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Plastic Molds Also Need To Exhaust
Jun 11, 2018

Core Tip: Exhaust of the injection mold is an important issue in mold design. Especially in the rapid injection molding, the exhaust requirements of the injection mold are more stringent.

 1. The source of gas in the injection mold.

(1) Air in the pouring system and mold cavity. (2) Some raw materials contain moisture that has not been removed by drying, and they are vaporized into water vapor at high temperatures. (3) Due to the high temperature at the time of injection molding, gas generated from the decomposition of certain unstable plastics occurs. (4) Gases generated by certain additives in plastic materials that volatilize or react with each other.

2, the harm of poor exhaust

The poor exhaust of the injection mold will bring a series of hazards to the quality of the plastic parts. The main performance is as follows:

(1) In the injection molding process, the melt will replace the gas in the cavity. If the gas is not exhausted in time, it will cause difficulty in the melt filling, resulting in insufficient injection volume to fill the cavity. (2) Excluding poor gas will form high pressure in the cavity, and infiltrate into plastic within a certain degree of compression, resulting in defects such as pores, voids, sparse tissue, and silver lines. (3) As the gas is highly compressed, the temperature in the cavity rises sharply, causing the surrounding melt to decompose and burn, causing local carbonization and scorching of the plastic parts. It occurs mainly at the junction of the two melts, at the gate flange. (4) The removal of gas is not smooth, so that the melt velocity entering each cavity is different, so flow marks and fusion marks are easily formed, and the mechanical properties of the plastic parts are reduced. (5) Due to the obstruction of the gas in the cavity, the filling rate will be reduced, affecting the molding cycle and reducing the taxation efficiency.

3 Distribution of air bubbles in plastic parts

(1) Bubbles generated by the accumulation of air in the cavity are often distributed on the opposite side of the gate. (2) Bubbles generated by decomposition or chemical reactions in plastic raw materials are distributed along the thickness of the plastic parts. (3) Bubbles generated by gasification of residual water in plastic materials are irregularly distributed on the entire plastic part.

From the distribution of air bubbles in the above plastic parts, not only the properties of the air bubbles can be judged, but also whether the exhaust parts of the mold are correct and reliable.