Paper to EUfoam Conference, Paris, 5th-8th July 2004
S F Bush with O K Ademosu
Background
Rotomoulding is a mechanically simple, low pressure plastic processing operation for forming relatively large hollow structures by rotating a mould containing polymer powder either about two perpendicular axes or about one axis combined with a reciprocating back and forth motion along the second perpendicular axis. Polyethylene (PE) has been the principal polymer powder used owing to its low cost and easy availability. However, PE has a relatively low inherent stiffness compared with other thermoplastics although these have higher material cost. The use of fibre reinforcements to increase stiffness has recently been described [Ref 1]. However, the prospect of multi-layer mouldings using polymer foams within the hollow rotomoulding artefact to provide thermal and sound insulation as well as increasing bending stiffness to weight ratio is potentially very attractive.
The principal polymer foam systems used for sandwich structures are based on polystyrene or polyurethane. Solid polystyrene foam is created from solid beads containing dissolved gas which is released over a defined temperature range. Solid polyurethane foams are formed by the rapid reaction of an isocyanate with polyol in the presence of a blowing agent. Both systems currently involve a second state of manufacture after the hollow mouldings have been removed from their moulds.
As previously described [Refs 2, 3] the UMIST ROTOFOAM© process enables the foaming step to proceed without demoulding first, either in parallel with skin formation or immediately after. A major objective is to produce ultra light integral foams so that the panel bending strength to weight ratio is maximised. This is important in a host of industrial and commercial applications.
References
[1] O K Ademosu and D R Blackburn, Fibre-impregnation in Rotational Moulding & its effect on Mechanical & Thermal Properties, Advances in Materials and Processing Technologies, 2003.
[2] S F Bush and O K Ademosu, Combined Foaming and Rotomoulding: the Rotofoam Process, Eurofoam, 2002.
[3] S F Bush and O K Ademosu, Combined foaming and rotomoulding in the Rotofoam Process, Polymer Processing Society 19th Annual Meeting, Melbourne, Australia, 7-10 July 2003.