Structural Foam Molding is, in essence, a low pressure injection molding process. It is used to process thermoplastics, such as high density polyethylene (HDPE), in a low pressure environment. Instead of using high pressures to force the molten polymer to fill up the cavity of the mold, structural foam molding relies on the foaming action caused by an inert gas distributed in the plastic material to facilitate the flow. Alternatively, foaming can also be created by the gases released by the decomposition of a chemical blowing agent added to the resin.
Due to the lack of high pressures during the molding process, structural foam parts generally have thicker wall sections. Also, structural foams have a significant density reduction, sometimes as large as 40%, from its base material. They typically exhibit excellent strength-to-weight ratio, improved thermal and acoustic insulation properties, but suffer from lower tensile strengths.
Compared with injection molding, structural foam molding can use cheaper and lighter molds (aluminum instead of tool steel), and it can be used to produce larger parts. The downside is lower production speed. The setup cost of structural foam molding is also lower, making it a viable alternative to injection molding for low-volume applications.
In additional to HDPE, other polymers commonly used in structural foam molding are:
Acrylonitrile Butadiene Styrene (ABS)
Polypropylene (PP)
Polyethylene (PE)
Other than needing thicker wall sections, the design considerations of structural foam molding is very similar to those of injection molding.                                    

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