The venting of injection mold is an important problem in the mold design, especially in the high speed injection molding, the venting of injection mold is stricter.
1) Air in the runner system and mold cavity.
2) Some plastic raw materials contain moisture that has not been dried out, and they are vaporized into water vapor at high temperatures.
3) Due to the high temperature during injection molding, some unstable plastic raw materials are decomposed into gases.
Venting System is critical to the quality and consistency of the finished product. Venting is required to allow air from the sprue, runner, and cavity to exit the tool as the melt flows into the cavity. Inadequate venting can result in short shots, poor surface appearance, and weak weld lines.
The main disadvantages are as follows:
1) In the injection process, 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 volume and cannot be filled with the mold cavity.
2) The gas trapped inside the mold cavity will form high pressure in the cavity and penetrate into the plastic at a certain degree of compression, resulting in quality defects such as air trap, voids.
3) Because the gas is highly compressed, the temperature in the cavity rises sharply, which causes the surrounding melt to decompose and burn, so that the plastic shell appears local carbonization and burning.
(4) The insufficient venting makes the speed of hot melt plastic entering each cavity differently, so it is easy to form flow marks, and reduce the mechanical properties of the plastic shell.
(5) Due to the obstruction of the gas in the cavity, the filling speed will be reduced and the injection molding cycle will be affected.
The common venting methods are to set up vent slot and use the mold parting surface to vent.
In addition to venting on the parting surface, the purpose of venting can also be achieved by setting a vent slot at the end of the material flow in the runner system and leaving a gap around the ejection pin.
1) Vents should be placed at the last place to fill and in areas where weld lines occur.
2) The typical vent size for GLS TPEs, is 0.0005" - 0.0010" (0.012 mm - 0.025 mm) with a 0.040" - 0.060" (10 mm - 15 mm) land.
3) Past the land, the vent depth should be increased to 0.005" - 0.010" (0.12 mm - 0.25 mm) to provide a clear passage for the air to exit the tool.
4) Venting in areas below the parting line can be accomplished by allowing the ejector pin to be 0,001 loose on each side.
