What to do if the air is not smooth in the injection molding

Posted by yy on December 18th, 2020

1. Gas is often generated in plastic injection molding. What is it caused by?

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

(2) Some raw materials contain moisture that is not removed by drying, and they will vaporize into water vapor at high temperatures.

(3) Due to the high temperature during injection molding, certain unstable plastics will decompose and produce gas.

 (4) Gas generated by volatilization or chemical reaction of certain additives in plastic raw materials.

2. At the same time, the reason for poor exhaust must be found out as soon as possible. Poor venting of the plastic injection molding will bring a series of harms to the quality of plastic parts and many other aspects.

The main manifestations are as follows:

(1) The melt will replace the gas in the cavity during the injection molding process. If the gas is not discharged in time, it will be difficult to fill the melt, resulting in insufficient injection volume to fill the cavity.

(2) The gas that is not smoothly removed will form high pressure in the cavity and penetrate into the plastic under a certain degree of compression, causing quality defects such as voids, pores, sparse tissue and silver streaks.

(3) Because the gas is highly compressed, the temperature in the cavity rises sharply, which in turn causes the surrounding melt to decompose and burn, causing local carbonization and scorching of the plastic parts. It mainly appears at the junction of two melts, at the gate flange.

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

(5) Due to the obstruction of the gas in the cavity, the filling speed will be reduced, the molding cycle will be affected, and the taxation efficiency will be reduced.

3. The distribution of bubbles in plastic parts

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

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

(3) The air bubbles generated by water vaporization remaining in the plastic raw materials are irregularly distributed throughout the plastic parts.