Polymer sintering and its role in rotational molding

Abstract

<p>Polymer sintering can be described as the formation of a homogeneous melt from the coalescence of powder particles under the action of surface tension. Sintering is a fundamental phenomenon in processes such as rotational molding and powder coating. Rotational molding uses plastic powder to produce hollow plastic parts. The porosity of the final part produced in rotational molding depends on the completion of polymer sintering and the removal of bubbles. The objectives of this work are to study the effects of the material properties as well as the molding conditions on the sintering rate and to develop an appropriate model for polymer sintering.</p> <p>An experimental study of polymer sintering has been carried out. A reliable method has been developed for the obsevation and the measurement of the coalescence rate for two particles. The material viscosity, elasticity, and particle size were found to affect the sintering process. Most of the results obtained corroborate observations in rotational molding experiments. The effect of the particle geometry on the sintering rate was found to be negligible. This results has to led to the study of the rotomoldability of micropellets. Numerical studies have revealed that the processing conditions are severe and probably affect the micropellet rheology, which in return affects the coalescence process.</p> <p>A mathematical model describing the complete polymer sintering process has been developed. The approach was similar to that of Frenkel (1945). For Newtonian fluids, the proposed model's predictions are very close to Hopper's theoretical model (Hopper, 1984) and to numerical results for viscous sintering (Jagota and Dawson, 1988, Van de Vorst, 1994). The proposed model is successful in predicting the sintering rate for most of the rotational molding grade polyethylene resins used. However, all Newtonian models predict a faster coalescence rate than that observed with the copolymer resins used in this study. This result indicates that factors other than the surface tension and the viscosity play a role in polymer sintering.</p> <p>The proposed model has been generalized to describe sintering for viscoelastic fluids. As a first approach, the convected Maxwell constitutive equations were used together with the quasi-steady state approximation. The viscoelastic sintering model is capable of predicting the sintering rate observed in this study and the trends reported in the literature for the coalescence of acrylic resins.</p> <p>The combination of the present experimental and modeling studies can be used for the selection of appropriate materials and for improvement of the rotational molding process.</p>