Water assisted injection molding is the newest way to mold hollow or partially hollow parts. It is similar to gas assist molding as a means to core out thick sections, but the advantage of water injection technology is direct cooling inside the part. The thermal conductivity of water is 40x greater and has the heat capacity 4x greater than gas.

The chief difference between water and gas is that water is not compressible. When injected into the melt, the leading edge of the water forms a solid boundary, or highly viscous membrane. This membrane forces molten material forward instead of the polymer forcing the water to the side.

The primary reason for this action is the higher viscosity and incompressibility of water compared with gas. This viscous front acts as a ram that cores out the part and the front also cools the melt as it is pushed down the mold cavity by the water pressure.