Zhejiang Shifang Pipe Industry Co.,Ltd

Zhejiang Shifang Pipe Industry Co.Ltd is one of the most professional manufacturer and exporter of roof drainage system..

Why do injection moulds have exhaust systems?
- Jan 08, 2019 -

The exhaust of injection moulds is an important problem in the design of moulds. Especially in rapid injection moulding, the exhaust requirements of injection moulds are more stringent.

(1) Source of Gas in Injection Mold

1) Air in pouring system and mould cavity.

2) Some raw materials contain water that has not been dried out. They are gasified into steam at high temperatures.

3) Gases produced by decomposition of unstable plastics due to excessive temperature during injection.

4) Gases generated by volatilization or chemical reaction of some additives in plastic raw materials.

(2) Harm of bad exhaust

Poor exhaust of injection moulds will bring a series of hazards to the quality of plastic parts and other aspects. The main performances are as follows:

1) In the process of injection, the melt will replace the gas in the cavity. If the gas is not discharged in time, it will cause the difficulty of filling the melt, resulting in insufficient injection volume and not filling the cavity.

2) Removal of unobstructed gas will form high pressure in the cavity, and infiltrate into the plastic under a certain degree of compression, resulting in porosity, voids, loose tissue, craze and other quality defects.

3) Because the gas is highly compressed, the temperature in the cavity rises sharply, and then causes the melt around to decompose and burn, resulting in local carbonization and scorching of the plastic parts. It occurs mainly at the confluence of two melts, * angle and gate flange.

4) Gas removal is not smooth, making the melt into the cavity speed is different, so easy to form flow marks and fusion marks, and reduce the mechanical properties of the plastic parts.

5) Because of the obstruction of gas in the cavity, the filling speed will be reduced, the forming cycle will be affected, and the production efficiency will be reduced.

(3) Bubble Distribution in Plastic Parts

The gas sources in the cavity are mainly divided into three categories: the air accumulated in the cavity; the gas generated by the decomposition of the raw materials; and the water vapor evaporated from the residual water in the raw materials. The positions of the bubbles produced by the different sources are also different.

1) The bubbles generated by the accumulated air in the die cavity are usually distributed in the opposite part of the gate.

2) The bubbles produced by decomposition or chemical reaction in plastic raw materials are distributed along the thickness of plastic parts.