Patent Application
20090040276
The present invention relates to the field of inkjet printers. In particular, the invention concerns printheads with heater elements that vaporize ink to eject an ink droplet from the nozzle.
The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference.
The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 to Stemme.
Thermal inkjet printheads are traditionally prone to overheating. The rapid successive vaporization of ink during printing can build up heat in the printhead. If too much builds up in the printhead, the ink will boil in an uncontrolled manner. This heat is removed from the printhead either by an active cooling system or with heats sinks and the use of small nozzle arrays. The overheating problem has limited the firing frequency of the nozzles and printhead size, both of which reduce the print speed.
The Applicant has developed a range of pagewidth printheads that overcome the problem of excess heat generation. The large pagewidth arrays and high nozzle firing frequencies provide print speeds in excess of 60 pages per minute at full color 1600 dpi resolution. These printheads avoid excess heat generation by reducing the energy used by the heaters to eject the drops of ink. The heat input to the printhead by the heaters is removed from the printhead by the ejected drops of ink.
One aspect of reducing the energy required to eject drops of ink is a reduction in the mass of the ejected drop, and hence the volume of the drop. The Applicant's ‘self cooling’ printheads eject drops of about 1 pl to 2 pl (pico-liters). Unfortunately drop volumes this small are susceptible to trajectory misdirection. The trajectory of the ejected drop is particularly sensitive to the nozzle geometry and the shape of the bubble generated by the heater element. It will be appreciated that any misdirection of the ejected ink drops is detrimental to print quality.
Fluidic symmetry around the heater is not possible unless the heater is suspended directly over the ink inlet. The Applicant has developed printheads with this arrangement (see U.S. Pat. No. 6,755,509 filed Nov. 23, 2002—Our Docket MJT001US), however there are production efficiencies and nozzle density gains available if multiple ink chambers are supplied from a single ink supply channel through the supporting wafer. This requires that the individual chambers are supplied with ink through lateral inlets—that is, inlets extending parallel to the planes of the heaters and the nozzles. As the heater is laterally bounded by the chamber walls except for the ink inlet, the bubble generated by the heater is distorted by this asymmetry. The inlet can be lengthened and or narrowed to increase its fluidic resistance to back flow caused by the bubble. This will reduce the fluidic asymmetry caused by the inlet but also increase the chamber refill times because of the higher flow resistance.
Accordingly, the present invention provides a printhead for an inkjet printer, the printhead comprising:
the ejection aperture has a centroid that is offset from the centroid of the two dimensional shape of the heater element in a direction parallel to the plane of the ejection aperture.
The invention is predicated on the realization that misdirected drop trajectories caused by asymmetries in the vapor bubble can be compensated for by offsetting the nozzle centroid from the heater centroid. The ordinary worker in this field will understand that the centroid is a point at the geometric centre of a two dimensional shape.
The vapor bubble generated by the heater can be asymmetrical because of the configuration of the heater relative to the nozzle and the ink inlet. As the bubble grows, it not only forces ink from the nozzle but also creates a small back flow of ink through the ink inlet. The back flow is usually negligible compared to the ink ejected because the fluidic drag resisting flow out of the inlet compared to flow out of the nozzle is very high. If the ink inlet is at the side of the chamber (that is, the inlet flow is parallel to the plane of the heater and the nozzle), the small back flow of ink allows the bubble to skew towards the ink inlet. The pressure pulse through the ink is likewise skewed and meets one side of the ejection aperture slightly before the other side.
Preferably, the printhead further comprising a plurality of chambers in fluid communication with each of the nozzles respectively, each of the chambers adapted to hold printing fluid in contact with each of the heater elements respectively, wherein the chamber has a printing fluid inlet that defines a fluid path that extends parallel to the plane of the heater element. In a further preferred form, the chambers defines walls extending generally transverse to the plane of the heater element, the walls surrounding the heater element except for an opening defining one end of the printing fluid inlet. In a particularly preferred form, the ejection aperture centroid is offset from the centroid of the two dimensional shape of the heater element in a direction away from the printing fluid inlet.
Preferably, the ejection aperture centroid is offset from the centroid of the two dimensional shape of the heater element in a direction parallel to the major axis of the ejection aperture.
Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:
Referring to
The heater 18 is a thin rectangular strip suspended as a beam over a trench 24 in the floor of the chamber 16. The centroid of the top surface rectangle shape of the heater 18 is simply the intersection of the rectangle's diagonals. The nozzle 14 is an ellipse so the centroid is simply the intersection of the major and minor axes. As described in the above referenced U.S. Ser. No. 11/246,687 (Our Docket MNN001US) filed Oct. 11, 2005 the roof layer 22 is formed by CVD of silicon nitride and the nozzles 14 subsequently etched. Hence the centroids of the nozzle and the heater are closely aligned.
In
The side 32 is spaced from the centre line 50 of the nozzle 14. The surface tension acting on the stem has a component acting normal to the centre line 50. As a result, the centre of mass 46 of the drop 28 is pulled away from the centre line 50 until the stem 30 breaks. The drop trajectory 48 now deviates from the centre line 50 by the angle A.
The invention takes the asymmetry of the bubble into account and offsets the heater and nozzle accordingly.
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