Calendering finish

Calendering and calendered finishes are considered in detail in another chapter of this book but it may be helpful here to include a brief review of the process and a comparison of its main features with extrusion. 

Smooth surfaces are normally estabflshed by calendering, a process which subjects the fabric at the nip point(s) of two or more roUs to the influence of controlled time, temperature, and pressure. When calendering is used as a thermal-bonding process, the roUs are of the same dimension and composition and are independently driven. However, when calendering is used as a fabric finishing operation, the roUs are frequently of different dimensions and composition and are not always independently driven. 

More specific recipes appear in Table 3. The ingredients are added to the elastomers on standard two-roU mills or in internal mixers. Finished compounds are readily extmded, calendered, or molded in standard equipment. Vulcanisation of extmdates is accompHshed in Hve steam autoclaves, Hquid salt baths, fluidized beds, and microwave equipment. 

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. 

Satinappretur, /. satin finish, glazed finish, satinieren, v.t. satin, glaze, gloss calender or supercalender (paper). 

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. 

Another processing method has been termed the solventless. process. In this method, a premix is made from NC fibers plus liquid NG and minor ingredients dispersed in water. The paste is centrifuged and air-dried. The remaining ingredients are tumble-blended and then calender-rolled into sheets called carpet rolls . These are then extruded into finished pellets or cylinders... 

A general term applied to the variety of operations required to convert a raw elastomer into finished products. See Calendering, Compounding, Curing, Extrusion, Mastication, Mixing, Spreading. 

Calendering. Passing a material (plastic, cloth) between rollers to make a thinner material or to give it a smooth finish. 

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. 

Tile is manufactured in several ways. In each method, a continuous sheet is formed gauge refinement and planishing are carried out in subsequent calendering steps. Stresses that could lead to poor dimensional stability are avoided. The efforts to prevent stresses are governed by formulation, stock and roll temperatures, conveyor speeds, etc. After the final calendering, a resin—polymer—wax finish is applied to the surface of the sheet which is then buffed before it moves to the punch press. Frame scrap and tile rejected because of defects are returned to the mixers and recycled. 

In the manufacture of fine rubber sheets, two essential qualities are required in the finished products, namely, uniformity of texture and even thickness. When sheets of only a few thousandths of an inch thick are required, great care must be taken to ensure this result. The first essential quality is achieved in an adjusted calender with all its bearings in good condition. The rolls must be... 

Fastness against finishing. This test is applied to those fabrics which are to be subjected to calendering in the hot or to gassing to bum off the down. The sample is either passed four or five times over a flame, or drawn slowly across the surface of a tube inside which steam passes, to ascertain if any change occurs in the colour of the fabric. 

An alternative method to apply starch to paper, especially to paperboard grades, is to use a water box at the calender. Water boxes are used for wet-finishing of paper-board prior to coating application. Use of starch in the water box will increase surface strength and lessen fiber reswelling after coating application. Dilute dispersions of oxidized starch are used, frequently in conjunction with a lubricant to prevent calender roll picking. 

Poly(vinyl chloride) films are produced in two main forms—unplasticized and plasticized—and over the years different machines have been manufactured to handle the two types. When calendering unplasticized PVC there is a tendency for small particles, usually referred to as crumbs , to fall away from the edges of the film and from the feed nip. Such crumbs then could fall on to the finished film, where they would stick and form defects. To avoid this, producers of unplasticized film usually prefer an L configuration in which the product travels up the stack and surface contamination of this kind is prevented. With plasticized PVC the problem of crumbs does not occur to any great extent and, as it is an advantage to have good access to the part of the calender where the finished film is made, an inverted L configuration is the most popular. 

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. 

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