Calendering roll bending

The calendering configuration of rolls may consist of two to at least seven rolls. The number of rolls and their arrangement characterizes them. Examples of the layout of the rolls are the true L , conventional inverted L , reverse fed inverted L , I , Z , and so on. The most popular are the four-roll inverted L and Z rolls. The Z calenders have the advantage of lower heat loss in the film or sheet because of the melts shorter travel and the machines simpler construction. They are simpler to construct because they need less compensation for roll bending. This compensation occurs because there are no more than two rolls in any vertical direction as opposed to three rolls in a four roll inverted L calender and so on. 

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

Figure 35.43 shows a general layout of a dual-purpose calender line for coating steel cord and textile cord. The heart of the production line is the four-roll calender in an S-configuration. Two mbber sheets are formed in the upper and lower nip. The thin sheets are guided to the middle nip and the cords are coated in the middle nip between the two rubber sheets. Generally outer roll bending is applied on rolls 2 and 3 to compensate the roll deflection caused by the nip force in nip 2. Rolls 1 and 4 can be set crossed respectively to rolls 2 and 3. 

The distortion suffered by calender rolls resulting from the pressure of the rubber running between them. If uncorrected the deflection produces a rubber sheet thicker in the middle than at the edges. See Camber, Crossed Axes and Roll Bending, all techniques to compensate for calender bowl deflection. 

The elastic properties of materials exert pressure on calender rolls which causes their deformation. This is controlled by honing the surface of rolls, roll bending, or cross-axes. Modem calender automatically adjusts the gaps based on the data from the gauges scanning the entire width of the material. 

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,... 

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... 

Variations in these multimillion-dollar calender lines are dictated by the very high forces exerted on the rolls to squeeze the plastic melt into thin film or sheet web constructions. High forces at least up to 6000 psi (41 MPa) could (if rolls were not properly designed and installed) bend or deflect the rolls, producing gauge variations such as a web thicker in... 

A fourth method, sometimes used, makes use of the cross axis principle. Consider the vertical centreline of the calender as the fixed reference point. The roll journals can be moved by an equal amount in opposite directions and put one roll out-of-line or cross the axis . A pair of rolls with crossed axes will have greater clearance at the edges than at the centre and a very small angle of cross will counter all the bend likely to be produced by the material. 

---