Rotational Molding

Rotational molding, also known as rotomolding or centrifugal casting, involves filling a mold cavity, generally with powder, and rotating the entire heated mold along two axes to uniformly distribute the plastic along the mold walls. This method is commonly used for making hollow parts, like blow molding, but is used either when the parts are very large (as in the case of kayaks, outdoor portable toilets, phone booths, and large chemical storage drums) or when the part requires very low residual stresses. Also, rotomolding is well suited, compared with blow molding, if the desired part design is complex or if it requires uniform wall thicknesses. Part walls produced by this method are very uniform as long as neither of the rotational axes corresponds to the centroid of the part design. The rotomolding operation imparts no shear stresses to the plastic, and the resultant molded article is therefore less prone to stress cracking, environmental attack, or premature failures along stress lines. Molded parts also are free of seams. Figure 1.41

shows a diagram of a typical rotational molding process. This is a relatively low-cost method, as molds are inexpensive and energy costs are low, thus making it suitable for short-run products. The drawback is that the heating and cooling times required are long, and therefore the cycle time is correspondingly long. High melt flow index PEs are often used in this process.

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