substrate 12 is moved by a conveyor 14 beneath the print head clusters in this embodiment conveyor 14 supports substrates as wide as one to four feet and is capable of moving at speeds as high as 1600 feet minute each of print head clusters 10 include an array of jetting assemblies each of which is connected to one or more ink reservoirs print head clusters 10 print text or images upon the substrate as it passes beneath the print head clusters print head clusters 10 are supported by and movable on one end of a mounting rack 16 positioned over conveyor 14 and attached to a support post 18 an enclosure 19 is positioned between print head clusters 10 and just above the surface of conveyor 14 to define a conditioning print zone 21 the conditioning print zone provides an environment favorable to the nozzles of the print head clusters such that their reliability is improved enclosure 19 includes an inlet 20 connected to a source of material to be introduced in the enclosure for example in one embodiment the source of material is vaporized water for providing a desirable level of humidity in the print zone providing an environment with greater humidity can extend time for the deposited ink to dry and also prevent the nozzles from becoming dry when a nozzle has not jetted ink for a substantial amount of time there is a risk that ink will dry and eventually the nozzle will become clogged a humid environment addresses this problem and can improve the u201c nozzle open time u201d the.