temperature control

1. Barrel temperature: The temperature to be controlled during 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 one mainly affects the flow and cooling of plastics. Each kind of plastic has different flow temperature. Due to different source or brand, the flow temperature and decomposition temperature of the same plastic are different. This is due to different average molecular weight and molecular weight distribution. The plasticizing process of plastic in different types of injection machines is also different, so the temperature of material barrel is also different.

2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel, which is to prevent the "salivation" that may occur when the molten material is in the straight-through nozzle. The nozzle temperature shall not be too low, otherwise the nozzle will be blocked due to the early solidification of the molten material, or the performance of the product will be affected due to the injection of the early solidification material into the mold cavity.

3. Mold temperature: mold temperature has a great impact on the internal performance and apparent quality of products. The mold temperature depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed, injection pressure, molding cycle, etc.)

pressure control 

The pressure in the process of injection molding includes plasticizing pressure and injection pressure, which directly affects the plasticizing of plastics and the quality of products.

1. Plasticizing pressure: (back pressure) When the screw injection machine is used, the pressure that the molten material at the top of the screw is subjected to when the screw rotates backward is called plasticizing pressure, also called back pressure. The pressure can be adjusted by the overflow valve in the hydraulic system. In injection, the size of the plasticizing pressure needs to change with the design of the screw, the requirements of the product quality and the type of plastic. If these conditions and the rotational speed of the screw remain unchanged, increasing the plasticizing pressure will strengthen the shearing effect, that is, it will increase the melt temperature, but it will reduce the plasticizing efficiency, increase the countercurrent and leakage flow, and increase the driving power.

In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color mixture uniform and the gas in the melt discharged. In general operation, the plasticizing pressure should be determined as low as possible under the premise of ensuring the good quality of the product. The specific value varies with the type of plastic used, but usually it rarely exceeds 20 kg/cm2.

2. Injection pressure: In current production, almost all injection machines have injection pressure of plunger or screw top to plastic

The applied pressure (converted from oil circuit pressure) shall prevail. The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the rate of mold filling of molten material and compact the molten material.

Forming cycle

The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts: molding cycle: 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 far as possible under the premise of ensuring quality. In the whole molding cycle, the injection time and cooling time are the most important, which have a decisive impact on the quality of products. The mold filling time in the injection time is directly inversely proportional to the mold filling rate. The mold 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 mold cavity, which accounts for a large proportion of the whole injection time, generally about 20-120 seconds (the extremely thick parts can be as high as 5-10 minutes). Before the melting material at the gate is frozen, the holding time has an impact on the dimensional accuracy of the product. If it is later, it has no impact. The holding time also has the most favorable value, which is known to depend on the material temperature, mold temperature and the size of the main runner and gate. If the size and process conditions of the sprue and gate are normal, the pressure value with the minimum fluctuation range of the shrinkage of the product is usually taken as the criterion. The cooling time mainly depends on the thickness of the product, the thermal and crystalline 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 will not change during demoulding. The cooling time is generally between 30 and 120 seconds. Too long cooling time is unnecessary, which will not only reduce the production efficiency, but also cause demoulding difficulties for complex parts, and even cause demoulding stress when forced demoulding. 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.