Abstract:
An image forming apparatus and an image forming method are provided. The image forming apparatus includes a conveyer belt, a print head, a cutter, at least first and second rollers, and a controller. The conveyer belt conveys print media. The print head prints an image on the print media conveyed by the conveyer belt. The cutter is disposed downstream of the print head to cut the print media. The rollers are serially arranged between the print head and the cutter, and are configured to rotate the conveyer belt in the conveyance direction to convey the print media toward the cutter. The controller individually controls rotation speeds of the rollers to create a difference in rotation speed between the first and second rollers sufficient to make the print media bend between the first and second rollers while the cutter cuts.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2013-046659, filed on Mar. 8, 2013, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Example embodiments of the present invention generally relate to an image forming apparatus and image forming method capable of printing an image on a roll of paper. 
         [0004]    2. Background Art 
         [0005]    Label printers are known as a form of image forming apparatuses that print text or images on the surface of various kinds of labels such as point of purchase (POP) labels, price tags, and bar-code labels. Such labels are used for products on the market. Usually, such labels are adhesive labels backed with “release paper” (also called a “liner” or “separator”). In recent years, there is a trend toward using “linerless label paper” without “release paper (liner)” in order to save resources. Such linerless label paper is rolled like sticky tape, and an image forming apparatus (printer) that prints text or images on the linerless label paper is referred to as a linerless label printer. 
         [0006]    Label printers are also known as printers that handle a roll of paper or other recording media in a similar manner to linerless label printers. In such label printers, a roll of labels is usually cut before text or images are to be printed thereon. This is because label printers can perform ordinary printing if the label is already cut before performing printing operation, which is practical and efficient. By contrast, it is desired that a label in a linerless label printer be cut on the downstream side of the conveyance path because it is difficult to peel off adhesive sheets. For this reason, a cutter is arranged on the downstream side of the conveyance path and a print head is arranged on the upstream side. Note that the sheet conveying process needs to be stopped when a roll label is to be cut, at least at a position where the roll label is cut, in both linerless label printer and label printer. 
       SUMMARY 
       [0007]    Disclosed embodiments provide an improved image forming apparatus and image forming method. The image forming apparatus includes a conveyer belt, a print head, a cutter, at least first and second rollers, and a controller. The conveyer belt conveys print media. The print head prints an image on the print media conveyed by the conveyer belt. The cutter is disposed downstream of the print head to cut the print media. The rollers are serially arranged between the print head and the cutter, and are configured to rotate the conveyer belt in the conveyance direction to convey the print media toward the cutter. The controller individually controls rotation speeds of the rollers to create a difference in rotation speed between the first and second rollers sufficient to make the print media bend between the first and second rollers while the cutter cuts. 
         [0008]    The image forming method includes conveying, on a conveyer belt rotated by at least first and second rollers, print media on which an image is formed by a print head, cutting the print media with a cutter positioned downstream of the print head, and controlling speed of the conveying, by individually controlling rotation speeds of the rollers to create a difference in rotation speed between the first and second rollers sufficient to make the print media bend between the first and second rollers while the cutting is being performed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings. 
           [0010]      FIG. 1  is a schematic diagram illustrating a basic configuration of a label printer. 
           [0011]      FIG. 2  is a perspective view illustrating a basic configuration of a label printer. 
           [0012]      FIG. 3  illustrates a configuration of the label printer according to a first example embodiment of the present invention. 
           [0013]      FIG. 4  illustrates another configuration of the label printer according to the first example embodiment of the present invention. 
           [0014]      FIGS. 5A and 5B  illustrate the bending permissible space of a roll label according to an example embodiment of the present invention. 
           [0015]      FIG. 6  illustrates how the bending direction of a sheet is controlled according to an example embodiment of the present invention. 
           [0016]      FIG. 7  illustrates how the driving speed of each roller is controlled while a printing operation is performed, according to the first example embodiment of the present invention. 
           [0017]      FIGS. 8A and 8B  illustrate the operation sequences of a printing controller, a cutter conveyance controller, and a cutter controller between the start and end of a printing operation, in the label printer according to the first example embodiment of the present invention. 
           [0018]      FIG. 9  illustrates the driving speed of each roller of an ink-jet printer that alternately performs a main scanning and a sub-scanning movement, according to a second example embodiment of the present invention. 
           [0019]      FIG. 10  illustrates how the driving speed of each roller is controlled while a printing operation is performed, according to a third example embodiment of the present invention. 
           [0020]      FIGS. 11A and 11B  illustrate the operation sequences of a printing controller, a cutter conveyance controller, and a cutter controller between the start and end of a printing operation in the label printer according to the third example embodiment of the present invention. 
           [0021]      FIG. 12  illustrates a configuration of the label printer according to a fourth example embodiment of the present invention. 
           [0022]      FIG. 13  illustrates the driving speeds of a conveyance roller R 2  and a cutter conveyance roller R 3  when the first cutter cuts each page according to the speed control illustrated in  FIG. 7 . 
           [0023]      FIG. 14  illustrates the driving speeds of a conveyance roller R 2  and a cutter conveyance roller R 3  when the first cutter cuts one page for every four pages. 
       
    
    
       [0024]    The accompanying drawings are intended to depict exemplary embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
       DETAILED DESCRIPTION 
       [0025]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0026]    In describing example embodiments shown in the drawings, specific terminology is employed for the sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have the same structure, operate in a similar manner, and achieve a similar result. 
         [0027]    Example embodiments of the present invention will be described below in detail with reference to the drawings. 
         [0028]      FIG. 1  is a schematic diagram illustrating a basic configuration of a label printer that serves as an image forming apparatus.  FIG. 2  is a perspective view illustrating a basic configuration of the label printer of  FIG. 1 . For clarity, in  FIG. 2 , a main cabinet, structural elements, and stays are not illustrated. 
         [0029]    A label printer  1  illustrated in  FIG. 1  is an ink-jet printer that performs printing by ejecting ink on a linerless label that does not have a release paper (i.e., liner). The label printer  1  includes a rolled label sheet (i.e., roll label)  40 , a print head  2  that ejects ink, and a carriage  3  provided with the print head  2  that moves in the main scanning direction as guided by a guide rail. Further, the label printer  1  includes a conveyance belt  4  that conveys label paper (print media) L fed from the roll label  40 , a fan  5  that adsorbs a label on the conveyance belt  4  while the label is being conveyed, and a cutter  6  that cuts the label paper L on which printing has been performed. Note that the surface of the conveyance belt  4  is coated with an anti-adhesive so as not to become sticky due to the adhesive of the label paper L. A counter roller  7  and a pressure roller  8  press the label paper L against the conveyance belt  4 , and conveyance force is given to the label paper L. 
         [0030]    The label paper L fed from the roll label  40  is sent beneath the carriage  3  by the conveyance belt  4 , which rotates as driven by a conveyance roller R 1 . The carriage  3  performs a printing operation by scanning the label paper L that has reached the area beneath the carriage  3 , in the direction perpendicular to the conveyance direction. After the printing operation, the label paper L is sent to an area under the cutter  6 , and the cutter  6  cuts the label paper L when the label paper L is at a specified length. The leading end of the label paper L on the downstream side of the cut position is ejected outside the main body, and the trailing end of the label paper L is stopped and held by an ejection roller  11 . It is to be noted that the printing operation of the print head  2  may be performed by employing a line head system instead of a system such as that illustrated in  FIG. 1 . 
       First Embodiment 
       [0031]      FIGS. 3 and 4  illustrate the configuration of a label printer according to a first embodiment. In addition to the basic configuration illustrated in  FIGS. 1 and 2 , an additional configuration is added before the cutter  6 . In the description of  FIGS. 3 and 4 , the use of a line head system is assumed. Accordingly, an element such as a carriage, which is illustrated in  FIGS. 1 and 2 , is omitted. In  FIGS. 3 and 4 , the roll label  40  is conveyed by the conveyance belt  4  that is stretched by the conveyance roller R 1  as a drive roller and a conveyance roller R 2  as a driven roller, and a printing operation is performed at a print position  30  beneath the print head  2 . The print head  2  is fixed as a line head, and ejects ink in a synchronized manner with the progression of a sheet. Alternatively, however, the print head  2  may be provided with a carriage that moves in the main scanning direction, as described with reference to  FIGS. 1 and 2 . 
         [0032]    A cutter conveyance roller R 3  (second roller) is arranged on the downstream side of the conveyance belt  4  (i.e., farther downstream of the conveyance roller R 2 ). In other words, the cutter conveyance roller R 3  is arranged near the cutter  6 . The cutter conveyance roller R 3  sends the label paper L to the cutter  6 . Thus, the conveyance unit of the label printer  1  is composed of the conveyance roller R 1 , the conveyance roller R 2  (first roller), the conveyance belt  4 , and the cutter conveyance roller R 3  (second roller). The sheet (label paper L) is a linerless paper, and thus its adhesive surface touches the conveyance belt  4 . The label paper L moves towards the cutter  6  while the conveyance roller R 1  is being driven, and the label paper L stops moving when the conveyance roller R 1  stops. The conveyance roller R 1  and the conveyance roller R 2  (first roller) that is driven by the conveyance roller R 1  are always driven while a printing operation is being performed. 
         [0033]    As illustrated in  FIG. 3 , the label printer  1  includes motors M 1 , and M 2 , and M 3  that drive the conveyance roller R 1 , the cutter conveyance roller R 3 , and the cutter  6 , respectively. Moreover, the label printer  1  includes a driver D that drives a piezoelectric actuator provided for the print head  2 . Alternatively, the conveyance roller R 2  may be configured to be a drive roller that is driven by the motor M 1 , and the conveyance roller R 1  may be a driven roller in the driving mechanism of the conveyance belt  4 . Moreover, the label printer  1  includes a controller Ctr that mainly controls the conveying and printing operation of the label paper L. The controller Ctr includes a printing controller  50 , a cutter conveyance controller  51 , and a cutter controller  52 . 
         [0034]    The printing controller  50  controls the motor M 1  and the driver D 1  to control the printing operation. The cutter conveyance controller  51  controls the motor M 2  to bend a label between the conveyance roller R 2  and the cutter conveyance roller R 3 . The cutter controller  52  controls the motor M 3  to operate the cutter  6 . The controller Ctr includes a CPU (central processing unit), a ROM (read only memory), and a RAM (random access memory), and implements the printing controller  50 , the cutter conveyance controller  51 , and the cutter controller  52  by reading and executing software in the CPU. 
         [0035]    The cutter conveyance roller R 3  is driven when a printing operation is progressing, but the cutter conveyance roller R 3  stops when a cutting operation is being performed. By contrast, the conveyance roller R 1  (and the conveyance belt  4 ) keeps moving, and thus a sheet is bent in bending permissible space  20  arranged between the conveyance roller R 2  and the cutter conveyance roller R 3 , as illustrated in  FIG. 4 .  FIG. 4  illustrates a state in which the label paper L is bent in the configuration of  FIG. 3 , where the label paper L is bent between the conveyance roller R 2  and the cutter conveyance roller R 3 . Because the label paper L may stick to other members, the bending permissible space  20  of the label printer  1  where bending is permissible is determined by the layout of the printer  1 . In  FIG. 4 , the horizontal and vertical dimensions of the bending permissible space  20  are L 1  and L 2 , respectively. 
         [0036]      FIGS. 5A and 5B  illustrate the bending permissible space  20  of the roll label  40 . As illustrated in  FIG. 5A , the horizontal and vertical dimensions of the bending permissible space  20  are defined by L 1  and L 2 , respectively, and the maximum length of a sheet permissible in the bending permissible space  20  is assumed to be L max . The minimum length of sheet L min  becomes equivalent to L 1  when no bending is present, that is, when the sheet is stretched between rollers R 2  and R 3 . Accordingly, the maximum permissible bending amount M is calculated by Formula 1 below. 
         [0000]        M=L   max   −L   min =√{square root over ( L   1   2 +(2 L   2 ) 2 )}− L   1   [Formula 1]
 
         [0000]    Here, the bending amount refers to the length of a sheet that is placed in the bending permissible space  20 . As illustrated in  FIG. 5B , assuming that the length of a sheet when the sheet is bent is L 2  and the length of a sheet becomes equivalent to L 1  when the sheet is not bent, the bending amount is expressed as “L 2 −L 1 ”. The maximum permissible bending amount is a threshold that is defined by a person who designed the product, and indicates the maximum value for permissible bending amount (i.e., the smallest value beyond which leads to conveyance failure). 
         [0037]    Excessive bending can be prevented by controlling the difference in linear velocity (i.e., difference in rotation speed) between the conveyance roller R 2  and the cutter conveyance roller R 3  to meet the following equation. 
         [0000]    
       
      
       V 
       diff 
       *T 
       cut 
       ≦L 
       max  
      
     
         [0000]    Here, V diff , T cut , and L max  indicate a difference in the linear velocity of rollers, the length of time during which a cut operation is performed, and the maximum length of a sheet permissible in bending permissible space, respectively. 
         [0038]      FIG. 6  illustrates how the bending direction of a sheet is controlled according to an example embodiment of the present invention. There are some cases in which bending is not easily achieved due to the inherent stiffness of some kinds of label paper L and consequently the label paper L bends upward.  FIG. 6  illustrates how the direction in which the label paper L is bent is controlled to the downward direction. More specifically, a bending-direction controlling roller R 4  is provided to bend a sheet downward (control the bending direction of a sheet) between the conveyance roller R 2  and the cutter conveyance roller R 3 . The bending-direction controlling roller R 4  bends the label paper L downward by using its own weight or downward spring-load. It is desired that the bending-direction controlling roller R 4  be fixed to the upper side of the sheet conveyance path when it is not used. 
         [0039]      FIG. 7  illustrates how the driving speed of each roller is controlled while a printing operation is performed, according to an example embodiment of the present invention. As illustrated in  FIG. 7 , the controller Ctr controls the driving speed of the conveyance roller R 2  and the cutter conveyance roller R 3  as follows. The conveyance roller R 2  may be directly driven, or may be indirectly driven via the conveyance belt  4 . Once a printing operation starts, the printing controller  50  and the cutter conveyance controller  51  start driving the conveyance roller R 2  and the cutter conveyance roller R 3 , and increase the driving speed of both the conveyance roller R 2  and the cutter conveyance roller R 3  to print speed V p . Next, the cutter conveyance controller  51  starts reducing the speed of the cutter conveyance roller R 3  at time t 1  in order to stop a sheet at the position where a cutting operation is performed. Then, the cutter conveyance controller  51  stops the cutter conveyance roller R 3  at time tb. 
         [0040]    The printing controller  50  starts reducing the speed of the conveyance roller R 2  at time ta, which is later than time t 1 , and reduces the speed to cutting operational speed V c  at time tb. The bending amount of the label paper L is controlled to be equal to or less than a threshold amount when the cutting operational speed V c  is employed. The speed of the conveyance roller R 2  is reduced at time ta, which is later than when the speed of the cutter conveyance roller R 3  is reduced at time t 1 , because a sheet stretches if the speed of the conveyance roller R 2  is reduced before the speed of the cutter conveyance roller R 3  is reduced. When the cutter conveyance roller R 3  stops moving and the conveyance roller R 2  reaches the cutting operational speed V c  at time tb, the cutter controller  52  instructs the cutter  6  to perform a cutting operation before time Tc passes. The cutter conveyance roller R 3  stops moving during the period of time Tc. When the cutting operation is complete, the printing controller  50  returns the speed of the conveyance roller R 2  to the print speed V p . 
         [0041]    Then, the cutter conveyance controller  51  returns the speed of the cutter conveyance roller R 3  to the print speed V p  by time t 2 , and increases the speed of the cutter conveyance roller R 3  to a bending straightening speed V r  that is faster than the print speed V p  by time td in order to straighten a sheet. After time t 2 , bending is straightened by the amount of the product of elapsed time and the difference in speed between the conveyance roller R 2  and the cutter conveyance roller R 3 . When the bending accumulated during the cutting operation is all straightened after time td, the cutter conveyance controller  51  returns the speed of the cutter conveyance roller R 3  to the print speed V p  by time t 3 . 
         [0042]    The print speed V p , the cutting operational speed V c , and the bending straightening speed V r  are described in detail below. 
         [0043]    The print speed V p  indicates the conveyance speed of a sheet when normal printing operation is performed, and is the regular printing speed of a printer as designed. The cutting operational speed V c  indicates the conveyance speed of the conveyance roller R 2  while a cutting operation is in progress. The cutting operational speed V c  should be such that the bending occurred during the time between t 1  and t 2  will be equal to or less than the maximum permissible bending amount M, calculated in Formula 1 as described above. 
         [0044]    More specifically, the bending amount of a sheet caused in a cutting operation is the accumulated value of the difference in speed between time t 1  at which the cutter conveyance roller R 3  starts reducing speed and time t 2  at which the speed of the cutter conveyance roller R 3  is increased after the cutting operation and exceeds the print speed V p . As the acceleration and deceleration curves or the like of the conveyance roller R 2  and the cutter conveyance roller R 3  are designed in advance, the cutting operational speed V c  is calculated by using Formula 2 below. 
         [0000]      ∫ t1   t2 ( V   2 ( t )− V   3 ( t )) dt≦M   [Formula 2]
 
         [0000]    Here, speed V 2 (t), V 3 (t), and M indicate the speed of the conveyance roller R 2  at time t, the speed of the cutter conveyance roller R 3  at time t, and the maximum permissible bending amount, respectively. 
         [0045]    Alternatively, the cutting operational speed V c  may be approximated from Formula 3 below, ignoring the acceleration and deceleration curve. 
         [0000]        V   c   *T   c   ≦M   [Formula 3]
 
         [0046]    The bending straightening speed V r  indicates the conveyance speed of the cutter conveyance roller R 3  when the bending portion of a sheet is to be straightened after the cutting operation. As the accumulated value of the difference in speed between the conveyance roller R 2  and the cutter conveyance roller R 3  during the time between times t 2  and t 3  is the amount of bending to be straightened, the bending straightening speed V r  can be calculated by Formula 4 as follows. 
         [0000]      ∫ t1   t2 ( V   2 ( t )− V   3(t) ) dt =∫   t2   t3 ( V   3(t)   −V   2(t) ) dt   [Formula 4]
 
         [0047]    Alternatively, the bending straightening speed V r  may be approximated from Formula 5 below, ignoring the acceleration and deceleration curve. 
         [0000]        V   c   *T   c   =V   r *( t 3− t 2)
 
         [0048]    As described above, the conveyance roller R 2  is not stopped even when a cutting operation is being performed by the cutter  6 . Accordingly, the printing operation (e.g., image forming process and sheet conveying process) does not stop while a cutting operation is in progress. 
         [0049]      FIGS. 8A and 8B  illustrate the operation sequences of the printing controller  50 , the cutter conveyance controller  51 , and the cutter controller  52  between the start and end of a printing operation in the label printer  1 , according to the first embodiment. As described above, the printing controller  50  controls the ejecting process at the print head  2 , the driving process of the conveyance roller R 2 , and the controlling process of the bending-direction controlling roller R 4 . The cutter conveyance controller  51  controls the driving process of the cutter conveyance roller R 3 . The cutter controller  52  controls the operation of the cutter  6 . 
         [0050]    (1) The printing controller  50  initiates a printing process, and increases the speed of the conveyance roller R 2  to the print speed V p  (S 001 ). The printing controller  50  instructs the cutter conveyance controller  51  to start a printing process, and the cutter conveyance controller  51  increases the driving speed of the cutter conveyance roller R 3  to print speed V p  (S 002 ). 
         [0051]    (2) When the type of a sheet (label paper L) is a specified type of sheet that requires the use of the bending-direction controlling roller R 4 , the printing controller  50  initiates the operation of the bending-direction controlling roller R 4  (S 003 ). Next, the printing controller  50  controls the print head  2  to eject ink on a sheet (S 004 ). 
         [0052]    (3) The cutter conveyance controller  51  monitors the position of a sheet, and detects a sheet that is approaching the position where a cutting process is performed (S 005 ). The cutter conveyance controller  51  provides the printing controller  50  with notification that a cutting operation will be performed, while stopping the cutter conveyance roller R 3  (S 006 ). 
         [0053]    (4) When such a cutting operation notification is received, the printing controller  50  decreases the speed of the conveyance roller R 2  to the cutting operational speed V c  (S 007 ). 
         [0054]    (5) When the cutter conveyance roller R 3  is stopped, the cutter conveyance controller  51  instructs the cutter controller  52  to perform a cutting operation, and the cutter controller  52  performs the cutting operation as instructed (S 008 ). 
         [0055]    (6) When the cutting operation is complete, the cutter controller  52  provides the cutter conveyance controller  51  and the printing controller  50  with notification that the cutting operation is complete (S 009  and S 010 ). 
         [0056]    (7) When the notification is received, the printing controller  50  increases the speed of the conveyance roller R 2  to the print speed V p  (S 011 ). The cutter conveyance controller  51  increases the speed of the cutter conveyance roller R 3  to the bending straightening speed V r  (S 012 ). Because the bending straightening speed V r  is faster than the print speed V p , the amount of bending accumulated due to the cutting operation is straightened. When the bending is straightened, the cutter conveyance controller  51  returns the speed of the cutter conveyance roller R 3  to the print speed V p  (S 013 ). 
         [0057]    (8) Every time a cutting process is performed, the steps (3) to (7) are repeated. 
         [0058]    (9) When the printing operation is complete, the cutter conveyance controller  51  conveys a sheet to the position at which the previous cutting process was performed, and stops the cutter conveyance roller R 3  (S 014 ). Subsequently, the cutter conveyance controller  51  instructs the cutter controller  52  to perform a cutting process, and the cutter controller  52  performs a final cutting process (S 015 ). 
         [0059]    (10) When the printing process is complete, the printing controller  50  conveys a sheet to the position at which the previous cutting process was performed and then stops the conveyance roller R 2  (S 016 ). When the bending-direction controlling roller R 4  was used in the cutting process, the bending-direction controlling roller R 4  is set to a non-use state (S 017 ). 
       Second Embodiment 
       [0060]    In the second embodiment, the speed of the conveyance roller R 2  and the cutter conveyance roller R 3  of  FIG. 7  is controlled, and it is assumed that a printer employs a line head system or the like. Alternatively, however, a configuration that is employed in various ink-jet printers, where a main scanning and a sub-scanning are alternately performed, may be employed. 
         [0061]      FIG. 9  illustrates the driving speed of the conveyance roller R 2  and the cutter conveyance roller R 3  of an ink-jet printer that alternately performs a main scanning and a sub-scanning movement, according to the second embodiment. In  FIG. 7 , the conveyance roller R 2  and the cutter conveyance roller R 3  operate at the print speed V p , which is slow. By contrast, a sheet is not conveyed while a main scanning is being performed in the configuration of  FIG. 9 , and thus the conveyance roller R 2  and the cutter conveyance roller R 3  stop moving while a main scanning is being performed. While a sub-scanning is being performed, the conveyance roller R 2  and the cutter conveyance roller R 3  operate to convey a sheet in a similar manner to each other. In  FIG. 7 , the speed of the conveyance roller R 2  is reduced to the cutting operational speed V c  while a cutting operation is being performed. By contrast, intervals where the conveyance roller R 2  is not driven are extended in the configuration of  FIG. 9 . Because the cutter conveyance roller R 3  does not operate while a cutting operation is being performed, bending occurs during the cutting operation. 
         [0062]    In the configuration of  FIG. 9 , the number of times a sub-scanning is performed during the cutting operation is restricted to satisfy Formula 6. The intervals where the conveyance roller R 2  is not driven are increased to satisfy Formula 6. 
         [0000]      (Number of times sub-scanning is performed)*(Amount of sheet conveyance due to sub-scanning)≦ M  (Maximum permissible bending amount)  [Formula 6]
 
         [0063]    After the cutting operation, a bending straightening operation is performed. In this configuration, the driving speed of the cutter conveyance roller R 3  and the period during which the cutter conveyance roller R 3  is driven are controlled to satisfy Formula 7 below. 
         [0000]      (Number of times sub-scanning (R2) is performed during cutting operation and bending straightening operation)*(Amount of sheet conveyance due to sub-scanning)=(Amount of sheet conveyance due to operation of cutter conveyance roller R3 during bending straightening operation)  [Formula 7]
 
         [0064]    The process returns to (1) (i.e., S 001  and S 002 ) after the bending straightening operation is completed, and the conveyance roller R 2  and the cutter conveyance roller R 3  operate to convey a sheet in a similar manner to each other. 
       Third Embodiment 
       [0065]    In the first embodiment, after the speed of the conveyance roller R 2  is reduced for a cutting operation, the speed of the cutter conveyance roller R 3  is immediately or shortly reduced to the cutting operational speed V c . However, when the printing quality of a printer with a line head system is affected (e.g., streaks appear) due to a change in the conveyance speed, the acceleration and deceleration of the conveyance roller R 2  may be performed during a period where no printing is performed in the interval between two pages. The length of time it takes to reach the interval between two pages after a sheet (i.e., label paper L) has reached a cutting position and the cutter conveyance roller R 3  has stopped varies every time depending on the page length or the like, and thus it is necessary to calculate a value for cutting operational speed V c ′ when the cutter conveyance roller R 3  stops. The cutting operational speed V c ′ is approximated by Formula 8 below. 
         [0000]      ( t 1′− t 1)* V   p +( t 2′− t 1′)* V   c   ′≦M   [Formula 8]
 
         [0066]    t 1 ′: time at which the speed of the conveyance roller R 2  is reduced 
         [0067]    t 2 ′: time at which cutting operation is complete and the speed of the conveyance roller R 2  is increased 
         [0068]    When the cutting operational speed V c ′ calculated by Formula 8 has a negative value, the printing operation is stopped or the conveyance roller R 2  is stopped as the cutter conveyance roller R 3  is stopped as in  FIG. 9  without waiting in the interval between two pages. 
         [0069]      FIGS. 11A and 11B  illustrate the operation sequences of each controlling element in the label printer  1 , according to the third embodiment. In the operation sequences of  FIGS. 11A and 11B , the following steps are performed. 
         [0070]    (1) The printing controller  50  initiates a printing process, and increases the speed of the conveyance roller R 2  to the print speed V p  (S 101 ). The printing controller  50  instructs the cutter conveyance controller  51  to start a printing process, and the cutter conveyance controller  51  increases the driving speed of the cutter conveyance roller R 3  to the print speed V p  (S 102 ). 
         [0071]    (2) When the type of a sheet (label paper L) is a specified type of sheet that requires the use of the bending-direction controlling roller R 4 , the printing controller  50  initiates the operation of the bending-direction controlling roller R 4  (S 103 ). Next, the printing controller  50  controls the print head  2  to eject ink on a sheet (S 104 ). 
         [0072]    (3) The cutter conveyance controller  51  monitors the position of a sheet, and detects a sheet that is approaching the position where a cutting process is performed (S 105 ). The cutter conveyance controller  51  calculates cutting operational speed V c ′ by using Formula 8 as above, and waits in the interval between two pages while performing a printing operation (S 106 ). 
         [0073]    (4) When the interval between two pages is reached, the printing controller  50  decreases the speed of the conveyance roller R 2  to the cutting operational speed V c ′ (S 107 ). The cutter conveyance controller  51  provides the printing controller  50  with notification that a cutting operation will be performed, while stopping the cutter conveyance roller R 3  (S 108 ). 
         [0074]    (5) When the cutter conveyance roller R 3  is stopped, the cutter conveyance controller  51  instructs the cutter controller  52  to perform a cutting operation, and the cutter controller  52  performs the cutting operation as instructed (S 109 ). 
         [0075]    (6) When the cutting operation is complete, the cutter controller  52  provides the cutter conveyance controller  51  and the printing controller  50  with notification that the cutting operation is complete (S 110  and S 111 ). 
         [0076]    (7) When the notification is received, the printing controller  50  increases the speed of the conveyance roller R 2  to the print speed V p  (S 112 ). The cutter conveyance controller  51  increases the speed of the cutter conveyance roller R 3  to the bending straightening speed V r  (S 113 ). Because the bending straightening speed V r  is faster than the print speed V p , the amount of bending accumulated due to the cutting operation is straightened. When the bending is straightened, the cutter conveyance controller  51  returns the speed of the cutter conveyance roller R 3  to the print speed V p  (S 114 ). 
         [0077]    (8) Every time a cutting process is performed, the steps (3) to (7) are repeated. 
         [0078]    (9) When the printing operation is complete, the cutter conveyance controller  51  conveys a sheet to the position at which the previous cutting process was performed, and stops the cutter conveyance roller R 3  (S 115 ). Subsequently, the cutter conveyance controller  51  instructs the cutter controller  52  to perform a cutting process, and the cutter controller  52  performs a final cutting process (S 116 ). 
         [0079]    (10) When the printing process is complete, the printing controller  50  conveys a sheet to the position at which the previous cutting process was performed and then stops the conveyance roller R 2  (S 117 ). When the bending-direction controlling roller R 4  was used in the cutting process, the bending-direction controlling roller R 4  is set to a non-use state (S 118 ). 
       Fourth Embodiment 
       [0080]      FIG. 12  illustrates a configuration of the label printer according to the fourth embodiment. If all the pages are cut by a single cutter in the cutting operation referred to in  FIG. 7 , the conveyance roller R 2  and the cutter conveyance roller R 3  are constantly accelerated and decelerated. This leads to low productivity. In order to address this problem, productivity may be improved by providing two cutters (i.e., first cutter  6   a  and second cutter  6   b ), as illustrated in  FIG. 12 . The first cutter  6   a  on the upstream side of the conveyance direction cuts one sheet for every certain number of sheets and the second cutter  6   b  on the downstream side of the conveyance direction cuts the rest of the sheets. 
         [0081]      FIG. 13  illustrates the driving speeds of the conveyance roller R 2  and the cutter conveyance roller R 3  when the first cutter  6   a  cuts each page according to the speed control illustrated in  FIG. 7 .  FIG. 14  illustrates the driving speeds of the conveyance roller R 2  and the cutter conveyance roller R 3  when the first cutter  6   a  cuts one page for every four pages. In the case of  FIG. 14 , the number of times the speed of the conveyance roller R 2  is reduced is decreased compared with the case of  FIG. 13 , and the printing speed improves accordingly. After sheets have passed the first cutter  6   a,  these sheets are no longer influenced by the printing operation. Thus, the sheets can be conveyed at high speed between the first cutter  6   a  and the second cutter  6   b,  and the second cutter  6   b  cuts the rest of the pages. 
       Fifth Embodiment 
       [0082]    In the first embodiment, the state of maximum bending as illustrated in  FIG. 5  is assumed, and the maximum permissible bending amount is calculated by using Formula 1. However, a margin may be provided as in Formula 1′ below. 
         [0000]        M=L   max   −L   min =√{square root over ( L   1   2 +(2 L   2 ) 2 )}− L   1   −m   [Formula 1′]
 
         [0000]    Here, “m” indicates a margin. Alternatively, a threshold may be calculated by performing an examination with an actual device, instead of using the assumption of  FIG. 5 . 
         [0083]    According to the configuration described above, efficient sheet conveyance and high-speed printing are achieved in an image forming apparatus that is provided with a cutter on the downstream side of a print head. 
         [0084]    Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.