Calendering pressurization

Table I lists the conductivities obtained with the stainless steel electrodes and the Ga-In electrode, at 13 8 MPa pressure, as well as Ga-In without pressure. It is clear, from the Ga-In results, that the effect of pressure accounts for a 2Q% increase in conductivity. The value obtained from the stainless steel electrode under pressure is lower than that obtained with Ga-In without pressure, this being the result of the difference in the electrode areas, as discussed earlier. Furthermore, the table of results clearly shows that the effect of surface smoothness, as a result of the different calendering pressures, has no significant effect on the measured bulk conductivity for the case of the stainless steel electrodes with 13-8 MPa pressure.

In manufacture, the abrasive grain is mixed with cmde mbber, sulfur, and other ingredients for curing, then passed through calender roUs to produce a sheet of desired thickness. The wheels are stamped from this sheet and heated under pressure to vulcanise the mbber. 

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. 

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. 

Example 4.9 A calender having rolls of diameter 0.4 m produces plastic sheet 2 m wide at the rate of 1300 kg/hour. If the nip between rolls is 10 mm and the exit velocity of the sheet is 0.01 m/s estimate the position and magnitude of the maximum pressure. The density of the material is 1400 kg/m and its viscosity is 10 Ns/m. ... 

A thin layer of a mix of natural rubber, sulfur, precipitated silica, water, and some additives, such as carbon black and vulcanizing agents, is extruded on a paper support belt, calendered, and vulcanized as a roll in an autoclave under elevated pressure and temperature ( 180 °C). A modi-... 

The purpose of this section is to explain in a qualitative way how the product quality is related to the calendering parameters. Therefore a simplified calender model is presented. The model describes the pressure buildup in the calender nip region as a function of compound viscosity, clearance, calender line speed, rolling bank height, as well as geometrical data. The general layout of a typical steel and fabric cord calender is explained by means of the result of the presented calender model. 

The calendering process can be described by means of a combined drag and pressure flow. The rotating rolls of a calender drag the mbber through the calender nip. The clearance as well as viscoelastic properties determine the sheet thickness. 

Let p(x) be the pressure as a function of the distance x from the calender nip perpendicular to the calender axes. The coordinate y describes the distance from the center of the clearance to the roll axis. The equation of motion in this case can be reduced to... 

The pressure through the calender nip is determined with this equation. The distance from the calender flow centerline to each roll surface is given by... 

The pressure buildup through the calender nip, which is the difference in pressure between the rolling bank surface and p(x) is given by the integral of Equation 35.15 ... 

Pressure between two calender rolls Calender line speed... 

The following diagrams show typical pressure curves in the flow region of the calender nip as a function of various parameters. The following parameters are used for the presented model calculations ... 

Figure 35.34 shows a slight dependency of the pressure buildup on the calender hne speed, which equals the circumferential roll speed. The general shape of the pressure curve can be understood as follows. A converging drag flow yields a pressure buildup until a barrier has been passed. The material left (=upstream) from the pressure maximum will take part in the roUing bank flow. The material between the pressure maximum and the clearance of the calender flows by means of the drag flow and pressure flow. Each material volume element wfll pass the clearance. At the position where the pressure vanishes the sheet will be taken apart from one of the rolls. 

Figures 35.35 through 35.37 show the dependency of the pressure buildup on the roUing bank content. The pressure curve and therefore the nip force will change dramatically with the mbber content in the nip region. A varying feeding of the calender wfll cause varying nip forces.

Pressure in a calender clearance Rolling bank height=10 cm... 

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

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