A Basic Understanding of Chill Rolls

Application: The fundamental knowledge from a basic understanding of roll design, function, and construction can help when purchasing a new roll, evaluating a new product or process, and troubleshooting.

A chill roll primarily acts as a heat transfer and finishing device in the extrusion of flat sheet and cast film and in coating operations. Although these processes are different, the design criteria are the same—roll load, face deflection, and heat removal rate. The functional aspects of a roll will vary considerably from process to process. The process and product predicate roll construction, materials, and surface finish. All these factors require consideration to achieve a properly designed and functioning chill roll.

The first primary design principle is roll load in pounds per linear inch. Roll load is the amount of load applied to a web by an air or hydraulic actuating system. Equations are available in the paper to calculate roll load. Calculation of deflection is also possible using an equation from the paper.

Heat Removal Rate
The primary purpose of a chill roll is to remove heat from the product before any downstream operations. Several key factors are necessary to determine the heat removal or cooling capabilities of a roll used in extrusion operations—polymer type, maximum extruder output, melt temperature at the die exit, desired temperature off the roll, and specific heat of the polymer. These factors allow calculation of the heat that a specific set of process conditions can remove. Before making a significant process change such as increasing total rate, heat removal rate therefore requires consideration. Heat removal rate is one important thermal factor. Another is coolant flow rate.

Coolant Flow Rate
The coolant flow rate determines the ability of a roll to remove the required heat and temperature variation across a roll face. Sufficient flow must occur for proper cooling. Coolant flow rate is therefore important to a chill roll. A flow meter allows comparison of actual flow rate to calculated for a determined heat removal rate.

A larger than expected temperature variation on an existing roll may indicate restricted flow in the roll or coolant system. Often, a large variation in roll face temperature will not allow the product to release properly from a roll. These locations are hot spots. They result from blockage in the cooling passages of a roll. An internal acid flush or removing the outer shell and cleaning the internal passages will remove the restrictions.

Construction
Three types of common chill roll construction are double shell spiral baffle, cooling can, and single shell. The cooling can is probably the oldest design and produces poor mechanical and thermal abilities. The single shell design has improved mechanical and thermal abilities. The internal spiral baffles provide support to the outer shell and improved coolant flow characteristics. Design of the baffles can provide increased coolant velocity through the roll to improve cooling efficiency.

Rolls must not only help size and cool the product but also provide a surface finish. The surface finish on a roll can have a high polish or be rough like sand paper. Each finish provides a desired outcome and purpose. Four common roll finishes are polished, matte, release, and engraved. The polished roll has use to produce sheet products that have a smooth, polished look. The matte finish is useful for releasing products from the roll or providing a textured surface. The release finish facilitates removal of product from a chill roll. Paper coating processors often use a glossy finish to provide release and obtain a smooth, printable surface.

Contact Mike Puhalla at mikep@newcas.com.


Subscribe to PFFC's EClips Newsletter