Mold temperature refers to surface temperature of mold cavity in contact with product. It has a great influence on filling fluidity of plastic melt, cooling rate of product, molding cycle and crystallization, orientation and shrinkage of product, and is an important factor related to quality of product.

During injection molding of thermoplastics, mold temperature must be controlled below plastic's heat distortion temperature or glass transition temperature (Tg) (see Table 1 for plastic's heat distortion temperature) to ensure that product has sufficient rigidity without deformation when demolding. Under this general principle, specific value of mold temperature should be determined according to characteristics of plastic, structural characteristics and usage requirements of product, and other molding process conditions.
Heat distortion temperature of commonly used thermoplastics

For crystalline plastics, mold temperature directly affects crystallinity and crystalline configuration of plastic. High mold temperature, slow cooling rate, fast crystallization rate, high hardness and high rigidity of product, but prolongs molding cycle and increases shrinkage rate of the product; low mold temperature, cooling rate is fast, crystallization rate is slow, crystallinity is low, and toughness of product is improved. However, when Tg of crystalline plastic products formed at low mold temperature is low, late crystallization will occur, resulting in post-shrinkage and performance changes of product.
For amorphous plastics, mold temperature mainly affects viscosity of melt, that is, mold filling rate. Usually, in the case of ensuring smooth filling of mold, try to use a low mold temperature, because low mold temperature can shorten cooling time, thereby improving production efficiency.
For thick-walled products, since filling time is long and cooling time is also long, in order to make cooling inside and outside same, prevent defects such as internal stress (such as dents, voids, etc.) caused by large temperature difference between inside and outside, mold temperature should also be appropriately higher. Table 2 shows relationship between thickness of PA1010 product and mold temperature. From data in the table, it can be seen that there is a positive relationship between the two. In addition, thin-walled products with large areas or large flow resistance also need to maintain a high mold temperature.
Relationship between PA1010 product thickness and mold temperature

For plastics with low melt viscosity (such as PE, PP, PVC, PS, PA, etc.), due to their good fluidity and easy mold filling, low mold temperature can be used during processing; while for plastics with high melt viscosity (such as P℃, polyphenylene ether, polysulfone, etc.), mold temperature should be higher, on the one hand, it can meet needs of mold filling, on the other hand, cooling rate of product can be adjusted, so that product can be cooled slowly and uniformly, stress can be fully relaxed, defects such as dents and cracks can be prevented.
Cooling of mold is generally water-cooled or oil-cooled. When mold temperature is controlled at 5-95℃, water-cooling should be used, and oil-cooling should be used at 95-120℃. In individual cases, it is also necessary to use resistance wires and resistance heating rods to heat mold to maintain a constant temperature of mold. Table 3 lists barrel temperature, nozzle temperature and mold temperature that can be used for some plastics for reference.
Barrel, nozzle and mold temperatures for some plastics