Page 331 - Plastics Engineering

3 14 Processing of Plastics
early machines did not have sufficient accuracy or control over such things as
cylinder temperature and the gap between the rolls. Therefore acceptance of the
technique as a viable production method was slow until the 1930s when special
equipment was developed specifically for the new plastic materials. As well as
being able to maintain accurately roll temperature in the region of 200°C these
new machines had power assisted nip adjustment and the facility to adjust the
rotational speed of each roll independently. These developments are still the
main features of modem calendering equipment.
Calenders vary in respect of the number of rolls and of the arrangement
of the rolls relative to one another. One typical arrangement is shown in
Fig. 4.57 - the inverted L-type. Although the calendering operation as illus-
trated here looks very straightforward it is not quite as simple as that. In the
production plant a lot of ancillary equipment is needed in order to prepare the
plastic material for the calender rolls and to handle the sheet after the calen-
dering operation. A typical sheet production unit would start with premixing
of the polymer, plasticiser, pigment, etc in a ribbon mixer followed by gelation
of the premix in a Banbury Mixer and/or a short screw extruder. At various
stages, strainers and metal detectors are used to remove any foreign matter.
These preliminary operations result in a material with a dough-like consistency
which is then supplied to the calender rolls for shaping into sheets.

Supply of
1 plastic

-
Sheet off to cooling
drums and wind-up
I

Fig. 4.57 'Qpical arrangement of calender rolls

However, even then the process is not complete. Since the hot plastic tends
to cling to the calender rolls it is necessary to peel it off using a high speed
roll of smaller diameter located as shown in Fig. 4.57. When the sheet leaves
the calender it passes between embossing rolls and then on to cooling drums
before being trimmed and stored on drums. For thin sheets the speed of the
winding drum can be adjusted to control the drawdown. Outputs vary in the
range 0.1-2 m/s depending on the sheet thickness.