EP0237787A2

EP0237787A2 - Method and apparatus for maintaining a substantially constant ink pressure at a remotely fed ink printhead

Info

Publication number: EP0237787A2
Application number: EP87101964A
Authority: EP
Inventors: Niels Nielsen
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1986-03-20
Filing date: 1987-02-12
Publication date: 1987-09-23
Also published as: JPS62231760A; EP0237787A3

Abstract

Pressure surging and variations in ink supply in a remote source reservoir ink jet printing system are eliminated in a unique method and implementing apparatus employing a simple elastically suspended reservoir. The weight of the reservoir is kept negligibly small in relation to the weight of the ink. The elastic constant is selected so that the reservoir rises as the ink supply diminishes maintaining the surface of the ink at a relatively constant height with respect to the printhead, independantly of the depth of the ink in the reservoir. This maintains the hydrostatic pressure head (slightly negative in this application) relatively constant in relation to the printhead which is supplied by the reservoir. Any elastic system operating in tension, compression, bending, torsion and so on is contemplated.

Description

Technical Field

This invention relates generally to printing apparatus for applying a fluid ink to a printing medium, such as a sheet of paper to form characters or designs thereon. More particularly, this invention relates to an arrangement for maintaining the fluid ink supply for a remotely fed type of reciprocating ink jet or printhead at a substantially constant pressure.

Background Art

Ink jet printers having reciprocating printhead assemblies have remote ink reservoirs. Ink is plumbed to the printhead carriage through a flexible hose or a system of flexible hoses leading from the remote reservoir. These plumbing systems are generally quite complex and delicate. The include priming pumps, sensors and pressure regulators as the minimal equipment needed to control the pressure of the ink at the printhead. Ink pressure control is required to prevent depriming of the printhead and in controlling ink drop velocity. The following patents disclose systems which are typical of such arrangements.
Patent 4,555,712 entitled "Ink Drop Velocity Control System" issued November 26, 1985, describes an arrangement for controlling ink drop velocity by measuring ink flow rates between two selected points. When flow rate deviation from a reference value is detected, ink velocity and/or ink pressure is altered to change the flow rate.
Patent 4,462,037 entitled "Ink Level Control for An Ink Jet Printer" issued July 24, 1984 employs a pump to maintain the ink at one level during printing for controlling hydrostatic pressure.
Patent 4,456,916, entitled "Ink Jet Cartridge with Hydrostatic Controller", issued June 26, 1984, compensates decreasing head height of the ink supplying an ink jet nozzle, using a cam and a leaf spring to elevate a float which displaces ink from a reservoir to the compartment connected to the ink jet nozzle to achieve optimum hydrostatic pressure.
Patent 4,433,341, entitled "Ink Level Control for Ink Jet Printer", issued February 21, 1984, uses a check valve between a reciprocating and a main stationary reservoir to control the direction and the flow of ink between the reservoirs. The check valve allows the ink to rise in the reciprocating reservoir during nonprinting.
Patent 4,432,005, entitled "Ink Control System for Ink Jet Printer", issued February 14, 1984, employs a pressurized primary reservoir to supply a secondary reservoir on the movable carriage of a printhead. A solenoid valve controls the transfer of ink. The solenoid valve is controlled by an optical sensor at the secondary reservoir which senses the level of ink thereat.
Patent 4,342,042, entitled "Ink Supply System for An Array of Ink Jet Heads", issued July 27, 1982, provides a primary ink supply source and a secondary ink supply reservoir which are connected in series to a transfer circuit which supplies ink upon demand to the ink jet of a printhead. The secondary reservoir has a flexible membrane as its upper surface which is sensed so as to monitor the quantity of ink therein. The secondary reservoir acts as a static pressure regulator for ink entering the ink jets.
Patent 4,319,254, entitled "Constant Head Magnetic Actuated Ink Jet Cartridge For Use In An Electrostatic Ink Jet Printer", issued March 9, 1982, employs a magnetically actuated piston to raise ink over a fixed height controlling wall to maintain ink at a relatively constant head in the area of the ink jet nozzle.
Patent 4,183,030, entitled "Ink Jet Recording Apparatus", issued January 8, 1980, and patent 4,215,353, entitled "Ink Jet Recording Apparatus with Trial Run At Side", issued July 29, 1980, each employ static pressure variations within an ink tube to control the operation of a compressed air source which applys compressed air to the ink tank to compensate for reduction of static pressure within the ink tank as the ink supply is depleted.
Patent 4,074,284, entitled "Ink Supply System and Printhead", issued February 14, 1978, employs a pressure sensor in a printhead for sensing changes in pressure in the system of ink supply to control a valve to supply ink to the system upon a predetermined reduction in ink pressure.
Patent 4,067,020, entitled "Noninterrupt Transfer System For Ink Jet Printer", issued January 3, 1978, provides an arrangement whereby the ink which is not used in printing may be returned to a reservoir from which it is withdrawn together with fresh ink in a manner so that ink pressure fluctuations together with shut downs for filling the reservoir are minimized.
The prior art referenced above in regulating ink pressure in a printing system, suggests the use of sensors, pumps, pistons, compressed air and so forth. The failure of these latter devices and techniques can have serious, if not catastophic, effects on the integrity of the print mechanism because of ink spillage problems.

Disclosure of the Invention

Pressure surging and variations in ink supply in a remote source reservoir system is eliminated in a presently preferred method and implementing embodiment of this invention, employing a simple elastically supported reservoir. The weight of the reservoir is kept negligibly small in relation to the weight of the ink maintained in the reservoir. The elastic constant is selected so that the reservoir rises as the ink supply diminishes maintaining the surface of the ink in the reservoir at a relatively constant height or elevation in relation to the printhead. This maintains the hydrostatic head substantially constant between a maximum and a selected minimum weight of ink in the reservoir. The hydrostatic pressure is selected to at least maintain priming pressure at the printhead. Any elastic system employing either a tension, compression or other elastic support for the reservoir is contemplated. It can be shown that in a system such as that described above, if the weight of the ink reservoir is kept negligibly small in relation to the weight of the ink in the reservoir, that the elastic constant is directly proportional to the cross sectional area of the reservoir.

Brief Description of the Drawing

Figure 1 is an elevational view partly in section schematically depicting a printing mechanism having a remote ink reservoir supported by an elastic tension member for maintaining a substantially constant hydrostatic ink pressure head; and
Figure 2 is a variation of the elastic support of Figure 1 employing an elastic member operating in compression for supporting the ink reservoir.

Best Modes for Carrying Out the Invention

The present invention describes a method for supplying ink to a reciprocating printhead from a remotely positioned reservoir containing ink. wherein a substantially constant pressure of the ink supply at the printhead is maintained. The method comprises the steps of supporting the ink reservoir for vertical bidirectional movement with the surface of the ink in the reservoir at a predetermined elevation relative to the printhead to provide a selected hydrostatic pressure at the printhead and thereafter moving the reservoir vertically as a function of the combined weight of the reservoir and the ink therein, to maintain the surface of the ink in the reservoir substantially at the desired predetermined elevation with respect to the printhead.
In a more specific aspect of the method of practicing this invention, the weight of the reservoir is negligible in relation to the weight of the supply of ink in the reservoir.
A presently preferred implementation and embodiment of the method described above is illustrated in Figure 1 which schematically illustrates an ink jet type of printing mechanism having a reciprocating type of printhead 1 and a remote reservoir 2 which is coupled to the printhead 1 by means of a flexible tube 3. The flexible tube 3 is connected between the drain at the bottom of the reservoir 2 and a priming cavity 4 forming a part of the printhead 1. Such a printhead may be either of the thermal jet ("bubble jet") type or peizoelectric type, both of which are well known in the art. Ink is supplied to an orifice plate 5 from the priming cavity 4, in an on demand basis, whereby minute ink drops 6 are ejected from the orifice plate 5 to impinge in selected patterns upon a sheet of paper 7 which is conventionally advanced over a paper guide 8 past the orifice plate 5. The reciprocating mechanism for the printhead 1 and the paper feed mechanism for the paper 7 are not shown in the interest of simplicity, since both are well known in the art. In practice, the printhead reciprocates horizontally and the paper feed is a vertical direction past the printhead.
The reservoir 2 is fabricated of a light weight thin wall material such as ABS (Acrylonitrile-Butadiene-Styrene), Polyphenylene Oxide, Polyphenylene Ether, with or without fillers or modifiers such as glass or carbon fibers. The weight of the reservoir together with the weight of any portion of the flexible tube 3 attached thereto is negligible in relation to the supply of ink always maintained within the reservoir. A bridging structure 2a bridging the upper side of the reservoir receives the bottom end of a tension spring 9 the upper end of which is attached to a fixed support 10.
Thus the reservoir with its ink supply is freely supported from the bottom end of the tension spring 9. By keeping the weight of the reservoir negligibly small in relation to the weight of the ink contained therein, the deflection of the spring can be regarded as a linear function of the weight of the ink. By this expedient, the hydrostatic head H measured between the fixed support 10 and the surface S of the ink can be maintained relatively constant, while the depth of the ink h within the reservoir 2 decreases as the ink supply diminishes.
To prevent the hydrostatic pressure from forcibly discharging ink from the orifices in the orifice plate 5, the surface of the ink in the reservoir is kept below the center line of the orifice array. This provides a nominal negative back pressure at the orifices. This back pressure is easily overcome by the capillarity of the orifices during normal operation, but is sufficient to prevent ink from oozing through the orifices when the printhead is not in use. The static pressure is usually between -1 inch and -4 inches of H₂O (vacuum).
Although the reservoir may have any desired cross sectional configuration, for the purposes of this discussion it is assumed the the reservoir is of circular cross section having a radius r. In the drawing the acceleration due to gravity is represented by the arrow labeled g. The density of the ink in the reservoir is designated p and the spring constant is called k.
Assuming the weight of the reservoir 2 is negligible,
then, for equilibrium,
or
Taking the derivitive of both sides
therefore
For constant
Therefore
where A is the cross sectional area of the reservoir
The limit of useful travel of the reservoir 2 is determined by the change in depth of the ink which is in the reservoir. An indication of a minimum level of ink in the reservoir which is to be maintained is obtained by means of a switch 11 which is actuated by a projection 2b projecting from the side of the reservoir 2. As the reservoir 2 rises, when the minimum level of ink is reached, a projection 2b closes the switch 11, activating an indicator I.
In applications where the reservoir is subject to lateral and vertical accelerations causing swinging and bouncing, simple restraints may be provided to prevent such excursions. One such restraint with respect to lateral excursions, may be a sleeve 12 within which the container 2 is loosely fitted. This sleeve is provided with a slot 12a through which the switch actuator 2b projects. In this arrangement if the container should ride against the inner face of this sleeve 12 vibration of the printing operation usually will be sufficient to prevent any frictional restraint from introducing error in the vertical movement of the reservoir 2 as the ink supply is reduced. Vertical excursions or bounce may be minimized by means of a conventional fluid damper (not shown) comprising a piston loosely fitted in a tube, or provided with an orifice, or both, and filled with a fluid medium. Such a damper will absorb any kinetic energy of the reservoir 2 in work done pumping fluid back and forth across the piston, with insignificant frictional restraint with respect to normal vertical movement of the reservoir 2 in maintaining the surface of the ink S in the chamber at a relatively constant elevation E with respect to the printhead 1.
The invention also may be practiced employing a compression spring 9a as indicated in Figure 2 at the bottom side of the reservoir 2. In this arrangement, the compression spring 9a supports the weight of the reservoir and the supply of ink therein. The reservoir may be horizontally restrained as indicated in Figure 1, if needed. Vertical stabilization, again, may be achieved by means of a fluid damper.
Thus the method and the apparatus for implementing the method according to this invention supplies ink to the printhead with essentially constant pressure without the use of pumps and switches and sensors and other equipment, by the simple expedient of employing an elastic member such as the coil springs of Figure 1 and 2 to hold the term H fixed, which in this application holds the surface of the ink S at a constant elevation below the printhead, the hydrostatic pressure head exerted by the reservoir does not change as the volume of ink in the reservoir changes. By the simple expedient of selecting a spring having a spring constant proportional to the cross sectional area of the chamber as shown in equation 7 above, the hydrostatic head H will not change as the fluid level h changes. Thus the pressure at the inlet to the priming cavity on the printhead is simply maintained.
Although conventional springs have been shown in implementing this invention in Figures 1 and 2, it will be appreciated that spiral springs, helical torsion springs and- other less conventional springs may be employed in implementing the invention. Similarly elastic members other than metal springs may be used. Either positive or negative ink pressures may be maintained at the printhead depending upon printhead requirements.

Industrial Applicability

This invention improves and simplifies the problem of pressure regulation in ink jet printing systems employing reciprocating printheads and remotely positioned reservoirs.

Claims

1. A printing mechanism, comprising:

a. a horizontally movable printhead requiring a fluid ink supply;

b. a reservoir containing a supply of fluid ink;

c. flexible tubing connecting said reservoir to said horizontally movable printhead to conduct fluid ink to said printhead; and

d. elastic means coupled to said reservoir and responsive to the weight of said reservoir and fluid ink supply for lifting said reservoir as said fluid ink supply is depleted, to maintain the surface of said fluid ink supply at a substantially constant elevation with respect to said printhead for maintaining ink pressure at said printhead substantially constant.

2. The invention according to claim 1, in which:

a. said elastic means comprises an elastic member having a substantially linear elastic rate per unit of displacement over the displacement range of said reservoir.

3. The invention according to claim 2, in which:

a. said elastic member is a metal spring.

4. The invention according to claim 3, in which:

a. said metal spring is a coil spring.

5. The invention according to claim 4, in which:

a. said coil spring is a tension spring.

6. The invention according to claim 4, in which:

a. said coil spring is a compression spring.

7. The invention according to claim 1, in which:

a. a minimal ink supply is maintained in said reservoir and the weight of said reservoir in relation to the weight of said minimal ink supply does not introduce a significant error in ink pressure control at said printhead.

8. The invention according to claim 1, in which:

a. said elastic means comprises an elastic member having an elastic constant proportional to the cross sectional area of said reservoir.

9. The method for supplying ink to a reciprocating printhead from a remotely positioned reservoir containing ink, comprising:

a. supporting said reservoir for vertical bidirectional movement, with the surface of the ink in said reservoir at a predetermined elevation with respect to said printhead to provide a selected hydrostatic pressure at said printhead; and

b. moving said reservoir vertically as a function of the combined weight of said reservoir and the ink therein to maintain the surface of said ink in said reservoir substantially at said predetermined elevation with respect to said printhead.

10. The method of claim 9, in which:

a. said predetermined elevation of the surface of said ink is below said printhead to maintain a slightly negative hydrostatic pressure at said printhead.

11. The method of claim 9, comprision:

a. establishing a minimum weight of ink which is to be maintained in said reservoir so that the weight of said reservoir is negligible in relation to the weight of said ink.