Injection molding is an engineering technology that involves transforming plastic into useful products that maintain their original properties. The important process conditions of injection molding are the temperature, pressure, and corresponding action time that affect the plasticizing flow and cooling.

1. Temperature control

1. Material barrel 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. Every type of plastic has a different flow temperature, and the flow temperature and decomposition temperature of the same plastic vary due to different sources or grades. This is because the average molecular weight and molecular weight distribution are different, and the plasticization process of plastic in different types of injection machines is also different. Therefore, the selection of barrel temperature is also different.

2. Nozzle temperature

The nozzle temperature is usually slightly lower than the maximum temperature of the barrel, in order to prevent the possible “drooling phenomenon” of molten material in the straight through nozzle. The nozzle temperature should not be too low, otherwise it will cause premature solidification of the melt and block the nozzle, or affect the performance of the product due to the injection of premature solidification material into the mold cavity.

3. Mold temperature

The temperature of the mold has a significant impact on the intrinsic properties and apparent quality of the product. The presence or absence of plastic crystallinity, the size and structure of the product, performance requirements, and other process conditions such as melt temperature, injection speed and pressure, molding cycle, etc determines the temperature of the mold.

2. Pressure control

The pressure during the injection molding process includes plasticizing pressure and injection pressure, which directly affect the plasticization of the plastic and the quality of the product.

1. Plasticizing pressure

When using a screw injection machine, the pressure experienced by 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 magnitude of this pressure can be adjusted through the relief valve in the hydraulic system. In injection, the magnitude of plasticizing pressure remains constant with the rotation speed of the screw. Increasing the plasticizing pressure will increase the temperature of the melt, but will decrease the plasticizing speed.

In addition, increasing plasticizing pressure can often make the temperature of the melt uniform, the mixing of pigments uniform, and the discharge of gases from the melt. In general operation, the determination of plasticizing pressure should be as low as possible while ensuring excellent product quality. The specific value varies with the type of plastic used, but usually rarely exceeds 20 kilograms per square centimeter.

2. Injection pressure

In current production, the injection pressure of almost all injection machines is based on the pressure applied to the plastic at the top of the plunger or screw (converted from oil pressure). The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, provide a rate of filling the mold with molten material, and compact the molten material.

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 labor productivity and equipment utilization. Therefore, in the production process, it is necessary to shorten the relevant time in the molding cycle as much as possible while ensuring quality. In the entire molding cycle, injection time and cooling time are the most important, as they have a decisive impact on the quality of the product. 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.

The holding time during injection refers to the pressure time on the plastic inside the mold cavity, which accounts for a large proportion of the entire injection time, generally about 2-120 seconds (up to 5-10 minutes for thick parts). The holding time before the molten material freezes at the gate has an impact on the accuracy of the product dimensions. The holding time also has an optimal value, which is known to depend on the material temperature, mold temperature, and the size of the main channel and gate.

If the dimensions and process conditions of the main channel and gate are normal, the pressure value that minimizes the fluctuation range of the product shrinkage rate is usually used as the standard. The cooling time is mainly determined by the thickness of the product, the thermal and crystallization properties of the plastic, and the mold temperature. The end point of the cooling time should be based on the principle of ensuring that the product does not change during demolding, usually between 5-120 seconds.

Excessive cooling time is unnecessary, as it not only reduces production efficiency, but also makes demolding difficult for complex parts, and even generates demolding stress when forcibly demolding. The other times in the molding cycle are related to whether the production process is continuous and automated, as well as the degree of integration.