Cars need to exhaust exhaust fumes and install exhaust systems, range hoods need to exhaust fumes and install exhaust systems, and air conditioners need to exhaust hot air in exchange for cold air during refrigeration, etc. Should exhaust be applied in the mold?
The special exhaust of injection mold is an important problem in mold design, especially in rapid injection molding, the exhaust requirements of injection mold are 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) The gas produced by the decomposition of some unstable plastics due to the high temperature during injection molding. (4) Gases generated by volatilization or mutual 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 performances 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 lead to difficulty in filling the melt, resulting in insufficient injection volume and failure to fill the cavity. (2) The elimination of gas will be in the form of high pressure, and to a certain extent compressed into the plastic, resulting in porosity, porosity, tissue thinning, crazing and other quality defects. (3) Because of the high compression of the gas, the temperature of the cavity increases rapidly, and then melts, decomposes and burns around, and local carbonization and combustion occur. It mainly occurs at the intersection of two melts, the door flange. (4) The elimination of gas is not smooth, which makes the melt flow rate of different types of cavities different, so it is easy to form flow marks and fusion marks, and the mechanical properties of plastic parts are reduced. Because the gas in the cavity is blocked (5), it can reduce the filling speed, affect the forming cycle, and reduce the tax efficiency.
3. Distribution of bubbles in plastic parts
(1) The bubbles produced by the accumulated air in the mold cavity are often distributed on the position opposite to the gate. (2) Bubbles produced by decomposition or chemical reaction in plastic raw materials are distributed along the thickness of plastic parts. (3) Bubbles generated by residual water gasification in plastic raw materials are irregularly distributed on the whole plastic part.
From the distribution of bubbles in the above plastic parts, we can not only judge the nature of bubbles, but also judge whether the exhaust part of the mold is correct and reliable.