Optimization and modelling of rotational molding process

International audienceRotational Molding is the best method for producing large hollow plastic articles without weld lines, such as kayak, tanks, large bins, etc. But it is a quite complex and empirical process. Constant quality in technical parts requires the mastery of the process by controlling on line the main physical phenomena. One of these of first importance is heat transfers. During the processing time, polymer powder melts, then the phenomena of particle coalescence and melt densification occur. After cooling, the molded part is obtained. The understanding of sintering phenomenon, linked to polymer structure, may explain surface defects and bubbles in rotationally molded parts. This presentation is divided into two parts: the first part deals with the relationship between the material structure, the process and the final properties; the second part deals with the modelization of heat transfers during the process. Firstly, material properties such as polymer structure, rheological parameters and surface tension were studied and linked to sintering kinetics. The sintering phenomenon was investigated, including coalescence and densification, in order to examine the effect of particle size and shape. Moreover, existing sintering models were compared to experimental data and were improved. Secondly, sintering kinetics were bound to final parts properties. Indeed, samples were molded with a pilot-scale rotational molding machine. On these samples, some defects were detected such as inner roughness and bubbles. Samples mechanical properties were also studied. The effect of particle size and shape on samples properties was examined taking into account the sintering. Quantitative relationships were stablished. This work enabled us to model the sintering phenomenon and to bind its kinetics with polymer structure, rheological properties and final parts properties. An experimental analysis of heat transfer in rotational molding process was also lead. By using an instrumented mold associated with an original radio transmission data acquisition system, we demonstrate that the rotational nature of the process implies complex heat transfer evolutions in the mold. The crystallization of the material was modeled with accuracy by coupling heat transfer equation to a kinetic model determined by calorimetry. Moreover, a thermal model was developped by using a static heated plate in order to validate the numerical results. This modelization took into account the sintering phenomenon