How to remove the dent mark of the mold?

How to remove the dent mark of the mold?

1. Reasons for dents

 The thickness of each part of the product is different

 Insufficient pressure inside the mold

 Insufficient mold cooling

 Deformation due to insufficient cooling time

 2. Related knowledge

a. In the process of production, dents are the most frequent occurrence of undesirable phenomena. The plastic injected into the mold shrinks in volume when it is cooled. The early cooling part, that is, the surface hardens first, and bubbles will be generated inside. The so-called dent is the slow cooling part produces a conspicuous concave surface in the direction of bubble shrinkage.

b. Materials with large shrinkage are also prone to dents. When changing the molding conditions to eliminate dents, the setting conditions should be set 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.

c. Because the dents are better to be inconspicuous, if they do not affect the appearance, they are deliberately processed into a corroded pattern on the mold, such as grains. Also, if the molding material is HIPS, it is also effective to lower the mold temperature to reduce the finish. However, once dents occur in these methods, it is difficult to repair polished products.

3.The solution
Instant: Increase injection pressure, extend injection holding time, reduce barrel temperature and mold temperature, and force cooling where dents are generated.
Short-term: Fill up the flow edge where the dent is generated. When the material in the dented area has a narrow place, make this part thick. 

Long-term: The thickness difference of the designed product should be completely avoided. Reinforcing ribs that are prone to dents, the long and narrow shape should be as short as possible

4.Differences in materials
Materials with large molding shrinkage have large dents. For example, PE, PP, even if only a slight reinforcement ribs, will produce dents.

5.References
When the temperature is reduced to no dents, if the material in the cavity is still under pressure, it should be considered that there will be no dents. The pressure in the mold of the material surrounding the mold is the static pressure, which is not always the same everywhere.
The pressure at the part close to the gate is high. If the material has a wide through edge, because of the transmission of pressure to all corners, the pressure difference between the part near the gate and the place far away from the gate is very small compared to the overall pressure, and no dent mark will occur. It is also possible to obtain products without residual internal stress. 
When part of the material flows into a difficult place, this place has high pressure, and the pressure in other places decreases and dents are generated. 

When changing the molding conditions, the combination of temperature, pressure, and time should be prepared in advance, and the results can be known early by performing them in order. First of all, after the time has become very long, it is easy to know every small change in pressure.