COMPARING COATING HEADS ON A PILOT COATER - A CALL FOR RELIABLE RESULTS

The study examines factors that affect evaluation of paper coating hea ds during pilot trials. While coating heads are often evaluated based on sheet gloss and smoothness, the small sample sizes obtained during pilot trials undermine the statistical reliability of these measuremen ts. Coating trials showed that the selection of a coating head indeed affects paper gloss and smoothness, but sheet moisture content, coat w eight, and color solids also affect gloss and smoothness. Thus these v ariables must be strictly controlled during pilot trials. The trials a lso showed that the choice of coating head has an effect on gloss unif ormity, with short-dwell coating heads producing less variation than t he other coating heads. However, coating coverage makes a significance contribution to both gloss level and gloss uniformity. The greater co ating coverage obtained with long-dwell coating heads produces higher gloss but also greater gloss variability than short-dwell heads. There are no accepted methods for measuring smoothness uniformity, but opti cal surface profilometry provided valid and useful information about t he structure of the coated surface.