Note: Descriptions are shown in the official language in which they were submitted.
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MONOLITHIC INK JET PRINTHEAD CHASSIS
Technical Field
The present invention relates to the
filed of continuous ink jet printers and, more
particularly, to a device that provides a
structurally sound and precision platform that
facilitates mounting of electrical, mechanical, and
fluidic ink-jet printhead components while keeping
size and part count to a minimum.
Backaround Art
High resolution ink-jet printheads use a
droplet emitter, known as a droplet generator, and a
plurality of droplet deflection electrodes, known as
a catcher/charge plate, in precise alignment to
create the "ink-jet" technology area of a printhead.
A bridge, structure, chassis, or even the catcher
itself is normally used as a rigid link to hold the
droplet generator and catcher/charge plate in
precise alignment, as is described and claimed in
U.S. Patent Nos. 5,455,611; 5,475,411; and
5,475,409. As these components must be critically
aligned, the aligned structure is normally designed
to be a customer replaceable service component.
Printheads for long array continuous ink
jet printers include, in addition to these
components, support electronics such as data
handling electronics including charge plate drivers,
fluid filters, and connections. These components
normally are attached to a second frame or chassis.
The structure which maintains the alignment of the
ink jet components is then attached to this second
chassis. The printhead covers are also attached to
this second chassis. The printhead is typically
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located at the print station by a combination of
features, some associated with the first chassis and
some associated with the second chassis.
The customer replaceable printheads for
short array printers, for example printers having a
print swath of about 1", typically do not include
many of these fluid and electronic support
components. Rather, they are attached to a
structure or chassis which is part of a print
station which is not customer replaceable. The
chassis or frame of the aligned customer replaceable
printhead also attaches to the chassis of the print
station. Covers and locating features for locating
the print station/printhead are attached to this
second chassis which is part of the print station.
The current art, which includes a bridge,
locking structure, or chassis to maintain alignment
of the droplet generator and catcher/charge plate,
and a second frame or chassis to hold the electronic
and fluid support components, exhibits many
undesirable traits and functional deficiencies. For
instance, the "foot-print" (square inch area around
print array that is dedicated to the chassis that
holds the droplet generator and catcher/charge plate
together) and surrounding geometry (also known as
the base) is described by dividing the square inch
area at the base by the print array length. The
foot-print ratio is 14 on current wide bar products
and is considered to be quite large. A low single
digit number is desirable.
Another problem with the current art is
that current continuous ink-jet printheads require a
plethora of discrete electrical, mechanical, and
fluidic connections to be made by the end user.
Prior art attempted to address this issue, but still
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required connections to be made in several planes.
This made printhead construction difficult.
A third problem with the current art is
that mechanical forces that are used to constrain
ink-jet printheads in their working environment can
be large. With the mix of locating features
attached to the ink jet alignment chassis and the
support component chassis, movement can and does
occur between the drop generator and the
catcher/charge plate. This is a very undesirable
because it will degrade printhead life and can cause
a complete printhead failure. Thermal gradients
between the droplet generator, catcher/charge plate,
and the ink jet alignment chassis can also cause
movement and shorten printhead life.
Yet another problem with the existing art
is that all continuous ink-jet printheads require
positive pressure purified air around the ink-jet
array which improves printhead life. This can prove
difficult to achieve in current art because of the
cavernous areas created by the large foot-print
described above. For the shorter array printhead,
the critical ink-jet components are exposed to a
dirty environment during handling and installation
since the covers are attached to the print station,
not the customer replaceable printhead.
The use of a two chassis system is also a
problem with the internal "eyelid" of a printhead.
The eyelid functions as a shutter for the ink-jet
array and needs to maintain a precise relationship
to other ink-jet components. The eyelid actuation
components have been attached to the support
component chassis. The alignment of the eyelid to
the ink jet components has been less than ideal as a
result. Current art continuous ink-jet printheads
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with two chassis Eras have produced a system having
over-constrained or complex mechanical interfaces.
An over-constrained printhead has undesirable
internal movement and can have a complex mechanical
interface which will lead to difficult mating
designs.
Yet another problem with the existing art
is that EMI shielding and "skins" are attached to
the support component chassis which also supports
the ink-jet alignment chassis. These shielding
covers have been found to distort the support
component chassis, which in turn can distort the
ink-jet alignment chassis, resulting in movement
between critical ink-jet components.
It is seen, therefore, that it would be
desirable to have an ink jet printhead chassis
system capable of overcoming the problems associated
with the prior art.
Summarv of the Invention
This need is met by the monolithic ink
jet printhead chassis that allows ink-jet printhead
mechanical, electrical, and fluidic functions to be
precisely and rigidly tied together.
In accordance with one aspect of the
present invention, the problems with the prior art
are overcome by using a single highly rigid frame or
chassis which serves to maintain the alignment of
the ink-jet components and to which all the
electronic and fluid components are attached. The
central location of this chassis serves to isolate
the electronic support components from the fluid
related components, minimizing the risk to the
electronic components due to a catastrophic failure
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of an ink jet component. It further serves to
improve the flow of clean air around the critical
ink jet components.
Accordingly, it is an advantage of the
present invention that it provides a structurally
sound and precision platform for the ink jet
printhead. It is a further advantage of the present
invention that the monolithic ink jet printhead
chassis facilitates mounting of electrical,
mechanical, and fluidic printhead components.
Finally, it is an advantage of the present invention
that it minimizes part numbers and structure size.
Other objects and advantages of the
invention will be apparent from the following
description, the accompanying drawing and the
appended claims.
Brief Description of the Drawing
Fig. 1 is an exploded view of a printhead
with monolithic printhead chassis, constructed in
accordance with the present invention.
Detailed Description of the Preferred Embodiments
The novel design of the present invention
enhances ink-jet performance, increases printhead
ruggedness, and serves as a precision chassis.
Referring to Fig. 1, a structural beam or monolithic
frame 10 has been created that is similar in length
to the drop generator 12 and catcher of assembly 14.
This beam 10, which comprises the monolithic frame,
has a fairly large vertical axis, as indicated by z-
axis direction arrow 16, so that the rectangular
moment of inertia is large. This beam attaches to
the top of the catcher charge plate assembly 14 and
its length along the z axis, perpendicular to the
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plane of the catcher, is quite large. One manner by
which the catcher-charge plate assembly can be
attached to the frame is described and claimed in
commonly assigned, co-pending patent application
Serial No. (Attorney Docket SDP216PA),
totally incorporated herein by reference.
The long z-axis length of the frame 10
makes the frame very stiff and, therefore, immune to
distortion caused by loads to the top surface. The
value of this will be discussed later. The beam
depth along the x axis indicated by arrow 18 is
shallow, yet sufficient as the external loads in
this direction are minimal. This allows for an
incredibly small foot print ratio, for example a
foot print ratio of only five.
The accessible, and rigid monolithic
printhead frame 10 solves the problem of different
electrical, mechanical, and fluidic connections
being in several different planes, which has
heretofore required the end user to make many
different connections. As a result of the large z
axis extension of the monolithic frame, the new
structure 10 has enough stiffness to allow all of
these connections simultaneously. The new structure
has the necessary rigidity as a result of locating
all these connection at the upper surface of the
frame, and orienting all these connections so that
the insertion forces are directed down, parallel to
the large z axis of the frame. Furthermore, by
locating these connections and the mounting features
at the top of the monolithic frame and locating the
mounting features for the charge plate/catcher
assembly and the droplet generator near the bottom
of the frame, the risk of the critical alignment of
the drop generator to the charge plate/catcher
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assembly being altered by a printhead installation
is minimized.
The force required to mate all the fluid
and electrical connections is applied to the
printhead by means of a lift mechanism with engages
dovetail grooves on each end of the monolithic
frame. Clearance holes are provided in the cover,
so that the lifting force is applied only to the
monolithic frame. The lifting mechanism is part of
a suitable printhead latch mechanism, such as is
described and claimed in commonly assigned, co-
pending application Serial No. (Attorney
Docket SDP208PA), totally incorporated herein by
reference.
To dock or mate printheads with other ink-
jet hardware, prior art used "tooling balls" and
"vees" that would contact multiple planes, and
rectangular shapes that would nest and couple with
multiple planes. This required complex mating
structures to be developed. The monolithic frame
structure 10 of the present invention allows the
printhead to be located to a rigid three area (flat)
reference plane 24 on the top of the monolithic
frame. This facilitates a simple tripod type mating
surface on the printhead docking station. Pins that
pilot into apertures machined into the monolithic
frame 10 are used to help guide the fluid and
electrical connection to engage properly. It should
be noted that the printhead cover 26 is not used to
locate or hold the printhead in place, as has been
required in the prior art. As mentioned earlier,
the cover has openings to allow the lift mechanism
to engage the frame directly, as can be seen in the
exploded view of Fig. 1. Furthermore, the cover has
clearance holes so that contact to the reference
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plane of the frame, not to the cover, determines the
printhead location. These printhead locating
features are also described more fully in commonly
assigned, co-pending application Serial No.
(Attorney Docket SDP208PA).
The drop generator is mounted to the frame
at 42 by means of freeze blocks 28. These freeze
blocks are attached to the drop generator by means
of thin wall tubing which is bonded into dovetail
grooves in the top of the drop generator. The
freeze blocks are typically cyanoacrylate bonded to
mating surfaces on the monolithic frame. These
freeze blocks and the manner in which they are
bonded to the frames are described and claimed in
commonly assigned, co-pending application Serial No.
(Attorney Docket SDP223PA), totally
incorporated herein by reference. This structure
has resulted in a printhead which is much more
rugged-than the prior art. Accelerations as small
as 15g's, for example, could cause permanent
deformation to the two chassis structures of the
prior art, producing alignment shifts of precision
parts. The monolithic frame 10 of the present
invention can sustain "g" loads as high as, for
example, 70g's, without any associated movement,
thereby providing a design which yields extremely
high shock load capacity for the continuous ink-jet
printhead.
With the structure and configuration
proposed by the present invention and illustrated in
Fig. 1, the ink-jet components are on one side 24 of
the frame 10, and the electronics and the their
required cooling system, including cooling device
30, are on the other side 20 of the frame 10. This
configuration simplifies assembly while allowing for
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a smaller printhead size. The centrally located
frame also isolates the electronic components from
the ink, In the prior art the electronics could be
exposed detrimentally to ink mist produced during
normal operation or to ink from a printhead failure.
Continuous ink-jet printheads run at high
frequencies and emit electrical "noise". EMI
shielding is required to contain this noise that
could cause failures in other sensitive electronic
devices. Current art has resolved this dilemma with
metal covers on the printhead. As the prior art did
not isolate the electronic components from the ink
jet ones, it was necessary to install these EMI
shielding covers after the printhead was completely
aligned, to provide the necessary access for
alignment. When cover pieces were fastened to the
printhead chassis, the chassis could be twisted,
causing undesirable movement in critical ink-jet
components. Furthermore, the numerous seams between
the cover pieces and the printhead chassis tended to
allow excessive electrical noise leak out of the
printhead.
These problems are resolved by the present
invention. The monolithic frame 10 of the present
invention is extremely rigid and resolves the
movement issues related to the prior art.
Furthermore, the cover, which is a one piece unit,
attaches to the printhead only at two locations on
the top of the frame. It therefore does not induce
any shift of the critical ink jet components. The
monolithic frame 10 bisects the printhead into two
sections, an electronics side 20 and an ink jet side
24. This allows the noise emitting "charge driver
board" 22 to be isolated by a wall of metal, i.e.,
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the frame 10. The single piece cover 26,
eliminating all seams, is attached to the monolithic
frame 10. Flexible EMI shielding gaskets or seals
between the frame and the cover and around the cover
ports complete the EMI shielding of the electronics
without affecting the alignment of the ink jet
components.
It is known in the art to be desirable for
continuous ink-jet printheads to maintain purified
positive air pressure around the print array. The
positive air chamber shape and size around the print
array affects the uniformity of this air flow. The
positive air chamber should closely match the array
in length and, if designed effectively, print head
life and performance will be greatly enhanced. The
monolithic frame 10 facilitates this by reducing the
volumetric area to be pressurized to 99 cubic inches
versus 717 cubic inches on current art. The primary
structure of the monolithic frame is also aligned
parallel to the array so that the air flow can be
more readily distributed down the length of the
array. The use of a single piece cover 26
eliminates the possibility of air leaks at any seams
so that the supplied air can be more effectively
used. This positive air chamber is described and
claimed in commonly assigned, co-pending application
Serial No. (Attorney Docket SDP222PA),
totally incorporated herein by reference.
Thermal gradients in the printhead
structure can cause differential movement in
critical ink-jet printhead hardware. In accordance
with the present invention, these thermal gradients
have been greatly reduced. This is accomplished by
circulating the printing fluid that normally flows
through the drop generator and catcher/charge plate
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assembly also through the monolithic frame 10. In
this way, the temperature of the frame 10 tracks
that of the drop generator and the catcher due to
the flow of the same ink through each component.
This is facilitated by the size and rigidity of the
frame 10 which allow it to serve as a fluid
manifold. The proximity of the frame 10 to the
print array also helps in reducing the thermal
gradients in the printhead. Ideally, the frame
should be made of a material having a similar
thermal expansion to that of the catcher charge
plate assembly and of the drop generator. In this
way distortions of the printhead caused by thermal
gradients are minimized.
A shutter device known as an "eyelid" 14
is used for maintenance and as an air flow control
tool. The eyelid registration to other ink-jet
printhead components is critical. In the prior art,
the eyelid was located though a sheet metal support
component chassis or with loosely tied linkage. As
a result of poor alignment, there have been problems
with leaky eyelid seals. The monolithic frame 10
has built in registration pins 32 and pads that are
rigidly coupled to other critical ink-jet
components. This facilitates improved registration
and rigidity which improves printhead performance
and reliability.
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.
