Rotational Molding of Industrial Parts with Polymer Materials

Rotomolding is a polymer forming process used to make hollow parts of small or large dimensions or still double-walled parts. Even if the principle of rotational molding is relatively simple it allows, however, the manufacture of complex parts. These two benefits are the key to its success. The first patent describing a machine that looks like rotational molding equipment was filed in 1935. The development of plastisols (PVC) in 1941 by Union Carbide enabled the first productions of polymer parts by rotational molding. With the industrial development of polyethylene in the 1950s and its availability in micronized shape around 1960, the industry has the ideal material for this process.

Rotomolding remained marginal for a long time. Indeed, it was considered slow (therefore reserved for small series) and as restrictive in the choice of polymers that could be used (mainly polyethylene). However, progress in the synthesis of new polymers as well as in the process control have made it possible to review this judgment. These recent developments have led designers to consider this technology as an alternative to extrusion blow molding.

While the essential elements of rotational molding and the basic principle have not changed since the 1950s, significant progress has been realized during the last decades in the following areas :

• process understanding and the related physical state changes of the polymer ;

• process monitoring and control ;

• machine build quality ;

• heating and cooling mode and performance ;

• mold quality ;

• new materials.

At present, new types of automated machines with improved heating and cooling system are becoming available and allow the manufacture of technical parts as for example multilayer structures with polymers from different families, or with a foamed layer sandwiched between two virgin polymer layers (polymer/foam/polymer), or fiber reinforced polymer parts.

In addition, the reactive rotational molding technique allows the use of thermosetting polymers (polyurethanes or polyepoxy) for the manufacture of high-performance parts. This opens the route to new markets for rotomolded products because the process can provide high quality, high yield, and competitively priced parts. This is why rotational molding can now take its rightful place alongside other processes, such as extrusion blow molding, thermoforming, and injection molding.