Rotational molding of integral skin cellular composites

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The market for reinforced, large-sized, complex-shaped, single-piece, foamed plastic articles that can achieve improved strength-to-weight ratios rapidly grows.  Rotational foam molding is the unique process that can respond to this demand.   The recently-developed single-charge concept for the manufacture of rotational moldings with a distinct non-foamed outer skin that encapsulates entirely a foamed core or layer is based on charging the mold with a mixture of predetermined quantities of both non-foamable and foamable resins simultaneously at the outset of the cycle, so that the use of drop boxes or plastic bags becomes unnecessary.  However, this processing concept suffers from inherently aggravating the fulfillment of three crucial processing goals in integral-skin rotational foam molding, such as: (i) making sure that the adhesion of the non-foamable thermoplastic resin to the internal surface of the mold always takes place prior to the thermal activation of the foaming resin (thereby avoiding skin protrusions), (ii) obtaining a bubble-free solid-skin layer with a uniform thickness, and (iii) developing a fine-celled foam core (or layer) with uniform cell size and cell density distributions on top of the skin layer.  Since molten plastics are rheologically complex materials that can exhibit both viscous flow and elastic recoil, i.e., their behavior is a function of both shear rate and a time parameter, their viscoelastic properties govern their flow behavior.  In addition, the viscoelastic and thermal behaviors of a particular polymeric material have a controlling effect on its foaming behavior.  To extract information about the rheological properties of a material, it is necessary either to measure the deformation resulting from a given force or measure the force required to produce a given deformation.  The contribution of each of these components varies with time, temperature, deformation, and rate of deformation.  Thus, to characterize, improve, and optimize the multilayer rotofoamability of skin-encapsulated polyolefins, a thorough study of their fundamental rheological and thermal properties as well as their corresponding mixtures with relevant foaming agents and additives would be of crucial importance.