How Do Dents And Pores On Injection Molded Products Form And How Do Solve Them

In the production process of plastic products, product dents, and pores are the most frequent occurrences of adverse phenomena. The plastic injected into the mold shrinks in volume when cooled, and the early cooling part, that is, the surface hardens first, and bubbles will form inside;

The dent is that the slow cooling part produces a conspicuous concave surface in the direction of the bubble shrinkage; the air hole refers to the material in the mold starting to solidify from the surface, and the material in the mold is relatively insufficient for the entire volume of the mold. Due to this reason, a hole in a vacuum state is generated, which generally occurs in the thick part of the product and at the injection port.

Materials with high shrinkage are also prone to dents. When changing the molding conditions to eliminate dents, the setting conditions should be set in the direction of smaller shrinkage. That is, the mold temperature and barrel temperature are lowered, and the injection pressure is increased, but it should be noted that this may cause residual internal stress.

Because it is better for the dent to be inconspicuous, so when it does not affect the appearance, it should be processed into a corroded pattern on the mold, such as striations, grains, etc.

If the molding material is impact-resistant polystyrene HIPS (a type of polystyrene PS), it is also effective to reduce the finish by reducing the mold temperature. However, once dents occur in these methods, it is difficult to repair polished products.

Pores in transparent products are a problem. Pores in opaque products are not an obstacle to use and should not be seen inside the product.

Due to the pores generated by moisture and volatiles, they generally diffuse to all parts of the product, and the shape of the pores is generally small.

1. The solution

Immediately: Increase the injection pressure, prolong the injection holding time, reduce the barrel temperature and mold temperature, fully dry the material when moisture and volatile matter are caused, and force cooling at the place where the dent occurs.

Short term: fill in the flow edge where the dent occurs. When the material in the place where the dent occurs passes through a narrow place, thickens this part.

Long-term: The difference in the thickness of the designed product should be completely avoided. For ribs that are prone to dents, the long and narrow shape should be kept as short as possible. Gates, sprues, runners, and nozzle holes should be enlarged. Improved exhaust.

2. References

1. Materials with large molding shrinkage also have large dents, such as polyethylene PE and polypropylene PP, even if there is only a slight reinforcement, dents will occur.

Material molding shrinkage PS0.002~0.006PP0.01~0.02PE0.02~0.05

2. When the temperature is reduced to no dents if the material in the cavity still has pressure, it should be considered that no dents will be produced. The pressure of the material surrounding the mold in the mold, that is, the static pressure, is not constant no matter where it is.

The pressure near the gate is high. If the material has a wide edge, because of the pressure transmission to all corners, the pressure difference between the near gate and the place far from the gate is small compared with the overall pressure, so no dents will occur, and products without residual internal stress can be obtained.

And when part of the material flows into a place where there is difficulty, there is high pressure in this place, and the pressure in other places is reduced to produce dents. This part of the high-pressure residue is the internal stress of the product is also large. In an ideal state, the temperature of the material rises with the temperature of the mold, the fluidity of the material is better, and the injection is also lower in the state of static pressure.

3. 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. First of all, after the time becomes very long, it is easy to know every small change in pressure. It should be noted that the results obtained when the temperature changes should be obtained after the injection of the material, and the results when the temperature is lowered before production.

4. To determine the cause of the pores, just observe whether the bubbles of the plastic product appear instantaneously when the mold is opened or after cooling. If they appear instantaneously when the mold is opened, it is mostly a material problem. If they appear after cooling, it is a problem with the mold or injection molding conditions.