Methods to Reduce the Number and Severity of Scratches Occurring While Blade Coating a Smooth Basesheet, 2002 Coating Conference Proceedings

G. Fugitt and R. Crabtree

One of Mead's mills was considering making equipment modifications that would significantly decrease the roughness of the paper it produced. One of the biggest concerns associated with smoother rawstock was coater runnability and, more specifically, an increase in blade scratches on the coater. A source of smooth base paper, comparable to what the modified equipment would produce, was available. A series of pilot coater trials were conducted to investigate the source of scratches, characterize the scratches, and evaluate a series of potential methods to reduce scratches. A total of 75 pilot coater runs were made with varying blade thickness, blade angle, coating solids, coating temperature, coat weight, and starch viscosity. For each condition, a roll of paper was produced at 3800 fpm (1160 m/min). The paper made between the 7th and 8th minute of running on the coater was analyzed for scratches. Each scratch was identified, measured, and judged as to whether it would be removed by supercalendering. In all, 285,000 feet (86.4 km) of paper were visually inspected and 5730 scratches were identified and measured. The cumulative length of the scratches was 218,000 ft (66.1 km) with the average scratch length being about 38 ft (11.5 m). The smallest scratch identified was only 0.25 inches (.63 cm) in length.

For one of the rolls made under control conditions, the beginnings and ends of scratches were located and characterized. Fibers in the coating were the predominant source of scratches. Blade thickness had the greatest influence on scratches, with increased thickness producing fewer scratches. Conventional wisdom, for scratches caused by particulate contamination, did not apply. Bent blade application actually increased the number of scratches. Other variables such as blade angle, coating solids, coating viscosity, and coating temperature had lesser, but significant effects on scratches.