Calenders, rolls

Tape may be made by a similar process. In this case the lubricant selected is a non-volatile oil. The preform is placed in the extruder and a rod extruded. The rod is then passed between a pair of calender rolls at about 60-80°C. The unsintered tape is often used for lapping wire and for making lapped tube. Sintering is carried out after fabrication. The current most important application of unsintered tape is in pipe-thread sealing. 

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. 

Considering unit width of the calender rolls the total throughput, Q, is given by... 

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... 

The resulting force on the calender roll can be estimated as follows ... 

Pressure between two calender rolls Calender line speed... 

FIGURE 35.35 Pressure buildup in the nip between two calender rolls. Rolling bank height = 10 cm. 

FIGURE 35.38 Pressure buildup in the nip region of two calender rolls as a function of the clearance setting. 

High pressures can be found in the calender nip region. The resulting force acts on the adjacent calender rolls and causes a deflection of the rolls and therefore a thickness gradient across the calendered product. The total displacement Y(z) of the roll surface caused by roll deflection due to this viscous force can be approximated by Kopsch. ... 

Figure 35.41 shows the effect of the cross axes setting on the clearance along the calender roll. The effect of the deflection of both rolls on the clearance is given for the reference simation as given before. The total width W of the roll is 1600 mm and the diameter D is 650 mm (inner diameter Dj = 350 mm). Z is the coordinate parallel to the rolls and originates at one of the calender roll edges. 

In modem calender lines both compensating devices can be found. It is common that wide calender rolls are crowned in such a way that under working conditions the crowning compensates the roll deflection due to high shear forces. Crowning specifications can be understood by means of the presented theory. Since the lowest nip forces can be achieved at high clearance settings, low line... 

FIGURE 35.41 Effect of the cross axes setting compensation method on the clearance in the total width of the calender roll in working condition. 

The distortion suffered by calender rolls resulting from the pressure of the rubber running between them. If uncorrected the deflection produces a rubber sheet thicker in the middle than at the edges. See Camber, Crossed Axes and Roll Bending, all techniques to compensate for calender bowl deflection. 

Arrow-shaped markings on the surface of calendered sheet mbber due either to the use of insufficiently warmed stock or to too low temperature of the calender rolls. 

The ratio of the relative surface speeds of adjacent mill or calender rolls usually given in the form 1.25 1, where the 1 represents the speed of the slower roll. 

An instrument for measuring temperatures, in the rubber industry the term is usually applied to an instrument for determining the surface temperature of mill and calender rolls, moulds, etc. The instrument is usually based on thermocouples or, where higher accuracy is required, platinum resistance thermometers. Infrared (IR) techniques are now used which have the advantage of non contact but require careful calibration for the emissivity of the surface. 

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. 

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. 

FIG G6 Strain gages oft loading screws show pressure between calender rolls... 

Figure 20 indicates the pyroelectric current in calender-rolled rigid poly (vinyl chloride) (PVQ (Furukawa and others, 1968). In Fig. 20, results are shown for fifteen specimens cut from a sheet of PVC film, with the polarity map as Fig. 21. The map shows a heterogeneity of polarity.

Fig. 20. Pyroelectric current of 15 specimens of poly(vinyl chloride) film cut out from a calender-rolled sheet as indicated in Fig. 21. Electrode area = 1 x 1 cm2 film thickness = 0.2 mm heating rate=6 K/min. Reproduced from Furukawa and others [J. Appl. Polymer Sci. 12,2675 (1968)] by permission of John Wiley Sons,...

The calendering machine of three, four or more hollow rolls has arrangements for heating by steam or hot water or cooling by refrigerated water. The widths of the calender rolls may be only six inches for a laboratory size calendering machine or up to hundred inches when heavy work and a large volume production are required. 

The contouring of the calender rolls by grinding in order to counteract the natural deflection and the deflection set up by the work being done with rubber stock load is a precision job and in most cases it is done by trial and error method. The nip pressure per inch width of the roll for most rubber compounds has been extensively studied by scientists in the Dunlop Rubber Company [ref Paper "Estimation of the shear deformation exerted by a roll-mill upon a rubber compound" by T.S. Ng, G. Angerer, Dunlop Forschung, Dunlop-Strasse 2, Hanau, Federal Republic of Germany, Presented at the Jahrestagung der... 

It is not possible to standardise operating temperatures for calender rolls. Although on each calender a set of conditions can be established for several runnings, this may vary approximately on calenders using different types of compounds. It is however a normal practice to operate with lower temperatures for those rolls around which the rubber bands rotate. 

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