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What you need to know about injection molding

2021-07-19 11:17:01

Injection molding, also known as injection molding , is a molding method of injection molding and molding. The advantages of injection molding method are fast production speed, high efficiency, automatic operation, wide variety of colors, simple shapes, large to small sizes, accurate product sizes, easy product updates, complex shapes, injection molding is suitable for mass production and complex shapes The products and other forming processing fields.

At certain temperatures, the fully melted plastic material is stirred with a screw, injected into the mold cavity under high pressure, and cooled and solidified to obtain a molded product. This method is suitable for mass production of parts with complex shapes and is one of the important processing methods.

Injection molding process

One, temperature control.

1) Barrel temperature:

The temperature that needs to be controlled during the injection molding process includes the cylinder temperature, the nozzle temperature and the mold temperature. The first two temperatures mainly affect the plasticization and flow of the plastic, and the latter temperature mainly affects the flow and cooling of the plastic. Each plastic has a different flow temperature. The flow temperature and decomposition temperature of the same plastic vary by source or brand. This is because the average molecular weight and molecular weight distribution are different, and the plasticization process of plastics in different types of injection molding machines is also different, so the temperature of the cylinder is also different.

2) Nozzle temperature:

The nozzle temperature is usually slightly lower than the upper limit temperature of the barrel to prevent the melt flow from the straight-through nozzle. The nozzle temperature should not be too low, otherwise it will cause the melt to prematurely congeal and block the nozzle, or the product performance will be affected by the premature condensing material being injected into the cavity.

3) Mold temperature:

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

Second, pressure control.

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

1) Plasticizing pressure:

(Back pressure) When using a screw-type injection molding machine, the pressure that the melt on the top of the screw receives when the screw rotates and retreats is called plasticizing pressure, which is also called back pressure. This pressure can be adjusted by the relief valve in the hydraulic system.

In the injection molding process, the size of the plasticizing pressure should be changed according to the screw design, product quality requirements and plastic type. If these conditions and the rotation speed of the screw remain unchanged, increasing the plasticizing pressure will enhance the shearing effect, that is, the melt temperature will increase, but the plasticizing efficiency will decrease, the backflow and leakage flow will increase, and the driving power will also increase.

In addition, increasing the plasticizing pressure can often make the melt temperature uniform, the pigments are mixed uniformly, and the gas in the melt can be discharged. Generally speaking, the plasticizing pressure should be determined under the premise of ensuring the high quality of the product. The lower the better, the specific value varies with the type of plastic used, but generally does not exceed 20 kg/cm².

2) Injection pressure:

In the current production, the injection pressure of almost all injection molding machines is applied to the plastic with a plunger or screw.

The applied pressure (converted from the oil line pressure) is a reference. The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, increase the speed of the molten mold, and compact the molten material.


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