First, the cause of the dent
1, the thickness of each part of the product is different
2, the mold is under pressure
3, mold cooling is not enough
4. Deformation due to insufficient cooling time
Second, the associated knowledge
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 is the first hardened surface, and the inside will generate bubbles. The so-called dent is cooling. The slow portion produces a conspicuous concave surface in the direction in which the bubble contracts.
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.
3, because the dent is not conspicuous, so if it does not affect the appearance, there is a deliberately processed on the mold into a corrosive appearance, such as striate, granular and so on. 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
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 there is a narrow place on the side of the material where the dent is generated, the side is thick.
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.
Fourth, the difference in materials
A material with a large mold shrinkage has a large dent. For example, PE and PP produce dents even with a slight rib.
Material molding shrinkage
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, 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.