Injection molding is an engineering technique that involves transforming plastics into useful products that retain their original properties. The important process conditions of injection molding are the temperature, pressure and corresponding action time that affect the plasticizing flow and cooling.
1. Temperature control
1. Cylinder temperature
The temperature that needs to be controlled in the injection molding process includes 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 has different flow temperature and decomposition temperature due to different sources or grades. This is due to the difference in average molecular weight and molecular weight distribution. Plastics in different types of injection The plasticizing process in the machine is also different, so the selection of barrel temperature is also different.
2. Nozzle temperature
The temperature of the nozzle is usually slightly lower than the maximum temperature of the barrel, which is to prevent the "salivation phenomenon" that may occur in the straight-through nozzle. The temperature of the nozzle should not be too low, otherwise it will cause premature solidification of the molten material and block the nozzle, or affect the performance of the product due to the injection of early solidification material 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 temperature of the mold depends on the presence or absence of plastic crystallinity, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.)
2. Pressure control
The pressure in the injection molding process includes plasticizing pressure and injection pressure, and directly affects the plasticizing of plastics and product quality.
1. Plasticizing pressure
(Back pressure) When a screw injection machine is used, the pressure on the molten material at the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted through the relief valve in the hydraulic system. In injection, the size of the plasticizing pressure is constant with the speed of the screw. When the plasticizing pressure is increased, the temperature of the melt will be increased, but the plasticizing speed will be reduced.
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color material can be mixed uniformly and the gas in the melt can be discharged. In general operation, the decision of plasticizing pressure should be as low as possible under the premise of ensuring good product quality. The specific value varies with the type of plastic used, but usually rarely exceeds 20 kg/cm2.
2. Injection pressure
In current production, the injection pressure of almost all injection machines is based on the pressure exerted on the plastic by the top of the plunger or screw (converted from the pressure of the oil circuit). The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, to give the molten material a filling rate and to compact the molten material.
3. Molding cycle
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The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. The molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring quality. In 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 pressure holding time in the injection time is the pressure time on the plastic in the cavity, which accounts for a relatively large proportion in the entire injection time, generally about 2 to 120 seconds (for extra thick parts, it can be as high as 5 to 10 minutes). Before the molten material at the gate is frozen, the holding time will affect the dimensional accuracy of the product. The holding time also has a sweet spot, which is known to depend on the material temperature, mold temperature, and the size of the sprue and gate.
If the size of the sprue and the gate and the process conditions are normal, usually the pressure value with the smallest shrinkage fluctuation range of the product shall prevail. The cooling time is mainly determined by the thickness of the product, the thermal properties and crystallization properties of the plastic, and the mold temperature. The end of the cooling time should be based on the principle of ensuring that the product does not change when it is demoulded, generally between 5 and 120 seconds.
Too long cooling time is unnecessary, which not only reduces production efficiency, but also makes demoulding difficult for complex parts, and even produces demoulding stress during forced demoulding. Other times in the molding cycle are related to whether the production process is continuous and automated, and the degree of automation.
