What to do if dents appear on plastic products？

I. Causes of dents in medical injection molding products

1. Different thicknesses of different parts of the product

2. Insufficient internal pressure of the mold

3. Insufficient cooling of the mold

4. Deformation caused by insufficient cooling time

II. Related knowledge

1. In the process of producing products, dents are the most frequent defective phenomenon. The plastic injected into the mold shrinks in volume during cooling. The early cooling part, that is, the surface, hardens first, and bubbles will be generated inside. The so-called dent is that the slow cooling part has 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 in the direction of small shrinkage. That is, the mold temperature and barrel temperature are reduced, and the injection pressure is increased, but it should be noted that this may cause residual internal stress.

3. Because dents are better to be inconspicuous, if it does not affect the appearance, there are intentional processing on the mold to make it look corroded, such as grainy, granular, etc. Also, if the molding material is HIPS, lowering the mold temperature to reduce the finish is also effective. However, once these methods have dents, it is difficult to repair the polished product.
Three, the solution
1. Immediate: increase the injection pressure, extend the injection holding time, reduce the barrel temperature and mold temperature, and force cooling where the dent occurs.
2. Short-term: fill the flow edge where the dent occurs. When there is a narrow space on the material side where the dent occurs, make this part thicker.
3. Long-term: The difference in the thickness of the designed product should be completely avoided. The ribs that are prone to dents should be as short as possible in a narrow and long shape.
Four, the difference in materials
Materials with large molding shrinkage also have large dents. For example, PE and PP will produce dents even with a slight rib.
Material molding shrinkage rate
PS0.002~0.006
PP0.01~0.02
PE0.02~0.05

V. Reference items:
1. When the temperature is reduced to the point where no dents are produced, if there is still pressure on the material in the mold cavity, 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 transmission of pressure to each corner, the pressure difference between the place near the gate and the place far from the gate is very small compared to the overall pressure, no dents will be produced, and a product with no residual internal stress can be obtained. When part of the material has difficulty flowing into a place, this place has high pressure, and the pressure in other places is reduced, which will produce dents. The residual high pressure in this part is that the internal stress of the product is also large. Under ideal conditions, the material temperature rises with the mold temperature, the material fluidity is better, and the injection is also lowered in the static pressure state.
2. When changing the molding conditions, the combination of temperature, pressure and time should be prepared in advance. If you follow the order, you can know the results early. First, after the time becomes very long, it is easy to know every small change in pressure. It should be noted that the result of temperature change should be obtained after the injection of the material and the temperature drops before the production.