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Although in-mold special effects are difficult to achieve during the rotomolding process, the technical challenge is being met through cooperation between rotomolders and resin and color blenders.
Ronotational molding (rotomolding) as a means of economicalproducing medium-to-large products has developed a substantial following. While the majority of rotomolded parts are made of polyethylene, a variety of thermoplastic resins can be used in the process.
Further, because rotomolding is a low-pressure method, tooling is less costly than the tooling for high-pressure processes. Because rotomolded resins are flowed rather than pressed into shape, residual stresses are minimized, even at corners. Finally, layers of different resins can be built up into a single wall; foam can be injected between walls; and crosslinking can be accomplished in one layer, but not others. Thus, a foam-reinforced, crosslinked canoe can feature high strength, impact resistance, and relatively low weight. Or a chemical tank exterior might be made of a highly UV-resistant material, while the inner lumen consists of a chemically resistant resin.
With the growth in rotomolding popularity has come a parallel growth on the technical side. Resin formulations, as well as formulating, blending, and rotomolding equipment, have been continually enhanced to the point that processes are fully controllable, and product quality and consistency are excellent. Over many years, a large library of resin and processing data has been accumulated, and a great deal of experimentation has resulted in the ability to mold even fairly complex shapes.
Color and other in-mold decoration and special effects for rotomolding, however, have lagged behind. Some of the reasons for this lag are inherent in the process. Resins for rotomolding undergo minimal shear forces during processing, so that shear-dependent effects are not available with normal rotomold processes. Missing from rotomolding is the high-work mixing and melting that occurs in injection-molding screws, as well as the shear forces created when molten resin traverses through sprues, gates, and mold sections.
Among the effects largely closed to rotomolding are some forms of marbling, brushed-metal effects, and frosted-glass effects. At the same time, certain deep effects are difficult, because wall thickness in rotomolded parts is achieved by repeated, relatively thin "washes" of molten resin as the mold rotates. Thus, large metallic flake or other effects dependent on random...