Patent Publication Number: US-6984025-B2

Title: Ink jet head

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet head for performing record by discharging ink to a recording medium. 
     2. Description of Related Art 
     In recent years, an ink jet recording apparatus has been widely used especially as an output device of a computer because a high definition character and an image can easily be obtained by means of the ink jet recording apparatus. Inter alia, the bubble jet system for discharging ink from nozzles by means of a sudden pressure change produced by boiling the ink in the nozzle rapidly has become the main stream of the ink jet recording apparatus since many nozzles can easily be arranged in a high density in a simple configuration by the bubble jet system. 
     Moreover, as the ink jet recording apparatus has been widely spread in recent years, demands for the performances of the ink jet recording apparatus, especially for the image quality thereof and the recording speed thereof, have been increased. For the improvement of the image quality, it is important to reduce the diameters of dots recorded on a recording medium (especially on a sheet of recording paper). The demand is remarkable in case of the record of images represented by a photographic image in comparison with character documents. For example, the resolution necessary for obtaining the beauty of characters or for resolving small characters in the record of a character document is within a range from 600 dpi to 1200 dpi, and it is consequently enough for obtaining the resolution that the diameters of dots of liquid droplets to be discharged are within a range from about 80 μm to 90 μm (about 30 pl in case of being expressed by the volume). 
     On the other hand, in case of performing image record, the resolution, for example, for expressing smooth gradation equivalent to that of a film photo is required to be within a range from 1200 dpi to 2400 dpi. If the diameters of dots of liquid droplets to be discharged are 40 μm (about 4 pl in case of being expressed by the volume) in case of record with the resolution mentioned above, it is required to use two kinds of inks having the densities of dyes different from each other by the degree from about ¼ to ⅙ properly according to the densities of images. If the diameters of dots of liquid droplets to be discharged are made to be as small as about 20 μm (0.5 pl in case of being expressed by the volume), both of the requirements for density in a high density part and for smoothness in a low density part can be satisfied without any conflict by means of a kind of ink of a single density. As described above, it is essential for obtaining an image quality equivalent to a film photo to achieve the reduction in size of the liquid droplets to be discharged. 
     An ink jet head configured to discharge small liquid droplets is required to increase the number of times of discharging liquid droplets per a unit time. Consequently, the amount of current flowing a heat generating member increases, which in turn generates a large voltage drop at a parasitic resistance in a wiring section up to the heat generating member. Thus, the ink jet head has a problem of a decrease of its discharge efficiency. For preventing the decrease of the discharge efficiency, a method for decreasing current values by increasing the resistance value of the heat generating member is effective. It can be considered to increase the resistance value of the material of the heat generating member as means for increasing the resistance. However, there is a limit in increasing the resistance value by changing the material of the heat generating member. Besides, if a new material is used, a necessity to examine the new material fully whether there is some functional problem or not is generated. The change of the material of the heat generating member is difficult to realize. Accordingly, the increase of the resistance can be realized by dividing the heat generating member into a plurality of pieces to be connected in series and by arranging the pieces in an ink flow passage. 
     However, it was found that a new problem is produced as another problem in case of arranging the heat generating member after dividing it into a plurality of pieces. 
     Since the structure of an ink jet head is fine, as shown in  FIGS. 10A and 10B , there is a case where the center of a heat generating member  1102  provided on a substrate  1101  and the center of a discharge port  1104  provided on a flow passage forming member  1103  are shifted from each other owing to the dispersion generated in a manufacturing process. A reference numeral  1105  designates an ink flow passage, and a reference numeral  1106  designates an ink feed passage. 
     SUMMARY OF THE INVENTION 
     The shifting of relative positions of the heat generating member  1102  and the discharge port  1104  is not so serious problem in a conventional single heat generating member  1102 . However, if the relative positions of the heat generating member  1102  and the discharge port  1104  are shifted from each other in the case where the heat generating member  1102  is arranged by being divided into a plurality of pieces, it can be found that a minute liquid droplet is placed at a position separated from the position of the main liquid droplet, which mars the image definition, as shown in FIG.  11 . In particular, since the misdirection of a discharge direction seriously affects an image in case of a smaller ink liquid droplet in comparison with a conventional ink liquid droplet, it is further required to make it difficult to generate the misdirection of the discharge direction in comparison with in the case of the prior art. 
     The inventor of the present invention found out that the misdirection of the discharge direction was caused by the dispersion of the resistances and the shapes of heat generating members provided in the same flow passage and by the minute dispersion of the performances such as the thicknesses of the heat generating members in case of using the plurality of heat generating members, and that an ink jet head could adopt a structure in which the misdirection of the discharge direction was easily affected according to the position of the discharge port. Then, the inventor investigated a configuration for achieving a suitable layout of the discharge port to the heat generating members. 
     Accordingly, the present invention aims to provide an ink jet head capable of discharging ink liquid droplets from a discharge port efficiently without any discharge direction shifts even if the center position of the discharge port and the center position of a pressure generating area are somewhat shifted from each other. 
     For achieving the object mentioned above, an ink jet head of the present invention includes a substrate provided with heat generating members for generating a bubble in ink on a surface of the substrate, a plurality of discharge ports for discharging the ink, the ports opposed to the surface of the substrate, and a plurality of ink flow passages communicating with the plurality of discharge ports to feed the ink, the ink jet head discharging the ink from the discharge ports by a pressure generated by generating the bubble, wherein a plurality of the heat generating members is provided in each of the ink flow passages, and the discharge port is arranged on an extension line extending from a center of a pressure generating area composed of the plurality of heat generating members toward the surface of the substrate in a normal direction; and a distance dhc between centers of each of two heat generating members arranged most apart from each other among the plurality of heat generating members is set to be larger than a diameter do of an aperture of the discharge port. 
     According to the ink jet head of the present invention, even if the center position of the discharge port and the center position of the pressure generating area are somewhat shifted from each other, the influence of the distribution of foaming in the plurality of heat generating members, and the possibility of touches of the liquid columns of the ink discharged through the discharge port to the side walls of the discharge port is remarkably decreased. Consequently, the main liquid droplets of the ink are discharged from the discharge port without any shifts of the discharge directions. Moreover, if the liquid columns do not touch the side wall surfaces of the discharge walls of the discharge port, the parts where the main droplets are separated from the liquid columns are fixed. Consequently, it becomes possible to stable the sizes of the main liquid droplets, namely the sized of the dots formed by the main droplets placed on a sheet of recording paper, or the like. 
     Moreover, by adopting the configuration in which these plural heat generating members are connected to each other in series electrically with wiring, a resistance value higher than that of a single heat generating member having the same size as that of the plural heat generating members can be obtained, which makes it possible to reduce the necessary current value. Consequently, if the speed of discharge operation is intended to be high as discharged liquid droplets become smaller, it is possible to suppress the increase of current quantities flowing through the heat generating members. Moreover, it is possible to suppress heat generation and voltage drops owing to the resistance of a wiring section up to the heat generating members, and further to suppress induction noises generated by large currents flowing through the wiring section. 
     Moreover, by adopting the configuration in which, when a shift quantity of the center of the discharge port to the extension line is designated by derr, the distance dhc, the diameter do of the aperture, and the shift quantity derr satisfy a relation: dhc&gt;do+derr×2, it becomes possible to place minute liquid droplets generated at separation portions between main liquid droplets and liquid columns at impact positions of the main droplets. Furthermore, it also becomes possible to stable the impact positions of the main liquid droplets. Consequently, the shapes and positions of dots formed by the placed liquid droplets can be stabled. 
     Moreover, by adopting the configuration in which at least two heat generating members among the plurality of heat generating members provided in each of the ink flow passages are arranged with a certain interval dhh with respect to a direction between partition walls partitioning each of the ink flow passages; and the interval dhh between two heat generating members adjoining to each other most apart from each other with respect to the direction between the partition walls among the plurality of heat generating members is twice or less as long as an interval dhn between each of the partition walls and the heat generating members adjoining the each of the partition walls, it is prevented that bubble remaining in ink stay in an area between the two heat generating members. Consequently, the stability of discharging ink is further heightened. 
     Moreover, an ink jet head of the present invention includes a substrate provided with heat generating members for generating a bubble in ink on a surface of the substrate, a plurality of discharge ports for discharging the ink, the ports opposed to the surface of the substrate, a plurality of ink flow passages communicating with the plurality of discharge ports to feed the ink, and a flow passage forming member provided on the surface of the substrate, the ink jet head discharging the ink from the discharge ports by a pressure generated by generating the bubble, wherein a plurality of the heat generating members is provided in each of the ink flow passages, and the discharge port is arranged on an extension line extending from a center of a pressure generating area composed of the plurality of heat generating members toward the surface of the substrate in a normal direction; and center lines of each of two heat generating members with respect to an ink flow direction are located at an outside of the discharge port projected above the pressure generating area, the heat generating members arranged most apart from each other with respect to the direction between partition walls partitioning each of the ink flow passages, the direction orthogonal to the ink flow direction flowing in each of the ink flow passages toward the pressure generating area, among the plurality of heat generating members. 
     According to the ink jet head of the present invention, even if the center position of the discharge port and the center position of the pressure generating area are somewhat shifted from each other, the deviations of the flight directions of the liquid droplets, which deviations can be produced by a heat generating member on one side of the two heat generating members, and the deviations of the flight directions of the liquid droplets, which deviations can be produced by the other heat generating member on the other side of the two heat generating members, are produced in the directions opposite to each other. Consequently, the deviations of the flight directions of the liquid droplets, which deviations can be produced by a heat generating member on one side, are cancelled by the deviations of the flight directions of the liquid droplets, which deviations can be produced by the other heat generating member on the other side. Therefore, the deviations of the flight directions of the liquid droplets can be reduced, and the discharge directions of the liquid droplets can be stabled. 
     Moreover, the configuration in which the bubble are debubbled without communicating with outside air through the discharge port may be adopted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a transparent plan view showing an arrangement relationship of an ink flow path, heat generating members and a discharge port in an ink jet head of a first embodiment of the present invention; 
         FIGS. 2A and 2B  are views showing a case where the center position of the discharge port is shifted from the center position of two heat generating members in the ink jet head shown in  FIG. 1 ,  FIG. 2A  is a plan view thereof, and  FIG. 2B  is a sectional view thereof; 
         FIG. 3  is a view showing the shape of a dot formed by a liquid droplet discharged from the ink jet head shown in  FIG. 1 ; 
         FIGS. 4A and 4B  are views showing an arrangement relationship of an ink flow passage, heat generating members and a discharge port of an ink jet head of a second embodiment of the present invention,  FIG. 4A  is a plan view thereof, and  FIG. 4B  is a sectional view thereof; 
         FIG. 5  is a transparent plan view showing an arrangement relationship of an ink flow passage, heat generating members and a discharge port of an ink jet head of a third embodiment of the present invention; 
         FIGS. 6A and 6B  are views showing a case where the center position of the discharge port in the ink jet head shown in  FIG. 5  is shifted from a point of symmetry of two heat generating members,  FIG. 6A  is a plan view thereof, and  FIG. 6B  is a sectional view thereof; 
         FIGS. 7A ,  7 B and  7 C are views showing a substantial part of an ink jet head according to a fourth embodiment of the present invention typically,  FIG. 7A  is a plan view thereof,  FIG. 7B  is a view for the illustration of the arrangement of discharge port columns, and  FIG. 7C  is a sectional view thereof; 
         FIGS. 8A ,  8 B and  8 C are views showing an example of an ink jet recording cartridge provided with the ink jet head shown in  FIGS. 7A ,  7 B and  7 C; 
         FIG. 9  is a schematic diagram showing an example of a recording apparatus capable of mounting an ink jet head of the present invention; 
         FIGS. 10A and 10B  are views showing an arrangement relationship of an ink flow passage, heat generating members and a discharge port of a conventional ink jet head,  FIG. 10A  is a plan view thereof, and  FIG. 10B  is a sectional view thereof; 
         FIG. 11  is a view showing the shapes of dots formed by liquid droplets discharged from the conventional ink jet head; and 
         FIG. 12  is a view showing distribution of printing misdirections in the first embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, the preferred embodiments of the present invention will be described by reference to the attached drawings. 
     First Embodiment 
       FIG. 1  is a transparent plan view showing an arrangement relationship of an ink flow path, heat generating members and a discharge port in an ink jet head of a first embodiment of the present invention. 
     The ink jet head of the present embodiment includes a substrate  1  provided with many heat generating members  2  on the surface thereof, and a flow passage forming member  3  provided on the substrate  1 . The flow passage forming member  3  includes partition walls  3   a  for partitioning the many heat generating members  2  into twos, and a ceiling wall  3   b  opposed to the substrate  1 . The partition walls  3   a  form a plurality of ink flow passages  5  for feeding ink into pressure generating areas composed of the two heat generating members  2  partitioned by the partitioned walls  3   a . Moreover, in each ink flow passage  5 , a discharge port  4  is formed in the ceiling wall  3   b  on an extension line extending from the center of a pressure generation area, composed of two heat generating members  2 , in the normal direction to the surface of the pressure generation area. Each ink flow passage  5  commonly communicates with an ink feed passage  6 . The ink fed from ink feed means such as an ink tank (not shown) to the ink feed passage  6  is adapted to be fed into each ink flow passage  5  from the ink feed passage  6 . 
     As described above, in the present embodiment, one pressure generation area composed of two heat generating members  2  is arranged in one ink flow passage  5  equipped with one discharge port  4 . Moreover, a distance dhc between the centers of the two heat generating members  2  in each pressure generation area is set to be larger than a diameter do of the aperture of the discharge port  4 . Thereby, even if the center position of the discharge port  4  is shifted from the center position of the heat generating members  2  at the time of the production of a recording head as shown in  FIG. 2A , the influence of the dispersion of foaming in the plurality of heat generating members  2  becomes less, and a liquid column also does not touch side wall surfaces of the discharge port  4 . Consequently, a main liquid droplet is discharged from the discharge port  4  without any shifting in its discharge direction. 
     Moreover, since the parts of the liquid columns at which the main droplets are separated from the liquid columns are made to be fixed when the liquid columns do not touch the side wall faces of the discharge port  4 , it is possible to stabilize the sizes of the main droplets, i.e. the sizes of dots formed by the impact of the main droplets onto a sheet of recording paper or the like. 
     Moreover, in the configuration in which the discharge port  4  is arranged almost right above the center position of the pressure generation area composed of the two heat generating members  2  as in the present embodiment, the center of the discharge port  4  is shifted from the center position of each of the heat generating members  2  (namely, the center of the discharge port  4  is located at a position shifted from the positions almost right above the centers of respective heat generating members  2 ) as shown in  FIGS. 2A and 2B . Consequently, the centers of air bubble generated by respective heat generating members  2  are out of the center of the discharge port  4 . Therefore, the nearest part of liquid surface formed by the ink in the ink flow passage  5  to the interface with the outside air (i.e. the center part of the discharge port  4 ) becomes apart from the parts at which the bubble have most grown (i.e. the parts almost right above the centers of respective heat generating members  2 ). Consequently, the timing at which the bubble communicate with the outside air is delayed in comparison with the case where the center of the heat generating member  2  coincides with the center of the discharge port  4 . Therefore, it becomes easy to form a state in which the bubble communicates with the outside air in the ink flow passage  5  as disclosed in Japanese Patent Laid-Open Application NO. 11-188870. 
     If the state in which the bubble communicate with the outside air in the ink flow passage  5  can be formed, a liquid column which extends from a position between the two heat generating members  2  through the discharge port  4  can be formed as shown in FIG.  2 B. Thereby, the discharge directions of the main liquid droplets can be regulated within a predetermined range. Then, it becomes possible to make the discharge directions of the main droplets further stable. 
     An example of the present embodiment was designed as follows. That is, the diameter do of the aperture of the discharge port  4  was made to be 11 μm; the width of each heat generating member  2  was made to be 12 μm; the length thereof was made to be 27 μm; the arrangement interval dhh of the two heat generating members  2  from each other was made to be 3 μm; and the distance dhc between the centers of the two heat generating members  2  was made to be 14 μm. Moreover, the height of the ink flow passage  5  was made to be 13 μm; and the thickness (the width between the surface touching the substrate  1  and the surface at which the discharge port  4  was opened) of the flow passage forming member  3  was made to be 25 μm. 
     The ink jet head configured as above was arranged at a position where the surface on which the discharge ports  4  of the recording head were opened was distant from a sheet of recording paper (not shown) by 2 mm. While the ink jet head was scanned at the speed of 15 inches (about 38 cm)/second, current pulses of 0.9 μs were flown through the heat generating members  2 . Thereby, ink droplets were discharged onto the recording paper. The operation was performed by means of several ink jet heads having different quantities derr of the relative misregistration of the center positions of the discharge ports  4  from the center positions of pressure generating areas composed of the two heat generating members  2 . 
     The relation between quantities derr of the relative misregistration of the center positions of the discharge ports  4  from the center positions of the pressure generating areas composed of the two heat generating members  2  and the shapes of dots of ink liquid droplets placed on the recording paper was analyzed on the basis of the ink liquid droplets placed on the recording paper. The analysis taught that the shapes of the dots became good shapes of dots without any satellite dots caused by minute liquid droplets to be generated at separation parts between the main liquid droplets and the liquid columns, as shown in  FIG. 3 , and that there was almost no dispersion of discharge directions, if the quantities derr of the relative misregistration were within a range smaller than 2 μm inclusive. However, if the quantities derr of the relative misregistration exceeded 2 μm, the satellite dots gradually became more distant from the dots of the main liquid droplets and the dispersion of the positions of placed liquid droplets became larger, as the quantities derr of the relative misregistration became larger. 
     Consequently, it was known that it was preferable to set the distance dhc between the centers of the two heat generating members  2  larger than the distance equal to (the diameter do of the aperture of the discharge port  4 )+(the quantity derr of relative misregistration×2). 
     Moreover, if the area generating no heat that is formed between adjoining heat generating members  2  is too wide, a bubble remaining ink stay in the area, and the remaining bubble absorbs a discharge pressure to be generated at the time of foaming. For preventing the phenomenon, it is preferable to set the interval dhh of the two heat generating members  2 , where no heat is generated, twice or less as long as the intervals dhn between the ends of respective heat generating members  2  which adjoin the partition walls  3   a  and the partition walls  3   a . To put it concretely, if the intervals dhn are about 2 μm, it is preferable to set the interval dhh is equal to or less than 4 μm. 
     The influences to printing in the present embodiment at the time when the distance dhc between the centers of respective heat generating members  2  is changed without changing the diameter do of the aperture of the discharge port  4  are fixed are illustrated in FIG.  12 .  FIG. 12  shows distributions of printing misdirections. The ordinate axis of  FIGS. 2A and 2B  indicates the number of heads, and the abscissa axis of  FIGS. 2A and 2B  indicates the quantity of maximum misdirections. As apparent from the figure, it is known that nozzles having larger misdirections increase as the distance dhc becomes smaller owing to the influence of alignment shifting. 
     Moreover, a judgment of these heads by means of a prescribed pattern for examining misdirections, satellites and the like showed the results such that the efficiency percentages of printing are 99% at dhc=15, 95% at dhc=13, 90% at dhc=10.5, and 85% at dhc=9. 
     It is known that the present invention is very useful from these results also. 
     Moreover, the present embodiment has the configuration in which the two heat generating members  2  having an elongated shape as described above are connected in series electrically with wiring. Thereby, resistance values from three and a half times to six times as high as the resistance value of the conventional heat generating members  1102  having comparatively large area shown in  FIGS. 10A and 10B  can be obtained. Consequently, it becomes possible to make necessary current values about half of the conventional ones. Thereby, the increases of the quantities of currents flowing through the heat generating members  2  can be suppressed even if the increase of the speed of the discharge operation of the ink jet head is achieved as the discharge liquid droplets become smaller. Furthermore, it is possible to suppress the generation of heat and voltage drops owing to the resistance of wiring sections up to the heat generating members  2 , and induced noises generated by large currents flowing through the wiring sections. 
     Incidentally, proposals of arranging divided heat generating members were submitted in the past in response to the electric request of suppressing the increase of the quantity of currents in the case where the increase of the speed of the discharge operation of the ink jet head is achieved as the discharge liquid droplets become smaller, and from the point of view of preventing the heat generating members from getting a shock owing to cavitation breakdowns, which are generated at the time when boiled bubble is collapsed by negative pressures in their insides. However, the present embodiment examined the optimum arrangement relationship of the heat generating members  2  to the ink flow passage  5  and the discharge port  4  from the point of view of how the plural heat generating members  2 , namely a plurality of pressure generating sources, arranged in one ink flow passage  5  influence discharge performances. Such an example has not proposed in the past. 
     Second Embodiment 
       FIGS. 4A and 4B  are views showing an arrangement relationship of an ink flow passage, heat generating members and a discharge port of an ink jet head of a second embodiment of the present invention.  FIG. 4A  is a plan view thereof, and  FIG. 4B  is a sectional view thereof. 
     As shown in  FIG. 4A , especially, the ink jet head of the present embodiment is provided with a pressure generating area composed of four-in-a-set heat generating members  2  in one ink flow passage  5 . Supposing that the ink flow direction in the ink flow passage  5  is an X direction and a direction orthogonal to the X direction is a Y direction, these heat generating members  2  are arranged in the way in which two of them are arrange in the X direction and two of them are arranged in the Y direction. Moreover, these heat generating members  2  are connected in series electrically by wiring. A discharge port  4  is arranged on an extension line extending from the center of the pressure generating area composed of the four heat generating members  2  in the normal direction to the surface of the-pressure generating area. 
     Also in the present embodiment, as is the case with the first embodiment, the distance dhc between the centers of the adjoining heat generating members  2  is set to be larger than the distance equal to (the diameter do of the aperture of the discharge port  4 )+(the quantity derr of relative misregistration×2), and the interval dhh of the heat generating members  2  is set to be twice or less as long as the intervals dhn between the ends of respective heat generating members  2  which adjoin the partition walls  3   a  and the partition walls  3   a.    
     According to the configuration of the present embodiment, liquid columns do not touch the side wall surfaces of the discharge port  4  even if the center position of the discharge port  4  to the center position of the pressure generating area is shifted not only in the Y direction, but also in the X direction. Consequently, main liquid droplets are discharged from the discharge port  4  without producing any shifts in their discharge directions. Furthermore, the sizes of the main droplets, i.e. the sizes of the dots formed by placed main droplets on a sheet of recording paper or the like, can be stabled. 
     As described above, the first embodiment adopts the configuration for producing its effect in the case where the center position of the discharge port  4  to the center position of the pressure generating area composed of the two heat generating members  2  is shifted in the Y direction. On the other hand, the present embodiment is configured to produce an effect in the case where the center position of the discharge port  4  to the center position of the pressure generating area is shifted not only in the Y direction, but also in the X direction. Consequently, the present embodiment can perform the discharge of liquid droplets further stably. 
     Incidentally, the ink jet head of the present invention can be applied not only to the case where two or four heat generating members  2  are provided in one ink flow passage  5  like the first and the second embodiments, but also to all of the cases where a plurality of (two or more) heat generating members  2  are provided in one ink flow passage  5 . 
     In the latter case, the distance dhc is defied as “a distance between the centers of the heat generating members arranged at the most distant positions from each other among a plurality of heat generating members”, and the interval dhh is defined as “an interval between two heat generating members adjoining to each other with the most distant space with regard to a direction between the partition walls partitioning the ink flow passage”. 
     Third Embodiment 
       FIG. 5  is a transparent plan view showing an arrangement relationship of an ink flow passage, heat generating members and a discharge port of an ink jet head of a third embodiment of the present invention. 
     As in the case with the first embodiment, the third embodiment is provided with two heat generating members  2  which have a slender shape and are arranged in one ink flow passage  5 . The other configurations of the recording head are also the same as those of the first embodiment. 
     In the present embodiment, the width of each heat generating member  2  was set to be 11 μm; the length thereof was set to be 27 μm; the interval dhh of the two heat generating members  2  was set to be 4 μm; and the distance dhc between the centers of the two heat generating members  2  was set to be 15 μm. Moreover, the diameter do of the aperture of the discharge port  4  was set to be 10.5 μm, and the height OH of the aperture plane of the discharge port  4  from the top surface of a substrate  1  was set to be 40 μm. 
     In the configuration in which the aperture plane of the discharge port  4  and the surface of the substrate  1  are comparatively distant from each other as mentioned above, a bubble boiled on the heat generating members  2  is again coagulated to be liquefied without communicating with the outside air. Consequently, according to the configuration, the ends of liquid droplets do not adhere to the wall surfaces of the discharge port  4  to the contrary in the case of the configuration in which a bubble boiled on the heat generating members  2  communicate with the outside air. Consequently, it becomes difficult to produce flights of minute liquid droplets constructed at the end parts into different directions from those of the main liquid droplets. 
     However, as shown in  FIGS. 6A and 6B , if the center position of the discharge port  4  is shifted from the center position of the pressure generating area composed of the two heat generating members  2 , the discharge directions of liquid droplets are easily influenced by a bubble generated by a heat generating member  2  on one side, which causes deviations in flight directions.  FIGS. 6A and 6B  are views showing a case where the center position of the discharge port  4  in the ink jet head shown in  FIG. 5  is shifted from a point of symmetry of the two heat generating members  2 .  FIG. 6A  is a plan view thereof, and  FIG. 6B  is a sectional view thereof. 
     The phenomenon in which the flight directions of liquid droplets are deviated by the shift of the center position of the discharge port  4  from the center position of the pressure generating area composed of the two heat generating members  2  as described above is especially easy to happen in case of discharging relatively small droplets, for example, equal to 5 pl or less owing to the following two primary factors. 
     As a first primary factor, it is cited that making the discharge port  4  smaller, which is necessary for discharging smaller liquid droplets, increases the fluid resistance of a pipe section including the discharge portion  4 , which in turn makes the discharge speed low to make the discharge operation of liquid droplets unstable. As means for preventing this phenomenon, it is also considerable to shorten the distance OH of the aperture plane of the discharge port  4  from the substrate  1  to decrease the resistance of the flow passage in the pipe section. However, the means lowers the commutation operation of ink which is an operation of the pipe section including the discharge portion  4 , and makes the liquid droplets discharged from the discharge port  4  be easily influenced by the bubble caused by the heat generating member  2  on one side. Consequently, the means makes the deviations produced in the flight directions of the liquid droplets larger on the contrary. 
     As a second principal factor, it can be cited that the movement of ink in the vicinity of the heat generating members  2  after the boiling of the ink easily produces differences according to positions to the heat generating members  2 , since the sizes of the heat generating members  2  preferable to discharge small liquid droplets is smaller than those of the heat generating members  2  preferable to discharge large liquid droplets, and since division of a heat generating member having a certain size into a plurality of pieces makes the size of each of the divided pieces further smaller. If the heat generating members  2  are relatively large, a little differences of the positions of ink to the heat generating members  2  do not influence the movement of the ink in the vicinity of the heat generating members  2 . However, the influences of the differences of the positions to the heat generating members  2  gradually become relatively larger as the sizes of the heat generating members  2  become smaller. Consequently, if the size of a heat generating member  2  becomes smaller, the discharge operation of liquid droplets becomes easy to be unequal. 
     The inkjet head of the present embodiment shown in  FIG. 5  was devised with attention to such matters. The distance dhc of the centers of the two heat generating members  2  is set so that the respective center lines of the two heat generating members  2  related to the X directions being the flow directions of the ink are located at positions outside of the discharge port  4  projected on the pressure generating area composed of the two generating members  2 , with putting the discharge port  4  between the center lines. Since, in this configuration, the deviations of the flight directions of liquid droplets to be generated by one side heat generating member  2  and the deviations of the flight directions of the liquid droplets to be generated by the other side heat generating member  2  are generated in directions opposite to each other, the deviations of the flight directions of the liquid droplets to be generated by one side heat generating member  2  are cancelled by the deviations of the flight directions of the liquid droplets to be generated by the other side heat generating member  2 . Consequently, the deviations of the flight directions of liquid droplets can be reduced, and it becomes possible to stable the discharge directions of the liquid droplets. 
     Incidentally, the operation of canceling the deviations of the flight directions of the liquid droplets can be obtained as long as the respective center lines of the two heat generating members  2  are located at the positions outside of the discharge port  4  projected on the two heat generating members  2  with putting the discharge port  4  between the center lines, even if the center position of the discharge port  4  is shifted from the center position of the pressure generating area composed of the two heat generating members  2 . 
     Fourth Embodiment 
       FIGS. 7A ,  7 B and  7 C are views showing a substantial part of an ink jet head according to a fourth embodiment of the present invention typically,  FIG. 7A  is a plan view thereof,  FIG. 7B  is a view for the illustration of the arrangement of discharge port columns, and  FIG. 7C  is a sectional view thereof. 
     As shown in  FIG. 7C , a recording head  300  of the present embodiment is provided with a substrate  17  including heat generating resistance devices  15   a  and  15   b  as energy conversion devices, and an orifice plate  16  including discharge ports  31  and ink flow passages  30  for feeding ink to the discharge ports  31 . 
     The substrate  17  is formed with a single crystal of silicon having a plane direction ( 100 ). On the top surface of the substrate  1  (connection surface with the orifice plate  16 ) are formed by means of a semiconductor process the heat generating resistance devices  15   a  and  15   b , driving circuits  33  composed of driving transistors and the like for driving these heat generating resistance devices  15   a  and  15   b , contact pads  19  connected with a wiring board, which will be described later, wiring  18  connecting the driving circuits  33  with the contact pads  19 , and the like. Moreover, the substrate  17  is therein provided with five through-holes formed by anisotropic etching in areas other than the areas in which the above-mentioned driving circuits  33 , the heat generating resistance devices  15   a  and  15   b , the wiring  18  and the contact pads  19 . These through-holes form ink feed ports  32  for feeding liquid to discharge port columns  21   a ,  21   b ,  22   a ,  22   b ,  23   a ,  23   b ,  24   a ,  24   b ,  25   a  and  25   b , which will be described later. Incidentally,  FIG. 7A  typically shows a state in which the substantially transparent orifice plate  16  is put on the substrate  17 , and the drawing of the above-mentioned ink feed ports  32  is omitted. 
     The discharge port columns  21   a ,  21   b ,  22   a ,  22   b ,  23   a ,  23   b ,  24   a ,  24   b ,  25   a  and  25   b  are coupled by the twos communicating with the same ink feed ports  32  to constitute five coupled discharge port columns  21 ,  22 ,  23 ,  24  and  25 . Among the coupled discharge port columns  21 ,  22 ,  23 ,  24  and  25 , an ink having a cyan (C) color is fed to the coupled discharge port columns  21  and  25 , an ink having a magenta (M) color is fed to the coupled discharge port columns  22  and  24 , and an ink having a yellow (Y) color is fed to the coupled discharge port column  23 . Moreover, in each coupled discharge port columns  21 ,  22 ,  23 ,  24  and  25 , the adjoining discharge port columns are shifted from each other by ta in the arrangement directions as shown in, for example,  FIG. 7B  with regard to the coupled discharge port column  23 . 
     The orifice plate  16  provided on the substrate  17  is formed with photosensitive epoxy resin. In the orifice plate  16 , the discharge ports  31  and the liquid flow passages  30  are formed correspondingly to the above-mentioned heat generating resistance devices  15   a  and  15   b  by, for example, the process described in Japanese Patent Laid-Open Application No. 62-264957. Hereupon, it is desirable for producing a cheap and precise recording head to produce the recording head in conformity with the process disclosed in Japanese Patent Laid-Open Application No. 9-11479. That is, first a silicon oxide film or silicon nitride film (not shown) is formed on the silicon substrate  17 ; then, the orifice plate  16  provided with the discharge ports  31  and the liquid flow passages  30  is formed on the film; and finally the silicon oxide film or the silicon nitride film at the parts where the ink feed ports  32  are formed is removed by the anisotropic etching. 
       FIGS. 8A ,  8 B and  8 C are vies showing an example of an ink jet recording cartridge equipped with the ink jet head shown in  FIGS. 7A ,  7 B and  7 C. 
     The recording head  300  provided with the substrate  17  and the orifice plate  16 , both described above, utilizes the pressure of the bubble produced by film boiling caused by the heat energy applied by the heat generating resistance devices  15   a  and  15   b  to discharge liquid such as ink from the discharge ports  31  for performing recording. As shown in  FIG. 8A , the recording head  300  is fixed on an ink flow passage forming member  12  for feeding ink to the ink feed ports  32 . Then, the contact pads  19  are connected with a wiring board  13 , and thereby the recording head  300  can receive drive signals and the like from a recording apparatus, which will be described later, when an electric connection portion  11  formed on the wiring board  13  is connected with an electric connection portion of the recording apparatus. 
     On the ink flow passage forming member  12 , a recording head  400  provided with discharge portion columns  40  and  41  for discharging black ink (Bk) is fixed besides the recording head  300  capable of discharging each ink of Y, M and C. A recording head cartridge  100  capable of discharging four color ink is formed by combining the recording heads  300  and  400 . 
       FIGS. 8B and 8C  are perspective views showing an example of the recording head cartridge  100  equipped with the recording head  300 . As shown in  FIG. 8C , the recording head cartridge  100  is provided with a tank holder  150  for holding ink tanks  200 Y,  200 M,  200 C and  200 Bk for feeding inks to the ink flow passage forming member  12 . 
     Referring to  FIGS. 7A ,  7 B and  7 C again, the recording head  300  of the present embodiment includes the one substrate  17  provided with  10  discharge port columns  21   a ,  21   b ,  22   a ,  22   b ,  23   a ,  23   b ,  24   a ,  24   b ,  25   a  and  25   b  and five slit-like ink feed ports  32 , and each discharge portion column of each coupled discharge column is arranged in a line on both sides along the longitudinal direction of the ink feed portions  32 . 
     The ink introduced into each of the ink feed ports  32  from each of the ink tanks  200 Y,  2000 M,  200 C and  200 Bk through the ink flow passage forming member  12  is fed to the obverse side of the substrate  17  from the reverse side thereof, and then is introduced to the discharge ports  31  through the ink flow passages  30  formed on the obverse side of the substrate  17 . The introduced ink is then discharged from the discharge ports  31  by the pressures of the bubble produced by being heated and boiled by the heat generating resistance devices  15   a  and  15   b  provided in the vicinity of each of the discharge ports  31  on the obverse of the substrate  17 . 
     As described above, inks of cyan (C), magenta (M), yellow (Y), magenta (M) and cyan (C) are fed to each of the ink feed ports  32  in order from the left side in FIG.  7 A. Consequently, it is four discharge columns  21   a ,  21   b ,  25   a  and  25   b  that discharge the cyan ink; it is four discharge columns  22   a ,  22   b ,  24   a  and  24   b  that discharge the magenta ink; and it is two discharge columns  23   a  and  23   b  that discharge the yellow ink. When the recording head  300  is scanned into the left direction of an arrow in  FIG. 7A , record is performed by discharging ink from the coupled discharge port columns  21 ,  22  and  23 . When the recording head  300  is scanned into the right direction of the arrow in  FIG. 7A , record is performed by discharging ink from the coupled discharge port columns  25 ,  24  and  23 . By adopting the configuration in which each color ink is fed to each discharge port column in such a way, the order of the superposition of ink colors on a recording medium becomes the same in both of the times of movements of the recording head  300  into the outward direction and the return direction in both cases where record is performed while the recording head  300  is moved into any of both directions of the arrow directions in FIG.  7 A. Consequently, it becomes possible to record a high quality image at a high speed without any color shading. 
     In the recording head  300  of the present embodiment, the coupled discharge port columns  21  and  25  for discharging the cyan ink and the coupled discharge port columns  22  and  24  for discharging the magenta ink composed of two discharge port columns having discharge ports different in the sizes of the liquid droplets to be discharged therefrom. That is, the coupled discharge port columns  21  and  25  for discharging the cyan ink are composed of the discharge port columns  21   a  and  25   a  for discharging relatively large liquid droplets and the discharge port columns  21   b  and  25   b  for discharging relatively small liquid droplets. Moreover, the coupled discharge port columns  22  and  24  for discharging the magenta ink are composed of the discharge port columns  22   a  and  24   a  for discharging relatively large liquid droplets and the discharge port columns  22   b  and  24   b  for discharging relatively small liquid droplets. 
     Correspondingly to this, a relatively large heat generating resistance device  15   a  is provided in each of the discharge ports in the discharge port columns  21   a ,  22   a ,  23   a  and  24   a  for discharging relatively large liquid droplets, and a relatively small heat generating resistance device  15   b  is provided in each of the discharge ports in the discharge port columns  21   b ,  22   b ,  23   b  and  24   b  for discharging relatively small liquid droplets. 
     According to the configuration described above, it becomes possible to perform high quality recording while keeping the high speed of recording operation by using the discharge ports to be used for recording properly like by the way in which the parts of an image to be recorded where highly precise recording is required are recorded by the use of the discharge ports  31   b  for discharging relatively small liquid droplets and the other parts are recorded by the use of the discharge ports  31   a  for discharging relatively large liquid droplets. For achieving the high image quality and the high speed at the best balance, it is preferable to set the ratios of the quantities (largeness) of the liquid droplets to be discharged from each discharge port in the discharge port columns  21   a ,  22   a ,  24   a  and  25   a  for discharging relatively large liquid droplets to the quantities (largeness) of the liquid droplets to be discharged from each discharge port in the discharge port columns  21   b ,  22   b ,  24   b  and  25   b  for discharging relatively small liquid droplets to be 2:1 or more. 
     Moreover, the coupled discharge port column  23  for discharging the yellow ink is composed of two discharge port columns  23   a  for discharging relatively large liquid droplets, and relatively large heat generating resistance devices  15   a , which are the same ones used in the discharge port columns  21   a ,  22   a ,  24   a  and  25   a , are provided in each discharge port in each of the discharge port train  23   a.    
     In the present embodiment, each discharge port  31   a  of the discharge port columns  21   a ,  22   a ,  23   a ,  24   a  and  25   a  for discharging relatively large liquid droplets is formed to be an ellipse sized to be 19.5 μm in the diameter in each ink flow direction in each of the ink flow passages  30  and to be 12 μm in the diameter in the direction orthogonal to the above-mentioned direction, and each discharge port  31   b  of the discharge port columns  21   b ,  22   b ,  23   b ,  24   b  and  25   b  for discharging relatively small liquid droplets is formed to be a circle having the diameter of 11 μm. In each of the ink flow passages  30  provided with discharge ports  31   a  for discharging relatively large liquid droplets, two heat generating resistance devices  15   a  having the width of 12 μm and the length of 28 μm are arranged with the interval of 4 μm from each other while the distance between the centers of them is set to be 16 μm. On the other hand, in each of the ink flow passages  30  provided with discharge ports  31   b  for discharging relatively small liquid droplets, two heat generating resistance devices  15   b  having the width of 12 μm and the length of 27 μm are arranged with the interval of 3 μm from each other while the distance between the centers of them is set to be 15 μm. Incidentally, the thickness of the flow passage forming member (orifice plate  16 ) is 25 μm, and the heights of the flow passages (the height from the surface of the substrate  17  to the aperture plane of the discharge ports  31   a  and  31   b ) are formed to be 13 μm commonly to both discharge ports  31   a  and  31   b.    
     The recording head  300  configured in the way described above stably discharge the liquid droplets of about 5 pl from the discharge ports  31   a  for discharging relatively large liquid droplets and the liquid droplets of about 2.5 pl from the discharge ports  31   b  respectively. Consequently, high quality images can be obtained owing to the superior impact precision and the dot shapes of the recording head  300 . 
     Incidentally, although the optimum configuration is described in the present embodiment, it is possible to change the kinds of inks to be fed from each ink feed port  32 , the ink feed ports  32  and the number of the discharge port columns suitably without being limited to the configuration described above. 
     Other Embodiments 
     Finally, a recording apparatus capable of mounting the ink jet heads or the recording head cartridges, both described in each embodiment described above, will be described by reference to FIG.  9 .  FIG. 9  is a schematic diagram showing an example of a recording apparatus capable of mounting an ink jet head of the present invention. 
     As shown in  FIG. 9 , the recording head cartridge  100  is exchangeably mounted in a carriage  102 . The recording head cartridge  100  is provided with a recording head unit and ink tanks. The recording head cartridge  100  is also provided with a connector (not shown) for transferring signals such as one for driving a head section and the like. 
     The recording head cartridge  100  is exchangeably mounted on the carriage  102  at a fixed position. The carriage  102  is provided with an electric connection section for transmitting driving signals and the like to each head section. 
     The carriage  102  is supported by guide shafts  103 , which is installed in the main body of the apparatus to extend in the main scanning direction (the arrow direction in the figure), in a manner capable of performing reciprocating movements while being guided by the guide shafts  103  along them. The carriage  102  is driven by a main scanning motor  104  through driving mechanisms such as a motor pulley  105 , a driven pulley  106 , a timing belt  107  and the like. The positions and the movements of the carriage  102  are also controlled by the components mentioned above. Moreover, a home position sensor  130  is provided on the carriage  102 . Thereby, by detecting that the home position sensor  130  on the carriage  102  has passed through the position of a shielding board  136 , it can be known that the carriage  102  has been located at the home position. 
     A recording medium  108  such as a sheet of record paper, a plastic thin board and the like is separated one by one from an automatic sheet feeder  132  to be fed by the driving of a paper feeding motor  135  to rotate pickup rollers  131  through gears. The recording medium  108  is conveyed (sub-scanning) through a position (print section) opposed to the surface of discharge ports of the head cartridge  100  by rotations of a conveyance roller  109 . The conveyance roller  109  is rotated by the driving force transmitted from an LF motor  134  through gears when the LF motor  134  is driven. At that time, the judgment whether the recording medium  108  has actually been fed or not, and the decision of the head position at the time of feeding are preformed at the point of time when the tip portion of the recording medium  108  in the conveyance direction has passed through a paper end sensor  133 . Moreover, the paper end sensor  133  is also used for detecting the position where the rear end of the recording medium  108  actually exists to calculate the present recording position finally on the basis of the position of the actual rear end. 
     Incidentally, the reverse side of the recording medium  108  is supported by a platen (not shown) for forming a flat print surface at the print portion. In this case, the recording head cartridge  100  mounted on the carriage  102  is held with the surface of the discharge ports projecting downward from the carriage  102  to be parallel to the recording medium  108 . 
     The recording head cartridge  100  is mounted on the carriage  102  with the arrangement direction of the discharge port columns crossing the scanning direction of the carriage  102 . Record on the recording medium  108  is performed by repeating the operation of performing record in the main scanning direction by scanning the recording head cartridge  100  while discharging ink from the discharge port columns and the operation of conveying the recording medium  108  in the sub-scanning direction by the record width of one scanning by means of the conveyance roller  109 . 
     As described above, the ink jet head of the present invention sets the distance dhc between the centers of each of two heat generating members arranged at positions farthest from each other among a plurality of heat generating members provided in each ink flow passage to be larger than the diameter do of the aperture of a discharge port. Consequently, even if the center position of the discharge port is somewhat shifted from the center position of a pressure generating area, liquid columns of ink to be discharged through the discharge port do not touch the side wall surfaces of the discharge port. Consequently, it is possible to discharge ink liquid droplets from the discharge port without any shifts of the discharge directions of the ink liquid droplets. Moreover, by adopting the configuration of connecting these plurality of heat generating members in series electrically with wiring, a resistance value higher than that of a one-body heat generating member having the same size of the plural heat generating members can be obtained, which makes it possible to reduce a necessary current value. Consequently, the discharge efficiency of the ink jet head can be heightened. 
     Moreover, in another ink jet head of the present invention, the center lines of respective two heat generating members with respect to an ink flow direction are located at the outside of a discharge port projected on a pressure generating area, which members are arranged at the most distant positions from each other with respect to the direction between partition walls partitioning each ink flow passage, which direction is orthogonal to the ink flow direction flowing in each ink flow passage toward the pressure generating area, among a plurality of heat generating members provided in each ink flow passage. Consequently, even if the center position of the discharge port and the center position of the pressure generating area are somewhat shifted from each other, the deviations of the flight directions of liquid droplets are reduced to make it possible to stable the discharge directions of the liquid droplets, since the deviations of the flight directions of the liquid droplets which deviations can be produced by a heat generating member on one side is cancelled by the deviations of the flight directions of the liquid droplets which deviations can be produced by the other heat generating member on the other side.