Rotational molding of integral skin cellular composites

可以提高强度重量比的增强的、大尺寸的、形状复杂的单件泡沫塑料制品的市场迅速增长。旋转泡沫塑料成型是能够响应这种需求的唯一工艺。最近开发的单次充填概念用于制造旋转模塑，其外层具有独特的非发泡外壳，完全包裹发泡的核心或层，其基础是在周期开始时同时向模具注入预定数量的非发泡和发泡树脂的混合物，因此不再使用投放箱或塑料袋。然而，该工艺概念固有地加剧了整皮旋转泡沫塑料成型中三个关键工艺目标的实现，例如：(I)确保非发泡热塑性树脂与模具内表面的粘合总是在发泡树脂的热激活之前发生(从而避免皮肤突出)，(Ii)获得均匀厚度的无气泡固体-皮肤层，以及(Iii)在皮肤层上开发具有均匀泡孔尺寸和泡孔密度分布的细孔泡沫芯(或层)。由于熔融塑料是既可呈现粘性流动又可呈现弹性反冲的流变性复杂材料，即其行为既是剪切速率的函数又是时间参数的函数，其粘弹性性质支配其流动行为。此外，特定聚合物材料的粘弹性和热行为对其发泡行为具有控制作用。为了提取关于材料的流变性的信息，测量由给定力引起的变形或测量产生给定变形所需的力是必要的。由于这些组分的贡献随着时间、温度、变形和变形速率的不同而不同。因此，为了表征、改善和优化皮肤包裹的聚烯烃的多层旋转发泡性能，深入研究它们的基本流变性和热学性能以及它们与相关发泡剂和添加剂的相应混合物将是至关重要的。

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.