Page 335 - Plastics Engineering

318 Processing of Plastics

4.6 Rotational Moulding
Rotational moulding, like blow moulding, is used to produce hollow plastic
articles. However, the principles in each method are quite different. In rotational
moulding a carefully weighed charge of plastic powder is placed in one half
of a metal mould. The mould halves are then clamped together and heated
in an oven. During the heating stage the mould is rotated about two axes at
right angles to each other. After a time the plastic will be sufficiently softened
to form a homogeneous layer on the surface of the mould. The latter is then
cooled while still being rotated. The final stage is to take the moulded article
from the mould.
The process was originally developed in the 1940s for use with vinyl plas-
tisols in liquid form. It was not until the 1950s that polyethylene powders
were successfully moulded in this way. Nowadays a range of materials such
as nylon, polycarbonate, ABS, high impact polystyrene and polypropylene can
be moulded but by far the most common material is polyethylene.
The process is attractive for a number of reasons. Firstly, since it is a low
pressure process the moulds are generally simple and relatively inexpensive.
Also the moulded articles can have a very uniform thickness, can contain rein-
forcement, are virtually strain free and their surface can be textured if desired.
The use of this moulding method is growing steadily because although the cycle
times are slow compared with injection or blow moulding, it can produce very
large, thick walled articles which could not be produced economically by any
other technique. Wall thicknesses of 10 mm are not a problem for rotationally
moulded articles.
There is a variety of ways in which the cycle of events described above
may be carried out. For example, in some cases (particularly for very large
articles) the whole process takes place in one oven. However, a more common
set-up is illustrated in Fig. 4.59. The mould is on the end of an arm which
first carries the cold mould containing the powder into a heated oven. During
heating the mould rotates about the arm (major) axis and also about its own
(minor) axis (see Fig. 4.60). After a pre-set time in the oven the arm brings the
mould into a cooling chamber. The rate of cooling is very important. Clearly,
fast cooling is desirable for economic reasons but this may cause problems
such as warping. Normally therefore the mould is initially cooled using blown
air and this is followed by a water spray. The rate of cooling has such a major
effect on product quality that even the direction of the air jets on the mould
during the initial gradual cooling stage can decide the success or otherwise
of the process. As shown in Fig. 4.59 there are normally three arms (mould
holders) in a complete system so that as one is being heated another is being
cooled and so on. In many machines the arms are fixed rigidly together and so
the slowest event (heating, cooling or charging/discharging) dictates when the
moulds progress to the next station. In some modem machines, the arms are