Ink Return System For A Multijet Ink Jet Printer

Abstract

A multi-jet, ink jet printer having deflection plates at a positive or negative potential with respect to ground which includes catch chambers for returning unused ink to an ink reservoir. Non-information bearing ink droplets are passed uncharged to a catch chamber and from there through a return conduit system to the ink reservoir. To help direct the ink droplets to the catch chambers, the catch chambers are mechanically biased toward the trajectory of the uncharged droplets by placing them at a slight angle with respect to the trajectory. To prevent short-circuiting of the deflection plates to ground potential, the return conduit system includes denebulization chambers which convert the returning ink stream into large drops thus developing a high resistance path between the deflection plates and the reservoir ink at ground potential.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of the ink jet printers and more particularly in ink return systems for such printers.

2. Description of the Prior Art

Ink jet printers have created much interest in the area of high speed printing. For example, in computer systems, it is often desirable to print out information at speeds of up to 4,000 characters per second. Ink jet printers have this capability and may be used in conjunction with or in place of conventional CRT type display units.

In ink jet printing, one or more ink jet producing nozzles, connected to an ink reservoir through a pressure pump, receive fluid ink under pressure and eject the ink in fine continuous sprays, each of which comprises a string of droplets. The continuous ink spray inherently breaks up into very small droplets. In some systems, the rate and size of these droplets are controlled by vibrating the nozzles. The particular vibration frequency is controlled to control the size and spacing of the droplets. The stream breaks up into droplets as it passes through a charging electrode. The potential on the charging electrode is varied in accordance with the information to be printed. As the droplets pass through the electrode, a charge is transferred to the individual droplets, this charge being a function of the potential applied to the charging electrode to thereby produce information bearing droplets. Between the charging electrode and the moving printing medium upon which the information bearing droplets impinge, there is generated a fixed electrostatic field which changes the trajectory of the droplets passing therethrough, in accordance with the charge thereon, whereby the droplets are directed to selected points on the moving printing medium. An example of such a printer is illustrated in the article by R.L. Gamblin et al, Electrostatic Ink Deflection Bar Code, Printer, IBM Technical Disclosure Bulletin, Volume 11, No. 9, May 1969, pages 1736-1737.

Since the flow of liquid ink is continuous, a catch basin must be used to gather unused ink. The unused ink is that ink which does not carry an information indicative charge. For the purpose of describing this invention, ink jet printers will be classified into two groups, those which position the catch basin between the deflection plates and the printing medium and those in which the catch basin is integral with the deflection plates. In the former type system, exemplified by U.S. Pat. No. 3,484,793, to G.A. W. Weigl, issued Dec. 16, 1969, the droplet trajectory length is long, creating problems of aerodynamic instability of the droplets.

A technique for improving droplet stability is to shorten the trajectory path. A convenient means for accomplishing this is to make the catch basin integral with the deflection plates thereby permitting the printing medium to be placed close to the deflection plates. Such a system is described in U.S. Pat. No. 3,512,173 to D.E. Damouth, issued May 12, 1970. In the Damouth system a pair of deflection plates are associated with each nozzle, with one of the plates being placed at ground potential. The grounded deflection plate has associated therewith an intercepting plate which together with the deflection plate forms a return channel for the unused ink. A high voltage source is coupled to the other deflection plate to provide a potential difference in the area of 3000 volts between the two plates. To direct non-information bearing ink droplets into the return channel, a uniform bias charge is applied thereto. Thus each droplet, whether it carries information or not, is provided with a charge. The problem with this technique is that the charge on the unused droplets affects the charge on the information bearing droplets. Further, by requiring one of the deflection plates to be at ground potential, a separate pair of deflection plates must be utilized with each nozzle. This requirement results from manufacturing difficulties associated with the positioning of the plurality of nozzles relatively close together. Still further, very high potential sources must be used with the system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved multi-jet, ink jet printing apparatus which includes a novel ink return system of the type which includes deflection plates formed with catch chambers integral therewith.

To accomplish the above objective, there is provided a multi-jet, ink jet printing apparatus wherein fluid ink stored in an ink reservoir is supplied under pressure to a plurality of nozzles to produce a continuous ink spray from each nozzle. A moving printing medium is positioned to receive the ink. Associated with each nozzle is a charging electrode and a pair of deflection plates. The ink ejected from the nozzles breaks up into small droplets as these droplets pass through a charging electrode. The charging electrodes are caused to assume a potential indicative of the information to be printed. When no printing is to take place, the charging electrode is placed at ground potential so that no charge is imparted to the droplets passing therethrough. As droplets pass through a charging electrode at a potential indicative of information to be printed, a charge is developed on the droplet, the charge being a function of the potential applied to the charging electrode. The droplets pass through an electrostatic field created by a pair of deflection plates whereby charged droplets impinge upon the printed medium at selected locations.

Non-information bearing droplets remain uncharged so that they do not interfere with the charge on the information carrying droplets. The deflection plates are formed with a hollow portion acting as a catch chamber for the uncharged, non-information bearing droplets. To assist in the catching of uncharged droplets, the deflection plates are disposed at a small angle with respect to the trajectory of these droplets.

Adjacent deflection plates are given respectively a positive and negative potential with respect to ground and adjacent ink jet nozzles share a common deflection plate thus reducing the total number of plates needed. For example, in a system utilizing 60 nozzles and 60 charging electrodes, only 61 deflection plates are required. In that none of the deflection plates is at ground potential, the magnitude of the potential applied to any one plate can be greatly reduced thus permitting the use of less expensive, lower potential sources. That is, instead of using for example a 3000 volt source coupled to a deflection plate, the other plate being at ground potential, one plate is coupled to a +1500 volt source while the other plate is coupled to a -1500 volt source.

An opening is provided at the end of each deflection plate to receive fluid collected by the catch chamber. The opening is coupled to a return conduit system for carrying the unused ink back to the ink reservoir. However, as the ink enters a catch chamber, it assumes a charge which corresponds to the potential on the plate. So that the returning fluid does not provide a low resistance path between the reservoir ink at ground potential, and the deflection plates, thereby presenting a short-circuit path from the deflection plate to ground, denebulization chambers are provided within the return conduit system. As used herein denebulization refers to the breaking up of a fluid stream into relatively large drops. These chambers convert the returning substantially continuous flow of ink into relatively large drops which present a high impedance to current seeking to flow between the deflection plates and the ink reservoir.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates the improved non-shorting ink return system of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the FIGURE, information received from information sources 15 is to be printed on a printing medium 3 by selectively deflecting inking droplets produced by a plurality of nozzles 6. To this end, ink 2, stored in ink reservoir 4 is supplied in a conventional manner to the nozzles 6. Ink pump 8 is provided to introduce the ink into the nozzles 6 under pressure. The ink forced into the nozzles 6 is then ejected therefrom toward the printing medium 3. Due to the natural instability of the liquid stream 10, the ink inherently breaks up into small droplets 12 a short distance from the nozzle. In the vicinity of the ink break-up there is positioned a charging electrode 14 coupled to an information signal source 15. The information signal source provides a potential on its associated charging electrode indicative of the information which is to be printed. In its simplest form, the information signal source may be a ramp generator selectively turned on or off. As the ink droplets 12 pass through a charging electrode 14, they assume a charge which is a function of the potential on the charging electrode. The ink stream 10 is continuous and when no information is to be printed on document 3, the potential on a charging electrode 14 is at ground and thus uncharged ink droplets emerge from the charging electrode. To collect the uncharged ink droplets, each deflection plate 17 is provided with a catch chamber 16 for receiving the uncharged droplets. Each nozzle 6 is mechanically aimed at a catch chamber 16. Two deflection plates are associated with each nozzle and create a fixed electrostatic field which alters the trajectory of charged droplets. One of these plates is at a potential positive with respect to ground while the other is at a potential negative with respect to ground. To aid in the catching process, the deflection plates are positioned at a slight angle with respect to the trajectory of the droplets. As illustrated in the FIGURE, each of the deflection plates 17 is positioned at an angle of approximately 3.degree. with respect to the trajectory of uncharged droplets.

Ink collected by the catch chambers 16 flows through openings 18 into either the return conduit 20 or 22 depending upon whether the fluid has emerged from a positive or negative potential deflection plate. The conduits may be formed of insulating material for safety reasons. However, as uncharged droplets enter a catch chamber, they assume a charge corresponding to the potential on that plate 17. Ink flows out of a deflection plate through return conduit 20 or 22 in the form of a substantially continuous flow which if it were connected would present a low resistance path between the deflection plate and the reservoir ink at ground potential. If such a low resistance path occurs, shorting out the deflection plates might result.

To prevent this, there are provided denebulization chambers 24, one coupled in line with each conduit 20 and 22. Each chamber, made of insulating material, is divided into two compartments 26 and 27. In the first compartment 26 there is provided material having a high surface tension such as metal wool. The returning ink stream enters this compartment and is emitted therefrom in large drops 28 which impinge upon a grounded conductive plate 30 in the second compartment 27. The conversion of the substantially continuous ink stream into separated large drops produces an extremely high impedance path to the deflection plate 17. In fact, the impedance between the ink reservoir 4 and the deflection plates increases to a value greater than 100 megohms. After impinging upon grounded conductive plate 30 the fluid passes through conduit 32 to the ink reservoir 4. In order to aid the return flow of ink there is provided the vacuum pump 36 to evacuate air in the reservoir 4.

To dry the denebulization chambers for preventing conduction of electrical current from taking place, each chamber 24 includes an aperture 34 through which dry air is forced using any suitable air pump (not shown). The introduction of the dry air dries the chamber thus decreasing the humidity in the chamber to prevent electrical current flow.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. In an electrostatically deflected ink-jet printer providing a continuous spray of ink droplets, a method for controlling the interception of said ink droplets with a printing medium comprising the steps of;

a. providing deflection plates at a non-zero potential for generating an electrostatic field, each plate including a catch-chamber,

b. imparting an electric charge to only those droplets which are to intercept said printing medium,

c. directing the uncharged droplets to a catch-chamber

d. breaking the ink received by the catch-chamber into drops to thereby cause the ink to form a high impedance return path to the deflection plates,

e. passing each drop over a ground plate whereby the charge acquired by the ink upon passing through the catch-chambers of the deflection plates is neutralized and

f. collecting the neutralized drops of ink in a reservoir.

2. The method of claim 1 further including the step of disposing said deflection plates at a slight angle with respect to the trajectory of the uncharged droplets.

3. In an electrostatically deflected ink-jet printer of the type including at least one nozzle for ejecting ink toward a printing medium, a charging electrode for selectively charging ink droplets, and an electrostatic field for deflecting the ink droplets from their trajectory an amount proportional to the charge thereon, the improvement comprising:

a. electrostatic deflection plate having a non-zero potential thereon for generating said electrostatic field, each of said deflection plates including a catch-chamber for intercepting and collecting substantially uncharged droplets, said collected droplets thereby assuming a charge dependent upon the potential of said plate,

b. a reservoir, held at substantially zero potential, for storing said ink to be fed through said at least one nozzle, and

c. a return conduit system for returning the said ink collected by said catch-chambers to said reservoir, said return conduit system comprising,

i. a container means, receiving said collected ink in a substantially continuous flow, for emitting said received collected ink in separate and discrete droplets which are relatively large compared to said ink droplets collected by said catch-chamber, to thereby result in a relatively large electrical impedance path through said ink in said return conduit system between said catch-chamber and said reservoir, and

ii. means insulating said container means from said reservoir.

4. The ink-jet printer of claim 3 wherein said deflection plates are disposed at a slight angle with resepect to the trajectory of said substantially uncharged droplets.

5. The ink jet printer of claim 3 wherein there are a plurality of nozzles, a pair of said deflection plates being associated with each nozzle, each deflection plate being either at a positive or negative potential with respect to ground, said conduit system comprising a first conduit coupled to the catch chambers of said negative potential deflection plates and a second conduit coupled to the catch chambers of said positive potential deflection plates, further including first and second of said containers for receiving ink from said conduit system, said first container receiving ink from said first conduit said second container receiving ink from said second conduit.

6. The ink jet printer of claim 5 wherein at least one plate of each pair of deflection plates associated with a nozzle forms one plate of another pair of deflection plates associated with another nozzle.

7. The ink-jet printer of claim 3 wherein each of said deflection plates is hollow, the hollow portion of the deflection plate forming the catch-chamber.

8. In an electrostatically deflected ink-jet printer of the type including at least one nozzle for ejecting ink toward a printing medium, a charging electrode for selectively charging ink droplets, and an electrostatic field for deflecting the ink droplets from their trajectory an amount proportional to the charge thereon, the improvement comprising:

a. electrostatic deflection plates having a non-zero potential thereon for generating said electrostatic field, each of said deflection plates being provided with a catch-chamber for intercepting substantially uncharged droplets;

b. an ink return conduit system receiving the ink collected by said catch-chamber,

c. a container for receiving ink from said conduit system in a substantially continuous flow, said container comprising a compartment including means for collecting the received ink and emitting it in the form of drops, so that the ink forms a high impedance return path to said deflection plates, said container further comprising another compartment including a grounded conductive plate for receiving said ink drops whereby the charge on the ink drops acquired upon passing through the catch-chambers of the deflection plates is neutralized, and

d. an ink collecting reservoir for receiving the ink drops after they pass through said container.

9. The ink-jet printer of claim 8 wherein said container is formed from non-conductive materials and wherein said compartment includes high surface tension material and said another compartment includes an inlet port for receiving dry air and an outlet port coupled to said reservoir.

10. The ink jet printer of claim 9 wherein said reservoir includes an air outlet port for connection to subatmospheric pressure and an ink outlet port for returning ink to said nozzle.