Rotational Moulding of Polyethylene Water Tanks in Australia.
The basic elements in manufacturing
polyethylene water tanks are polyethylene
powder, water tank moulds and gas fired ovens. The original process was
discovered many years ago, but only reached its potential following the
commercial production of polyethylene.
In Australia, the basic raw material used to rotomould water tanks is Low Density Polyethylene (LDPE) which is sourced from the Qenos Olefins plant in Port Botany, New South Wales. This manufacturing plant uses ethane as its source material. The ethane is piped almost 1400 kms from Santos’ processing facilities at Moomba in outback South Australia.
LDPE has a density range of 0.910 – 0.940 g/cm3 with a high degree of short and long chain branching which gives molten LDPE its unique and desirable flow properties making it very suitable for the manufacture of water tanks. LDPE is created by a process of free radical polymerization.
The molecular structure of ethylene is shown on the left below and polyethylene’s molecular structure is on the right.
Rotomoulding is one of a number plastics moulding techniques, with others being blow and injection moulding. In the rotomoulding process, a measuured quantity of powdered polyethylene resin is placed into the tank mould. The powder has been compounded and mixed to include the colour which is desired, UV stabilizer to ensure the tank does not fade when exposed to solar rays, and a release agent to assist in removing the tank from its mould.
Once the mould is removed from the oven, it must be cooled to allow the polyethylene to set and to shrink slightly so that the tank can be removed from the mould. This is a critical step in ensuring a perfect tank. When the mould has cooled sufficiently to be handled the mould is opened and the tank is removed. Then the process is repeated by adding polyethylene polymer powder to the mould.
Tank manufacturers try to manufacture as many water tanks as possible without having to change the mould for one of a different size as each mould change costs them lost production. The rotomoulding process is ideally suited to the manufacture of hollow vessels with relatively small production runs.