Abstract:
A priming system for ink jet printers includes an ink tank, an ink supply line, an ink bypass line and a valving arrangement which alternately permits either pressurized ink to be supplied to a remote printhead for printing purposes or unpressurized ink to be drawn to the printhead by use of a vacuum source applied to the bypass line.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 08/904,658 filed Aug. 1, 1997. 
    
    
     BACKGROUND OF THE INTENTION 
     This invention relates to ink jet printers. More particularly, it relates to continuous jet ink jet printers, which are used for marking alpha-numeric characters and the like on substrates. Typically, these commercial printers are used for applying date codes, place of manufacture codes and related information on products as they are manufactured. Such printers must be versatile, working in fairly hostile industrial environments, and quite reliable as down time is particularly unwelcome as it affects the output of the factory. Such ink jet printers are typically housed in a cabinet at a location some distance removed from the actual site of the printing. A printhead is connected to the printing cabinet by an umbilical duct which carries ink to and from the printhead as well as the electrical signals required to operate the printhead. 
     When installing a new printer or when servicing the printhead, it is necessary to prime the printer, that is to fill the ink supply line with ink and to remove as much, if not all, of the trapped air as possible. Failure properly to prime and remove air is a major cause of misprinting. Because of the use of the umbilical this priming process and the related purging and cleaning operations can be quite time consuming and presently require substantial manual intervention by a skilled technician as described hereafter. Obviously, the elimination or reduction of this downtime and the requirement for skilled, manual servicing are desirable goals. It is accordingly an object of the present invention to reduce the time and frequency of operator intervention to set up an ink jet printer. 
     It is a further object of the invention to provide an automated system for self-priming which will remove substantially all of the air entrapped in the ink supply line, valves and nozzles associated with the printhead. 
     It is a further object of the invention to provide an automated system for priming of an ink jet printer and purging of air. 
     Another object of the invention is to provide a safer and more efficient method for recovering the fluids used during the cleaning and priming operations. These and other objects of the invention will be apparent from the remaining portion of the specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a typical ink jet printhead being disassembled to permit manual cleaning and priming. 
     FIG. 2 is a bottom view of the printhead of FIG. 1 showing the manual adjustments which need to be made to prepare the printhead for printing. 
     FIG. 2A is a perspective view of the printhead, its cover removed, showing the attachment of a bleed tube to the ink valve for purposes of flushing the printer with cleaning solution prior to priming. 
     FIG. 3 is a perspective view illustrating the manner in which the flushing and priming procedures are accomplished in the prior art. 
     FIG. 4 shows an inkjet printer according to the present invention operating in the normal printing mode. 
     FIG. 5 is a view of the system of FIG. 4 in the auto prime mode according to the present invention. 
     FIG. 6 shows an alternative ink jet printer according to the present invention. 
    
    
     SUMMARY OF THE INVENTION 
     The printer nozzle valve is fitted with an ink bypass line to a source of vacuum. When it is desired to purge and/or prime the ink supply line and nozzle, the air pressure to, or the pump from, the ink supply tank is turned off and vacuum is applied to the bypass line. This sucks ink or solvent from the ink supply tank through the ink line into the nozzle valve and back to a reservoir for reuse or, alternatively, to an ink trap. Because positive pressure is not used to drive the ink through the system, air is not compressed in the ink line or nozzle where it may become temporarily trapped. In addition, any air which is already present in the ink line, tends to be removed by virtue of the fact that a vacuum is used to draw the fluid through the system. After the purging and priming process is complete, the vacuum source is switched out of the system and air pressure is thereafter used to pressurize the ink supply tank to provide pressurized ink to the nozzle for printing. Alternatively, a pump may be used to pressurize the ink supplied by the ink supply tank. All of this is accomplished without the need for manual disassembly of the printhead. Nor is it necessary manually to realign the ink stream within the printhead as is the case with the prior art procedures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As illustrated in FIGS.  1 - 3 , the start-up and priming of a typical ink jet printer requires extensive manual preparation. The printhead  1  is connected to the printer electronics and ink supply by an umbilical  2  in which the ink supply and return lines and electrical wires are contained. A cover  3  is disposed over the printhead. Once the cover is removed, as by the screw shown in FIG. 1, it is possible to service the components of the printhead. Adjustment of the ink stream so that ink drops which are not electrically charged pass into a catcher for return to the ink system, is effected by proper use of adjustment screws  4  and  5  (FIG.  2 ). When it is desired to purge and/or prime the printer system, thereby to clear the ink supply line of any entrapped air or to prime the system for start up after a major service or upon installation, the printhead cover must be removed and a bleed tube  6  (FIG. 2A) attached to an ink valve bleed port  7  located on the nozzle valve  15 . All of these steps are accomplished by a technician who requires a fair degree of skill in order to prepare the printhead for this process without damage to the delicate printhead components. Once the bleed tube has been attached to the nozzle valve bleed port, the system is ready to be purged and/or primed. For that purpose, the printhead is usually positioned as shown in FIG. 3 on a fixture located above a service tray  8  into which the solvent and/or ink is directed during the purging or priming process. During the purging process, cleaning fluid or make-up solution is fed under positive pressure through the ink supply to the nozzle valve bleed port. Thereafter when it is desired to refresh the system, adjustment screw  5  is operated to raise the ink stream above its normal position so that the drops do not enter the catcher. Instead, they are directed into the service tray  8 . 
     Because the umbilical duct supplying the printhead can be from five to twenty-five feet in length, priming the printhead and purging it of air is an important task. In the prior art system as illustrated in FIG. 3, pressurized ink from a reservoir is fed to the nozzle to drive air out of the line. When mostly ink appears at the printhead outlet, the process is stopped. Because the prior art purging process is accomplished by applying ink under positive pressure, air compresses into the pockets inside the various components both in the umbilical duct ink line and the printhead components causing bubbles of air to become trapped. During normal printer operation, this trapped air can dislodge and cause print quality problems and printer faults if they lodge in the nozzle cavity thereby altering the nozzle resonance which is critical to the formation of correctly sized and spaced ink drops. 
     As is also apparent from FIGS.  1 - 3 , such prior art systems lack a convenient way to capture and recycle the solvent and/or ink used to flush and prime the system. The method illustrated in FIGS.  1 - 3  can require thirty to sixty minutes to complete depending upon the diameter of the nozzle orifice used in the printhead. After completing a system flush or refresh in present systems, the skilled technician must then take the now primed printhead and realign the ink stream, again using the adjustment screws  4  and  5  so that the stream of uncharged drops will enter the ink catcher at the proper location. This ensures capture of unused drops and also ensures that charged drops will be accurately and properly deflected above the catcher and on to a substrate to be marked. The cover is replaced and the printer placed in service. 
     In contrast to the prior art techniques illustrated in FIGS.  1 - 3 , the present invention permits purging and priming of an ink jet printer in a considerably shorter time frame, without the need for redirecting fluid away from the catcher and with very little operator intervention. Additionally, because the system priming is performed under vacuum, air entrapment is virtually eliminated. 
     Referring to FIGS. 4 and 5 an ink jet printer according to the improvements of the present invention is illustrated. FIG. 4 illustrates the system during normal operation where ink is being used for printing on a substrate. FIG. 5 illustrates the same system during a refresh, purge and/or priming operation. Housed in the ink cabinet  9  is an ink supply  10  and a solvent or make-up supply  13 . Ink and make-up solvent are provided to a reservoir  11  contained within the ink cabinet. The liquid in reservoir  11  is provided to a pressurized ink tank  12  by means of a pump  40  and line  42 . The level of fluid in the tank is monitored by switches  44  and  46  which signal full and low conditions respectively. 
     Tank  12  is pressurized above atmospheric pressure for normal printing operations by an air source  45  via an electronically controlled solenoid valve  30 , via line  47 , and pressure regulator  48 . Pressurized air is also supplied to the by-pass shut-off valve  31  via line  50 . 
     The output from the pressurized ink tank  12  is supplied via line  14  (the printhead umbilical) to the printhead nozzle valve  15  and then to the nozzle  16 . As is well known in this art, the pressurized ink is expelled through the nozzle via an outlet orifice to form a stream of ink. As the ink passes through the orifice, a stimulation energy is applied thereto causing the ink stream to break up into droplets shortly after leaving the nozzle. As the droplets form, certain of them are electrically charged for deflection onto a substrate. The remaining, uncharged drops, pass into an ink catcher  52  for eventual return via line  54  to the reservoir  11 . Line  54  is under vacuum pressure (below atmospheric pressure) for that purpose. The vacuum pressure is supplied from the Venturi Aspirator source  55  via line  56  and reservoir  11 . 
     As illustrated in FIG. 4 the normal printing conditions cause pressurized ink in tank  12  to flow via line  14  through the nozzle valve  15  to the nozzle  16  and out of the nozzle via the outlet orifice. Uncharged drops are returned via line  54  to the reservoir  11 . This fluid flow path is emphasized by use of heavy lines in the drawing. During such operation, the solenoid valve  30  is open so that air pressure from source  45  is supplied via line  47  to pressurize tank  12  and via line  50  to the shut-off valve  31  thereby keeping it closed. An alternative construction, suitable for use with the present invention, provides a nozzle  16  which has incorporated therein the equivalent of nozzle valve  15 . That is, the nozzle includes the valve function at its input end. 
     Referring to FIG. 5, the system is illustrated operating in its purge, prime and refresh mode. In this mode, which is initiated by the change of state of solenoid valve  30 , no air pressure is supplied to the tank  12 . Thus, pressure source  45  is blocked by closing solenoid valve  30 . This also blocks air pressure from shut-off valve  31  permitting this valve to open. Under these circumstances, the fluid in tank  12  is not pressurized. Nevertheless, it flows through ink line  14  to nozzle valve  15  (or a nozzle with an integral valve). From there, it enters the ink bypass line  18  connected thereto which conveys it, via the now open valve  31 , to a diverter valve  34 . Diverter valve  34  permits the fluid in line  18  to be provided to an ink trap  60  or back to reservoir  11 . This permits the ink to be reused if desired or discarded by diverting it to the ink trap  60 . The vacuum required to suck ink from tank  12  through the line  14  and nozzle valve  15  and back to the reservoir  11  is supplied via line  56  from the vacuum source  55 . 
     When operating as illustrated in FIG. 5, it will be apparent that the principal objects of the invention are achieved. It is no longer necessary to manually disassemble the printhead by removing its cover (FIG.  1 ), nor is it required to manually attach a bleed tube to the bleed port of the nozzle valve  15  as was the case with the manual procedure of the prior art (FIG.  2 A). In addition, it is no longer necessary to readjust the ink stream to direct it above the catcher during the process as illustrated for the prior art system in FIGS. 2 and 3. All of these steps and the resultant cost, time and inconvenience are avoided by the present invention. Instead, whenever it is desired to refresh, purge and/or prime the ink system, all that is necessary is that the ink solenoid valve  30  be operated to shut off the air supply to the pressurized ink tank  12 . This in turn opens the bypass shut-off valve  31  and allows the vacuum source to suck ink from the tank  12  via line  14 , through the nozzle valve  15  and ink bypass line  18 . This quickly and efficiently purges any trapped air in the ink line, primes the ink line  14  so that the printer is ready for printing and returns the ink used for this purpose to the reservoir  11  for further use during printing or, if desired, for example in the event that the ink has become contaminated, diverts it to an ink trap  60 . 
     In an alternative ink jet printer shown in FIG. 6, an ink pump  101 , vacuum pump  107  and ink reservoir  109  have been added and the shop air source  45 , solenoid valve  30 , pressure regulator  48 , pressurized ink tank  12 , vacuum source  55  and reservoir  11  of FIGS. 4 and 5 have been removed. The components of the printheads that are the same in FIG.  6  and FIGS. 4 and 5 are denoted using the same reference numerals. 
     In this presently preferred embodiment, when operating in its printing mode, the following settings are adopted by the printer of FIG.  6 : ink pump  101  is on; bypass valve  103  is closed; ink return valve  105  is open; and vacuum pump  107  is on. Thus, ink pump  101  pressurizes the ink between it and the nozzle valve  15 , opening the nozzle valve and forcing ink out through the orifice of nozzle  16 . Unused ink enters ink catcher  52 , and is drawn by the vacuum generated by vacuum pump  107 , along ink return line  54  and through ink return valve  105  to return to ink reservoir  109 . Since bypass valve  103  is closed, the suction developed by vacuum pump  107  is not applied to ink bypass line  18 . 
     When operating in its priming mode, the following settings are adopted by the printer of FIG.  6 : ink pump  101  is off, bypass valve  103  is open; ink return valve  105  is closed; and vacuum pump  107  is on. Thus, vacuum pump  107  draws ink from reservoir  109  through ink pump  101  (pump  101  may be a gear pump), through the nozzle valve  15 , along ink bypass line  18 , and through bypass valve  103  to return reservoir  109 . Since ink return valve  105  is closed, the suction developed by vacuum pump  107  is not applied to ink return line  54 . 
     As indicated, a significant advantage of the invention is that by using a vacuum source to pull ink through the system instead of pressurized air to push the ink, as is the case during printing, no additional air is entrained in the ink supply and there is none to become entrapped within the ink system or the printhead. Furthermore, during the priming operation, any trapped air which is present will tend to be removed ensuring superior printing operation. 
     While preferred embodiments of the present invention have been illustrated and described, it will be understood by those of ordinary skill in the art that changes and modifications can be made without departing from the invention in its broader aspects. Various features of the present invention are set forth in the following claims.