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 plastic 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 | 