Calendering pressure force

The passage of hot melt between the rolls creates a pressure forcing them apart and the calculation of tolerances using a statistical model has been described (417). There is a rolling bank of feed material created in the gap or nip set between the first pair and also the second pair of rolls. Passage of the material is controlled by roll temperature, surface finish and the ratio of the roll speeds at the nip. The final calender nip controls film thickness. 

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,... 

Figures 35.35 through 35.37 show the dependency of the pressure buildup on the roUing bank content. The pressure curve and therefore the nip force will change dramatically with the mbber content in the nip region. A varying feeding of the calender wfll cause varying nip forces.

High pressures can be found in the calender nip region. The resulting force acts on the adjacent calender rolls and causes a deflection of the rolls and therefore a thickness gradient across the calendered product. The total displacement Y(z) of the roll surface caused by roll deflection due to this viscous force can be approximated by Kopsch. ... 

The correction of calender bowl deflection by application of hydraulic pressure to counteract the forces tending to produce deflection. 

Of importance to the mechanical design of the calendering system and to the prediction of the film thickness uniformity is the force separating the two rolls, F. This is computed by integrating the pressure over the area of interest on the surface of the roll... 

Calendering problem with a Newtonian viscosity polymer. A calender system with R = 10 cm, w = 100 cm, h0 = 0.1 mm operates at a speed of U =40 cm/s and produces a sheet thickeness In = 0.0218 cm. The viscosity of the material is given as 1000 Pa-s. Estimate the maximum pressure developed in the material, the power required to operate the system, the roll separating force and the adiabatic temperature rise within the material. 

Figure 6.22 depicts schematically the flow configuration. Two identical rolls of radii R rotate in opposite directions with frequency of rotation N. The minimum gap between the rolls is 2H0. We assume that the polymer is uniformly distributed laterally over the roll width W. At a certain axial (upstream) location x = X2 (X2 < 0), the rolls come into contact with the polymeric melt, and start biting onto it. At a certain axial (downstream) location x A), the polymeric melt detaches itself from one of the rolls. Pressure, which is assumed to be atmospheric at X2, rises with x and reaches a maximum upstream of the minimum gap location (recall the foregoing discussion on the pressure profile between non-parallel plates), then drops back to atmospheric pressure at X. The pressure thus generated between the rolls creates significant separating forces on the rolls. The location of points A i and X2 depends on roll radius, gap clearance, and the total volume of polymer on the rolls in roll mills or the volumetric flow rate in calenders. 

The thickness of the calendered product must be uniform in both the machine and cross-machine directions. Any variation in gap size due to roll dimensions, setting, thermal effects, and roll distortion due to high pressures developing in the gap, will result in product nonuniformity in the cross-machine direction. Eccentricity of the roll with respect to the roll shaft, as well as roll vibration and feed uniformity, must be tightly controlled to avoid nonuniformity in the machine direction. A uniform empty gap size will be distorted in operation because of hydrodynamic forces, developed in the nip, which deflect the rolls. The resulting product from such a condition will be thick in the middle and thin at the edges, as shown in Fig. 15.2. 

Calendering of Polymers The Newtonian Haskell Model A 0.2-m-diameter, 1-m-wide, equal-sized-roll calender operates at a speed of 50 cm/s. At a gap separation of 0.02 cm, it produces a 0.022-cm-thick film. Assuming a Newtonian viscosity of 104 poise, calculate in the last nip (a) the maximum pressure (b) the separating force and (c) estimate the mean temperature rise. 

Separating Force between Rolls in an Experimental Calender A cellulose acetate-based polymeric compound is calendered on a laboratory inverted, L-shaped calender with 16-in-wide rolls of 8 in diameter. The minimum gap between the rolls is 15 mil. The sheet width is 15 in. Calculate the separation force and the maximum pressure between a pair of rolls as a function of exiting film thickness, assuming that film thickness equals the gap separation at the point of detachment. Both rolls turn at 10 rpm. The polymer at the calendered temperature of 90°C follows a Power Law model with m = 3 x 106 dyne.s"/cm2 and n = 0.5. [Data based partly on J. S. Chong, Calendering Thermoplastic Materials, J. Appl. Polym. Sci., 12, 191-212 (1968).]... 

Shear rates during calendering range typically from 10 to 10 sec. The pressure and forces encountered within the nip region are affected by the characteristics of the material (rheology,... 

Ultimately, equations are obtained for various process parameters, such as Pniaxj maximum pressure Z, the power F, force separating rolls and Q, the volumetric calendering rate). These are given in the following equations ... 

A calendering system (R of 0.17 m. Ho of 0.006 m, roll speed of 0.1 m/s) processes a material whose viscosity is 1 X 10 N s/m. For this system, find the sheet thickness, maximum pressure, and roll-separating force. 

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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