Note: Descriptions are shown in the official language in which they were submitted.
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SERt~L lNK JET PR~Nl`ING II~D
The present invention relatcs to a serial ink jet
printing head for liquid, electrically conductive ink,
comprising an electrically insulating container for the ink,
having a nozzle for the selective discharge of partic]es on ink,
an electrode in contact with thc ink and a co~ ter-electro(ie
adjacent to the nozzle, the discharge being caused by an e~ect-
rical voltage pulse between the counter-electrode and the
electrode.
In known printers of the above-indicated type, the
container is connected by means of conduits to a larger-capacity
tank which is disposed at a certain distance from the printing
location. In the case of printers in which the head is mounted
on a movable carriage, the tank is disposed on the fixed part of
the machine and is connected to the container by way of flexible
conduits of substantial length. A pump is required to pass the
ink from the tank to the head, so that theprinter is expensive to
produce and complicated in operation.
The objecl; of the present invention is to provide an
ink jet printing head which does not require a separate tank and
which can be easily replaced when the ink is exhausted.
In meeting this object, the invention provides a
printing head which is characterised in that the container is
closed andcomprises means for creating within the container a
depression such as to permit the formation of a concave meniscus
of ink in ;the nozzle, until the ink in the container isexhausted.
The invention wi]l now be described in mone detail, by
way of e~ample, with reference to the accompanying drawings, in
which:
Fig. 1 is a view in longitudinal section of a printer
incorporating a printing head in accordance with a first
embodiment of the invention,
Fig. 2 is a plan view of the printing head partly in
section taken along line II-II in Fig. 1,
Fig. 3 is a view in partial section taken along line
III-III in Fig. 1,
Fig. 4 is a longitudinal section of a printer incorporating
a printing head in accordance with another embodiment of the
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invention,
Fig. S is a view in horizontal section taken along line
V-V in Fig. 4,
Fig. 6 is a view in section taken along line VI-VI in
Fig. 4, and
Fig. 7 is a front view on an enlarged scale of the
nozzle-bearing plate of the printing head.
Referring to Fig. 1, a support bar 10 acts as a platen
bar for a sheet of paper 11 which is displaced vertically to
permit the printing of dots at successive elementary rows, for
example for alphabetic printing in a dot-matrix format.
The printer comprises an ink jet printing head 12 whi.ch
is mounted on a carriage 13 which is movable transversely with
an alternating movement in manner known per se. The head 12
essentially comprises a container 14 of insulating material, for
the ink 16 which is electrically conductive. The container 14
is closed towards the support 10 by a plate 17 in which a
nozzle 18 is disposed (see Fig.3), for the discharge of particles
of ink 16. The ink is in electrical contact with an electrode
41 which will be seen in greater detail hereinafter, being
connected to the outside of the container 14 tsee Fig. 1).
The printer comprises an electrical control circuit 21
which is capable of producing a voltage pulse between the
electrode and a counter-electrode 22 which is adjacent to the
nozzle 18. A state of electrical and thermal e~citation is then
generated at the meniscus which is formed by the ink 16 in the
nozzle 18, such as to cause a plurality of particles of ink to be
discharged through the nozzle 18.
Thecarriage 13 (see Fig. 1) substan-tially corresponds
to the carriage indicated at 51 in the above-quoted patent
application, and comprises a projection portion 24 by means of
which it normally rests against the support 10, under the force
of a leaf spring 26.
The container 14 comprises two apertured flange portions
27 (see Fig. 2) for removably connecting-it to the carriage 13 b~
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means o~` two screws 28.
In accordance with a first embodiment of the invention,
the container 14 has`a capacity of about 8 - 10 cm3 of ink 16
and essentially comprises a rigid box construction 29 of a
substantially square base configuration. The box 29 has an
appendix portlon 31 which is connected to a front closure block
member 32 carrying the plate 17. Ihe block member 32 divides
the container14 in such a way as to form a first cavity 33
which is disposed between the plate 17 and a surface 34 of the
block member 32, which i~ parallel to the plate 17. The
spacing between the surface 34 and the plate 17 of the same
order of magnitude as the thickness ofthe plate 17. Preferably,
that spacing is about 0.4 mm. The block member 32 also forms
within the container 14, another cavity35 which is much larger
in capacity than the cavity 33 and which essentially forms the
tank for the ink 16.
The portion 31 is provided with an aperture 36 of
substantially rectangular shape, which is disposed in line with
a cavity 40 in the block member 32, wh,ich, by means of three
passages in the block member 32, forms a commuliication between
the cavity 33 and the cavity 35. A first passage is formed by
an aperture 37 disposed above the nozzle 18 (see Fig. 3), while
the other two passages are formed by two conduits 38 and 39 which
are disposed in the same horizontal plane parallel to the nozæle
18 and equally spaced therefrom. The conduits 38 and 39 are
associated with a region (see Fig. 1) of the cavity 40, which is
lower than the bottom of the box 29, and thus permits all the
ink 16 to pass into the cavity 33. The aperture 37 (see Fig. 3)
is oblong and extends at least over a length equal to the distance
between the two conduits 38 and 39.
Embedded in a position associated with one of the two
conduits 38 and 39 (in Fig. 2, the conduit 38) in the block
member 32 is the electrode 41 in the form of a metal stud or pin
having a head 42 which is arranged to engage a contact 43 which
is fixed on the carriage 13, in turn connected to the negative
terminal of the control circuit 21. It will be clear therefore
that the electrode 41 remains in contact with the ink 16,
substantially until the ink 16 is used up. On the other hand,
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the counter-electrode 22 en~ages a contact l~4 on the carriage
13, which in t,urn is connected to the positive terminal of the
control circuit 21 (s~e Fig. 1). The box 29 is closed upwardly
by a substantially flat cover 45 which is of insulating
material and which' is welded or bonded to the edge of the box 29.
The cover 45 is provided in its central part with a
circular aperture 46; disposed in line with the circular
aperture 46 is a sleeve portion 47 which is provided on its
outside with two grooves 48. A disc 49 can be fitted into the
sleeve portion 47 and act~ as a plug for the box 29 to which it
is connected by means of atubular bag-like diaphragm 51 which
is imperviousboth to air and to the ink 16. The diaphragm 51 is
sealingly secured to the sleeve portion 47 by means of two
annular rings 52 which are disposed ln the groove 48. The disc
49 is urged upwardly by a coil compression spring 53 which permits
expansion of the cavity 35, maintaining a certain depression
in the cavity.
The head 12 forms an easily replaceable ink cartridge,
90 that it can~be supplied as a dispo,~able ink container. The
container 14 of the head 12 is filled `up to the level indicated
in Fig. 1, and then the diaphragm 51 is fixed to the sleeve
portion'47, holding the spring 53 in a compressed condition.
The spring 53, when thus compressed,urges the disc 4g upwardly
and produces a predetermined depression of the order of 3/lOOth of
an atmosphere (about 3 kPa) in the space above the ink 16. That
depression on the one hand permits the ink 16 to form the meniscus
within the nozzle 18 at a substantially constant position, while
on the other hand it prevents the ink 16 from escaping`from the
nozzle 18. When the head 12 is mounted on the carriage 13, with
the electrode 41 and th'e counter-electrode 22 being energized
with voltage pulses fromthecircuit 21, a condition of excitation
is generated in the nozzle 18, such as to produce a spray of ink
particles 16 towards the paper 11, as described in the above-
quoted patent application. The flow of electric current in the
ink in the nozzle 18 generates in the cavity 33 a series of small
bubbles which tend to move upwardly. By way of the aperture 37,
the bubbles pass into the cavity 35 and then into the region 54
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above the ink, whereby the internal pressure in the region 54
tends to increase gradually. The spring 53 then raises the
disc 49, causing an expansion of the volume of the region 54
and thus maintaining a certain depression within the container
1~. The magnitude of that depression however gradually decreases
down to a value of about l/lOOth atmosphere (about 1 kPa).
When the ink in the container 14 is on the point of being
used up, it still rises by capillary action in the cavity 33,
forming the meniscus 23. When the meniscus 23 of ink is no
longer formed, the spring ~3 takes up its position of maximum
extension. The cartridge 12 is then totally exhausted and
must be replaced in order to effect further printing.
Figs. 4 to 7 show another embodiment of the invention
in which the components generally indicated in the embodiment
shown in Figs. 1 to 3 are denoted by the same reference numeral.
The carriage 13 is now formed by a block 56 of insulating plastics
material, of substantially prismatic shape. Internally, the
block 56 defines a cavity 57 of inverted frustoconical shape with
its axis slightly inclined towards the paper 11, relative to
the vertical. '~
The block 56 integrally carries a sleeve portion 59 with
a horizontal axis. The block is slidable by means of the
sleeve portion 59 on a fixed transverse guide 61. The block 56
also has a recess 62 by means of which it is guided with a great
deal of clearance on a second transverse guide 63. A bow-shaped
leaf spring 64 tends to cause tlie block 56 to rotate in a clock-
wise direction about the guide 61, in a similar manner as
described above in relation to the spring 26 shown in Fig. 1.
The carriage 13 is displaced transversely in known manner by
means of a flexible cable connected to a projection portion 65 on
the block 56.
The container 14 of the head 12 is formed by a box
structure 66 of non-conducting material which has a substantially
rectangular base and a wall portion 67 which is directed towards
the paper support, in the form of a pyramid with a sub-
horizontal axis, which is inclined with respect to the vertical
large base of the pyramid. Provided on the small base of the
pyramid is a seat means 68 for accommodating the plate 17 of the
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nozzle 18. A substantially rectangular recess 69 in the outsi-le
surface of the wall portion 67 forms a first cavity 710f the
container, between !the plate 17 and the front surface of the
recess 69. The thickness of the cavity 71 is of the same order
of magnitude as the thickness ofthe plate 17, preferably 0.4 mm.
The box 66 also forms a second cavity 72 which has a
capacity of about 8 cm3 and is therefore much larger than the
cavity 71, thus forming the tank for the ink 16. The cavity
72 communicates with the cavity 71 by means of a C-shaped
aperture having a hori~ontal portion 73 (see Fig; 6) above the
nozzle 18 and two vertical portions 74 and 75 beside the nozzle
18, the portions 74 and 75 being equally spaced from the nozzle
and extending to the lowest point of the cavity 72. In
particular, the bottom 76 (see Fig. 4) of the box 66 comprises a
portion 77 adjacent to the wall portion 67, which is slightly
inclined towards thewall portion 67 to form the above-mentioned
lowest point. The horizontal portion 73 of the aperture, which
forms a communication between the cavity 71 and the cavity 72,
is inclined, at a larger angle than the portion 77, upwardly and
towards the centre of the box 66 to promote discharge of the
bubbles which are formed during the printing operation.
At thecentre, the box 66 carries a small pillar or
column 78 which extends over the entire height of the box and
which has an axial bore 79. The upper end of the pillar 78
has a transverse recess 80 ~see also Fig. 5).
Provided on the underneath surface fothe bottom 76 of
the box 66 (see ~ig. 1) is a circular seat 81 in which there is
disposed a helical compression spring 82, the turns of which
progressively decrease in diameter in such a way that, when the
spring 82 is totally compressed, all the turns are disposed in the
same plane and the spring 82 is contained within the seat 81.
Also fixed in the seat 81 is a flexible diaphragm 83 which
is impervious with respect to air and the ink. The diaphragm 83
is of substantially frustoconical shape and has a concertina-like
closable side surfaceO The diaphragm 83 is fixed in position by
means of a ring 84 which is welded or bonded to the edge of the
seat 81. The diaphragm 83 is ofgreater thickness at the small
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base 85 of` the conical configuration, whereby the diaphragm
is substantially rigid and serves as a support for the spring 82.
The diaphragm 83 is ~apable of forming a cavity 89 which is
expandable downwardly and is capable of being accommodated in the
cavity 57 in the block 56.
The container 14 also has a cover 87 which is welded or
bonded to the edge of the box 66 after the box has been filled
with ink up to the level indicated in Fig. 4.
The region 88 above the level of ink in the box
communicates with the ca~vity 89 between the bottom 76 of the box
66 and the diaphragm 83 by way of the bore 79 and the recess 80.
In its lower part, the container 14 has an electrode 91
disposed at a position associated with the portion 74 of the
aperture between the cavity 71 and the cavity 72, whereby it
remains in contact with the ink l6 substantially until the ink
is exhausted. An end portion 92 of the electrode 91 extends
to the outside of the bottom 76 and is arranged to engage a
contact 93 carried by the carriage 13 when the head 12 is mounted
thereon. s~
The carriage 13 also carries a second electrical
contact 94 which is arranged to engage the counter-electrode
22 of the head 12. The two contacts 93 and 94 are in turn
connected in known manner to the control circuit 21.
For the purposes of mounting the head 12 on the carriage
13, the box 66 carries, in its~lower part, two curved projection
portions 96 which are arranged to engage into two complementary
recesses 97 in the block 56. A latching projection 98 on the box
66 is arranged to engage with a spring action over a shoulder 99
on the block 56.
Finally, the plate 17 is formed of alumina and is 0.2 mm
in thickness and is entirely covered over its outside surface and
over the lower part 95 of its inside surface, by a layer of
conducting material which is applied by the thick film method.
When the plate 17 is covered in that manner, it is pierced by a
laser beam to produce the nozzle 18 while the layer of
conducting material forms the counter-electrode 22. Finally, two
strips 100 of an irsulating wear-resistant material, for example
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glass, are applied to the layer 22 on the plate 17. The strips
100 (see Fig. 7) are parallel to the lines of printing on the
paper 11 and are about 0.1 mm in thickness in such a way that,
by bearing against the paper 11 during the printing operation,
they define the spacing between the nozzle 18 and the paper 11,
similarly as described above in regard to the projection portions
24 on the carriage 13 shown in Fig. 1. When the plate 17 is
prepared in the above-described manner (see Fig. 4), it is welded
or bonded to the seat 68 on the wall portion 67 of the box 66.
In a similar man~er as described above wit`h regard to the
embodiment shown in Figs. 1 to 3, the box 66 is filled with ink
16 to the level indicated in Fig. 4, and closed, for example by
welding the ring 84 in place, holding the spring 82 in a compressed
condition. That therefore produces a depression in the cavity
89 and thus, by way of the bore 79 and the recess 80, in the
region 88, ensuring that the meniscus is formed in the nozzle 18.
When the box 66 is mounted by a spring latch action on
the block 56, the spring 64 causes thestrips 100 to bear against
the paper 11. When the circuit 21 is selectively energi~ed, a
spray of ink for printing a dot is produced by way of the nozzle
18. The small bubbles which are formed as a result of the flow
of currentare discharged towards the cavity 72 by way of the
portion 73 of the aperture. Therefore, in the region 88 and thus
also in thecavity 89, the pressure progressively increases,
causing the compression spring 82 to extend, which`causes
expansion of the cavity 89 defined by the diaphragm 83. When
the ink 16 is exhausted, the spring 82 is completely extended and
the head 12 has to be replaced.
It should be noted that, if any portion of ink 16 passes
through the bore 79 into the caivty 89, it cannot return to the
nozzIe 18, so that it cannot be recovered. In order to avoid
ink being drained off in that manner, for example during the
storage and transportation operations before the assembly is
used on the printer, the cover 87 may be formed with a flexible
cover portion (not shown), at the location of the recess 80. The
flexible cover portion is caused to adhere to the recess 80 by
means of a strip or tongue portion which is stuck to the rigid
part of the cover 87, thus preventing the ink from draining away.
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After the box 66 has been fitted onto the block 56, the strip or
tongue portion is pulled off, whereby the flexible cover portion
can come out of the r'ecess 80 which then establishes the
communication between the region 88 and the cavity 89.
It will be appreciated that various modifications and
improvements may be made in the above-described heads, without
departing from the scope of the invention. For example, it is
possible to provide other systems for removably mounting the
head 12 or other means for unsealing the box 66 of a fresh
head when it is fitted o~to the machine.
Finally~ it is possible to provide mutliple heads with
various containers in various compartments for containing inks
of various colours which are to be used alternatively or in
combination.
