Bubble dissolution in molten polymers and its role in rotational molding

The presence of bubbles is inherent in the rotational molding process and can often cause poor quality in rotomolded parts. In an effort to gain a fundamental understanding of bubble removal in polymer melts, a model that includes diffusion, surface tension, and viscosity effects has been implemented. This work has been complemented with experiments performed in a heated chamber, which enabled the visualization of the bubble formation and removal process. It was concluded that bubble removal is mainly diffusion controlled and is not influenced significantly by melt viscosity when the latter lies within ranges typical for rotational molding resins. Air concentration in the bulk of the polymer melt and initial bubble size were found to be of great importance. The application of pressure after the polymer has melted accelerates the dissolution rate, owing to an increased driving force for diffusion.