Mounted within a substantially hollow supporting and containing housing are tubes which communicate at one end with an ink reservoir and whose opposite ends are aligned with respective ink discharge nozzles provided in a plate member which faces the printing surface in use. The plate is mounted on the housing so as not to touch the ends of the tubes and with the interposition of yieldable sealing members. The tubes are mounted in the housing with the aid of a positioning tool the shape of which reproduces essentially the shape of the plate. A resin is subsequently poured into the housing and, after hardening, constitutes an elastic mass for retaining the tubes in the housing.