Why is an exhaust system necessary for injection molds?

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 present in the pouring system and mold cavity.

2) Some raw materials contain moisture that has not been dried out, which vaporizes into water vapor at high temperatures.

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

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

(2) The 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 mold 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 mold cavity.

2) Removing the obstructed gas will create high pressure inside the mold cavity and infiltrate 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 gas, the temperature inside the mold cavity rises sharply, causing the surrounding melt to decompose and burn, resulting in local carbonization and burning of the plastic parts. It mainly appears at the confluence of two melts, at the corner and gate flange.

4) The poor removal of gas results in different melt velocities entering each cavity, making it easy to form flow marks and fusion marks, and reducing the mechanical properties of the plastic parts.

5) Due to the obstruction of gas in the mold cavity, the filling speed will be reduced, affecting the molding cycle and lowering production efficiency.

(3) Distribution of bubbles in plastic parts

The sources of gas in the mold cavity are mainly divided into three categories: air accumulated in the mold cavity; Gas generated from the decomposition of raw materials; The residual water vapor in the raw materials evaporates, and the location of the bubbles generated varies depending on the source. Why is it necessary to design an exhaust system for injection molds.

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

2) The bubbles generated by decomposition or chemical reactions in plastic raw materials are distributed along the thickness of the plastic part.

3) The bubbles generated by residual water vapor in plastic raw materials are irregularly distributed throughout the entire plastic part.