Calendering configurations

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. 

The use of the vinyl polymer in fluid form offers distinct advantages to the manufacturer of vinyl products. The most important obvious advantage is the relatively lower cost of liquid processing equipment and accessories as compared to those required for processing of solid or powder thermoplastics. For example, a spread coating line, for example, knife or roll coater, even with appropriate unwind and takeup accessories, is less expensive than a banbury-mixer-mill calender configuration. 

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. 

This basic operation utilises a two-bowl calender in horizontal or vertical configuration. The feed material, either in strip or pig form, is fed into one side of the nip and is squeezed by the bowls, thereby emerging as a sheet which is pulled from the bowl by some manual or mechanical means or supported by a liner cloth. Because of their versatility, three-bowl calenders are now more widely used for sheeting as well as other basic calendering operations. Thickness control is accomplished by use of the adjustable nips and may be further refined by automatic control systems using thickness sensors. It should be noted here that the force required in the nip to flatten the feed material causes deflection of the bowls, however slight. If some corrective steps are not taken, the product thickness will vary across the sheet, resulting in excessive variations of the product and possibly excessive use of expensive materials. In order to overcome these problems, three basic techniques are used to achieve uniform product thickness ... 

The number of rolls, their sizes and their configuration characterize the calender. Some possibilities are indicated below, without claiming to be exhaustive ... 

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. 

The flow configuration of building block 3, of two non-parallel plates in relative motion, shown in Fig. 6.19, was analyzed in detail in Example 2.8 using the lubrication approximation and the Reynolds equation. This flow configuration is not only relevant to knife coating and calendering, but to SSEs as well, because the screw channel normally has constant-tapered sections. As shown in Fig. 6.19, the gap between the plates of length L is Ho and II at the entrance and exit, respectively, and the upper plate moves at constant velocity Vo-... 

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. 

Poly(vinyl chloride) films are produced in two main forms—unplasticized and plasticized—and over the years different machines have been manufactured to handle the two types. When calendering unplasticized PVC there is a tendency for small particles, usually referred to as crumbs , to fall away from the edges of the film and from the feed nip. Such crumbs then could fall on to the finished film, where they would stick and form defects. To avoid this, producers of unplasticized film usually prefer an L configuration in which the product travels up the stack and surface contamination of this kind is prevented. With plasticized PVC the problem of crumbs does not occur to any great extent and, as it is an advantage to have good access to the part of the calender where the finished film is made, an inverted L configuration is the most popular. 

BR is extruded and calendered. Processing properties and performance properties are related to polymer configuration cis- or trans-... 

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.

The best known characteristic of factice, before the invention of partially crosslinked Superior Processing grades of NR and their synthetic equivalents, was the property of conferring dimensional stability to extruded and calendered stocks. Extruded items such as tubes, hose, cables and automotive window seals must retain their configuration during vulcanization either in open steam or during vulcanization by low-pressure processes such as LCM. In this area factice acts as a non-thermoplastic additive. 

The third step, calendering, converts the roofing compound into a uniform continuous sheet that has almost the final desired thickness and width for a roofing panel. Calendering consists of placing the bulk compounded material onto mill rolls (set of from 3 to 4) which grab the material and convert it into a sheet configuration. Such sheets, when formed at moderate speeds, have few residual stresses. The product of this process step is a roll of thin (0.13 inch), flexible rubber that can be trimmed with scissors or knives to fit into any desired mold. 

Details concerning the rolls and roll covers, the roll configuration and other important components of the calenders will be treated below. 

BR is extruded and calendered. Processing properties and performance properties are related to polymer configuration cis- or trans- stereoisomerism, MW and MWD, degree of crystallization (DC), degree of branching, and Mooney viscosity. Broad MWD and branched BR tend to mill and process more easily than narrow MWD and more linear polymer.Lower Mooney viscosity enhances processing. BR is blended with other synthetic rubbers such as SBR to combine BR properties with millability and extrudability. 

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