Metal is non-porous: Unlike other substrates like wood, cardboard, and paper, metal does not absorb the ink applied to its surface. Accordingly, solvent-based inks are needed for this application, as they dry quicker than water-based inks—producing a sharp image on non-porous substrates. http://www.westwu-machine.com/automatic-marking-machine/steel-pipe-weighing-lengthing-marking-machine/