Patent Publication Number: US-6705691-B2

Title: Ink-jet printing method and ink-jet printer

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ink-jet printing method and ink-jet printer for printing an image on a printing medium by driving print elements of a printhead and ejecting ink in accordance with an image signal. 
     BACKGROUND OF THE INVENTION 
     Conventionally, as printers for printing images on printing media (to be referred to as printing sheets hereinafter) by selectively driving print elements in accordance with print signals input from external devices such as host computers, printers based on the wire dot scheme, thermal transfer scheme, ink-jet schemes, and the like are known. Of these printers, an ink-jet printer, which incorporates an ink-jet printhead to print images by discharging ink from orifices (nozzles) of the printhead, can print high-resolution images, and is inexpensive. Owing to these advantages, this printer has recently attracted a great deal of attention, and is increasingly used in various fields. There is increasing demand for an ink-jet printer for color printing or gray-scale printing, in which a plurality of printheads, each having a plurality of ink channels and print elements with discharge energy generating elements arrayed at a fine pitch, are arranged in a direction (main scanning direction) perpendicular to the array direction (sub-scanning direction) of the plurality of print elements, and an image is printed by scanning these printheads in the main scanning direction. 
     In the above printhead, heating resistors serving as discharge energy generating elements are arranged at positions corresponding to the respective nozzles, and heat energy is generated by flowing a current in heating resistors. A liquid is then discharged from the corresponding nozzles by using the heat energy, thereby printing an image. Since today&#39;s demands for high-density, high-speed printing are especially high, a plurality of lines are generally printed by one scanning operation of the printhead in the main scanning direction. Therefore, a printhead having many heating elements arranged at a high density is used. 
     When high-density, high-speed printing is performed, neighboring nozzles of the printhead are driven at very short time intervals. For this reason, ink discharged from a given nozzle tends to be influenced by a pressure wave produced by ink discharged from adjacent nozzles. Consequently, the amount, discharge speed, and the like of ink discharged from the respective nozzles become unstable, resulting in a deterioration in the quality of printed images. 
     In addition, if a printing sheet is checked by an electrostatic chuck method in conveying the printing sheet, ink droplets flying from the printhead are charged before they reach the printing sheet, as shown in FIG.  12 . As a consequence, ink droplets flying nearby repel each other and their flying directions interfere with each other. As a result, the landing position of each ink droplet on the printing sheet deviates from the correct position. This will degrade the quality of an image printed on the printing sheet, thus posing a serious problem. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above prior art, and has as its object to provide an ink-jet printing method and ink-jet printer which can print a high-quality image by eliminating the mutual influences of neighboring print elements, which is occurred in an apparatus for conveying a recording sheet by using an electrostatic chuck method. 
     It is another object of the present invention to provide an ink-jet printing method and ink-jet printer which can print a high-quality image by eliminating the influences of ink droplets discharged from neighboring print elements (nozzles). 
     It is still another object of the present invention to provide an ink-jet printing method and ink-jet printer which eliminate the influences of ink droplets discharged from neighboring print elements (nozzles) and increase the capacity of a power supply for driving a printhead. 
     It is still another object of the present invention to provide an ink-jet printing method and ink-jet printer in which the print elements of a printhead are formed into a plurality of groups, and the groups are time-divisionally driven, thereby eliminating, at the current driving timing, the influences of pressure waves generated by print elements which discharged ink at a driving timing preceding the current driving timing. 
     It is still another object of the present invention to provide an ink-jet printing method and ink-jet printer which can print a high-quality image by eliminating the influences of ink droplets discharged from neighboring print elements (nozzles) even when a printing medium is conveyed by the electrostatic chuck method. 
     In order to attain the above described objects, an ink-jet printer of the present invention prints an image on a printing medium by driving print elements of a printhead and ejecting ink in accordance with an image signal. The printer comprises: division means for dividing a timing of driving a plurality of print elements of the printhead in accordance with an image signal into a plurality of driving timings; selection means for selecting one of print element groups, of a plurality of print elements of the printhead, which are spaced apart from each other at predetermined intervals corresponding to the number of driving timings; driving means for energizing and driving the print element group selected by the selection means in accordance with the image signal at one of the plurality of driving timings; driving control means for causing the selection means to select a next print element group by shifting a position of the print element selected by the selection means by a predetermined amount, after driving is performed by said driving means, and causing the driving means to drive the print element group; and control means for causing the driving control means to repeatedly drive until a plurality of print elements of the printhead are selected by said selection means and driven at the plurality of driving timings. 
     An ink-jet recording apparatus of the present invention records an image on a recording medium by driving recording elements of a recording head and ejecting ink in accordance with an image signal. The apparatus comprises: 
     conveyance means for conveying the recording medium by an electrostatic chuck method; and selection means for selecting recording elements which are separately located from each other among a plurality of recording elements of the recording head, as a group, that are substantially simultaneously driven; wherein the selection means selects the recording elements which are separated, such that a deterioration in image quality due to a landing position offset by an electrostatic power from said conveyance means can be suppressed. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the descriptions, serve to explain the principle of the invention. 
     FIG. 1A is a perspective view showing a printhead unit according to an embodiment of the present invention, and FIG. 1B is an enlarged sectional perspective view of a printhead portion of the unit; 
     FIG. 2 is a circuit diagram of a driving circuit for the printhead according to this embodiment of the present invention; 
     FIG. 3 is a circuit diagram of a driving element according to this embodiment of the present invention; 
     FIG. 4 is a schematic view for explaining a driving sequence in a printhead unit according to the embodiment of the present invention; 
     FIG. 5 is a block diagram showing the arrangement of an ink-jet printer according to the embodiment of the present invention; 
     FIG. 6 is a flow chart showing control processing in the control unit of the ink-jet printer according to the embodiment of the present invention; 
     FIG. 7 is a schematic view for explaining a driving sequence in a printhead unit according to the embodiment of the present invention; 
     FIG. 8 is a flow chart showing control processing in the control unit of the ink-jet printer according to the embodiment of the present invention; 
     FIG. 9 is a schematic perspective view of an ink-jet printer according to the embodiment of the present invention; 
     FIG. 10 is a graph for explaining the offset amounts of dot positions in the ink-jet printer; 
     FIG. 11 is a graph for explaining how ink is sprayed by an ink-jet printer according to the embodiment of the present invention; 
     FIG. 12 is a schematic view for explaining how ink droplets are sprayed from a conventional printhead; 
     FIG. 13 is a schematic perspective view of an ink-jet printhead according to the embodiment of the present invention; 
     FIG. 14 is a sectional view schematically showing the ink discharging mechanism of the ink-jet printhead according to the embodiment of the present invention; 
     FIGS. 15A to  15 C are views for explaining the ink-jet printhead according to the embodiment of the present invention, in which FIG. 15A is a schematic plan view of the printhead, FIG. 15B is a sectional view taken along a line A—A in FIG. 15A, and FIG. 15C is a sectional view taken along a line B—B in FIG. 15A; 
     FIG. 16 is a circuit diagram showing the circuit arrangement of an ink-jet head board according to the embodiment; 
     FIG. 17 shows an equivalent circuit of the ink-jet print head for modifying the distances between neighboring nozzles that were driven simultaneously; and 
     FIGS. 18-21 show views for explaining a relationship between a surface potential of sheet and variances of ink-jetted positions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     [First Embodiment] 
     FIG. 1A is a perspective view of a full-line type printhead unit  2100  according to the first embodiment of the printhead. FIG. 1B is an enlarged sectional perspective view of a printhead portion of this embodiment. 
     Referring to FIGS. 1A and 1B, heat energy generating elements (heating resistors)  2009  are arranged on a print element board  2001 , and nozzles (ink orifices: print elements) and a ceiling plate  2005  forming an ink chamber  2008  are arranged on the heat energy generating elements  2009 . In addition, driving elements  2004  for driving the heat energy generating elements  2009  are mounted on the print element board  2001 . The driving elements  2004  supply electric energy to the heat energy generating elements  2009  via an interconnection pattern (not shown) formed on the print element board  2001 . The printhead having this arrangement is fixed on a base plate  2002 , together with a printed board  2003 . In this case, the printhead and printed board  2003  are electrically connected to each other via bonding wires  2006 . An electric connector  2007  for inputting external electrical signals is mounted on the printed board  2003 . Ink used for printing is supplied into the ink chamber  2008  via an ink tank and ink supply tube (not shown). In printing, driving signals corresponding to print signals input through the electric connector  2007  are sent to the driving elements  2004  via the bonding wires  2006 . As a consequence, the heat energy generating elements  2009  are driven by electrical pulse signals output from the driving elements  2004 . Bubbles are then formed in the ink in the nozzles  2010 , and ink droplets are discharged from ink  2010 . 
     FIG. 2 is a view showing the circuit wiring of the printhead unit  2100  according to this embodiment. 
     In this embodiment, 28 driving elements  2004  (IC 1  to IC 28 ) are used, and 256 heat energy generating elements  2009  are driven by one driving element  2004 . These 28 driving elements  2004  are grouped into a total of seven blocks each consisting of four driving elements (ICi to ICi+3). Print data signals (SI 1  to SI 7 ), a data signal transfer clock (CK), a latch signal (LT), and signals EA, EB, EC, and EG (to be described later) are input to each block. Signals (SEL 1  to SEL 7 ) for chip-enabling the driving elements  2004  belonging to the respective blocks are respectively input to the blocks. Signals (ENB 1  to ENB 28 ) for determining the pulse widths of electrical pulses for driving the heat energy generating elements  2009 , signals D 1 -A 1  to D 1 -A 28  and D 1 -C 1  to D 1 -C 28 , and power supply lines VDD, L-GND, and P-GND are input to each driving element  2004  via the corresponding interconnections (not shown). 
     FIG. 3 is a block diagram showing the arrangement of each driving element  2004  in this embodiment. 
     A data signal (SI) is sequentially transferred and stored in a 256-bit shift register  301  in synchronism with a data transfer clock (SCKI: CK in FIG.  2 ). The 256-bit data stored in the shift register  301  is sent to a 256-bit latch register  302  and stored therein in accordance with a latch signal (LT*: “*” indicating a negative-logic signal). All signals EA*, EB*, EC*, and EG* are negative-logic (low true) signals, which are input to a 3-8 decoder  303  to perform distributed driving eight times. The signals stored in the latch register  302  are selectively output to a driver  304  in units of eight blocks. Each signal selected in this manner drives a transistor corresponding to the heat energy generating element in accordance with a signal ENB (ENBI) for determining the width of a pulse for driving the heat energy generating element  2009 , thereby driving the heat energy generating element  2009 . Note that each of the signals EA*, EB*, and EC* is a 1-bit signal. These signals determine which one of outputs (terminals  1  to  8 ) from the decoder  303  are to be set at high level. The signal EG* is a signal for enabling an output from the decoder  303 . 
     In this embodiment, 7,168 nozzles are arranged in one printhead unit  2100  at a density of 600 dpi (42.5-μm intervals), which are driven at a driving frequency of 4 kHz. The heat energy generating element  2009  is an electric resistor having a size of about 20 μm×80 μm and a resistance of about 55 Ω. When a voltage pulse of about 10 to 12 V (pulse width: about 3 μs) is supplied to this heat energy generating element  2009 , ink near the heat energy generating element  2009  is heated to form a bubble, thereby discharging ink from the nozzle. At this time, a current of about 200 mA instantaneously flows in the single heat energy generating element  2009 . The ink bubble formed by the heat energy generating element  2009  upon application of a pulse signal has a maximum volume about 12 μs after the application of the pulse signal to the heat energy generating element  2009 . Thereafter, the ink bubble starts shrinking, and disappears about 25 μs after the application of the pulse. 
     FIG. 4 is a view for explaining the ink discharge timing of the printhead unit  2100  of this embodiment. 
     In the first embodiment, all the nozzles (7,168) arranged on the printhead unit  2100  are formed into eight (=N) groups, and time-divisional driving is performed in units of groups by using the above signals EA*, EB*, and EC*. In printing, first of all, ink is discharged from the 1st, 9th, 17th, . . . , 7,162nd (a total of 896) nozzles belonging to the first group. At this time, an instantaneous current of about 200 mA flows in the single heat energy generating element  2009 . In this case, since a maximum of 896 heat energy generating elements  2009  are simultaneously turned on, the total instantaneous current is about 180 A at maximum. Ink is then discharged from the 5th, 13th, . . . , 7,165th nozzles belonging to the second group. Subsequently, ink is sequentially discharged from the nozzles belonging to the third, fourth, . . . , eighth groups in the same manner. In this case, the nozzles of the groups driven at successive timings are spaced apart from each other by N/2 dots (4 dots in this case) or {N/2)−1} dots (3 dots in this case). For example, the 5th nozzle belonging to the second group is spaced apart from each of the 1st and 9th nozzles belonging to the first group by (N/2=) 4 dots, and is spaced apart from each of the 2nd and 10th nozzles belonging to the third group, which is driven afterward, by {(N/2)−1 =} 3 dots. Setting the distance between the nozzles belonging to the groups driven at successive timings to N/2 bits or {(N/2)−1} will reduce the influences of the pressure waves of ink droplets discharged from given nozzles at a timing immediately before the current timing on ink droplets discharged from nozzles at the current timing. 
     In this embodiment, the time interval (to be referred to as a group delay time td) between successive ink discharge timings at which nozzle groups are driven is set to about 28 μs. To form an image with one pass, a driving period T of a head and a group count N must satisfy 
     
       
         td≦T/N  
       
     
     To reduce the influences of pressure waves generated by nozzles which have discharged ink at a timing immediately before the current timing and stabilize an ink discharge speed and ink discharge amount, the group delay time td must be longer than at least a time tmax (about 12 μs) between the instant at which an electric pulse is applied to the heat energy generating element  2009  and the instant at which a formed bubble reaches its maximum volume: 
     
       
         tmax&lt;td  
       
     
     In addition, the group delay time td is preferably longer than a time tb (about 25 μs) taken for the formed bubble to shrink. Therefore, we have 
     
       
         tb&lt;td  
       
     
     FIG. 5 is a block diagram showing the arrangement of an ink-jet printer having the full-line type printhead according to the first embodiment of the present invention. 
     Referring to FIG. 5, reference numeral  500  denotes a control unit including a CPU  510  such as a microprocessor, a program memory  511  storing control programs executed by the CPU  510 , a RAM  512  which is used as a work area when the CPU  510  executes processing and temporarily stores various data, and the like. Reference numeral  2100  denotes the printhead unit described above; and  501 , a motor driver for controlling the rotation of a sheet feed motor  502  on the basis of an instruction from the control unit  500 , thereby conveying a printing sheet used for printing. 
     FIG. 6 is a flow chart showing control processing in the ink-jet printer according to the first embodiment. A control program for executing this processing is stored in the program memory  511 . 
     In step S 1 , print data is input from an external device such as a host computer. After 1-line (7,168 pixels) data is created, the flow advances to step S 2  to send out the created image data to the shift register  301  of each driving element of the printhead unit  2100  in synchronism with the clock signal CK. When the 1-line print data is stored in each of the shift registers  301  of IC 1  to IC 28 , the flow advances to step S 3  to output a latch signal (LT*) to latch the print data in the latch register  302  of each driving element. The flow then advances to step S 3  to convey a printing sheet by rotating the sheet feed motor  502  and by using an electrostatic chuck method (to be described later). When the printing sheet reaches a print position, the flow advances to step S 4 . In step S 4 , all the selection signals SEL 1  to SEL 7  for selecting the first to seventh blocks are set at high level. In step S 5 , all the group selection signals EA*, EB*, and EC* are set at “1” (selecting the first group). The flow then advances to step S 6  to set heat signals (ENB 1  to ENB 28 ) at high level. With this operation, the heating resistors of the first group in FIG. 4 are driven to print by using ink discharged from the nozzles of the first group. 
     The flow then advances to step S 7  to check whether 1-line printing is complete. If NO in step S 7 , the flow advances to step S 8  to wait for a predetermined period of time (group delay time td). The flow then advances to step S 9  to update the group selection signals EA*, EB*, and EC* described above and select the second group (EA*=0, EB*=EC*=1). The flow advances to step S 6  to set heat signals (ENB 1  to ENB 28 ) at high level and print by using the next nozzle group in the same manner as described above. When groups are sequentially selected in steps S 7  to S 9  and printing by the eighth group (EA*, EB*, EC*=0) is complete, the flow advances to step S 10  to check whether 1-page printing operation is complete. If YES in step S 10 , this processing is terminated. If NO in step S 10 , the flow advances to step S 11  to convey the printing sheet by, for example, one dot corresponding to the resolution by rotating the sheet feed motor  502 . The flow then returns to step S 3 . In this case, reception of data from the host or the like, creation of printing data, transfer of the printing data to the shift register  301 , and the like are executed in the background during printing of a previous line. By outputting a latch signal in step S 3 , printing data of the next line is latched by the latch register  302 . 
     As described above, the nozzles of the printhead are formed into N groups and time-divisionally driven to reduce the influences of pressure waves generated by nozzles which have discharged ink at a preceding timing on ink discharge amount and ink discharge speed, thereby stably discharging ink. This makes it possible to improve the print quality. 
     A characteristic feature of this embodiment is that when time-divisional driving described above is performed, the intervals between nozzles that simultaneously discharge ink are so set as to prevent static electricity produced in conveying a printing sheet by the electrostatic chuck method from affecting a printed image. This embodiment will be described below. 
     [Second Embodiment] 
     In this embodiment, a printhead unit has nozzles arranged at a pitch of 42.5 μm, i.e., at a higher density than in the first embodiment. In this embodiment, as in the first embodiment, when a condition under which the ink droplet landing position offset amount became ½, i.e., 21.25 μm or less, the nozzle pitch of 42.5 μm or less was obtained, the obtained condition was that the distance between adjacent nozzles that were simultaneously turned on should be set to 300 μm or more. On the basis of this result, the number (N) of groups for divisional driving was set to 8 in a printhead having a nozzle resolution of 600 dpi (nozzle pitch p=42.5 μm) according to this embodiment. 
     In addition, according to this embodiment, in consideration of the time interval between the discharge timings of nozzles belonging to groups which are adjacent to each other in an ink discharge sequence, a group delay time td is set to be sufficiently longer to prevent the ink droplets discharged from the nozzles belonging to the groups adjacent to each other in the ink discharge sequence from mutually interfering with their flying directions due to an electrostatic field until they land on a printing sheet  1005 . 
     This operation will be described with reference to FIG.  11 . 
     FIG. 11 shows a state wherein ink droplets  3001 ,  3002 , and  3003  discharged from the printhead are flying before they land on the printing sheet  1005  in the second embodiment. A horizontal distance L between the ink droplet  3001  from a nozzle belonging to the first group and the ink droplet  3002  from a nozzle belonging to the second group can be expressed by 
     
       
           L=P·N/ 2  
       
     
     where N is the number of groups for divisional driving, and P is the nozzle pitch. 
     A vertical distance VH between them can be expressed by 
       V   1   =V·td    
     where V is the flying speed of ink, and td is the group delay time. 
     A linear distance L 1  between the ink droplet  3001  from the nozzle belonging to the first group and the ink droplet  3003  from the nozzle belonging to the second group is given by 
     
       
           L   1   ={V   2   ·td   2 +( N·P/ 2) 2 } 
       
     
     In an electrostatic field, a force F 1  that ink droplet  3001  receives from the ink droplet  3002  is proportional to the square of this linear distance L 1 , and hence can be given by 
     
       
           F   1   =α·L   1   2   =α{V   2   ·td   2 +( N·P/ 2) 2 } 
       
     
     where α is a constant. Of the force F 1 , only a horizontal component F 1   x  influences the landing position of the ink droplet  3001 . In this case, the component F 1   x  is given by              F1x   =         F1   ·   cos                   θ1     =       α   ·     (     L1   2     )     ·                (     NP   /   2     )          L1                   =       α   ·     (     NP   /   2     )            √     {         V   2     ·     td   2       +       (     N   ·     P   /   2       )     2       }                               
     Likewise, consider the force that the ink droplet  3002  from the nozzle belonging to the second group receives from the ink droplet  3003  from the nozzle belonging to the third group. The horizontal distance between the ink droplet  3002  and the ink droplet  3003  is given by either (N/2−1)·P or (N/2+1)·P. With regard to the respective expressions, horizontal components F 2   x  and F 3   x  that are received in an electrostatic field are given by 
     
       
           F   2   x=[αP·{ ( N/ 2)−1 }]{V   2   ·td   2 +{( N/ 2)−1} 2   ×P   2 ] 
       
     
     
       
           F   3   x=[αP·{ ( N/ 2)+1 }]{V   2   ·td   2 +{( N/ 2)+1} 2   ×P   2 ] 
       
     
     F 3   x  is the largest among F 1   x , F 2   x , and F 3   x.    
     The horizontal distance between ink droplets from nozzles belonging to the same group can be expressed by N·P, and a force F 0  that each ink droplet receives from another ink droplet while they fly is given by 
     
       
         
           F 
           0 
           =α·N 
           2 
           ·P 
           2  
         
       
     
     According to the above equalities, a condition for setting the above component F 3   x  to F 0  or less is given by 
     
       
         [ V   2   ·td   2 +{( N/ 2)+1) 2   ×P   2 ] 2   ×P   2 ]1≦2 N   2   ·P /( N+ 2)  
       
     
     When a driving method satisfying: 
     
       
           N·P&gt; 300  
       
     
     
       
         [ V   2   ·td   2 +{( N/ 2)+1)} 2   ×P   2 ]≦2 N   2   ·P /( N+ 2) Ps for  
       
     
     
       
         V=10 [m/S],  td= 28 [μs], N=8 , P  42.5×10 −6  [m] 
       
     
     was actually taken, ink landing position offsets due to an electrostatic field fell within 15 μm, and good print quality was obtained. 
     As described above, according to the third embodiment, an ink-jet printer is provided, which can minimize the landing position offset of each ink droplet due to an electrostatic field to realize excellent printing when the printhead described in the first and second embodiment is mounted in an ink-jet printer using the electrostatic chuck method. 
     [(Third Embodiment] 
     FIG. 7 is a view for explaining the third embodiment of the present invention. As in the first embodiment, in the third embodiment, the nozzles of a printhead unit  2100  are formed into eight groups to be time-divisionally driven, and it is determined the intervals between nozzles that are simultaneously driven in consideration with an effect of the electrostatic chuck method. The third embodiment differs from the first embodiment in that the nozzles belonging to each group of the printhead unit  2100  are further grouped into seven blocks, i.e., the first to seventh blocks, and the nozzles belonging to the same group are further time-divisionally driven. 
     As shown in FIG. 7, ink is discharged from the nozzles belonging to the first block of the first group, and then ink is discharged from the nozzles belonging to the second block of the first group with a delay of about 4 μs. Subsequently, the nozzles belonging to the third to seventh blocks of the first group are sequentially driven with a delay of 4 μs to discharge ink. Note that each group is selected by signals EA*, EB*, and EC* like those described above, and each block is selected by signals SEL 1  to SEL 7 . 
     When printing by the nozzles belonging to the first group is completed in this manner, ink is discharge from the nozzle belonging to the first block of the second group. By dividing the driving timing of the 896 nozzles belonging to the same group into seven timings, the number of heat energy generating elements  2009  simultaneously driven can be further decreased to 128. As a consequence, since a current of about 200 mA instantaneously flows in the signal heat energy generating element  2009 , the sum of currents that instantaneously flows in the elements can be reduced to about 25.6 A at maximum. 
     This processing is shown in the flow chart of FIG.  8 . Since the arrangement of the ink-jet printer of the third embodiment is the same as that of the first embodiment, a description thereof will be omitted. The same reference numerals as in the flow chart of FIG. 6 denote the same part in FIG. 8, and a description thereof will be omitted. 
     In the third embodiment, the first block is selected (SEL=1, SEL 2  to SEL 7 =0) in step S 4 - 1  after step S 3 . In step S 5 , the first group is selected by setting the signals EA*, EB*, and EC*=(1, 1, 1). In step S 6 , ENB 1  to ENB 28  are output to drive the heating resistors. In step S 6 - 1 , the flow waits for 4 μs. The flow then advances to step S 6 - 2  to check whether printing by all the blocks belonging to the first group is complete. If NO in step S 6 - 2 , the flow advances to step S 6 - 3  to output a selection signal SELi (I=1 to 7) for selecting the next block. When printing by the nozzles belonging to the first group is complete, the flow advances to step S 7  to check whether printing of one line (by the nozzles belonging to the first to eight groups) is complete. If NO in step S 7 , the flow advances to step S 8 . If YES in step S 7 , the flow advances to step S 10 . 
     As described above, according to the third embodiment, the nozzles belonging to the same group are further grouped into a plurality of blocks, and time-divisional driving is performed in units of bocks, thereby reducing the maximum current instantaneously flowing in the printhead. This makes it possible to reduce the load imposed on the head power supply, power supply capacitor, and the like and more stably discharge ink. 
     FIG. 9 is a view for explaining a color ink-jet printer  1200  designed to electrostatically convey a printing sheet according to the present invention. The color ink-jet printer  1200  of the this embodiment incorporates four printhead units  2100  identical to those described above. Each printhead unit  2100  in this embodiment has the same arrangement as that described above except that the nozzle pitch is set to 63.5 μm. Yellow, magenta, cyan, and black inks are respectively supplied to the four printhead units  2100 . These printer units print color images by using these four colors. A printing sheet  1005  stacked on a paper tray  1004  is conveyed by a sheet convey belt  1002 . When the printing sheet  1005  passes under the color printhead units  2100 , a color image is printed on this sheet by using inks discharged from the respective printhead units  2100 . The printing sheet  1005  on which the color image is printed in this manner is stacked on a paper discharge tray  1003 . 
     The sheet convey belt  1002  is looped around a sheet convey belt roller  1001 . Electrodes  1012  are arranged on this sheet convey belt  1002  to reliably convey the printing sheet  1005 . Feed portions  1013  are arranged at end portions of the electrodes  1012 . Charge supply brushes  1011  made of a conductive material and arranged on a charge supply unit  1010  for applying a high potential to the electrodes  1012  are in contact with the feed portions  1013 . By applying a high potential to the charge supply unit  1010 , the printing sheet  1005  is electrostatically chucked and conveyed. 
     In this case, the printhead unit  2100  described above is mounted in the color ink-jet printer  1200  designed to convey a sheet by such an electrostatic chuck method. 
     As described above, when printing is performed by the ink-jet scheme on the sheet convey system using this electrostatic chuck method, ink droplets flying nearby influence their flying directions owing to an electrostatic field, resulting in a deterioration in print quality. 
     Before the printhead unit  2100  of this embodiment was designed, the printhead unit  2100  having a driving circuit capable of independently driving heat energy generating elements  2009  disposed in the respective nozzles was formed first, as shown in FIG. 17, and the relationship between the distances between neighboring nozzles that were driven simultaneously, the voltage applied to the electrodes  1012 , and the offset amounts of printed dots was examined on experiment. In this examination, a printhead unit having 512 nozzles arranged at a pitch of 63.5 μm was used. FIG. 10 shows the examination result. 
     Referring to FIG. 10, the abscissa represents the distance between adjacent nozzles from which ink droplets are simultaneously discharged; and the ordinate, the ink landing position offset amount on a printing sheet. 
     As shown in FIG. 10, in the 2,000-V range, even with a change in potential applied to the sheet surface, if adjacent nozzles were spaced apart from each other by 300 μm or more, the ink position offset amount was 15 μm or less. In this case, the ink position offset was hardly recognized. 
     Images were actually printed under the same conditions as in the above experiment, and the resultant print quality was evaluated. FIG. 18 shows the result. A criterion for this image quality evaluation was set such that an image on which the occurrence of streaks due to ink droplet landing position offsets was not recognized was regarded as good “◯”, and an image on which streaks were produced was regarded as poor “X”. FIG. 19 shows the evaluation results, which are superimposed on plotted points under the same conditions as in FIG.  10 . Referring to FIG. 19, the print quality evaluation results “◯” and “X” are written on the upper right corners of the respective plotted points. Obviously from FIG. 19, image evaluations were “◯”, i.e., image quality was good in the range in which the print offset amount was ½, i.e., 31.75 μm or less the nozzle pitch of 63.5 μm or less. 
     In designing a printhead unit having a nozzle pitch of 70 μm on the basis of the above experiment results, the distance between adjacent nozzles that are turned on at the same time when the landing position offset amount became ½70 μm or less, i.e., 35 μm or less was obtained by experiment. The distance between nozzles was set to 140 to 420 μm, and the sheet surface potential was set to 0 to 3 kV. Under these conditions, a landing position offset was measured 10 times, and the measured values were averaged. 
     As shown in FIG. 20, it was found that when the distance between adjacent nozzles was 140 μm, the landing position offset amount was 35 μm or more at a sheet surface potential of 2 kV or more, whereas when the distance was 280 μm or more, the landing position offset amount could be suppressed to 35 μm or less at a sheet surface potential of 3 kV. In addition, when images were actually printed under the same conditions as described above, and the resultant image quality was evaluated, it was confirmed that good print quality could be obtained when the landing position offset amount was 35 μm or less. 
     On the basis of the above result, according to this embodiment, an ink-jet printer could be provided, which suppressed a deterioration in image quality due to landing position offsets by using a printhead unit in which the distance between adjacent nozzles that were simultaneously turned on was set to 280 μm. 
     Furthermore, in the printhead and block driving arrangement shown in FIGS. 1 to  4  described above as well, the distance between adjacent nozzles that were simultaneously driven was set to 340 μm to ensure good images even when the sheet surface potential was set to 2 kV, thereby obtaining good images without any streak irregularity. 
     [Fourth Embodiment] 
     FIGS. 13 to  15 C are views for explaining an ink-jet printhead according to the fourth embodiment of the present invention. FIG. 13 is a schematic perspective view of the ink-jet printhead according to the fourth embodiment. FIG. 14 is a sectional view schematically showing the ink discharging mechanism of the ink-jet printhead. FIG. 15A is a schematic plan view of the ink-jet printhead. FIG. 15B is a sectional view taken along a line A—A in FIG.  15 A. FIG. 15C is a sectional view taken along a line B—B in FIG.  15 A. 
     In the ink-jet printhead according to the fourth embodiment shown in FIG. 13, a plurality of orifices  202  for discharging ink are formed in that surface portion of a print element board  201  which is located near its middle portion. Printing is performed by using ink droplets discharged from these orifices  202 . 
     As shown in FIGS. 14 and 15A to  15 C, heaters  204  corresponding to the respective orifices  202  are formed on the print element board  201 . These heaters  204  are energized to generate heat to form ink bubbles. Ink as a printing liquid is discharged by the resultant kinetic energy. 
     Wires run from the heaters  204  to the mount portions of driving elements  205  on the print element board  201  and are electrically connected to the driving elements  205  mounted on the mount portions. The driving elements  205  are connected to the print element board  201  via an anisotropic conductive film by a COB (Chip On Board) method. In addition to transistor circuits, logic circuits for driving transistors are mounted on the driving elements  205 . A signal for driving the logic circuit is connected to a flexible film  206  via the print element board  201 . This flexible film  206  is connected to a circuit board  207  (FIG. 15A) made of a composite material such as glass epoxy. An electric connector  208  (FIG. 15B) for receiving external electrical signals is mounted on the circuit board  207 . 
     If the electric connection portions of the driving elements  205  and flexible film  206  are exposed, ink droplets scattered from the orifices  202 , ink bouncing off a sheet, and the like adhere to the electrodes. As a consequence, the electrodes and underlying metal corrode. To prevent this, the electric connection portions are coated with a silicon sealant (not shown) having excellent sealing properties and ion-blocking properties and are sealed. 
     A common liquid chamber  210  (not shown) for holding ink is formed on the lower surface of the print element board  201  by using a print element board holding member  211  and support member  212  so as to have a length almost equal to the length of an array of a plurality of orifices  202 . A slit  203  (FIG. 15C) for supplying ink from the lower surface side to the upper surface side is formed in the print element board  201 . This common liquid chamber  210  communicates with ink supply ports  215  and  216 . In ink discharging operation, ink is supplied from an ink tank (not shown) outside the ink-jet printhead via these two ink supply ports  215  and  216 . 
     In filling this ink-jet printhead with ink, the ink is flowed from the ink supply port (inlet)  215  with pressure, and the air in the common liquid chamber  210  is purged mainly through the ink supply port (outlet)  216 , thereby filling the common liquid chamber  210  with the ink without any bubbles. This operation is continued until the common liquid chamber  210  is completely filled with the ink. Meanwhile, ink containing air bubbles is discharged from the ink supply port (outlet)  216 . This ink is returned into an ink tank (not shown) located upstream the ink supply port (inlet)  215 , thus realizing an ink supply flow path arrangement designed to circulate ink. 
     FIG. 16 is a view showing the circuit arrangement of an ink-jet printhead board according to the fourth embodiment. FIG. 16 shows an example of a driving circuit using a driving IC in which each driving transistor does not have a one-to-one correspondence with a shift register and latch. 
     As shown in FIG. 16,  256  drivers are used in a driving transistor  1600  per IC, whereas a shift register  1601  and latch  1602  each have a 16-bit configuration. Image data (SI) are serially transferred to the shift register  1601 , and 16-bit data is transferred to the shift register  1601  and held therein. Thereafter, this 16-bit data is stored in the latch  1602 . Each output from the 16-bit latch is connected to a corresponding one of 16 signal lines, and ANDed with an output signal from a decoder  1603 , which is externally controlled/input, by an AND circuit  1604 . An AND circuit  1605  further ANDs an output signal from the AND circuit  1604  and an ENB signal (ENB 0 , ENB 1 ) for determining the width of a pulse for driving the transistor. The driver circuit  1600  is driven by an output signal from the AND circuit  1605 . 
     When image data is to be actually printed, first of all, the image data are sequentially input to the 16-bit shift register  1601 . When 16-bit image data are transferred, this image data is latched in the latch circuit  1602 . Signals BE 0 * to BE 3 * (* represents a negative-logic signal) are input to the decoder  1603  to set only the first output of the decoder  1603  at high level, while the remaining outputs are set low level (BE 0 * to BE 3 *=1). When the signal ENB is applied in this state, the 1st transistor element, 17th transistor element, 33rd transistor element, . . . are driven, and ink is discharged from the corresponding nozzles. 
     As in the above case, the signals BE 0 * to BE 3 * are set to (1110) to set only the ninth output of the decoder  1603  at high level, with the remaining outputs being set at low level. When the signal ENB is applied as in the above case, the 9th transistor element, 25th transistor element, 41st transistor element, . . . are driven, and ink is discharged from the corresponding nozzles. By sequentially switching the signals BE 0 * to BE 3 * input to the decoder  1603 , the corresponding nozzles are driven, for example, in the following sequence, thus discharging ink:                                          1      st     ,     17      th     ,     33      rd     ,   …                 9      th     ,     25      th     ,     41      st     ,   …                       2      nd     ,     18      th     ,     34      th     ,   …                       10      th     ,     26      th     ,     42      nd     ,   …                   ⋮                     16      th     ,     32      nd     ,     48      th                                
     By sequentially driving the nozzles in this manner, this embodiment can be applied to the present invention in the same manner as in the embodiments described above. 
     The present invention has exemplified a printer based a system, which comprises means (e.g., an electrothermal transducer or laser) for generating heat energy as energy utilized upon ink discharge, and causes a change in state of an ink by the heat energy, among the ink-jet printers. However, the same effects as those described above can also be obtained in an ink-jet print system based on a piezoelectric scheme like, for example, the one described in Japanese Patent Laid-Open No. 6-6357. According to this system, a high-density, high-definition print operation can be realized. 
     As for the typical structure and principle, it is preferable that the basic structure disclosed in, for example, U.S. Pat. No. 4,723,129 or 4,740,796 be employed. The above method can be adopted in both a so-called on-demand type apparatus and a continuous type apparatus. In particular, a satisfactory effect can be obtained when the on-demand type apparatus is employed because of the structure in which one or more drive signals, which rapidly raise the temperature of an electrothermal converter disposed to face a sheet or a fluid passage which holds the fluid (ink) to a level higher than levels at which film boiling takes place are applied to the electrothermal converter in accordance with print information so as to generate heat energy in the electrothermal converter and to cause the heat effecting surface of the printhead to take place film boiling so that bubbles can be formed in the fluid (ink) to correspond to the one or more drive signals. The growth/shrinkage of the bubble will cause the fluid (ink) to be discharged through a discharging opening so that one or more droplets are formed. If a pulse shape drive signal is employed, the bubble can be grown/shrunk immediately and properly, causing a further preferred effect to be obtained because the fluid (ink) can be discharged while revealing excellent responsibility. 
     It is preferable to use a pulse drive signal disclosed in U.S. Pat. No. 4,463,359 or 4,345,262. If conditions disclosed in U.S. Pat. No. 4,313,124 which is an invention relating to the temperature rise rate at the heat effecting surface are employed, a satisfactory print result can be obtained. 
     As an alternative to the structure (linear fluid passage or perpendicular fluid passage) of the printhead disclosed in each of the above inventions and having an arrangement that discharge ports, fluid passages and electrothermal converters are combined, a structure having an arrangement that the heat effecting surface is disposed in a bent region and disclosed in U.S. Pat. No. 4,558,333 or 4,459,600 may be employed. In addition, the following structures may be employed: a structure having an arrangement that a common slit is formed to serve as a discharge section of a plurality of electrothermal converters and disclosed in Japanese Patent Laid-Open No. 59-123670; and a structure disclosed in Japanese Patent Laid-Open No. 59-138461 in which an opening for absorbing pressure waves of heat energy is disposed to correspond to the discharge section. 
     As a full-line type printhead having a length corresponding to the maximum width of a recording medium on which printing can be performed by a printer, a printhead configured to satisfy the requirement for the length by a combination of a plurality of printheads as disclosed in the above specification or a printhead integrated as a single printhead may be used. 
     In addition, the invention is effective for a printhead of the freely exchangeable chip type which enables electrical connection to the printer main body or supply of ink from the main device by being mounted onto the apparatus main body, or a printhead of the cartridge type having an ink tank provided integrally on the printhead itself. 
     It is preferred to additionally employ the printhead restoring means and the auxiliary means provided as the component of the present invention because the effect of the present invention can be further stabilized. Specifically, it is preferable to employ a printhead capping means, a cleaning means, a pressurizing or suction means, an electrothermal converter, an another heating element or a pre-heating means constituted by combining them and a pre-ejection mode in which ejection is performed before actual printing ejection in order to stably print. 
     In addition, the printer of the present invention may be used in the form of a copying machine combined with a reader, and the like, or a facsimile apparatus having a transmission/reception function in addition to a printer integrally or separately mounted as an image output terminal of information processing equipment such as a computer. 
     The present invention can be applied to a system constituted by a plurality of devices (e.g., host computer, interface, reader, printer) or to an apparatus comprising a signal device (e.g., copying machine, facsimile machine). 
     The objects of the present invention are also achieved by supplying a storage medium, which records a program code of a software program that can realize the functions of the above-mentioned embodiments to the system or apparatus, and reading out and executing the program code stored in the storage medium by a computer (or a CPU or MPU) of the system or apparatus. 
     In this case, the program code itself read out from the storage medium realizes the functions of the above-mentioned embodiments, and the storage medium which stores the program code constitutes the present invention. 
     As the storage medium for supplying the program code, for example, a floppy disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, nonvolatile memory card, ROM, and the like may be used. 
     The functions of the above-mentioned embodiments may be realized not only by executing the readout program code by the computer but also by some or all of actual processing operations executed by an OS (operating system) running on the computer on the basis of an instruction of the program code. 
     Furthermore, the functions of the above-mentioned embodiments may be realized by some or all of actual processing operations executed by a CPU or the like arranged in a function extension board or a function extension unit, which is inserted in or connected to the computer, after the program code read out from the storage medium is written in a memory of the extension board or unit. 
     As has been described above, according to this embodiment, a high-quality image can be printed by eliminating the influences of ink droplets discharged from adjacent nozzles. 
     The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made.