1. The influence of mold temperature on product appearance
Higher temperature can improve the fluidity of the resin, which usually makes the surface of the part smooth and shiny, especially to improve the surface beauty of glass fiber reinforced resin parts. It also improves the strength and appearance of the fusion line.
For the etched surface, if the mold temperature is low, the melt is more difficult to fill to the root of the texture, making the surface of the product appear shiny, and the real texture of the mold surface cannot be “transferred”. It can be improved after the mold temperature and material temperature are increased. Make the product surface get the ideal etching effect.
2. Influence on the internal stress of the product
The formation of internal stress in molding is basically caused by different thermal shrinkage rates during cooling. When the product is molded, its cooling gradually extends from the surface to the inside. The surface first shrinks and hardens, and then gradually goes to the inside. In this process, due to shrinkage The difference in speed results in internal stress.
When the residual internal stress in the plastic part is higher than the elastic limit of the resin, or under the erosion of a certain chemical environment, cracks will occur on the surface of the plastic part. Research on PC and PMMA transparent resins shows that the residual internal stress is in a compressed form on the surface layer and a stretched form in the inner layer.
The surface compressive stress depends on the cooling condition of the surface. The cold mold quickly cools down the molten resin, which makes the molded product produce higher residual internal stress. The mold temperature is the most basic condition to control the internal stress. A slight change of the mold temperature will greatly change its residual internal stress. Generally speaking, the acceptable internal stress of each product and resin has its minimum mold temperature limit. When molding thin walls or longer flow distances, the mold temperature should be higher than the minimum for general molding.
3. Improve product warpage
If the mold cooling system is not properly designed or the mold temperature is not properly controlled, insufficient cooling of the plastic part will cause the plastic part to warp and deform.
For the control of the mold temperature, the temperature difference between the male mold and the female mold, the mold core and the mold wall, and 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 and shrinkage speed of each part of the mold. After demolding, it tends to bend in the traction direction on the higher temperature side to offset the difference in orientation shrinkage and avoid warping and deformation of the plastic part according to the law of orientation.
For a plastic part with a completely symmetrical structure, the mold temperature should be kept consistent accordingly, so that the cooling of all parts of the plastic part is balanced.
4. Affect the molding shrinkage rate of the product
The low mold temperature accelerates the molecular “freezing orientation”, which increases the thickness of the frozen layer of the melt in the mold cavity. At the same time, the low mold temperature hinders the growth of crystals, thereby reducing the molding shrinkage of the product. On the contrary, if the mold temperature is high, the melt will cool slowly, the relaxation time will be long, the orientation level will be low, and it will be beneficial to crystallization, and the actual shrinkage of the product will be greater.
5. Affect the heat distortion temperature of the product
Especially for crystalline plastics, if the product is molded at a lower mold temperature, the molecular orientation and crystals are instantly frozen, and under a higher temperature use environment or secondary processing conditions, the molecular chain will be partially rearranged And the process of crystallization makes the product deform at even much lower than the heat distortion temperature (HDT) of the material.
The correct way is to use the recommended mold temperature close to its crystallization temperature to make the product fully crystallized in the injection molding stage to avoid this post-crystallization and post-shrinkage in a high-temperature environment.
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 molding, secondary processing and final use of the product cannot be underestimated.
Post time: Nov-09-2021