Patent Publication Number: US-5896149-A

Title: Ink jet type recording head having a flow passage substrate with a stepped configuration and recesses formed in a surface thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an ink jet type recording head of an on-demand system which jets ink droplets in response to printing signals to form dots on a recording medium. 
     2. Description of the Prior Art 
     An ink jet type recording head comprises a flow passage unit and a displacement effecting unit. The flow passage unit comprises: a nozzle plate having nozzle openings; a flow passage forming substrate having pressure generating chambers which are communicated with the nozzle openings, a common ink chamber and ink supply inlets; and a vibrating member which sealingly covers the flow passage forming substrate, and which inflates and deflates the pressure generating chambers, being externally displaced. The displacement effecting unit is adapted to apply mechanical energy to the pressure generating chambers through the vibrating member to jet ink. 
     In the ink jet type recording head, each of the ink supply inlets greatly affect the picture quality of the resultant print as well as each of the nozzle openings, and is an important element which changes the flow passage impedance ratio of the ink supply inlet and the nozzle opening, and the absolute value of the flow passage impedance. Hence, the dimensions and the flow resistances of the ink supply inlet and the nozzle opening greatly affect various characteristics of the recording head such as an ink-droplet jetting speed, a quantity of ink droplet, and an ink-droplet jetting frequency. 
     This problem may be solved by a technique which has been disclosed, for instance, by Japanese Patent Application No. Hei. 5-229114. That is, by anisotropic-etching a silicon monocrystal wafer, the pressure generating chambers, the common ink chamber, and the ink supply inlets of the flow passage substrate can be formed with high dimensional accuracy. 
     On the other hand, in a recording head for high-density printing, the pressure generating chambers are large in aspect ratio, and walls 45 (FIG. 7) separating the pressure generating chambers from one another are thin, so that the bonding areas for bonding the nozzle plate and the vibrating member to the flow passage forming substrate are extremely small. 
     In an example of the conventional recording head described above, as shown in FIG. 7, a bonding region 41 on the side of the nozzle openings 40 is large, 800 μm×141 μm, while bonding regions around ink supply inlets 43, which are formed on one side of the flow passage forming substrate which is opposite to the other side where the pressure generating chambers 42 are formed, are each small, 25 μm×200 μm. The sum of those small bonding regions 44 around the ink supply inlets is also extremely small, about one-twentieth (1/20) of the bonding region 41 provided near the nozzle openings 40, and therefore it is difficult to obtain a sufficient adhesive strength on the side of the ink supply inlets. 
     Accordingly, at the junctions of the flow passage forming substrate, the nozzle plate and the vibrating member which form the ink flow paths, those components may be separated from one another for instance because of the non-uniform application of the adhesive agent; and the ink may leak from one of the pressure generating chambers into another. Thus, the conventional recording head is low in reliability. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the invention is to provide an ink jet type recording head in which, while the ink supply inlets are controlled in flow resistance, the nozzle plate, the flow passage forming substrate, and the vibrating member are bonded together with a sufficiently high adhesive strength. 
     There is provided an ink jet type recording head comprising: a nozzle plate which has a nozzle opening for jetting ink droplets; a flow passage forming substrate laminated on the nozzle plate and having a pressure generating chamber which is communicated with the nozzle opening, a common ink chamber for supplying ink to the pressure generating chamber and an ink supply inlet through which the ink chamber are communicated with the pressure generating chamber; a vibrating member laminated on the flow passage forming substrate; and a displacement effecting section for inflating and deflating the pressure generating chamber through the vibrating member; wherein the ink supply inlet in the flow passage forming substrate has a plurality of steps so that the ink supply inlet gradually expands towards the pressure generating chamber. 
     With the recording head, bonding regions are obtained between the common ink chamber and the pressure generating chamber which are large in opening area, which provide a sufficiently high adhesive force which resists against a shearing stress produced by the displacement effecting means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view showing an ink jet type recording head; 
     FIG. 2 is a sectional view of part of a recording head according to a first embodiment of the present invention; 
     FIG. 3 is a sectional view showing a state of the recording head at room temperature which has been heated so as to join a head frame and a displacement effecting unit together; 
     FIG. 4 is a front view showing the first embodiment of a flow passage forming substrate which is formed by cutting a silicon monocrystal wafer; 
     FIG. 5 is an enlarged diagram of the flow passage forming substrate, mainly showing pressure generating chambers and parts concerning the latter; 
     FIG. 6(a) is an enlarged diagram of a second embodiment of the flow passage forming substrate, mainly showing pressure generating chambers and parts concerning the latter, and FIG. 6(b) is a sectional diagram taken along line A--A in FIG. 6(a); and 
     FIG. 7 is an enlarged front view of an example of a flow passage forming substrate in a conventional ink jet type recording head, mainly showing pressure generating chambers and parts concerning the latter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a perspective view showing the structure of an ink jet type recording head. FIG. 2 is a longitudinal sectional view showing the structure of a pressure generating chamber in the recording head according to a first embodiment of the present invention. Fundamentally, the ink jet type recording head comprises two units, namely, a flow passage unit 1 and a displacement effecting unit 2. The flow passage unit comprises three elements; more specifically, it comprises: a nozzle plate 11; an ink flow passage forming substrate having pressure generating chambers 13, a common ink chamber 14, and ink supply inlets 15; and a vibrating member 17 which is made of a re:sin film, a SUS plate or the like. 
     The flow passage forming substrate 16 has arrays of pressure generating chambers 13 which are arranged at equal intervals in correspondence to nozzle openings 12, 12, 12 . . . of the nozzle plate 11. The pressure generating chambers 13 are communicated through the respective ink supply inlets 15 to the common ink chamber 14. 
     The nozzle plate 11, the flow passage forming substrate 16, and the vibrating member 17 are combined together, thus providing the flow passage unit 1. More specifically, the nozzle plate 11 is bonded liquid-tight to one side of the flow passage forming substrate 16 through an adhesive layer 26, while the vibrating member 17 is bonded liquid-tight to the other side of the flow passage forming substrate 16 through an adhesive layer 27. 
     On the other hand, the displacement effecting unit 2 is designed as follows. The unit 2 includes arrays of displacement effecting sections 18 which are adapted to inflate and deflate the pressure generating chambers 13 to jet ink droplets. The displacement effecting sections 18 are arranged in arrays in the same direction as the pressure generating chambers 13 (in the direction of the arrow X in FIG. 1), and cantilevered to a base stand 19 in such a manner that their ends abut against island-like protrusions 24 which are elastically deformed for inflating and deflating the pressure generating chambers 13. 
     The displacement effecting sections 18 are each designed as follows. Its free end portion is an active section which comprises a piezo-electric layer 20 in which electrode layers 21 and 22 are alternately arranged. The electrodes 21 are drive electrodes provided respectively for the displacement effecting sections, and the electrodes 22 are parallel-connected to one another as common electrodes. Those electrodes 21 and 22 are connected through lead frames 23 to an external drive circuit (not shown). 
     The above-described flow passage unit 1 and displacement effecting unit 2 are secured to a head frame 3 with an adhesive agent 25 in such a manner that the ends of the displacement effecting sections 18 abut on the island-like protrusions 24 formed on the vibrating member 17 in the regions which are confronted with the pressure generating chambers 13. 
     During printing, the displacement effecting sections 18 inflate and deflate the pressure generating chambers 13 to jet ink droplets from the nozzle openings 12, and therefore a considerably great reaction force acts on the displacement generating unit 2. Hence, in order to bond the base stand 19 supporting the displacement effecting sections 18 to the head frame 3, the adhesive agent 25 must be great in the force of adhesion and must resist fatigue. The adhesive agent of this type is low in cure rate at room temperature, thus lowering the work efficiency in the recording head assembling operation. In order to overcome this difficulty, the adhesive agent is heated to a degree of about 60° C. to increase the cure rate. 
     On the other hand, for convenience in manufacture, the head frame 3 is formed by injection molding macromolecular material, and the displacement effecting sections 18 are formed by using ceramics such as PZT (piezoelectric electrostriction element) showing piezoelectric characteristics. 
     As described above, the head frame 3 and the displacement effecting sections 18 are formed by using different materials, namely, macromolecular material and ceramics. Hence, at room temperature, the head frame 3 and the displacement effecting sections 18 are different in length because the room temperature is different from the temperature provided when the base stand 19 is bonded to the head frame, and these elements 3 and 18 are different in thermal expansion coefficient. This difference in length acts as a pressing force collectively near the pressure generating chambers 13 which abut on the ends of the displacement effecting sections 18. As a result, The flow passage forming substrate 16, the vibrating member 17, and the nozzle plate 11 are pushed upwardly, or deformed, as shown in FIG. 3. Because of this deformation, a load is applied to the flow passage forming substrate 16, the vibrating member 17, and the nozzle plate 11 to act, as a shearing force, on the adhesive agent layers 26 and 27 through which those components 16, 17 and 11 are combined together. 
     FIG. 4 shows an embodiment of the flow passage forming substrate 16 which overcomes the above-described difficulty. In FIG. 4, reference character S designates a silicon monocrystal wafer 200 to 300 μm in thickness which has a (110) crystal face. In the wafer S, a plurality of pressure generating chambers 13, and a common ink chamber 14 are formed, and furthermore ink supply inlets are formed as through-holes. The wafer S thus processed is cut out to the size of the aimed ink jet type recording head. 
     FIG. 5 shows the ink supply inlets 15 and the pressure generating chambers 13 of the flow passage forming substrate 16, and their relevant parts. In the recording head, the flow resistance of each of the ink supply inlets 15 should be substantially equal that of each of the nozzle openings 12 and much higher than that of each of the pressure generating chambers 13. Hence, each of the ink supply inlets 15 is much smaller in sectional area than each of the pressure generating chamber 13. 
     That is, in a concrete embodiment of the flow passage forming substrate, the width w1 of the pressure generating chamber 13 is of the order of 100 μm, and the width w2 of the narrowest part 15a of the ink supply inlet 15, which substantially determines the flow resistance of the ink supply inlet 15, is of the order of 25 μm. 
     In addition, the common ink chamber 14 is shifted outwardly, and the distance L for each of the ink supply inlets 15 and the respective pressure generating chamber 13 is increased when compared with that in the conventional flow passage forming substrate. That is, in the embodiment, the distance is of the order of 600 μm which is three times that in the conventional flow passage forming substrate. The width of the ink supply inlet 15 is increased stepwise from the narrowest part 15a towards the pressure generating chamber 13. In the example, the ink supply inlet 15 includes three steps 15b, 15c and 15d being larger towards the pressure generating chamber, that is, the width of the ink supply inlet 15 is increased to 100 μm which is substantially equal to the width of the pressure generating chamber 13. 
     In FIG. 5, reference numerals 28 and 29 designate small recesses or grooves formed in the adhesion regions. Those recesses or grooves absorb an excess of adhesive agent which is going to flow over the substrate during the substrate bonding operation; that is, they serve as adhesive agent absorbing means for preventing the adhesive agent from flowing into the pressure generating chamber and the ink supplying inlets. 
     Thus, between the common ink chamber 14 which, in the flow passage forming substrate 16, is largest in opening area and the pressure generating chambers 13 which are ranked next to the common ink chamber 14 in opening area, a bonding region having some width, 600 μm, is obtained without adversely affecting the function of the ink supply inlets 15. In the recording head of the invention, the length of the ink supply inlet region is larger as much as L2 than that L&#39; (FIG. 7) of the conventional ink supply inlet, and therefore the bonding region is about twice as large as that in the conventional recording head. 
     In order to confirm the effects of the above described increase in bonding region, the temperature of the recording head was changed from a heating temperature of 60° C. used for hardening the adhesive agent to a temperature of -30° C. to which it may be cooled for instance during transportation by airplane, and the adhesive agent layers 26 and 27 of the flow passage forming board, the nozzle plate, and the vibrating member were inspected. However, none of the layers 26 and 27 were peeled off them. 
     The width of each of the ink supply outlets 15 are gradual y larger from the ink chamber 14 towards the pressure generating chamber 13. That is, the width change between the ink supply outlets 15 and the pressure generating chamber 13 is relaxed as compared with that of the conventional recording head. Hence, in the case where, in order to regain the ink jetting performance, the ink is forcibly discharged by the application of a negative pressure to the nozzle openings 12, the formation of eddies in the boundary between the ink supply inlet and the pressure generating chamber is sufficiently suppressed, so that bubbles near the boundary can be also readily discharged from the nozzle opening 12. 
     The above described embodiment is the recording head of the type that one end portion of each of the pressure generating chambers is communicated with the respective nozzle opening, and the ink is supplied from one side of the pressure generating chamber. However, the invention is not limited thereto or thereby. That is, the recording head may be modified as shown in FIGS. 6(a) and 6(b). This is a second embodiment of the invention. In the second embodiment, a nozzle opening 31 is located at the center of each of the pressure generating chambers 30; and ink supply inlets 32 and 33, and common ink chambers 34 and 35 are provided on both sides of the pressure generating chambers 30, so that the recording head is improved in ink supplying performance; that is, it is operated at high speed. Thus, the second embodiment is more noticeable in effect than the first embodiment. 
     More specifically, in the case where the two common ink chambers 34 and 35 are arranged on both sides of the pressure generating chambers 30, the bonding regions around the pressure generating chambers 30 are very small. However, in the second embodiment, this problem is solved as follows. In each of the ink supply inlets, the narrowest parts 32a and 33a are provided on the sides of the common ink chambers, and are communicated through long passageways including a plurality of steps 32b, 32c, 33b and 33c with the pressure generating chamber 30. Hence, in the second embodiment, while the ink supply inlets 32 and 33 are maintained most suitable in flow resistance for the ink jetting operation, the bonding region can be increased, that is, the adhesive strength can be increased. In addition, both ends of the pressure generating chambers receive equally the load applied thereto. This feature eliminates the difficulty that the adhesive agent layers peel off the flow passage forming unit at a particular point or points. 
     In the case where the ink supply inlets 32 and 33 are formed on both sides of the pressure generating chambers 30, the flow resistance of each of the ink supply inlets must be higher than in the case where each pressure generating chamber has only one ink supply inlet. However, if the narrowest parts are decreased in width to increase the flow resistance, then it is difficult to make the dimensions of the ink supply inlets high in accuracy. Therefore, in this case, it is preferable that the depth d of the ink supply inlets is made smaller by half-etching. 
     As described above, an ink jet type recording head according to the present invention comprises: a nozzle plate which has a nozzle opening for jetting ink droplets; a flow passage forming substrate laminated on the nozzle plate and having a pressure generating chamber which is communicated with the nozzle opening, a common ink chamber for supplying ink to the pressure generating chamber and an ink supply inlet through which the ink chamber are communicated with the pressure generating chamber; a vibrating member laminated on the flow passage forming substrate; and a displacement effecting section for inflating and deflating the pressure generating chamber through the vibrating member; wherein the ink supply inlet in the flow passage forming substrate has a plurality of steps so that the ink supply inlet gradually expands towards the pressure generating chamber. 
     Hence, bonding areas can be obtained between the common ink chamber and the pressure generating chamber which are large in opening area, which provides an adhesive force which resists; against the shearing force produced by the displacement effecting means.