Mounted within a substantially hollow supporting and containing housing (6) are tubes (15) which communicate at one end with an ink reservoir (4) and whose opposite ent' are aligned with respective ink discharge nozzles (5) pro vided in a plate member (23) which faces the printing surfac in use. The plate (23) is mounted on the housing (6) so as no to touch the ends of the tubes (15) and with the interposition of yieldable sealing members. The tubes (15) are mounted in the housing (6) with the aid of a positioning tool the shape of which reproduces essentially the shape of the plate (23). A resin (18) is subsequently poured into the housing (6) and, after hardening, constitutes an elastic mass for retaining the tubes in the housing.