THE FORMATION AND REMOVAL OF GAS-BUBBLES IN A ROTATIONAL MOLDING GRADE OF PE

As part of a research programme to investigate the rotational moulding process, a series of tests were conducted to examine the mechanisms by which bubbles form in, and are removed from, polymer melts. It was found that air gets trapped between powder particles and the size and density of the bubbles depends on factors such as the particle size distribution and melt flow index of the plastic. Once formed, the bubble remains static in the melt but decreases in size by a process of diffusion. The rate of decrease in size is dependent on temperature, initial bubble size, rate of heating and location of a bubble in the melt. From measurements on the bubble size it has been possible to obtain an expression to quantify the decrease in bubble size with time.