Abstract:
A droplet generator for a continuous stream inkjet print head has an elongate cavity for containing the ink and nozzle orifices in a wall of the cavity for passing the ink from the cavity to form jets. The nozzle orifices extend along the length of the cavity. An actuator is disposed so as to address the face of the cavity opposite the wall to vibrate the ink in the cavity such that each jet breaks up into ink droplets at substantially the same predetermined distance from the wall. The actuator has a plurality of constituent sub-actuators each capable of independent vibration. The combined coverage of the face of the cavity by the sub-actuators is such that the jet break up can be achieved without resonance of the cavity in its dimension from the wall to the face.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a droplet generator for a continuous stream ink jet print head. 
   More particularly the invention relates to such a generator comprising: an elongate cavity for containing the ink; nozzle orifices in a wall of the cavity for passing ink from the cavity to form jets, the nozzle orifices extending along the length of the cavity; and an actuator disposed on the opposite side of the cavity to the nozzle orifice wall for vibrating the ink in the cavity such that each jet breaks up into ink droplets at substantially the same predetermined distance from the wall, i.e. such that there is uniform jet break up. 
   Droplet generators of the above kind are disclosed in U.S. Pat. No. 4,587,528 and U.S. Pat. No. 5,502,473. However, in order to achieve uniform jet break up, these generators must be operated at a frequency at which the ink cavity is resonant in its vertical dimension, i.e. from the nozzle orifices to the actuator. In other words, it is necessary that at operating frequency an integer number of half wavelengths fit precisely into this dimension. Consequently, a very high accuracy is required of the physical dimensions of the structural components of the generator. Further, very little stray is permitted in operating parameters of the generator such as ink composition and temperature. 
   WO-A-98/51503 also discloses a droplet generator of the above kind, which generator is able to achieve the required uniform jet break up without resonance of the ink cavity in its vertical dimension. This is done by arranging for the actuator to address the full or complete area of the face of the ink cavity opposite the nozzle orifice wall. The actuator addresses the ink in a piston-like manner, i.e. all points across the actuator face that addresses the ink vibrate vertically in phase and with the same amplitude. A drawback with this droplet generator is that it is difficult to achieve the required precise piston-like motion of the actuator at all points along the actuator&#39;s length. This problem increases the longer the ink cavity (typically 50 mm and above) and the higher the frequency of operation (typically 100 kHz and above). 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided a droplet generator for a continuous stream ink jet print head comprising: an elongate cavity for containing the ink; nozzle orifices in a wall of said cavity for passing ink from the cavity to form jets, said nozzle orifices extending along the length of said cavity; and actuator means disposed so as to address the face of said cavity opposite said wall to vibrate the ink in the cavity such that each said jet breaks up into ink droplets at substantially the same predetermined distance from said wall, said actuator means comprising a plurality of constituent sub-actuators each capable of independent vibration, the combined coverage of said face of the cavity by said sub-actuators being such that the said jet break up can be achieved without resonance of said cavity in its dimension from said wall to said face. 
   Preferably, each said sub-actuator includes a holding-part whereby it is held by said generator, the dimension of said holding-part in a direction along the cavity being less than the dimension in the same direction of an ink addressing face of the sub-actuator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A droplet generator in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIGS. 1 and 2  are side and end views respectively of the generator; 
       FIG. 3  is a perspective view of a plurality of sub-actuators of the generator secured within a sealing and holding plate of the generator; 
       FIG. 4  is a perspective view of one of the sub-actuators; 
       FIG. 5  is a perspective view of a first alternative sub-actuator to the one of  FIG. 4 ; 
       FIG. 6  illustrates incorporation of the first alternative sub-actuator into the droplet generator; and 
       FIGS. 7 and 8  are perspective views of second and third alternative sub-actuators respectively. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1  to  4 , the generator comprises a polyetheretherketone manifold  1 , a nozzle carrier  3  secured to the underneath of manifold  1 , a top plate  5 , and, held by plate  5 , an actuator in the form of six sub-actuators  7 . Nozzle carrier  3  comprises a stainless steel element  9  defining therein a ‘V’ cross-section channel, and, bonded to element  9 , a stainless steel foil sheet  11 . Sheet  11  contains a line of nozzle orifices  12 , and is so bonded to element  9  that this line runs along the length of the open apex of the ‘V’ cross-section channel of element  9 . Each sub-actuator  7  comprises a piezoelectric driver  13 , a stainless steel head  15  below driver  13 , and a brass tuning mass  17 . 
   An elongate ink cavity  19  is defined by the lower faces  21  of sub-actuators  7 , and the interior faces  23 ,  25  of element  9  which define the ‘V’ cross-section channel of element  9 . ‘O’ rings  27 ,  29  provide an ink seal respectively between element  9  and manifold  1 , and between manifold  1  and top plate  5 . Channels (not shown) are provided in manifold  1  to communicate with cavity  19  to supply ink under pressure thereto and bleed air/ink therefrom. 
   A lower part  31  of the head  15  of each sub-actuator  7 , forms a shoulder  33  with the remainder of the sub-actuator, and this lower part, as it approaches ink cavity  19 , flares outwardly to each side along the length of the cavity. This part  31  of each sub-actuator  7  will be referred to as the foot of the sub-actuator. The ink in cavity  19  is addressed by the lower face  21  of the foot  31  of each sub-actuator. Each lower face  21  comprises an elongate rectangle. 
   In the droplet generator, sub-actuators  7  are disposed so that their rectangular lower faces  21  lie in the same plane, in line, along cavity  19 , the longer sides of each face extending along the cavity, the shorter sides being closely adjacent and parallel. Each end of cavity  19  is overlapped by the sub-actuator  7  disposed thereat. This overlap is referenced  35 . Sub-actuators  7  overlap the sides of cavity  19  along its entire length. This overlap is referenced  37 . 
   The remainder of each sub-actuator  7  above foot  31  is in the form of a cylinder  39 . Each cylinder  39  is push fitted into a respective hole in top plate  5 , which hole has a diameter slightly less than that of the cylinder. Plate  5  is made of a compliant material so as to form an ink tight seal around each cylinder  39 . Such sealing obviates the need for ‘O’ rings or gaskets, the use of which would be relatively complex given the number of sub-actuators and the tight space requirement. Plate  5  should not be so compliant that it distorts under the pressure applied to ink cavity  19 . Further, plate  5  should hold sub-actuators  7  sufficiently securely that they are not pushed out under ink cavity pressure. Plate  5  is suitably made of a plastics material such as polyetheretherketone or Delrin. When fitted, the shoulder  33  of each sub-actuator abuts against the bottom face of plate  5 . Such abutment further prevents the pushing out of sub-actuators  7  under ink cavity pressure. Such abutment may be dispensed with, i.e. cylinder  39  may extend lower than the bottom face of plate  5 . 
   The length of the lower face  21  of each sub-actuator  7  is significantly greater than the diameter of each cylinder  39 . This enables only a narrow gap  41  to be present between faces  21 , whilst at the same time providing sufficient separation of the cylinders  39  to enable proper individual securing of sub-actuators  7  in top plate  5 . In this regard, if the holes in plate  5  are too close together, the distortion in plate  5  caused by the push fitting will be communicated from one hole to the next causing failure of the push fit seals. Further, if the holes are too close together, in operation of the generator, too much vibration will be communicated from one sub-actuator to the next. 
   At the frequency of operation of the generator, each sub-actuator  7  has a vertical thickness resonance at which all points across its lower face  21  vibrate vertically in phase and with the same amplitude, i.e. At which lower face  21  is driven in contact with the ink in cavity  19  in piston-like manner. Each sub-actuator  7  is held by top plate  5  at a position along its length corresponding or close to a stationary node in its resonant vibration. Sub-actuators  7  are driven in synchronism so that they behave collectively as a single piston-like actuator having a lower face that extends the full length and width of ink cavity  19 , i.e. To cavity  19  sub-actuators  7  appear as a single piston actuator extending its fill length and width. 
   Cavity  19  is shaped so as to provide a steady and essentially unidirectional flow of ink to nozzle orifices  12 . The reducing surface area in the direction of wave travel (i.e. From the lower faces  21  of sub-actuators  7  to nozzle orifices  12 ) causes an increased acoustic pressure at the apex of the ‘V’ cross-section channel as compared to that at lower faces  21 . 
   The advantage of the simulation by sub-actuators  7  of a single piston actuator covering the full cavity, is that the cavity need not be resonant in its vertical dimension at operating frequency. This facilitates a relaxation in the required accuracy of the physical dimensions of the structural components of the generator. It also permits a greater stray in operating parameters of the generator such as ink composition and temperature. 
   The combined coverage of the top face of cavity  19  by sub-actuators  7  must be sufficiently great that non-resonant operation of the cavity is feasible. If the coverage is not enough, it will not be possible to achieve the same acoustic pressure at all nozzle orifices along the cavity, without operation of the cavity at resonance. Thus, the gaps  41  between lower faces  21  of sub-actuators  7  must be sufficiently narrow (significantly less than a quarter wavelength in the ink), and the sides and ends of cavity  19  must be sufficiently closely approached by sub-actuators  7 , ideally overlapped thereby. In this regard, and as explained previously, in the generator described by way of example, gaps  41  are able to be narrow by virtue of each sub-actuator lower face  21  being significantly longer than the diameter of each sub-actuator cylinder  39 . Further, by having an ink interface in the gap  41  between sub-actuator feet  31 , truly independent sub-actuator vibration is still possible at very small inter-sub-actuator distances. This is to be compared with a more solid interface where truly independent vibration breaks down at appreciably longer inter-sub-actuator distances. 
   It is advantageous to use a number of independently vibratable sub-actuators rather than a single actuator. When using a single actuator it is difficult to achieve the required precise piston-like motion at all points along the single actuator&#39;s length. When using a number of sub-actuators a finer control is possible whereby the vibration of each sub-actuator can be individually adjusted or trimmed so that at all points along the cavity there is the required piston-like motion. In particular, all sub-actuators  7  would be driven with the same frequency, and the phase and amplitude of the driving signal supplied to each sub-actuator would be adjusted, as necessary, so that the sub-actuators all vibrate in phase and with the same amplitude. This compensates for slight differences in the resonant frequencies of the sub-actuators due to manufacturing tolerances. 
   Referring now also to  FIGS. 5 and 6 , first alternative sub-actuator  51  has a foot  53  which, as it approaches ink cavity  55 , tapers in its dimension across the cavity. This is to be compared to sub-actuator  7  of  FIGS. 1  to  4 , the foot  31  of which is of constant dimension across the ink cavity. Otherwise, sub-actuator  51  is the same as sub-actuator  7 . The inner surfaces  57  of manifold  59  slope correspondingly to the taper of foot  53 . 
   Otherwise, the incorporation of alternative sub-actuator  51  in a droplet generator is the same as for sub-actuator  7 . It is to be noted from  FIG. 6  that the overlap  61  by sub-actuator  51  of the sides of ink cavity  55  is much reduced as compared to this overlap by sub-actuator  7 . 
   Referring also to  FIG. 7 , second alternative sub-actuator  71  has a foot  73  which, as it approaches the ink cavity, is of constant dimension both along and across the cavity. Thus, the gap between the feet  73  of sub-actuators  71  is of constant size over the height of the feet. 
   Referring also to  FIG. 8 , third alternative sub-actuator  81  has a foot  83  which, as it approaches the ink cavity, flares outwardly both in its dimension along the cavity and in its dimension across the cavity. When alternative sub-actuator  81  is incorporated in a droplet generator, the inner surfaces of the generator manifold to each side of the cavity, slope correspondingly to the flare of feet  83  across the cavity, i.e. the cavity sides become further apart the lower the position in the cavity.