In structural foam molding, the resin is injected into the mold cavity as a short shot. The addition of gas during injection expands and drags the melted resin throughout the mold until it is filled. Because the foam is doing the work, cavity pressure remains low — which translates into a low-stress part and lower clamp tonnage requirements.

Structural foam molding presses are similar to traditional injection molding machines, but larger in size and accompanied by a gas injection apparatus. Like standard injection molding, the Thermoplastic resin is fed by a hopper into a barrel and heating screw prior to injection. At this stage, structural foam molding takes on a life of its own with the introduction of inert gas or a blowing agent that produces CO2.

The inert gas, usually Nitrogen, is introduced to the melted resin just before injection. This gas expands within the mold cavity to aid plastic flow, thus requiring only a partial short-shot of resin. The foaming process and low pressure combine to fill the mold with plastic. Against the walls of the mold, the plastic cells freeze to develop a smooth, rigid outer skin on the part.

Once the cavity is filled the part begins to cool, the material in the core of the walls develops a porous cellular structure. The solid outer skin and porous interior give the part its strength and structural integrity.

Nominal wall thickness for structural foam parts ranges from 0.150-0.500 inches, with typical thickness being 0.250 inches. Due to the introduction of inert gas and internal cellular structure, structural foam parts can have 10-15% less density than traditional injection molded parts. This density reduction results in lower resin usage and costs.

Gas Assist, Gas Counter Pressure, and External Gas Molding

Gas assist, gas counter pressure, and external gas molding are complementary processes to structural foam molding. In these processes, specialized nozzles, inlets, and channels are incorporated into the part and tool design.

During injection, inert pressurized gas is introduced using a specialized nozzle or gas pin. With the resin shot completed, the pressurized gas continues to work. The gas controls the flow front and creates bubbles inside or around the part to manipulate the skin and resin throughout the mold cavity. Once the part has cooled to form a strong outer skin, the gas pressure is vented before the mold is opened and the part is ejected. Hollow sections can be achieved within a part using gas assist, gas counter pressure, and external gas.