Embossing calenders

The objective of embossing is to give the paper a three-dimensional pattern. This is achieved by means of a single nip calender. 

There are three different embossing methods, namely matrix embossing, flat-back embossing and union embossing. 

Union embossing calenders differ from the a.m. machines in so far as they consist of two rigid rolls of the same diameter. The rolls mesh. The distance between the two roUs is always adjusted in such a way that it is identical with the thickness of the web to be embossed. The result of union embossing is a web having a corrugated shape. 

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Mechanical softening treatments may involve fibre displacement, web buckling, and affect the absorbency of the resulting structure. Embossing, calendering surface glazing and other mechanical treatments can be utilized to modify web properties and the related absorbency characteristics. 

Figure 1.9 Schematic diagram of an inverted L-type calendering process. 1—calender 2—embossing calender 3—thickness gauges 4—water-cooled train 5—windiq> accumulator 6—windup station. (Rom Ref. 175.)...

Specific terms have been designated according to the function and composition of various roUs. Steel roUs that impose pressure, transmit heat, and emboss a pattern onto the fabric are known as pattern roUs. Flexible surface roUs that transport the fabric and permit pressure transmission to the fabric are termed bowl roUs or bowls. Bowl roUs are usually larger in diameter than pattern roUs. The material used to make these types of roUs is chosen according to the depth of surface smoothness to be placed on the fabric being calendered, and must be compatible with the pattern roU. Cellulose pulp, cotton, wool, cotton—wool mixtures, com husk, and various polymer materials are used as fillers for the roU surface compound. 

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 various processes carried out on a calender. See Coating, Doubling, Embossing, Frictioning, Profiling and Sheeting. 

In a calender line, the polymer melt is transformed into films and sheets by squeezing it between pairs of co-rotating high-precision rollers. Calenders are also used to produce certain surface textures which may be required for different applications. Today, calendering lines are used to manufacture PVC sheet, floor covering, rubber sheet, and rubber tires. They are also used to texture or emboss surfaces. When producing PVC sheet and film, calender... 

In the early days a calender train for plastics was a very basic affair, consisting of a line of small rollers to transport the film to an embossing nip, and a set of three or four larger cooled rollers to bring it to a crude wind-up. Probably there would be only three motors to drive the whole train and very little adjustment of speed ratios would be possible—a far cry from modern equipment, in which there may be twenty motors for a train. 

Embossing film in line with the calender has several advantages while giving the surface finish required it removes most of the heat remaining in the film and—since the nip of the embossing unit is the only point in the line between calender and wind-up where a film is gripped positively—it provides a single position at which fine adjustments may be made in the tension on the train. 

Creases formed in the film prior to the emboss nip will become welded into it as longitudinal faults. Such creases usually are caused by release of film from the calender roll in the form of an extreme arc, or by a total failure to control the tension between the calender and the emboss nip. Momentary contact of the film with the surface of the stripping rolls also may cause creases. 

A surface pattern can be applied by the calender roll or by a post-embossing operation. Feeding hot sheet to a cold embossing roll works well. The pattern is measured by a profilometer. 

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