature and mold temperature, and force cooling in the place where the dent is generated.
3-2 Short-term: Fill the edge where the dent is generated. When there is a narrow place on the edge of the material where the dent is generated, make this part of the edge thick.
3-3 Long-term: The difference in the thickness of the design product should be completely avoided. Stiffeners that are prone to dents should be as short as possible in shape

4.the difference in materials

4-1 Molding materials with large shrinkage and large dents. For example, PE and PP can cause dents even if they are slightly stiffened.

5. References:

5-1 When the temperature is lowered to the point where no dents are generated, if there is still pressure on the material in the mold cavity, it should be considered that no dents will occur. The pressure of the material surrounding the mold in the mold is the static pressure, which is not necessarily the same everywhere.

If the pressure near the gate is high, if the edge of the material is wide, the pressure difference between the near gate and the place far away from the gate is small compared to the overall pressure due to the transmission of pressure to all corners, and it is possible to obtain a product with no residual internal stress. When some materials flow into difficult places, there is a high pressure in this place, and dents will occur when the pressure in other places decreases. The residue of this part of the high pressure is also the internal stress of the product.

In the ideal state, the material temperature rises with the mold temperature, the material has better fluidity, and the injection also bottoms out in the static pressure state.

5-2 When changing molding conditions, the combination of temperature, pressure, and time should be pre-made and performed in order, so that the results can be known as soon as possible. First of all, it is easy to see small changes in stress after a long period of time. It should be noted that the results obtained when the temperature changes should be produced after the injection and after the temperature has dropped.