Rotational molding is a process for transforming plastic materials used to produce hollow parts of small or large dimensions or even double-walled parts. The principle of rotational molding is relatively simple but this process is capable of manufacturing complex parts. These two advantages are the key to its success.
The first patent describing a machine that closely resembles today’s rotational molding equipment was filed in 1935. The introduction of plastisols (PVC) in 1941 by Union Carbide enabled the first developments in rotational molding. The industrial use of polyethylene in the 1950s and its availability in micronised form around 1960 provided industry with the ideal material for this process.
The process 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 might be used (mainly polyethylene). However, progress in the synthesis of new polymers, as well as in the means of controlling the process, have made it possible to review this judgment. This is why recent developments have led designers to consider this technology as an alternative to blow molding.
While the essential elements of rotational molding and the basic principle have not changed since the 1950s, significant progress has since been made in the following areas:
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understanding of the process and of the change in state of the material during the process;
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process monitoring and control;
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the build quality of the machine;
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heat transfer performance;
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heating and cooling modes;
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the quality of the mould;
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new materials.
At the present time, new types of machines (automated), molds (with the direct heating system), and materials are becoming available and make it possible to manufacture various technical parts such as, for example, parts made of reinforced polymers (filled) or multilayer parts (polymer/polymer, polymer/foam/polymer) with polymers from different families.
In addition, the reactive rotational molding technique allows the use of thermosetting polymers (polyurethanes or polyepoxy) for the manufacture of high performance parts.
Significant areas of the market are opening up to 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.