Calendering pressure profiles

Figure 7.66 Pressure profile between rolls in calendering. From Z. Tadmor and C. G. Gogos, Principles of Polymer Processing. Copyright 1979 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

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. 

Experimental measurements of pressure profiles in calenders were conducted by Bergen and Scott (34). A strain gauge transducer was embedded in the surface of one of the 10-in-diameter rolls, and traces were recorded at various conditions corresponding to both calendering and roll milling. 

Fig. 6.27 Comparison between experimental pressure profile for plasticized thermoplastic resin (34) and theoretical pressure profiles for n — 1 and n — 0.25 calculated by Kiparissides and Vlachopoulos (35). The theoretical curves were calculated both by finite element method and analytically by way of Gaskell type models, as discussed in this section, giving virtually identical results. [Reprinted by permission from C. Kiparissides and J. Vlachopoulos, Finite Element Analysis of Calendering, Polym. Eng. Set, 16, 712-719 (1976).]...

Fig. 15.3 Pressure profiles in the calender gap at various cylinder axial positions, with rigid PVC (Vestolit Z 1877) at equal roll speeds of 5cm/s and roll temperature of 185°C minimum gap, 0.6 mm roll diameter, 30 cm width, 50 cm. Note the drop in pressure in the cross-machine direction with distance from the centerline, which drops to zero at the end of the rolled web. [Reprinted by permission from W. Unkruer, Doctoral Thesis, KV, Technischen Hochschule, Aachen, 1970.]...

Surface profiles of both sides of a calendered and an uncalendered sheet were recorded with a Talysurf 5 instrument (Rank Taylor Hobson, England), and are reported in Fig. 13. The surface is different in each case, but the bulk conductivity is not significantly altered. The pressure applied to the electrodes within the cell has not changed the surface profile of the sheet for the calendered sample. Although a difference is observed for the uncalendered sample, the surface is not as smooth as that of the calendered sheet after pressure is applied. 

Figure 13. Surface profiles for a paper sheet with and without calendering, and before and after pressure. (Reproduced, with permission, from Ref. 16. Copyright 1981, American Institute of Physics.)...

Rubber Separators A thin layer of a mix of natural rubber, sulfur, precipitated siHca, water, and some additives, such as carbon black and vulcanizing agents, is extruded on a paper support belt, calendered, and vulcanized as a roll in an autoclave under elevated pressure and temperature ( 180°C). A modified process extrudes and calenders a ribbed profile and crosslinks the rubber separator by irradiation. 

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