How To Remove Mold Dents

Publish Time: 07/01 2021 Author: Site Editor Visit: 17933

Causes of 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

Related knowledge

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 it is cooled. The early cooling part, that is, the surface hardens first, and bubbles will be generated inside. The so-called dents are The slow cooling part produces a conspicuous concave surface in the direction of the bubble shrinkage.
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 where the shrinkage is small. 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.
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, grains, etc. 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.

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 (Figure A). When there is a narrow place on the edge of the material where the dent is generated, make this part thick (Figure B).
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

The difference in materials

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

References

When the temperature is lowered to the point that no dents are produced, if the material in the cavity is still under pressure, it should be considered that no dents will be produced. The pressure in the mold of the material surrounding the mold is the static pressure, which is not always the same everywhere. The pressure of the part close to the gate is high. If the material has a wide through edge, because of the transmission of pressure to each corner, the pressure difference between the part near the gate and the place far away from the gate is very small compared with the overall pressure, and no depression 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 is reduced, which will cause dents. This part of the residual high pressure is the internal stress of the product. In an ideal state, the material temperature rises with the mold temperature, the material fluidity is better, and the injection becomes bottom in the static pressure state.
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 proceeding in order. First of all, after the time has become very long, it is easy to know every small change in pressure. It should be noted that the result when the temperature changes is obtained, and the result should be the result when the temperature drops after the injection of the material.

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