Why set the exhaust system for injection molds?

- Apr 17, 2018-

         The exhaust of the injection mold is an important issue in the mold design, especially in the rapid injection molding, the exhaust requirements for the injection mold are more stringent.

(1) The source of the gas in the injection mold.        1) Air in the gating system and mold cavity.        2) Some raw materials contain moisture that has not been removed by drying. They are vaporized to steam at high temperatures.        3) Due to the high temperature during the injection molding, some of the plastics with unstable properties undergo gas decomposition.        4) Gases produced by volatilization or chemical reaction of certain additives in plastic materials. (2) Harmful exhaust gas         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) During the injection molding process, the melt will replace the gas in the cavity. If the gas is not discharged in time, it will cause difficulty in the melt filling, resulting in insufficient injection volume and failure to fill the cavity.        2) Excluding poor gas will form high pressure in the cavity, and infiltrate into the plastic within a certain degree of compression, resulting in air holes, voids, loose tissue, silver lines and other defects.        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 corner and at the gate flange.        4) The removal of gas is not smooth, so that the melt velocity entering the various cavities 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 production efficiency. (3) Distribution of air bubbles in plastic parts          The sources of gas in the cavity are mainly divided into three categories: the air accumulated in the cavity; the gas produced by the decomposition in the raw material; the water vapor that evaporates the residual water in the raw material, and the location of the bubbles generated due to the different sources is also different.

          1) Bubbles generated by the accumulation of air in the cavity are often distributed on the opposite side of the gate.           2) The bubbles generated by decomposition or chemical reaction in plastic raw materials are distributed along the thickness of the plastic parts.          3) Bubbles generated by vaporization of residual water in plastic raw 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.