Rotational Molding
“Rotational molding or moulding is a versatile process for creating many kinds of mostly hollow items, typically of plastic. The phrase is often shortened to rotomolding or rotomoulding.” (Wikipedia 8/13/2009)
“A heated mold causes the material within to melt and form a puddle at the bottom of the mold cavity. The mold is then slowly rotated (usually around two perpendicular axes) causing the melted material to flow into to the mold and stick to its walls. In order to maintain even thickness throughout the part, the mold continues to rotate during the cooling phase. The process was applied to plastics in the 1940s but in the early years was little used because it was a slow process restricted to a small number of plastics. Over the past two decades, improvements in process control and developments with plastic powders have resulted in a significant increase in usage.” (Wikipedia, Rotational Molding, 1/28/2011)
“In rotational molding or rotomolding, the product or molded object is formed inside a closed mold or cavity while the mold is rotating bi-axially in a heating chamber. There are typically four steps, or stages, in a rotational molding process: loading, molding (or curing), cooling, and unloading. In the loading stage, either liquid or powdered plastic, which may be a thermoplastic, is charged into a hollow mold. The mold is closed, rotated about two orthogonal axes, and moved into a heating chamber or oven for the molding or curing stage. In the oven, heat penetrates the mold, causing the plastic to melt, adhere to, and sinter onto the mold surface. The mold continues to rotate during heating, and the plastic gradually becomes distributed evenly on the mold walls through gravitational force. As the cycle continues, the plastic melts completely, forming a homogeneous layer of molten thermoplastic on the interior surfaces of the mold. While continuing to rotate, the mold is moved out of the oven to a cooling area or chamber for the cooling stage, where the plastic is cooled to the point that the molded object will retain its shape. During cooling, the molded object typically shrinks away from the mold. In the unloading stage, rotation of the mold stops, and the mold is opened to remove the molded object.” (Riggins, Dobson and Moore, US Patent 7,833,459; 11/16/2011)
Recent US Patents
10/4/2011
8,029,718
Process for forming multi-layer molded articles
O’Brien, Clay and Partridge of Arkema, Pennsylvania, developed a rotomolding process for forming a multi-layer article of two different polymers with different melting points in which an outer layer polymer powder is added to a rotational molder along with an inner layer polymer powder that is separated from the outer layer powder by a release film. The release film has a particular melting point that is greater than that of the outer layer polymer and preferably between the melting points of the inside layer and outside layer polymers. In one embodiment, the release layer forms an enclosure or bag in which the inner layer (higher melting point) powder is enclosed. In another embodiment the release film forms a barrier between an insulated cavity and the mold. The process provides a means for a "one-shot" rotomolding method. (RDC 10/5/2011)
Recent Journal Articles
12/3/2010
Characterization and modeling of sintering of polymer particles
( 2784–2792) Journal of Applied Polymer Science 119 #5 (2010)
Asgarpour et al modified the model for sintering during rotational molding based on surface tension and viscosity to include the effect of neighboring particles. The result is a model able to predict multi-grain sintering. (RDC 12/9/2010)