Calendering shear thinning

A first model of the calender nip flow has been presented by ArdichviUi. Further on Gaskefl presented a more precise and well-known model. Both models are very simplified, which yields that the flow is Newtonian and isothermal, and they predict that the nip force is inversely proportional with the clearance. Since mbber materials show a shear thinning behavior Ardichvilli s model seems not to be very realistic. The purpose of this section is to present a calender nip flow model based on the power law. The model is stiU being considered isothermal. Such a model was first presented by McKelvey. ... 

Figure 6.36 Calendered thickness as a function of fed sheet thickeness for a Newtonian fluid and a shear thinning fluid with n = 0.25.

A 15 cm diameter, 2 m wide calendering system is used to produce a PVC sheet with a shear thinning power law viscosity with a consistency index, m=17,000 Pa-sn and a power law index, n=0.26. The roll speed, U=6 m/s and the distance between the nips is 5 mm. 

Modeling the calendering process for Newtonian and shear thinning polymer melts. Using the RF method, Lopez and Osswald [5] modeled the calendering process for Newtonian and non-Newtonian polymer melts. They used the same dimensions and process conditions used by Agassant et al. [1], schematically depicted in Fig. 11.17. 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

Calenders with at least four to six rolls are used to fabricate thin rigid sheet where the extra nips greatly improve the surface finish of the sheet. The more popular are the four-roll inverted L calender and Z calender. The Z calender has the advantage of lower heat loss in the film or sheet because of the melts shorter travel and the machines simpler construction. They are simpler to construct because they need less compensation for roll bending. This compensation occurs because there are no more than two rolls in any vertical direction as opposed to three rolls in a four roll inverted L type calender. The speed of the calendering rolls usually differs. They operate at different speeds to provide the best performance of the melt, particularly the required shearing action (Chapter I). 

---