Z calender

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. 

Fig. 15.1 (a) A four-roll, inclined Z calender for double casting of tire cord, (b) A four-roll,... 

Z calender A four-bowl calender in which rolls 1 and 2 are situated in a horizontal plane. Roll 3 is below roll 2, 1 and 4 are therefore offset. Also called Z-cylinder. See calender. 

Z-Calender n. A calender with four rolls arranged so that, as the web of material (fabric) passes through them, the cross section of its path has a shape resembling the letter Z. 

Elden, R.A. and Swann, A.D, Calendering of Plastics, Plastics Institute Monograph, Iliffe, 1971. Rosenzweig, N., Markis, M and Tadmar, Z., Wall Thickness Distribution in Thermoforming Polym. Eng. Sci. October 1979 Vol 19 No. 13 pp 946-950. 

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. ... 

By crossing the axes of both rolls in the surface of the calendered sheet the effect of roll deflection can be compensated partially. If Co represents the cross axes setting displacement the resulting clearance Yca(z) is given by... 

Figure 35.41 shows the effect of the cross axes setting on the clearance along the calender roll. The effect of the deflection of both rolls on the clearance is given for the reference simation as given before. The total width W of the roll is 1600 mm and the diameter D is 650 mm (inner diameter Dj = 350 mm). Z is the coordinate parallel to the rolls and originates at one of the calender roll edges. 

A more complex form of coating calender is the four-bowl Z or L arrangement. A four-bowl calender can simultaneously apply a rubber coating onto both sides of a fabric. In effect, the No. 1 and 2 bowls and the No. 3 and 4 bowls form pairs where two rubber sheets are produced. The sheets are then laminated to a substrate between the No. 2 and 3 bowls. Very sophisticated devices are usually incorporated into the calender design to control thickness and width of the individual sheets and the resulting laminate. 

Figure 7.64 The nip area of the two roll geometry in calendering. From Z. Tadmor and C. G.

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.

There is little work of a fundamental nature done that helps towards a better understanding or control of calendering of rubber. Most theoretical studies were concerned with the pressure developed when a stock of thick viscous rubber sheet material is reduced to a thinner sheet by passage between rolls. Calenders with 3, 4 or more rolls might be assembled to quite different configurations. Calenders in L shaped, F shaped, I shaped or Z shaped configurations with roll diameters of 168 mm, 250 mm and 350 mm are available on the market. 

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.]...

Basically, a viscous bank of excess thermoplastic or rubber is pressure-formed between a pair of parallel co-rotating rolls to form a thin film or a thicker sheet. Most calenders consist of four rolls in L- or Z-shape, plus additional feed rolls and post-calender laminating, patterning, and/or wind-up rolls. 

Calenders vary in respect to the number of rolls and their arrangements. Examples of the layout of the rolls are the true L, conventional inverted L, reverse fed inverted L, 1, Z, and so on. These large diameter heated rolls have the function to convert the high viscosity plastic melt into film or sheet. Figures 9.2 and 9.3 provide examples of lines. 

The Z-type roll arrangements followed developments in offset rolls. This design eliminated the floating No. 3 roll on a calender fitted with bearings. Each roll can be preloaded on to its bearings at a point that is... 

Figure 14.2. Arrangements of calender rolls, a -inverted L, b - inclined Z.

Calenders having a Z-arrangement have also been installed. This arrangement permits a compact design and makes a calender with only two rolls in any one plane so that the loads developed in one working bank do not affect the gauge of subsequent passes. 

To manufacture EPDM sheeting/membrane, pre-warmed, plasticised EPDM compound at a temperature of 80-100 °C is used for calendering in a 3 roll in-line T configuration rubber calender machine or in a 4 bowl Z configuration rubber calender machine. 

A calender consists of two or more heated rolls with different configurations depending on the product to be manufactured. All different roll configurations (I, L, F, Z, etc. [2]) used for PVC can also be optimized for the processing of PP. The purpose of the heated rolls in the calender is to convert the molten material into a product that has the desired thickness. The thickness decrease is achieved by squeezing the molten plastic through the nips between the heated rolls. The differences in rotation speeds and directions of the rolls drive the molten material to pass the calender [2, 4]. 

Fig. 15. Calendering-roll configurations (a) three-roll superimposed type (b) four-roll inverted-L type (c) four-roll Z type.

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 ... 

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