COMBINED FOAMING AND ROTOMOULDING : THE ROTOFOAM PROCESS
Stephen BUSH
UMIST Centre for Manufacture
England UK
Keywords: L.175, ROTOFOAM
ABSTRACT OF PAPER FOR PPS-19
Melbourne, Australia July 7-10, 2003
For Session 5 or 12
COMBINED FOAMING AND ROTOMOULDING:-
THE ROTOFOAM PROCESS
by
S F Bush and O K Ademosu
Polymer Engineering Research Laboratory
Centre for Manufacture
UMIST, P.O.Box 88, Manchester M60 1QD
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 have a wide range of applications ranging from relatively dense foams used in shoe soles where their friction properties are the best of any common material, to low density foams used 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.
Rotomoulding is an established process for forming relatively large hollow structures by rotating a mould containing polymer powder (typically polyethylene) either about two axes or about one axis combined with a rocking back and forth motion along another axis. The powder is thus 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 which 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.
Up to now, if such a rotomoulded hollow form needed to be foam-filled (e.g. 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 demoulded, then, as a comparatively costly second operation, taken to another station where it is filled with polyurethane foam. The UMIST Rotofoam process allows the foaming step to proceed at the same time as the moulding step, giving 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 set up, the principal materials used – polyethylene and pentane-propelled polystyrene – are considerably cheaper than self-skinning polyurethanes, and without their chemical hazards.
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, cold store doors, harbour fenders and pallets, all made on our industrial collaborators’ plants will illustrate the results obtained in practice from this new industrial process. A further variant – Rotofil - in which long glass fibre filaments are distributed into the polyethylene skin will also be briefly described.