Initial technology of adjusting machine for injection moulding machine: Injection moulding is an engineering technology, which involves transforming plastics into useful products and maintaining their original properties. The important technological conditions of injection moulding are the temperature, pressure and corresponding action time which affect the plasticizing flow and cooling.
- Temperature Control
- Barrel temperature: The temperature of barrel, nozzle and mould should be controlled during injection moulding. The first two temperatures mainly affect the plasticization and flow of plastics, while the second one mainly affects the flow and cooling of plastics. Each kind of plastic has different flow temperature. The same kind of plastic has different flow temperature and decomposition temperature because of its different origin or brand. This is due to the different distribution of average molecular weight and molecular weight. The Plasticization Process of plastic in different types of injection machines is also different, so the selection of barrel temperature is also different.
- Nozzle temperature: nozzle temperature is usually slightly lower than the maximum temperature of the barrel, which is to prevent the “salivation” phenomenon that may occur in the through nozzle. The nozzle temperature should not be too low, otherwise it will cause premature solidification of the melt and block the nozzle, or the performance of the product will be affected by the injection of premature solidification into the cavity.
- Mould temperature: Mould temperature has a great influence on the intrinsic performance and apparent quality of products. Mold temperature depends on the crystallinity of plastics, the size and structure of products, performance requirements, and other technological conditions (melt temperature, injection speed, injection pressure, injection cycle, etc.).
- Pressure Control
The pressure in the injection process includes plasticizing pressure and injection pressure, which directly affects the plasticization of plastics and the quality of products.
- Plasticizing Pressure: (Back Pressure) When a screw injection machine is used, the pressure on the top of the screw when the screw rotates backward is called plasticizing pressure, also known as back pressure. The magnitude of this pressure can be adjusted by the relief valve in the hydraulic system.
In injection, the magnitude of plasticizing pressure is unchanged with the speed of screw. When the plasticizing pressure is increased, the melt temperature will be increased, but the plasticizing speed will be reduced. In addition, increasing plasticizing pressure can make melt temperature uniform, color mixing uniform and discharge gas from melt. In general operation, the lower the plasticizing pressure is, the better the quality of the product is guaranteed. The specific value of plasticizing pressure varies with the type of plastics used, but it is seldom more than 20 kg/cm2.
- Injection pressure: In the current production, the injection pressure of almost all injection machines is based on the pressure (converted by oil pressure) applied to the plastic at the top of the plunger or screw. The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, to give the filling rate of melt and to compact the melt.
- Forming cycle
The time required to complete an injection moulding process is called the moulding cycle, also known as the moulding cycle. It actually includes the following parts:
Molding cycle: Molding cycle directly affects labor productivity and equipment utilization. Therefore, in the process of production, on the premise of guaranteeing the quality, we should shorten the relevant time in the forming cycle as far as possible. Injection time and cooling time are the most important factors in the whole moulding cycle, which have a decisive impact on the quality of products. The filling time in injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds.
Pressure holding time in injection time is the pressure time on the plastic in the cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (extra-thick parts can be up to 5-10 minutes). Before the freezing of the melt at the gate, the holding time has an effect on the dimensional accuracy of the product, but in the future, it has no effect. Pressure holding time also has a maximum value, which is known to depend on material temperature, die temperature and the size of main runner and gate. If the sizes of the main runner and gate and the process conditions are normal, the minimum pressure value of the shrinkage fluctuation range is usually obtained.
The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastics, and the temperature of the mould.
The end point of cooling time should be based on the principle of ensuring that no change occurs during demoulding. The cooling time is generally between 30 and 120 seconds. It is unnecessary to have too long cooling time, which not only reduces the production efficiency, but also makes demoulding difficult for complex parts, and even causes demoulding stress when demoulding is forced. The other time in the forming cycle is related to the continuity and automation of the production process and the degree of continuity and automation.