Coat weight control in bent blade mode

Abstract

Bent blade metering is the most common method used in board coating. As board machine speeds today have increased, new requirements have been set on blade geometry and accuracy of blade adjustments. A good understanding of the blade parameters and physical phenomena around the blade tip area is, therefore, necessary. Automatic coat weight profile adjustment using feedback from the coat weight measurement system is a great tool to improve the cross-direction coating uniformity and reel condition. In bent blade coating, however, the parameters for adjusting the cross-direction profilers need to be modified from those used in stiff blade coating. At high board coating speeds with bent blade, good coat weight control will require tuning of the blade parameters. To be able to reach a given coat weight at a high speed with a low tip angle, a high mechanical force is needed to control the high dynamic forces that are created in the blade tip area. The mechanical force can be increased either with a thicker blade or with a shorter free stick-out (i.e., shorter distance between the blade support bar and the blade tip). A higher tip angle can be used to decrease the dynamic forces to keep the coat weight in target; however, coating coverage and uniformity of the coating layer will then be reduced. Furthermore, blade wear is more severe at higher tip angles. For cross-direction profiling, a shorter free stick-out is suggested as a better approach than use of a thicker blade. The coat weight profile response and accuracy improves with a shorter stick-out as thinner blade with more flexibility can be used. Three alternative coat weight control methods for bent blade operation are discussed: blade load adjustment, beam angle adjustment, and load adjustment at constant tip angle. For effective coat weight cross-direction controls, the blade operating mode and parameters need to be chosen in accordance with the speed and coat weight range at which the coater is operating.