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Injection molding process

Release time:2021-08-30

Injection molding is an industry with wide knowledge, strong technology and strong practicality. Plastic raw materials, toner, nozzle materials, molds, injection molding machines, peripheral equipment, fixtures, sprays, various auxiliary materials and packaging materials are used in the injection production process, which brings a lot of workload to the management of injection molding. Compared with other industries or departments, the injection molding workshop has higher requirements for managers at all levels.

Injection molding production requires 24-hour continuous operation. Generally, the working mode is double-sided, three knife shift or three shift system. There are many positions in the injection molding workshop, and the division of labor is complex. The skill requirements of different positions are also different. In order to make the production and operation of the injection molding workshop smoothly, it is necessary to manage the personnel, materials, equipment, tools, etc. involved in all links and positions, including the operation and coordination management of raw material room, waste room, batching room, production site, post-processing area, tool room, semi-finished product area, office and other areas.

Injection molding processing type:

1. Rubber injection molding: rubber injection molding is a production method of injecting rubber directly into the mold from the barrel to stop vulcanization. The advantages of rubber injection molding are: although it is an intermittent operation, the molding cycle is short, the production efficiency is high, the blank preparation process is omitted, the labor intensity is small, and the product quality is excellent.

2. Plastic injection molding: plastic injection molding is a method of plastic products. Molten plastic is injected into the mold of plastic products by pressure and cooled to form various plastic parts. There are special mechanical injection molding machines for injection molding. The commonly used plastic is polystyrene. The resulting shape is usually the final product, and no other treatment is required before installation or use as the final product. Many details, such as protrusions, ribs, and threads, can be molded in one injection molding step.

Influencing factors:

Due to the volume change caused by crystallization in the process of plastic injection molding, the pressure during injection molding is high, the melt viscosity difference is small, the interlayer shear stress is small, and the elastic rebound after demoulding is large, so the shrinkage of injection molding is also OK. If the dosage is reduced, the material temperature during injection molding is high, the shrinkage is large, but the directionality is small.

When the pressure increases during pressurized injection molding, the molten material is subject to greater shear and fluidity, especially PE and POM are more sensitive, so the pressure should be adjusted during injection molding to control the fluidity during molding. During the molding process, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled, and the filling conditions can be adjusted appropriately to meet the molding needs [1].

Injection molding process:

Injection molding is a processing method used to mass produce some parts with complex shapes. Specifically, it refers to injecting the heated molten material into the mold cavity through high pressure, cooling and curing to obtain the molded product.

1、 Temperature control

1. Barrel temperature: the temperature to be controlled during injection molding includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastic, and the last two temperatures mainly affect the flow and cooling of plastic. Each plastic has a different flow temperature. For the same kind of plastic, due to different sources or brands, its activity temperature and synthesis temperature are also different. This is due to the difference between uniform molecular weight and molecular weight dispersion. Different types of plastic injection machines have different plasticizing processes, so the temperature of the barrel is also different.

2. Nozzle temperature: the nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent melt "salivation" that may occur in the straight through nozzle. The temperature of the nozzle should not be too low, otherwise the nozzle will be blocked due to premature solidification of the melt, or the product performance will be affected due to premature solidification of the material injected into the cavity.

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

2、 Pressure control

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

1. Plasticizing pressure: (back pressure) when using the screw injection machine, the pressure acting on the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. The pressure can be adjusted by the overflow valve in the hydraulic system. During injection, the plasticizing pressure needs to change with the design of screw, the requirements of product quality and the type of plastic. If these conditions are the same as the screw speed, increasing the plasticizing pressure will strengthen the shear effect, that is, it will increase the melt temperature, but it will reduce the plasticizing efficiency, increase countercurrent and leakage, and increase the driving force.

In addition, increasing the plasticizing pressure can often make the melt temperature uniform, the pigment mixed evenly, 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 product quality. The specific value varies according to the type of plastic used, but usually rarely exceeds 20 kg/cm ²。

2. Injection pressure: in current production, the injection pressure of almost all injection machines is based on the pressure (converted from oil pressure) exerted by the top of the plunger or screw on the plastic. The function of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, so that the molten material has the speed of filling the mold, and the molten material is compacted.

3、 Forming cycle

The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. Its theory includes the following parts: forming cycle: forming cycle directly affects labor consumption rate and equipment application rate. Therefore, in the process of consumption, the relevant time in the molding cycle should be shortened as far as possible on the premise of ensuring the quality.

In the whole molding cycle, with the importance of injection time and cooling time, they 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, and the mold filling time in consumption is generally about 3-5 seconds.

The pressure holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (for extra thick parts, it can be as high as 5-10 minutes). Before the molten material at the gate is sealed and frozen, the holding time has an impact on the dimensional accuracy of the product, but it has no impact in the future. Pressure holding time also has a value, which is known to depend on material temperature, mold temperature, and the size of sprue and gate. Assuming that the size and process conditions of the sprue and gate are normal, the pressure value with a small shaking range of the product shrinkage is usually used.

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 point of the cooling time should be based on ensuring that the product will not cause changes during demoulding. The cooling time is generally between 30~120 seconds. It is unnecessary to have a long cooling time, which will not only reduce the consumption efficiency, but also make demoulding difficult for complex parts, resulting in demoulding stress when forced demoulding.

The other time in the molding cycle is related to the continuity and automation of the consumption process and the level of continuity and automation.

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