Three Major Processes of Injection Molding

Ⅰ. Injection molding 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 caused by the difference in average molecular weight and molecular weight distribution. The plasticizing process of plastics in different types of injection machines 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 cavity.

3. Mold temperature

The temperature of the injection molding mold 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.

Ⅱ. Injection molding 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

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 molding injection, the size of the plasticizing pressure needs to be changed with the design of the screw, the requirements of product quality and the type of plastic. If these conditions and the speed of the screw are unchanged, increasing the plasticizing pressure will strengthen The shear effect will increase the temperature of the melt, but will reduce the efficiency of plasticization, increase the counterflow and leakage, and increase the driving power. 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 plasticizing pressure should be determined as low as possible on the premise of ensuring the quality of the product.

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 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.

Ⅲ. Injection molding 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, each 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 20 to 120 seconds. Before the molten material at the gate is frozen, the holding time will affect the dimensional accuracy of the product, but it will have no effect if it is later. The holding time also has an optimal value, 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 demolded. The cooling time is generally between 30 and 120 seconds. If the cooling time is too long, it is not necessary, which will not only reduce production efficiency, but also affect complex parts. It causes difficulty in demoulding, and even demoulding stress will be generated when demoulding is forced. Other times in the molding cycle are related to whether the production process is continuous and automated and the degree of continuous and automated.