Technical Field
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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
[0014] 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
[0015]
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
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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).
[0022] 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.
[0023] 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
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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
[0029] This invention improves and simplifies the problem of pressure regulation in ink
jet printing systems employing reciprocating printheads and remotely positioned reservoirs.
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.