Bubble removal in rotational molding

Abstract

Closed form solutions have been obtained for bubble dissolution in typical polymer melts encountered in rotational molding. The solutions are in excellent agreement with experimental data available in the literature. Using these solutions, it is shown that under typical rotational molding conditions the polymer melts may be almost saturated. As a result, bubble shrinkage occurs over long periods. Depending on the degree of saturation, surface tension may contribute substantially to the concentration gradient that drives bubble shrinkage. It is also shown that a pressure increase imposed on a nearly saturated polymer melt leads to a steep concentration gradient at the bubble/melt interface that can cause extremely fast bubble shrinkage. Applied to the rotational molding process, such a pressure increase can result in substantial cycle‐time shortening through elimination (or reduction) of the currently used excessive heating. A further benefit may be that additional resins, which at present cannot be used because of oxidation at sustained high‐temperatures, can become available to the rotational molding industry. Under the under‐saturated conditions created by a pressure increase, the effect of surface tension on the rate of bubble shrinkage is negligible.

Study on processs and impact strength for a rotationally molded truck fender

“…Thus, it has reached a wide application [3,4], and also drawn more attentions from researchers to further optimize its fabrication process as well as the formed part's performance. In this regard, Spence and Gogos did some investigations on the bubble defect in a rotationally molded part [5,6]. Lim et al adopted numerical analysis method to predict the temperature distribution in the mold [7].…”

The effect of pressure variations on the formation of gas inclusions in the rotational molding process

“…These cavities form due to gas, which is trapped during sintering [2,[9][10][11][12]. The time to remove these inclusions is, among others, determined by the size of the inclusions and thereby by the amount of gas within the inclusions [2,[9][10][11][12].…”

Viscoelastic modelling of gas pore collapse during polymer sintering

“…Viscoelastic relaxation has a pronounced effect on the time scales of the shrinkage process in the simulation but does not alter the equilibrium pore radius. A first attempt at including gas diffusion was made motivated by the works of Kontopoulou and Vlachopoulos [4] and Gogos [5]. Numerical solutions of a simplified model showed that, depending on the time scales of the diffusion process, diffusion can have a significant influence on viscoelastic polymer sintering and that the pressure inside the pore reaches high values without diffusion or with slow diffusion.…”

“…Separate models for the diffusive dissolution of gaseous pores in polymer melts have been presented [4], [5]. These do however either model flow processes as purely viscous or neglect them completely.…”