Abstract:
An ink jet apparatus offers enhanced print quality and is highly conducive to mass-production. The apparatus has a piezoelectric ceramic arrangement including a plurality of ink channels filled with ink. The ink channels are defined by walls and correspond to nozzles. The walls are deformed selectively by a piezoelectric effect to vary the volumes of the selected ink channels to jet out the ink through the corresponding nozzles. The center position of each of the nozzles is allowed to deviate from the middle between one wall of the corresponding ink channel and the other wall opposite thereto by half the distance between the two walls minus half the diameter of the nozzle within the half distance. This arrangement reduces the number of defective ink jet printer heads in the manufacturing phase and allows the ink jet apparatus to offer enhanced print quality. Such an apparatus is also highly conducive to mass-production with no need for specialized adjustments of individual nozzles.

Description:
This is a Continuation of application Ser. No. 08/148,507 filed Nov. 8, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet apparatus having a piezoelectric ceramic arrangement comprising ink channels separated by walls and filled with ink. The walls of the ink channels are selectively deformed by the piezoelectric effect to vary the volumes of the channels so that the ink therein will be jetted out through the nozzles corresponding to those channels. 
     2. Description of the Related Art 
     The above type of ink jet apparatus has been conventionally used in ink jet printers. A first example of a typical prior art ink jet apparatus is outlined below with reference to FIG. 6. In FIG. 6, each ink chamber 70 is formed by a housing 72 including side walls 74 equipped with piezoelectric ceramic devices 76. Electrodes 77 are furnished on both sides of each piezoelectric ceramic device 76 and are connected to a driving circuit 79. 
     When the driving circuit 79 applies a driving voltage to the electrodes 77, the piezoelectric ceramic device 76 attached thereto is deformed accordingly. With the piezoelectric ceramic device 76 deformed, the side wall 74 is deformed into a shape indicated by the dashed line in FIG. 6, thereby reducing the volume of the ink chamber 70. The reduction in the volume of the ink chamber 70 causes the ink therein to jet out as an ink droplet 80 through a nozzle 82. 
     Thereafter, with the driving voltage removed, the piezoelectric ceramic device 76 returns to its original shape and thus increases the volume of the ink chamber 70. The increase in the volume of the ink chamber 70 introduces additional ink thereinto through an ink conduit 84. Typically, ink jet printers comprise numerous ink chambers 70 when manufactured. 
     A second example of a typical prior art ink jet apparatus is outlined below with reference to FIG. 7. In FIG. 7, an ink jet printer head 1 comprises a piezoelectric ceramic plate 2, a cover plate 3, a nozzle plate 31 and a substrate 41. 
     The piezoelectric ceramic plate 2 has a plurality of grooves 8. Side walls 11 defining the grooves 8 are polarized in the direction shown by arrow 5. The grooves 8 are of a uniform depth and are parallel. The grooves 8 become gradually shallower as they approach an edge 15 of the piezoelectric ceramic plate 2. Near the edge 15, the grooves 8 merge into shallow grooves 16. Inside the grooves 8, the upper half of each of the side walls 11 is covered with a metal electrode 13 formed thereon by sputtering or by other suitable processes. Inside the shallow grooves 16, the entire side walls and the bottoms are covered with metal electrodes 9 deposited by sputtering or by other suitable processes. In this arrangement, the metal electrodes 13 formed on both sides of each groove 8 are connected electrically to the metal electrodes 9 furnished over the shallow grooves 16. 
     The cover plate 3 is made of ceramic or plastic resin. On the cover plate 3, ink inlet ports 21 and manifolds 22 are formed by grinding or by cutting. The side of the piezoelectric ceramic plate 2 having the grooves 8 formed thereon is bonded using epoxy resin adhesive or the like to the side of the cover plate 3 having the manifolds 22 machined thereon. Covering the top of the grooves 8 in this manner with the plate 3 forms a plurality of ink channels 12 (FIG. 9) spaced apart crosswise in the ink jet printer head 1. Each ink channel 12 has a rectangular cross section and is long and narrow in shape. All ink channels 12 are filled with ink during operation. 
     The nozzle plate 31 is attached to one end of the piezoelectric ceramic plate 2 and the cover plate 3. The nozzle plate 31 has nozzles 32 formed in the positions corresponding to the ink channels 12. The nozzle plate 31 is made of plastic such as polyalkylene (e.g., ethylene), terephthalate, polyimide, polyether imide, polyether ketone, polyether sulfone, polycarbonate or cellulose acetate. 
     A substrate 41 is bonded using epoxy resin adhesive or the like to the surface opposite to the side of the piezoelectric ceramic plate 2 having the grooves 8. The substrate 41 has conductive layer patterns 42 corresponding to the positions of the ink channels 12. The metal electrodes 9 at the bottoms of the shallow parallel grooves 16 are connected to the conductive layer patterns 42 by conductors 43 deposited by wire bonding. 
     The control section of the prior art apparatus is described with reference to FIG. 8, which is a schematic diagram of the control section. Each of the conductive layer patterns 42 on the substrate 41 is connected individually to an LSI chip 51. Also connected to the LSI chip 51 are a clock line 52, a data line 53, a voltage line 54 and a grounding line 55. Given continuous clock pulses through the clock line 52 as well as data from the data line 53, the LSI chip 51 decides through which of the nozzles 32 ink droplets are to be jetted out. Based on its decision, the LSI chip 51 selectively applies the voltage V of the voltage line 54 to the conductive layer patterns 42 connected to the metal electrodes 13 that belong to the target ink channels 12. The LSI chip 51 also applies a zero voltage of the grounding line 55 to those conductive layer patterns 42 connected to the metal electrodes 13 associated with ink channels 12 that are not targetted for jetting. 
     The operation of the ink jet printer head 1 is described with reference to FIGS. 9 and 10. Suppose that, given appropriate data, the LSI chip 51 decides that an ink droplet is to be jetted out from an ink channel 12b. Then, a positive driving voltage V is applied to metal electrodes 13e and 13f while metal electrodes 13d and 13g are connected to ground. This develops a driving electric field in a side wall 11b in the direction of arrow 14b and another driving electric field in a side wall 11c in the direction of arrow 14c, as illustrated in FIG. 10. Because the directions 14b and 14c of the driving electric fields are each perpendicular to the direction of polarization 4, the side walls 11b and 11c are deformed rapidly toward the inside of the ink channel 12b due to the piezoelectric thickness slip effect. The side wall deformation reduces the volume of the ink channel 12b and rapidly raises the ink pressure therein. The resulting pressure wave causes an ink droplet to be jetted out through the nozzle 32 (FIG. 7) connected to the ink channel 12b. 
     One disadvantage of the prior art ink jet apparatus is that the nozzle 32 is located inside each ink channel 12, i.e., the position of the nozzle 32 is not fixed definitively relative to the ink channel 12. The center positions of the nozzles 32 may or may not be aligned with the centers of the ink channels 12. The flow rate of ink at the center of each ink channel 12 can differ from the flow rate off the center thereof. The differences in ink flow rate can translate into different jet speeds of ink droplets from the nozzles, causing the print speed to vary from one ink jet printer head 1 to another. Such speed discrepancies can render the ink jet printer head 1 nonconducive to mass-production. 
     On the same ink jet printer head 1, the center positions of individual nozzles 32 may or may not be aligned with the centers of the corresponding ink channels 12. This means that the jet speed of ink droplets can vary from nozzle to nozzle 32 on the same printer head. The jet speed discrepancies can lead to a serious deterioration of print quality with the ink jet printer head 1. In some cases, the printer head does not function outright. As a result, there can be many defective ink printer heads if the printer heads are mass-produced. 
     On the prior art ink jet apparatus, the nozzles 32 are thought to jet out ink droplets in more stable quantities and at more stable speeds when narrowed progressively (i.e, tapered) toward the ink jetting side from the ink channel side than when shaped otherwise (i.e., widened progressively toward the ink jetting side from the ink channel side, or bored straight to have the same diameter at both the ink jetting side and the ink channel side). Conventionally, however, there is no strictly determined ratio of the sectional area of the nozzle on the ink jetting side to that on the ink channel side. That ratio can be disproportionately large on one ink jet printer head and inordinately small on another. As a result, the jet speed of ink droplets, and hence the print speed, may vary from one ink jet printer head 1 to another. Such speed discrepancies can lead to a deterioration of print quality with ink jet printer heads, rendering them nonconducive to mass-production. 
     On the same ink jet printer head 1, the ratio of the sectional area of the nozzle on the ink jetting side to that on the ink channel side can vary significantly from one individual nozzle to another. This means that the jet speed of ink droplets can vary from nozzle to nozzle on the same printer head. The jet speed discrepancies can lead to a serious deterioration of print quality with the ink jet printer head 1. In some cases, the printer head does not function outright. As a result, there can be many defective ink printer heads if the printer heads are mass-produced. 
     Furthermore, with the prior art ink jet apparatus, there is no definitely determined taper angle θ at which the diameter of the nozzle 32 is narrowed progressively from the ink channel side toward the ink jetting side. The taper angle can be disproportionately large on one ink jet printer head and inordinately small on another. As a result, the jet speed of ink droplets, and hence the print speed, may vary from one ink jet printer head 1 to another. Such speed discrepancies can lead to a deterioration of print quality with ink jet printer heads, rendering them nonconducive to mass-production. 
     On the same ink jet printer head 1, the nozzle taper angle can vary significantly from one individual nozzle to another 32. This means that the jet speed of ink droplets can vary from nozzle to nozzle 32 on the same printer head. The jet speed discrepancies can lead to a serious deterioration of print quality with the ink jet printer head 1. In some cases, the printer head does not function outright. As a result, there can be many defective ink printer heads where the printer heads are mass-produced. 
     With ink jet apparatuses, the minimum sectional area of the nozzle 32 must be greater than the maximum projected area of solid ink particles to ensure good ink jet performance. Conventionally, however, there is no fixedly determined ratio of the minimum sectional area of the nozzle to the maximum projected area of solid ink particles. Whereas there is little problem if the sectional area of the nozzles on their ink jetting side is sufficiently large compared with the maximum projected area of solid ink particles, an insufficient sectional area of the nozzles can change the jet speed of ink droplets due to the friction between the nozzles and solid ink particles. The varying jet speeds can also vary the print speed from one ink jet printer head 1 to another. Such speed discrepancies can lead to a deterioration of print quality with ink jet printer heads, rendering them nonconducive to mass-production. 
     On the same ink jet printer head 1, some nozzles 32 may have insufficient sectional areas compared with the maximum projected area of solid ink particles while others have sufficient sectional areas. This means that the jet speed of ink droplets can vary from one individual nozzle 32 to another on the same printer head. The jet speed discrepancies can lead to a serious deterioration of print quality with the ink jet printer head 1. In some cases, the printer head does not function outright. As a result, there can be many defective ink printer heads if the printer heads are mass-produced. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary object of the present invention to overcome the above and other deficiencies and disadvantages of the prior art and to provide an ink jet apparatus that offers enhanced print quality and is highly conducive to mass-production. 
     According to one aspect of the invention, an ink jet apparatus has a piezoelectric ceramic arrangement including a plurality of ink channels filled with ink, the ink channels being formed by walls and corresponding to nozzles. The walls are selectively deformed by the piezoelectric effect to vary the volumes of the selected ink channels to jet out the ink inside through the corresponding nozzles. The center position of each of the nozzles is allowed to deviate from the middle between one wall of the corresponding ink channel and the other wall opposite thereto by half the distance between the two walls minus half the diameter of the nozzle within each half distance of the channel. 
     In a preferred structure according to the invention, the nozzles are furnished on a nozzle plate that is positioned so that the nozzles will correspond to the ink channels. 
     According to another aspect of the invention, the ratio of the sectional area of each of the nozzles on the ink jetting side to that on the ink channel side ranges from 1:2 to 1:50. 
     According to a further aspect of the invention, the minimum sectional area of each of the nozzles is at least four times the maximum projected area of solid ink particles. 
     According to an even further aspect of the invention, the taper angle θ, at which the diameter of each of the nozzles is narrowed progressively from the ink channel side toward the ink jetting side, ranges from five to 30 degrees. 
     In operation, the jet speed of ink droplets is increased significantly with the inventive ink jet apparatus in any one of the following four cases. First, the jet speed of ink droplets increases when the center position of each of the nozzles is allowed to deviate from the middle between one wall of the corresponding ink channel and the other wall opposite thereto by half the distance between the two walls minus half the diameter of the nozzle within the half distance. Second, the jet speed of ink droplets increases when the ratio of the sectional area of each of the nozzles on the ink jetting side to that on the ink channel side ranges from 1:2 to 1:50. Third, the jet speed of ink droplets increases when the minimum sectional area of each of the nozzles is at least four times the maximum projected area of solid ink particles. Further, the jet speed of ink droplets increases when the taper angle θ at which the diameter of each of the nozzles is narrowed progressively from the ink channel side toward the ink jetting side ranges from five to 30 degrees. 
    
    
     These and other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial enlarged sectional front view showing where the center of each nozzle is located in an ink jet apparatus embodying the invention; 
     FIG. 2 is a graph depicting typical relations between the nozzle center position and the ink jet speed in connection with the embodiment; 
     FIG. 3 is a graph illustrating typical relations between the ratio of the sectional area of each nozzle on the ink jetting side to that on the ink channel side on the one hand, and the ink jet speed on the other in connection with the embodiment; 
     FIG. 4(A) is a partial sectional side view portraying the nozzle taper angle in the preferred embodiment; 
     FIG. 4(B) is a graph indicating typical relations between the nozzle taper angle and the ink jet speed in connection with the embodiment; 
     FIG. 5 is a graph exhibiting typical relations between the ratio of the sectional area of each nozzle on the ink jetting side to the maximum projected area of solid ink particles on the one hand, and the ink jet speed on the other in connection with the embodiment; 
     FIG. 6 is a partial cross-sectional view of the first example of the typical prior art ink jet apparatus; 
     FIG. 7 is an exploded perspective view of the second example of the typical prior art ink jet apparatus; 
     FIG. 8 is a schematic sketch of the control section of an ink jet printer head in the second prior art example; 
     FIG. 9 is a front cross-sectional view of the ink jet printer head in the second prior art example; and 
     FIG. 10 is a front cross-sectional view showing the operation of the ink jet printer head in the second prior art example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the invention will now be described with reference to the accompanying drawings. The basic constitution and workings of the embodiment are the same as those of the second prior art example and will not be discussed further. 
     Described first is where the center of each nozzle 32 is to be located with respect to the corresponding ink channel 12. In developing the apparatus, the inventor measured the jet speeds of ink droplets while varying the center position of the nozzle 32 relative to the ink channel 12. Specifically, a strobe light device was made to emit light at varying intervals so that the jetted ink droplets would synchronize with the light emissions. The light emission intervals in synchronism with the jetted ink droplets were measured and translated into the jet speeds of the ink droplets. FIG. 2 depicts typical relations between the position of the center of each nozzle 32 and the ink jet speeds measured in the above manner. 
     FIG. 2 plots measurements from two cases, one in which the diameter D1 of the nozzle 32 was 30 μm and the other in which the diameter D2 of the nozzle 32 was 45 μm. The two cases shared the same setup wherein the length L between two side walls 11 (i.e., width of the ink channel 12) was 90 μm and the height of the walls was 300 μm. The center of the nozzle 32 was 300 μm high. During the measurement of ink jet speeds using the setup above, the center of the nozzle 32 was moved away from the middle between one side wall 11 of the ink channel 12 and the other side wall opposite thereto, in units of 7.5 μm, i.e., 1/12 of the distance between the two side walls. 
     As shown in FIG. 2, where the diameter D1 of the nozzle 32 was 30 μm, the measured jet speeds of ink droplets were the highest and were relatively stable as the center of the nozzle 32 was moved away from the inter-wall middle by 15.0 μm to 75.0 μm. Where the diameter D2 of the nozzle 32 was 45 μm, the measured jet speeds of ink droplets were the highest and were relatively stable as the center of the nozzle 32 was moved away from the inter-wall middle by 22.5 μm to 67.5 μm. 
     It follows that, in more general terms, the jet speed of ink droplets is the highest and is relatively stable when the center of the nozzle 32 is allowed to deviate from the middle of the two adjacent side walls 11 by half the distance between the two walls minus half the diameter D of the nozzle 32 within each half distance of the channel. 
     Although not shown, the height of the center of the nozzle 32 from the channel bottom was also varied as the nozzle center was moved away from the inter-wall middle in units of 7.5 μm for measurement of ink jet speeds. The trend obtained was the same as that in effect where the center of the nozzle 32 remained unchanged in height. That is, the center position of the nozzle 32 with respect to the ink channel 12 was not affected by how high or how low the nozzle 32 was located relative to the channel bottom. 
     In other experiments, the inventor adopted other diameters D of the nozzle 32 to measure the jet speeds of ink droplets while the nozzle center position was varied crosswise between the two adjacent side walls 11 as well as vertically within the ink channel 12. The trend obtained was the same as that in effect where the diameter D1 was 30 μm and where the diameter D2 was 45 μm. 
     This means that the jet speed of ink droplets is the highest and relatively stable when the center of the nozzle 32 is allowed to deviate from the middle of the two adjacent side walls 11 by half the distance between the two walls minus half the diameter D of the nozzle 32 within the half distance (i.e., the hatched range 90 in FIG. 1). 
     The amount of deformation in each side wall 11 having the above-described size was about 30 μm, a very small quantity compared with the size of the nozzle 32. Fluctuations in the deformed amount of the side walls are thus negligible relative to the size of the nozzle 32. That is, the same trend as that shown in FIG. 2 is in effect regardless of the variations in the deformation of the side walls 11. 
     If the nozzles 32 are formed on the nozzle plate 31 in such a manner that the center of each nozzle will be located within the allowable range designated above, the jet speed of ink droplets is kept high and relatively stable. This means that the accuracy with which to position the center of each nozzle 32 need not be very high. If the nozzle plate 31 is mounted in such a manner that the nozzles 32 thereon will have their centers positioned as required above, the jet speed of ink droplets is also held high and relatively stable. Thus, the accuracy with which to position the center of each nozzle 32 with respect to the ink channel 12 need not be very high. The centers of individual nozzles 32 may deviate from the middle between two adjacent side walls as long as each nozzle center is located within a certain range with respect to each ink channel 12. This kind of constitution reduces the number of defective ink jet printer heads in the manufacturing phase. Thus, the ink jet apparatus according to the invention offers enhanced print quality and is highly conducive to mass-production with no need for specialized adjustments of individual nozzles. 
     The shape of the nozzle 32 will be described next. It is generally considered that the nozzle jets out ink droplets in more stable quantities and at more stable speeds when narrowed progressively (i.e, tapered). Specifically, the nozzle is narrowed toward the ink jetting side. Such a tapered nozzle jets ink droplets in more stable quantities and speeds from the ink channel side than when shaped otherwise (i.e., widened progressively toward the ink jetting side from the ink channel side, or bored straight to have the same diameter at both the ink jetting side and the ink channel side). 
     The inventor measured the jet speeds of ink droplets while varying the ratio of the sectional area of each nozzle 32 on the ink jetting side to that on the side of the ink channel 12. The method of measuring the ink jet speeds was the same as that described earlier. The measurements taken are plotted in FIG. 3. 
     FIG. 3 illustrates typical relations between the ratio of the sectional area of each nozzle 32 on the ink jetting side to that on the ink channel side on the one hand, and the ink jet speed on the other. In this case, the diameter of the nozzle 32 on the ink jetting side was fixed to 30 μm while the nozzle diameter on the side of the ink channel 12 was varied. The center of each nozzle 32 was positioned in the approximate middle of the ink channel 12 as it was flanked by side walls. The ink channel 12 was 90 μm in width and 300 μm in height. 
     As depicted in FIG. 3, the measured jet speeds of ink droplets were relatively high and stable when the ratio of the sectional area of each of the nozzles 32 on the ink jetting side to that on the side of the ink channels 12 ranged from 1:2 to 1:50, and preferably from 1:2 to 1:15. 
     Similar results were obtained when the center position of the nozzle 32 was moved within the allowable range designated earlier. Furthermore, similar trends were acquired when the diameter of each nozzle 32 was varied on the ink jetting side. 
     When the ratio of the sectional area of each of the nozzles 32 on the ink jetting side to that on the side of the ink channels 12 ranges from 1:2 to 1:50, the measured jet speeds of ink droplets are high and stable. This affords enhanced degrees of freedom to the designing of nozzles 32. Even if manufacturing error causes some deviations in the shape of each nozzle 32, the speeds of ink droplets jetted from each nozzle 32 remain stable as long as the ratio of the sectional area of each nozzle on the ink jetting side to that on the ink channel side ranges from 1:2 to 1:50. This kind of arrangement reduces the number of defective ink jet printer heads in the manufacturing phase. Thus, the ink jet apparatus according to the invention offers enhanced print quality and is highly conducive to mass-production with no need for specialized adjustments of individual nozzles. 
     Next, the inventor measured the jet speeds of ink droplets while varying the taper angle θ illustrated in FIG. 4(A). The method of measuring the ink jet speeds was the same as that described above. The results are plotted in FIG. 4(B). 
     FIG. 4(B) indicates typical relations between the nozzle taper angle θ and the ink jet speed in effect when the diameter of each nozzle 32 was fixed to 30 μm on the ink jetting side while the taper angle θ was varied. In this setup, the ratio of the sectional area of each nozzle 32 on the ink jetting side to that on the ink channel side ranged from 1:1.5 to 1:15. The center of each nozzle 32 was positioned in the approximate middle of the ink channel 12 as it was flanked by two side walls. Each ink channel 12 was 90 μm wide and 300 μm high. 
     As shown in FIG. 4(B), the measured jet speeds of ink droplets were relatively high and stable when the taper angle θ of the nozzle 32 ranged from five to 30 degrees, and preferably from five to 20 degrees. 
     Measurements were also taken while the diameter of each nozzle 32 on the ink jetting side was varied. The results obtained were the same. Furthermore, measurements were taken while the center of each nozzle 32 was moved away from the inter-wall middle within the allowable range stipulated above. The results acquired were also the same. 
     When the taper angle Θ of each nozzle 32 ranges from five to 30 degrees, the jet speeds of ink droplets are held high and stable. This affords enhanced degrees of freedom to the designing of nozzles 32. Even if manufacturing error causes some deviations in the shape of each nozzle 32, the speeds of ink droplets jetted from each nozzle 32 remain stable as long as the nozzle taper angle Θ is between five and 30 degrees. This kind of constitution reduces the number of defective ink jet printer heads in the manufacturing phase. Thus the ink jet apparatus according to the invention offers enhanced print quality and is highly conducive to mass-production with no need for specialized adjustments of individual nozzles. 
     Described below is the significance of the relation between the maximum projected area of solid ink particles and the minimum sectional area of the nozzle 32 with respect to the ink jet speed. The inventor measured the jet speeds of ink droplets using various kinds of ink containing different sizes of solid particles. The results were plotted in FIG. 5. 
     FIG. 5 shows typical relations between the ratio of the sectional area of each nozzle 32 on the ink jetting side to the maximum projected area of solid ink particles on the one hand and the ink jet speed on the other. The relations were measured when the diameter of each nozzle 32 was fixed to 30 μm on the ink jetting side while the maximum size of solid ink particles was varied. In this setup, the ratio of the sectional area of each nozzle 32 on the ink jetting side to that on the ink channel side ranged from 1:1.5 to 1:15. The taper angle θ of the nozzle 32 ranged from five to 20 degrees. The center of each nozzle 32 was positioned in the approximate middle of the ink channel 12 as it was flanked by two side walls. Each ink channel 12 was 90 μm wide and 300 μm high. 
     As illustrated in FIG. 5, the measured ink jet speeds were high and stable when the sectional area of the nozzle 32 on the ink jetting. side was at least four times the maximum projected area of solid ink particles and preferably at least 16 times the maximum projected area of solid ink particles. The measured jet speeds of ink droplets were seen deteriorating appreciably when the sectional area of the nozzle 32 on the ink jetting side was less than four times the maximum projected area of solid ink particles. The deterioration is attributable to the friction that occurs when solid ink particles come into contact with the nozzle 32 while ink droplets are being jetted out. 
     In the above setup, measurements were also taken while the center position of each nozzle 32 was moved within the allowable range designated above. The results obtained were the same. Furthermore, measurements were taken while the diameter of each nozzle 32 was varied on the ink jetting side. The results acquired were also the same. 
     In the setups used for the measurement above, the minimum sectional area of the nozzle 32 happened to be the sectional area thereof on the ink jetting side. However, the trend obtained was the same as that in FIG. 5 regardless of where the minimum sectional area was located throughout the nozzle 32. 
     As described, when the minimum sectional area of each nozzle 32 is at least four times the maximum projected area of solid ink particles, the measured jet speeds of ink droplets remain stable. This makes it possible for a single ink jet printer head 1 to jet out a plurality of kinds of ink containing different sizes of solid ink particles. From another point of view, nozzles 32 of the same shape may be used on a plurality of ink jet printer heads 1 addressing a plurality of kinds of ink for color printing. This feature contributes to boosting the level of productivity in manufacturing ink jet printer heads. With the minimum sectional area of each nozzle 32 least four times the maximum projected area of solid ink particles, the jet speeds of ink droplets are kept stable and high. This affords enhanced degrees of freedom to the designing of nozzles 32. Even if manufacturing error causes some deviations in the shape of each nozzle 32, the speeds of ink droplets jetted from each nozzle 32 remain stable as long as the minimum sectional area of each nozzle 32 is at least four times the maximum projected area of solid ink particles. This kind of arrangement reduces the number of defective ink jet printer heads in the manufacturing phase. Thus, the ink jet apparatus according to the invention offers enhanced print quality and is highly conducive to mass-production with no need for specialized adjustments of individual nozzles. 
     The jet speeds of ink droplets were then measured with the same arrangement and workings as those of the first example of the prior art ink jet apparatus. The results obtained yielded the same trends as those shown in FIGS. 2, 3, 4(B) and 5. The measurements were also taken using the same setup except that the width and height of each ink channel 12 were varied. The acquired results also provided the same trends as those in FIGS. 2, 3, 4(B) and 5. Furthermore, the results were found to be the same when the nozzles 32 were made rectangular or elliptical in cross section instead of round. 
     As described, the ink jet apparatus according to the invention keeps the jet speed of ink droplets high and stable in any one of the four cases. The first case is when the center position of each of the nozzles is allowed to deviate from the middle between one wall of the corresponding ink channel and the other wall opposite thereto by half the distance between the two walls minus half the diameter of the nozzle within the half distance. The second case is when the ratio of the sectional area of each of the nozzles on the ink jetting side to that on the ink channel side ranges from 1:2 to 1:50. The third case is when the minimum sectional area of each of the nozzles is at least four times the maximum projected area of solid ink particles. The fourth case is when the taper angle Θ at which the diameter of each of the nozzles is narrowed progressively from the ink channel side toward the ink jetting side ranges from five to 30 degrees. All ink jet apparatuses adopting any of the above arrangements provide uniform jet speeds of ink drops jetted out of their nozzles and hence ensure high levels of print quality. Because specialized adjustments of individual nozzles are not needed, the inventive ink jet apparatus is highly conducive to mass-production. 
     While preferred embodiments of the invention have been described using specific terms, such description is for illustrative purposes only. It is understood that changes and variations may be made without departing from the spirit or scope of the invention defined by the following claims.