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Flat Surfaces: Rotomoulded parts having large flat areas should be avoided as
they tend to distort. If absolutely necessary they should be designed with reinforcing
ribs, domes, curves, contours, grids, textures and patterns on flat surface. Corners
and their radius can affect the flatness of adjacent surfaces as differential cooling
rates can cause the corner angle to distort.
Mould Venting: Venting is required to release build-up gas in the heating cycle. A
vent reduces flash and piece distortion, also prevents blowouts caused by pressure
and permits the use of thinner-walled moulds. Depending on mould size vents can
range from 1/8 " to 2 " inside diameter. Since vent leave holes in the moulded parts it
should be located in area that is to be cut out of the finished part or in an area where a
patch closing hole does not reduce the aesthetic value of the end product. Also
location of vent be such that that water does not enter the mould while cooling and
cause problems
Mould Release: Moulds designed with little or no draft angle, it is important to
condition the moulds with a release agent.
Other factors of consideration are material used, mesh size, whether dry blended
colours or compounded, heating and cooling methods, additives incorporated.
Charge weight calculation for desired wall thickness:
To get the charge weight for desired wall thickness it is necessary to work out the
volume of material in end product and multiply it by density of the plastic. The volume
of plastic is obtained by subtracting the volume of air space inside the product form
volume of inside of the mould.
Alternative way to estimate the wall thickness is to take the volume of the part as the
surface area of the inside of the mould multiplies by the wall thickness of the part.
The Charge weight is then given by following equation:
Weight of plastic = surface area of moulding x thickness of
moulding x density of plastic.

Advantages of Rotational Moulding Process:
Design freedom over other moulding processes
Hollow, totally enclosed items as well as pieces with openings can be made
Costs for moulds and tooling are relatively low
Allows moulded threads and mould-in inserts
Product is virtually stress free
Quick colour changes – No purging required
There is little or no waste due to resin scrap
Very large parts with thin walls possible
Wall thickness & piece weight can be controlled
Similar or different parts can be moulded on a single spindle together
Double wall constructions are feasible
Suitable for both low-volume prototypes and high-volume production run
Disadvantages: Long moulding cycles, limited choice of material, Lower precision

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