简体中文
Due to the volume change caused by crystallization in the plastic injection molding process, the pressure during injection molding is high, the melt viscosity difference is small, the interlayer shear stress is small, and the elastic rebound after demolding is large. Therefore, if the injection molding volume is reduced, it will shrink The rate can also be that the material temperature is high during injection molding, the shrinkage rate is large, but the directionality is small.
When the pressure rises during the injection molding process, the molten material will withstand greater shear force and fluidity, especially PE and POM are more sensitive, so the pressure during the injection molding process should be adjusted to control the fluidity during the molding process . In the molding process, you can also control the material temperature, mold temperature, pressure in the injection molding process, injection speed and other factors to appropriately adjust the filling conditions to meet the molding requirements.
Injection molding is a processing method used for mass production of some complex-shaped parts. Specifically, it means that the material melted by heat is injected into the cavity by high pressure, and after cooling and solidification, a molded body is obtained.
Temperature control
1. Barrel temperature: The temperature that needs to be controlled during the injection molding process includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of the plastic, while the latter two temperatures mainly affect the flow and cooling of the plastic. Each plastic has a different flow temperature. For the same plastic, due to different sources or grades, its flow temperature and decomposition temperature are also different. This is due to the difference in average molecular weight and molecular weight distribution. The plasticizing process in the machine is also different, so the temperature of the barrel is also different.
2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent "salivation" from occurring in the straight-through nozzle. The temperature of the nozzle should not be too low, otherwise it will cause the melt to solidify prematurely and block the nozzle, otherwise the performance of the product will be affected due to the premature solidification injected into the cavity.
3. Mold temperature: Mold temperature has a great influence on the internal performance and appearance quality of the product. The mold temperature depends on the crystallinity of the plastic, the size and structure of the product, performance requirements and other process conditions (melting temperature, injection speed and pressure, molding cycle, etc.).
Pressure control
The pressure in the injection molding process includes plasticization pressure and injection pressure, which directly affects the plasticization of plastics and product quality.
1. Plasticizing pressure: (back pressure) When using a screw injection machine, when the screw rotates backwards, the pressure on the melt at the top of the screw is called plasticizing pressure, also known as back pressure. The size of the pressure can be adjusted by the overflow valve in the hydraulic system. During the injection process, the size of the plasticizing pressure needs to be changed according to the screw design, product quality requirements and the type of plastic. If these conditions are the same as the screw speed, increasing the plasticizing pressure will enhance the shearing function, that is, increasing the temperature of the melt, but will reduce the plasticizing efficiency, increase the reverse flow and leakage, and increase the driving force.
In addition, increasing the plasticizing pressure can usually make the temperature of the melt uniform, the mixing of the pigments and the discharge of the gas in the melt. In general operation, under the premise of ensuring product quality, the decision of plasticizing pressure should be as low as possible. The specific value varies with the type of plastic used, but usually rarely exceeds 20 kg/cm².
2. Injection pressure: In the current production, the injection pressure of almost all injection machines is based on the pressure (converted from oil pressure) exerted by the plunger or the top of the screw on the plastic. The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, give the melt filling rate and compact the melt.