5 points of the role of mold temperature on the quality control of injection molded parts

Mold temperature refers to the surface temperature of the mold cavity which is in contact with the products during the injection molding process. Because it directly affects the cooling rate of the products in the mold cavity, thus has a great impact on the inner performance and appearance quality of the products.

1. The influence of mold temperature on product appearance

Higher temperature can improve the fluidity of resin, which usually makes the surface of the parts smooth and shiny, especially to improve the surface aesthetics of glass fiber reinforced resin parts. It also improves the strength and appearance of the fusion line.

As for the etched surface, if the mold temperature is low, it is difficult to fill the melt to the root of the texture, which makes the surface of the product appear shiny and "transfer" the real texture of the mold surface, and after increasing the mold temperature and material temperature, it can make the surface of the product get the ideal etching effect.

2. Influence on the internal stress of the product

The formation of internal stress is basically due to the different heat shrinkage rates during cooling, when the product is formed, its cooling is gradually extended from the surface to the interior, the surface first shrinks and hardens, and then gradually to the interior, in the process due to the difference between the speed of shrinkage and internal stress.

When the residual internal stress inside the plastic part is higher than the elastic limit of the resin, or under the erosion of a certain chemical environment, the surface of the plastic part will be cracked. Studies on PC and PMMA transparent resins show that the residual internal stresses are compressive in the surface layer and tensile in the inner layer.

The surface compressive stress depends on the cooling condition of the surface, and a cold mold causes the molten resin to cool down rapidly, resulting in a high residual internal stress in the molded part. The mold temperature is the most basic condition to control the internal stress, and a slight chance of mold temperature will have a great change on its residual internal stress. Generally speaking, each product and resin has its own minimum mold temperature limit for acceptable internal stress. When molding a thin wall or long flow distance, the mold temperature should be higher than the minimum limit in general molding.

3. Improve product warpage

If the cooling system of the mold is not designed properly or the mold temperature is not controlled properly, insufficient cooling of the plastic part will cause a warpage of the plastic part.

For the mold temperature control, according to the structural characteristics of the product to determine the temperature difference between the Yang mold and the Yin mold, mold core and mold wall, mold wall and insert, so as to use the different cooling and shrinkage speed of each part of the control mold, the plastic part tends to bend to the higher temperature side of the traction direction after demolding, to offset the orientation shrinkage difference and avoid warping deformation of the plastic part according to the law of orientation.

For the shape and structure of completely symmetrical plastic parts, the mold temperature should be maintained accordingly, so that the cooling of each part of the plastic part is balanced.

4. Affect the shrinkage rate of the product

Low mold temperature makes the molecules "freeze orientation" faster, making the melt in the cavity of the freezing layer thickness increase, while the low mold temperature hinders the growth of crystallization, thus reducing the molding shrinkage of the product. On the contrary, if the mold temperature is high, the melt cools slowly, the relaxation time is long, the orientation level is low, and at the same time, it is conducive to crystallization, and the actual shrinkage rate of the product is larger.

5. Influence the heat deflection temperature of the product

Especially for crystalline plastics, if the product is molded at a lower mold temperature, the molecular orientation and crystallization are instantly frozen, and when a higher temperature is used or under secondary processing conditions, its molecular chains will partially rearrange and crystallize the process, making the product deform at even much lower than the material's heat deflection temperature (HDT).

The correct approach is to produce the product at the recommended mold temperature close to its crystallization temperature so that the product is fully crystallized during the injection molding stage, avoiding such post-crystallization and post-shrinkage at high temperatures.

In conclusion, mold temperature is one of the most basic control parameters in the injection molding process and is also a primary consideration in mold design. Its influence on the molding, secondary processing, and end-use process of the product cannot be underestimated.