Structural foam molding is a low-pressure method of processing thermoplatics. Inert gas, usually nitrogen, inserted into the resin causes bubbles to form and create a foaming action that helps fill a molding cavity. Gases created by decomposition of a chemical blowing agent is another way foaming occurs in this process. The process creates rigid parts with hard surfaces.
      Common Polymers
A common misconception is that only high density polyethylene (HDPE) is used in this process. HDPE is most frequently used, but other plastics are also employed. These include acrylonitrile butadiene styrene (ABDE), polypropylene (PP) and polyethylene (PE).
      Key Element
Low pressure is crucial to structural foam molding because, as resin flows through a mold, surface cells burst to form solid skins which produce thicker and sturdier walls on the products that are manufactured. Foaming agents are held in check by an injection screw until resin enters the mold. The mold's configuration works in conjunction with the low pressure. At this point, the foaming action produced by the inert gases is inserted into the molten resin, making it flow much further than what typically occurs in injection molding processes. The cellular structure of parts manufactured in this manner is similar to wood.
      Advantages
Structural foam molding provides several advantages for manufacturers, Set-up costs are lower than those for injection molding. Lower pressure allows molding equipment and tooling to be produced more economically. At the same time, larger molds may be produced, resulting in larger parts. Lower internal pressures also contribute to a heavier cellular structure, which makes a sturdier product.
      Shrinkage
Resins are injected into molds at 500 degrees F, then rapidly cooled to 100 to 140 degrees F. This cooldown causes the piece to set. Some in-mold shrinkage occurs, dependent on several factors including size and type of machine producing the product, thickness of molded section, degree and direction of material used and the temperature of the mold itself.
Immediately following release from the mold, structural foam parts can shrink from 1.5 to 4 percent from their original size. Shrinkage of an additional 0.5 percent continues to occur over the next 48 hours, as noted by Creative Techniques. Miniscule reduction in size continues throughout the life of the part.
      Uses
Structural foam parts may be used in place of wood, concrete, fiberglass and metal. According to Structural Foam Network, weight of the end product can be as much as 20 percent less than one made via injection molding. Black is the most common color used in manufacture, but a wide range of hues may be used.