The injection molding process can be roughly divided into the following six stages:
Clamping, injection, pressure holding, cooling, demold, product removal
Repeat the above process and the product can be periodically produced in batches. The molding of thermosetting plastics and rubber also applies the same process. The barrel temperature is lower than that of the thermoplastic while the injection pressure is higher and the mold is heated, the material injected in the mold needs to be cured or vulcanized, and then demold when it is hot.
The current trend of processing technology is developing in the direction of high-tech, including: micro injection molding, high-fill composite injection molding, water-assisted injection molding, mixed use of various special injection molding processes, foam injection molding, mold technology, simulation technology, etc.
The process of injection molding
Temperature control:
1. Barrel temperature: The temperature to be controlled during the injection molding process is the barrel temperature, nozzle temperature and mold temperature. The first two types primarily affect the plasticization and flow of the plastic, while the latter one primarily affects the flow and cooling of the plastic. Every plastic has a different flow temperature. The flow and decomposition temperatures of the same plastic may be different due to different sources or grades. This is because of the difference in average molecular weight and molecular weight distribution. When plastics are injected in different types of machine, the plasticizing process is also different, so the selected barrel temperature is also different.
2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum barrel temperature. It is to prevent the “flowing phenomenon” of the melt 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 condensate materials 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.
pressure control
The injection molding process includes two kinds of pressure, plasticizing pressure and injection pressure, and it directly affects the plasticization of plastics and product quality
1. Plasticizing pressure: (back pressure) When using a screw type injection machine, the pressure that the top melt of the screw receives when the screw rotates backward is called plasticizing pressure, also known as back pressure. This pressure can be adjusted by a relief valve in the hydraulic system. During injection, the plasticizing pressure will change depending on the design of the screw, the quality of the product and the type of the plastic. If these conditions and the rotation speed of the screw are constant, increasing the plasticizing pressure will enhance the shearing. It will increase the temperature of the melt, but reduce the efficiency of plasticization, and increase the backflow, leakage and the driving power.
In addition, increasing the plasticizing pressure often makes the temperature of the melt and the mixing of the colorant uniform, and discharges the air from the melt. Generally, in the operation, the plasticizing pressure should be kept as low as possible under the premise of ensuring the quality of the product. The specific value varies with the type of plastics used, but rarely exceeds 20 kg/cm 2.
2. Injection pressure: In the current production, almost all the injection pressure of the injection machine is based on the pressure that the plunger or screw top applied to plastic. 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.
molding cycle:
The time required to complete an injection molding process is 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, 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 3-5 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 20-120 seconds (the extra-thick parts can be as high as 5-10 minutes). Before the melting 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, and it depends on the material temperature, the mold temperature and the size of the main runners and gates. If the dimensions 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 cooling time should be based on the principle of ensuring that the product does not cause changes during demolding. It is generally between 30 and 120 seconds. It is not necessary to prolong the cooling time as it will not only reduce the production efficiency, but also complicate demolding process, and even generate mold release stress when the mold is forcedly released. Other times in the molding cycle are related to the continuity and automation of the production process, as well as the degree of continuity and automation.