Rotational molding continued

Figure 7.81 Schematic diagram of rotating mold system used in continuous extrusion blow molding. Reprinted, by permission, from A. B. Strong, Plastics Materials and Processing, 2nd ed., p. 489. Copyright 2000 by Prentice Hall, Inc.

Vinyls Excellent wealherability, superior electrical properties, excellent moisture and chemical resistance, self-extinguishing injection molding Continuous laminating Rotational molding... 

Most practitioners deflne the flow behavior of polymers based on the melt flow index however, this property is not entirely relevant to the rotational molding process because it is essentially a shear-free and pressure-free process. The use of zero-shear viscosity has been proposed as a better way to assess the coalescence behavior of resins. Resins with lower zero-shear viscosity coalesce at a faster rate and can thus be processed using a shorter molding cycle.The coalescence of individual powder particles is initiated as the particles stick and melt onto the mold surface or melt front. As the melt deposition process continues, pockets of air remain trapped between partially fused particles and lead to the formation of bubbles. In the rotational molding process, the coalescence of particles occurs at a temperature range close to the melting point of the material thus, from a processing standpoint, low values of zero-shear viscosity at low temperatures (i.e., close to the temperature at which the particles adhere to the mold surface) are preferable. 

Examples in these and other categories could be multiplied indefinitely, but these few will suffice. The fields of compounding, additives, blends, grafts, and composites all see continual small advances that eventually add up to major gains and will undoubtedly continue to do so. Similarly, new applications and new shaping techniques will continue to be developed (e.g., rotational molding is old hat but not exhausted, solid-phase forming has yet to come into its own, and polyolefin foams have enormous potentials that have scarcely been tapped). 

FIGURE 2.78 Basics of continuous-type three-arm rotational molding machine. 

After the main resin system has gelled, a chemically resistant topcoat may be applied while the mold is still rotating. Mold rotation is continued until all the resin has cured. 

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