Causes and solutions for dents and pores in plastic products

First, the cause of dents and pores in plastic products .
1. There are moisture and volatiles in the material.
2. The thickness of the product is uneven and the difference is too large.
3. Insufficient pressure in the mold, insufficient injection volume, pressure, time, speed and cycle.
4. The mold is not sufficiently cooled, resulting in deformation due to insufficient cooling time.
Second, related knowledge
1. In the production process, product dents and pores are the most frequent occurrences in 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, so-called dents. That is, the slow cooling portion produces a conspicuous concave surface in the direction in which the bubble shrinks; the so-called air hole means that the material in the mold solidifies from the surface, and the material is relatively insufficient for the entire volume in the mold, and for this reason, a vacuum state hole is generated. It usually occurs in places where the product is thick and at the injection opening.
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 when it does not affect the appearance, the mold is processed into a corrosive appearance, such as striate, granular and so on. If the molding material is impact-resistant polystyrene HIPS (a type of polystyrene PS), 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.
4, transparent products have pores become a problem, opaque products have pores without barriers to use and products should not be seen.
5. The pores due to moisture and volatiles generally diffuse into all parts of the product, and the shape of the pores is generally small.
Third, the solution of plastic products to produce dents and pores
1. Instant: Increase the injection pressure, extend the injection pressure holding time, reduce the barrel temperature and mold temperature, and dry the material when moisture and volatiles are caused, and forcibly cool 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 passes through a narrow place, 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. The gate, main flow, split runner, and nozzle hole should be enlarged to improve exhaust.
Fourth, due to material differences
A material having a large mold shrinkage property is also large indentation, for example, polyethylene PE or polypropylene PP, and even if a slight reinforcing rib is used, a dent is generated.


Mold shrinkage







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