Technical Info South Africa
calendering process for plastics and calendered PVC sheeting
Calendering is a finishing process applied to textiles and plastic. During calendering rolls of the material are passed between several pairs of heated rollers, to give a shiny surface. Extruded PVC sheeting is produced in this manner as well other plastics. Calendering is a final process in which heat and pressure are applied to a fabric by passing it between heated rollers, imparting a flat, glossy, smooth surface. Lustre increases when the degree of heat and pressure is increased. Calendering is applied to fabrics in which a smooth, flat surface is desirable, such as most cotton, many linens and silks, and various man-made fabrics.
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.
