Gas-assisted injection molding, “GAIN”, is a thermoplastic molding process which provides stress-free large parts with a class “A” surface and vitually no sink marks. “GAIN” is a low-pressure molding process, compared to conventional injection molding. In this process, a controlled volume of inert gas is injected into the plastic after it enters the mold. The gas does not mix with the plastic, but remains in the middle of the thicker sections of the molding. By controlling the gas pressure, the quantity of plastic injected into the mold (short shot), and the rate of gas flow, a predetermined network of hollow interconnecting channels is formed within the molded part. The gas pressure remains constant in the network of hollow channels throughout the molding. This compensates for the tendency of the plastic to shrink at the thicker areas of molding, preventing warpage and reducing stress. The gas pressure is relieved just prior to opening the mold. Because of the relatively low injection pressure, large parts can be molded with substantial reductions in clamp tonnage.
The “GAIN” process is particularly well suited for large exterior automotive body panels such as fenders, applique panels or fascia. The primary advantages derived from gas-assisted injection molding are: (1) reduces molding stress and warpage, (2) eliminates the need for costly hot runners, (3) reduces clamp tonnage, and (4) allows reductions in wall thickness and part weight. Furthermore, gas-assisted molding is ideally suited for parts using “difficult to flow” engineering thermoplastic resins, including fiber-filled materials; and, tubular sections are possible which can increase the strength and rigidity of the molded parts. For most large automotive panels, gas-assisted injection molding will increase productivity and return on investment, will reduce tooling costs and production costs, and may substantially reduce operating expenses by enabling the production of larger parts on smaller tonnage machines.