Injection molding is an engineering technology. It involves the transformation of plastics into useful products that can maintain their original properties. The important process conditions for injection molding are the temperatures, pressures, and corresponding action times that affect plasticizing flow and cooling.
First, temperature control
1, barrel temperature
The injection molding process requires controlled temperatures such as barrel temperature, nozzle temperature, and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter temperature mainly affects the flow and cooling of plastics. Each plastic has a different flow temperature, the same plastic, due to different sources or grades, the flow temperature and decomposition temperature are different, due to the difference in the average molecular weight and molecular weight distribution, plastic in different types of injection The plasticization process inside the machine is also different, so the choice of cylinder temperature is not the same.
2, the nozzle temperature
The nozzle temperature is usually slightly lower than the maximum barrel temperature. This is to prevent the "flow phenomenon" that may occur in the through-flow nozzle. The nozzle temperature should not be too low, otherwise it will cause early coagulation of the melt and block the nozzle, or affect the performance of the product due to the early injection into the mold cavity.
3, mold temperature
The mold temperature has a great influence on the intrinsic performance and apparent quality of the product. The mold temperature is determined by the presence or absence of plastic crystallinity, product size and structure, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).
Second, pressure control
In the injection molding process, pressure includes plasticization pressure and injection pressure, and directly affects the plasticization of plastics and the quality of products.
1, plasticization pressure
(back pressure) When a screw type injection machine is used, the pressure at the top of the screw when the screw rotates backwards is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the relief valve in the hydraulic system. In the injection, the plasticizing pressure is constant with the rotation speed of the screw. When the plasticizing pressure is increased, the temperature of the melt is increased, but the speed of plasticization is reduced. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the mixing of the colorant and the discharge of the gas in the melt. In general operation, the determination of plasticizing pressure should be as low as possible on the premise of ensuring good quality of the product. The specific value varies depending on the type of plastic used, but usually it rarely exceeds 20 kg/cm2.
2, injection pressure
In current production, almost all injector injection pressures are based on the pressure exerted by the top of the plunger or screw on the plastic (converted from the oil pressure). The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the cartridge to the cavity, to give the rate of filling of the melt, and to compact the melt.
Third, the molding cycle
The time required to complete an injection molding process is called the molding cycle, also called the molding cycle. The molding cycle directly affects the labor-to-labor rate and equipment utilization. Therefore, in the production process, all relevant time in the molding cycle should be minimized under the premise of ensuring quality. During the entire molding cycle, the injection time and cooling time are the most important and they have a decisive influence on the quality of the product. The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3 to 5 seconds.
The holding time in the injection time is the pressure time for the plastic in the cavity. The proportion of the time spent in the entire injection time is relatively large, generally about 20 to 120 seconds (extra-thick workpieces can reach up to 5 to 10 minutes). Before the melt is frozen at the gate, how much pressure holding time will affect the accuracy of the product size. The dwell time also has the best value, which is known to depend on the material temperature, mold temperature, and the size of the main channel and gate. If the dimensions and process conditions of the main sprue and the gate are normal, it is usually the pressure that results in the smallest range of fluctuations in the shrinkage of the product. The cooling time is mainly determined by the thickness of the product, the thermal and crystallization properties of the plastic, and the temperature of the mold. The end of the cooling time should be based on the principle of ensuring that the product does not cause changes during demolding, generally between 30 and 120 seconds. The long cooling time is not necessary, which not only reduces the production efficiency, but also results in difficulty in demoulding the complex parts, and may even cause mold release stress when forcibly demolding. Other times in the molding cycle are related to whether the production process is continuous and automated and the extent of the two.
