Rotational Moldings

This method, like blow molding, is used to make hollow one-piece TP parts. RM consists of charging a measured amount of TP into a warm mold cavity that is rotated in an oven about two axes. In the oven, the heat penetrates the mold, causing the plastic, if it is in powder form, to become tacky and adhere to the mold female cavity surface, or if it is in liquid form, to start to gel on the mold cavity surface. Since the molds continue to rotate during the heating cycle, the plastic will gradually become distributed on the mold cavity walls through gravitational force. As the cycle continues, the plastic melts completely, forming a homogeneous layer of molten plastic. After cooling, the molds are opened and the parts removed.

RM can produce quite uniform wall thicknesses even when the product has a deep draw of the parting line or small radii. The liquid or powdered plastic used in this process flows freely into corners or other deep draws upon the mold being rotated and is fused/melted by heat passing through the mold's wall.

This process is particularly cost-effective for small production runs and large product sizes. The molds are not subjected to pressure during molding, so they can be made relatively inexpensively out of thin sheet metal. The molds may also be made from lightweight cast aluminum and electroformed nickel, both of them light in weight and low in cost. Large rotational machines can be built economically because they use inexpensive gas-fired or hot air ovens with the lightweight mold-rotating equipment.

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