Gravure printing transfers ink from small wells or cells that are engraved into the surface of a cylinder. The cylinder rotates through a fountain of ink. The ink is wiped from the surface by a doctor blade. The cup-like shape of each cell holds the ink in place as the cylinder turns past the doctor blade.
The example below shows a single quadrangular cell and a series of 3D images of a single pyramidal gravure cell being filled. Notice in the 3D images that the coating bead advances due to the action of the blade faster than the contact line can move into the cell. The result is a trapped air bubble, which partially ruptures leaving a partially filled cell.