The Calendering Process

Calender sizes range up to 90 cm (36 in) in diameter and 250 cm (97 in) wide, with polymer throughputs up to 4000 kg/h. The surface temperature of the rolls is carefully controlled by using drilled rolls—that is, axially drilled holes all around the periphery—in which a temperature-controlling liquid is circulated. 

The calendering process is commonly used for shaping high melt viscosity thermoplastic sheets and is particularly suitable for polymers susceptible to thermal degradation or containing substantial amounts of solid additives. This is because the calender can convey large rates of melt with a small mechanical energy input (compared to an extruder). 

Principles of Polymer Processing, Second Edition, by Zehev Tadmor and Costas G. Gogos Copyright 2006 John Wiley Sons, Inc. 

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. 

Three common methods, which are commonly referred to as roll-crown, roll-crossing, and roll-bending, are employed to compensate for this deformation. Roll-crown indicates that the roll diameter at the center is slightly greater than at the edges. In principle, by applying an appropriate roll diameter and profile, roll deflection can be exactly 

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Basically the calendering process is used in the production of plastic films and sheets. It converts plastic into a melt and then passes the pastelike mass through roll nips of a series of heated and rotating speed-controlled rolls into webs of specific thickness and width. The web may be polished or embossed, either rigid or flexible (9). One of its sheets major worldwide markets is in credit cards. At the low cost side these lines can start a million. A line, probably the largest in the world... 

The product quality, however, is related to the calendering process. Thickness gradients across the calendered product, thickness variations along the product, as well as cord density distribution determine the quahty of the product and are tremendously related to the process parameters. 

The calendering process can be described by means of a combined drag and pressure flow. The rotating rolls of a calender drag the mbber through the calender nip. The clearance as well as viscoelastic properties determine the sheet thickness. 

From the diagrams in Figures 35.34 through 35.40 it might become clear that all parameters will have a tremendous influence on the calendering process and therefore on the calendered product. The calender line speed seems to have the least influence on the calendering performance. 

There are various textbooks available on the calendering process,which is referred for an extensive explanation. In this section the impact of the presented theory on the general layout of a dual-purpose calendering line for textile cord and steel cord coating is considered. 

The accumulator in the upstream is designed in such a way that the calendering process can mn steadily while splicing the tail end of a fabric with the lead end of the fabric of the next roU. The accumulator in the downstream takes care for a constant process during the exchange procedure at the winding station. 

The advantage of calendering over direct sheet extrusion is that a complicated and expensive die is not required. Some additional mixing can be done as part of the calendering process, as well, and a surface finish can be applied by the final roller, if desired. Sheet up to a few millimeters thick and a meter wide can be made using calendering. 

The calendering process and its conditions are developed or modified according to the requirements of subsequent operations and the purpose for which the sheet is used. Thus for sheets which are to be open cured, such as in chemical plant lining and custom built items such as inflatables and ebonite pipes, roll coverings for paper and steel mills, the calendering needs to be more exact than the sheets which are used for blank preparation for molding of... 

As discussed in Chapter 3, the calendering process is used to squeeze a mass of polymeric material through a set of high-precision rolls to form a sheet or film. In this section, we will derive the well known model developed by Gaskell [11] and by McKelvey [15]. For the derivation, let us consider the notation and set-up presented in Fig. 6.26. 

All the above problems relate to the calendering process where a large mass of polymer melt is fed into the calender. In some industrial applications, a finite polymer sheet of thickness hf is fed to the calendering rolls, as depicted in Fig. 6.35. 

Figure 6.34 Comparison between experiments and theoretical predictions of maximum pressure between the rolls during the calendering process of an unplasticized PVC film. A power law index, n, of 0.1505 and a consistency index, m, of 155.2 kPa-s were used in the power law model of the viscosity.

Modeling the calendering process for Newtonian and shear thinning polymer melts. Using the RF method, Lopez and Osswald [5] modeled the calendering process for Newtonian and non-Newtonian polymer melts. They used the same dimensions and process conditions used by Agassant et al. [1], schematically depicted in Fig. 11.17. 

A comprehensive mathematical model of the calendering process should consist of a coupled hydrodynamic and roll structural analysis, heat transfer in the deforming polymer... 

Fig. 15.7 Iterative computational scheme used by Mewes et al. [Reprinted by permission from D. Mewes, S. Luther, and K. Riest, Simultaneous Calculation of Roll Deformation and Polymer Flow in the Calendering Process, Int. Polym. Process., 17, 339-346 (2002).]...

The characteristics of lubricants, their effects during plastics processing and their influence in the calendering process are discussed in depth. Attention is paid to the different intemal/external behaviour of lubricants, viscosity reduction by internal lubricants, fusion delay by external lubricants, shear liquefaction by lubricants and the suitability of various lubricants for the manufacture of calendered PVC films in relation to melt elasticity, release effect, flow and plate-out. 

A wide variety of plastics can be used although about 80 percent is PVC and 15 percent ABS. (Calendering consumes about 6 percent of total plastics consumption for all processes.) Other plastics used are HOPE, PP, and styrenes. The basic plastic limitation of the calendering process is the need to have a sufficiently broad melt index to allow a heat range for the process. This permits the material to have a relatively high viscosity in the banks of the calender (banks indicating where two rolls meet, or the nip of the rolls). As a result of the viscosity, a shear effect can be developed throughout the process, and especially between the calender rolls. Thus, the calender forms... 

Factice is produced from fatty oils such as linseed oil, castor oil, soybean oil, or rape seed oil. To obtain brown factice, the oil is heated with sulfur to 130-160 C for 6- 8 h. This vulcanization gives a soft, crumbly, elastic product with 5%-20% sulfur. White factice is obtained by vulcanization of the oil with S2CI2 at room temperature. It contains 15%-20% sulfur and is not elastic. Both types of factice are used as cheap bulking materials in rubber articles, and improve the calendering processing of natural rubber. 

The calendering process also increases the density of the fabric (especially needle felts) the pore size is reduced and dust penetration into the body of the needle felt thus restricted. In practice, the polymer type largely dictates the surface temperature of the heated bowl whilst the pressure, which may be up to 300 da N m and speed are adjusted to achieve the desired density and air permeability. 

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