Plastic molding processing technology of raw materials

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: injection molding process needs to control the temperature of the barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of the plastic, while the latter temperature mainly affects the flow and cooling of the plastic. 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, nozzle temperature: nozzle temperature is usually slightly lower than the maximum barrel temperature, this is to prevent the melt flow in the straight-through nozzle may occur "flow phenomenon." The nozzle temperature should not be too low, otherwise it will cause the premature coagulation of the melt and plug the nozzle, or affect the performance of the product due to the early injection into the cavity.

3, mold temperature: mold temperature on the product's intrinsic performance and apparent quality. 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, plasticizing pressure: (back pressure) When using a screw type injection machine, the pressure on the top of the screw when the screw rotates back 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 decision 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 it usually rarely exceeds 20 kg/cm2.

2. Injection pressure: In the current production, almost all the injection pressure of the injection machine is based on the pressure exerted by the plunger or the top of the 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. It actually includes the following parts: Molding cycle: The molding cycle directly affects labor productivity 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 3-5 seconds.

The pressure holding time in the injection time is the pressure time for the plastic in the cavity, and the proportion of the plastic injection time is relatively large, generally about 20-120 seconds (extra-thick workpieces can be as high as 5 to 10 minutes). Before the melt is frozen at the gate, how much pressure holding time will have an effect on the accuracy of the product size, and if it is later, it has no effect. The dwell time also has the most favorable value. It is known that it depends 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.