How to deal with dents in plastic products?

一、Causes of dents

1. The thickness of each part of the product is different.

2.The internal pressure of mold is insufficient.

3.Mold cooling is not enough.

4.Deformation due to insufficient cooling time.

二、The associated knowledge

1. In the process of production,  the dent is the most frequent occurrence of the bad phenomenon. When the plastic injected into the mould is cooled, volume shrinkage occurs. The early cooling part (surface)hardens first, and bubbles will occur inside. The so-called dents are that the slow cooling part produces conspicuous concave surface in the direction of bubble shrinkage.

2.Materials with large shrinkage are prone to dents. When the molding conditions are to be changed to eliminate the dents, the setting conditions should be set in a direction in which the shrinkage is small. That is, the mold temperature,the barrel temperature are lowered, and the injection pressure is increased,but it should be noted that residual internal stress may be caused thereby.

3. It's better if the dent is not conspicuous.Therefore, if it does not affect the appearance, it is intentionally processed into corrosion patterns on the die,such as grain, granular and so on. Also, if the forming material is HIPS,reducing the mold temperature to reduce the smoothness is also effective. But once these methods produce dents, it is difficult to repair polished products.

三、Solutions

1.Instant: Increase the injection pressure, extend the injection pressure holding time, reduce the barrel temperature and mold temperature, and force cooling in the place where the dent is generated.

2.Short-term: fill the flow edge where the dent is created. When the material in the place where the dent is generated has a narrow space, the portion is thickened.

3.Long-term: The difference in thickness of the design product should be completely avoided. Ribs that are prone to dents should be as short as possible.

四、The difference in materials

 1.The material with large shrinkage in moulding also has large dent.For example, PE, PP, even if only slightly reinforced, will produce dents.

Material molding shrinkage

   PS 0.002~0.006

   PP 0.01~0.02

   PE 0.02~0.05

五、References

1.When the temperature is lowered to no dents, if there is pressure in the material in the cavity, it should be considered that no dents will occur.The pressure within the mold, ie, the static pressure, of the material surrounding the mold is not necessarily anywhere.The pressure near the gate is high. If the material is wide, because the pressure to the corners is transmitted, the pressure difference between the near gate and the gate is not much different from the pressure of the whole. It is also possible to obtain a product which does not leave internal stress.When part of the material flows into a difficult place, there is a high pressure in this place, and the pressure in other places is lowered to cause dents. This part of the high pressure residue is also the internal stress of the product. In the ideal state, the temperature of the material rises with the temperature of the mold,the fluidity of the material is good, and the injection also changes to the bottom in the static pressure state.

2. When changing the molding conditions, the combination of temperature, pressure and time should be prepared in advance and in order, the results can be known early. First, after the time has become very long, it is easy to know the small changes in the pressure. It should be noted that the result when the temperature is changed should be the result after the injection of the material, and then the temperature is lowered and then produced.