2. Associated knowledge

2-1 In the process of producing products, dents occur more frequently among the bad phenomena. The plastic injected into the mold shrinks in volume when it is cooled. The early cooling part, that is, the surface hardens first, and bubbles are formed inside, so-called dents. It is the slow cooling part that creates a conspicuous concave surface in the direction in which the bubble shrinks.

  2-2 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 small shrinkage. That is, the mold temperature and barrel temperature decrease, and the injection pressure increases, but it should be noted that residual internal stress may be caused by this.

  2-3 Because the dents are better to be inconspicuous, if they do not affect the appearance, they are deliberately processed on the mold to corrode, such as grains, grains, etc. Also, if the molding material is HIPS, it is also effective to lower the finish by lowering the mold temperature. However, once a dent occurs in these methods, it is difficult to repair the polished product.