Nip region

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. 

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.

FIGURE 35.37 Pressure buildup in the nip region between two calender rolls. Rolling bank height = 5 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. ... 

Only in the calender area the cord tension is high enough to resist the pressure caused by the mbber flow in the calender nip region. Therefore, a constant cord density can be maintained by means of locally high cord tension. 

For any given size of mill there is a minimum volume of polymer per unit width of roll, below which no dispersive mixing occurs. Increasing the amount of material above this minimum level increases the high shear zone in the polymer, increasing dispersive mixing. There is an upper limit however, above which a circulatory flow develops at the entrance to the nip region and no further improvement occurs. 

The viscosity of the polymer or mix also controls the level of the shear stresses developed in the nip region. The level of temperature in the rubber mass, by its effect on viscosity, will also influence the level of shear stresses developed. 

Two-roll mills have been analyzed in terms of the pressure distribution and velocity profiles created between the rolls [95], the shear imposed on fluid elements exposed to these conditions in the nip region [129] and their resulting efficiency as dispersive mixing devices [130,131]. An earlier mathematical model was proposed to describe the dispersive mixing process of carbon black in rubber on roll mills, through consideration of agglomerate size distribution and... 

Attempts to correlate structure and spectra in the case of di- and polysubstituted compounds have again been largely unsuccessful. The spectra of various dialkylbenzo[6]thiophenes are very similar to those of monoalkyl derivatives, except that the bathochromic shift relative to benzo[6]thiophene is more pronounced.104 134-187 It is claimed that the sharp peaks in the 290-305 nip. region of the spectra of 5-substituted 2,3-dialkylbenzo[6]thiophenes are replaced by points of inflection in similar 6-substituted compounds, allowing such isomers to be distinguished.81104 136 Several 2-acetyl- and 3-acetyl-... 

Figure 6.26 Schematic diagram of a two roll calendering system in the nip region.

Figure 6.28 Comparison of theoretical and experimental pressure profiles [15], The experiments were performed by Bergen and Scott [2] with roll diameters of 10 in, a gap in the nip region of 0.025 in and a speed U =5 in/s. The measured viscosity was 3.2 x 109 P (Poise, 1 P=0.1 Pa-s).

Fig. 6.22 The nip region of the two-roll geometry, with radii R. A rectangular coordinate system is placed at the midplane in the gap between the rolls connecting the two roll centers.

The numerical analysis domain can be reduced to the one-pitch segment shown in Fig. 10.19 and further reduced to the midregion containing the intermesh zone, based on the assumption that the flow is fully developed far from the intermeshing zone. In other words, the channel flow region, which is far from the nip region, is omitted. The boundary conditions used were the flow at cross sections A and B are fully developed and obtained... 

With regard to constitutive equations, White (13) notes that, in view of the short residence time of the polymer in the nip region (of the order of magnitude of seconds), it would be far more realistic to use a constitutive equation that includes viscoelastic transient effects such as stress overshoot, a situation comparable to that of squeezing flows discussed in Section 6.6. 

An estimate of the fraction, f, of material lost from the nip region. Typically f is in the range 0.05 to 0.25 depending on machine design and wear. 

He provided the technical rationale to calculate the nip pressures in the nip region. He showed that material trapped in a volume between arc-length segments AL must be compressed to volume Vg between the same arc-length segments. The relationship requires that the bulk densities Je in volumes Va, Ve be related by Eq. (3) (Fig. 3) ... 

Fig. 13 Pattern of gas escape from roll nip region. (From Ref., courtesy of the Institute for Briquetting and Agglomeration.)...

The final feed screw will determine the force with which the powder is delivered to the nip region of the compactor This again can result in starving the rolls or precompacting the powder. The rate should be high enough to provide sufficient friction with the roll so it can pull the material in yet not so high as to bog down the rolls. 

Nip Region Powder begins to pre-densify as it is fed into the roll gap region where powder particles arc rearranged and densificd, and plastic deformation occurs. 

The Galerkin finite element method is successfully applied to flow in a relatively simple element of roll coating symmetric film-splitting in the nip region between smooth, rigid, counterrotating rolls. 

In roll coating arrangements of the sort shown in Figure 1, liquid is carried by the pick-up roll into the nip region between it and the applicator roll. Beyond the nip the liquid is, in effect, stretched and split into two films, each carried off... 

Friction between the roller, die, and material as well as interparticle friction in the mass to be pelleted are responsible for the pull of feed into the nip region and for densification. Smooth surfaces may result in slip and low interparticle resistance to flow will result in a more or less pronounced tendency of the mass to avoid the squeeze (back-flow), thus reducing densification and potentially choking the machine (see above and Figure 321). While the first problem can... 

The frictional and deformation characteristics of the feed play a decisive role in the ability of a pellet press to pull the material into the nip region between the roller(s) and die, compress it, and extrude agglomerates of good quality. A measure for acceptable feed consistency is the slip between the roller(s) and die. Therefore, a recent patent proposes to measure the slip or its rate of change and correct the feed composition based on this information. 

Figure 2. Schematic of the nip region indicating the charging of the toners and the magnetic field lines when a pole is in the nip.

Shear rates during calendering range typically from 10 to 10 sec. The pressure and forces encountered within the nip region are affected by the characteristics of the material (rheology,... 

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