Calendering processes

Calendering processes, of great importance in the production of sheet materials from PVC compounds, are little used with polyethylene because of the difficulty in obtaining a smooth sheet. Commercial products have, however, been made by calendering low-density polymer containing a small amount of a peroxide such as benzoyl peroxide to give a stiff but crinkly sheet (Crinothene) which was suitable for lampshades and other decorative applications. 

Vinyl backed packaging tapes are quite popular in Europe. The vinyl backing is commonly manufactured using a calendering process. Printability and smooth unwind of the tape rolls are amongst the desirable features of these tapes. 

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... 

A FUNDAMENTAL APPROACH OF THE CORD CALENDERING PROCESS 35.5.1 Introduction... 

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. 

In fact a parabolic surface of the rolls in the flow region is assumed, which is valid as long as the rolling bank diameter is small compared with the roll diameter. This is normally the case in a calendering process. 

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. 

A calendering process in which hot soft unvulcanised rubber compound is applied to a fabric. The cloth passes between the bottom and middle bowls of a three-bowl calender it travels at the speed of the bottom bowl and the faster middle bowl shears the rubber into the pores of the fabric. 

A calendering process in which rubber compound (suitably warmed) is passed through the nip between two bowls set at a specified distance apart to produce a comparatively thin sheet of rubber. The name sheeting is also given to the vulcanised product. 

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... 

Figure 3.64 Schematic of a typical calendering process (Berstorff GmbH, Germany).

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.

Figure 11.17 Schematic diagram of a calendering process fed with a finite sheet.

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. 

O.A. Estrada, I.D. Lopez-Gomez, and T.A. Osswald. Modeling the non-newtonian calendering process using a coupled flow and heat transfer radial basis functions collocation method. Journal of Polymer Technology, 2005. 

Mixing Mechanisms Consider a calendering process for manufacturing PVC floor covering. The line consists of a ribbon-type blender in which the PVC is dry-blended, and an internal mixer that feeds a SSE equipped with a static mixer, feeding the first nip of the calender. What types of mixing mechanisms does the polymer experience Specify the locations at which each mechanism occurs. 

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). 

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).]...

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