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Step 3: Cooling before unloading: While continuing
the rotation, the machine moves the mould out of the
oven and into the cooling chamber. Cooling is done with
air, or a mixture of air and water, which cools the mould
and the layers of molten plastic material. Cooling allows
the polymer to solidify to retain shape and shrink slightly
so that it can be easily removed from the mould,
however care be taken as rapid cooling causes the
polymer to shrink too fast and warps the part. Then the
mould is taken to the
loading and unloading
station.
Step 4: Unloading/Demoulding
when the polymer has cooled sufficiently to retain its
shape and be easily handled, the mould is opened and
the product removed. The moulding process can then
be repeated by adding the polymer powder to the
mould. The product then undergoes secondary
operations.

It is important to understand that rotational moulding does not rely on centrifugal
forces to throw the plastic against the mould wall. The speeds of rotation are slow,
and the powder undergoes a regular tumbling and mixing action. The ratio of the
speeds about the two axes has a major influence on the thickness distribution of the
plastic on the mould.
Effectively the powder lies in the bottom of the mould and different points on the
surface of the mould come down into the powder pool. The regularity with which this
happens depends on the speed ratio that is the ratio of the major (arm) speed to the
minor (arm) speed. The most common speed ratio is 4:1 because this gives a
uniform coating of the inside surface of most mould shapes. For some moulded
parts it is necessary to reverse the direction of rotation occasionally to ensure that
all areas of the mould surface have access to the powder pool.
Cycle Time: Many variables effects cycle time in roto moulding process. It varies
from 4 minutes for liquid PVC to around 60 minutes for 20000 Litres tank moulding.
Secondary Operations: After part has been removed from mould, wide range of
secondary operations can take place depending on product end application.
Parting lines are trimmed, sawed, milled and/or spin welded and part can be flame
polished to give it shine.
How inserts work in rotational moulded parts: Inserts are loaded into the tool
and held in place mechanically with a fastener. During the moulding process resin
totally encapsulates the insert. The fastener is then removed from the tooling
leaving the encapsulated insert within the finished plastic part.
Types of Rotational Moulding Machines: The different types of rotational
moulding machines in commercial use are:

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