Optimisation of bubble size in rotationally moulded parts

Rotational moulding is one of the most important methods of manufacture of hollow plastic products. However, there are several unsolved problems that confound the overall success of this technique, including surface pin holes and internal bubbles of moulded parts, caused by inappropriate mould design and processing conditions. In this report, an L'18 experimental matrix design based on the Taguchi method was conducted to optimise the bubble size of rotationally moulded parts. Experiments were carried out on a laboratory scale biaxial rotation moulding unit. The polymeric material used to mould the parts was linear low density polyethylene. After moulding, the size of the bubbles on the surface of moulded parts was characterised by an image analysis system. For the factors selected in the main experiments, the cooling conditions and the particle size of the material were found to be the principal factors affecting the bubble size of rotationally moulded thermoplastics. In addition, mould pressurisation helped decrease the size of the bubbles. The bubble size of moulded parts was not affected by the water content of the polymeric powder, but increased with the viscosity of the materials.