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Calendering is a method used to produce plastic film and sheet.
It would appear, therefore, to be in direct competition with extrusion.
But this is not true. In fact it finds its main applications in
the production of PVC sheet since with calendering there is much
less risk of material degradation. Some film is also produced in
this way from ABS, cellulosics, and toughened polystyrene.
Basically the process consists a plastic formulation
being passed between the nip of heated rolls. Usually four rolls
with three nips are employed although a varierty of combnation is
possible. The first nip controls the feed rate, the second nip acts
as a metering unit and the third nip controls the sheet thickness.
The last roll may be matt, glossy, or embossed to provide the disired
surface finish on the product. The other calender rolls must be
highly polished and concentric to a very high degee of accuracy
and hence, the equipment is expensive. Indeed since there is very
little mixing during calendering the total plant requires very much
more than the calender. A lot of preparatory work on the feedstock
needs to be done before it reaches the calender rolls. This includes
mixing, blending, gelling, and straining the formulation of resin,
lubricants, plasticizers, stabilizers, and pigments. Also, since
repairs to rolls can be very costly, it is essential to remove any
foreign particles before the strip feed passes into the calendar.
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