Machine calenders

Machine calenders consist of two or more hard rolls and are practically always installed on-line. They are primarily used for paper that only requires moderate finishing or for pre-calendering grades that need further finishing treatment in order to obtain higher gloss and/or smoothness. Machine calenders are equipped with overall deflection or zone-controlled deflection rolls (Fig. 6.69). 

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In 1798, Nicolas Louis Robert invented the paper machine. It took more than fifty years, however, until calenders were installed in the paper machine. These calenders - also called machine calenders - consisted of at least two hard rolls. A decade later the supercalender appeared with a large number of alternating hard... 

Hence, the introduction of the Swimming RoU in the 1950s by Kusters was of decisive importance for the further development of both the machine calender and the supercalender. This roll consists of a fixed shaft with a shell rotating around it. Between the shaft and the shell is an oil-filled chamber. By adjusting the oil pressure in this chamber the shape of the roll shell can be changed (Fig. 6.66). Thus the operating window of calenders was suddenly expanded. 

Machine calenders and supercalenders with width about 5000 mm and above are today equipped with zone-controlled rolls as standard. Narrower machines still use the simpler overall control type rolls. 

Fig. 6.69 Machine calender, schematic and photo of mounting (source Voith).

Woodfree uncoated papers include, above all, office papers, which are in turn subdivided into writing and printing papers as well as copy papers. In the past, these papers were not supercalendered. Simple machine calendering was considered satisfactory. However, the demands made on the surface quality of these papers have now increased considerably. 

Calendering can achieve surprising accuracy on the thickness of a sheet. Typically the tolerance is 0.005 mm but to achieve this it is essential to have very close control over roll temperatures, speeds and proximity. In addition, the dimensions of the rolls must be very precise. The production of the rolls is akin to the manufacture of an injection moulding tool in the sense that very high machining skills are required. The particular features of a calender roll are a uniform specified surface finish, minimal eccentricity and a special barrel profile ( crown ) to compensate for roll deflection under the very high presurres developed between the rolls. 

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. 

A machine for masticating rubber, mixing rubber compounds and for warming up rubber compounds prior to calendering, extrusion, etc. See Open Mill. 

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. 

Hose constmcted by wrapping the reinforcing fabric and the calendered cover on to the inner tube which has been fitted over a mandrel of the desired diameter. The application of the fabric, the cover and of the wrapper may be done either by hand or by machine. 

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. 

Control of product gauge is of considerable importance for the majority of products produced from calenders and various means have been used to achieve this from the early days of the machine. Inaccuracies in gauge on calendered products can not only mean out of specification goods being produced, but also one can be giving away free of charge considerable quantities of expensive compounds (unless the product is sold by weight, of course). 

Early thickness control systems relied on the calender operator setting up the machine correctly by the use of hand-held thickness gauges, with the consequent slowness of response time during set-up and running before any desired modification to gauge could be achieved. It was usual to run the calender at a low speed to set-up the required dimensions for the product and then to speed up the... 

Calenders are very expensive and specific machines that are used only for high-output production. 

Calender A precision machine equipped rolls with three or more heavy, internally heated or cooled (or both), revolving in opposite directions, used for continuously sheeting and plying up elastomeric compounds, and frictioning or coating with elastomeric compounds. 

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