Foamless ramps for controlling the flow of ink to eliminate foam in an ink tank

An ink tank apparatus having a reservoir for containing ink, is improved to control or eliminate the harmful effects of foaming, misting and splatter of ink in the ink tank. A fluid pump delivers ink to a printer system, having an associated reservoir, and a vacuum pump acts on the reservoir to return unused ink from the printer to the reservoir. The returning ink is caused to enter the reservoir at an angle so the returned ink tends to stay near a surface of the reservoir ink level. In particular a ramp or series of ramps are provided along which the returning ink travels. Furthermore, the returned ink is ported into a confined inlet chamber from which the returned ink drains into the ink tank, and the returned ink is ported into this confined inlet chamber so as to produce a vortex-like flow in the inlet chamber. A drain port from the inlet chamber can be restricted to reduce pulsations in the returned flow, and baffles can be used to inhibit the flow of mist to the vacuum port of the ink tank.

TECHNICAL FIELD
 The present invention relates to continuous ink jet printers and, more
 particularly, to an anti-foaming design of a fluid reservoir of a
 continuous ink jet printer.
 BACKGROUND ART
 In continuous ink jet printing, ink from an ink tank is supplied under
 pressure to a manifold that distributes the ink to a plurality of
 orifices, typically arranged in linear array(s). The ink is expelled from
 the orifices in jets which break up due to surface tension in the ink into
 droplet streams. Ink jet printing is accomplished with these droplet
 streams by selectively charging and deflecting some droplets from their
 normal trajectories. The deflected or undeflected droplets are caught and
 returned to the ink tank, while the others are allowed to impinge on a
 printing surface.
 Ink is returned from the printhead to the ink tank by holding the ink tank
 under vacuum. As the ink is drawn through the ink return lines from the
 catcher or the drop generator, air can also be drawn in as well. This
 ink-air mixture flows through the ink return lines in a combination of
 plug and surge flow.
 If these fluid lines are ported into the top of the ink tank, the ink
 returning to the tank can splash and spray down on the ink in the tank.
 This can generate large amounts of foam in the tank. If the foam build up
 is too large, some of this foam can be sucked out the vacuum port of the
 tank. This foam can foul up the vacuum pump or the vacuum control system,
 if allowed to pass through them. To avoid this problem the prior art has
 employed three strategies. First, one can try to develop low foaming or
 non-foaming ink. This typically requires the addition of an anti-foaming
 agent into the ink. It has been found that such chemicals can have a
 detrimental effect on the ink-paper interaction, hurting print quality and
 on runnability of the ink jet system. As a result this is not a desirable
 option. Second, one can place a foam carry over jar in the vacuum line,
 between the ink tank and the vacuum pump or vacuum control system. This
 system is still not trouble free. Under some conditions, the foam can fill
 not only the ink tank but also the carry over jar. This allows the foam to
 enter the vacuum pump and control system. Even without filling both the
 ink tank and carry over jar, the foam can be harmful to the vacuum system.
 When the bubbles at the surface of the foam burst, they send a fine spray
 of ink mist or droplets into the air. This fine mist can also be drawn
 into the vacuum pump, causing it damage. The third strategy to deal with
 foam has been to reduce the amount of foam which is generated.
 Since the foam had been produced by the splashing and spraying of ink onto
 the surface of the ink, the prior art attempted to port the ink return
 lines into the ink tank below the surface of the ink in the tank. As the
 return lines also carry air in, along with the ink, numerous large bubbles
 are formed as the air enters the tank. As a result a large amount of foam
 is produced by the this porting option as well.
 Another prior art option, has been to port the return lines into the fluid
 tank above the ink level, but aim the fluid ports directly at the tank
 walls. The fluid then flows down the tank walls. This greatly reduces the
 amount of foam produced. While beneficial, this option is not sufficient.
 The rapidly moving layer of ink flowing down the tank walls, which results
 from directing the fluid flow at the walls, enters the bulk fluid in the
 tank at high velocity. This can cause air to be dragged or entrained down
 into the bulk of the ink. The entrained air bubbles in the ink can be
 drawn into the ink pump that is ported out of the bottom of the tank. Such
 air in the ink pumped to the printhead can adversely affect the operation
 of the ink jet printer.
 An addition failing of this prior art design of directing the returning
 fluid ports is the result of the non-uniform flow out of these ports. The
 two phase (ink and air) flow in the return tubes tends to be a combination
 of plug and surge flow. As a result of the plug and surge flow in the
 return lines, some foam and a spray of mist can be produced as the ink-air
 mixture exits the fluid port and splashes into the walls of the tank. This
 can still produce the vacuum system errors found in the earlier prior art.
 It is seen that there exists a need for better means for controlling or
 eliminating the harmful effects of foam in the ink tank without the need
 for chemical defoamers and without the problem of air bubbles being
 entrained into the bulk fluid by the rapid flow of ink down the tank
 walls.
 SUMMARY OF THE INVENTION
 It is the object of the present invention to provide a foamless ink tank
 which eliminates foaming in the ink tank. This allows ink formulators to
 formulate ink without having to include defoaming chemicals in the ink
 composition. The present invention therefore results in improved print
 capability and fewer problems with crooked jets. Furthermore, eliminating
 the ink maintains the controls, pumps and filters in better print quality
 condition. Finally, the foamless ink tank design of the present invention
 allows for a smaller ink tank since there is no need to allow excess room
 for the foam to collect.
 In accordance with one aspect of the present invention, an ink tank
 apparatus having a reservoir for containing ink, is improved to control or
 eliminate the harmful effects of foaming, misting and splatter of ink in
 the ink tank. A fluid pump delivers ink to a printer system, having an
 associated reservoir, and a vacuum means acts on the reservoir to return
 unused ink from the printer to the reservoir. The returning ink is caused
 to enter the reservoir at an angle so the returned ink tends to stay near
 a surface of the reservoir ink level. In particular a ramp or series of
 ramps are provided along which the returning ink travels. Furthermore, the
 returned ink is ported into a confined inlet chamber from which the
 returned ink drains into the ink tank, and the returned ink is ported into
 this confined inlet chamber so as to produce a vortex-like flow in the
 inlet chamber. A drain port from the inlet chamber can be restricted to
 reduce pulsations in the returned flow, and baffles can be used to inhibit
 the flow of mist to the vacuum port of the ink tank.
 Other objects and advantages of the invention will be apparent from the
 following description and the appended claims.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention proposes a technique for preventing ink
 dissemination, such as ink foam, mist, and spray, from entering the
 vacuum, and further to prevent entrained air from entering the fluid pump.
 In accordance with the present invention, ink from an ink tank or
 reservoir is pumped by an ink pump to a printhead and uses vacuum on the
 reservoir to return the unused ink from the printhead to the reservoir.
 The tank is constructed to minimize or prevent the harmful effects of
 foaming of the ink in the reservoir by employing one or more mist
 barriers, ink ramps, submerged ink ramps, and a tubular ink inlet that
 reduces ink velocity.
 It was seen in the prior art, that causing the return fluid to flow down
 the wall the ink tank produced less foam than allowing the ink to flow,
 spray or splash down on the surface of the ink in the tank. The rapid flow
 of fluid down the wall of the tank, however, provided the ink with too
 much vertical momentum as it entered the bulk fluid. As a result, air
 tended to get entrained and drawn down into the bulk fluid.
 The interrelationship between components associated with ink tank 10 can be
 seen in FIG. 1. One problem with the current art is that bubbles in ink
 tank 10 send mist and splatter into the air when they burst, with air flow
 in the direction of arrow 12 and ink flow in the direction of arrow 14.
 The ink tank 10 has an associated ink pump 16, and provides ink to and
 receives ink from a printhead 18. Small bubbles break up violently when
 leaving the end of inlet tube 20 entering the tank 10. This causes an air
 borne mist that travels through vacuum solenoid 22 and into vacuum pump
 24. In addition, with the prior art, large bubbles can form inside tank 10
 and lay on the ink surface. When these large bubbles break, large droplets
 are sent across the tank 10 and generate a fine mist which may get sucked
 out of vacuum line 26.
 Bubbles, mist and splattering can also be caused by the impact of high
 velocity ink jet entering tank 10, and the impact of high velocity air
 entering the ink tank 10. With prior art configurations, high velocity
 exit air to the vacuum pump 24 sucks mist and splatter out of the tank 10,
 to a carry over jar (not shown) through the vacuum servo 22.
 In accordance with the present invention, ink disturbance problems are
 eliminated by introducing certain novel structural features to the ink
 tank. The present invention proposes an ink tank constructed to prevent
 the formation of bubbles, foam, mist, and splatter associated with ink
 tanks in the existing art. The ink tank 10 illustrated in FIG. 2 is
 constructed in accordance with the teachings of the present invention. The
 present invention proposes several novel features which can be applied
 individually or simultaneously to eliminate or reduce the adverse effects
 of foaming in the ink tank of an ink jet printhead.
 Prior art ink tanks reduced the generation of foam by directing the
 returning ink flow down the walls of the ink tank. While an effective foam
 reducer, this resulted in air being entrained down into the ink tank
 produced by the large vertical velocity of the fluid down the wall into
 the bulk ink. To maintain the foam reducing benefit of flow down the wall
 without the disadvantage of the air entrainment, the present invention
 causes the fluid to flow down a ramp 40 into the bulk fluid. The normal
 reservoir ink level in the tank is at the middle float switch 81b which is
 shown in the center of the tank. The ink tank employs three float switches
 81a, 81b, and 81c, shown in FIG. 2. These float switches are mounted on a
 shaft installed in the tank by means of port 39, in FIG. 3B. The trip
 point for the lower float switch 81a is used to detect dangerously low ink
 levels. The refilling of the ink tank with make-up fluids to replace ink
 used for printing or due to evaporation is controlled by the middle float
 switch 81b. The upper float switch 81c is used to detect dangerously high
 ink levels. The slope of the ramp greatly reduces the velocity of the
 fluid as it enters the bulk ink. With the reduced vertical flow, the
 returned flow tends to flow across the surface of the fluid in the ink
 tank rather than down into it. In this way, the problem of air being
 entrained into the ink, from which entrained air bubbles in the ink can be
 drawn into the ink pump via port 64 at the bottom of the tank, is
 eliminated.
 The prior art produced a lot of spray and splash as the ink entered the ink
 tank. This was produced by the plug and surge flow in the return fluid
 lines. The splashing and spraying of the prior art tended to produce mist
 which was detrimental to the vacuum system. The present invention deals
 with this problem by improving the design of entrance port 30 to the tank.
 Rather than port the return lines 20 directly to the tank, the return
 lines open into an inlet chamber 42 which is small compared to the tank
 but still significantly larger than the cross section of the return lines
 30. This entrance chamber 42 is shown in FIGS. 2, 3A and 3B. In a
 preferred embodiment, this entrance chamber resembles a portion of a
 donut-shaped volume 41 in the top of the ink tank. Fluid enters this
 volume through a number of ink return ports 30 as seen in FIG. 3A, with a
 port 39 for a float switch illustrated in FIG. 3B. The fluid exits this
 chamber 42 through a restricted exit port 43 shown in FIG. 2. The wall 4,
 which contains this restricted exit is shown in FIG. 3A. Another section
 of this donut shaped volume 41 in the top of the tank forms an exit
 chamber 80 associated with the vacuum port of the tank.
 This entrance chamber 42 acts as an impulse reducer. Much like an
 automobile muffler helps to dissipate impulses from the engine by
 enlarging the diameter of the exhaust line and by also providing some
 restriction to flow, the entrance chamber helps to reduce the impulses
 from the ink return lines by providing an enlarged volume and having a
 restricted exit. As slugs of air, mixed with the ink enter this entrance
 chamber they expand into the volume of the entrance chamber. The
 restriction of the exit from this chamber serves to reflect some of the
 pressure pulse produced by this expanding air back into the entrance
 volume. In this way the impulses from the returning flow are effectively
 dissipated in the entrance chamber.
 The chamber 42 is designed to be roughly circular in cross section. The ink
 return ports 30 are not centered over the entrance chamber, but rather
 direct the incoming fluid down the wall. In this way the fluid tends to
 stay attached to the wall, rather than spattering into it. The incoming
 flow also tends to set up a vortex inside the inlet chamber. The vortex
 also aids in separating the ink and air flows. Any splatter and mist
 produced by ink entering this chamber is tends to be confined in the
 entrance chamber, minimizing the risk of the spray entering the vacuum
 system.
 Fluid leaves the inlet chamber through the restricted exit located in wall
 47 of FIG. 3A. This restricted exit 43, shown in FIG. 2 is in the shape of
 a vertically elongated port. The ink which has separated from the air,
 flows out through the bottom of restricted exit 47 in FIG. 3A, while air
 can flow out through the upper portion at restricted exit 43 in FIG. 2,
 without being remixed with the ink. The ink exiting from the restricted
 exit of the inlet chamber is directed at adjacent surface 62, from which
 it begins flowing down the ramp 40 to the level of the ink. As shown in
 FIG. 2, the flow in the inlet chamber is clockwise, as indicated by arrow
 48. The slope of the ramp is such that the fluid descends the ramp in a
 counter-clockwise direction, as indicated by arrow 46. This reversal of
 the flow direction as the fluid exits the chamber through the restricted
 exit and strikes the adjacent surface 62 helps to slow the flow of ink
 down the ramp.
 In addition to minimizing the formation of foam, and the splatter of ink
 entering the ink tank, the present invention also protects the vacuum
 system by preventing airborne mist or spray from being drawn into the
 vacuum port 36. This is done in part by using some of the donut shaped
 volume 41 at the top of the tank as an exit chamber for the ink tank as
 indicated in FIG. 3B. This exit chamber 80 is the section of the
 donut-shaped volume 41 between the two walls 45 shown in FIG. 3A. Air,
 exiting the ink tank through the vacuum port 36, enters this exit chamber
 80 through an opening or port 78, shown in FIG. 3A in the bottom wall of
 this chamber. The air pass must then through exit chamber to reach the
 vacuum port 36. In this way both the end walls 45 and the bottom wall 90
 exit chamber serve as baffles to produce a circuitous pass for the air
 flow leaving the ink tank. While the air can make the turns with ease, the
 heavier mist tends to strike the walls, from which it can flow back down
 to the level of ink in the tank. Similarly, the ramps 40 down which the
 ink flows also serve as baffles 40 for the air flow in the tank to impede
 mist from reaching the vacuum port. As will be obvious to those skilled in
 the art, other or additional mist barriers can be employed to enhance the
 advantages provided thereby.
 The invention has been described in detail with particular reference to
 certain preferred embodiments thereof, but it will be understood that
 modifications and variations can be effected within the spirit and scope
 of the invention.