Abstract:
A nozzle plate apparatus for an inkjet printer has a nozzle aperture ( 8 ) which includes a plurality of elements ( 9 ) sub-dividing the aperture into a plurality of smaller apertures.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to nozzle plate for an inkjet printer having a nozzle aperture through which ink is dispensed. 
     It is known in the field of fluid mechanics and in inkjet printing technology that, if a liquid is held in an container and that container has a hole in it with the hole below the level of the surface of the liquid, the liquid will not necessarily leak from the container depending upon the parameters of the liquid and the hole. The parameters which determine whether or not the liquid leaks from the container or not are the size of the hole, the surface tension of the liquid, the surface energy of the material in which the hole is formed, the vertical height of the liquid above the hole and the force of gravity. In inkjet printer systems, for example the so-called “BubbleJet” printers, additional means are used to control the pressure which the liquid exerts on the hole, typically in the form of an open cell foam structure. The surface tension force of the liquid acting in the capillaries of the foam at the liquid/air interface on the external surface of the foam can generate a back pressure which is able to balance the weight of the liquid in the foam. 
     There can be occasions when it is beneficial to have a large hole or slit in an inkjet printer, but where it is not feasible to use an open cell foam structure in order to exert a back pressure. For example, WO-A-93-11866, PCT/GB95/01215 and WO-A-94-18011 all disclose printing methods where a large slit may be useful. 
     It is desirable therefore to provide an alternative mechanism for ink retention. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a nozzle plate apparatus for an inkjet printer having a nozzle aperture which includes a plurality of elements sub-dividing the aperture into a plurality of smaller apertures. 
     Preferably, the nozzle aperture includes a material disposed within the aperture and comprising a plurality of strands. 
     The aperture may be an elongate slit. 
     A particular feature of the invention is that one can sub-divide an aperture in a liquid containing vessel of dimensions which would otherwise normally allow liquid to escape due to gravity. Sub-dividing the aperture as proposed by the present invention effectively produces a number of small orifices of dimensions which allow the surface tension of the liquid to prevent the liquid from escaping. The invention also allows what is, in effect, a local reservoir of fluid to be retained close to the ejection location which can be used to improve the supply of fluid to the ejection location and the supply of charged particles to the ejection location. This is of use in printers of the type to which the invention relates as these are capable of operating at very high speeds. 
     The strands dividing the aperture into a plurality of apertures may be provided by the walls of a foam structure deposited in the nozzle aperture or else by individual filaments arranged substantially orthogonal to a major axis of the aperture, for example. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     One example of a nozzle plate assembly according to the present invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of an array type inkjet printhead; 
     FIG. 2 is a close-up view of the nozzle plate aperture; 
     FIGS. 3 and 4 are cross-sectional views through alternative embodiments; and 
     FIG. 5 is a partial perspective view of a portion of a further printhead incorporating ejection apparatus according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures illustrate printers of the type generally described in the above mentioned patent specifications. 
     In the printhead of FIGS. 1 to  4 , the printhead  1  has a flow of ink, in this case, an ink having particles which are dispensed according to the method described in WO-A-93-11866, the ink  2  flowing around an angled path  3 , 4  behind nozzle plate component  5 , 6 . The nozzle plate  6  contains a series of protruding electrodes  7  which are spaced apart from one another as seen in FIG.  2  and which project from a slit-like aperture  8 . 
     Between each of the electrodes  7  are providing nylon filaments  9  which sub-divide the slit  8  between the two component parts  5 ,  6  of the nozzle plate into segments corresponding to each of the electrodes  7 . 
     FIG. 2 shows how, under the action of the surface tension of the liquid, plural liquid menisci are formed, from which the ink is dispensed, in use, as described in the above mentioned patent specifications. 
     The example shown in FIGS. 3 and 4 has a slot  8  which is partially sub-divided by walls  10  which extend part of the way across the slot. FIG. 4 illustrates the internal structure of the foam  11  indicated generally in FIG.  3 . Between each pair of walls is an electrode  7  and in the space between the free ends of the walls  10  and the other side of the slot  8  there is provided Basotect foam  11 . The foam  11  prevents the liquid from escaping and provides a plurality of strands  12  which, when taken in association with the walls, divide the slit into a plurality of apertures  13 . FIG. 4 illustrates the strands  12  in relation to their depth in the figure by way of the darkness of the strands, darker strands being nearer the surface of the cross-section. 
     In a modification of this construction the foam could be replaced by individual strands of the type for example as shown in FIG.  2 . 
     Another example is illustrated in FIG.  5 . FIG. 5 illustrates part of an array-type printhead  1 , the printhead comprising a body  2  of a dielectric material such as a synthetic plastics material or a ceramic. A series of grooves  103  are machined in the body  2 , leaving interposing plate-like lands  104 . The grooves  3  are each provided with a ink inlet and ink outlet (not shown, but indicated by arrows I &amp; O) disposed at opposite ends of the grooves  103  so that fluid ink carrying a material which is to be ejected (as described in our earlier applications) can be passed into the grooves and depleted fluid passed out. 
     Each pair of adjacent grooves  103  define a cell  105 , the plate-like land or separator  104  between the pairs of grooves  103  defining an ejection location for the material and having an ejection upstand  106 ,  106 ′. In the drawing two cells  105  are shown, the left-hand cell  5  having an ejection upstand  6  which is of generally triangular shape and the right-hand cell  105  having a truncated ejection upstand. Each of the cells  5  is separated by a cell separator  107  formed by one of the plate-like lands  104  and the corner of each separator  107  is shaped or chamfered as shown so as to provide a surface  108  to allow the ejection upstand to project outwardly of the cell beyond the exterior of the cell as defined by the chamfered surfaces  108 . A truncated ejection upstand  106 ′ is used in the end cell  105  to reduce end effects resulting from the electric fields which in turn result from voltages applied to ejection electrodes  109  provided as metallised surfaces on the faces of the plate-like lands  104  facing the ejection upstand  106 ,  106 ′ (ie. the inner faces of each cell separator). The ejection electrodes  109  extend over the side faces of the lands  104  and the bottom surfaces of the grooves  103 . The precise extent of the ejection electrodes  109  will depend upon the particular design and purpose of the printer. 
     FIG. 5 illustrates two alternative forms for side covers of the printer, the first being a simple straight-edged cover  111  which closes the sides of the grooves  103  along the straight line as indicated in the top part of the figure. A second type of cover  112  is shown on the lower part of the figure, the cover still closing the grooves  103  but having a series of edge slots  13  which are aligned with the grooves. This type of cover construction may be used to enhance definition of the position of the fluid meniscus which is formed in use and the covers, of whatever form, can be used to provide surfaces onto which the ejection electrode and/or secondary or additional electrodes can be formed to enhance the ejection process. Additionally, the fingers  115  between the edge slots  113  serve to reduce the overall size of the aperture between the opposed covers  111 ,  112 , thus acting in accordance with the invention, to sub-divide the aperture into smaller apertures. 
     In all the examples referred to above, sub-dividing the basic aperture into plural smaller ones allows a larger basic aperture to be used (without the risk of leakage) which, in turn, allows an increased migration of material for ejection within the liquid in the device.