Structural Foam Molding is a low-pressure injection molding process that is capable of producing very large structural parts. The molten plastic material is injected into a mold after being mixed with a blowing agent or high-pressure gas. This produces bubbles in the plastic causing it to foam. The foam retains the properties of the plastic but weighs less because of reduced density.
Structural foam molding is similar to the injection molding process, structural foam molding is a low pressure method of processing thermoplastics, with the most commonly used resin being HDPE (high density polyethylene). The end product is typically a rigid part with a relatively hard surface. Structural foam should not be confused with expanded polystyrene (EPS), which can be associated with the white disposable foam blocks that package and protect new appliances and electronics.
The key element in structural foam molding is low pressure. Unlike conventional injection molding, which utilizes extremely high pressures to force materials into a mold's cavity, structural foam molding takes advantage of a part's configuration, its generally thick wall sections (that act as runners), and the foaming action (supplied by either chemical reaction within the resin blend, or the introduction of a compressed, inert gas into the mold), to allow the molten resin to flow much further, and with far lower pressure, than the typical injection molding process would allow.
Typical structural foam molds are produced in aluminum, the foaming of the plastic causes a swirling finish on the surface of the plastic part. Under pressure inside the injection screw, the foaming agents do not expand. When the melted plastic enters the mold cavity, foaming occurs. As the foaming plastics fill the foam mold cavity, the wall of the part solidifies against the cold mold wall. A thin layer of plastic solidifies without foaming along the mold wall. This thin layer forms a skin structure over the foamed inter core. The thin solid wall is supported by the interior cellular foamed structure. A part produced this way results in a heavy cellular structure similar to wood products. Structural foam molding is a common process.
After molding, parts will shrink 1.5 – 4 % of the mold size and will continue shrink another .5% during the first 48 hrs, the shrinkage abides, but continues at minuscule levels for the life of the part, but the part size in a constant flux as ambient temperatures change, oils and chemicals act on the plastic, etc. Thermal expansion of vinyl siding requires ½” expansion gaps to accommodate movement as the size changes with temperatures.
When plastic parts are injected into a mold they are at 500°F and are rapid cooled to 100-140°F. Once released from the mold, the part will deform from the mold shape due to residual stresses from the cooling process. The differences between the dimensions of the mold and of the molded article produced therein from a given material vary according to the design and operation of the mold. The differences vary with the type and size of molding machine, the thickness of molded sections, the degree and direction of flow or movement of material in the mold, the size of the nozzle, sprue, runner, and gate, the cycle on which the machine is operated, the temperature of the mold, Mold shrinkage (in-mold shrinkage or moldedpart shrinkage are more accurate terms), although a volume phenomenon, usually refers to the difference between the linear dimension of the mold at room temperature and that of the molded part at room temperature within forty-eight hours following ejection.