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ROTATIONAL MOULDING
Rotational moulding, known also as rotomoulding or rotocasting, is a highly versatile
method for making precise, stress and seam free hollow plastic components at
comparatively low cost. If liquid materials are used, the process is often called slush
moulding. It is a virtually shear-free and pressure-free process.

In this process, finely ground powders are heated in a rotating mould until melting or
fusion occurs. The melted or fused resin uniformly coats the inner surface of the
mould. When cooled, a hollow finished part is removed. The wall thickness
uniformity and part weight can be easily maintained. There is very little waste of
material due to scrap and moulds are relatively less expensive. The process
provides for highly complex geometric shapes, deep draws and zero draft that can
be incorporated in the product. No other hollow parts process has the design
freedoms and potential of rotational moulding. The process utilizes plastic ground
into powder form typically 35 mesh. More often the material is Polyolefin although
other materials like PVC and Nylon are also used.
The Process takes place in 4 steps:
Step 1: Mould Preparation: The hollow mould is
charged with a pre-determined quantity of polymer
powder. It may be pre-compounded for desired colour.
The mould is often mounted onto a steel grid (spider) on
a machine arm. The oven is preheated by hot air or gas
flame heaters to maintain temperature around 300 °C,
depending on the polymer used. The mould is then
closed, locked and loaded into the oven.

Step 2: Heating and fusion of resin: Inside the oven the
mould is bi-axially rotated (i.e. rotated around two axes) at
slow speed as the polymer melts and coats the inside of the
mould. The mould is rotated within the oven until all the
plastic material has been picked up by the hot inside
surfaces of the cavity and densifies into a uniform layer.
Timing is very critical as more time will degrade polymer
and impact its properties whereas less time will not
uniformly melt material and have bubbles and non uniform thickness in product.

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