Mold temperature refers to the surface temperature of the mold cavity that is in contact with the product during the injection molding process. Because it directly affects the cooling rate of the product in the mold cavity, it has a great influence on the intrinsic performance and appearance quality of the product.
1. Effect of mold temperature on product appearance
The higher temperature can improve the fluidity of the resin, which will usually make the surface of the product smooth and glossy, especially to improve the surface aesthetics of glass fiber reinforced resin parts. At the same time, it also improves the strength and appearance of the fusion line.
For the etched surface, if the mold temperature is lower, the melt is harder to fill in the root of the texture, making the surface of the product look bright, and the 'transfer' is less than the actual texture of the mold surface. After raising the mold temperature and material temperature, Makes the surface of the product ideal for etched lines.
2. Effect on the stress in the product
The forming of the internal stress is basically caused by different thermal shrinkage rates during cooling. After 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, due to the shrinkage. The difference between the speed and the internal stress.
When the residual internal stress in the plastic part is higher than the elastic limit of the resin or under certain chemical environment, the surface of the plastic part will be cracked. Studies on PC and PMMA transparent resin show that residual internal stress is on the surface. The layer is compressed and the inner layer is stretched.
The surface compressive stress depends on the surface cooling conditions. The cold mold rapidly cools the molten resin, which causes the molded product to have a high residual internal stress. The mold temperature is the most basic condition for controlling the internal stress, and the mold is slightly changed. Temperature, its residual internal stress will be greatly changed. In general, the acceptable internal stress of each product and resin has its lowest mold temperature limit. When the thin wall or long flow distance is formed, The mold temperature should be higher than the minimum when molding.
3. Improve product warping
If the design of the cooling system of the mold is unreasonable or the temperature of the mold is improperly controlled, the plastic parts will be insufficiently cooled, which will cause the plastic parts to warp and deform.
For mold temperature control, the temperature difference between male and female molds, mold cores and mold walls, mold walls and inserts should be determined based on the structural characteristics of the products. After demolding, it tends to bend toward the traction direction of the side with higher temperature to compensate for the difference in orientation shrinkage, and to avoid warping deformation of plastic parts according to the orientation law.
For plastic parts with a fully symmetrical body structure, the mold temperature should be kept consistent and the cooling of all parts of the plastic parts should be balanced.
4. Affect the molding shrinkage of products
The low mold temperature accelerates the 'freeze orientation' of the molecules, so that the thickness of the frozen layer in the melt in the mold cavity increases, while at the same time the low mold temperature prevents the growth of crystals, thereby reducing the molding shrinkage of the product. On the contrary, when the mold temperature is high, the melt is melted. Slow cooling, long relaxation time, low level of orientation, beneficial to crystallization, and high actual shrinkage of the product.
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 crystallization are instantaneously frozen, and the molecular chain is partially rearranged when used in a relatively high temperature use environment or secondary processing conditions. And the crystallization process makes the product deform even below the heat distortion temperature (HDT) of the material.
The correct method is to use the recommended temperature close to the crystallization temperature of the mold production, so that the product in the injection molding stage will be fully crystallized, to avoid this in the high temperature environment after the crystallization and post shrinkage.
In short, the mold temperature is one of the most basic control parameters in the injection molding process. At the same time, it is also a primary consideration in the mold design. Its influence on the molding, secondary processing and final use of the product cannot be underestimated.