The exhaust of injection mold is an important problem in mold design, especially in rapid injection molding, the exhaust requirement of injection mold is more strict.
1. Source of gas in injection mold.
1) Air in the gating system and mold cavity.
2) Some raw materials contain water that has not been removed by drying, and they are vaporized into steam at high temperature.
3) Due to the high temperature during injection molding, some unstable plastics decompose and produce gas.
4) The gas produced by volatilization or chemical reaction of some additives in plastic raw materials.
2. Hazards of poor exhaust
Poor exhaust of injection mold will bring a series of hazards to the quality of plastic parts and many other aspects. The main manifestations are as follows:
1) In the process of injection molding, the melt will replace the gas in the cavity. If the gas is not discharged in time, it will make the melt filling difficult, resulting in insufficient injection volume and unable to fill the cavity.
2) High pressure will be formed in the mold cavity when the gas is not smooth, which will penetrate into the plastic under a certain degree of compression, resulting in porosity, cavity, loose tissue, craze and other quality defects.
3) Due to the high compression of the gas, the temperature in the mold cavity rises sharply, which causes the decomposition and burning of the surrounding melt, resulting in local carbonization and scorching of the plastic parts. It mainly occurs at the confluence of two melts, * angle and gate flange.
4) Due to the poor gas removal, the melt velocity entering each cavity is different, so flow marks and fusion marks are easy to form, and the mechanical properties of plastic parts are reduced.
5) Due to the obstruction of gas in the cavity, the filling speed will be reduced, the molding cycle will be affected and the production efficiency will be reduced.
3. Distribution of bubbles in plastic parts
There are three main sources of gas in the mold cavity: the air accumulated in the cavity; the gas produced by decomposition in the raw material; and the residual water vapor in the raw material. Due to different sources, the positions of bubbles are different.
1) The air bubbles generated by air accumulation in the mold cavity are usually distributed in the position opposite to the gate.
2) The bubbles produced by decomposition or chemical reaction in plastic raw materials are distributed along the thickness of plastic parts.
3) The bubbles produced by the residual water gasification in the plastic raw materials are irregularly distributed on the whole plastic part
From the distribution of bubbles in the plastic parts mentioned above, not only the properties of bubbles can be judged, but also whether the exhaust part of the mould is correct and reliable.