Definition of coating thickness
In defining coating thickness, we have to differentiate between
- Theoretical wet film thickness
- Practical wet film thickness and
- Dry film thickness
Theoretical wet film thickness
The theoretical wet film thickness is the height of the applicator blade above the substrate.
Practical wet film thickness
However, when the applicator is drawn across the substrate a shearing effect is observed in the film and this is dependent on the flow properties of the coating material and the speed with which the applicator is moved. The effect of this is that the practical wet film thickness is less than the theoretical. (In general, practical wet film thickness = approx. 60 - 70% of theoretical wet film thickness).
Dry film thickness
The resulting dry film thickness depends on the reduction in volume during drying and is generally approx. 40 - 50% of the practical wet film thickness.
ERICHSEN Film Applicators
If the storage space for the coating material on a bar film applicator is wider than the recess through which the coating material flows, the edge zone of the applied film will be thicker by an uncontrolled amount. This comes into contact with the blade guide and so an unknown measurement will be added to the theoretical film thickness. This source of inaccuracy is eliminated on ERICHSEN Film Applicators (see Fig. 1).
Fig. 1: Film Applicators
