Absorption Driver
Force applied to a liquid in contact with a porous substrate creates a pressure differential that drives the rate of vehicle separation. In modern printing, dynamic capillary pressure controls how quickly ink solvents penetrate the coating layer of paper or board as the web moves through the press. This pressure changes rapidly as the liquid and the substrate make initial contact under force.
The rate of penetration determines whether the pigment remains on the surface or sinks too deep.
Nip Mechanism
Nip pressure initially forces the ink vehicle into the largest pores of the paper, but as the mechanical pressure decays, capillary action becomes the primary transport mechanism. During this phase, dynamic capillary pressure is driven by the surface tension of the fluid and the effective radius of the pores. Because the contact time in a high-speed flexographic or offset press is often less than ten milliseconds, the fluid cannot reach an equilibrium contact angle.
The dynamic nature of the wetting angle during this brief interval restricts the volume of liquid that can be absorbed before the paper leaves the printing zone. This fast transition prevents the ink from spreading excessively across the surface.
Coating Performance
Deep penetration of the fluid vehicle reduces the optical density of the print. If the dynamic capillary pressure is too high, ink show-through occurs on the reverse side of the paper. Papermakers adjust the pore size of paper coatings using fine minerals to control these capillary effects.
Proper balance ensures that the ink dries quickly without compromising the final image resolution.
