The exhaust of injection mold is an important problem in the mold design, especially in the rapid injection molding, the requirement of the exhaust of injection mold is more strict.
(1) The source of gas in the injection mold.
1) Air in the casting system and mold cavity.
2) Some of the raw materials contain water that has not been dried out. They are vaporized at high temperatures to form water vapor.
3) The gas produced by the decomposition of some unstable plastics due to the high temperature during injection molding.
4) The gas produced by the volatilization or mutual chemical reaction of some additives in the plastic raw materials. Why should the exhaust system be set up in the injection mold?
(2) The harm of poor exhaust
The bad exhaust of injection mold will bring a series of harm to the quality of plastic parts. The main manifestations are as follows:
1) In the process of injection molding, the melt will replace the gas in the mold cavity. If the gas is not discharged in time, it will cause the melt filling difficulty, resulting in insufficient injection amount and unable to fill the mold cavity.
2) Remove the gas in the cavity will form a high pressure, and in a certain degree of compression into the plastic interior, resulting in pores, cavities, loose tissue, silver stripes and other quality defects.
3) As the gas is highly compressed, the temperature in the mold cavity rises sharply, and then causes decomposition and burning of the surrounding melt, leading to local carbonization and charring of the plastic parts. It mainly appears in the confluence of two melts, * Angle and gate flange.
4) Poor gas elimination makes the melt speed into 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 gas in the mold cavity, the filling speed will be reduced, the molding cycle will be affected, and the production efficiency will be reduced.
(3) Bubble distribution in plastic parts
The source of gas in the mold cavity is divided into three types. Gases produced by decomposition in raw materials; The residual water vapor in the raw material, due to the different sources of the bubble location is also different injection mold why to set the exhaust system mold design.
1) Air bubbles generated by air accumulation in the mold cavity are usually distributed on the part opposite to the gate.
2) The bubbles produced by the decomposition or chemical reaction in the plastic materials are distributed along the thickness of the plastic parts.
3) Bubbles from residual water gasification in the plastic raw materials are irregularly distributed on the whole plastic parts.
From the distribution of bubbles in the above plastic parts, not only the nature of bubbles can be judged, but also can judge whether the exhaust part of the mold is correct and reliable.