As a rule of thumb, the wet thickness of a coating is estimated to be half of the gap between blade edge and substate surface in the blade coating or similar coating process.
Publication (FIBRES & TEXTILES in Eastern Europe 2010, Vol. 18, No. 1 (78) pp. 79-83.) gives a good calculation for the above statement.
The above figure schematically shows the process of blade coating (symbols showing in the figure). In a simplified view, the wet coating thickness is determined by 5 variables: (1) knife-substrate distance (h), (2) substrate speed (u), (3) viscosity(η), (4) density (ρ) and (5) surface tension (σ) of the coating material. Assuming the coating material is moving in the x direction, the following equation can describe the process.
For Newtonian liquid coating materials, the speed is obtained by equation by integration of the above equation:
Therefore, the total amount of coating material, Q, going through the gap in units of length and time can be calculated by the following equation:
The wet thickness W is Q/u and it can be calculated by the following equations.
Therefore, the wet film coating can be about half of the blade-substrate gap for a quick rough estimation.