Shoe Making Technology: Exhaust From Injection Moulds

(1) sources of gases in injection moulds.

1) air in the gating system and mold cavity.

2) some raw materials contain moisture that has not been dried up, and they are vaporized into steam at high temperatures.

3) because of the high temperature of the injection molding, the gas produced by the decomposition of some plastics with unstable properties.

4) the volatilization or chemical reaction of some additives in plastic raw materials.

(2) hazards caused by poor exhaust

The poor exhaust of injection mold will bring a series of hazards to the quality of plastic parts and many other aspects.

The main findings are as follows:

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

2) the excluded gas will form a high pressure in the cavity, and infiltrate into the inner part of the plastic under a certain degree of compression, resulting in porosity, cavity, loose texture, crazing and other quality defects.

3) because the gas is highly compressed, the temperature inside the cavity rises sharply, which causes the surrounding melt to decompose and burn, resulting in partial carbonization and scorching of the plastic parts.

It occurs mainly at the junction of two melts, corners and gate flanges.

4) the removal of gas is not smooth, so that the melt velocity entering the cavity is different, so it is 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 mold cavity, the filling speed will be reduced, the molding cycle will be affected, and the production efficiency will be reduced.

(3) distribution of bubbles in plastic parts

There are three main sources of gas in the cavity.

1) the bubbles generated by the accumulated air in the mold cavity are often distributed on the location opposite to the gate.

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

3) the bubbles generated by the gasification of residual water in plastic raw materials are distributed on the whole plastic parts irregularly.

Judging from the distribution of bubbles in the above plastic parts, we can not only judge the nature of bubbles, but also judge whether the exhaust parts of the molds are correct and reliable.