Patent Publication Number: US-6341836-B1

Title: Water-repellent coating and method for forming same on the surface of liquid jet

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to compositions of water-repellent coatings on the surface of liquid jet nozzles and in particular a nozzle plate for an inkjet printer. 
     Among inkjet heads, those using a piezo-electric element have recently become more and more popular for their high energy-efficiency, etc. This kind of inkjet head typically comprises a piezo-electric element, one common ink chamber with ink supplied from outside and stored therein, a plurality of pressure chambers connected to the piezo-electric element and a nozzle plate connected to the pressure chambers so that a nozzle is connected to each pressure chamber. Each pressure chamber that is connected each corresponding ink feed path to the common ink chamber receives ink from the common ink chamber, increases an internal pressure by utilizing a deformation of the piezo-electric element, and thereby jets ink from the nozzle. 
     On the surface of the nozzle plate (opposite to the pressure chamber) a water-repellent coating is typically formed around the nozzle. The water-repellent coating has the following exemplary effects. First, the water-repellent coating serves to stabilize a flying direction of ink jetted from the nozzle. Without the water-repellent coating, onto the nozzle plate surface is adhered the ink spouted from the pressure chamber, the ink adhered onto the nozzle plate like this pulls the next ink jetted continuously, and thereby bends the flying direction of ink and prevents from flying straight in a desired direction. Secondly, the water-repellent coating serves to smooth a wiping process. After a printing operation is completed, the inkjet head usually undergoes a backup process that eliminates dirt from the nozzle. In the backup process, a suction pump contacts the nozzle and sucks out dirt therein, and at the same time the ink in the nozzle adheres onto the surface of the nozzle plate. Thus, the wiping process that a wiper such as rubber blade, etc. wipes ink on the nozzle surface follows. In that event, without the water-repellent coating, the ink adhered onto the nozzle plate surface after the backup process could not successfully be wiped out and would remain on the nozzle plate surface. Consequently, the subsequently flying direction of ink is bent and printing quality is adulterated with impure or diluted color if the remaining ink is different in color from the subsequently flying ink. 
     For the forgoing effects, it is inevitable for inkjet head to form the water-repellent coating. In addition, a conventional water-repellent coating has a fluoric polymer of high water repellency as a main ingredient. 
     However, the fluoric polymer is soft and less adhesive to a substrate, and thus is likely to flaw, abrasion or scratch (i.e. low wiping-resistant); therefore, its anticipated water repellency can not be continuously maintained. Accordingly, it has been desired to form a water-repellent coating that has a fluoric polymer as a main ingredient and is continuously usable about one hundred thousand times. 
     Conventionally, it has been suggested for example that a fluoric polymer is plated and a subsequent heating process melts the fluoric polymer adhered onto the plated surface, forms a coating and thereby improves its wiping resistance. This process, however, involves a problem that the coating, even if it is formed, is worn out shortly by a plural of frictions and its water repellency lowers. On the other hand, it has also been suggested to form a member around the liquid jet in concave shape and avoid the fluoric coating around the nozzle from scratched by a friction. The unleveling process, however, increases its person-hours and costs. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful water-repellent coating and method of forming the water-repellent coating in which the above disadvantages are eliminated. 
     More specifically, it is an exemplified object of the present invention to provide a water-repellent coating that has higher wiping resistance relative to a wiper and is formed by a more simplified process than was previously possible, and a method of forming such a water-repellent coating. 
     In order to achieve the above object, a print head according to claim  1  comprises a nozzle plate having a nozzle which jets ink, and a water-repellent coating that is formed on the nozzle plate as a substrate around the nozzle and comprises a hard body and a fluoric polymer formed by a plating process. According to the print head claimed in claim  1 , water repellency of the fluoric polymer works against liquid like ink, etc. jetted from the nozzle, and the hard body enhances wiping resistance of the fluoric polymer. 
     A print head as set forth in claim  2  that depends upon claim  1  comprises a water-repellent coating including the hard body in a flat shape. Thus, according to the print head claimed in claim  2 , the hard body is less likely to fall off than a spherical shaped one and serves to maintain wiping resistance for a long time. A print head as set forth in claim  3  that depends upon claim  1  comprises a water-repellent coating including the hard body having a major axis of 1 μm or smaller in its particle diameter. According to the print head claimed in claim  3 , the hard body having a big particle diameter never prevents a nozzle plate surface from being smoothly wiped. A print head as set forth in claims  4  and  5  that depends upon claim  1  comprises a water-repellent coating having the hard body including a boron nitride boron nitride single crystal. Therefore, according to the print head claimed in claims  4  and  5 , the boron nitride or boron carbide single crystal intrinsically having the advantage of a flat shape requires no additional process to deform the hard body into a flat shape. A print head as set forth in claims  6  and  7  that depends upon claim  1  comprises a water-repellent coating employing an electrolytic or electroless plating process as the plating process. Accordingly, the print head claimed in claims  6  and  7  has the advantage of requiring no special plating process. 
     A recording device as set forth in claim  8  comprises a print head and a driving device which drives the print head wherein the print head includes a nozzle plate having a nozzle which jets ink and a water-repellent coating which is formed on the nozzle plate as a substrate around the nozzle and comprises a hard body and a fluoric polymer formed by a plating process. According to the recording device claimed in claim  8 , water repellency of the fluoric polymer works against liquid like ink, etc. jetted from the nozzle, and the hard body enhances wiping resistance of the fluoric polymer. 
     A method of forming a water-repellent coating as set forth in claim  9  comprises the steps of forming on a nozzle plate a first resist which is open only around a nozzle of the nozzle plate, forming a first layer of a plated fluoric polymer by a first plating process via the first resist, forming a second resist, adding a hard body to a first layer by a second plating process, and removing the first and second resists. According to the method of forming a water-repellent coating claimed in this claim, the hard body is allowed to protrude from the water-repellent coating surface because the water-repellent coating is formed on the nozzle plate as a substrate. A method claimed in claim  10  that depends on claim  9  further comprises the step of heating the water-repellent coating until its water repellency becomes enough to make a contact angle of ink containing 10% of alcohol 60 degrees or larger. By the heat treatment, the fluoric polymer melts and taking in the additive hard body. 
     Other objects and further features of the present invention will become readily apparent from the following description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic sectional view for explaining a composition of a water-repellent coating of the present invention. 
     FIG. 2 is a schematic sectional view illustrating a variation of the water-repellent coating shown in FIG. 1 or a state after a predetermined period of use. 
     FIG. 3 is an enlarged view of a portion circled in a solid line in FIG.  2 . 
     FIG. 4 is an aschematic sectional view for explaining a composition of a water-repellent coating having a spherical hard body relative to the water-repellent coating in FIG. 1 having a flat hard body. 
     FIG. 5 is a schematic sectional view for explaining a state in which the spherical hard body in FIG. 4 is fallen down. 
     FIG.  6 A-FIG. 6E are flow sectional diagrams for explaining one example of a method of forming the water-repellent coating shown in FIG.  1 . 
     FIG.  7 A-FIG. 7E are flow sectional diagrams for explaining another example of a method of forming the water-repellent coating shown in FIG.  1 . 
     FIG. 8 is an exploded perspective view of a completed inkjet head  300 . 
     FIG. 9 is a partially enlarged side view of an inkjet head  300 . 
     FIG. 10 is a perspective overview of a recording device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 1 to  3  inclusive, a description will be given of a water-repellent coating  100  according to the present invention. FIG. 1 is a schematic sectional view for explaining a composition of a water-repellent coating  100  of the present invention. FIG. 2 is a schematic sectional view of a water-repellent coating  100   a  showing an exemplified variation or a state after a predetermined period of use. FIG. 3 is an enlarged view of a portion circled in a solid line in FIG.  2 . In each drawing, those elements designated by the same reference numeral denote the same elements, and a duplicate description thereof will be omitted. Those elements designated by the same reference numeral with a variety of alphabetical letters attached thereto denote the same kinds of elements but are distinguished from each other by alphabets and are comprehensively designated by simple reference numerals. 
     The water-repellent coatings  100  and  100   a  are, for example, 1 through 2 μm thick and are formed around a nozzle  12  on the surface of a nozzle plate  10 . FIG. 1 is an enlarged sectional view around a nozzle (hole)  12  applicable to a print head  300  which will be described later (e.g. piezo-type inkjet head and a bubble jet-type inkjet head). The nozzle plate  10  comprises the nozzles  12  each having a straight portion  14  and a taper portion  16 , to the number corresponding to a predetermined resolution. The nozzle  12  does not necessarily include both of the straight portion  14  and the taper portion  16  but may include only one of them. A portion defined by the straight portion  14  is an opening portion  18  of the nozzle  12  where a meniscus  20  of ink is formed. The nozzle plate  10  is connected to a pressure chamber plate  30 , and the pressure chamber plate  30  is provided with an ink chamber as will be described later. 
     The water-repellent coatings  100  and  100   a  comprise a fluoroplastic coating  102 , a fluoroplastic particle  104 , a nickel base  106  and a flat hard body  108 . The water-repellent coating  100  shown in FIG. 1 is different from the water-repellent coating  100   a  shown in FIG. 2 in whether the flat hard body  108  is partially protruded from the fluoroplastic coating  102 . 
     The water-repellent coatings  100  and  100   a  are characteristically formed on the nozzle plate  10  as a substrate. Therefore, this invention does not adopt such a method, for example, that the water-repellent coating and the nozzle plate  10  are formed in this sequence on a plane and then the plane is removed. Because this method makes the hard body  108  unable to protrude from the fluoroplastic coating  102  as shown in FIG. 2. A projection structure of the hard body  108  as shown in FIG. 2 is suitable for preventing the fluoroplastic coating  102  from being scratched by a friction of a wiping blade (wiper) and making it possible to maintain ink water repellency for a long period. 
     For the fluoroplastic coating  102  and the fluoroplastic particle  104 , tetrafluoroethylene resins, tetrafluoroethylene-hexafluoropropylene copolymerization resins, trifluoroethylene chloride resins, fluorovinylidene resins, fluorovinyl resins, PTFE, FEP, ETFE, PFA, PCTFE and PVDF are usable either singly or in the form of a mixture of two or more of them. Their average particle diameters should preferably be less than 150 μm and in particular ranging from 0.05 to 20 μm. In addition to the above fluoroplastic particles, as needed, other inorganic or organic precipitation polymer particulates may be formed together. 
     The nickel base  106  as a plating coating is added to improve adhesion. Other than nickel may be employed copper, silver, zinc, tin, cobalt and such nickel alloys as a nickel-cobalt alloy, a nickel-phosphorus alloy and a nickel-boron alloy, etc. The plating coating can be formed, for instance, by using the electrolytic plating solution or electroless plating solution in which PFA is suspended. The electrolytic plating solutions according to a variety of metal plating coatings to be deposited may be selected from an electrolytic nickel plating solution such as the Watts bath, a chloride-rich bath, a nickel sulfamate bath and a nickel borofluoride bath, etc.; an electrolytic cobalt plating solution such as a cobalt sulfate bath and a cobalt chloride bath, etc.; an electrolytic copper plating solution such as a copper sulfate bath and a copper borofluoride bath, etc.; an electrolytic lead/tin plating solution such as a lead sulfate bath, a tin sulfate bath and a lead borofluoride bath, etc. It is however preferable to employ a sulfamic acid bath having a sulfamic acid ion content of more than 0.5 mol, more desirably more than 0.8 mol especially in the light of their properties that form more precipitation and resist agitation. The electroless plating solutions may be selected from an electroless nickel plating solution, an electroless cobalt plating solution and an electroless copper plating solution, etc. using a boron compound such as a hypophosphate and a dimethyl borazon, etc. as a reducing agent. 
     The hard body  108  has a higher hardness than a fluoric polymer and a flat shape. The hard body  108  should preferably be as water-repellent, wiping-resistant and frictionless as possible. Even though the hard body  108  has low water repellency, a heat treatment as will be described later melts the fluoric polymer, covers the hard body  108 , and thereby maintains the water repellency. The hard body  108  is added so as to promote the wiping resistance of the fluoric polymer against the wiper. Its flat shape aims at enhancing an anchor effect into the plating coating. A more specific description is now given to the enhanced anchor effect. To illustrate, suppose that a spherical hard body  208  (e.g. having more than 1 μm in diameter) is dispersed in the water-repellent coating  200  (e.g. of about 1 μm in thickness), as shown in FIG.  4 . If the water-repellent coating  200  is wiped on its surface by a wiper  5 , the spherical hard body  208  other than having more than half of its diameter embedded in the water-repellent coating  200  is fallen down as shown in FIG. 5, so that the wiping resistance of the water-repellent coating lowers to such a level as that of the water-repellent coating having no hard body  208 . In FIG. 5, a mark left by the hard body  208  is indicated with  209 . 
     As the hard body  108 , are usable, for example, BN (boron nitride), boron carbide, silicon carbide, titanium carbide, tungsten carbide, graphite fluoride, alumina, glass and ceramics, etc. The boron nitride and boron carbide are suitable for the water-repellent coating of this invention in that they are dealt with in a single crystal and the single crystal is not spherical in crystal structure (the boron nitride is flat). Particularly, the boron nitride, which is used for reducing friction of a bearing, is suitable for improving sliding properties of the electroless nickel coating and increasing strength of the fluoroplastic coating  108 . When the alumina, glass, ceramics are used, they should be deformed in a flat shape. BN added for this is, for example, some 5 g/l or 10 g/l, preferably 20 g/l. 
     Since the additive hard body  108  is less water-repellent than the fluoric polymer, the water-repellent surface should be covered as widely with the fluoric polymer as possible. Therefore, it is necessary to increase a water-repellent portion of liquid contact surface by heating and melting the fluoric polymer after plated so as to taking in the additive. 
     Referring now to FIG.  6 A-FIG. 6E, a description will be given of a method of manufacturing a nozzle plate with a water-repellent coating as shown in FIGS. 1 and 2. Hereupon, FIG.  6 A-FIG. 6E are flow sectional diagrams for explaining one exemplified method of the water-repellent coating  100  shown in FIG. 1 or the water-repellent coating  100   a  shown in FIG.  2 . First, as shown in FIG.  6 (A), a nozzle plate substrate  10  of a stainless steel (SUS316) plate of 100 μm through 300 μm thickness is processed by stamping, etching, electrical discharge machining and laser machining, etc. and is provided with a nozzle  12 . To illustrate, assume that a conic nozzle  12  is made by stamping, a straight portion  14  being 40 μm thick and 20 μm length, and a taper portion  16  has a taper angle of 20 degrees. A nozzle plate surface  22  is roughly ground to remove burrs left by the processing but the burrs are not completely removed. 
     Next, as shown in FIG.  6 (B), corrosion-resistant polymer resin as a resist is filled in the processed nozzle  12 . A photosensitive liquid resist is usable as a resin member in contemplation of its subsequent removal and its machinability. This example utilizes a dry film resist (DFR)  24  of a curing acrylic resin. The DFR  24  becomes a viscid liquid by adding a sufficient heat and is easily filled in the nozzle  12 . Further, in terms of removal, water soluble DFR which may be easily removed with alkaline water solution is available. 
     As shown in FIG.  6 (C), the nozzle plate surface  22  is drenched in a stainless etching solution and etched. On the nozzle plate surface  22  there exist burrs left by the processing or rough grinding of the nozzle  12 , but can easily be removed by etching process. This makes it possible to omit a final finishing grinding step in processing the nozzle plate  10 , and enables a cost-reduction. In addition, a chemical grinding means, if used, may reduce a mechanical stress applied to the nozzle substrate  10  and may improve processing accuracy. The etching depth is 10 μm and the length of straight portion  14  is 10 μm. 
     Thereafter, a water washing, an electrolytic defatting, a water washing, an acid washing and a strike Ni plating are processed, and a water-repellent coating  100  is formed on the nozzle plate surface  22  with a Ni precipitation plating as shown in FIG.  6 (D). The water-repellent coating  100  has the thickness not exceeding the height of the protruded DFR  24 . Then, the nozzle plate  10  is drenched in an alkaline water solution, the DFR  24  is removed as shown in FIG.  6 (E), and the nozzle plate  10  with a water-repellent coating  100  becomes completed. When materials as having difficulty in being etched, such as ceramics, glass, etc. are used as the nozzle plate  10 , the grinding process (FIG.  6 (C)) may be substituted by a physical means using a sandblast. In that event, a sandblast-resistant DFR  24  that includes a polyurethane resin other than an acryl resin as usual ingredients (e.g. BF series made by Tokyo Ohka Kogyo Co., Ltd.) may be employed. The physical grinding means is also applicable to a nozzle plate substrate  10  made of metal. 
     Like this, the water-repellent coating  100  on the nozzle plate surface  22  by Ni precipitation plating is formed along a projected portion of DFR  24 , preventing dropping into the nozzle  12 , and maintains the size accuracy of the nozzle  12  and the water-repellent coating  100 . For example, in FIG. 1, the water-repellent coating  100  is formed so that it permits dropping by making its diameter φ 2  within 3% range of the diameter φ 1  of an opening  18 . This 3% difference is for the purpose of arranging the opening of the water-repellent coating  100  and the opening  18  of the nozzle plate on almost the same side. This arrangement can prevent a deviation of ink dots, stabilize flying ink direction and provide high quality images. 
     Referring next to FIG.  7 A-FIG. 7E, a description will be given of another method of manufacturing the nozzle plate  10  having the water-repellent coating  100 . The process shown in FIG.  7 A-FIG. 7E, is a variation of the process of FIG.  6 (C) and those that follow, and it is to be construed that the process indicated in FIG.  7 (A) follows the process indicated in FIG.  6 (B). As shown in FIG.  7 (A), on the nozzle plate surface  22  is formed a liquid resist or a DFR coating  26  capable of alkaline development and removal, and then the exposure and development with a mask pattern eliminate coatings around the opening  18  on the nozzle plate surface  22 . Next, as shown in FIG.  7 (B), the nozzle plate substrate  10  is drenched in an etching solution and the surface of the opening the coating  26  is ground. The etching depth can be adjusted by altering etching conditions. By adjusting the depth, the length of the straight portion  14  and the projection amount of the DFR  24  are adjusted. 
     As shown in FIG.  7 (C), the coating  26  is removed with strong alkaline solution. In this case, the DFR  24 , which is an alkaline-resistant resist, is not eliminated and remains. After that, a water washing, an acid washing, an electrolytic defatting, a water washing and a strike Ni plating are processed. Subsequently, as shown in FIG.  7 (D), Ni precipitation plating is processed on the nozzle plate surface  22  and the water-repellent coating  100  is formed. The coating thickness is so adjusted as does not exceed the projection amount of the DFR  24 . Thereafter, as shown in FIG.  7 (E), the DFR  24  are removed and eliminated with solution development-type resist removal solution. 
     The above manufacturing method can also provide a nozzle plate  10  having an accurate sized water-repellent coating  100 , as in FIG.  6 . This method, particularly as using DFR  24  as a resist member, only necessitates a heating process where an exposure process may be omitted, and is applicable at one step from the back of the nozzle plate substrate  10 , whereby reducing manufacturing costs. 
     Description will be given of a method of manufacturing a water-repellent coating  100  shown in FIG. 1 or a water-repellent coating  100   a  shown in FIG.  2 . First, in order to form a water-repellent plating coating only on the surface of the nozzle plate  10 , other portions are masked so as not to adhere the coating. In this step, the nozzle plate  10  as a substrate is laminated at the side on which a pressure chamber  30  is formed with an alkaline development-type dry film (this exemplified embodiment utilizes α-450 made by Tokyo Ohka Kogyo Co., Ltd.) on conditions of 120° C., 2.5 kgf/cm, 0.5 m/min. This allows the dry film to break in to the taper portion  16  and the straight portion  14  of the nozzle  12 . Moreover, the resist flows out of ink jet opening of the nozzle, covering a portion around the edge of the nozzle opening of a width of 1 μm, and then the resist is hardened by a double-sided exposure. 
     On the other hand, in order to form a water-repellent coating with a single crystal BN (boron nitride) added thereto, prepare a fluoroplastic containing Ni plating solution (made by Hikifune Co., Ltd.) to which a BN with longitudinal particle size of 1 μm or smaller (particles of more than 1 μm being crushed to this size) is added at the rate of 20 g/l and coat a water-repellent plating to the nozzle plate  10  masked as described above. 
     The nozzle plate  10 , made of stainless steel (SUS430), is drenched in 10% hydrochloric acid for three minutes, washed in water to remove an oxidized coating and is strike Ni plated to improve its plating adhesion. 
     The specification of the strike Ni plating is as follows. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 (1) 
                 Bath composition 
                   
                   
               
               
                   
                   
                 nickel chloride (NiCl2. 6H 2 O) 
                 220 
                 g/l 
               
               
                   
                   
                 hydrochloric acid (HCl 35%) 
                 45 
                 g/l 
               
            
           
           
               
               
               
               
            
               
                   
                 (2) 
                 Temperature 
                 room temperature 
               
               
                   
                 (3) 
                 Electrode 
               
               
                   
                   
                 titanium basket (150 × 30 × 250 mm) 
               
               
                   
                   
                 electrolytic nickel (ø 1B × 10 mm) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 (4) 
                 Current density 
                 2 
                 A/dm 2   
               
               
                   
                   
               
            
           
         
       
     
     After one-minute plating by using this strike Ni plating solution, the nozzle substrate is drenched in a water-washing bath and immediately commences a water-repellent plating process. The specification of the water-repellent plating is as follows. 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 (1) 
                 Solution composition 
                   
                   
               
               
                   
                 nickel sulfamate 
                 420 
                 through 480 g/l 
               
               
                   
                 nickel chloride 
                 40 
                 through 50 g/l 
               
               
                   
                 boric acid 
                 30 
                 through 40 g/l 
               
               
                   
                 PTFE 
                 40 
                 through 50 g/l 
               
               
                   
                 BN 
                 20 
                 g/l 
               
               
                   
                 PH 
                 4.0 
                 through 4.4 
               
               
                 (2) 
                 Temperature 
                 42° 
                 C. 
               
               
                 (3) 
                 Electrode 
               
               
                   
                 titanium basket (150 × 30 × 250 mm) 
               
               
                   
                 electrolytic nickel (ø 1B × 10 mm) 
               
               
                   
                 diaphragm 
               
               
                 (4) 
                 Current density 
                 2 
                 A/dm 2   
               
               
                   
               
            
           
         
       
     
     The nozzle substrate is plated for three minutes by using this water-repellent plating solution. After washed in water, it is drenched in a NaOH (3 wt %) solution, removes a resist, and then after water washing and drying processes, makes PTFE into a coating adhered as a plating by a heating process at 350° C. for thirty minutes. The plated coating, as shown in a photograph attached herewith, has BN particles scattered thereon, whereby preventing a convex portion of the BN particles from being scratched even though an outermost fluoric coating is scratched by friction and abrasion of a wiper (rubber blade), so that a water-repellent effect can be maintained. 
     Referring next to FIGS. 8 and 9, a description will be given of an inkjet head  300  of the present invention. FIG. 8 is an exploded view of the completed inkjet head  300  and FIG. 9 is a partially enlarged side view of the inkjet head  300 . As seen from FIG. 8, the inkjet head  300  of the present invention comprises a pressure chamber plate  310 , a piezo-electric element  320 , a nozzle plate  330 , a resin film  340  and a protective layer  350 . The nozzle plate  330  corresponds to the nozzle plate  10  shown in FIG.  1  and the pressure chamber plate  310  corresponds to the pressure chamber plate  30  shown in FIG.  1 . The pressure chamber plate  310 , the resin film  340  and the protective layer  350  are aligned with each other at a nozzle connection surface  360  that is a surface to which a surface  330   a  of the nozzle plate  330  is connected. In other words, the front surface  310   a  of the pressure chamber plate  310 , a front surface  340   a  of the resin film  340  and a front surface  350   a  of the protective layer  350  form the flat nozzle connection surface  360 . 
     The pressure-chamber plate  310  has the desired number (four in FIG. 8 for description purposes) of pressure chambers  312  and ink introduction channels  314  and a common ink chamber  316  in an approximately rectangular parallelepiped glass plate. Each pressure chamber  312  receives and accommodates ink, and jets the ink from a nozzle  332  connected to an opening  312   a  as its internal pressure increases. The internal pressure changes according as the piezo-electric block  321  just under the pressure chamber  312  deforms, as will be described later. The pressure chamber  312  is formed as an approximately rectangular parallelepiped space by a concave groove on the pressure chamber plate  310  and the elastically deformable resin film  340 . The common ink chamber  316  supplies ink to each pressure chamber  312  via the corresponding ink introduction channel  314 . A bottom of the common ink chamber  316  is defined by the resin film  340  so as to absorb sudden internal pressure changes, and connected to an ink feed device (not shown) at a side surface  310   b  of the pressure chamber plate  310 . The common ink chamber  316  supplies a necessary amount of ink to the pressure chamber  312  via the ink introduction channel  314  when the pressure chamber  312  returns to the original state after the chamber  312  contracts, receives pressure and jets ink. 
     The resin film  340  defines one surface of each of the pressure chambers  312 , the common ink chamber  316  and each of the ink introduction channels  314 , and serves to transmit a deformation of each piezo-electric block  321  which will be described later to the corresponding pressure chamber  312  and to prevent ink in the pressure chamber  312  from penetrating into grooves  323  in the piezo-electric element  320 . The resin film  340  is, for example, approximately 16 μm thick and the order of Gpa adhesive. The resin film  340 , which is a member that forms one surface of the pressure chamber  312 , may be replaced with an elastic metal thin film. 
     The piezo-electric element  320  has layered structure having a plurality of (four in FIG. 1 for description purposes) piezo-electric blocks  321  which are divided by parallel grooves  323  which extend from a front surface  320   a  to a rear surface  320   b . Internal electrodes  322  and  324  are provided between layers of piezoelectric blocks  321 , and the internal electrode  322  is connected to an external electrode  326  and the internal electrode  324  is connected to an external electrode  328 . FIG. 8 shows only one external electrode  328  for illustration purposes. As shown in FIG. 9, an active area  325  is a portion where the internal electrodes  322  and  324  overlap each other in direction A, and each piezo-electric block  321  deforms in the active area  325 . The length of each active area  325  is adjustable depending upon a pressure to be applied to the pressure chamber  312 . Since the active area  325  is spaced at a predetermined distance from the nozzle connection surface  360 , even when the piezo-electric blocks  321  deform, such deformation does not affect the adhesion between the piezo-electric element  320  and the protective layer  350  at the nozzle connection surface  360 . 
     The external electrode  326  is an electrode layer that is evaporated onto an entire surface of the front surface  320   a  of the piezo-electric element  320 , and an external electrode commonly used for all the piezo-electric blocks  321 . The external electrode  326  is grounded. On the contrary, the external electrode  328 , which is provided on the rear surface  320   b  of the piezo-electric element  320 , is however an electrode layer which is not evaporated onto an entire surface of the rear surface  320   b  and is independently provided only on a portion corresponding to each piezo-electric block  321 . The external electrode  328  has no potential unless electrified, but may apply a positive voltage to the internal electrode  324 . 
     Due to such a structure, each piezo-electric block  321  of the piezo-electric element  320  does not deform when no voltage is applied to the external electrode  328 , since both potentials of the internal electrodes  322  and  324  remain zero. On the other hand, when a voltage is applied from the external electrode  328 , each piezo-electric block  321  may deform in the direction A (longitudinal direction) in FIG. 8, independent of the other piezo-electric blocks  321 . In other words, the direction A is the polarization direction of the piezo-electric blocks  321 . When the electrification to the external electrode  328  stops, that is, when the piezo-electric element  320  is discharged, the corresponding piezo-electric block  321  returns to the original state. 
     The piezo-electric element  320  of this embodiment is made of a plurality of green sheets  327 . Each green sheet  327  is blended with solvents such as a ceramic powder, etc., kneaded into a paste and then formed to be a thin film having a thickness of about 50 μm by a doctor blade. 
     Among these green sheets, a pattern of the internal electrode  322  is printed and formed onto one surface of each of three green sheets, a pattern of the internal electrode  324  is printed and formed onto one surface of each of another three green sheets, and no internal electrode is formed onto the remaining sheets. Each of the internal electrodes  322  and  324  is printed by blending alloy powder of silver and palladium with a solvent into a paste to apply for its pattern formation. 
     Then, the three sheets including the internal electrode  322  and the three sheets including the internal electrode  324  are alternately stuck together, and thereafter the remaining six sheets are also stuck together. Thereby, the layered structure of the piezo-electric element  320  is formed as shown in FIG.  9 . The green sheets that include none of the internal electrode  322  or  324  are stuck at a lower portion (in FIG. 9) of the piezo-electric element  320  and form a base part. These layered green sheets are sintered. Then, at least six sheets are partially cut by a diamond cutter from the front surface  320   a  to the rear surface  320   b , whereby a plurality of the piezo-electric blocks  321  divided by the grooves  323  is formed. Lastly, the external electrodes  326  and  328  are formed by vacuum evaporation at the front surface  320   a  and the rear surface  320   b . It is possible to form the grooves  323  before sintering. The completed piezo-electric element  320  is submitted to a characteristic test by applying a voltage to the external electrodes  326  and  328 , and malfunctioning ones are eliminated. 
     The inkjet head  300  shown in FIG. 8 further comprises the protective layer  350 . The protective layer has useful effects as will be explained later, but there is a choice whether the protective layer is provided. 
     The protective layer  350  is a thermosetting epoxy adhesive member having an approximately rectangular parallelepiped shape with a thickness of about 50 μm, and connected via a surface  350   b  to the front surface  320   a  of the piezo-electric element  320  (external electrode  326 ). The materials for the protective layer  350 , however, are not limited to this type. For example, an epoxy filler member, an acrylic resin, a polyethylene resin or the like are usable for the protective layer  350 . The protective layer  350  in the actual inkjet head  300  does not have a rectangular parallelepiped shape in the strict sense of the term, and an interface between the protective layer  350  and the piezo-electric element  320  is not clear or simple as shown in FIGS. 8 and 9 by the external electrode  326  and the surface  350   b . The protective layer  350  partially penetrates through the grooves  323  into the piezo-electric element  320  before heatedly solidifying. Accordingly, it is preferable that the protective layer  350  is made of insulators so as to prevent a short circuit of the internal electrodes  322  and  324 . The protective layer  350  of this embodiment is applied to the piezo-electric element  320  (external electrode  326 ) all over the front surface  320   a , but may, if necessary, be applied partially. 
     The protective layer  350  spaces the piezo-electric element  320  about 50 μm apart from the nozzle connection surface  360 . If ink leaked from the pressure chamber  12  and penetrated into the piezo-electric element  320 , the ink would penetrate into the piezo-electric element  320  mainly along the nozzle connection surface  360 . However, the protective layer  350  spaces from the nozzle connection surface  360  the piezo-electric element which has been conventionally located on the nozzle connection surface  360 , and thereby prevents the ink from penetrating into the piezo-electric element  320  and short-circuiting the internal electrodes  322  and  324 . 
     Moreover, the protective layer  350  shields the grooves  323 . If ink leaked and penetrated into the piezo-electric element  320 , the ink would penetrate mainly from an opening  312   a  of the pressure chamber  312 , running along the nozzle connection surface  360 , through the grooves  323  into the piezo-electric element  320 . However, the protective layer  350  does shield the grooves  323  against or from the nozzle connection surface  360 , and thereby prevents the ink from penetrating into the groove  323  from somewhere in the neighborhood of the front surface  320   a  of the piezo-electric element  320  and short-circuiting the internal electrodes  322  and  324 . 
     In addition, the protective layer  350  also has the effect of protecting the piezo-electric element  320  from being destroyed by polishing in a polishing process for forming the nozzle connection surface  320   a  among various steps of manufacturing the inkjet head. Consequently, the polishing process never causes any removing crack and chip-off of the piezo-electric element  320 . The external electrode is never cut off. Furthermore, the pressure chamber plate  310 , which is made of glass, is rather strong, and thereby enables such a high polishing speed as to shorten the polishing time down to about one-fifth in comparison with conventional manufacturing methods. 
     The nozzle plate  330  is made of metal, e.g. stainless steel, etc. Each nozzle  332  may be formed, as described above with reference to FIG. 6, with a punch using a pin or the like, preferably into a conic shape (or as showing a tapering section) spreading from the front surface  330 b toward the rear surface  330   a  of the nozzle plate  330 . To obtain such a conic shaped nozzle  332  is one of the reasons why the pressure chamber plate  310  and the nozzle plate  330  are not formed in one but the nozzle plate  330  is adhered to the pressure chamber plate  310 . In this embodiment, the nozzle  332  is about 80 μm in diameter at the rear surface  330   a  and about  25  through 35 μm at the front surface  330   b . The present invention is also applicable to such an inkjet head that a nozzle thereof is formed, for example, above the pressure chamber plate  310  shown in FIG. 8, unlike the inkjet head  300 . 
     On the surface (front surface)  330   b  of the nozzle plate  330 , at least around the nozzle  332 , is formed the water-repellent coating  100 . Of course, the water-repellent coating  100  may be formed all over the front surface  330   b . The water-repellent coating serves to stabilize a wiping operation, which will be described later, and to provide a high quality image. It is to be construed that the water-repellent coating should be located differently to accompany the nozzle where the nozzle of the inkjet head is formed, for example, above the pressure chamber plate  310  shown in FIG.  8 . 
     In the inkjet head  300 , each external electrode  328  independently applies a voltage the internal electrode  324  of the piezo-electric block  321 , and each piezo-electric block  321  independently deforms in the direction A in FIG. 1, bending the resin film  340  in the direction A and compressing the corresponding pressure chamber  312 . This compression results in jetting ink from the pressure chamber  321  through the corresponding nozzle  332 . When the electrification from the external electrode  328  stops, the resin film  340  and the piezo-electric block  321  returns to the original states by discharging. At that time, the internal pressure of the pressure chamber  312  reduces and ink is supplied from the common ink chamber  316  through the ink introduction channel  314  to the pressure chamber  312 . 
     Although this embodiment uses the piezo-electric element  320  that deforms in the longitudinal direction, but another embodiment may use one that deforms in the lateral direction. Further, the present invention is not limited to the piezo-type inkjet head employing the piezo-electric element but applicable to the bubble-type inkjet head. 
     Referring next to FIG. 10, a description will be given of an inkjet printer  400  provided with the inkjet head  300  of the present invention. In each drawing, those members designated by the same reference numeral denote the same members, and a duplicate description will be omitted. FIG. 10 schematically illustrates an embodiment of the color inkjet printer (recording device)  400  to which the inkjet head  300  of the present invention is applied. In a housing  410  of the recording device  400 , a platen  414  is rotatably provided. 
     In a recording operation, the platen  412  is driven to intermittently rotate by a driving motor  414  and send recording paper P at a predetermined feed pitch in the arrow direction W. In the housing  416  of the recording device, a guide rod  416  is provided in parallel to and above the platen  412 . 
     A carriage  418  is mounted to an endless driving belt  420  that is driven by the driving motor  422  reciprocating for scanning along the platen  412 . 
     The carriage  412  is mounted with a black recording head  424  and a color recording head  426 . The color recording head  426  may comprises three parts. The black recording head  424  is removably provided with a black ink tank  428 , and the color recording head  426  is removably provided with color ink tanks  430 ,  432  and  434 . The inkjet head  300  of the present invention is applicable to such recording heads  424  and  426 . 
     Needless to say, the black ink tank  428  accommodates black ink and the color ink tanks  430 ,  432  and  434  accommodate yellow ink, cyan ink and magenta ink respectively. 
     While the carriage  418  reciprocates along the platen  412 , the black recording head  424  and the color recording head  426  are driven based on image data received from a word processor and a personal computer, etc., predetermined characters, images and the like are recorded on recording paper P. When the recording operation is suspended, the carriage  418  is returned to its home position and this home position is provided with a nozzle maintenance mechanism (or backup unit)  436 . 
     The nozzle maintenance mechanism  436  is provided with a movable suction cap (not shown) and a suction pump (not shown) connected to the movable suction cap. When the recording heads  224  and  226  are placed at the home position, the suction cap is adsorbed to the nozzle plate of each recording head and the nozzle of the nozzle plate is sucked. This mechanism prevents the nozzle from being plugged. After that, a wiping unit (also not shown) wipes out the nozzle plate  330   b  with a wiper. On that occasion, the water-repellent coating  100  wipes out ink on the nozzle plate surface  330   b  completely, and the hard body  108  in the water-repellent coating  100  prevents the water-repellent coating from being destroyed or otherwise. 
     Although the preferred embodiments of the present invention have been described above, it is to be understood that various modifications and changes may be made in the present invention without departing from the spirit and scope thereof. 
     According to the water-repellent coating as set forth in claim  1 , the water repellency of its fluoric polymer works well serving to provide a high quality image, and its hard body enhancing the wiping resistance of the fluoric polymer guarantees to continuously provide the high quality image. According to the water-repellent coating as set forth in claim  2 , its flat hard body is not so vulnerable to friction or likely to fall off compared with a spherical hard body, and therefore can keep its wiping resistance for a long time. According to the water-repellent coating as set forth in claim  3 , its flat body having a big particle diameter never prevents the nozzle plate surface from being smoothly wiped. According to the water-repellent coating as set forth in claims  4  and  5 , the flat hard body applicable to claim  1  is easily obtainable. The water-repellent coating as set forth in claims  6  and  7  can be easily formed without any special plating process. The recording device as set forth in claim  8  including the same water-repellent coating as claimed in claims  1  through  7  have the same effect as these claims. 
     The method of forming the water-repellent coating as set forth in claim  9  enables the hard body to protrude from the water-repellent coating surface; therefore, the wiping resistance of the water-repellent coating is advantageously enhanced. According to the method of forming the water-repellent coating as set forth in claim  10 , the fluoric polymer melts by the heat treatment taking in the additive hard body whereby sufficient water repellency is expected even on the surface of the intrinsically low water-repellent hard body.