The application of back pressure can ensure that when the screw is rotated and reset, it can generate enough mechanical energy to melt and mix the plastic.

Back pressure also has the following uses:

① Exhaust volatile gases, including air, from the outside of the shooting cylinder;

② Mix the additives (such as toner, color, antistatic agent, talc, etc.) with the melt evenly;

③ homogenize the molten material flowing through the length of the screw;

④ Provide uniform and stable plasticized material to obtain accurate weight control of the finished product.

The selected back pressure value should be as low as possible, as long as the plastic melt has a proper density and uniformity, there are no air bubbles, volatile gases and incompletely plasticized plastic in the melt.

The use of back pressure increases the pressure temperature and melt temperature of the injection molding machine. The increase is related to the set back pressure value. However, it should be noted that too high back pressure causes the melt temperature in the shot barrel to be too high, which is destructive to the injection molding of heat-sensitive plastics.

And too high back pressure will also cause the screw to be too large and irregular offside, making the shot volume extremely unstable. How much offside is affected by the viscoelastic properties of plastics.

The more energy stored in the melt, the screw will suddenly jump backwards when it stops rotating. Some thermoplastics have a stronger beating phenomenon than other plastics, such as LDPE, HDPE, PP, EVA, PP / EPDM composites and PPVC Compared with GPPS, HIPS, POM, PC, PPO-M and PMMA, it is more prone to beating.

In order to obtain the best production conditions, it is important to set the correct back pressure of the injection mold.

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