Ink jet printhead having a preloaded check valve

A preloaded check valve disposed in a valve body between an ink reservoir and a printhead has a valve opening pressure or "cracking" pressure at least sufficient to overcome the hydrostatic pressure due to gravity of ink in the reservoir. This prevents depriming or leakage of ink at the orifices of the orifice plate forming part of the ink ejecting mechanism. Ejection of ink during printing creates a pressure differential across the valve sufficient to cause it to open and replenish the ink supply at the ink ejecting mechanism.

TECHNICAL FIELD 
This invention relates generally to ink jet printheads, and more 
particularly to an ink jet printhead having an ink reservoir in which 
provision is made to prevent ink leakage from the orifice plate in normal 
handling and in use. 
BACKGROUND ART 
Portable or disposable ink jet printheads having attached ink reservoirs 
require provision to contain the ink during handling as well as in use. 
Without some provision for containing the ink when the orifice plate is 
face down, only the surface area mechanics at the orifices operate to 
contain the ink and this is usually inadequate. 
Various techniques for preventing ink leakage at the orifice plate have 
been proposed. Probably the simplest is to apply a pressure sensitive 
adhesive tape to the orifice plate to seal the orifice. However, the 
removal of such tape places the delicate orifice plate at risk to damage 
and may induce ink leakage at one or more of the orifices. Additionally, 
the adhesive material if in contact with the orifice plate adjacent to and 
over the orifices may contaminate the orifices or change the 
characteristics of the orifice plates sufficiently to degrade print 
quality. 
Foam is also used in the reservoir to retain the ink. Hereagain, surface 
energy mechanics is a factor in retaining ink in the interstices or the 
cells in the foam. Pressure reductions when ink is ejected by the ink head 
are usually sufficient to maintain an uninterrupted ink supply at the 
orifice plate. Volumetric effeciency however in the use of foam is only 
about 60 to 65 percent in most applications. 
U.S. Pat. No. 4,509,062 entitled "Ink Reservoir With Essentially Constant 
Negative Back Pressure", issued Apr. 2, 1985 and assigned to the assignee 
of this invention adresses this problem in an arrangement which maintains 
a substantially constant negative back pressure slightly greater than the 
maximum anticipated ink hydrostatic head. This negative back pressure is 
maintained by the utilization and maintenance of a nonlinear force in an 
elastic section of the ink reservoir of the printhead. 
DISCLOSURE OF THE INVENTION 
While the approach of U.S. Pat. No. 4,509,062 above offers a viable 
solution to the problem of ink leakage, an arrangement in accordance with 
the present invention and providing a positive seal of the ink reservoir 
is to be preferred. When the reservoir is sealed the hydrostatic head at 
the orifice plate is reduced. Now only the hydrostatic head of the small 
ink prime in the cavity between the reservoir and the orifice plate is 
effective to exert ink pressure at the orifices, significantly reducing 
the probability of ink leakage thereat. 
In implementing this approach to resolving the ink leakage problem, this 
invention provides a valve body having an opening therethrough. A 
reservoir is connected to one side of the valve body to supply ink to said 
opening and a printing substrate assembly comprising a substrate having an 
orifice plate thereon is sealed to the other end of the valve body 
defining a small cavity between the substrate and the valve body adjacent 
the opening. The ink prime is contained in this cavity. 
An elastically loaded valve member seals the end of the opening at the 
cavity. The elastic loading establishes a value of opening pressure of the 
valve greater than the maximum anticipated hydrostatic pressure due to 
accelerating forces acting on the ink and the ink reservoir. When the 
printhead is operated to eject ink, pressure in the ink prime cavity 
drops. The pressure differential across the valve exceeds the valve 
opening pressure and the ink is supplied to the ink priming cavity. Thus, 
a continuous supply of ink is maintained for the printhead while the 
hydrostatic forces at the orifice plate under quiesent conditions are due 
only to the hydrostatic head of the ink in the cavity.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, the printhead comprises a valve body 1 having an 
integrally formed or separately attached rigid ink reservoir 3. Such 
assemblies are usually molded from a plastic material which has good 
dimensional stability. In this case such a material may be sufficiently 
transparent so that the ink level in the reservoir 3 may be visually 
monitored. Openings 5 formed in the valve body 1, extending between the 
upper and lower valve body faces 7 and 9, as viewed, provide ink flow 
between the reservoir 3 and the printing substrate assembly 11. The 
printing substrate assembly 11 is sealed in the open end of the bottom of 
the valve body 1 in a position spaced from the lower face 9, defining a 
small cavity 10 which must be primed with ink for the printing substrate 
assembly to function properly in ejecting ink to impinge upon paper 13 
during a printing operation. Only the relative positions of the printhead 
body and the paper are shown, it being understood that both the paper and 
the printhead body are selectively moved in orthogonal paths during a 
printing operation. The printhead assembly comprises a substrate 11a and 
an orifice plate 11b. Thermal excitation is used to eject ink. Thermal ink 
jet printheads are described in the Hewlett Packard Journal, May 1985, 
Vol. 36, No. 5, beginning on page 4 which material is incorporated herein 
by reference. The invention is not limited however to this specific type 
of printhead assembly. The invention is equally useful in applications 
involving other methods for ejecting the ink. 
A valve 15 is employed to control the transfer of ink between the reservoir 
3 and the cavity 10. The valve illustrated is a rubber diaphram type of 
valve functioning as a check valve to prevent depriming of the cavity 10. 
It comprises an elastic umbrella shaped diaphram 15a having a peripheral 
edge which seats upon the surface 9 and provides a peripheral seal 
surrounding the openings 5 through the valve body. The valve 15 comprises 
an integrally formed valve stem having an enlarged base section on the 
inner side of the umbrella shaped diaphram and an enlarged end 15c. The 
enlarged end 15c is forced through a central hole in the valve body, 
projecting through the upper end as viewed to engage the upper surface of 
the valve body. The enlarged base section 15b engages the lower face of 
the valve body. The peripheral edge of the diaphram is seated and sealed 
against the surface 9. This deflects the diaphram 15a and preloads its 
peripheral edge against the surface 9. In this position, the preloading 
establishes a valve opening pressure or valve cracking pressure in excess 
of the hydrostatic pressure resulting at least from the maximum depth of 
ink in the reservoir 3. 
Each time the printing head assembly 11 is fired to eject ink onto the 
paper 13, the pressure in the cavity is reduced. The diaphram valve 15 
opens replenishing the ink in the primary cavity 10 for that which has 
been ejected. 
The use of the check valve such as the diaphram valve is advantageous in 
that the check valve action prevents deprimes. Further, neither the 
introduction of foam in the reservoir nor the use of the elastic bladder 
is necessary to prevent gravitationally induced ink leakage through the 
orifices. Further, there is an easy visual indication of the ink supply in 
the reservoir if the reservoir has clear body walls. 
The use of a rigid reservoir as seen in FIG. 1 offers certain advantages in 
that the ink can be "loose". If the printhead is filled and primed with 
ink as manufactured, the air hole 3a in the cover may be sealed with a 
tape which is removed after the printhead is installed in the printing 
mechanism. Alternatively a check valve may be used for this purpose. 
The invention may be practiced with other than a rigid reservoir such as 
that shown in FIG. 1. FIG. 2 shows the use of a "limp" bladder 17 
installed as the reservoir. The use of the bladder eliminates the need for 
a tape seal or a check valve on the air return vent 3a, as seen in FIG. 1. 
The limp bladder is sealed to the upper body projection of the valve, in 
FIG. 2, at the time of manufacture. 
Only one type of check valve has been illustrated herein. It will be 
appreciated however, that any type of check valve may be employed which 
can be preloaded in the closed position. Other typical valves may include, 
but without limitation, reed valves, spring loaded ball valves, and duck 
billed valves for example. In all cases, the valve preload derived opening 
pressure or cracking pressure is chosen to exceed the gravity head by an 
appropriate safety margin to account for manufacturing tolerances, but not 
to exceed an opening pressure requirement which would interfere with the 
ejection of ink when the printhead is fired. 
INDUSTRIAL APPLICABILITY 
The invention is applicable in any printhead where depriming or ink leakage 
must be prevented.