Shoe Calendering Process Compared to Conventional Coated Paper Board Precalendering Methods, 2001 Coating Conference Proceedings

Matti Lares, Stefan Kuni--Surface smoothness and structure are essential properties of high quality coated board grades. To achieve this, high smoothness and uniformity of the baseboard are a precondition. Surface properties have normally been achieved by using a Yankee cylinder or wet-stack calendering. Both of these methods are today reaching their limits, due to increasing machine speeds that cause runnability and capacity problems.

In the 1990s, hot soft calendering was suggested as a solution to this problem. However, in practice, it has been shown that the hot soft precalendering method cannot produce high quality coated board equal in surface structure, bulk and visual appearance to that produced by the Yankee cylinder.

The new shoe calendering process opens up further possibilities to solve these problems. By extending the nip dwell time and reducing compression pressure, structural changes in the board can be reduced drastically without compromising the surface properties. The presence of a moisture gradient is an important factor, as in all thermo-calendering processes.

The pilot work is divided into two parts. In the first part, the results of pilot machine calendering, soft calendering and shoe calendering are compared. Both the baseboard and final coated products are evaluated regarding their surface and bulk properties, offset printability and visual appearance. The main objective was to find possible correlations between printability and other surface properties of the board. The results showed that the Bendtsen roughness measured before coating gave the best correlation. In the second part of this study, the results of pilot shoe calendering are compared with those achieved in production by the Yankee cylinder. They show that the shoe calendering process can produce quality that is equal to that of the Yankee cylinder.