Rotational moulding is a process ideally suited to producing hollow plastic products. By using additional post-moulding operations, complex components can be produced enabling the process to compete effectively with other moulding and extrusion practices. Rotational moulding differs from other processing methods in that the heating, melting, shaping, and cooling stages all occur after the polymer is placed in the mould, therefore no external pressure is applied during forming. This provides the following advantages:
• Economically produced large products
• Minimum design constraints
• Very low residual stresses
• No polymer weld lines
The rotomoulding process
1 Charging
A pre-determined amount of polymer powder is placed in the mould. With the powder loaded, the mould is closed, locked and loaded into the oven. The powder can be pre-compounded to the desired colour.
2 Heating and fusion
Once inside the oven, the mould is rotated around two axes, tumbling the powder, the process is not a centrifugal one. The speed of rotation is relatively slow, less than 20 rev/min. The ovens are heated by convection, conduction and, in some cases, radiation. As the mould becomes hotter the powder begins to melt and stick to the inner walls of the mould. As the powder melts, it gradually builds up an even coating over the entire surface.
3 Cooling
When the melt has been consolidated to the desired level, the mould is cooled either by air, water or a combination of both. The polymer solidifies to the desired shape.
4 Demoulding
When the polymer has cooled sufficiently to retain its shape and be easily handled, the mould is opened and the product removed. At this point powder can once again be placed in the mould and the cycle repeated.
The rotomoulding advantage
Balmoral rotationally moulded tanks
Balmoral rotationally moulded tanks are rotationally moulded in high density polyethylene providing a one piece, seamless tank with no major structural welds and no inherent stresses. The manufacturing process is automated providing a high quality and consistent product. Moulded-in features enhance flexibility and performance.
Conventional "fabricated" thermoplastic tanks
Usually manufactured from a flat sheet rolled into a cylinder with vertical weld(s). Top and base are then welded on from flat sheet. Strengthening is often required with further layers of sheet being added. The integrity of the tank is directly related to the quality of welds and the personnel responsible for the process. Because the tank is a load bearing structure, subject to cycle loading, potential for tank failure exists if high stress occurs at the weld points. Impact damage or stress caused by temperature or imposed loads such as mixers, heaters, personnel, etc, can add to risk of tank failure.
