[0001] The present invention relates generally to ink jet printers, and more specifically
to an ink jet print head of the type wherein liquid is discharged through axially
aligned rear and front channels under the combined effects of electric field and air
pressure gradients
[0002] An ink jet print head of the type as shown and described in United States Patent
4,403,234 comprises a front nozzle member secured to a housing to define a laminar
airflow chamber. The housing is formed with a rear channel axially aligned with a
front channel provided in the front nozzle member. The rear channel is connected by
an electrically conductive pipe to a liquid supply to create a meniscus at the exit
end of the rear channel. The conductive pipe is connected to a signal source to charge
the liquid in the rear channel with respect to the front channel so that an electric
field gradient is established between the meniscus and the front channel. The airflow
chamber is connected to a pressurized air supply to produce an air pressure gradient
between the exit ends of the rear and front channels. Owing to the combined effects
of the field and pressure gradients, the meniscus is pulled forward and ejected through
the front channel to a writing surface.
[0003] However, the meniscus is very sensitive to disturbance generated when the print head
scans across the writing surface and becomes unstable when it returns to the original
shape after ejection of a droplet.
[0004] Patents Abstracts of Japan, Vol. 9, No. 57 (M-363) (1780) shows an ink jet printer
of the type concerned in the present application where an air supply is turned sharply
prior to passage through an outlet and an electric field is in use applied in the
region where the ink drop meniscus is formed.
[0005] EP-A-109755 is concerned with manufacture of a nozzle for an ink jet printing head
using etching techniques.
[0006] According to the present invention there is provided an ink jet printer comprising
a source of pressurized air, a liquid container, an ink jet print head comprising
a front nozzle member with a front channel, a housing secured to said front nozzle
member, a rear nozzle member having a forwardly projecting nozzle substantially corresponding
in radial dimensions to said front channel and defining with said housing a liquid
chamber which is connected to said container and further defining with said front
nozzle member a laminar airflow chamber, said rear nozzle member having a rear channel
extending forwardly from the liquid chamber in axial alignment with said front channel
to form a meniscus at the front end thereof, said airflow chamber being connected
to said air source for directing air to a point between said front and rear channels
so that it makes a sharp turn at the entry into said front channel creating a sharp
pressure gradient along a path between the exit ends of said front and rear channels
and a dead air region adjacent the exit end of said rear channel, an annular electrode
attached to the front surface of said front nozzle member, means for applying an electric
potential between said electrode and the liquid in said liquid chamber to establish
an electric field gradient between said front channel and said meniscus to cause it
to extend to and be partially expelled through said front channel, means for connecting
said liquid container to said air source so that in the absence of said electric field
liquid pressure in said rear channel is statically balanced with combined forces of
air pressure acting on said meniscus and the surface tension of the liquid, characterised
in that a portion of said electrode, inner walls of said front channel and a portion
of the rear surface of said front nozzle member which surrounds the entrance of said
front channel comprises a material having a liquid repellant characteristic for repelling
said liquid.
[0007] The nozzle member can be made by a method involving the steps of:
a) etching a first surface of a substrate to a predetermined depth according to a
first pattern to form a projecting nozzle having a nozzle opening therein, said substrate
being composed of a photosensitive glass or a laminate of an insulator and a layer
of a material dissimilar to said insulator, said layer forming said first surface;
and
b) etching a second, opposite surface of said substrate to a predetermined depth to
form a channel extending to and axially aligned with said nozzle opening.
[0008] The formation of the dead air region in the nozzle of the printer of the invention
causes the meniscus to convex, producing a high concentration of electric field and
reducing the minimum voltage required to tear it apart into a droplet.
[0009] The two-step etching process is advantageous in reducing the time taken to produce
the projecting nozzle since it minimizes deviations in nozzle-opening size which might
occur as a result of the tendency of the substrate material to erode sideways on a
single substrate. Furthermore, the bore at the rear of the nozzle opening can be appropriately
dimensioned so that its transverse cross-section is larger than that of the nozzle
opening and hence to reduce the resistance it offers to liquid passing therethrough.
[0010] The present invention will be described in further detail by way of example, with
reference to the accompanying drawings, in which:
Fig. 1 is a block diagram of an ink jet printer incorporating a print head of the
present invention;
Fig. 2 is an illustration of details of a portion of the print head of Fig. 1;
Fig. 3 is an illustration useful for describing the advantageous effect of the projecting
nozzle of the invention;
Figs. 4A to 4F are illustrations of various modifications of the rear nozzle plate;
Figs. 5A to 5G are illustrations of steps for fabricating a rear nozzle plate of the
print head according to the invention;
Fig. 6 is an illustration of a modified step of Fig. 5C;
Figs. 7A and 7B are illustrations of a further modification of Fig. 5C;
Figs. 8A to 8F are illustrations of a second method for fabricating the rear nozzle
plate;
Figs. 9A to 9F are illustrations of a third method for fabricating the rear nozzle
plate;
Fig. 10 is a cross-sectional view of a rear nozzle plate manufactured according to
the present invention.
Figs. 11A to 11C are cross-sectional views of embodiments in which ink-repellant layers
are formed on the nozzle members; and
Figs. 12A and 12B are illustrations of apparatus for depositing an ink-repellant layer
on a nozzle member.
[0011] Referring now to Fig. 1, there is shown an ink jet print head and its associated
devices according to a preferred embodiment of the invention. The print head 1 comprises
a front nozzle panel 2 having a front channel 3. The front nozzle plate 2 is formed
of insulative material and secured to a rear housing 4 of insulative material. The
rear housing is formed with a liquid chamber 5 to hold ink therein supplied from an
ink container 6 through electrically conductive pipe 6a. The liquid chamber 5 is defined
at the front with a rear nozzle plate 7 having a projecting nozzle 8. A rear channel
9 extends from the liquid chamber 5 through the projecting nozzle 8 in axial alignment
with the front channel 3 to allow ink in liquid chamber 5 to lead therethrough to
form a meniscus at the extreme end. Front nozzle plate 2 defines with rear nozzle
plate 7 a disc-like, laminar airflow chamber 10a of an air chamber 10 and defines
with rear housing 4 an annular portion 10b.
[0012] A ring electrode 11 encircling the front channel 3 is secured to the outer surface
of front nozzle plate 2. A voltage is applied across electrode 11 and pipe 6a from
a signal source 12 to establish an electric field gradient between electrode 11 and
the liquid in rear channel 9.
[0013] A pressurized air supply source 13 is connected by a pipe 14 to the air chamber 10
to generate an airflow in the annular air chamber portion 10b to cause it to spiral
in a laminar flow through the disk-like chamber portion 10a to front channel 3 and
thence to the outside. The airstream makes a sharp turn at the entry to front channel
3 creating a sharp pressure gradient along a path between the front ends of rear channel
9 and front channel 3. Pressurized air is also supplied through a regulator valve
15 to the ink container 6. Valve 15 is adjusted so that in the absence of a voltage
on electrode 11 the liquid pressure in rear channel 9 is statically balanced with
the combined forces of air pressure acting on the meniscus and its surface tension.
In response to the application of a voltage to electrode 11, the liquid in rear channel
9 is electrostatically charged and pulled forward under the influence of electric
field gradient. The liquid is elongated into a pencil-like shape under the pressure
of air ejected through the front channel 3 and ejected to a writing surface.
[0014] As best seen in Fig. 2, the projecting nozzle 8 has an outer diameter slightly smaller
than the diameter of front channel 3 and extends forward from the nozzle plate 7 by
a distance B. Airstream is narrowed as it passes through the space between the front
and rear channels and creates a dead air region immediately adjacent the front end
of rear channel 9. On the other hand, the liquid in rear channel 9 wets the front
surface of the nozzle 8 and tends to disperse outward. However, further dispersion
of the liquid beyond the outer edge of rear nozzle 8 is prevented by a force exerted
thereupon by the airstream moving past that outer edge, causing the liquid to slightly
bulge forward. In the absence of electric field, the high pressure in the dead air
region causes the meniscus at the front end of rear channel 9 to assume a convexed
shape as shown at 8a and stabilizes it against external disturbance.
[0015] When the ring electrode 11 is impressed with a voltage, the meniscus is elongated
rapidly, forming a slope portion 8b extending from the outer edge of rear nozzle 8
to a narrow, pencil-like portion 8c, as shown at Fig. 3. The formation of convexed
meniscus 8a concentrates the electric field thereon and reduces the minimum voltage
required to tear it apart into droplets. Because of the presence of the dead air region,
the meniscus quickly returns to the original state after ejection of ink.
[0016] In a preferred embodiment, the front surface of the nozzle 8 is roughened to present
a small angle of wet to liquid to allow the meniscus to easily wet the front surface
of nozzle 8. The small wet angle reduces the response time of the print head and increases
the amount of liquid to be ejected per unit time.
[0017] It is preferable that the axial dimension B of the rear nozzle 8 and the outer diameter
Dr of rear nozzle 8 satisfy the following relations:
4L/5 < B < L/20
Df < Dr < Df/4
where, L = spacing between front and rear nozzle plates 2 and 7, and Df = diameter
of front channel 3.
[0018] Experiments confirmed that under like operating factors the print head of the present
invention operates with a minimum pulse duration which is 1/10 of the minimum pulse
duration of the prior art and is immune to vibrations in a range which is ten times
greater than the prior art.
[0019] Various preferred forms of the rear nozzle plate are shown in Figs. 4A to 4F. The
variations shown at Figs. 4A to 4D are advantageous to further increase meniscus stability
and improve meniscus response characteristic. This is accomplished by increasing the
contact area of the rear nozzle front end face with liquid. In these variations, the
rear channel 9 has a front portion passing through nozzle 8 and a rear portion passing
through nozzle plate 7.
[0020] In Fig. 4A, the rear channel 9 has a front portion 9A′ having a part-spherical surface
and a cylindrical rear portion 9A˝. The rear channel 9 in Fig. 4B has a frusto-conically
shaped front portion 9B′ and a rear portion 9B˝. In Fig. 4C, rear channel 9 has a
front portion 9C′ having a larger transverse cross-sectional area than a rear portion
9C˝. This increases the amount of liquid to be contained in the nozzle 8. The rear
channel 9, Fig. 4D, has a front portion 9D′ having a staircase cross-section and a
cylindrical rear portion 9D˝, the staircase portion increasing its diameter with distance
away from the rear portion 9D˝.
[0021] In the embodiments of Figs. 4A and 4B, the liquid being ejected forms a large angle
of wet contact with the surface of the front portions 9A′, 9B′ as compared with the
embodiment of Fig. 1 and is thus given a greater liquid retaining force with which
the meniscus is more stabilized against external vibrations which might otherwise
cause it to break. In the embodiments of Figs. 4C and 4D, front portions 9C′ and 9D′
serve as reservoirs to hold a greater amount of liquid therein to increase liquid
ejection capability.
[0022] In Fig. 4E, rear nozzle 8 is formed with an annular groove 80 to entrap liquid which
might spill over the edge of the nozzle if an excessive amount of force is externally
applied to the print head. The annular groove may be provided around the nozzle 8
as shown at 81 in of Fig. 4F.
[0023] Description will now be given to a method for fabricating a rear nozzle plate with
reference to Figs. 5A to 5G.
[0024] Illustrated at 21 in Fig. 5A is a photosensitive glass which is composed of a SiO₂-Aℓ₂O₃-Li₂O
glass containing CeO₂ and Ag₂O. A photomask 22 having a plurality of ring-shaped opaque
portions 22a (only one of which is shown for simplicity) in a transparent area 22b
is placed on the upper surface of the glass 21. The photosensitive glass 21 is subject
to an imagewise radiation of ultraviolet light through the mask 22 to cause portions
21b underlying the transparent portion 22b to provide the following reaction:
Ce³⁺ + A⁺ + ultraviolet light ― Ce⁴⁺ + Ag⁰
The glass is then subject to a primary heat treatment so that the silver content of
the compound becomes colloidal and then subject to a secondary heat treatment to form
crystals Li₂O-SiO₂ around silver colloids. The Li₂O-SiO₂ crystals are etched away
to a predetermined depth. This leaves an upper portion of the amorphous region to
serve as a rear nozzle 21a as shown in Fig. 5B. This etching process is preferably
accomplished by applying a layer of hydrofluoric acid resistant material to the lower
surface of the glass and submerging it into an aqueous hydrofluoric acid solution.
Suitable material for the hydrofluoric acid resistant layer is a paraffin-containing
material available from Sou Denshi Kogyo Kabushi Kaisha under the trademark of "Electron
Wax". The wax is applied at a temperature of 70°C and removed by immersing it in a
trichloroethylene solution agitated at an ultrasonic frequency.
[0025] In Fig. 5C, a photoresist layer 24 is coated on the lower surface of the glass 21
and a photomask 25 having a plurality of opaque portions 25a is placed on the photoresist
24 so that opaque portion 25 aligns with corresponding the nozzle 21a. The diameter
of the opaque portion 25a is greater than the inner diameter of, but smaller than
the outer diameter of, the nozzle 21a. The photoresist is exposed to ultraviolet imagewise
radiation through the mask 25. Unexposed portions are etched to form a plurality of
holes 24a each being concentrical with the nozzle 21a as shown at Fig. 5D.
[0026] A hydrofluoric acid resistant layer 26 is then formed over the entire upper surface
of the glass 21 so that it fills the space within the projecting nozzle 21a as shown
in Fig. 5D. The glass substrate is immersed in an aqueous hydrofluoric acid solution
to etch the portions of the glass above the hole 24a to thereby produce a bore 27
extending across the thickness of the glass 21. The photoresist 24 is removed after
it is carbonized in a plasma and the layer 26 is removed by immersing the glass in
a trichloroethylene solution agitated at an ultrasonic frequency (Fig. 5E). Since
the nozzle 21a remains amorphous, it is preferable that the glass be flooded with
ultraviolet light and heat-treated in a manner similar to that described in connection
with the step of Fig. 5A to crystallize the amorphous channel portions 21a. This crystallization
process causes the whole glass 21 to homogenize as shown at Fig. 5G and increases
its mechanical strength. The glass 21 is then cut into individual nozzle plates.
[0027] It is seen that nozzle portion 21a and hole 27 are created by etching the glass in
opposite directions. Although the amorphous region of the glass has a tendency to
erode at a rate substantially 1/20 of the rate at which the crystalline region erodes,
the method of the invention keeps the glass 21 from being subject to a prolonged single
etching process and thus prevents it from being excessively eroded sideways. It is
possible to produce a rear nozzle plate with a nozzle 21a having an outer diameter
of 100 micrometers with an error of ± 2 micrometers, an inner diameter (at the forward
end) of 40 micrometers with an error of ± 2 micrometers and an axial dimension of
35 micrometers. In this case, the hole 27 has a depth of 130 micrometers. Although
it has a small thickness in radial directions, the nozzle 21a has a sufficient rigidity
to retain its shape for an extended period of time. The glass-formed nozzle plate
7 has another advantage in that it is chemically resistant to ink and free from swelling.
[0028] In the process step shown in Fig. 5C, incident ultraviolet light that penetrates
the photoresist 24 is reflected irregularly at different depths of the crystallized
portions of the glass and part of the reflected light enters undesired portions of
the photoresist 24, causing the boundary between the light-exposed and non-exposed
areas to blur. For this reason, a light-shielding layer 16 is provided between the
lower surface of glass 21 and photoresist 24 as shown in Fig. 6. The light-shielding
layer 16 is formed by vacuum-evaporating a hydrofluoric acid resistant material such
as gold on the glass until it attains a thickness of 1 to 2 micrometers. After being
exposed to ultraviolet imagewise radiation, the photoresist 24 is removed followed
by the removal of gold layer 16 using aqua regia. Alternatively, the lower surface
of glass 21 is roughened by etching as shown in Fig. 7A. The photoresist layer 24
is applied on the roughened surface (Fig. 7B). Most of the ultraviolet light penetrating
the photoresist 24 is reflected at the roughened surface, whereby the light entering
the undesired portion of the photoresist 24 is negligible. The roughened surface presents
an increase in contact area between the glass 21 and photoresist 24 so that the latter
is firmly adhered to glass 21.
[0029] Figs. 8A to 8F are illustrations of a second preferred method of fabricating the
rear nozzle plate 7. In the first step, an insulative substrate 31 of ceramic or glass
is prepared (Fig. 8A). On the substrate 31 is deposited a layer 32 of a material which
is dissimilar to the underlying substrate. This material is chemically resistant to
ink but can easily be eroded by an etchant. Suitable materials for the layer 32 are
copper, aluminum, gold, platinum, chrome, molybdenum, photosensitive glass as mentioned
previously, and photosensitive resin. Such metal is deposited by electroplating and
the nonmetal material can be deposited using a suitable adhesive. A photoresist layer
33 is applied on the layer 32. The photoresist 33 is exposed to ultraviolt imagewise
radiation through a photomask 34 having transparent portion 34a in the shape of a
ring in the opaque background. The unexposed portions of the photoresist 33 are removed
to create a photoresist ring 33a on the layer 32 as shown in Fig. 8B. An etching resistant
coat 35 is applied on the lower surface of substrate 31. The substrate 31 is then
immersed in an etching solution to remove the portions of the layer 32 which are unoccupied
by the photoresist ring 33a. If the layer 32 is composed of gold or platinum, aqua
regia can be used as the etching solution. The photoresist ring 33a is then removed
by carbonizing it in a plasma followed by the removal of the etching resistant layer
35 to thereby form a nozzle 32a (Fig. 8C).
[0030] In Fig. 8D, photoresist is applied to the lower surface of substrate 31 to form a
layer 36 which is flooded with an ultraviolet imagewise radiation through a photomask
37 having an opaque portion 37a masking the portion directly below the nozzle 32a
in a manner similar to the step shown in Fig. 5C. A hydrofluoric acid resistant layer
38 of the material as used in the layer 26, Fig. 5D, is applied entirely over the
upper surface of substrate 31 so that the space within the nozzle 32a is filled (Fig.
8D), which is followed by the immersion of the substrate into a photoresist etching
solution to remove the unexposed portion of photoresist layer 36 to form a hole 36a
(Fig. 8E). The substrate is then immersed in an aqueous hydrofluoric acid solution
to form a hole 31a, Fig. 8F, that extends through the thickness of substrate 31, followed
by the removal of layers 36 and 38. The method of Figs. 8A to 8F is advantageous for
applications in which it is desired to select a suitable material for the projecting
nozzle portion 32a having a sufficient surface roughness to retain the meniscus which
may be different from the surface roughness of the substrate 31.
[0031] Figs. 9A to 9F illustrate a further manufacturing process in which the steps of Fig.
5A is initially performed to crystallize portions of a glass substrate 41 that surround
a cylindical amorphous portion. The step shown at Fig. 9A follows. This step is similar
to the step of Fig. 5B with the exception that the etching process is carried out
on opposite surfaces of the glass substrate 41 to form a pair of nozzles 41a and 41b.
Since the upper nozzle 41a is produced out of the region which is located closer to
the photomask than is the lower nozzle 41b, the former has a more sharply defined
boundary with the sourrounding area than the latter. In Fig. 9B, the upper surface
of substrate 41 is entirely coated with a hydrofluoric acid resistant layer 42 so
that it fills the space within the nozzle 41a. The lower surface is coated with a
layer 43 over areas outside of the lower nozzle 41b. The layer 43 may be formed of
the same wax as used in Fig. 5D. The lower nozzle portion 41b has a greater surface
roughness on its side wall than on its upper face. The difference in surface roughness
prevents the paraffin layer 43 from spreading beyond the upper edge of the nozzle
portion 41b. The substrate is then immersed in an aqueous hydrofluoric acid solution
of 5% concentration which is maintained at a temperature lower than 34°C to create
a hole 41c within the amorphous cylinder that extends between nozzles 41a and 41b
(Fig. 9C). In this process, etching solution tends to permeate through the boundary
between the nozzle 41b and surrounding layer 43 to cause erosion to occur along that
boundary. The substrate can be etched for a period of 35 minutes at a solution temperature
of 20°C to remove a volume to a depth of 170 micrometers with a diameter of about
50 micrometers. Due to sideways erosion, the hole 41c is tapered upward.
[0032] Layers 42 and 43 are removed in a solution of trichloroethylene agitated at ultrasonic
frequency (Fig. 9D). The lower surface of the substrate is lapped to present a flat
surface (Fig. 9E). The substrate 41 is then subject to ultraviolet radiation and then
heated in the same manner as in Fig. 5G to crystallize the amorphous region (Fig.
9F).
[0033] The hydrofluoric acid resistant layer 43 may alternatively be formed of epoxy resin
adhesive which is a mixture of Epicoat 828 as a principal component and Epicure Z
as a curing agent (both being the trademarks of Shell Chemicals). The photosensitive
glass substrate 41 is heated to a temperature of 40°C to apply Epicoat 828 to a thickness
of 5 micrometers and then allowed to half-cure for a period of 50 hours at room temperature
to prevent intrusion of Epicoat into the nozzle 41b. This is followed by a full curing
process in which the substrate is maintained at a temperature of 70°C for a period
of 60 minutes. The epoxy resin layer 43 can be removed in an oxygen plasma environment.
In comparison with the method involving the use of the wax, the epoxy resin layer
43 is favored in terms of its excellent adherence to the underlying glass substrate
and strength. Due to the high strength, undesired erosion around the nozzle 41b can
be minimized.
[0034] In the process of Figs. 9A to 9F just described, the ultraviolet imagewise radiation
process is performed only on one surface of the photosensitive glass substrate, whereas
in the previous methods the radiation process is performed on opposite sides of a
substrate. The process of Figs. 9A to 9E eliminates misregistration which might occur
between the two photomasks used on opposite sides of the substrate.
[0035] As seen in Fig. 10, typical dimensions of a rear nozzle manufactured according to
Figs. 9A to 9E measure F=170 µm, E=30 µm, D1=45 µm, D2= 50 µm and D3= 90 µm. Due to
the single imagewise radiation, the nozzle opening 41c is precisely aligned with the
nozzle opening 41d in the nozzle 41a.
[0036] Since the first etching process involved in forming the rear nozzle openings on one
surface of the substrate is performed in a much smaller period of time than is taken
to perform the second etching process on the opposite side and since dimensional variations
between different nozzles increase as a function of time taken to perform the etching
process, the method of the present invention ensures quantity manufacture of nozzle
plates with a precisely dimensioned nozzle opening. Furthermore, the second etching
process can be effected for a desired length of time to take advantage of the sideway
etching tendency of the photosensitive glass substrate so that the transverse cross-section
of the rear hole 41c can be made greater than that of the nozzle 8 opening 41d to
reduce its flow resistance to liquid.
[0037] It is found that the configuration of the ink meniscus on the projecting nozzle 8
is affected by the electric field distribution, the viscosity of the ink of typically
oily material, the transient pressure variations in the projecting nozzle 8 and in
the air chamber 10 and the size of the meniscus which is affected by the voltages
applied to the electrodes. As a result, the ink tends to be deflected out of the intended
trajectory as it is discharged from the projecting nozzle 8. This results in a buildup
of an ink layer on the walls adjacent to the projecting nozzle 8. Since the ink is
conductive, the electric field will be seriously deformed to worsen the out-of-the-path
deflection problem.
[0038] It is therefore preferable that portions of the adjacent walls where the ink particles
are likely to hit be rendered ink-repellant. Since the tendency of a material to become
wet depends on the roughness of its surface, it is effective to polish a portion 2a
of the front nozzle plate 2 surrounding the front channel 3 to a mirror-finish.
[0039] Figs. 11A to 11C are illustrations of preferred embodiments for eliminating the deflection
problem. In Fig. 11A, the inner surface of the front nozzle plate 2 is coated with
a thin layer 50 of an ink-repellant material (which is also oil-repellant) such as
ethylene tetrafluoride resin which is typically available as Teflon, a trademark of
Du Pont, or a fluoride-containing polymer available as a mixture of liquids known
under the trademark Fluorad FC-721 and FC-77 of 3M Corporation. Due to the reduced
wetness, any amount of ink deposited on layer 50 is expelled to the outside by the
air passing over the surface of the layer 50.
[0040] In Fig. 11B, the fluoride-containing polymer liquid mentioned above is sprayed on
the inner surface of the front nozzle member 2 so that an ink-repellant layer 51 is
formed on the inner wall of a forwardly tapered front channel 3 as well as on the
inner surface of the member 2. Since Fluorad has a surface tension of 11 to 12 dynes/cm,
a satisfactory level of repulsiveness can be obtained. On the surface of the rear
nozzle member 7 is preferably deposited an ink-repellant layer 52 formed of a mixture
of fluoride-containing diamine and epoxy resin. Specifically, after forming a coat,
the mixture is cured by heating it at 150°C for 1 to 5 hours. The same level of repulsiveness
as ethylene tetrafluoride can be obtained. Since the outer wall of the projecting
nozzle 8 and the area surrounding the foot of the nozzle 8 have a surface roughness
greater than that of the front end of the projecting nozzle 8 due to the etching process
mentioned previously, the repellant layer 52 can be easily formed excepting the front
end of the nozzle. In the emodiment of Fig. 11B, the ink tends to extend to the perimetry
of the front end face of the projecting nozzle 8 due to the low wet contact angle
with glass with which it is formed. Therefore, a relatively large meniscus 53 will
thus be formed. An electrode 54 may be provided on the rear surface of the rear nozzle
member 7.
[0041] An ink-repellant layer 55 may also be formed on the front end face of the projecting
nozzle 8 as shown in Fig. 11C. This layer is formed by spraying the fluoride-containing
polymer liquid mentioned above. Due to repelling action, the ink is confined within
the inner perimetry of the coat on the front end face, a relatively small meniscus
56 will be formed. Because of an increased field concentration on the meniscus 56
a lower threshold voltage is required for dischaging the ink through nozzle 8 than
is required with the previous embodiment. Front nozzle member 2 is preferably coated
with an ink-repellant layer 57 which extends outwardly to enclose the electrode 11.
The front-wall coating is to repel the ink particles which might return to the front
member 2 by turbulence caused by the air ejected at high speeds from the channel 3.
[0042] Ink-repellant materials that can be advantageously employed in the present invention
include:
(a) fluoride-containing polymer such as polytetrafluoroethylene, fluorinated ethylene-propylene
copolymer, polychlorotrifluoroethylene, polyvinylfluoride, tetrafluoroethylene perfluoroalkylvinylether
copolymer, polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene
copolymer, epoxy resin mixed with fluoride-containing diamine, or fluoride-containing
alkyl silane;
(b) inorganic fluoride-containing compound such as calcium fluoride and graphite fluoride;
(3) silicone polymer of the type which is composed of a Si-O bond and is capable of
being cured at room temperatures or silicone polymer of the type which is cured at
elevated temperatures; and
(4) a copolymer of fluoride-containing polymer and silicone polymer such as:
[0043] Ink-repellant material is successfully deposited on the front and rear nozzle plates
by means of apparatus shown in Figs. 12A and 12B.
[0044] In Fig. 12A, a mount 60 includes an annular groove 61 on the upper surface in which
a seal 62 is fitted. Mount 60 is formed with a negative pressure chamber 63 which
communicates through a pipe 64 to a suction pump 65. Nozzle member 2 or 7 is placed
on the mount 60. Seal 62 provides an air-tight sealing contact to allow air to be
admitted into the chamber 63 exclusively through the channel 3 (or 9). The speed of
the air passing through the channel is controlled by a pressure regulator 66 located
in the pipe 64. Ink-repellant material is sprayed by a spray gun 67 to the nozzle
member to form an ink-repellant layer 69 thereon. Due to the air flowing in the same
direction as the direction of movement of the sprayed particles, the latter is carried
by the air and forms a thin film on the inner wall of the channel. Otherwise, the
sprayed material would clog the channel.
[0045] Apparatus shown in Fig. 12B is useful for forming the ink-repellant layer only on
the surface portion of the nozzle member. A mount 70 has an annular groove 71 in which
is provided a seal 72 and a positive pressure chamber 73. A holding member 74 is detachably
secured to the mount 70 by screws 75 to hold the nozzle plate in between. Holding
member 74 is formed with a window 76. Chamber 73 is connected by a pipe 77 to a pressure
pump 78 to produce a positive pressure in the chamber 73 and eject air to the outside
through the channel of the nozzle member, the speed of airflow in the channel being
controlled by a pressure regulator 79. Ink-repellant material is sprayed by a spray
gun 80 to form an ink-repellant layer 81 within the window 76. Since the direction
of movement of air through the channel is opposite to the direction of movement of
the sprayed material, the latter is deposited only on the surface portion of the nozzle
plate and is prevented from clogging the channel.