Modelling of an innovative liquid rotational moulding process

Abstract

An innovative liquid rotational moulding process aiming at using standard injection-moulding polymer grades, shortening cycle times and at accessing more higher-end markets is studied. In this new process, the polymer is melted beforehand in a conventional injection unit and injected into the mould rotating around two axes. This work focus on modelling the liquid polymer flow occurring in this process, as well as providing guidance on the choice of process parameters. First, a stability criterion for rimming flow is derived, defining the process and material parameters space available for a functional process. From this criterion, it appears that initial thickness, and density have a destabilising effect, while liquid viscosity and angular velocity have a stabilising effect. Next, using Lax-Friedrichs scheme, the one-dimensional transient gravity driven liquid film is numerically modelled, linking the total extent of spreading to the frequency of orientation reversals and to the major and minor angular velocities ratio : the closer this ratio to one, the larger the extent of spreading. Then, Computational Fluid Dynamics (CFD) simulations are carried out to take into account three-dimensional features in the moulding of a cube part. Finally, comparison with trials performed on the moulding of the same cube part validates entirely this approach. In particular, some peculiar features of spreading can only be explained and reproduced when employing three-dimensional CFD simulations.