Using calender rolls with unequal speeds or diameters to process materials, providing superior control over sheet curvature, thickness, and surface properties.
To enable asymmetric calendering, the traditional balanced configuration of the calender nip is intentionally disrupted by altering three primary parameters:
- Rotational Speed (Kinematic Asymmetry) – involves operating adjacent rollers at different surface velocities. The speed differential is defined as the “friction ratio,” which introduces controlled shear energy and frictional forces into the material. This speed parameter is adjusted depending on the application to achieve outcomes like intense homogenization, breakdown of polymers, or the safe detachment of ultra-thin films from roll surfaces.
- Roll Radii (Geometric Asymmetry) – this setup uses rollers that possess different diameters. Changing the radii alters the effective geometry of the nip, which directly influences the pressure distribution and separating forces acting on the material.
- Surface Temperature (Thermal Asymmetry) – entails maintaining the rollers at differing temperatures. While temperature differences can be used to manipulate a material’s rheological and aesthetic properties, this parameter must be carefully managed. Excessive thermal asymmetry causes non-uniform softening of the material, which can lead to thickness fluctuations, uneven surface gloss (the “yin-yang effect”), and defects like “orange peel” or localized sticking.