Mold temperature refers to the external surface temperature of the mold cavity that is in contact with the article during plastic injection molding. Because it directly affects the cooling rate of the plastic parts in mold cavity, it has a great influence on the intrinsic performance and appearance quality of the injection molded plastic parts.

Effect of mold temperature on the appearance of injection molded plastic parts
Higher temperatures can improve the activity of the resin, which generally results in a smooth, shiny appearance of the injection molded plastic parts, especially in the aesthetics of glass-reinforced resin parts. It also improves the strength and surface of the blending line.

On the etched surface, if the injection mold temperature is relatively low, the melt is more difficult to fill the root of the texture, making the appearance of the plastic product appear bright.

Effect on internal stress of the injection molded plastic parts
The composition of the injection molding internal stress is basically formed by the different heat shrinkage rate during cooling. When the plastic part is formed, its cooling is gradually extended from the outside to the inside, and the appearance first shrinks and hardens. Then gradually goes to the inside and produces internal stress in the process due to shrinkage difference.

When the residual internal stress in the plastic part is higher than the elastic limit of the resin or under the corrosion of a certain chemical environment, there will be crack marks on the appearance of the plastic part. A study of PC and PMMA transparent resin revealed that the residual internal stress was in the compacted form on the outer surface and the inner layer was in the stretched form.

The external compressive stress depends on the external cooling condition, and the cold mold rapidly cools the molten resin, so that the injection molded part generates a high residual internal stress. Mold temperature is the most fundamental condition for controlling the internal stress. A slight change in injection mold temperature will greatly change the residual internal stress. In general, the endurance of each plastic product and resin has its lowest mold temperature limit.

Mold temperature has effect on improve the warpage of injection molded parts
If the design of cooling system of plastic injection mold is not reasonable or the mold temperature is improperly controlled, the lack of cooling of the injection molded plastic parts will cause warping deformation of the plastic parts.

Regarding the mold temperature control, the temperature difference between the male mold and the female mold, the core and the cavity, should be affirmed according to the structural characteristics of the plastic parts, so as to control the difference in the cooling shrinkage speed of each part of the injection molded part. The plastic part tends to bend toward the higher temperature side to overcome the difference in orientation shrinkage and prevent the plastic parts from warping and deforming according to the orientation law after demolding.

Mold temperature affects the molding shrinkage of the injection molded plastic parts
The low mold temperature accelerates the “freezing orientation” of the molecules, so that the thickness of the frozen layer of the melt in mold cavity increases, and the mold temperature lowers the growth of the crystal, thereby reducing the molding shrinkage of the injection molded plastic parts. On the contrary, when the mold temperature is high, the melt cooling is slow, the relaxation time is long, the degree of orientation is low, and the crystallization is favored, and the practical shrinkage rate of the plastic parts is large.

Mold temperature affects the heat distortion temperature of the injection molded plastic parts
Especially with regard to crystalline plastics, if the plastic part is molded at a lower mold temperature, the orientation and crystallization of the molecules are frozen when they are entangled. When a higher temperature environment or secondary processing conditions, the molecular chains will stop partially rearranging.

The correct approach is to use the recommended mold temperature near its crystallization temperature to allow the plastic part to be crystallized during the injection molding stage to prevent post-crystallization and post-shrinkage in high temperature environments.

In short, mold temperature is one of the most fundamental control parameters in the injection molding process, and it is also the primary consideration in mold design. Its influence on the injection molding, secondary processing and final application of the injection molded plastic parts cannot be underestimated.