Calendering crowned rolls

Calender to be equipped with crowned rolls, external roll bending, and adjustable cross axes setting (controlled)... 

Calendering can achieve surprising accuracy on the thickness of a sheet. Typically the tolerance is 0.005 mm but to achieve this it is essential to have very close control over roll temperatures, speeds and proximity. In addition, the dimensions of the rolls must be very precise. The production of the rolls is akin to the manufacture of an injection moulding tool in the sense that very high machining skills are required. The particular features of a calender roll are a uniform specified surface finish, minimal eccentricity and a special barrel profile ( crown ) to compensate for roll deflection under the very high presurres developed between the rolls. 

In modem calender lines both compensating devices can be found. It is common that wide calender rolls are crowned in such a way that under working conditions the crowning compensates the roll deflection due to high shear forces. Crowning specifications can be understood by means of the presented theory. Since the lowest nip forces can be achieved at high clearance settings, low line... 

Crown n, (1) Of a calender roll, a gradual, small increase in the diameter of a roll toward the center to compensate for the slight deflection due to bending of the roll under pressure. (2) The much ore convex surface of a transmission-belt pulley designed to keep the belt running in the center of the pulley. 

Roll Bending n In calendering of sheet, the practice of applying a bending moment to the ends of the calender rolls that opposes the bending caused by the pressure forces as the plastic is squeezed between the rolls. The object is to produce sheet whose thickness varies minimally across its width. See also Crown and Roll Crossing. 

The rolls are typically 4-10 ft. wide. The gap between rolls becomes progressively smaller as melt moves from 1-2, 2-3, and 3-4 rolls, and the final thickness is controlled by the last gap. The forces generated are immense rolls are crowned in the middle to compensate for the pressure and some systems employ roll bending to control the forces. Rolls turn at a differential rate to produce shear film exits the calender at 80-180 ft./min, and pounds per hour... 

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