Injection molding is an engineering technique that involves transforming plastic into useful and original products. The important process conditions for injection molding are the temperature, pressure and corresponding individual time of action that affect plasticizing flow and cooling.
First, temperature control
1, barrel temperature
The temperatures that need to be controlled during the injection molding process are barrel temperature, nozzle temperature, and mold temperature. The first two temperatures primarily affect the plasticization and flow of the plastic, while the latter temperature primarily affects the flow and cooling of the plastic. Each plastic has a different flow temperature. The same plastic has different flow and decomposition temperatures due to different sources or grades. This is due to the difference in average molecular weight and molecular weight distribution. Plastics are injected in different types. The plasticizing process in the machine is also different, so the temperature of the selected cylinder is also different.
2, nozzle temperature
The nozzle temperature is usually slightly below the maximum temperature of the barrel, in order to prevent the "flowing phenomenon" that can occur in the straight-through nozzle. The nozzle temperature should not be too low, otherwise it will cause early condensation of the melt to block the nozzle, or affect the performance of the product due to the injection of the early aggregate into the cavity.
3, mold temperature
The mold temperature has a great influence on the intrinsic properties and apparent quality of the product. The temperature of the mold depends on the presence or absence of crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).
Second, pressure control
The pressure during the injection molding process includes plasticizing pressure and injection pressure, and directly affects the plasticization of plastics and the quality of products.
1. Plasticizing pressure
(Back pressure) When a screw type injection machine is used, the pressure that the top melt of the screw receives when the screw rotates backward is called plasticizing pressure, which is also called back pressure. This pressure can be adjusted by a 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 plasticizing speed is reduced. In addition, increasing the plasticizing pressure often makes the temperature of the melt uniform, the mixing of the colorant uniform, and the discharge of the gas in the melt. In general operation, the determination of plasticizing pressure should be as low as possible to ensure the quality of the product. 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 by the plunger or the top of the screw on the plastic (converted from the oil pressure). The injection pressure plays a role in injection molding by overcoming the flow resistance of the plastic from the barrel to the cavity, giving the melt filling rate and compacting the melt.
Third, the molding cycle
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 the labor rate and equipment utilization rate. Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring the quality. Injection time and cooling time are of the utmost importance throughout the molding cycle 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 of the plastic in the cavity, which accounts for a large proportion in the whole injection time, generally about 20 to 120 seconds (the extra-thick parts can be as high as 5 to 10 minutes). Before the sealing of the melt at the gate, the amount of dwell time has an effect on the dimensional accuracy of the product. The holding time also has the best value, which is known to depend on the material temperature, the mold temperature, and the size of the main runners and gates. If the size and process conditions of the main runners and gates are normal, it is usually the value of the pressure at which the range of fluctuation of the shrinkage of the product is the smallest. The cooling time is mainly determined by the thickness of the product, the thermal properties and crystallization properties of the plastic, and the temperature of the mold. The end point 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. It is not necessary to cool the cooling time too much, which not only reduces the production efficiency, but also causes difficulty in demolding the complicated parts, and even releases the mold release stress when forcibly releasing the mold. Other times in the molding cycle are related to whether the production process is continuous and automated, and the extent of the two processes.
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