Optimization of the rotational moulding process for polyolefins

Hollow plastic parts can be made by a number of manufacturing methods, but only rotational moulding offers the ability to create one-piece, stress-free parts with attractive economics. However, the fundamental nature of rotational moulding is such that cycle times are long. Consequently, the plastic is subjected to relatively high temperatures, in the presence of air, for excessive periods of time. This can lead to thermal and oxidative degradation at the inner free surface of the plastic, resulting in a deterioration in the performance of the moulded part. The industry relies heavily on the experience of operators to establish the best processing conditions to avoid degradation, but this is problematic and inefficient. Unfortunately, automatic process control is difficult owing to the complex rotation of the mould. Recent developments highlighting the importance of the peak air temperature inside the mould have been an important step forward, but ever-increasing technical demands on moulders make it clear that more sophisticated process control is needed. It is known that the processing conditions that lead to degradation vary with factors that affect the heating rate, such as the type of mould used and the thickness of the end-product. In the work reported here, a method is proposed for predicting the onset of degradation, on the basis that this occurs when the concentration of antioxidant in the polymer reaches zero. Good agreement has been obtained between the experimental and predicted optimum processing temperature for polyethylenes stabilized with different antioxidant systems. A procedure is described for identifying the best rotational moulding conditions so that more efficient manufacturing methods can be achieved.