First, the cause of the dent
1-1 Different thickness of each part of the product
1-2 Insufficient pressure inside the mold
1-3 mold cooling is insufficient
1-4 Deformation due to insufficient cooling time
Second, the associated knowledge
2-1 In the process of producing products, the dent is the most frequent occurrence of the bad phenomenon. The plastic injected into the mold shrinks when it cools. The early cooling part, the surface is first hardened, and the inside will generate bubbles. The so-called dent is The slow cooling portion produces a conspicuous concave surface in the direction in which the bubble contracts.
2-2 Materials with large shrinkage are also 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.
2-3 Because the dent is not conspicuous, if it does not affect the appearance, it is intentionally processed into a corrosive pattern on the mold, such as striate, granular, and the like. Also, if the molding material is HIPS, it is also effective to lower the mold temperature to lower the finish. However, once these methods have dents, it is difficult to repair the polished products.
Third, the solution
3-1 Instant: Increase the injection pressure, extend the injection dwell 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 flow edge where the dent is created. When there is a narrow place on the side of the material where the dent is generated, the side is thick.
3-3 Long-term: The difference in thickness of the design product should be completely avoided. Ribs that are prone to dents, and the shape should be as short as possible
Fourth, the difference in materials
4-1 Materials with large mold shrinkage and large dents. For example, PE and PP produce dents even with a slight reinforcement.
V. References:
5-1 When the temperature is lowered to no dent, if there is pressure in the material in the cavity, it should be considered that no dents will occur. The pressure within the mold surrounding the mold, i.e., the static pressure, is not necessarily anywhere.
The pressure near the gate is high. If the material is wide, the pressure difference between the near gate and the gate is small, and the difference between the pressure of the near gate and the gate is small. 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 drop in other places will cause dents. This part of the high pressure residue is also the internal stress of the product.
In an 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.
5-2 When changing the molding conditions, the combination of temperature, pressure, and time should be prepared in advance, and 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 after the temperature is lowered.
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