Materials

While the rotational moulding process boasts many attractive features, it also has some limitations. The most significant is the limited availability of suitable materials. This is mainly due to the severe time-temperature demands placed on the polymer and the relatively small market for non-polyolefins. When special resins are available, their prices are high due to the development costs and the additional expense of small-scale grinding.

Rotational moulders use several different materials, but most products made with this process use polyethylene in powder or pellet form. Depending on the region, this material is either supplied as pellets that are ground into powder, coloured, and prepared to specification by the rotational moulder, or as ready-to-use powder. Powders can be formulated with flame retardants, UV resistance, and other specific additives.

Currently, polyethylene in its various forms represents about 85% to 90% of all polymers used in rotational moulding. Cross-linked grades of polyethylene are also common. PVC plastisols make up about 12% of global consumption, while polycarbonate, nylon, polypropylene, and other materials constitute the remainder. PE for the process can be pulverised into powder at room temperature, with particle sizes ranging from less than 150 microns to about 500 microns. The particle size distribution is crucial to the final product’s appearance. PE must flow easily during the process, measured by a Melt Flow Index (MFI). High MFI materials have low viscosity, and for rotomoulding, an MFI range of 2-8 is desirable, depending on product specifications.

PEs for rotational moulding come in various densities, including Low Density (LDPE), Linear Low (LLDPE), Medium (MDPE), and High (HDPE). The material's density affects strength, stiffness, chemical resistance, and impact vulnerability.

Rotomoulders conduct several tests on materials, including dry flow, bulk density, wall thickness, and drop impact. For more information about these tests and how they are performed, check out the ARMA Material Testing Guide.