The temperature to be controlled in plastic injection molding products mainly includes barrel temperature, nozzle temperature and mold temperature.
When determining the temperature of barrel, it should be ensured that the plastic is plasticized evenly and can be filled smoothly without causing plastic degradation. The determination principle of barrel temperature is as follows.
1. For plastic injection molding in plastics with poor thermal stability, such as PVC, POM, the barrel temperature should be lower. For plastics with wide heating temperature range and large melting viscosity, the barrel temperature should be higher. For the same type of plastics with high average molecular weight and narrow molecular weight distribution, the barrel temperature should be high, and vise versa.
For plastics with plasticizer, lubricant and other additives, because of its low viscosity and good fluidity, the barrel temperature should be relatively low. For plastics with glass fiber or other solid filler, the barrel temperature should be relatively high. In short, the barrel temperature of plastic injection molding should consider the thermal stability and rheological properties of molding materials.
On the basis of reasonable selection, maintain stability in the whole molding process. To avoid the overheating decomposition of materials, in addition to the selection and control of the highest temperature of the barrel, we should also strictly control the residence time of materials in the barrel, which is particularly important for plastics with poor thermal stability. The residence time of the material in the barrel depends on the relative ratio of the actual injection volume to the maximum injection volume of injection molding machines, the plasticizing ability of injection molding machines and other factors.
2. The barrel temperature determination of plastic injection molding is also related to molding products and mold structure. Generally, when injection molding products is with thin wall, long flow, complex structure and metal inserts, the barrel temperature should be higher. Otherwise, it may cause defects of products such as incomplete mold filling.
3. In the case of the same other conditions for plastic injection molding, the type of injection molding machines is different, and the barrel temperature should also change accordingly. For example, the barrel temperature of a screw injection molding machine is 10-20℃ lower than that of a plunger injection molding machine.
4. The barrel temperature of plastic injection molding interacts with other process conditions. Sometimes changing the barrel temperature will have the same effect as changing other process conditions. For example, in order to improve the filling flow length of melt, the method of increasing the barrel temperature can be adopted, and the injection pressure and injection speed can be correspondingly increased. Therefore, the barrel temperature determination of plastic injection molding must be combined with other technological conditions.
5. The barrel temperature distribution of plastic injection molding is usually gradually increased from the hopper to the nozzle to ensure the steady rise of plastic temperature and achieve the purpose of uniform plasticizing. In some special cases, this distribution can be changed. For example, the high water content in molding material will make rear barrel temperature slightly high to facilitate the discharge of water vapor. The screw injection molding machine can produce a lot of shear heat, so in order to prevent material decomposition, the front barrel temperature can be slightly lower than the middle barrel temperature.