Water-Injection Molding vs. Gas-Assisted Molding

Gas-assisted injection molding is a low-pressure process utilizing nitrogen gas to apply uniform pressure throughout the molded plastic part. By displacing molten plastic from thicker sections of the part toward areas in the cavity that are last to fill, nitrogen gas pressure creates channels within the part. Through the gas channels, pressure is transmitted evenly across the part, eliminating warping, sink marks, and internal stress. As a result, reductions in clamp tonnage, cycle time, and part weight are realized, while the strength and rigidity of the part is increased.
Gas-assisting allows molders the freedom to design plastic parts without restrictions and limitation inherent to conventional plastic injection molding. When it comes to the disadvantages of gas-assisted molding, because gas can branch into multiple flow streams, it can create an undesirable fingering effect in your products. It also has the potential to roughen up the inner surface of whatever you’re molding with if not handled correctly, resulting in a product lacking visual appeal.

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. The front also cools the melted plastic as it is pushed down the mold cavity by the water pressure. That cooling effect can have a big impact on production. Because of the heavier density of the water compared against the lighter density of the gas, you can be assured of a more uniform interior surface in your final product when it is created with waster-assisted molding. The finer level of control and the ability to cool parts as they’re formed mean that water-assisted injection is a great option to speed up production while lowering costs.