Rotational molding involves powder mixing, heating and melting of powder particles to form a homogeneous polymer melt, as well as cooling and solidification. The densification of a loose powder compact into a homogeneous melt occurs over a wide range of conditions as the material passes from a solid state into a melt state. The numerical simulation of the non‐isothermal melt densification in the rotational molding process is presented in this work. The simulation combines heat transfer, polymer sintering and bubble dissolution models, and is based on an idealized packing arrangement of powder particles. The predictions are in general agreement with experimental observations presented in the literature for the rotational molding of polyethylenes. The simulation allows for systematic and quantitative studies on the effect of molding conditions and material properties on the molding cycle and molded part density. Results indicate that the densification process is primarily affected by the powder characteristics, which are accounted for in terms of the particle size and the particle packing arrangement. The material rheological properties become increasingly important as the powder characteristics lessen in quality. The simulation demonstrated that while certain combinations of processing conditions help reduce the molding cycle, they have a detrimental effect on the densification process.
We investigate the effects of high shear rate and a melt‐insensitive, organophosphate nucleating agent on shear‐induced crystallization of four isotactic polypropylenes using a sliding plate rheometer under isothermal, low‐supercooling conditions. We used a bifurcated optical fiber probe to measure light intensity and a shear stress transducer to monitor the simultaneous viscoelastic response to small‐amplitude oscillatory shear. The two techniques complement each other; at early times of crystallization, large attenuation in the light intensity is observed, whereas during the later stages, a major change in the viscoelastic response occurs due to the growing volume fraction of spherulites. In contrast to quiescent crystallization, the nucleation pathway of nucleated polymers after a brief, strong shear is little influenced by the nucleating agent but strongly affected by molecular weight. The early kinetics of non‐nucleated polymers is more strongly enhanced by shear than that of nucleated polymers. Increasing either shear rate or strain accelerates crystallization, and we found the product of shear rate and strain to be useful for correlating our data. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers
A key factor that limits the production rate in film blowing is bubble instability. Although processing conditions play an important role, it is well known that some resins are more resistant to instabilities than others. It is clear that long-chain branching enhances stability, but it is not currently possible to model the process with sufficient accuracy to establish a quantitative relationship between rheological properties and blown film processability. It would thus be useful to be able to compare the processabilities of film resins by means of a laboratory measurement. We compared two laboratory tests that may be helpful in evaluating the ability of a resin to resist instabilities in the film blowing process. One of these was a film resin tester designed to simulate some aspects of the film blowing process, and the other was an extensional rheometer. We used a set of polyethylene resins that had been used previously in an extensive study of blown film stability. The extensional rheometer clearly shows the superiority of low-density polyethylene but is not able to distinguish among polymers of other types. The melt tester, on the other hand, is sensitive to differences among linear polymers.
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