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, that is, the surface first hardens, and the inside generates bubbles. The so-called dent is cooling. The slow part produces a conspicuous concave surface in the direction of bubble contraction.
2, materials with large shrinkage are also prone to dents. When changing the molding conditions to eliminate dents, the setting conditions should be set to a direction of small shrinkage. That is, the mold temperature, the barrel temperature is lowered, and the injection pressure is increased. However, it should be noted that residual internal stress may be caused by this.
3, because the dent is not conspicuous, so if it does not affect the appearance, it is intentionally processed on the mold into a corrosive appearance, such as striate, granular, etc. Also, if the molding material is HIPS, use lower Mold temperature to reduce the finish is also effective. However, if 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, forced cooling in the place where the dent is generated.
2, short-term: fill the flow edge in the place where the dent is generated. When there is a narrow place in the material where the dent is generated, the thickness of this part is thick.
3, long-term: should completely avoid the difference in the thickness of the design. The ribs are prone to dents, the narrow shape should be as short as possible.
Fourth, the difference in materials
For materials with large shrinkage and shrinkage, the dents are also large. For example, PE, PP, even if only a slight rib is used, it will produce dents.
Material molding shrinkage
PS0.002~0.006
PP0.01~0.02
PE0.02~0.05
Five, reference items:
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 dent will occur. The pressure of the material surrounding the mold in the mold is the static pressure, no matter what. The place is not necessarily the same. The pressure near the gate is high. If the material is wide, because of the pressure transmission to each corner, the pressure difference between the near gate and the distance from the gate is different from the pressure of the whole. When it is small, it will not produce dents, and it will be able to obtain products without residual internal stress. When some materials are in a place where there is difficulty flowing, there is high pressure in this place, and the pressure drop in other places will cause dents. This part of the high pressure residue is the internal stress of the product. In the ideal state, the material temperature rises with the mold temperature, the material fluidity is better, and the injection is also in the static pressure state.
2. When changing the molding conditions, the combination of temperature, pressure and time should be prepared in advance, and the results can be known in advance. The first time the time becomes very long, the small changes in pressure are easy to know. It should be noted that the result of the temperature change should be the result after the injection of the material, and then the temperature is lowered and then produced.