Calendering defects

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. 

J. F. Agassant and P. Avenas, Calendering of PVC—Forecast of Stresses and Torques, paper presented at the 2nd Int. Symp. on PVC, Lyon, France, 1976 also, J. L. Bourgeois and J. F. Agassant, Calendering of PVC—Defects in Calendered PVC Films and Sheets, paper presented at the 2nd Int. Symp. on PVC, Lyon, France, 1976. 

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. 

Circulation within the bank depends upon the position of the currents and these are not necessarily constant across the width of a calender. Examination of a bank may reveal sluggish areas, some rotating in opposite directions, or what appears to be spiralling (the last, because of local variation in the temperature of the stock, often will result in flow marks in the film). Defects of this nature in the flow are influenced by the composition, and it may be necessary to adjust the amount of lubricant however, if a mix is long-established the indication probably is that the conditions of calendering have drifted away from the optimum—and appropriate corrections must be made. The size of a bank can be altered by adjusting the previous nip or by changing the speed ratio of the rolls, and often this will rectify a fault. 

Table 14.1. Typical defects in calendering related to plasticizers...

Controlled rheology grades of PP have been used in the production of calendered PP films [29]. Compared with conventional reactor grade material, the reduced molecular weight distribution of PP provides better flow and a slow rate of crystallisation, giving better processibility and films free from surface defects. However, the optical quality of the calendered film does not reach that of extruded film. The poor thermal oxidation resistance of PP in the presence of air is the major impediment in use of PP as a calendering material. 

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