Gas (Water) Assisted Injection Molding

- Oct 13, 2018-

Gas-assisted injection molding is one of the most important developments in injection molding technology since the advent of reciprocating screw injection molding machines. It produces a hollow section inside the injection molded part by high-pressure gas, utilizes the volumetric pressure of the gas, reduces the residual internal stress of the product, eliminates the surface shrinkage of the product, reduces the material used, and shows the superiority that the conventional injection molding cannot match. The gas-assisted injection process consists of three main phases:

The initial stage is melt injection. At this stage, the plastic melt is injected into the cavity, which is the same as the conventional injection molding, but the melt is only filled with 60%-95% of the cavity, and the specific injection amount varies depending on the product.

The second stage is gas injection. At this stage, a high pressure inert gas is injected into the melt core, and the melt front continues to flow forward under the pressure of the gas until it fills the entire cavity. The melt flow distance is significantly shortened during gas-assisted injection molding, and the melt injection pressure can be greatly reduced. The gas can be introduced into the workpiece from the main flow channel or directly from the cavity through the gas injection component. Since the gas has the characteristic of always selecting the direction of resistance (high temperature, low viscosity), it is necessary to specifically design a gas passage in the mold.

The third stage is gas holding pressure. This stage allows the part to cool while maintaining gas pressure. Further, the gas isotropic pressure is applied to the inside of the workpiece uniformly. And through the gas expansion to supplement the volume shrinkage (secondary penetration) caused by the melt cooling solidification, to ensure that the outer surface of the product is close to the mold wall.