Abstract:
A printing apparatus and preliminary discharge control method capable of minimizing the influence of a preliminary discharge operation on printing and keeping a short printing time are disclosed. In an inkjet printing apparatus for printing image data on a printing medium by discharging ink from a printhead mounted on a carriage which scans back and forth parallel to the printing medium, an ink receptor having at least the same length as the scanning range of the printhead is formed below a printing position at which the printhead prints image data on a printing medium. When this inkjet printing apparatus is to perform preliminary discharge, an edge of a printing medium is sensed during the forward and backward scans of the carriage, and the carriage mounting the printhead is moved to be separated a predetermined distance from the sensed edge of the printing medium. At this position separated by the predetermined distance, the printhead is controlled to perform a preliminary discharge operation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an inkjet printing apparatus and a preliminary discharge control method. 
     BACKGROUND OF THE INVENTION 
     An inkjet printer forms an image by discharging an ink droplet from a discharge orifice formed in a printhead and sticking the discharged ink droplet onto a printing sheet. 
     A printhead usually has a plurality of discharge orifices. If a discharge orifice not frequently used is present in these discharge orifices during printing, the ink viscosity in this discharge orifice rises owing to water evaporation to clog this discharge orifice. To avoid this phenomenon, ink droplets corresponding to a predetermined number of dots are discharged, at predetermined time intervals or in accordance with the calculated use ratios of the ink discharge orifices, while printing is executed, into an ink receptor (to be referred to as a preliminary discharge receptor) formed outside the scanning region of the printhead, thereby holding the state of each discharge orifice and its vicinity constant. This operation is called a preliminary discharge operation. 
     FIG. 7 is a perspective view showing an outline of the arrangement of a conventional inkjet printer. 
     Referring to FIG. 7, reference numeral  101  denotes a printing sheet;  102 , an automatic sheet feeding mechanism which feeds stacked printing sheets one by one into the printer and, as printing progresses, discharges each printing sheet forward (in the direction of an arrow);  103 , a carriage; and  104 , an ink cartridge containing a printhead which is so mounted as to discharge ink downward (toward the printing sheet). 
     The carriage  103  on which the ink cartridge  104  is mounted moves back and forth parallel to the printing sheet in a direction perpendicular to the conveyance direction of the printing sheet. An image is formed by discharging ink stored in the ink cartridge  104  toward the printing sheet. 
     Reference numeral  105  denotes a carriage motor; and  106 , a driving belt. The carriage  103  is interlocked with the carriage motor  105  by the driving belt  106 . When the carriage motor  105  is driven forward and backward, the carriage  103  moves forward and backward. Guide shafts  107  and  108  for mounting the carriage  103  support the forward and backward movements of the carriage  103  mounting the ink cartridge  104 . 
     Reference numerals  109  and  110  denote preliminary discharge receptors. In a preliminary discharge operation, the carriage  103  moves to a position above the preliminary discharge receptor  109  or  110 , and the printhead discharges ink droplets corresponding to a predetermined number of dots. 
     In this conventional inkjet printer, as can be seen from FIG. 7, a preliminary discharge receptor is generally formed on one or both sides of the scanning region of the printhead. When a preliminary discharge operation is to be executed, the printhead is moved from the printing end position to the position above the preliminary discharge receptor. 
     FIG. 8 is a schematic view showing the control of a printing operation and a preliminary discharge operation in this conventional inkjet printer. 
     As shown in FIG. 8, when an image such as “A . . . A” is to be printed on the printing sheet  101 , the carriage  103  mounting the printhead explained in FIG. 7 scans within the range of a line  301  from a point x to a point y. This line  301  is made up of a printing region in which an image is formed by discharging ink onto the printing sheet, and acceleration (deceleration) regions a and b. To form an image in the printing region, the carriage  103  must be driven at a constant speed. When the carriage  103  is at rest at a point  302 , this carriage  103  is accelerated in the region a to move at the constant speed from a point  303 . Also, to stop the carriage  103 , which has moved at the constant speed to a point  304 , at a point  305 , this carriage  103  is decelerated in the region b. 
     When the image is to be printed by moving the carriage  103  in the opposite direction from the point y to the point x, the region b is an acceleration region, and the region a is a deceleration region. Therefore, printing region+region a+region b is the carriage moving range corresponding to a printing operation of one scan. 
     On the other hand, when a preliminary discharge operation is to be executed, the carriage  103  is moved to the position above the preliminary discharge receptor  109  or  110  to discharge ink. That is, when preliminary discharge is to be performed for the preliminary discharge receptor  109 , the carriage  103  is moved to a point  307 . A region d indicates a moving region from the carriage stop position to the preliminary discharge receptor  109 . When preliminary discharge is to be performed for the preliminary discharge receptor  110 , the carriage  103  is moved to a point  306 . A region c indicates a moving region from the carriage stop position to the preliminary discharge receptor  110 . 
     In the above prior art, however, if the width of the printing sheet is smaller than the maximum width of the scanning region, the actual scanning region of the printhead narrows in accordance with the width of the printing sheet. This increases the moving distance from the end of the scanning region to the preliminary discharge receptor when a preliminary discharge operation is to be performed. This undesirably prolongs the printing execution time. 
     FIG. 9 is a schematic view showing the control of a printing operation and a preliminary discharge operation of the conventional inkjet printer, when the width of a printing sheet is much smaller than the scan enable region. As is apparent from the comparison of FIGS. 8 and 9, the width of a printing sheet  400  is much smaller than the scan enable region shown in FIG.  8 . 
     When an image such as “A . . . A” is to be printed on this printing sheet  400 , the scanning range of the carriage  103  mounting the printhead explained in FIG. 7 is the range indicated by a line  401  from a point x′ to a point y′. This line  401  is made up of a printing region in which an image is formed by discharging ink onto the printing sheet, and acceleration (deceleration) regions a′ and b′. To form an image in the printing region, the carriage  103  must be moved at a constant speed. When the carriage  103  is at rest at a point  402 , this carriage  103  is accelerated in the region a′ to move at the constant speed from a point  403 . Also, to stop the carriage  103 , which has moved at the constant speed to a point  404 , at a point  405 , this carriage  103  is decelerated in the region b′. 
     When the image is to be printed by moving the carriage  103  in the opposite direction from the point y′ to the point x′, the region b′ is an acceleration region, and the region a′ is a deceleration region. Therefore, printing region+region a′+region b′ is the carriage moving range corresponding to a printing operation of one scan. 
     When a preliminary discharge operation is to be executed, the carriage  103  is moved to the position above the preliminary discharge receptor  109  or  110  to discharge ink. That is, when preliminary discharge is to be performed for the preliminary discharge receptor  109 , the carriage  103  is moved to a point  407 . A region d′ indicates a moving region from the carriage stop position to the preliminary discharge receptor  109 . When preliminary discharge is to be performed for the preliminary discharge receptor  110 , the carriage  103  is moved to a point  406 . A region c′ indicates a moving region from the carriage stop position to the preliminary discharge receptor  110 . 
     As can be seen by comparing FIGS. 8 and 9, the width of the printing region decreases because the width of the printing sheet decreases. However, since the position of the preliminary discharge receptor  109  remains unchanged, the region d′ shown in FIG. 9 which is the moving region, from the carriage stop position to the preliminary discharge receptor  109 , for performing a preliminary discharge operation is longer than the region d shown in FIG.  8 . Therefore, although the printing region is narrowed, the carriage moving distance necessary to execute a preliminary discharge operation during a printing operation increases. This requires an extra time whenever a preliminary discharge operation is performed, prolonging the time to the completion of the printing operation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a printing apparatus and preliminary discharge control method capable of minimizing the influence of a preliminary discharge operation on printing and keeping a short printing time. 
     According to one aspect of the present invention, the foregoing object is attained by providing an inkjet printing apparatus for printing on a printing medium by discharging ink, on the basis of printing data, from a printhead mounted on a carriage which moves back and forth on the printing medium. The apparatus includes preliminary discharge control means and preliminary discharge position designating means. The preliminary discharge control means controls the printhead to perform preliminary discharge, by which no printing is performed, between scanning actions of the carriage. The preliminary discharge position designating means designates a position at which the preliminary discharge is controlled by the preliminary discharge control means to be performed by the printhead. The preliminary discharge position designating means variably designates a preliminary discharge position, which is a predetermined distance away from an edge of the printing medium, as the position at which the preliminary discharge is to be performed, in accordance with a position of the edge of the printing medium. 
     The printing is desirably performed by forward and backward scans of the printhead. 
     The apparatus can further comprise an ink receptor for receiving ink discharged by preliminary discharge by the preliminary discharge control means, the ink receptor having a length not less than the length of a scanning range of the printhead. Alternatively, the apparatus can further comprise a plurality of ink receptors for receiving ink discharged by preliminary discharge by the preliminary discharge control means, in accordance with the width of the printing medium in the scanning direction. 
     The apparatus desirably further comprises sensing means for sensing an edge of the printing medium. 
     The preliminary discharge position designating means can designate a preliminary discharge position such that preliminary discharge is performed in a position separated a predetermined distance from the position of an edge of the printing medium, regardless of this edge position of the printing medium. The designated predetermined distance when the ink discharge frequency of the printing is relatively high is desirably made longer than that when the ink discharge frequency is relatively low. 
     When performing preliminary discharge after printing in a forward or backward scanning direction, the preliminary discharge control means desirably so controls the printhead as to move in the forward or backward scanning direction, after the printing is completed, and to perform a preliminary discharge operation. 
     The preliminary discharge position designating means can designate a preliminary discharge position in accordance with information about an edge of the printing medium sensed by the sensing means after the printing is performed. 
     The apparatus desirably further comprises storage means which, when preliminary discharge is to be performed after the printhead prints in a first direction of the forward and backward scans of the printhead, stores information about an edge of the printing medium sensed by the sensing means after the printing is completed, and, when preliminary discharge is to be performed after the printhead prints in a second direction opposite to the first direction, stores information about an edge of the printing medium sensed by the sensing means after the printing is completed. 
     The preliminary discharge control can be so performed that the preliminary discharge is performed, while the printhead is scanning, in a position farther from the printing medium than the position designated by the preliminary discharge position designating means, or that the preliminary discharge is performed before or after the scanning directions are switched. 
     Preferably, the printhead is an inkjet printhead for discharging ink by using thermal energy, and comprises an electrothermal converter for generating thermal energy to be applied to ink. 
     According to another aspect of the present invention, the foregoing object is attained by providing a preliminary discharge control method of an inkjet printing apparatus for printing on a printing medium by discharging ink, on the basis of printing data, from a printhead mounted on a carriage which moves back and forth on the printing medium. The method includes a preliminary discharge position designation step of designating a position at which preliminary discharge, by which no printing is performed, is performed between scanning actions of the carriage and a preliminary discharge control step of controlling the printhead to perform the preliminary discharge. The preliminary discharge position designation step variably designates a preliminary discharge position, which is a predetermined distance away from an edge of the printing medium, as the position at which the preliminary discharge is to be performed, in accordance with the position of the edge of the printing medium. 
     In accordance with the present invention as described above, when an inkjet printing apparatus for printing on a printing medium by discharging ink, on the basis of printing data, from a printhead mounted on a carriage which moves back and forth on the printing medium is to perform preliminary discharge, a position at which preliminary discharge, by which no printing is performed, is performed between scanning actions of the carriage is designated in accordance with the position of an edge of the printing medium in the scanning direction, and the printhead is caused to perform the preliminary discharge. 
     The invention is particularly advantageous since preliminary discharge can be performed in a designated position regardless of the width of a printing medium. 
     Accordingly, even when the width of a printing medium is smaller than the maximum width of a carriage scanning region, preliminary discharge can be performed with a minimum necessary carriage movement, without moving the carriage to an edge of the carriage scanning region unlike in conventional apparatuses. This can minimize the time for the carriage movement and therefore minimize the printing execution time. 
     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 description, serve to explain the principles of the invention. 
     FIG. 1 is a perspective view showing an outline of the arrangement of an inkjet printer as a representative embodiment of the present invention; 
     FIG. 2 is a side sectional view in which a printhead and its vicinity of the inkjet printer explained in FIG. 1 is cut in the conveyance direction of a printing sheet; 
     FIG. 3 is a block diagram showing the configuration of a control circuit of the inkjet printer; 
     FIG. 4 is a schematic view showing the control of a printing operation and a preliminary discharge operation in the inkjet printer shown in FIG. 1, when the width of a printing sheet is much smaller than a scan enable region; 
     FIG. 5 is a schematic view showing an operation of sensing the right- and left-hand edges of a printing sheet by using a sheet sensor; 
     FIG. 6 is a flow chart showing the control of a preliminary discharge operation; 
     FIG. 7 is a perspective view showing an outline of the arrangement of a conventional inkjet printer; 
     FIG. 8 is a schematic view showing the control of a printing operation and a preliminary discharge operation in the conventional inkjet printer; and 
     FIG. 9 is a schematic view showing the control of a printing operation and a preliminary discharge operation in the conventional inkjet printer, when the width of a printing sheet is much smaller than a scan enable region. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. 
     In this specification, “print” not only represents the formation of significant information such as characters and graphics, but also broadly represents the formation of images, figures, patterns, and the like on a printing medium, or the processing of the medium, regardless of whether they are significant or insignificant and whether they are so visualized as to be visually perceivable by humans. 
     Also, a “printing medium” not only represents a paper sheet used in common printing apparatuses, but also broadly represents materials, such as cloth, a plastic film, a metal plate, glass, ceramics, wood, and leather, capable of accepting ink. 
     Furthermore, “ink” (to be also referred to as a “liquid” hereinafter) should be extensively interpreted similarly to the definition of “print” described above. That is, “ink” represents a liquid which, when applied onto a printing medium, can form images, figures, patterns, and the like, can process the printing medium, and can process ink (e.g., can solidify or insolubilize a coloring agent contained in ink applied to the printing medium). 
     FIG. 1 is a perspective view showing an outline of the arrangement of an inkjet printer as a representative embodiment of the present invention. The same reference numerals as in FIG. 7 of the prior art denote the same components in FIG. 1, and a detailed description thereof will be omitted. That is, only the characteristic elements of this embodiment will be explained. 
     In FIG. 1, reference numerals  109   a  and  110   a  denote preliminary discharge receptors having the same width as a scan enable region. These preliminary discharge receptors are open in an entire portion below a printing sheet shown in FIG.  1 . That is, the preliminary discharge receptors  109   a  and  110   a  actually communicate with each other. 
     In a preliminary discharge operation (a discharge operation for maintaining a printhead without performing printing) of this embodiment, therefore, after a carriage  103  mounting a printhead  104  passes by an edge of a printing sheet  101 , ink droplets corresponding to a predetermined number of dots are discharged into the preliminary discharge receptor  109   a  or  110   a  in a portion where the printing sheet  101  does not exist. 
     FIG. 2 is a side sectional view of the inkjet printer of FIG. 1 in the vicinity of the printhead taken along the conveyance direction of a printing sheet. 
     In FIG. 2, the same reference numerals as in FIGS. 1 and 7 denote the same parts. As shown in FIG. 2, an image is formed on the printing sheet  101  by discharging ink, stored in an ink cartridge  104  mounted on the carriage  103 , downward from a printhead  104 N attached to the lower portion of this ink cartridge  104 . Referring to FIG. 2, the carriage  103  moves back and forth, into and out from the paper, and the printing sheet  101  moves in the direction of an arrow as printing progresses. 
     Also, a sheet sensor  111  is attached to the carriage  103  and moves parallel to the printing sheet  101  along with the movement of the carriage  103 . While moving like this, the sheet sensor  111  senses a position where the printing sheet is present and a position where it is absent, thereby sensing a point at which the sheet present state changes to the sheet absent state (or vice versa), as a printing sheet edge. This sheet sensor  111  is, e.g., an optical sensor, and can find the presence of a printing sheet by irradiating it with light and sensing the reflected light. This sensor can also be an interruptive sensor having a pair of a light emitting portion and a light receiving portion arranged to sandwich a printing sheet. 
     A control configuration for executing printing control of the above apparatus will be described below. 
     FIG. 3 is a block diagram showing the configuration of a control circuit of the inkjet printer. In this FIG. 3 showing the control circuit, reference numeral  1700  denotes an interface for inputting a printing signal;  1701 , an MPU;  1702 , a ROM for storing control programs executed by the MPU  1701 ;  1703 , a DRAM for saving various data (e.g., the printing signal and printing data supplied to the printhead  104 N); and  1704 , a gate array (G.A.) for controlling the supply of printing data to the printhead  104 N. This gate array  1704  also controls data transfer between the interface  1700 , the MPU  1701 , and the RAM  1703 . 
     Reference numeral  105  denotes a carriage motor for conveying the printhead  104 N;  1709 , a conveyor motor for conveying printing sheets;  1705 , a head driver for driving the printhead  104 N; and  1706  and  1707 , motor drivers for driving the conveyor motor  1709  and the carriage motor  105 , respectively. 
     The operation of the above control configuration will be explained below. A printing signal is input to the interface  1700  and converted into printing data between the gate array  1704  and the MPU  1701 . The motor driver  1706  and  1707  are driven, and the printhead  104 N is driven in accordance with the printing data supplied to the head driver  1705 , thereby printing the data. 
     The DRAM  1703  stores data indicating the coordinates of the two ends, in the carriage scanning direction, of a printing sheet sensed by the sheet sensor  111 . This data is used in carriage movement control during preliminary discharge to be described later. 
     FIG. 4 is a schematic view showing the control of a printing operation and a preliminary discharge operation in the inkjet printer shown in FIG. 1, when the width of a printing sheet is much smaller than a scan enable region. As shown in FIG. 4, the width of a printing sheet  100  is much smaller than a maximum scan enable region (SMAX). 
     When an image such as “A . . . A” is to be printed on this printing sheet  100 , the carriage  103  mounting the printhead explained in FIGS. 1 and 2 prints the image by moving back and forth within the range of a line  501  from a point x″ to a point y″. This line  501  is made up of a printing region in which an image is formed by discharging ink onto the printing sheet, and acceleration (deceleration) regions a″ and b″. 
     To form an image in the printing region, the carriage  103  must be driven at a constant speed. When the carriage  103  is at rest at a point  502  in FIG. 4, this carriage  103  is accelerated in the region a″ to move at the constant speed from a point  503 . Also, to stop the carriage  103 , which has moved at the constant speed to a point  504 , from the point  504 , this carriage  103  is decelerated to a point  505  in the region b″. When the image is printed by moving the carriage  103  from the point y″ to the point x″, the region b″ is an acceleration region, and the region a″ is a deceleration region. 
     Accordingly, printing region+region a″+region b″ is the carriage moving range corresponding to a printing operation of one scan. 
     When a preliminary discharge operation is to be executed, the carriage  103  is moved to the position above the preliminary discharge receptor  109   a  ( 110   a ) having the width covering the whole maximum scan enable region, and ink is discharged. 
     When preliminary discharge is to be performed at the right-hand edge of the printing sheet  100  in FIG. 4, the carriage  103  is moved to a point  506 . Since the preliminary discharge receptor  109   a  ( 110   a ) covers the entire scanning region of the carriage  103 , a preliminary discharge operation is possible provided that the ink discharge orifices of the printhead  104 N are outside the edge of the printing sheet. However, a preliminary discharge operation must be performed in a position separated from the printing sheet to a certain extent, so that ink discharged during the operation does not stick to the printing sheet. For this reason, a preliminary discharge operation is executed by moving the carriage  103  to the point  506 . A region c″ indicates a moving region from the carriage stop position (point  502 ) to the designated preliminary discharge position (point  506 ). 
     When preliminary discharge is to be performed at the left-hand edge of the printing sheet  100  in FIG. 4, the carriage  103  is moved to a point  507 . Similar to the preliminary discharge performed at the right-hand edge of the printing sheet, a preliminary discharge operation must be performed in a position separated from the printing sheet to a certain extent, so the carriage  103  is moved to the point  507 . A region d″ indicates a moving region from the carriage stop position (point  505 ) to the designated preliminary discharge position (point  507 ). 
     As can be seen by comparing the moving region d″ to the position of preliminary discharge shown in FIG. 4 with the moving region d′ to the position of preliminary discharge shown in FIG. 9 of the prior art, the moving region d″ is shorter than the moving distance d′. This shortens the time necessary for the carriage movement required to execute a preliminary discharge operation during printing. Consequently, the time to the completion of printing can be further shortened. 
     The control of a preliminary discharge operation using a preliminary discharge receptor having a width covering the entire scanning region of the carriage  103  explained above will be described below with reference to FIGS. 5 and 6. 
     FIG. 5 is a view showing how to sense the coordinates of the right- and left-hand edges of a printing sheet in order to move the printhead to a preliminary discharge position by using the sheet sensor  111 . 
     FIG. 6 is a flow chart showing the control of a preliminary discharge operation. 
     This flow chart explains control when the printhead  104 N prints data by discharging ink while moving in the direction of an arrow shown in FIG. 5 and, after that, preliminary discharge is performed at the left-hand edge of scanning. 
     First, in step S 10 , the carriage  103  mounting the printhead  104 N starts moving in the direction of the arrow from a point P 1  shown in FIG.  5 . At this time, the sheet sensor  111  has not sensed the printing sheet  101  yet. 
     Next, in step S 20 , the sheet sensor  111  senses the printing sheet  101  when the carriage  103  mounting the printhead  104 N moves to the left in FIG. 5 to reach a position P 2 . Since the sheet absent state changes to the sheet present state, it is determined that the position P 2  is the right-hand edge of the printing sheet  101 . After that, in step S 30  the printhead  104 N discharges ink to print data. Although the printhead  104 N further moves to the left in FIG. 5 during this printing, the sheet sensor  111  keeps sensing the presence of the printing sheet  101 . This state of sensing continues until the carriage  103  mounting the printhead  104 N reaches a position P 4  from the position P 2  via a position P 3 . 
     In step S 40 , the sheet sensor  111  can no longer sense the printing sheet  101  when the carriage  103  mounting the printhead  104 N has reached the position P 4 . Since the sensor status changes from the sheet present state to the sheet absent state, it is determined that the position P 4  is the left-hand edge of the printing sheet  101 . The coordinates of this point P 4  are stored in the DRAM  1703 . 
     A preliminary discharge operation is started from this point. That is, in step S 50 , the carriage  103  mounting the printhead  104 N is further moved to the left in FIG. 5 to a position P 5 , so that preliminarily discharged ink droplets do not stick to the printing sheet. At the positions P 4  and P 5 , the sheet sensor  111  cannot sense any sheet because the printing sheet  101  does not exist below the sensor any longer. Note that the distance between these positions P 4  and P 5  corresponds to the moving region d″ shown in FIG.  4 . 
     In step S 60 , the printhead  101 N performs preliminary discharge at the position P 5 . 
     In the above explanation, preliminary discharge is performed after the printhead  104 N has printed data by moving in the direction of the arrow shown in FIG.  5 . However, similar control is performed even when preliminary discharge is performed after the printhead has printed data by moving in the opposite direction. In this case, the coordinates of the position P 2  are stored in the DRAM  1703 , and preliminary discharge is performed at a point (e.g., the position P 1 ) separated a predetermined distance from the position P 2 . 
     Also, the coordinate system for specifying the position of the printhead is a one-dimensional coordinate system which extends along the carriage moving direction and has a predetermined position (e.g., the home position of the carriage) as its origin. 
     In the above-mentioned embodiment, therefore, on the basis of the coordinates of the right- and left-hand edges of a printing sheet sensed by the sheet sensor, the printhead is separated from the printing sheet, i.e., from the sensed coordinates, by a distance necessary for preliminary discharge, and then preliminary discharge is performed. Accordingly, even when the width or position of the printing sheet changes, a preliminary discharge operation can be executed in an optimum position. This can minimize the carriage movement required for the preliminary discharge operation. 
     Consequently, the printing time can be minimized. 
     Furthermore, the printing sheet sensor attached to the carriage senses an actual printing sheet width, and the sensed coordinate position is stored. This makes it possible to accurately control the moving distance from the printing sheet edge to the preliminary discharge operation execution position. Hence, more accurate control by which the scanning region of the printhead is minimized can be performed. 
     In the above embodiment, a preliminary discharge position is separated a predetermined distance from the edge of a printing sheet. However, the present invention is not limited to this embodiment. For example, preliminary discharge can also be performed while the printhead is scanned farther from this position separated by the predetermined distance. Alternatively, preliminary discharge can be performed before or after the scanning directions of the printhead are switched in a position farther from the position separated by the predetermined distance. When this is the case, ink can be dispersed without being concentrated to a predetermined position of the ink receptor for receiving preliminary discharge. 
     The above designated predetermined distance can also be changed in accordance with the printing conditions. For example, the higher the discharge frequency of preliminary discharge, the further the ink scatters to the surroundings. Therefore, in this case it is possible to designate a longer distance from the edge of a printing sheet than when the discharge frequency is relatively low. 
     Furthermore, in the above embodiment, the preliminary discharge receptor having a width covering the entire scanning region of the carriage is taken as an example. However, the present invention is not restricted to this embodiment. For example, if printing sheets of several fixed forms (e.g., A3, A4, B4, and B5 sheets) are used as printing sheets, a preliminary discharge receptor need not have a width covering the whole scanning region. That is, it is possible to form a plurality of preliminary discharge receptors corresponding to positions separated a predetermined distance from the edges of these fixed-form sheets. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.