Profiles will have dents in the production process, so what is the reason for the dents? What is the solution?
Causes of dents: different thickness of parts of products, insufficient internal pressure of dies, insufficient cooling of dies, and deformation due to insufficient cooling time
Associated knowledge
In the process of producing products, dents are the most frequent adverse phenomena. When the plastic injected into the mould is cooled, volume shrinkage occurs. The early cooling part, i.e. the surface hardens first, and bubbles will appear inside. The so-called dent is that the slow cooling part produces conspicuous concave surface in the direction of bubble shrinkage.
Large shrinkage materials are also prone to dents. When changing the forming conditions to eliminate dents, the setting conditions should be set in the direction of small shrinkage. That is, the die temperature and the barrel temperature decrease, and the injection pressure increases, but it should be noted that the residual internal stress may be caused by this.
Because the indentation is better not to be conspicuous, if it does not affect the appearance, it is intentionally processed into corrosion patterns on the die, such as grain, granular and so on. Also, if the forming material is HIPS, reducing the mold temperature to reduce the smoothness is also effective. But once these methods have dents, it is difficult to repair polished products.
Resolvent
Instant: Increase injection pressure, prolong injection holding time, reduce barrel temperature and die temperature, and force cooling in the place where dents occur.
Short-term: Fill in the runner where the dent occurs (Fig. A). When there is a narrow area along the edge of the material where the dent occurs, the edge is thicker (Fig. B).
Long-term: The thickness difference of design products should be completely avoided. Reinforcement tendons prone to dents should be as short as possible in narrow and long shapes (Fig. C).
Material differences
The material with large shrinkage in moulding also has large dents.
For example, PE, PP, even if only slightly reinforced, will produce dents.
Molding shrinkage of materials PS0.002~0.006PP0.01~0.02PE0.02~0.05
Matters needing attention
When the temperature decreases to zero, if there is pressure on the material in the die cavity, it should be considered that no dent will occur. The pressure of the material surrounding the die, i.e. the static pressure, is not necessarily anywhere in the die.
If the pressure near the gate is high, if the material passes wide, because of the pressure transfer to each corner, the pressure difference between the near gate and the place far from the gate is very small compared with the whole pressure, there will be no dent, and the products without residual internal stress will be obtained.
When part of the material flows into a difficult place, there is high pressure in this place, and when the pressure in other places decreases, there will be dents. This part of the high pressure residue is the internal stress of the product is also large. In the ideal state, the material fluidity is better when the temperature of the material rises with the temperature of the die, and the ejection also changes to the bottom in the static pressure state.
When changing the forming conditions, the combination of temperature, pressure and time should be prepared in advance, and the results can be known in order. After a long time, it's easy to know when the pressure changes slightly. It should be noted that when the temperature changes, the results should be obtained after injection material, and when the temperature drops, the results should be reproduced.
