Calendering of rubber

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. 

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. 

The amount of rubber compound running at the nip or nips of a calender during the operations of ffictioning, sheeting, coating or profiling. 

A heavy type of open mill with deeply corrugated rolls, used for the preliminary breakdown of rubber compound prior to the processes of extrusion and calendering. 

Difference in physical properties along and across a section of rubber compound resulting from the passage of the rubber through a mill nip, a calender nip or through an extruder die. See Calender Grain. 

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. 

In calendering the term indicates poor extensibility of rubber sheet usually due to the use of insufficiently warmed compound. 

The process of controlled, gentle warming of unvulcanised calendered sheet rubber to reduce its anisotropy and prevent subsequent distortion of articles made from the sheet and cured by embedding in chalk in open steam. 

A set of knives fitted to a mill to cut off a strip of rubber compound for continuous feeding of an extruder or calender. Similar knives may be fitted to a calender roll to produce strip or sheet rubber of specified width. 

A diagram showing how rubber sheet, fabric, linings, etc., are let off through tensioning devices into calenders, spreading machines, etc., and finally taken up on box rollers. Direction of travel and position of rubber nips are usually included in such diagrams. 

An alternative method to that of using a spreading machine or a calender for the initial preparation of fabrics for application of rubber (for composite product assembly) can be by the use of dip coaters. Application of rubber compound by this method ensures a better penetration of the fabric interstices than can be achieved by conventional frictioning techniques using calenders. More delicate fabrics which would not be strong enough for calender application can also be treated with rubber by this technique. 

Calendering involves passing unformed or extruded rubber through a set or sets of rolls to form sheets or rolls of rubber product. The thickness of the material is controlled by the space between the rolls. The calender may also produce patterns, double the product thickness by combining sheets, or add a sheet of rubber to a textile material. The temperature of the calender rolls is controlled by water and steam. Products produced by this process include hospital sheeting and sheet stock for other product fabrication. 

In the production of fine rubber sheets or coated rubber sheets, the main equipment used is a calendering machine. Though rubber extruding machines which are later developments are used for manufacturing some sheet profiles, the calendering machines are the ones used widely in the rubber industry for the manufacture of rubber and coated rubber sheets. Calenders are conveniently used for the continuous production of rubber sheets from different types of rubbers. 

The achievement of rubbering is also dependent upon the condition of the textile material. Unless a perfectly flat uniformly tensioned sheet is passed through the fabric nip, the calender operator must make frequent adjustments to the roll setting to prevent either fabric crushing or loose topping. 

Calender - A machine equipped with two or more heavy, internally heated or cooled rolls, used for the continuous sheeting or plying up of rubber compounds. 

Grain - The unidirectional orientation of rubber or filler particles occurring during processing (extrusion, milling, calendering) resulting in anisotropy of a rubber vulcanizate. 

Silicone rubber compounds can be molded, extruded and calendered. The vulcanisation of rubber compounds occurs in two stages 1) in a press or steam boiler at high pressure and 120-150 °C 2) by thermostatic control at atmospheric pressure and 200-250 °C. 

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