Rubber calendering

Change in dimensions of an unvulcanised rubber (calendered sheet or extruded section) on cooling from the processing temperature. Also the volume contraction of a moulded rubber product on cooling from vulcanising temperature. See Coefficient of Thermal Expansion (Volumej. Shrinking... 

Calendering The rubber calender is essentially a device for forming rubber sheeting of continuous length and uniform thickness. It is also used extensively for frictioning or skim coating of fabric/cord/wires with rubber for making... 

Calendering. Rubber calenders consist of at least three rolls, which can be adjusted for gap, speed, and temperature. Calendering can be used for forming sheets of uncured... 

Production of sheeting of such thickness presents great problems on more conventional equipment. Normal two- or three-bowl rubber calenders can produce satisfactory sheet at thicknesses of 1-2 mm maximum. At greater thicknesses blisters become a problem. If very thick sheets (7 mm and above) are attempted there is an additional problem with air inclusions in the body of the sheet, as well as surface blisters. The traditional method for obtaining very thick sheets has been to ply together many thin ones, a troublesome and expensive procedure. An Alternative process is to extrude a tube which is slit to form a sheet this process is used for comparatively low outputs and widths (maximum 500 kg hr , 1000 mm). 

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. 

Rubber calenders consist of at least three rolls which can be adjusted for gap, speed and temperature. They are used to form rubber sheeting to required lengths and thickness for subsequent building operations. They are also used for frictioning or skim coating fabrics. Cord and wire are coated in this way for making plies used in the construction of tires and conveyor belts. 

Gut Rubber. To produce cut mbber thread, smoked mbber sheet or crepe mbber is milled with vulcanizing agents, stabilizers, and pigments. This milled stock is calendered into sheets 0.3—1.3 mm thickness, depending on the final size of the mbber thread desired. Multiple sheets are layered, heat-treated to vulcanize, then sHt into threads for textile uses (Fig. 2). Individual threads have either square or rectangular cross-sections. 

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

Fig. 3. Two routes to rubber/resin tapes 1, Traditional calender-mill and 2, solution polymer method for high performance tape.

The lining sheet is prepared by calendering in thin plies up to 1 mm in thickness, followed by combining the individual layers on the calender to form a finished sheet, usually between 3 mm and 6 mm thick. Alternatively, lining sheet can be manufactured by the roller die process in which the sheet is extruded and then passed through rollers to give the finished thickness. Extrusion is also used for the preparation of unvulcanised rubber tubes suitable for lining pipes up to 200 mm in diameter. 

A thin layer of a mix of natural rubber, sulfur, precipitated silica, 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 modi-... 

The calender was developed over a century ago to produce natural rubber products. With the developments of TPs, these multimillion dollar extremely heavy calender lines started using TPs and more recently process principally much more TP materials. The calender consists essentially of a system of large diameter heated precision rolls whose function is to convert high viscosity plastic melt into film, sheet, or coating substrates. The equipment can be arranged in a number of ways with different combinations available to provide different specific advantages to meet different product requirements. Automatic web-thickness profile process control is used via computer, microprocessor control. 

Recent Developments in Rubber Mixing and Cord Calendering in Tire Production... 

This chapter deals with recent developments of rubber mixing and calendering. Rubber extm-sion is discussed elsewhere in this book. Although it dominates mbber mixing in the last ten years, the mixing of silica-filled tread compounds is only slightly discussed in this chapter, since another chapter in this book totally covers the mixing of silica compounds. 

Constant rubber temperature with which the calender is fed... 

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. 

G. Capelle Calendering technology, in A.K. Bhowmick, M.M. Hall, and H.A. Benarey (ed) Rubber Products Manufacturing Technology, Marcel Dekker, New York/Basel/Hong Kong, 1994. 

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