On-chip fluid recirculation pump for micro-fluid applications
Lexmark International, Inc.
There have been a lot of developments of recirculation architectures within printheads since the launch of the Xaar system. Already we have reviewed Canon and Hewlett-Packard’s proposals for thermal ink jet recirculation, and here is Lexmark’s.
Recirculation systems enable fresh ink to be brought to the actuator chambers and nozzles. This is particularly important for fixed array printheads. With scanning heads, ink can be ejected from each nozzle during the period when the head is outside of the substrate width. This is not possible for fixed arrays.
With small nozzle sizes and relatively volatile aqueous inks, evaporation of the ink solvent from the nozzle may cause an increase in local ink viscosity and hence a deterioration in jetting properties within a second or two of becoming idle. To maintain nozzle performance this means ensuring each nozzle fires drops within that period to bring fresh ink to the nozzle.
Nozzle failures are often more noticeable for fixed arrays as it is not possible to cover up defects by multiple passes. Therefore it is necessary to be even more careful in keeping nozzles working with a fixed array. To keep nozzles working, drops can be fired onto web substrates between pages. The amount of ink consumed can be considerable. Lexmark estimates heavy users, printing large multi-page jobs, may lose 15% of the ink to nozzle maintenance. However light users printing only short jobs infrequently may consume 80% of the ink on maintenance. As well as excess ink consumption the printhead life is shortened, as only perhaps 20% of the actuator cycles are used for printing. The use of recirculation within the printhead can therefore eliminate a lot of external nozzle maintenance, ink wastage and increase the working life of the printhead.
Various schemes are proposed, but all involve the passage of ink through the actuator chamber from one side to the other, driven by a pump. Here the ink flows in from the manifold via inlets 60, through channels 65 to heater chambers 22. Heaters 12 generate bubbles to fire drops from nozzles 24. The ink circulation path continues via channels 75 and through outlet 60, driven by pump 50. The pump is another heater, similar to the drop generators, but of course with no corresponding nozzle.
When the pump heater is actuated, ink flows preferentially through the large area outlet rather than back to the heater chambers. When the drop generator heaters are actuated, ink preferentially flows from the nozzle rather than back to the inlet or to the recirculation outlet.
Note that the flow doesn’t have to be continuous. The printer can determine which nozzles have not been fired for the past one or two seconds, and then drive the pumps corresponding to just those nozzles.