DE3586998T2 - INK-JET PRINTER. - Google Patents

Description

The present invention relates generally to inkjet printers and more particularly to an inkjet printhead of the type in which a liquid is ejected through axially aligned rear and front channels by the combined effects of the electric field and the air pressure gradient.

An ink jet printhead of the type shown and described in U.S. Patent 4,403,234 includes a front nozzle element secured to a housing to form a laminar flow air chamber. The housing is formed with a rear channel aligned with a front channel provided in the front nozzle element. The rear channel is connected to a liquid supply by an electrically conductive tube to create a meniscus at the exit end of the rear channel. The electrically conductive tube is connected to a voltage source to charge the liquid in the rear channel with respect to the front channel so that an electric field gradient is created between the meniscus and the front channel. The air flow chamber is connected to a source of compressed air to create an air pressure gradient between the exit ends of the rear and front channels. Due to the overlapping effects of the field and pressure gradients, the meniscus is pulled forward and ejected through the anterior canal onto a writing surface.

However, the meniscus is very sensitive to disturbances generated when the print head scans across the writing surface and becomes unstable when it returns to its original shape after a drop is ejected.

The Patent Abstract 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 sharply deflected before passing through an outlet, and an electric field is applied during operation in the region where the ink drop meniscus is formed.

EP-A-109755 is concerned with the manufacture of a nozzle for an ink jet print head using etching processes.

Patent Abstract of Japan, Vol. 6, No. 201 discloses an ink nozzle plate for an ink jet printer in which the inner wall of the ink nozzle and the front surface of the nozzle plate around the ink nozzle are lined with ink repellent material.

According to the present invention, an inkjet printer is provided with the following components:

a compressed air source;

a liquid container;

an inkjet printhead comprising a front nozzle element having a front channel, a housing attached to the front nozzle element, a rear nozzle element defining with the housing a liquid chamber connected to the reservoir and defining with the front nozzle element a laminar flow air chamber, the rear nozzle element including a forwardly projecting nozzle and a rear channel extending forwardly from the liquid chamber in axial alignment with the front channel, the front and rear nozzle elements are both formed as a flat disc and mounted opposite one another, and the forwardly projecting nozzle is in the form of a ring which projects from the flat disc rear nozzle element as an extension of the rear channel to form a meniscus at a front end of the extension, the annular projection substantially corresponding in radial dimensions to the front channel, the air flow chamber being connected to the air source for directing the air to a point between the front and rear channels so that it undergoes a sharp turn at the entrance to the front channel which creates a strong pressure gradient on the way between the front ends of the front and rear channels and, as a result of the strong pressure gradient, a region of still air around the meniscus;

an annular electrode secured to the front end of the anterior channel, means for applying an electrical potential between the electrode and the liquid in the liquid chamber to create an electrical field gradient between the anterior channel and the meniscus to cause the meniscus to partially exit through the anterior channel; and

a device for connecting the liquid container to the air source so that in the absence of the electric field gradient the liquid pressure in the rear channel is in static equilibrium with the superimposed forces of the air pressure acting on the meniscus and the surface tension of the liquid;

a first liquid-repellent layer covering the front and rear end portions of the front channel and inner walls of the front channel; and

a second liquid-repellent layer covering the front end and the outer walls of the annular projection.

The nozzle element can be manufactured by a method comprising the steps of:

a) etching a first surface of a substrate to a predetermined depth according to a first pattern to form a protruding nozzle having a nozzle opening therein, wherein the substrate is composed of a photosensitive glass or of a laminate of an insulating agent and a layer of a material dissimilar to the insulating agent, the layer forming the first surface; and

b) etching a second, opposite surface of the substrate to a predetermined depth to form a channel extending to and axially aligned with the nozzle orifice.

The formation of the still air region in the nozzle of the printer of the invention causes the meniscus to become convex, which creates a high electric field strength and reduces the minimum voltage required to break the meniscus into a drop.

The two-stage etching process is advantageous for reducing the time required to produce the protruding nozzle, as it minimizes variations in the nozzle orifice size that could occur as a result of the tendency of the substrate material to erode at the sides on a single substrate. Furthermore, the bore at the rear end of the nozzle orifice can be appropriately dimensioned so that its cross-section is larger than that of the nozzle orifice and by Resistance is reduced, allowing the fluid to flow through.

The present invention will be further described in detail by 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 a detailed illustration of a print head section of Fig. 1;

Fig. 3 is a diagram useful for describing the advantageous effect of the above nozzle of the invention;

Fig. 4A to 4F are illustrations of various modifications of the rear nozzle disk;

Figs. 5A to 5G are illustrations of manufacturing steps of a rear nozzle disk 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;

8A to 8F are illustrations of a second method of manufacturing the rear nozzle disk; 9A to 9F are illustrations of a third method of manufacturing the rear nozzle disk;

Figure 10 is a cross-sectional view of a rear nozzle disk made according to the present invention.

Figs. 11A to 11C are cross-sectional views of embodiments in which the ink-repellent layers are formed on the nozzle elements; and

Fig. 12A and 12B are illustrations of an apparatus for applying an ink repellent layer to a nozzle element.

Referring 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 includes a front nozzle disk 2 having a front channel 3. The front nozzle disk 2 is made of an insulating material and is secured to a rear housing 4 of insulating material. The rear housing is formed with a liquid chamber 5 for receiving ink supplied thereto from an ink container 6 through an electrically conductive tube 6a. The liquid chamber 5 is formed at the front with a rear nozzle disk 7 having a protruding nozzle 8. A rear channel 9 projects from the liquid chamber 5 through the protruding nozzle 8 in axial alignment with the front channel 3 so that the ink in the liquid chamber 5 can be passed through to form a meniscus at the outer end. The front nozzle disk 2 defines with the rear nozzle disk 7 a disk-like laminar flow air chamber 10a of an air chamber 10 and defines with the rear housing 4 an annular section 10b.

A ring electrode 11 surrounding the front channel 3 is attached to the outer surface of the front nozzle disk 2. A voltage from a voltage source 12 is applied between the electrode 11 and the tube 6a so that an electric field gradient is generated between the electrode 11 and the liquid in the rear channel 9.

A source of compressed air 13 is connected to the air chamber 10 by a tube 14 to create an air flow in the annular air chamber section 10b and cause it to wind in a laminar flow through the disk-like chamber section 10a to the front channel 3 and outwardly. The air flow undergoes a sharp turn at the entrance to the front channel 3 which creates a strong pressure gradient in a path between the front end of the rear channel 9 and the front channel 3. Compressed air is also supplied to the ink container 6 through a regulator valve 15. The valve 15 is adjusted so that in the absence of voltage at the electrode 11 the fluid pressure in the rear channel 9 is statically balanced with the superimposed forces of the air pressure acting on the meniscus and its surface tension. In response to the application of a voltage to the electrode 11, the liquid in the rear channel 9 is electrostatically charged and drawn forward under the influence of the electric field gradient. The liquid is stretched into a pencil-like shape by the pressure of the air expelled through the first front channel 3 and expelled onto a writing surface.

As best seen in Fig. 2, the protruding nozzle 8 has its own outside diameter which is slightly smaller than the diameter of the front channel 3 and extends forward from the nozzle disc 7 by a distance B. The air flow is constricted as it enters the space between the front and the rear channel, creating a region of still air immediately adjacent to the front end of the rear channel 9. On the other hand, the liquid in the 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 the rear nozzle 8 is prevented by a force applied by the air stream moving past the outer edge and forcing the liquid to bulge slightly. In the absence of an electric field, the high pressure in the region of still air causes the meniscus at the front end of the rear channel 9 to assume a convex shape as shown at 8a, stabilizing it against external disturbance.

When a voltage is applied to the ring electrode 11, the meniscus quickly stretches and forms an inclined portion 8b extending from the outer edge of the rear nozzle 8 to a narrow pencil-like portion 8c as shown in Fig. 3. The formation of a convex meniscus 8a concentrates the electric field thereon and reduces the minimum voltage required for division into droplets. Due to the presence of the region of still air, the meniscus quickly returns to its original state after ink ejection.

In a preferred embodiment, the front surface of the nozzle 8 is roughened to create a small wetting angle for the liquid so that the meniscus can easily wet the front surface of the nozzle 8. The small wetting angle reduces the response time of the print head and increases the amount of liquid ejected per unit time.

It is best if the axial distance B of the rear nozzle 8 and the outer diameter Dr of the rear nozzle 8 satisfy the following relationships:

4L/5 < B < L/20

Df < Dr < Df/4

where L is the distance between the front and rear nozzle discs 2 and 7 and Df is the diameter of the front channel 3.

Experiments confirmed that under similar operating conditions, the printhead of the present invention operates at a minimum pulse width that is 1/10 of the prior art minimum pulse width and is insensitive to vibrations over a range ten times larger than the prior art.

Various preferred shapes of the rear nozzle disk are shown in Figures 4A to 4F. The variations shown in Figures 4A to 4D are advantageous for further increasing the meniscus stability and improving the response characteristics of the meniscus. This is achieved by increasing the contact area of the front end of the rear nozzle with the liquid. In these variations, the rear channel 9 has a front portion passing through the nozzle 8 and a rear portion passing through the nozzle disk 7.

In Fig. 4A, the rear channel 9 has a front portion 9A' with a part-spherical surface and a cylindrical rear portion 9A''. The rear channel 9 in Fig. 4B has a frusto-conical front portion 9B' and a rear portion 9B''. In Fig. 4C, the rear channel 9 has a front portion 9C with a larger cross-sectional area than a rear portion 9C''. This increases the amount of liquid trapped in the nozzle 8. The rear channel 9, Fig. 4D, has a front Section 9D' with a stepped cross-section and a cylindrical rear section 9D'', the stepped section increasing its diameter with increasing distance from the rear section 9D'.

In the embodiments of Figs. 4A and 4B, the ejected liquid forms a large wetting contact angle with the surface of the front portions 9A', 9B' compared to the embodiment of Fig. 1 and is consequently subjected to a larger liquid restoring force, with which the meniscus is more stabilized against external vibrations which could otherwise cause it to disintegrate. In the embodiments of Figs. 4C and 4D, the front portions 9C' and 9D serve as containers to store a larger amount of liquid therein and to increase the liquid ejection capability.

In Fig. 4E, the rear nozzle 8 is formed with an annular groove 80 to catch the liquid that might leak over the edge of the nozzle if an excessive force is applied to the print head from the outside. The annular groove may be formed around the nozzle 8 as shown at 81 in Fig. 4F.

Now, a description will be given of a method of manufacturing a rear nozzle disk with reference to Figs. 5A to 5G.

Depicted by reference numeral 21 in Fig. 5A is a photosensitive glass composed of a SiO2-Al2O3-Li2O glass containing CeO2 and Ag2O. A photomask 22 having a plurality of annular opaque portions 22a (only one of which is shown for simplicity) in a transparent region 22b is applied to the upper surface of the glass 21.

The photosensitive glass 21 is exposed to imagewise irradiation with ultraviolet light through the mask 22 to cause the following reaction to occur in the sections 21b lying under the transparent section 22b:

Ce³&spplus; + A&spplus; + Ultraviolet light - Ce&sup4;&spplus; + Ag&sup0;

The glass is then subjected to a first heat treatment so that the silver content of the compound becomes colloidal and then to a second heat treatment so that Li₂O - SiO₂ crystals form around the silver colloids. The Li₂O - SiO₂ crystals are etched away to a predetermined depth. This leaves an upper portion of the amorphous region which serves as a rear nozzle as shown in Figure 5B. This etching process is preferably accomplished by applying a layer of hydrofluoric acid resistant material to the lower glass surface and immersing it in an aqueous hydrofluoric acid solution. One material suitable as a hydrofluoric acid resistant layer is a paraffin-containing material available from Sou Denshi Kogyo Kabushi Kaisha under the trade name "Electron Wax". The wax is applied at a temperature of 70ºC and removed by immersion in a trichloroethylene solution stirred at an ultrasonic frequency.

In Fig. 5C, the lower surface of the glass 21 is coated with a light-resistant layer 24, and a photomask 25 having a plurality of light-resistant portions 25a is applied to the light-resistant layer 24 so that the light-resistant portion 25a is aligned with the nozzle 21a. The diameter of the light-resistant portion 25a is larger than the inner diameter but smaller than the outer diameter of the Nozzle 21a. The photoresist layer is exposed to imagewise ultraviolet irradiation through the mask 25. Unexposed portions are etched to form a plurality of holes 24a each concentric with the nozzle 21a as shown in Fig. 5D.

A hydrofluoric acid resistant layer 26 is then formed over the entire upper surface of the glass 21 so as to fill the space within the protruding nozzle 21a as shown in Fig. 5D. The glass substrate is immersed in an aqueous hydrofluoric acid solution to etch the glass portions above the opening 24a to thereby produce a bore 27 extending through the thickness of the glass 21. The photoresistant layer 24 is removed after carbonization in a plasma, and the layer 26 is removed by immersing the glass in a trichloroethylene solution stirred 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 in Figure 5A so that the amorphous channel portions 21a crystallize. This crystallization process causes the entire glass 21 to become homogenized as shown in Figure 5G and to increase its mechanical strength. The glass 21 is then cut into individual nozzle disks.

It can be seen that the nozzle portion 21a and the opening 27 are made by etching the glass in opposite directions.

Although the amorphous region of the glass tends to erode at a rate substantially 1/20 of the rate at which the crystalline region erodes, the process of the invention preserves the glass 21 from being subjected to a single prolonged etching process and thus prevents it from eroding excessively sideways. It is possible to manufacture a rear nozzle disk with a nozzle 21a having an outside diameter of 100 micrometers, with a tolerance of ± 2 micrometers, an inside diameter (at the front end) of 40 micrometers, with a tolerance of ± 2 micrometers, and with an axial thickness of 35 micrometers. In this case, the opening 27 has a depth of 130 micrometers. Although it has a small thickness in radial directions, the nozzle 21a has sufficient rigidity to maintain its shape for a long time. The nozzle disk 7 formed of glass has a further advantage in that it is chemically resistant to ink and does not swell.

In the process step shown in Fig. 5C, incident ultraviolet light penetrating the photo-insensitive layer 24 is irregularly reflected at different depths of the crystallized portions of the glass, and part of the reflected light penetrates into undesired portions of the photo-insensitive layer 24, causing the boundary between exposed and unexposed areas to be blurred. For this reason, a light-shielding layer 16 is provided between the lower surface of the glass 21 and the photo-insensitive layer 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 reaches a thickness of 1 to 2 micrometers. After being exposed to imagewise ultraviolet irradiation, the photo-insensitive layer 24 is removed, followed by removal of the gold layer 16 by using aqua regia. Alternatively, the lower surface of the glass 21 is roughened by etching as shown in Fig. 7A. The light-insensitive layer 24 is deposited on the roughened surface (Fig. 7B). Most of the ultraviolet light penetrating the photo-insensitive layer 24 is reflected by the roughened surface, whereby the light penetrating the undesired portion of the photo-insensitive layer 24 is insignificant. The roughened surface represents an increase in the contact area between the glass 21 and the photo-insensitive layer 24, so that the latter adheres firmly to the glass 21.

8A to 8F are illustrations of a second preferred method of manufacturing the rear nozzle disk 7. In the first step, an insulating substrate 31 of ceramic or glass (Fig. 8A) is prepared. On the substrate 31 is deposited a layer 32 of a material different from the underlying substrate. This material is chemically resistant to ink, but can be easily eroded by an etchant. Suitable materials for the layer 32 are copper, aluminum, gold, platinum, chromium, molybdenum, photosensitive glass as previously mentioned, and photosensitive resin. Such metal is deposited by electroplating, and the non-metallic material can be deposited using a suitable adhesive. A photoinsensitive layer 33 is deposited on the layer 32. The photo-insensitive layer 33 is exposed to image-wise ultraviolet irradiation through a photomask 34 having a transparent portion 34a in the form of a ring in the opaque background. The unexposed portions of the photo-insensitive layer 33 are removed to create a ring 33a of the photo-insensitive layer on the layer 32 as shown in Fig. 8B. An etch-resistant layer 35 is applied to the lower surface of the substrate 31. The substrate 31 is then immersed in an etching solution to expose the portions of the Layer 32 which is not protected by the ring 33a of the light-insensitive layer. If the layer 32 is composed of gold or platinum, aqua regia can be used as the etching solution.

The ring 33a of the photo-insensitive layer is then removed by carbonization in a plasma, followed by removal of the etch-resistant layer 35, thus forming a nozzle 32a (Fig. 8C).

In Fig. 8D, a photo-insensitive layer is applied to the lower surface of the substrate 31 to form a layer 36 which is exposed to imagewise ultraviolet irradiation through a photomask 37 having an opaque portion 37a shielding the portion directly under the nozzle 32a in a manner similar to the step shown in Fig. 5C. A hydrofluoric acid resistant layer 38 of the material used in layer 26, Fig. 5D, is applied to the entire upper surface of the substrate 31 so as to fill the area inside the nozzle 32a (Fig. 8D), followed by immersing the substrate in a photo-insensitive etching solution to remove the unexposed portion of the photo-insensitive layer 36 to form an opening 36a (Fig. 8E). The substrate is then immersed in an aqueous hydrofluoric acid solution so as to form an opening 31a, Fig. 8F, extending through the thickness of the substrate 31, followed by the removal of the layers 36 and 38. The process of Figs. 8A-8F is advantageous for applications in which it is desired to select a suitable material for the protruding nozzle portion 32a with sufficient surface roughness, which may be different from the surface roughness of the substrate 31, to obtain the meniscus.

9A to 9F illustrate another manufacturing process in which the steps of Fig. 5A are initially performed to crystallize portions of a glass substrate 41 surrounding a cylindrical amorphous portion. The step shown in Fig. 9A follows. This step is similar to the step shown in Fig. 5B except that the etching is performed on the opposite surfaces of the glass substrate 41 to form a pair of nozzles 41a and 41b. Since the upper nozzle 41a is formed outside the region closer to the photomask than the lower nozzle 41b, the former has a sharper boundary with the surrounding region than the latter. In Fig. 9B, the upper surface of the substrate 41 is completely coated with a hydrofluoric acid resistant layer 42 so that it fills the space inside the nozzle 41a. The lower surface is coated with a layer 43 outside the areas of the lower nozzle 41b. The layer 43 can be formed from the same wax as used in Fig. 5D. The lower nozzle portion 41b has a larger surface roughness on its side walls than on its upper side. 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 a 5% aqueous hydrofluoric acid solution maintained at a temperature of less than 34°C to create an opening 41c within the amorphous cylinder extending between nozzles 41a and 41b (Fig. 9C). In this process, the etching solution tends to penetrate into the interface between nozzle 41b and surrounding layer 43, causing ablation to occur along this interface. The substrate may be etched for a period of 35 minutes at a solution temperature of 20°C to remove a volume to a depth of 170 microns with a diameter of about 50 microns. As a result of the lateral ablation, the opening 41c is tapered upward.

Layers 42 and 43 are removed in a solution of trichloroethylene stirred at ultrasonic frequency (Fig. 9D). The bottom surface of the substrate is lapped to create a flat surface (Fig. 9E). The substrate 41 is then exposed to ultraviolet irradiation and then heated in the same manner as shown in Fig. 5G so that the amorphous region crystallizes (Fig. 9F).

The hydrofluoric acid resistant layer 43 may also be made of epoxy resin adhesive which is a mixture of Epicoat 828 as the main component and Epicur Z as a curing agent (both are 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 microns and then allowed to semi-cure for 50 hours at room temperature to prevent Epicoat from entering the nozzle 41b. This is followed by a complete curing process in which the substrate is held at a temperature of 70°C for 60 minutes. The epoxy resin layer 43 may be removed in an oxygen plasma environment. In comparison with the method involving the use of wax, the epoxy resin layer 43 is preferred because of its excellent bonding to the underlying glass substrate and strength. Due to the high strength, the undesirable abrasion around the nozzle 41b can be minimized.

In the method of Figs. 9A to 9F just described, the ultraviolet imagewise irradiation process is carried out only on one surface of the photosensitive glass substrate, whereas in the previous methods the irradiation process is carried out on the opposite sides of a substrate. The method according to Figs. 9A to 9E eliminates a misregistration that occurs between the two Photomasks used on opposite sides of the substrate.

As can be seen in Fig. 10, the typical dimensions of a rear nozzle made according to Figs. 9A to 9E are F = 170 µm, E = 30 µm, D₁ = 45 µm, D₂ = 50 µm and D₃ = 90 µm. As a result of a single imagewise exposure, the nozzle opening 41c is precisely aligned with the nozzle opening 41d in the nozzle 41a.

Since the first etching involved in forming the rear nozzle orifices on one surface of the substrate is performed in a much shorter time than that required for the second etching on the opposite side, and since the dimensional variations between the various nozzles increase as a function of the time of the etching, the method of the present invention ensures mass production of nozzle disks with a precisely dimensioned nozzle orifice. Furthermore, the second etching can be performed for a desired time to take advantage of the tendency of the photosensitive glass substrate to etch sideways, so that the cross-section of the rear orifice 41c can be made larger than that of the nozzle orifice 41d to reduce its flow resistance to the liquid.

It has been found that the arrangement of the ink meniscus at the protruding nozzle 8 is determined by the electric field strength distribution, the ink viscosity of the typically oily material, the pressure surge changes in the protruding nozzle 8 and in the air chamber 10 and the meniscus size generated by the voltage applied to the electrodes. As a result, the ink tends to be deflected from the intended path when it is ejected from the protruding nozzle 8. This results in the build-up of an ink layer on the walls adjacent to the protruding nozzle 8. Since the ink is conductive, the electric field is seriously deformed, so that the path deflection problem becomes greater.

It is therefore best that the portions of the adjacent walls where the ink particles are likely to deposit are made ink-repellent. Since the tendency of a material to be wetted depends on the roughness of its surface, it is effective to polish a portion 2a of the front nozzle disk 2 surrounding the front channel 3 to a high gloss.

Figures 11A through 11C are illustrations of preferred embodiments for eliminating the deflection problem. In Figure 11A, the inner surface of the front nozzle disk 2 is lined with a thin layer 50 of an ink repellent material (which is also oil repellent), such as ethylene tetrafluororesin typically available as Teflon, a trademark of Du Pont, or a fluoride-containing polymer known as a liquid blend under the trademark Fluorad FC-721 and FC-77 of 3M Corporation. Due to the reduced wetting, any amount of the ink deposited on the layer 50 can be expelled outwardly by the air passing over the surface of the layer 50.

In Fig. 11B, the above-mentioned fluoride-containing polymer liquid is sprayed onto the inner surface of the front nozzle element 2, so that an ink-repellent layer 51 is formed on the inner wall of a forwardly tapered front channel 3 as well as on the inner surface of the element 2. Since the fluorad has a surface tension of 11 to 12 dynes/cm, a satisfactory level of repellency can be achieved. On the surface of the rear nozzle element 7, an ink-repellent layer 52 consisting of a mixture of fluoride-containing diamine and epoxy resin is preferably applied.

Specifically, after forming a coating, the mixture is cured by heating at 150°C for 1 to 5 hours. The same level of repellency as with ethylene tetrafluoride can be achieved. Since the outer wall of the protruding nozzle 8 and the area surrounding the base of the nozzle 8 have a surface roughness greater than that of the front end of the protruding nozzle 8 due to the aforementioned etching process, the repellent layer 52 can be easily formed except for the nozzle front part. In the embodiment of Fig. 11B, the ink tends to extend to the periphery of the front surface end of the protruding nozzle 8 due to the low wetting contact angle with the glass of which it is formed. Therefore, a relatively large meniscus 53 is formed. An electrode 54 may be provided on the rear surface of the rear nozzle 7.

An ink repellent layer 55 may also be formed on the front end face of the protruding nozzle 8 as shown in Fig. 11C. This layer is formed by spraying the above-mentioned fluoride-containing polymer liquid. As a result of the expulsion process, in which the ink is confined within the inner periphery of the coating on the front end face, a relatively small meniscus 56 is formed. Due to an increased field strength at the meniscus 56, a lower bias voltage is required to eject the ink through the nozzle 8 than in the previous embodiment. required. The front nozzle element 2 is preferably coated with an ink repellent layer 57 which extends outwardly to enclose the electrode 11. The coating of the front wall serves to expel from the channel 3 the ink particles which might return to the front element 2 as a result of the turbulence caused by the air being thrown out at high speeds.

Ink-repellent materials that can be advantageously used in the present invention include:

(a) a fluoride-containing polymer such as polytetrafluoroethylene, fluorinated ethylene-propylene-containing copolymer, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether-containing copolymer, polyvinylidene fluoride, ethylene-tetrafluoroethylene-containing copolymer, ethylene-chlorotrifluoroethylene-containing copolymer, epoxy resin mixed with fluoride-containing diamine or fluoride-containing alkylsilane;

(b) inorganic fluoride-containing compound such as calcium fluoride and graphite fluoride;

(3) silicone polymer of the type composed of a Si-O compound and curable at room temperature, or silicone polymer of the type curable at elevated temperature; and

(4) a copolymer of fluoride-containing polymer and silicone polymer, such as:

Ink repellent material is successfully applied to the front and rear nozzle disks by a device shown in Figs. 12A and 12B.

In Fig. 12A, a socket 60 includes an annular groove 61 on the surface into which a gasket 62 is inserted. The socket 60 is formed with a vacuum chamber 63 which is connected to a suction pump 65 through a pipe 64. The nozzle element 2 or 7 is mounted on the socket 60. The gasket 62 creates an airtight sealing contact so that the air can be admitted into the chamber 63 exclusively through the channel 3 (or 9).

The speed of the air flowing through the channel is controlled by a pressure regulator 66 arranged in the pipe 64. Ink repellent material is sprayed onto the nozzle element by a spray gun 67 to form an ink repellent layer 69 thereon. As a result of the air flowing in the same direction as the direction of movement of the sprayed particles, the latter are transported through the air and form a thin film on the inner wall of the channel. Otherwise, the sprayed material would clog the channel.

The device shown in Fig. 12B is useful for forming the ink repellent layer only on the surface portion of the nozzle member. A socket 70 has an annular groove 71 in which a gasket 72 is inserted and a pressure chamber 73. A holding member 74 is detachably attached to the socket 70 by screws 75 to hold the nozzle disk between them. A holding member 74 is formed with a window 76. The chamber 73 is connected by a pipe 77 to a pressure pump 78 to create a positive pressure in the chamber 73 and to force air out through the channel of the nozzle member, the speed of the air flow in the channel being controlled by a pressure regulator 79. Ink repellent material is sprayed with a spray gun 80 to form an ink repellent layer 81 within the window 76. Since the direction of movement of the air through the channel is opposite to the direction of the sprayed material, the latter is deposited only on the surface portion of the nozzle disk and prevents the channel from sticking.

Claims (3)

1. Inkjet printer with the following components: a compressed air source (13); a liquid container (6); an inkjet print head (1) comprising a front nozzle element (2) with a front channel (3), a housing (4) attached to the front nozzle element, a rear nozzle element (7) defining with the housing a liquid chamber (5) connected to the liquid container (6) and a laminar flow air chamber (10, 10a) with the front nozzle element, the rear nozzle element having a forwardly projecting nozzle (8) and a rear channel (9) extending forwardly from the liquid chamber axially aligned with the front channel, the front and rear nozzle elements each being attached to a first disk and opposite one another, the forwardly projecting nozzle being in the form of a ring extending from the rear nozzle element formed as a flat disk as an extension of the rear channel to form a meniscus protruding from a front end of the extension, the annular projection (8) substantially corresponding in radial dimensions to the front channel, the air flow chamber being connected to the air source to direct air to a point between the front and rear channels so that it undergoes a sharp turn at the entrance to the front channel which creates a pressure gradient on the way between the front end of the front and rear channels which creates an area of still air as a result of the sharp pressure drop around the meniscus; an annular electrode (11) mounted at the front end of the front channel, means (6a,12) for applying an electrical potential between the electrode and the liquid in the liquid chamber to create an electrical field gradient between the front channel and the meniscus and to partially expel the meniscus through the front channel; and means (15) for connecting the liquid container to the air source so that in the absence of the electric field gradient the liquid pressure in the rear channel is in static equilibrium with the combined forces of the air pressure acting on the meniscus and the surface tension of the liquid; a first liquid-repellent layer covering front and rear end portions of the front channel and inner walls of the front channel; and a second liquid-repellent layer covering the front end and outer walls of the annular projection. 2. The ink jet printer of claim 1, wherein the first liquid repellent layer substantially further covers an entire rear surface of the front nozzle member and the second liquid repellent layer substantially further covers an entire front surface of the rear nozzle member. 3. Inkjet printer according to claim 1 and 2, characterized in that the material is composed of a fluoride-containing polymer, an inorganic fluoride-containing compound, silicone polymer and a copolymer of fluoride-containing polymer and silicone polymer.