5 points effect of mold temperature on quality control of injection molded parts

Effect of mold temperature on product appearance

Higher temperatures improve the fluidity of the resin, which generally results in a smooth, shiny surface, especially for the surface aesthetics of glass-reinforced resin parts. It also improves the strength and appearance of the fusion line.

For the etched surface, if the mold temperature is lower, the melt is more difficult to fill the root of the texture, so that the surface of the product appears bright, "transfer" can not be the real texture of the mold surface, and the mold temperature and material temperature can be improved. Make the surface of the product get the desired etch effect.

Effect on internal stress of the product

The formation of the internal stress of the molding is basically caused by the different heat shrinkage rate during cooling. When the product is formed, its cooling is gradually extended from the surface to the inside, and the surface first shrinks and hardens, and then gradually goes to the inside, in the process due to shrinkage. The difference between the speed and the slowness produces internal stress.

When the residual internal stress in the plastic part is higher than the elastic limit of the resin or under the attack of a certain chemical environment, cracks may occur on the surface of the injection molded part. Studies on PC and PMMA transparent resins have shown that the residual internal stress is in a compressed form on the surface layer and the inner layer is in a stretched form.

The surface compressive stress depends on the surface cooling condition, and the cold mold rapidly cools the molten resin, thereby causing the molded article to generate a high residual internal stress. The mold temperature is the most basic condition for controlling the internal stress. A slight change in the mold temperature will greatly change the residual internal stress. In general, the acceptable internal stress for each product and resin has its lowest mold temperature limit. When forming a thin wall or a long flow distance, the mold temperature should be higher than the minimum of the general molding.

Improve product warpage

If the cooling system design of the mold is not reasonable or the mold temperature is improperly controlled, the plastic parts are insufficiently cooled, which will cause the plastic parts to warp and deform.

For the control of the mold temperature, the temperature difference between the male mold and the female mold, the core and the mold wall, the mold wall and the insert should be determined according to the structural characteristics of the product, so as to control the difference in the cooling shrinkage speed of each part of the molded part, the plastic part After demolding, it tends to bend toward the higher temperature side to overcome the difference in orientation shrinkage and avoid warping deformation of the plastic parts according to the orientation law.

For plastic parts with completely symmetrical structure, the mold temperature should be consistent, so that the cooling of all parts of the plastic parts is balanced.

Affect the molding shrinkage of the product

The low mold temperature accelerates the "freezing orientation" of the molecules, increasing the thickness of the frozen layer of the melt in the cavity, while the low mold temperature hinders the growth of the crystal, thereby reducing the mold shrinkage of the article. On the contrary, when the mold temperature is high, the melt is cooled slowly, the relaxation time is long, the orientation level is low, and the crystallization is favored, and the actual shrinkage rate of the product is large.

Affect the heat distortion temperature of the product

Especially for crystalline plastics, if the product is molded at a lower mold temperature, the orientation and crystallization of the molecules are instantaneously frozen. When a higher temperature environment or secondary processing conditions, the molecular chains are partially rearranged. And the crystallization process causes the product to deform even at temperatures well below the material's heat distortion temperature (HDT).

The correct approach is to use the recommended mold temperature close to its crystallization temperature to allow the product to be fully crystallized during the injection molding stage to avoid post-crystallization and post-shrinkage in high temperature environments.

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