Combined Foaming and Rotomoulding: the Rotofoam Process
July 7th, 2002
Paper to Eurofoam 2002 Conference, 7th-10th July 2002
S F Bush with O K Ademosu
Abstract
Polymer foams are widely used in the manufacturing and construction industries to provide thermal insulation and/or to increase bending stiffness to weight ratios. The principal polymer foam systems used are expanded polystyrene and polyurethane. The former is commonly found in polystyrene cups where the combination of heat insulation and adequate stiffness are a great improvement over the PVC alternative. Polyurethane foams are used in shoe soles where their friction properties are the best of any common material, and in refrigeration where special formulations provide a “self-skinning” property. In this case, the reaction of the isocyanate and polyol constituents is controlled by the catalyst used to ensure that for a given blowing agent, a solid (closed cell) surface is obtained next to the metal of the refrigerator cavities being fitted. It is thus a highly convenient manufacturing process.
Rotomoulding is an established process for forming relatively large hollow structures by rotating a mould containing polymer powder (typically polyethylene) about two axes. The powder is distributed more or less uniformly over the mould surface by a combination of gravity and centrifugal forces. If the mould temperature is set with appropriate regard to the melting temperature range of the polymer powder, a viscous molten layer is built up next to the mould surface. The free surface of this layer gradually extends inward. Reducing the mould temperature to below the polymer melting point will give a solid skin which allows the hollow form to be removed. Footballs, and hollow planking are made this way. Dustbins are made by cutting one end off and using it as the lid. In the UMIST laboratory, a two axial rotational moulding rig was designed by W G Neilson.
Up to now, if a rotomoulded artefact is needed to be foam-filled to promote its strength and stiffness without sacrificing too much the insulating properties of a hollow interior, the hollow form has had to be made first in one operation, then, as a comparatively costly second step, removed from the mould and filled with polyurethane foam. The UMIST Rotofoam process allows the foaming step to proceed at the same time as the moulding step, giving a a solid outer skin of one material and a foamed interior made of another. Besides the sharp reduction in overall process time, and the avoidance of the special equipment needed in the present foaming setup, the principal materials used – polyethylene and expanded polystyrene – are considerably cheaper than self-skinning polyurethanes, and without their chemical hazards. A composite variant has also been devised [Ref 1].
The paper will describe experiments on the Rotofoam process at both the laboratory scale and full-scale as rotation speeds, feed materials and temperature-time profiles are varied. Large bore steam pipe insulators, damage resisting post covers and cold store doors all made on our industrial collaborator’s plant (Micropol Ltd in Stalybridge) will illustrate the results obtained in practice from this new industrial process.
Reference
[1] O K Ademosu and D R Blackburn, “An Investigation of the Production of Rotationally Moulded Composites”, 9th International Eurofoam Conference on Fibre Reinforced Composites, 26th-28th March 2002.