David mixes resin to make the other mold half to be used for rotocasting after it has cured.

Rotocasting, also known as rotomolding or centrifugal casting, is a method for forming thermoplastic resins where the molten material solidifies in and conforms to the shape of the inner surface of a heated, rapidly rotating container.

In industrial rotocasting, the metal being cast is thrown toward the walls of the mold and forms the casting after hardening. The centrifugal method is widely used in industries for the production of hollow castings with free surfaces, such as cast-iron and steel pipe, rings, sleeves and cowlings.

It’s a unique plastic molding process used primarily to create seamless, stress-free, hollow one-piece items. It’s a high-temperature, low-pressure manufacturing method that combines heat and bi-axial rotation. Typical molded parts from industrial rotocasting can include containers, tanks, children’s toys, medical and industrial equipment, and automobile parts.

Proponents of the rotational molding process point to its low production cost and unlimited design possibilities. It’s a competitive alternative to blow molding, thermoforming, and plastic injection molding. It produces little waste since the required weight of plastic to produce the part is placed inside the mold.

In industrial rotocasting, the process begins with filling a hollow mold with a quantity of powder resin (polymer). More often than not this powdered resin is polyethylene, although other compounds such as polypropylene, PVC, and nylons can also be used. The mold is then heated at high temperatures in an oven and bi-axially rotated as the polymer melts and coats the inside of the mold.

In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase and also during the cooling phase to avoid sagging or deformation.

Although it may seem like a very machine-intensive process, makers and tinkerers alike are able to perform the same process at home with what we call it the home rotocasting — equivalent to both resin casting and industrial rotocasting. When built at home, rotocasting is when you use a resin casted part, and place it in a home rotocasting tool, spinning it around while gravity makes the mold conform to the walls of the piece, making a hollowed out part.

A rotocasted part uses less material to make a larger part, allows for a reduction in weight and amount of material required to make the part.

Home rotocasting is particularly useful for cosplayers and fandoms. Check out this video on a home rotocasted iron man leg.

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