Patent Publication Number: US-8985740-B2

Title: Inkjet printing apparatus and control method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet printing apparatus and a method for controlling an inkjet printing apparatus. 
     2. Description of the Related Art 
     In inkjet printing apparatuses, ink is sometimes deposited to the surface of a print head (hereinafter also referred to as an “orifice face”) having nozzles (ejection ports) formed therein and, therefore, normal ejection is interfered. In printing apparatuses that form an image using a plurality of types of ink that react to one another or in printing apparatuses that forms an image using reaction liquid and ink, the ink may be firmly deposited onto an orifice face, and it may be difficult to remove the deposited ink. In addition, in printing apparatuses that solidifies ink using an ultraviolet ray, a microwave, or heat in order to improve the fastness of the ink, the same issue arises. To address such an issue, a solvent inkjet printer or a UV-curable inkjet printer is sometimes used. In some cases, such a printer requires a maintenance operation performed by a user. 
     Examples of the maintenance method include (1) wiping away ink by sliding a wiper or a blade on an ejection surface and (2) absorbing ink by urging a porous sheet-like purge member having ink absorbency against an ejection surface. The sheet-like purge member is also referred to as a “web”. Hereinafter, the sheet-like purge member is simply referred to as a “sheet member”. The above method (2) is described in Japanese Patent Laid-Open No. 2003-300329. In the technology described in Japanese Patent Laid-Open No. 2003-300329, after the purge operation is performed, a predetermined amount of the sheet member is wound and collected. 
     According to the above-described method (2), accumulated ink and dust particles deposited on the orifice face can be removed. If a recording medium is paper, a paper fiber that generates an undesired ink line on the recording medium can be also removed. According to the above-described method (1), if a wiper is used, wet ink spreads after the ejection surface is wiped. In contrast, if a wiping mechanism is used, wiping of thickening ink generated by heat and evaporation may be difficult. The method (2) can address such issues. Accordingly, the ink ejection performance can be more consistently maintained or recovered. 
     If the orifice face of the print head is cleaned using the method (2), that is, by using a sheet member, the following issue arises: if the amount of winding is set to a small value, the entire portion of the sheet member used is not collected, in some cases. Accordingly, the portion of the sheet member used for cleaning the orifice face may be reused in the next cleaning operation and, thus, the cleaning effect may be decreased. In contrast, if the amount of winding is set to a large value, the portion of the sheet member used can be more reliably collected. However, it is likely to collect an unused portion. In addition, since the use amount of the sheet member increases, the use efficiency of the sheet member is decreased. 
     SUMMARY OF THE INVENTION 
     The present invention provides an inkjet printing apparatus capable of sufficiently purging ink deposited on the orifice face of the print head using a sheet member and limiting a portion of the sheet member used to an optimal amount. 
     According to an embodiment of the present invention, an inkjet printing apparatus includes a print head having at least one nozzle array disposed in an orifice face, where the nozzle array includes a plurality of nozzles that are arranged in a predetermined direction and that eject ink, a scanning unit configured to cause the print head to scan in a direction perpendicular to the predetermined direction, a purge unit disposed in the vicinity of one end portion of a scanning area of the scanning unit, where the purge unit includes a sheet member configured to purge ink deposited on the orifice face of the print head and a winding unit configured to wind the sheet member thereon, and a control unit configured to control, on the basis of the number of ink ejections from the print head executed between a first purging operation performed by the purge unit and a second purging operation subsequently performed after the first purging operation is performed, an amount of winding of the sheet member wound by the winding unit after the second purging operation is completed. 
     According to the embodiment, ink deposited on the orifice face of the print head can be sufficiently purged using the sheet member. In addition, the amount of the sheet member used for purging can be set to an optimum amount. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  are schematic illustrations of a whole printing apparatus and the internal mechanism of the printing apparatus according to an embodiment of the present invention. 
         FIG. 2  is a schematic illustration of a recovery unit. 
         FIGS. 3A and 3B  are schematic illustrations of a purge unit. 
         FIG. 4  is an overall control diagram of the printing apparatus. 
         FIG. 5  illustrates the procedure of a recovery sequence. 
         FIG. 6  is a flowchart of the procedure of a print sequence. 
         FIGS. 7A to 7F  are schematic illustrations of the operation performed by the purge unit. 
         FIGS. 8A and 8B  are schematic illustrations of a purge operation sequence according to a first exemplary embodiment of the present invention. 
         FIGS. 9A to 9C  illustrate wiping traces appearing on a sheet member in Example 1 and Comparative examples 1 and 2, respectively. 
         FIGS. 10A and 10B  are schematic illustrations of a purge operation sequence according to a second exemplary embodiment of the present invention. 
         FIGS. 11A to 11C  illustrate wiping traces appearing in a sheet member in Example 2 and Comparative examples 3 and 4, respectively. 
         FIGS. 12A and 12B  illustrate a block driving method according to a fourth exemplary embodiment of the present invention. 
         FIG. 13  is a schematic illustration of a recovery unit and a purge unit according to a fifth exemplary embodiment of the present invention. 
         FIGS. 14A to 14D  are schematic illustrations of the operation performed by the purge unit according to the fifth exemplary embodiment. 
         FIGS. 15A to 15E  are schematic illustrations of the operation performed by the purge unit according to the fifth exemplary embodiment. 
         FIGS. 16A and 16B  are schematic illustrations of a purge operation sequence according to the first exemplary embodiment. 
         FIGS. 17A to 17C  illustrate wiping traces appearing in a sheet member in Example 5 and Comparative examples 5 and 6, respectively. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. The configuration of an inkjet printing apparatus according to an exemplary embodiment of the present invention and the print operation performed by the inkjet printing apparatus are described first. 
     (1-1) Configuration of Inkjet Printing Apparatus 
       FIG. 1A  is a schematic illustration of the inkjet printing apparatus according to the present exemplary embodiment of the present invention.  FIG. 1B  is a schematic perspective view of a print head  9  mounted in a carriage  2  of the inkjet printing apparatus illustrated in  FIG. 1A .  FIG. 1C  is a schematic illustration of an internal mechanism of  FIG. 1A . As illustrated in  FIG. 1A , an inkjet printing apparatus  1  is a printer of a serial scan type. The inkjet printing apparatus  1  forms an image by scanning (main scanning) the print head  9  over a recording medium P in a direction (a main scanning direction) orthogonal to a direction in which the recording medium P is conveyed. 
     The configuration of the inkjet printing apparatus  1  and the print operation performed by the inkjet printing apparatus  1  are briefly described next with reference to  FIG. 1A . The recording medium P held by a spool  6  is conveyed onto a platen  4  by a sheet feed roller (not illustrated). The sheet feed roller is driven by a sheet feed motor (not illustrated) via a gear. A carriage motor (not illustrated) moves a carriage unit  2  to scan along a guide shaft  8  extending in a main scanning direction S at a predetermined conveyance position. During the scan, ink is ejected from a nozzle (an ink ejection port) of the print head  9  removably mounted in the carriage unit  2  at a point in time based on a position signal received from an encoder  7 . Printing is performed on a certain printing width of the recording medium P that is equal to the nozzle array length. Thereafter, the recording medium P is conveyed, and printing is performed on the next region of the recording medium P having the printing width. 
     (1-2) Structure of Print Head 
     As illustrated in  FIG. 1B , the print head  9  has chips  11  to  16  that are arrayed up side by side thereon in the main scanning direction S. In addition, the chips  11  to  16  are disposed so as to be parallel to one another. Each of the chips  11  to  16  has a plurality of nozzle arrays that can eject ink of one of a plurality of different hues (colors having different densities). That is, the chips  11  to  16  include nozzle arrays  11   a  to  16   a  each having a plurality of nozzles, respectively. The ink colors are, for example, black (Bk), light cyan (Lc), cyan (C), light magenta (Lm), magenta (M), and yellow (Y). That is, each of the nozzle arrays  11   a  to  16   a  ejects ink having a color assigned thereto among the plurality of colors. Each of the nozzles has an energy generating device (a print element) that generates energy for ejecting ink when power is applied thereto. Examples of the energy generating device include a heating element (a heater) that generates thermal energy when power is applied thereto and a piezoelectric element. When a heating element is used as the energy generating device, a bubble is formed in the ink due to heat generated by the heating element. Thus, a pressure is generated, and the ink is ejected from an ejection port due to the pressure. Each of the nozzle arrays  11   a  to  16   a  receives ink supplied from one of ink introducing units  23  via an ink flow passage formed in the print head  9 . Each of the ink introducing units  23  receives ink from an ink tank (described in more detail below) via a tube. 
     (1-3) Recovery Unit 
     The carriage unit  2  stops at a home position and a back position before printing begins or during printing as necessary. As illustrated in  FIG. 1C , the home position is located within a scanning area of the carriage unit  2  and outside an area where printing is performed on a recording medium. A recovery unit  22  including caps and a wiper is disposed in the vicinity of the home position. In contrast, a purge unit  30  is disposed in the vicinity of the back position. 
       FIG. 2  is a schematic perspective view of an exemplary configuration of the recovery unit  22 . A cap  27  is supported by a lifting mechanism (not illustrated) so as to be raised and lowered. The plurality of nozzles are capped at a high position of the caps  27 . In this manner, during a non-print operation, the nozzles can be protected, or the nozzles can be subjected to a suction recovery operation. During a print operation, the cap  27  is lowered to a low position at which the cap  27  does not interfere with the operation performed by the print head  9 . In order to remove ink deposited on the orifice face, a rubber wiper  26  supported by a wiper holder  25  wipes away the ink by sliding on the orifice face. 
     A suction pump  29  secures the caps  27  on the orifice face to form a closed space and generates a negative pressure inside the caps  27 . In this manner, ink is loaded into the print head  9  and the nozzle. In addition, for example, dirt and dust, deposits, and air bubbles present in the ejection port or an ink channel inside the ejection port can be removed by suction. In the example illustrated in  FIG. 2 , the suction pump  29  configured as a tube pump is employed. The suction pump  29  includes a member having a curved surface that holds a flexible tube  28  (at least part of the flexible tube  28 ) therealong, a roller that can urge the flexible tube  28  against the member, and a rotatable roller supporting portion that supports the roller. By rotating the roller supporting portion in a predetermined direction, the roller rotates on the member having the curved surface formed thereon while crushing the flexible tube  28  therebetween. As a result, a negative pressure is generated in the closed space formed in the cap  7  so that the ink is sucked from the ejection port and is delivered from the cap  27  into the tube  28  or the suction pump  29 . Thereafter, the delivered ink is transferred to an appropriate member (e.g., a waste ink absorber). 
     In addition to performing such suction recovery, the suction pump  29  can be actuated to remove ink that remains in the cap  27  through a preliminary ejecting operation performed with the cap  27  facing the orifice face. That is, by actuating the suction pump  29  when the amount of ink stored in the cap  27  by preliminary ejection has reached a predetermined value, the ink stored in the cap  27  can be transferred to the waste ink absorber via the tube  28 . 
     (1-4) Purge Unit 
     The ink deposited on the surface of the head may include an altered component that is difficult to remove by only wiping. For example, such a case may occur when the ink contains a low-boiling, highly-volatile solvent (e.g., low molecular alcohol, such as IPA, ketones, such as MEK, or esters, such as ethyl acetate) or when the ink contains a lot of polymers for dispersing pigments. In addition, such a case may occur when the pigments of ink have weak dispersiveness and, thus, easily agglomerate. In general, such ink has initial viscosity that is the same as another type of ink and, therefore, any problem does not arise. However, if the ink is concentrated by evaporation, the viscosity is increased more than that of another type of ink. Thus, it is difficult to remove the ink by wiping, as compared with widely used ink. 
     For ink having a functionality that responds to a change in accordance with evaporation, the cleaning performance is extremely deteriorated, as compared with the above-described ink having a viscosity that simply increases in accordance with evaporation. Examples of such ink include ink having a phase change in accordance with evaporation or application of heat and ink that causes dispersion breakdown or that is solidified when the density increases due to evaporation. 
     If it is difficult to wipe away ink by using an existing wiping mechanism or if ink residue remaining after wiping is easily solidified, the head may be wiped using a sheet member. Examples of such a sheet member include a porous urethane foam, a melamine foam, and a non-woven fabric of polyolefin, PET, or nylon. 
       FIG. 3A  illustrates the structure of the purge unit  30 . The purge unit  30  includes a sheet member  31 , a pressing member  33 , a winding roller  32  that winds (collects) the sheet member  31 , and a supply roller  34 . The sheet member  31  pre-contains impregnating fluid. An example of the impregnating fluid is liquid, such as water, surfactant, or solvent. 
     During a purge operation, the print head  9  moves to a position immediately above the purge unit  30  that is positioned in a non-printing area. At the same time, as illustrated in  FIG. 3B , the pressing member  33  moves upward, and the unwound part of the sheet member  31  is maintained at a height of an orifice face  19  of the print head  9  (refer to  FIGS. 7A to 7F ). Subsequently, the print head  9  moves towards the part of the sheet member  31  held by the pressing member  33 . The sheet member  31  is brought into contact with the orifice face  19  of the print head  9 . In this manner, ink deposited on the orifice face  19  is absorbed by the sheet member  31 . After the purge operation performed on the orifice face  19  is completed, the pressing member  33  is lowered, as illustrated in  FIG. 3A . The winding roller  32  rotates so as to wind the sheet member  31  and, thus, an unused part of the sheet member  31  having no absorbed ink is fed out of the supply roller  34 . 
     Such a purge operation can be performed for each of the scanning operations. However, the purge operation may be performed once every several scanning operations. As in the present exemplary embodiment, the purge unit can be used together with an existing wiper. However, only a purge operation using a purge member may be performed. 
     (2) Configuration of Control System 
       FIG. 4  illustrates an exemplary configuration of a control circuit (a control unit  130 ) of the printer according to the present exemplary embodiment. A programmable peripheral interface (PPI)  101  receives a command signal and a printing information signal including print data transmitted from a host computer  100 . Thereafter, the PPI  101  transfers the command signal and the printing information signal to a microprocessor unit (MPU)  102 . The PPI  101  transmits status information regarding the printer to the host computer  100  as needed. In addition, the PPI  101  performs an I/O operation with a console  106  and receives signals input from a sensor group  107 . The console  106  includes a setting input unit used for a user to input a variety of settings and a display unit that displays a message to the user. The sensor group  107  includes a home position sensor that detects whether the carriage unit  2  or the print head  9  is located at the home position and a capping sensor. 
     The MPU  102  controls all of the units of the printer in accordance with the procedure corresponding to a control program (described in more detail below) stored in a control read only memory (control ROM)  105 . A random access memory (RAM)  103  is used as a work area of the MPU  102 . The RAM  103  stores the received signals and temporarily stores various data. A font generation ROM  104  stores pattern information, such as characters and symbols corresponding to code information. The font generation ROM  104  outputs one of the pieces of the pattern information corresponding to input code information. A print buffer  121  temporarily stores the print data loaded into the RAM  103 . The print buffer  121  has a capacity for several print lines. In addition to the above-described control program, the control ROM  105  can store constant data corresponding to data to be used in a control process (described in more detail below) (e.g., data required for determining whether to execute the purge operation). Such memory elements are controlled by the MPU  102  via an address bus  117  and a data bus  118 . 
     A capping motor  113  acts as a drive source for lifting and lowering the caps  27 , moving the wiper holder  25 , and actuating the suction pumps  29 . Motor drivers  114 ,  115 , and  116  drive the capping motor  113 , a carriage motor  3 , and a sheet feed motor  5 , respectively, under the control of the MPU  102 . 
     A sheet sensor  109  detects the presence of a recording medium, that is, whether a recording medium is fed to a position at which the print head  9  can print data. A head driver  111  drives the print elements of the print head  9  in accordance with the printing information signal. A power supply unit  120  is disposed to supply power to each of the units. The power supply unit  120  includes an AC adaptor and a battery serving as a power supply device. 
     In a printing system including the above-described printer and the host computer  100  that supplies the printing information signal to the printer, when the host computer  100  transmits print data, the host computer  100  adds a predetermined command to the head of the print data. The print data are transmitted via, for example, a parallel interface port, an infrared port, or a network. For example, the following commands are added:
         the type of a recording medium on which data are to be printed (e.g., plain paper, an OHP sheet, or glossy paper and, in addition, a special type of a transfer printing film, thick paper, or banner paper)   the size of the recording medium (e.g., A0, A1, A2, B0, B1, or B2)   a print quality (e.g., draft, high quality, medium quality, enhancement of a particular color, or monochrome/color)   a sheet feed path determined by the form or the type of a sheet feed unit (e.g., an automatic sheet feeder (ASF), manual, a sheet feed cassette  1 , or a sheet feed cassette  2 )   ON/OFF of automatic detection of an object.
 
If a configuration in which process liquid for improving the fixing property of ink on the recording medium is applied is employed, information used for determining whether the application is activated may be transmitted as a command.
       

     Using such a command, the printer reads, from the control ROM  105 , data required for printing and performs a print operation on the basis of the data. Examples of the data include the number of print passes used when multipass printing is performed, an ink volume to be ejected per unit area of the recording medium, and information used to select one of printing methods. In some cases, the following data is additionally included: the type of mask for thinning data applied when multipass printing is performed, a drive condition of the print head  9  (e.g., the shape and the time length of a drive pulse applied to the print element), a dot size, a conveyance condition of a recording medium, and a carriage speed. 
     (3) Control Procedure 
     (3-1) Recovery Sequence 
       FIG. 5  illustrates the recovery sequence (i.e., the sequence of cleaning the print head  9 ). The sequence starts when “soft-on” is executed or when a print start command is input from the host computer  100  (step S 1 ). As used herein, the term “soft-on” refers to a secondary power-on operation for making printer functions actually executable after a primary power-on is performed. In step S 2 , a current time Ta is read first. Subsequently, in step S 3 , a time Tb at which the previous recovery process (the previous cleaning process) was performed is read. In step S 4 , the elapsed time (the cleaning interval T) is computed. In step S 5 , it is determined whether the cleaning interval T exceeds a predetermined threshold value U. If the cleaning interval T exceeds the predetermined threshold value U, a cleaning operation, that is, a preliminary ejection operation and a wiping operation are performed in step S 6 . After the cleaning operation is completed, the time Tb is set to the current time Ta in step S 7  and, simultaneously, the printer is placed in an operable condition (step S 8 ). 
     The cleaning interval T can be computed by acquiring the current time Ta using a calendar function provided by the MPU  102  or another appropriate unit and reading the time Tb stored in, for example, a register area of the RAM  103 . Alternatively, by resetting and restarting a timer, such as a programmable interval timer (PIT) each time the cleaning operation is performed, the cleaning interval T can be obtained. 
     (3-2) Print Sequence 
       FIG. 6  illustrates a print sequence. When the printer is stopped, the print head  9  or the orifice face  19  of the nozzle is capped, as described above. Accordingly, before printing begins, the cap is removed so that the print head and the carriage unit  2  can perform a scan operation. For example, it is determined whether the cap is placed on the print head  9  or the nozzle on the basis of the detection result of a sensor (step S 9 ). If the cap is placed on the print head  9  or the nozzle, the cap  27  is lowered and removed from the print head  9  or the nozzle (step S 10 ). The sensor can be installed as one of the sensors in the sensor group  107 . 
     In step S 11 , before printing begins, the recovery sequence illustrated in  FIG. 5  is performed as needed, and a recording medium is fed (step S 12 ). The recording medium is conveyed to the print start position. Thereafter, it is determined whether data required for one scan has been accumulated in the print buffer  121  (step S 13 ). If, in step S 13 , it is determined that data required for one scan has been accumulated in the print buffer  121 , the processing proceeds to a purge operation step (step S 14 ), where the purge operation described in Section (1-4) is performed. 
     The operation performed by the purge unit is described next with reference to  FIGS. 7A to 7F . As illustrated in  FIG. 7A , upon completion of the operation performed by the print head  9  for one scan, the print head  9  moves towards a position at which the print head  9  faces the purge unit  30 . If the print head  9  moves past a point above the platen  4 , the pressing member  33  is lifted, as illustrated in  FIG. 7B . The rewound part of the sheet member  31  is maintained at a height of the orifice face  19  of the print head  9 . Subsequently, as illustrated in  FIG. 7C , the print head  9  moves towards the sheet member  31  held by the pressing member  33  and the sheet member  31  is urged against the orifice face  19  of the print head  9 . In this manner, the orifice face  19  of the print head  9  that faces the sheet member  31  slides on the sheet member  31  and is cleaned by the sheet member  31 . As illustrated in  FIG. 7D , the pressing member  33  is maintained at the height of the orifice face  19  until the print head  9  has moved past the pressing member  33 . As illustrated in  FIG. 7E , after the purge operation is completed, the pressing member  33  is lowered to the height of the platen  4 . Thereafter, as illustrated in  FIG. 7F , the winding roller  32  rotates so that the unwound part of the sheet member  31 , that is, the part of the sheet member  31  that is used for cleaning the orifice face  19  is wound around the winding roller  32 . Simultaneously, an unused part of the sheet member  31  is fed from the supply roller  34 . 
     The purge operation illustrated in  FIGS. 7A to 7F  may be performed for each of the scans, as illustrated in  FIG. 6 . Alternatively, the purge operation may be performed once every several scans or at a predetermined timing determined by the number of ejections and a timer. The carriage unit  2  is moved by the carriage motor  3  to perform a scanning operation and a print operation for the accumulated data (step S 15 ). Subsequently, it is determined whether the operations are completed for data of one page of the recording medium and whether the recording medium is to be output in accordance with a sheet ejection command (step S 16 ). If it is determined that the recording medium is to be output, the recording medium is output (step S 22 ). Thus, the sequence is completed. 
     After a print operation for one scan is completed (step S 15 ) or if, in step S 13 , it is determined that sufficient data has not been accumulated, the processing proceeds to step S 16  and, subsequently, a print data accumulation wait time Tw for the scan is read (step S 17 ). This wait time can be obtained by, for example, using the above-described timer and measuring an elapsed time since when, in step S 13 , it is determined first that data has yet not been accumulated for each of the scans. Subsequently, it is determined whether the print data accumulation wait time Tw exceeds a predetermined time Tcap (step S 18 ). If it is determined that the print data accumulation wait time Tw does not exceed the predetermined time Tcap, it is further determined that the print data accumulation wait time Tw exceeds a predetermined time Ti (step S 20 ). If it is determined that the print data accumulation wait time Tw does not exceed the predetermined time Ti, the processing returns to step S 13 . After the data for one scan is printed, the determination of step S 13  is made for print data for the next scan. 
     If, in step S 18 , it is determined that the print data accumulation wait time Tw exceeds the predetermined time Tcap, a capping operation is performed in step S 19 . Thereafter, the processing returns to step S 13 , where the processing waits until the sufficient amount of data are accumulated. 
     However, if, in step S 20 , it is determined that the print data accumulation wait time Tw exceeds the predetermined time Ti, the processing proceeds to step S 21 , where preliminary ejection is performed. Thereafter, the processing returns to step S 13 , where the processing waits until a sufficient amount of data is accumulated. The relationship between the predetermined time Ti and the predetermined time Tcap can be determined as follows: Ti&lt;Tcap. 
     According to the exemplary embodiments, in the purge operation (step S 14 ) illustrated in  FIG. 6 , the amount of winding of the sheet member  31  (the length of part of the sheet member  31  to be wound after the orifice face  19  of the print head  9  is cleaned) is set to an appropriate amount in accordance with the use condition of the sheet member  31 . Accordingly, the design is such that the amount of winding of the sheet member  31  is changeable in accordance with the number of ink ejections executed in the print operation performed between the immediately previous two purge operations. The exemplary embodiments are described in more detail below. 
     First Exemplary Embodiment 
     A first exemplary embodiment of the present invention is described below with reference to the above-described configuration. In the present exemplary embodiment, the purge operation is performed for each of the scanning operations. The schematic purge sequence (step S 14 ) in the print sequence illustrated in  FIG. 6  and the flowchart of the purge sequence are illustrated in  FIGS. 8A and 8B . 
     If data for one scan is accumulated in the print buffer  121  after a print process begins, measurement of the number of ink ejections (D) from all of the nozzles during the print operation is started (step S 23 ). Immediately after the measurement is started, the print operation for one scan is started (step S 15 ). The number of ink ejections (D) can be measured using an ejection count measuring unit on the basis of, for example, print data stored in the print buffer  121  and font data stored in the font generation ROM  104 . The ejection count measuring unit includes the MPU  102  that measures the number of ejections. The ejection count measuring unit further includes a counter that is set in, for example, the RAM  103  and that stores the number of ink ejections (D). 
     After a one-scan operation is completed, the purge operation (step S 25 ) illustrated in  FIGS. 7A to 7E  is performed. Part of the sheet member  31  used in the purge operation is collected through the winding operation illustrated in  FIG. 7F  (step S 26 ). The amount of winding (X) is selected from Table 1 in accordance with the number of ink ejections (D) measured by the ejection count measuring unit during the print operation for a predetermined area range (for one scan in this case) corresponding to the purge operation performed immediately before the winding step. As used herein, the term “number of ink ejections (D)” refers to the number of ink ejections between the last but one purge operation and the last purge operation when this winding step is considered as a reference. In the winding step performed after the first one-scan operation performed first after the printer is turned on, the number of ink ejections (D) is the number of ejections executed from when the printed is turned on to when the first purge operation is performed. However, the relationship between the number of ejections and the amount of winding (X) is not limited to the three-step relationship illustrated in Table 1. The relationship may be a two-step relationship or a four-step or more step relationship. Alternatively, the relationship may be determined so that the amount of winding (X) continuously increases with increasing number of ejections. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Number of Ejections: D 
                 Amount of Winding: X 
               
               
                   
                   
               
             
            
               
                   
                 D &lt; 1 × 10 8   
                 3 mm 
               
               
                   
                 1 × 10 8  ≦ D &lt; 5 × 10 8   
                 5 mm 
               
               
                   
                 5 × 10 8  ≦ D 
                 7 mm 
               
               
                   
                   
               
            
           
         
       
     
     The amount of winding is controlled so as to increase as the number of ejections increases. This is because the amount of ink mist increases as the number of ejections increases and, therefore, the amount of mist deposited on the orifice face  19  of the print head  9  increases. If the orifice face  19  having a large amount of mist deposited thereon is cleaned, the mist purged onto the sheet member  31  bleeds. Accordingly, the width of a wiping trace on the sheet member  31  increases. As used herein, the term “wiping trace” refers to an ink line produced on the sheet member due to the purge operation. However, by increasing the amount of winding, the whole area in which the mist bleeds can be collected. Thus, the surface of the sheet member after the winding operation is performed can be used as an unused part. In contrast, when the orifice face  19  having only a small amount of mist is cleaned, the width of the wiping area is small, since the mist in the wiping area of the sheet member  31  negligibly bleeds. Thus, by reducing the amount of winding, excessive winding of the sheet member can be prevented. 
     That is, when an image having a low density (i.e., a high throughput oriented image) is printed, the amount of winding of the sheet member is decreased since the number of ejections is small. In contrast, when a high density image (i.e., a high quality oriented image) is printed, the amount of winding of the sheet member is increased since the number of ejections is large. 
     Subsequently, the counter storing the number of ejections is reset (step S 27 ). Thereafter, it is determined whether print data is present (step S 13 ). If print data is present, measurement of the number of ejections is started again (step S 23 ), and the above-described process flow is repeated. However, if print data is not present, the print process is completed. As described above, the present exemplary embodiment is characterized in that a predetermined length of the sheet member is wound immediately after the purge operation is performed, and the length is predetermined in accordance with the number of ejections, as illustrated in  FIG. 8A . In this manner, ink deposited on the orifice face of the print head can be sufficiently purged. In addition, the length of the sheet member wound (used) can be set to an optimal value. 
     Example 1 
       FIG. 9A  illustrates an example of wiping traces (areas used for the purge operation) of the sheet member  31  after the sequence illustrated in  FIGS. 8A and 8B  is performed. In  FIG. 9A , the winding direction is indicated by an arrow. Thus, the trace generated in the first purging operation is leftmost. Since the number of ink ejections (D) is greater than or equal to 5×10 8  in first three scans and the eighth scan, the amount of winding is 7 mm, according to Table 1. In addition, since the number of ink ejections (D) is less than 1×10 8  in each of the fourth to seventh scans and ninth scan, the amount of winding is 3 mm, according to Table 1. 
     Comparative Example 1 
     The amount of winding is fixed to 3 mm for each of the scans, and a print operation that is similar to that in Example 1 is performed.  FIG. 9B  illustrates wiping traces appearing on the sheet member  31  at that time. Since, in the first three scans and the eighth scan, the amount of winding is small after the scan is completed, the wiping operation starts at used part of the sheet member  31  in the second to fourth scans and the ninth scan. Therefore, the wiping traces overlap each other. 
     Comparative Example 2 
     The amount of winding is fixed to 7 mm for each of the scans, and a print operation that is similar to that in Example 1 is performed.  FIG. 9C  illustrates the wiping traces appearing on the sheet member  31  at that time. In each of the fourth to seventh scan, since the amount of winding is large after the scan is completed, the distance between the adjacent wiping traces is large. Therefore, a large unused area appears between the areas used for the wiping operation. 
     Table 2 summarizes the results of Example 1 and Comparative examples 1 and 2. In Example 1, the amount of winding is varied in accordance with the number of ejections. Accordingly, unlike Comparative example 1 in which the wiping traces on the sheet member  31  overlap each other, the purge performance is not degraded. In addition, the amount of used part of the sheet member  31  is made smaller than that of Comparative example 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Amount Used 
                 Overlap of Wiping Traces 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Example 1 
                 43 mm 
                 No 
               
               
                 Comparative Example 1 
                 27 mm 
                 Yes 
               
               
                 Comparative Example 2 
                 63 mm 
                 No 
               
               
                   
               
            
           
         
       
     
     Second Exemplary Embodiment 
     Like the first exemplary embodiment, the purge operation is performed for each of the scans. According to the present exemplary embodiment, the sequence in the wiping operation (step S 14 ) in the print sequence illustrated in  FIG. 6  differs from that of the first exemplary embodiment. The schematic sequence in the wiping operation (step S 14 ) and the flowchart of the sequence are illustrated in  FIGS. 10A and 10B . 
     If data for one scan is accumulated in the print buffer  121  after the print process begins, measurement of the number of ink ejections (D 1 ) is started (step S 28 ). Immediately after the measurement is started, the print operation for one scan is started (step S 15 ). After the one-scan operation is completed, part of the sheet member  31  used in the previous purge operation is collected through the winding operation illustrated in  FIG. 7F  (step S 29 ). The amount of winding (X 2 ) is selected from Table 1 in accordance with the number of ink ejections (D 2 ) measured by the ejection count measuring unit during the print operation for a predetermined area range (an area for one scan in this case) corresponding to the purge operation performed immediately before the winding step. The number of ink ejections (D 2 ) is the same as the number of ejections executed from when the purge operation before last is performed to when the last purge operation is performed. 
     The amount of winding is controlled so as to increase as the number of ejections increases. This is because the amount of mist increases if the number of ejections increases and, therefore, the amount of mist deposited on the orifice face  19  of the print head  9  increases. Accordingly, the amount of bleeding of ink in the wiping trace increases and, thus, the width of the wiping trace increases. After the winding operation is completed, the purge operation illustrated in  FIGS. 7A to 7E  is performed (step S 30 ). Subsequently, the counter storing the number of ejections (D 2 ) is reset (step S 31 ). Thereafter, it is determined whether print data is present (step S 13 ). 
     If print data is not present, the print operation is completed. However, if print data is present, measurement of the number of ejections (D 2 ) is started (step S 32 ). Immediately after the measurement is started, a print operation for one scan is started (step S 15 ). After the print operation for one scan is completed, part of the sheet member  31  used for the previous purge operation is collected through the winding operation illustrated in  FIG. 7F  (step S 33 ). The amount of winding (X 1 ) is selected from Table 1 in accordance with the number of ink ejections (D 1 ) measured by the ejection count measuring unit during the print operation for a predetermined area range corresponding to the purge operation performed immediately before the winding step. After the winding operation is completed, a purge operation illustrated in  FIGS. 7A to 7E  is performed (step S 34 ). Subsequently, the counter storing the number of ejections (D 1 ) is reset (step S 35 ). Thereafter, it is determined whether print data is present (step S 13 ). If print data is present, measurement of the number of ejections (D 1 ) is started again (step S 28 ), and the above-described process flow is repeated. However, if print data is not present, the print process is completed. 
     As described above, the present exemplary embodiment is characterized in that as illustrated in  FIG. 10A , the sheet member is wound immediately before the purge operation is performed, and the length of the sheet member wound is determined in accordance with the number of ejections executed from when the last but one purge operation is performed to when the last purge operation is performed. In this manner, ink deposited on the orifice face of the print head can be sufficiently purged. In addition, the length of the sheet member used for the purge operation can be set to an optimal value. 
     Note that according to the present exemplary embodiment, part of the sheet member  31  used is wound immediately before the purge operation is performed. Accordingly, the purge operation can be performed immediately after the sheet member  31  is fed out of the supply roller  34 . 
     Furthermore, according to the present exemplary embodiment, the winding operation can be performed at any time between the previous purge operation and the next purge operation. That is, according to the first exemplary embodiment, only one counter for storing the number of ejections is provided. Accordingly, the counter needs to be reset before a print operation begins and, therefore, the winding operation that uses a counter value needs to be started before the print operation begins. Thus, in reality, as illustrated in  FIG. 8A , the point in time at which the winding operation is started is limited to a point in time immediately before the print operation begins. In contrast, according to the present exemplary embodiment, a counter that operates during a print operation and a counter that is referenced during the winding operation are provided. Thus, the winding operation can be performed regardless of whether the print operation is being performed. At least two counters are needed. However, three or more counters may be provided. At that time, it is only required that the numbers of ink ejections measured in any two continuous print operations be stored in two different counters among the above-described at least two counters. 
     Example 2 
       FIG. 11A  illustrates an example of a wiping trace (an area used for a purge operation) appearing on the sheet-like member  31  when the sequence of processes illustrated in  FIGS. 10A and 10B  is performed. In  FIG. 11A , the winding direction is indicated by an arrow. Thus, the trace generated in the first purging operation is leftmost. Since the number of ink ejections (D) is greater than or equal to 5×10 8  in each of the fourth, sixth, and eighth scans, the amount of winding is 7 mm, according to Table 1. In addition, since the number of ink ejections (D) is greater than or equal to 1×10 8  and less than 5×10 8  in each of the first three scans and the eleventh and twelfth scans, the amount of winding is 5 mm, according to Table 1. Furthermore, since the number of ink ejections (D) is less than 1×10 8  in each of the fifth, seventh, ninth, tenth, thirteenth, and fourteenth scans, the amount of winding is 3 mm, according to Table 1. 
     Comparative Example 3 
     The amount of winding is fixed to 5 mm for each of the scans, and a print operation that is similar to that in Example 2 is performed.  FIG. 11B  illustrates the wiping trace appearing on the sheet member  31  at that time. Since, in the fourth, sixth, and eighth scans, the amount of winding is small after the scan is completed, the purge operation starts from a used part of the sheet member  31  in each of the fifth, seventh, and ninth scans. Therefore, the wiping traces overlap each other. 
     Comparative Example 4 
     The amount of winding is fixed to 7 mm for each of the scans, and a print operation that is similar to that in Example 2 is performed.  FIG. 11C  illustrates the wiping trace appearing on the sheet member  31  at that time. In each of the first three scans and the fifth, seventh, and ninth scans, since the amount of winding is large after the scan is completed, the distance between the adjacent wiping traces is large. Therefore, a large unused area appears between the areas used for the purge operations. 
     Table 3 summarizes the results of Example 2 and Comparative examples 3 and 4. In Example 2, the amount of winding is varied in accordance with the number of ejections. Accordingly, unlike Comparative example 3 in which the wiping traces on the sheet member  31  overlap each other, the purge performance is not degraded. In addition, the amount of a used part of the sheet member  31  is made smaller than that of Comparative example 4. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Amount Used 
                 Overlap of Wiping Traces 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Example 2 
                 71 mm 
                 No 
               
               
                 Comparative Example 3 
                 75 mm 
                 Yes 
               
               
                 Comparative Example 4 
                 105 mm  
                 No 
               
               
                   
               
            
           
         
       
     
     Third Exemplary Embodiment 
     According to the present exemplary embodiment, the amount of winding of the sheet member  31  is controlled while taking into account the mist depositability on the print head due to a factor other than the number of ink ejections. Hereinafter, description of the same or similar element and function to that of the first or second exemplary embodiment is not repeated. 
     According to the present exemplary embodiment, when the amount of winding of the sheet member  31  is selected from Table 1 in accordance with the number of ejections, the mist depositability on the print head in accordance with the distance between the print head and the recording medium (the print head-to-recording medium distance) and the humidity in the vicinity of the apparatus is taken into account. Thus, more accurate control can be performed. The number of ejections that takes into account the mist depositability is computed as follows:
 
Number of ejections that takes into account the mist depositability=(number of ejections between purge operations)×(print head-to-recording medium distance coefficient)×(humidity coefficient).
 
Note that only one of (print head-to-recording medium distance coefficient) and (humidity coefficient) may be used. The terms in the equation are described below.
 
Print Head-to-Recording Medium Distance
 
     According to the present exemplary embodiment, the print head-to-recording medium distance can be varied using a carriage lifting mechanism (not illustrated). The print head-to-recording medium distance can be switched to the following five positions in accordance with the type of the recording medium or the environmental condition or by a user: “low”, “moderately low”, “normal”, “moderately high”, and “high”. 
     As the print head-to-recording medium distance increases, the amount of mist deposited on the print head increases. This is because if the print head-to-recording medium distance increases, a mist particle having a low mass or a low ejection speed does not reach the recording medium and is easily suspended in air (or is easily deposited on the surface of the print head). Accordingly, in the present exemplary embodiment, as illustrated in Table 4, the number of ejections measured between the purge operations is multiplied by a coefficient predetermined in accordance with the print head-to-recording medium distance (i.e., a distance coefficient). Thus, the number of ejections is weighted. In this manner, the amount of winding of the sheet member  31  is controlled. More specifically, the number of ejections is multiplied by a distance coefficient to obtain the number of ejections corrected by the distance coefficient. Thereafter, a winding unit of the sheet member  31  is controlled on the basis of the corrected number of ejections. 
                         TABLE 4               Print Head-to-Recording Medium   Print Head-to-Recording       Distance   Medium Distance Coefficient                                        low (small)   0.4       moderately low (moderately small)   0.6       normal   0.8       moderately high (moderately large)   1       high (large)   1.2                    
Humidity Around Apparatus
 
     The amount of mist deposited on the print head increases depending on the use environment of the apparatus. In particular, as the humidity decreases, the amount of mist deposited on the print head increases. This is because if the humidity decreases, evaporation of ejected ink is facilitated and, thus, the mass or the ejection speed of the mist particle decreases. The mist particle having a low mass or a low ejection speed does not reach the recording medium and is easily suspended in air (or is easily deposited on the surface of the print head). Accordingly, in the present exemplary embodiment, the humidity in the vicinity of an inkjet print head unit is measured by a humidity measuring unit (a humidity sensor, not illustrated). The number of ejections is multiplied by a humidity coefficient corresponding to the number of ejections illustrated in Table 5. Thus, the number of ejections measured between the purge operations is weighted. In this manner, the amount of winding of the sheet member  31  is controlled. More specifically, the number of ejections is multiplied by a humidity coefficient to obtain the number of ejections corrected by the humidity coefficient. Thereafter, a winding unit of the sheet member  31  is controlled on the basis of the corrected number of ejections. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Humidity 
                 Humidity Coefficient 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 to 30% 
                 1 
               
               
                   
                 30% to 60% 
                 0.9 
               
               
                   
                 60% to 
                 0.8 
               
               
                   
                   
               
            
           
         
       
     
     According to the present exemplary embodiment, the amount of winding of the sheet member  31  is controlled in accordance with the number of ejections multiplied by the distance coefficient and a humidity coefficient while taking into account the mist depositability on the print head. Accordingly, degradation of the image quality due to, for example, ink ejection failure or color mixture can be prevented. In addition, the length of the sheet member  31  used can be reduced. In the present exemplary embodiment, the length of the sheet member  31  used can be reduced more than in the first and second exemplary embodiments. 
     Fourth Exemplary Embodiment 
     According to the present exemplary embodiment, when the print head is driven using a block driving method, the amount of winding of the sheet member  31  is controlled by taking into account a unique characteristic in that the amount of generated mist significantly varies in accordance with the order in which the blocks are driven. Hereinafter, description of the same or similar element and function to that of the first, second, or third exemplary embodiment is not repeated. 
     In the block driving method, all of the nozzles of the print head do not eject ink at the same time. Ejection is sequentially performed on a block basis, where the block includes a predetermined number of nozzles. The block driving method has an advantage that the power required for ejection at one time is small, for example. 
     However, depending on the configuration of a print head, the sequence of driving blocks (the block driving sequence) may have an impact on the image quality and the amount of generated mist. According to the present exemplary embodiment, to address such an issue, one of two different block driving sequences is selected in accordance with a print mode. 
     According to the present exemplary embodiment, 1280 nozzles are divided into 32 blocks each including 40 nozzles. The blocks are driven at 32 different points in time in a time division manner. In a poster/picture mode in which a poster or a picture is mainly printed, it is desirable that a block driving sequence A be employed. In contrast, in a line drawing mode in which a line drawing, such as a computer aided design (CAD) drawing, is mainly printed, it is desirable that a block driving sequence B be employed. The block driving sequences A and B are illustrated in  FIGS. 12A and 12B , respectively. In  FIGS. 12A and 12B , the nozzles having nozzle numbers 1 to 64 are arranged in a nozzle arrangement direction in the order from the nozzle number 1 to the nozzle number 64. In the following description, a nozzle having a nozzle number N (N is an integer greater than or equal to 1) is referred to as an “#N nozzle”. 
     Nozzles 1 to 32 of the block driving sequence A are discussed first. The #1 to #32 nozzles are arranged up side by side, and ink is sequentially ejected in the order from the #1 to #32 nozzles. In the same manner, ink is sequentially ejected from the #33 to #64 nozzles. In such a case, the difference between the time when the #32 nozzle ejects ink and the time when the #33 nozzle ejects ink is large. This slight time difference may cause ink dot misalignment. However, the amount of mist is smaller than in the block driving sequence B. 
     Nozzles 1 to 32 of the block driving sequence B are discussed next. Unlike the block driving sequence A, ink is sequentially ejected from #2, #4, . . . , and #32 nozzles. Subsequently, ink is sequentially ejected from #1, #3, . . . , and #31 nozzles. In the same manner, ink is sequentially ejected from the #33 to #64 nozzles. Thus, the difference between the times at which adjacent nozzles eject ink is substantially constant. Consequently, the difference between the time when the #32 nozzle ejects ink and the time when the #33 nozzle ejects ink is small. Accordingly, ink dot misalignment negligibly occurs. However, the amount of mist is larger than in the block driving sequence A. 
     The difference between the amounts of mist in the block driving sequences A and B results from the following reasons. In the block driving sequence A, a nozzle is affected by ejection performed by the adjacent nozzle, and the meniscus condition of the nozzle immediately before ejection tends to be unstable. Thus, the ejection speed is decreased and, therefore, the amount of mist is smaller than in the block driving sequence B. In contrast, in the block driving sequence B, a nozzle is negligibly affected by ejection performed by the adjacent nozzle, and the meniscus condition of the nozzle immediately before ejection tends to be stable. Thus, the ejection speed is increased and, therefore, the amount of mist is larger than in the block driving sequence A. 
     According to the present exemplary embodiment, such characteristics are utilized. That is, the block driving sequence A is employed for the poster/picture mode in which multi-pass printing is mainly performed, since ink dot misalignment caused by a slight difference between ejection times of two blocks is unnoticeable. In contrast, in a line drawing mode in which low pass printing is mainly performed, the block driving sequence B is employed, since ink dot misalignment caused by a slight difference between ejection times of two blocks is unnoticeable even in the low pass printing. 
     According to the present exemplary embodiment, by taking into account the difference between the mist depositabilities in the two block driving sequences, the actual number of ejections is obtained as follows:
 
The number of ejections that takes into account mist depositability=(the number of ejections between purge operations)×(print head-to-recording medium distance coefficient)×(humidity coefficient)×(ejection time difference coefficient).
 
     The ejection time difference coefficient is illustrated in Table 6. The ejection time difference coefficient increases with decreasing maximum value of a time difference between ejection times of two adjacent nozzles (in the block driving sequence A, the #32 nozzle and the #33 nozzles generate the maximum value of a time difference between ejection times). That is, the ejection time difference coefficient decreases with increasing maximum value of a time difference between ejection times of two adjacent nozzles. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Block Driving Sequence 
                 Ejection Time Difference Coefficient 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 A 
                 0.3 
               
               
                   
                 B 
                 1 
               
               
                   
                   
               
            
           
         
       
     
     According to the present exemplary embodiment, in order to take into account the mist depositability on the print head, the number of ejections is multiplied by the ejection time difference coefficient in addition to the distance coefficient and the humidity coefficient to obtain the “number of ejections that takes into account mist depositability”. However, it is not necessary to use all of the coefficients. Any appropriate combination of the coefficients may be used. The amount of winding of the sheet member  31  is controlled in accordance with the “number of ejections that takes into mist depositability”. Thus, degradation of the image quality caused by ink ejection failure and color mixture can be prevented. In addition, the length of the sheet member  31  used can be reduced. According to the present exemplary embodiment, the length of the sheet member  31  used can be more reduced than in the first to third exemplary embodiment. 
     The amount of mist deposited on the print head is also affected by the ink ejection volume and the type of ink. Accordingly, the amount of winding of the sheet member  31  may be controlled by multiplying the number of ejections measured between purge operations by coefficients predetermined in accordance with the ink ejection volume and the type of ink (i.e., an ink ejection volume coefficient and an ink type coefficient). Tables 7 and 8 illustrate the ink ejection volume coefficient and the ink type coefficient, respectively. The ink ejection volume coefficient decreases with decreasing ink ejection volume in one ejecting operation performed by the nozzle and increases with increasing ink ejection volume. The ink type coefficient is set in accordance with the type of ink. All of a total of five coefficients (i.e., these two coefficients and the above-described distance coefficient, the humidity coefficient, and the ejection time difference coefficient) may be used, or any appropriate combination of the coefficients may be used. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 Ejection Volume 
                 Ejection Volume Coefficient 
               
               
                   
                   
               
             
            
               
                   
                 5.0 pL 
                 1.0 
               
               
                   
                 6.0 pL 
                 1.1 
               
               
                   
                 8.0 pL 
                 1.2 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 Ink Type 
                 Ink Type Coefficient 
               
               
                   
                   
               
             
            
               
                   
                 Ink A 
                 1.1 
               
               
                   
                 Ink B 
                 1.0 
               
               
                   
                 Ink C 
                 0.9 
               
               
                   
                   
               
            
           
         
       
     
     Fifth Exemplary Embodiment 
     According to the present exemplary embodiment, a configuration that differs from the configuration described in Section 1-3 with reference to  FIG. 1C  is described. That is, the configuration according to the present exemplary embodiment differs from the configuration in which the recovery unit  22  is disposed on the home position side and the purge unit  30  is disposed on the back position side. As illustrated in  FIG. 13 , the recovery unit  22  and the purge unit  30  are disposed on the home position side and are fixed onto a movable table  61 . That is, the sheet member  31  is disposed on the side the same as the recovery unit  22  in the main scanning direction S of the print head  9  and is supported by the movable table  61  that is common to the recovery unit  22 . The movable table  61  moves along a slide guide  62  in a direction that is perpendicular to the main scanning direction S of the carriage unit  2  (i.e., a conveyance direction L). Accordingly, the purge unit  30  performs a purge operation over the orifice face  19  of the print head  9  in parallel to the nozzle arrays (in the conveyance direction L). The operation is illustrated in  FIGS. 14A to 14D  and  FIGS. 15A to 15E .  FIGS. 14A to 14D  illustrate the operations performed by the recovery unit  22  and the purge unit  30  viewed in the direction XIV illustrated in  FIG. 13 , and  FIGS. 15A to 15E  illustrate the operations viewed in the direction XV illustrated in  FIG. 13 . 
     As illustrated in  FIG. 14A , before printing begins, the cap  27  is removed from the print head  9  or the carriage unit  2 , and the print head  9  and the carriage unit  2  are ready for perform a scanning operation in the main scanning direction S. It is determined whether data needed for one scan has been accumulated in the print buffer  121 . If it is determined that data needed for one scan has been accumulated, the carriage unit  2  performs a print operation for one scan by moving from the home position to the back position, as illustrated in  FIG. 14B . When the print operation starts, the movable table  61  moves in the conveyance direction L. As illustrated in  FIG. 14C , the purge unit  30  is located on the extension line of the platen  4 . The recovery unit  22  retracts beneath the guide shaft  8 . This operation is performed in order to perform preliminary ejection above the purge unit  30 . Thereafter, it is determined whether data needed for one scan has been accumulated in the print buffer  121 . If it is determined that data needed for one scan has been accumulated, the carriage unit  2  performs a print operation for one scan by moving from the back position to the home position. Subsequently, a purge operation is performed. More specifically, as illustrated in  FIG. 14D , after the print operation is completed, the carriage unit  2  is moved to a position at which the carriage unit  2  faces the purge unit  30 . After the carriage is stopped, the purge operation is performed. 
     The purge operation is described in more detail below with reference to  FIGS. 15A to 15E .  FIG. 15A  illustrates a condition that is the same as the condition of  FIG. 14A , that is, the condition in which the cap is removed and, thus, the print head  9  or the carriage unit  2  is scannable.  FIG. 15B  illustrates a condition that is the same as the condition of  FIG. 14D , that is, the condition in which the carriage unit  2  has moved to the position at which the carriage unit  2  faces the purge unit  30 . After the carriage unit  2  stops above the purge unit  30 , the pressing member  33  is lifted. As illustrated in  FIG. 15C , the pressing member  33  is stopped and maintained at a height of the orifice face  19  of the print head  9 . Thereafter, as illustrated in  FIG. 15D , the movable table  61  moves in the conveyance direction L and, thus, the orifice face  19  of the print head  9  is slid and scrubbed by the sheet member  31  in a direction parallel to the nozzle arrays. In this manner, purging is performed. At that time, the pressing member  33  is maintained at the height of the orifice face  19  until the print head  9  moves past the pressing member  33 . After the purge operation is completed, the pressing member  33  is lowered to the height of the platen  4 , as illustrated in  FIG. 15E . The winding roller  32  rotates and winds the used part of the sheet member  31  therearound. At the same time, an unused part of the sheet member  31  is fed from the supply roller  34 . 
     The purge operation performed in the above-described configuration is described in more detail below. According to the present exemplary embodiment, the purge operation is performed for each of back-and-forth scans. A schematic purging sequence (step S 14 ) in the print process illustrated in  FIG. 6  and the flowchart of the print process are illustrated in  FIGS. 16A and 16B , respectively. After the print process is started and data needed for one scan is accumulated in the print buffer  121 , measurement of the number of ink ejections (D) is started for each of the chips  11  to  16  corresponding to different colors (step S 36 ). Immediately after measurement starts, a print operation for one scan is started (step S 15 ). 
     After the print operation for one scan is completed, a purge operation illustrated in  FIGS. 15A to 15D  is performed (step S 37 ). Part of the sheet member  31  used in the purge operation is collected through a winding operation illustrated in  FIG. 15E  (step S 39 ). The amount of winding (X) is determined by computing the maximum value of the number of ejections (Dmax) from the total number of ink ejections (D) performed by each of the chips corresponding to the different colors and measured until the purge operation is performed and selecting a value corresponding to the computed Dmax from Table 9 (step S 38 ). That is, the control unit computes a total number of ejections (D) performed by the nozzles during a predetermined range of the print operation for each of the colors and computes the maximum value from among the total numbers of ejections (D) as the maximum value of the number of ejections (Dmax). When a plurality of the ejection ports are assigned to each of the colors, the total number of ink ejections (D) is computed as a total number of ejections performed by the plurality of the ejection ports for the color. In contrast, when a plurality of nozzle arrays for the same color are disposed in a head, the total number of ink ejections (D) is computed as a total number of ejections performed by each of the nozzle arrays. The control is performed so that the amount of winding (X) increases with increasing number of ejections. This is because the amount of mist increases as the number of ejections increases and, therefore, the amount of mist deposited on the orifice face  19  of the print head  9  increases. If the orifice face  19  having a large amount of mist deposited thereon is cleaned, the purged mist in the wiping trace on the sheet member  31  bleeds. Accordingly, the width of the wiping trace of the sheet member  31  increases. However, by increasing the amount of winding, the whole area in which the mist bleeds can be collected. Thus, the surface of the sheet member after the winding operation is performed can be used as an unused part. In contrast, when the orifice face  19  having only a small amount of mist is purged, the mist in the wiping trace of the sheet member  31  negligibly bleeds and, therefore, the width of the wiping trace is small. Thus, by reducing the amount of winding, excessive winding of the sheet member can be prevented. 
     Subsequently, the number of ink ejections (D) is reset (step S 40 ). Thereafter, it is determined whether print data is present (step S 41 ). If print data is present, measurement of the number of ejections is started again (step S 36 ), and the above-described process flow is repeated. However, if print data is not present, the print process is completed. As described above, the present exemplary embodiment is characterized in that the number of ejections (D) is measured for each of the ejection ports corresponding to the different colors, the maximum value of the number of ejections (Dmax) is computed from the number of ejections (D) for each of the ejection ports corresponding to the different colors immediately after a purge operation is performed, and a predetermined length of the sheet member  31  equal to the length corresponding to Dmax is wound. Furthermore, according to the present exemplary embodiment, by moving the movable table  61  in the conveyance direction L of the recording medium P, a capping operation and a purge operation for the orifice face can be selectively performed. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                 Maximum Value of Number of 
                   
               
               
                   
                 Ejections: Dmax 
                 Amount of Winding: X 
               
               
                   
                   
               
             
            
               
                   
                 Dmax &lt; 1 × 10 6   
                 3 mm 
               
               
                   
                 1 × 10 6  ≦ Dmax &lt; 5 × 10 6   
                 5 mm 
               
               
                   
                 5 × 10 6  ≦ Dmax 
                 7 mm 
               
               
                   
                   
               
            
           
         
       
     
     Example 5 
       FIG. 17A  illustrates a wiping trace appearing in the sheet member  31  when the sequence illustrated in  FIGS. 16A and 16B  is performed. In  FIG. 17A , the winding direction is indicated by an arrow. Thus, the wiping trace generated in the first purging operation is uppermost. Since the number of ink ejections (D) for each of all colors is less than 1×10 6  and, thus, Dmax is less than 1×10 6  in each of the first scan, the fourth scan, and sixth scan, the amount of winding is 3 mm, according to Table 9. In addition, since Dmax is greater than or equal to 1×10 6  and less than or equal to 5×10 6  in the second scan, the amount of winding is 5 mm, according to Table 9. Since Dmax is greater than or equal to 5×10 6  in each of the third and fifth scans, the amount of winding is 7 mm, according to Table 9. 
     Comparative Example 5 
       FIG. 17B  illustrates a wiping trace appearing in the sheet member  31  when the amount of winding is fixed to 3 mm for each of the scans and a print operation that is similar to that in Example 5 illustrated in  FIG. 17A  is performed. Since the amount of winding is small after each of the second, third, and fifth scans is completed, the wiping operation for each of third, fourth, and sixth scans starts from used part of the sheet member  31 . Therefore, the wiping traces overlap each other. 
     Comparative Example 6 
       FIG. 17C  illustrates a wiping trace appearing in the sheet member  31  when the amount of winding is fixed to 7 mm for each of the scans and a print operation that is similar to that in Example 5 illustrated in  FIG. 17A  is performed. Since the amount of winding is large after each of the first, second, and fourth scans is completed, the distance between the adjacent wiping traces is large. Therefore, a large unused area appears between the wiping traces. 
     Table 10 summarizes the results of Example 5 and Comparative examples 5 and 6. In Example 5, the amount of winding is varied in accordance with the maximum value of the number of ejections (Dmax) from the ejection ports corresponding to each color. Accordingly, unlike Comparative example 5 in which the wiping traces on the sheet member  31  overlap each other, the purge performance is not degraded. In addition, the length of the sheet member  31  used can be made smaller than that of Comparative example 6. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 10 
               
               
                   
                   
               
               
                   
                 Amount of Sheet 
                 Overlap of 
               
               
                   
                 Member Used 
                 Wiping Traces 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Example 5 
                 28 mm 
                 No 
               
               
                   
                 Comparative Example 5 
                 18 mm 
                 Yes 
               
               
                   
                 Comparative Example 6 
                 42 mm 
                 No 
               
               
                   
                   
               
            
           
         
       
     
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-073289 filed Mar. 28, 2012 and No. 2012-275991 filed Dec. 18, 2012, which are hereby incorporated by reference herein in their entirety.