The technique

Rotational moulding is based on the principle of having the material deposited by gravity on the surface of the mould, and not by the effect of centrifugal force.

The rotational moulding consists in putting a predetermined quantity of polymer powder, granulated or liquid, inside a hollow shell-shaped mould. Then, the mould is heated and rotated time around two principal axes at the same, in order for the polymer to adhere to the walls of the mould by melting. Still rotating, the mould is cooled, so that the polymer can solidify in the desired shape. When the plastic is hardened enough, the rotation is suspended to permit the extraction of the plastic product from the mould. This process takes place at low rotational speeds, typically at 4-20 rev/min. The flexibility of this process also means freedom in design.

The moulds

The moulds used in rotational moulding are characterized by a shell-shaped structure.  They are usually made up of two half-shells, but more complex pieces may require moulds consisting of three or more parts. Along the coupling lines of the parts, moulds are closed with clamps.

The advantages

Rotational moulding is an atmospheric pressure process allowing producing stress-free pieces. The molten material is not subject to stresses while taking its final shape and this represents one of the biggest pluses of the rotational moulding, if compared to other manufacturing methods of plastic pieces. In addition, since there are no forces acting on the molten plastic during its shaping, the rotational moulds can have thin walls and can be produced at relatively low costs.

The rotational moulding is also a production technique with a low environmental impact because the fusion of the powders takes place without a direct contact with flames or heat sources.

Moreover, Niteko’s production system allows additional energy savings by means of a special regenerative unit that makes the reactive energy – which is produced during the braking phase of the engines through unbalanced operations or quick shutdowns – available for users on board the machine. At the end of the firing cycle, when the doors of the oven open to change the carts, the sudden shutdown of the ventilation system generates a large amount of reactive energy. Thanks to the energy recovery system, it is possible to shut down the ventilation system in a few seconds, with a drastic reduction of hot air leakage from the oven. The temperature loss is only 30°-40° and the return to the temperature set for the moulding takes place in a very short time, with clear reductions in cycle times and in consumptions.

Main pluses of rotational moulding:

• The hollow body is made in a single piece with no weld lines or joints
• The piece is essentially stress-free
• The moulds are relatively low cost
• The manufacture of the moulds takes relatively short times
• The walls have a fair thickness uniformity
• It is possible to change the distribution of wall thickness without the need to modify the mould
• Short production runs are economically viable
• There is no material wastage because the full charge of material is normally used to produce the piece
• It is possible to produce multilayer pieces, including those in foamed plastics
• It is possible to simultaneously mould different type of products on the same machine
• Relative ease of co-moulding inserts inside the piece
• It is possible to co-mould quality graphics in the moulded piece