Rotational moulding - a simplified theory

This paper considers a number of theoretical approaches to the description of the various and complex issues, which are involved in rotational moulding. Experimental rheological measurements are presented to illustrate the changes, which can occur in typical rotational moulding grade polymers when subjected to shear and also when held at an elevated temperature for a sustained period of time. The influence of temperature on the viscosity is indicated to be an important factor in bubble release and maintenance of wall thickness in thick-walled mouldings. Although it is desirable to have a low viscosity in the melt to aid bubble release and hence reduce the time that the polymer spends in the melt phase, the problem of maintaining contact with the mould wall can arise in thick moulded sections.