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Calendering is a process that usually uses four
heated rolls rotating at slightly different speeds. Again the material is
fed into the rolls, heated and melted, and then shaped into sheet or film.
PVC is the most commonly calendered material.
The molten material is fed tothe calender rolls from a
Banbury mixer and two-roll mill system, or from a large extruder.The major
plastic material that is calendered is PVC. Products range from wall
covering andupholstery fabrics to reservoir linings and agricultural
mulching materials.Owing to the large separating forces developed in the
calender gap, the rolls tend tobend. This may result in undesirable
thickness variations in the finished product.Compensations for roll
deflections are provided by using crowned rolls having a largerdiameter in
the middle than at the ends or by roll bending or roll skewing.
Calender installations require large initial
capital investment. Film and sheet extrusion are competitive processes
because the capital investment for an extruder is only afraction of the cost
of a calender. However, the high quality and volume capabilities of
calendering lines make them far superior for many products.Calendering in
principle is similar to the hot rolling of steel into sheets. It is
interesting to note that strip casting of semi-solid alloys can be modeled
with the help of thehydrodynamic lubrication approximation for a power-law
viscosity model, just like plasticscalendering. The process of calendering
is also used extensively in the paper industry.
Advantages:
achievable tolerance: ± 0.005 mm!
very little thermal degradation |
Molding
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