Analysis of venting and air bubbles in transparent PMMA injection mold

The requirements for PMMA injection molds are relatively high, and the surface should be high-gloss and there can be no process defects, but PMMA products are generally thicker, so there are often trapped air and product bubbles in the injection mold.

How to control exhaust?
(1) The air stored in the pouring system and the mold cavity;
(2) Some raw materials contain moisture that has not been 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) Some additives in plastic raw materials volatilize, or gas generated by mutual chemical reaction.

The reason for the poor exhaust of PMMA injection molds needs to be found out as soon as possible. Poor venting of injection molds will bring a series of hazards to the quality of plastic parts and other aspects, mainly in:
(1) During the injection molding process, the melt will replace the gas in the cavity. 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 cavities, pores, loose 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 and the flange of the gate;

(4) The gas is not removed smoothly, which makes the speed of the melt entering each cavity different. Therefore, 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 gas in the PMMA injection mold cavity, the filling speed will be reduced, the molding cycle will be affected, and the molding efficiency will be reduced.

In acrylic plastic parts, the main distribution of bubbles are:
(1) The bubbles generated by the air accumulated in the cavity are often distributed on the position opposite to the gate;
(2) The bubbles generated by decomposition or chemical reaction in the 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.