The exhaust of injection molds is an important issue in mold design, especially in rapid injection molding, where the exhaust requirements for injection molds are more stringent.

(1) The source of gas in injection molds.

1) The air stored in the pouring system and mold cavity.

2) Some raw materials contain water that has not been dried out and vaporized into water vapor at high temperatures.

3) Due to the high temperature during injection molding, some unstable plastics decompose into gases.

4) Why do injection molds need to be equipped with exhaust systems for gases generated by volatilization or chemical reactions of certain additives in plastic raw materials? Why do injection molds need to be equipped with exhaust systems.

(2) Hazards of poor exhaust

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

1) During the injection molding process, the melt will replace the gas in the cavity. If the gas is not discharged in a timely manner, it will cause difficulty in filling the melt, resulting in insufficient injection volume and inability to fill the cavity.

2) Excluding unobstructed gas will form high pressure in the mold cavity and penetrate into the plastic to a certain degree of compression, causing quality defects such as pores, voids, loose tissue, and silver lines.

3) Due to the high compression of the gas, the temperature inside the mold cavity rises sharply, causing the surrounding melt to decompose and burn, resulting in local carbonization and charring of the plastic parts. It mainly appears at the confluence of two molten streams, at the corner and gate flange.

4) The unobstructed removal of gas results in different velocities of the melt entering each mold cavity, leading to the formation of flow and fusion marks and a decrease in the mechanical properties of the plastic parts.

5) Due to the obstruction of gas in the mold cavity, it can reduce the filling speed, affect the forming cycle, and reduce production efficiency.

(3) The distribution of bubbles in plastic parts

The sources of gas in the mold cavity are mainly divided into three categories: the air accumulated in the mold cavity; Gas generated by decomposition in raw materials; The residual water vapor in the raw material evaporates, and the positions of bubbles generated due to different sources are also different. Why do injection molds need to be designed with exhaust systems.

1) The bubbles generated by the accumulation of air in the mold cavity are often distributed opposite the gate.

2) Bubbles generated 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 irregularly distributed throughout the entire plastic part.