Why do injection molds have exhaust systems

The exhaust of injection mold is an important problem in mold design, especially in injection molding, the exhaust requirements of injection mold are more strict.

(1) Source of gas in injection mold

1) Air in the gating system and mold cavity.

2) Some raw materials contain water that has not been removed by drying. They are gasified into water vapor at high temperature.

3) The gas produced by the decomposition of some unstable plastics due to the high temperature during injection molding.

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

(2) Hazards of poor exhaust

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

1) In the process of injection molding, the melt will replace the gas in the cavity. If the gas is not discharged in time, it will make it difficult to fill the melt, resulting in insufficient injection volume and unable to fill the cavity.

2) Poor gas removal will form high pressure in the mold cavity and penetrate into the plastic under a certain degree of compression, resulting in quality defects such as pores, cavities, loose tissue, crazing and so on.

3) Because the gas is highly compressed, the temperature in the mold cavity rises sharply, which leads to the decomposition and burning of the surrounding melt, resulting in local carbonization and charring of the plastic parts. It mainly appears at the confluence of the two melts, * angle and gate flange.

4) The poor removal of gas makes the melt velocity entering each cavity different, so it is easy to form flow marks and fusion marks, and reduce the mechanical properties of plastic parts.

5) Due to the obstruction of gas in the cavity, the mold filling speed will be reduced, the forming 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: the air accumulated in the cavity; Gas generated by decomposition in raw materials; The position of bubbles produced by the evaporation of residual water in raw materials is also different due to different sources.

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

Bubbles generated by decomposition or chemical reaction in plastic raw materials are distributed along the thickness of plastic parts.