Abstract:
A nozzle cleaning device for an ink jet printer containing a plurality of ink reservoirs, the cleaning device including a pressure source and a valve mechanism for selectively coupling the pressure source to the ink reservoirs, wherein the valve mechanism comprises a valve housing, and a piston movable within the valve housing and defining therewith a first pressure chamber and a second pressure chamber, said first and second pressure chamber being connected to the pressure source such that during idle conditions, the pressure source is in communication with the ink reservoirs through said first pressure chamber, and during cleaning conditions and through the movement of the piston, the pressure source is in selective communication with individual ink reservoirs through said second pressure chamber.

Description:
This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 04075043.2 filed in The European Patent Office on Jan. 14, 2004, which is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a nozzle cleaning device for an ink jet printer having a plurality of ink reservoirs, the cleaning device including a pressure source and a valve mechanism for selectively coupling the pressure source to the ink reservoirs. 
   An ink jet printer typically contains a plurality of printheads, e.g. at least one for each color in the case of a multi-color printer. Each printhead has an array of nozzles through which ink droplets can be jetted out onto a recording medium, and also ink reservoirs that are in communication with each of the nozzles of the nozzle array through respective ink channels. In or adjacent to each of the ink channels an actuator, e.g. a piezoelectric actuator is provided which may be energized in order to generate a pressure wave in the liquid ink in the ink channel, so that an ink droplet is expelled from the associated nozzle. 
   In some known ink jet printing systems, the ink reservoir is maintained under a slight subatmospheric pressure in order to prevent ink from leaking out of the nozzles. 
   Since the minute nozzles tend to become clogged with impurities, it is necessary to clean the nozzles from time to time. This is normally achieved by moving the printhead to a cleaning station and then applying a positive pressure to the ink reservoir in order to flush the nozzles with ink from the ink reservoir. The cleaning station includes a gutter which faces the nozzles of the printhead for collecting the ink that is ejected out of the nozzles in the flushing step, and may also include a wiper for removing remnants of ink and dust from the nozzle face of the printhead. Another purpose of such a cleaning or flushing procedure is to remove air bubbles that may have entered into the ink channels and which can have an adverse effect on the jetting properties of the nozzles. 
   In order to limit the required capacity of the pressure source and the dimensions of the cleaning station, it is preferable that the printheads are not cleaned simultaneously but are cleaned individually by applying pressure only to one ink reservoir at a time when the nozzles connected to this particular ink reservoir are in a position facing the ink collecting system of the cleaning station. This also has the advantage that the amount of waste ink can be reduced by performing the cleaning process only for the printhead or printheads that actually need a cleaning treatment. In this situation, however, a valve mechanism is required for connecting the pressure source selectively to the ink reservoir of the printhead that is being cleaned. Such a valve mechanism adds to the complexity and costs of the system as a whole, in particular since valves of the direct-driven type ought to be employed in view of the generally small differential pressures involved. 
   U.S. Pat. No. 6,095,633 discloses a cleaning device of the type described above, in which a separate valve is provided for each of the ink reservoirs in order to controllably connect the valves to a common pressure source. 
   U.S. Pat. No. 4,870,431 discloses a cleaning device for a hot-melt ink jet printer having a rotating printhead with nozzles arranged at the outer periphery thereof for printing onto a sheet of recording paper that has been wrapped around the printhead. Here, the interior of the printhead is subdivided into a plurality of ink reservoirs each having the cross-sectional shape of a sector of a circle. Each reservoir has a supply opening in its top face, so that solid pellets of ink may be dropped into the ink reservoir where the ink is molten. By rotating the printhead, each reservoir opening may be brought into a position where it is aligned with and sealingly connected to an air supply system. When the printhead is locked in this position, pressurized air is supplied into the ink reservoir for flushing the nozzles connected thereto. The rest of the top face of the printhead is covered by a stationary lid for closing off the openings of the ink reservoirs that are not in the cleaning position. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a nozzle cleaning device wherein the valve mechanism has a simple construction and can be manufactured and assembled at low costs. 
   According to the present invention, the nozzle cleaning device includes a valve housing having a wall with a plurality of outlet ports, each of which is connected to one of the ink reservoirs, and a piston movable in the valve housing and defining therewith a first pressure chamber and an inlet port connected to a pressure source and surrounded by a seal which defines, together with the wall of the valve housing a second pressure chamber adapted to be brought into communication with a selected one of the outlet ports through the movement of the piston. 
   The ink reservoirs may be constantly connected to the respective outlet ports of the valve housing, and a pressure suitable for flushing the nozzles connected to an individual ink reservoir may selectively be applied by moving the piston into a position in which the seal surrounds the outlet port to which that particular ink reservoir is connected. The seal will thus disconnect that outlet port from the first pressure chamber and will define the second pressure chamber that is connected to the pressure source through the inlet port of the piston, over an area of the wall of the valve housing which includes only the one outlet port. 
   The device according to the present invention is thus adaptable to a large variety of ink jet printers and permits a high degree of design choice as regards the arrangement of the printheads and the cleaning station and the manner in which the printheads are moved into and out of the cleaning station. The valve mechanism has a compact and simple construction with only a single movable member, i.e. the piston. 
   The first pressure chamber may be held at atmospheric pressure and will then only have the function to prevent foreign matter from entering through the outlet ports into the ink reservoirs. However, it is a particular advantage of the present invention that the first pressure chamber may be radially used for controlling the pressure inside of the ink reservoirs that are not being cleaned. Thus, for example, a single pressure control device is sufficient for maintaining the ink reservoirs of all printheads at a slight subatmospheric pressure in order to prevent the leakage of ink from the nozzles, unless the nozzles of a particular printhead are to be cleaned. 
   In the cleaning process, the piston may be moved steadily such that the seal passes sequentially over the outlet ports in the wall of the valve housing, so that pressure pulses of predetermined length are successively applied to the individual ink reservoirs. 
   In a particularly preferred embodiment, the movement of the piston in the valve housing may be coupled with the movement of the printheads relative to the cleaning station, so that the pressure pulses will be applied at appropriate timings, i.e. when the printheads are in the correct cleaning position. The coupling of the movement of the piston with the movement of the printheads may be achieved through mechanical means such as gears, rack-and-pinion devices and the like. 
   The movement of the piston in the valve housing may be a linear displacement, a rotation or a combination of both. 
   In a particularly preferred embodiment, the piston is moved linearly in a direction in parallel with the movement of a carriage carrying the printheads relative to the cleaning station. Then, the mechanical coupling may be achieved simply by elastically biasing the piston towards one end position and utilizing the relative movement between the carriage and the cleaning station for pushing the piston through the valve housing against the biasing force. To this end, the valve mechanism may be mounted on the carriage carrying the printheads, so that one end of the piston or a piston rod engages a stationary stop when the carriage reaches the cleaning position. As an alternative, the valve mechanism may be mounted to be stationary relative to the cleaning station, and an abutment on the carriage may be used for actuating the piston. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic view of the cleaning device according to the present invention in a state immediately before a cleaning operation starts; 
       FIG. 2  is a schematic view similar to  FIG. 1  but showing a state in which a single printhead is being cleaned; and 
       FIG. 3  is a schematic, longitudinal section through a valve mechanism of the device according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As is shown in  FIG. 1 , an ink jet printer to which the present invention is applicable includes a carriage  10  that is movable, for example, linearly along a guide rail  12  so as to scan a sheet of recording paper. The drawing shows only an end portion of the guide rail  12  outside of the area of the recording paper, so that the recording paper is not visible. 
   A number of printheads  14 , four in this example, are mounted side by side on the carriage  10  and have nozzle faces  16  arranged in a common plane and facing downward in the drawing so as to oppose the recording paper when the carriage  10  scans the paper. Each printhead  14  further has an ink reservoir  18 , and the ink reservoirs of the different printheads  14  contain liquid inks of different colors. 
   The part of the printer that has been shown in  FIG. 1  accommodates a cleaning device for cleaning the nozzles in the nozzle faces  16  of the printheads  14  by flushing the nozzles with liquid ink from the ink containers  18 . In order to cause the ink to flow out of the nozzles of the printhead, the cleaning device utilizes a pressure source  20  and a distributor valve  22  serving as a valve mechanism for supplying compressed air from the pressure source  20  to the individual ink reservoirs  18 , so that the ink will be forced out through the nozzles. The ink that has been jetted out of the nozzles in this way is captured and collected in a gutter  24  which is arranged to face the nozzle face  16  of the printhead that is currently being cleaned. Optionally, the gutter  24  may be combined with a wiper assembly for wiping off remnants of ink from the nozzle faces  16  while the carriage  10  moves past the gutter. 
   The distributor valve  22  includes an outer tube serving as a valve housing  26  and an inner tube serving as a piston  28  that is slidably accommodated in the valve housing. Both, the valve housing  26  and the piston  28  are connected to the pressure source  20  through flexible tubes  30 ,  32  and  34 , respectively, which have a sufficient length and flexibility to follow the path of travel of the carriage  10  along the guide rail  12 . Through the tubes  30 ,  32  that are connected to the valve housing  26 , the pressure source  20  supplies a slight subatmospheric pressure in the order to magnitude of about −1000 Pa, for example. This pressure is normally maintained in the ink reservoirs  18  of the printheads  14  when the printheads are operating or at idle, in order to prevent the ink from leaking out of the nozzles. The tube  34  is connected to one end of the piston  28  and supplies a pulse of positive air pressure, e.g. in the order of 10 kPa, individually and successively to the ink reservoirs  18  of the various printheads  14  at appropriate timings so as to flush the nozzles of each printhead when this printhead moves past the gutter  24 . 
   The supply of air to the ink reservoirs  18  of the individual printheads  14  is controlled by the movement of the piston  28  relative to the valve housing  26 , and this movement is controlled by the movement of the carriage  10  relative to the guide rail  12 . 
   As is shown in  FIG. 1 , the piston  28  projects beyond the ends of the valve housing  26  on both sides, and stops  36 ,  38  are provided on both ends of the piston. In the condition shown in  FIG. 1 , the carriage  10  is just approaching the cleaning device in the direction of the arrow, and the piston  28  is held in a rest position in which the stop  36  engages the left end of the valve housing  26 . The piston is held in this rest position by a compression spring  40  that biases the opposite end of the piston  28  away from the valve housing  26 . The stop  38  at this end of the piston just about reaches an abutment  42  that is held stationary relative to the guide rail  12 . When the carriage  10  moves further to the right in  FIG. 1 , the stop  38  engages the abutment  42 , and the piston  28  is pushed through the valve housing  26 . This is illustrated in  FIG. 2 , where the carriage  10  has been moved further to a position in which the nozzle face  16  of the third printhead faces the gutter  24 . 
   The internal construction of the distributor valve  22  is shown in  FIG. 3 . Here, the tubular valve housing  26  is shown to have seal rings  44  at both ends, by which the piston  28  is slidingly guided in the valve housing, so that an annular first pressure chamber  46  is formed between the piston  28  and the internal wall of the valve housing  26  and is closed-off at both ends by the seal rings  44 . This first pressure chamber  46  has four outlet ports  48  that are axially aligned in the circumferential wall of the valve housing  26  and are shaped as connectors to be respectively connected to one of the ink reservoirs  18  of the printheads  14 . Further, the valve housing  26  has two connectors  50  which are arranged near the opposite ends of the valve housing and serve to connect the low-pressure tubes  30  and  32  to the first pressure chamber  46 . 
   The tubular piston  28  is closed by the stop  38  at one end, whereas the other end forms a connector  52  for the high-pressure tube  34 . In the axial center of the piston  28 , two seal rings  54  are fixed on the outer surface of the piston. Together, these seal rings  54  form a seal which separates an annular second pressure chamber  56  from the first pressure chamber  46 . The seal rings  54  also divide the first pressure chamber  46  into two separate compartments, which is the reason for providing two connectors  50 , one for each compartment. 
   An inlet port  60  of the second pressure chamber  56  is formed in the peripheral wall of the piston  28  in a section between the two seal rings  54 . In the condition shown in  FIG. 3 , the inlet port  60  and the second pressure chamber  56  are aligned with the third outlet port  48 , so that the high pressure supplied via the tube  34  will be forwarded through the interior of the piston  28 , the inlet port  60 , the second pressure chamber  56  and the outlet port  48  into the ink reservoir  18  of the third printhead. This corresponds to the condition shown in  FIG. 2 , where the third printhead faces the gutter  24 . 
   In the condition shown in  FIG. 1 , when the piston  28  is in its rest position, the inlet port  60  would be in a position near the right connector  50 , and in this position the second pressure chamber  56  is connected to none of the outlet ports  48  nor to the connector  50 . In  FIG. 3 , this position of the inlet port  60  is indicated by an arrow A. 
   When the piston  28  is in the rest position, and the carriage  10  approaches the abutment  42 , the second pressure chamber  56  moves past the outlet ports  48 , so that the high pressure is applied successively to the ink reservoirs of the printheads  14  in synchronism with the movement of the printheads past the gutter  24 . A second series of cleaning steps will be performed in reserve order when the carriage moves away from the abutment  42  in the reverse stroke. Thus, a two-step cleaning process will automatically be performed separately for each of the printheads  14  whenever the carriage  10  is moved to the end portion of the guide rail  12  where the cleaning device is located. 
   In the example shown in  FIG. 3 , the piston  28  has additional seal rings  62  at both ends. These seal rings  62  will enter into the valve housing  26  and engage the seal rings  44  of the valve housing when the piston reaches one of its two end positions. It should be noted that the seal rings  54  delimiting the second pressure chamber  56  are movable with the piston  28 , whereas the seal rings  44  are stationary relative to the valve housing  26 . In the process of assembly, the seal rings  44  which are arranged close to the ends of the valve housing  26  may be inserted after the piston  28  carrying the seal rings  54  has been inserted into the valve housing but before the stops  36 ,  38  are mounted on the piston. 
   It will occur to a person skilled in the art that the embodiment described above may be modified in various ways. For example, the distributor valve  22  may be arranged stationary relative to the guide rail  12  and the pressure source  20 , if the printheads  14  are connected to the valve housing  26  through respective flexible tubes. The abutment  42  would then be replaced by an abutment arranged appropriately on the carriage  10 . As another alternative, the piston  28  of the distributor valve may be driven by any suitable drive mechanism such as a rack-and-pinion assembly or the like which itself is driven synchronously with the movement of the carriage. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.