Lateral Force Under the Blade Tip in Bevelled Blade Coating, 1991 Coating Conference Proceedings

Even if remarkable advances have been made in our understanding of the blade coating process (1,2,3,4,5,6,7,8,9,10), the proposed theories do not satisfactorily account for all the observed phenomena. In theoretical treatments of blade coating the main emphasis has been on the normal force, that is the force perpendicular to the paper surface transmitted from the blade tip via the coating layer or directly to the base paper. The lateral force has either been omitted, assumed negligible or it has been implicitly assumed that there is a fixed relationship between normal force and lateral force which depends only on the blade angle.

It is by no means obvious, however, that the lateral force is negligible and can be omitted. The viscosity of a coating colour can be rather high, especially in transient phenomena, and a preliminary calculation indicates that the lateral force transmitted by the shear stress in the coating layer under the blade could be significant.

The goal of this investigation was to clarify the role of lateral force in blade coating with the main emphasis in SDT application.

It is perhaps pertinent to clarify what we mean by the lateral force. Figure 1 is a schematic representation of the force equilibrium in the blade tip. The force applied by bending the blade is balanced by three forces:

1. The normal force perpendicular to the bevel surface

2. The lateral force under the blade parallel with the bevel surface

3. The impact force caused by the change in coating colour momentum.

Lateral force in this investigation means force no. 2. It is transferred through the coating layer to the base paper and has a balancing force on the paper surface. Force no. 3 is not transferred to the base stock and is balanced by stresses in the blade.