Modelling of sintering during rotational moulding of the thermoplastic polymers

This paper concerns the study of sintering phenomenon during rotational molding of polypropylene(PP),Polyvinylidenefluoride (PVDF) and Polymethyl methacrylate (PMMA). First, the coalescence (first step of sintering) of two grains has been followed. Bellehumeur's model has been tested as a model to explain this phenomenon. In order to study the effect of neighboring grains on coalescence of two grains, a third grain has been put in contact with these two grains. For modeling the phenomenon in this case, Bellehumeur's model has been modified by a geometric parameter called Farz Factor (FF), being this model validated by experimental test. Concerning densification, two different stages have been observed. In the first stage, before welding of the grains and formation of interphases between them, the grains are not stuck yet. The air trapped between the grains escapes through free ways between grains. This first step of densification is directly related to the coalescence where the density of the polymer varies very quickly. A new tridimensional model, based on a Body Centered Tetragonal (BCT) configuration, has been proposed to explain the densification during this first stage. In the second stage, the migration of air is controlled by diffusion.