The design of cooling system is very important in injection molding. This is because the molding of plastic products only cools to certain rigidity, plastic products then release from the plastic mould to avoid the deformation due to external force. Since the cooling time accounts for about 70% ~ 80% of the whole injection molding cycle, the design of a good cooling system can greatly shorten the molding cycle time, improve the injection production rate and reduce the cost. Improper design of the cooling system will make the cooling time longer, even more increasing the cooling time will intensify the warping deformation of plastic products.
According to the experiment, the heat transferred from the melt to the plastic mould is generally divided into two parts, a part of 5% by radiation and convection transfer to the atmosphere and the remaining 95% is from the melt into the mold. Due to the effects of water cooling, heat in the plastic mold cavity spread to the cooling water pipe, through convection heat carried away by the coolant. The remained heat that is not being cooled by water continues to be conducted to the mold, which is dispersed in the air.
The cycle of injection molding is made up of the close time, filling time, holding time, cooling time and ejection time. Cooling time takes the largest proportion of molding cycle, about 70% ~ 80%. So the cooling time will directly affect the plastic product molding cycle time and output rate. The temperature of the plastic products in the release stage should be cooled below to the temperature of the thermal deformation of the plastic products, in order to prevent the distortion caused by the residual stress and the external force.
Factors affecting the cooling rate of the products:
Plastic product design: plastic product wall thickness, the greater the thickness of the product is, the longer the cooling time will be. Generally, the cooling time is approximately proportional to the square of the thickness of the plastic products, or is proportional to the 1.6 time of the maximum flow channel diameter. That is, the thicknesses of the plastic product double the cooling time increase by 4 times.