DEVELOPMENT OF SUBAMBIENT PRESSURES AND CAVITATION IN THE BLADE COATING PROCESS

The flow behavior during blade coating is calculated with a finite ele ment method for conditions which resemble paper coating. The calculati ons include a free surface and shear-thinning theology. A subambient p ressure, caused by the rearrangement of the velocity field, is calcula ted near the blade exit. This pressure can approach the vapor pressure of water for industrial conditions and may induce cavitation in this region. The value of this exit pressure decreases for increasing web s peeds and viscosities and for decreasing coat weights. Shear-thinning behavior of the fluid causes a drop in this exit pressure. Conditions of these kinds are known to promote operational problems during paper coating which suggests that cavitation may be an underlying mechanism for these problems.