Mill Experiences with a New Improved Short Dwell Coating Head, 2002 Coating Conference Proceedings

T. Kiiha, P. Hakanen, J. Kangas and H. Sunde

Short dwell coating heads are mainly being used today in production of lightweight coated (LWC) papers. Many world-class machines produce these grades at speeds over 1500 mpm. It has been shown in many instances that short dwell streaking deteriorates the quality of short dwell coated sheets at those speeds. Short dwell streaking is mainly associated with flow instability of the coating color in the coating pond that faces the paper web.

Short dwell streaking is manifested in a coated sheet as random variations of coat weight. These short dwell streaks are clearly evident when looking through the coated sheet with a back light. However, it is quite difficult to quantify their severity. The most objective method of measuring is a variability analyzer. It has been verified in many past publications that small-scale residual variation of coated paper ash content can be used as a quantitative measure of streaking tendency.