Patent Publication Number: US-9896298-B2

Title: Apparatus and method for cutting sheet

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional Application of U.S. patent application Ser. No. 12/965,734, filed Dec. 10, 2010, now abandoned, which claims the benefit of Japanese Patent Application No. 2010-087892 filed Apr. 6, 2010. Each of U.S. patent application Ser. No. 12/965,734 and Japanese Patent Application No. 2010-087892 is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an apparatus and method for cutting a sheet for use in an image forming apparatus capable of obtaining cut sheet products by supplying a continuous sheet. 
     Description of the Related Art 
     In known image forming apparatuses capable of obtaining cut sheet products from a continuous sheet, a plurality of processes are performed including image formation and cutting from sheet supply to completion. The sheet is subjected to various processes while being conveyed, in which the sheet conveying speed are changed from one process to another. 
     In particular, in conventional upstream and downstream processes including a cutting process, it is necessary to change the conveying speed or to stop the conveyance depending on the situation due to differences in processing speed required in halting the sheet for cutting and in the upstream and downstream processes. 
     A photo-printing apparatus disclosed in Japanese Patent Laid-Open No. 1-99049 is provided in view of the problem that, in the flow of printing a sheet, cutting the sheet, and conveying the sheet to a developing process, the conveying speed is low and constant, while at the printing process, the conveying speed is high and intermittent. Japanese Patent Laid-Open No. 1-99049 discloses a method for coping with the difference in conveying speed by providing a conveying-speed adjusting unit capable of controlling the nip and separation of the sheet behind the cutting unit, instead of a conventional loop-like storage portion. 
     In this type of image forming apparatus, the need for enhancing the performance, such as increasing the speed and reducing the size, is always present as an object, also the need for apparatus specifications, such as controlling the conveying speed by easily coping with mixture of products of different lengths as a requirement. 
     SUMMARY OF THE INVENTION 
     The present invention provides, among other things, a sheet cutting apparatus in which slack in a sheet generated at halting of the sheet during cutting can be quickly removed, and even if the length of the sheet to be cut varies, a conveying unit can be driven depending on the length. 
     According to an aspect of the present invention, there is provided a sheet cutting apparatus including a first conveying unit that conveys a sheet; an upstream conveying unit that is disposed upstream in the conveying direction of the first conveying unit and that conveys the sheet at a first conveying speed; a first cutting unit that is disposed downstream in the conveying direction of the first conveying unit and that cuts the sheet; a second conveying unit that is disposed downstream in the conveying direction of the first cutting unit and that conveys the sheet; a third conveying unit that is disposed downstream of the second conveying unit and that conveys the sheet; and a detecting unit that is disposed between the second conveying unit and the third conveying unit and that detects the sheet. The first cutting unit cuts the sheet in a state in which the first conveying unit and the second conveying unit are halting and the upstream conveying unit is conveying the sheet at the first conveying speed; and after the sheet is cut, the first conveying unit conveys the sheet at a second conveying speed higher than the first conveying speed to reduce the slack of the sheet formed between the upstream conveying unit and the first conveying unit during the halting; and after the sheet is cut, the second conveying unit conveys the cut sheet at the downstream side at a third conveying speed higher than the first conveying speed; after the sheet is cut, if the third conveying unit is nipping the sheet during cutting of the sheet, the third conveying unit conveys the sheet at the third conveying speed, and if the third conveying unit is not nipping the sheet during cutting of the sheet, the third conveying unit is driven to convey the sheet at the third conveying speed or a fourth conveying speed after the detecting unit detects the sheet. 
     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 
         FIG. 1  is a schematic sectional view illustrating the internal configuration of a printer that accommodates a sheet cutting and conveying mechanism according to an embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a control unit. 
         FIG. 3  is a schematic diagram illustrating the operation of the printer accommodating the sheet cutting and converting mechanism according to an embodiment of the present invention. 
         FIG. 4  is a schematic diagram illustrating the configuration of a cutter included in the sheet cutting and conveying mechanism according to an embodiment of the present invention. 
         FIG. 5  is a schematic diagram illustrating the configuration of a sheet cutting and conveying mechanism of a first embodiment. 
         FIG. 6  is a block diagram illustrating the control configuration of the sheet cutting and conveying mechanism according to an embodiment of the present invention. 
         FIG. 7  illustrates an example of images formed on a yet-to-be-cut continuous sheet corresponding to the sheet cutting and conveying mechanism of the first embodiment. 
         FIGS. 8A to 8E  are schematic diagrams illustrating a process, in stages, in which a sheet is cut and conveyed by the sheet cutting and conveying mechanism of the first embodiment. 
         FIG. 9  is a chart illustrating a diagram when a sheet is cut and conveyed by the sheet cutting and conveying mechanism of the first embodiment. 
         FIG. 10  is a flowchart of the operation of the sheet cutting and conveying mechanism according to an embodiment of the present invention. 
         FIG. 11  is a flowchart of the operation of a conveying roller pair R 1  of the sheet cutting and conveying mechanism according to the first embodiment. 
         FIG. 12  is a flowchart of the operation of a conveying roller pair R 2  of the sheet cutting and conveying mechanism according to the first embodiment. 
         FIG. 13  is a flowchart of the operation of a conveying roller pair R 3  of the sheet cutting and conveying mechanism according to the first embodiment. 
         FIG. 14  is a flowchart of the operation of a conveying roller pair R 4  of the sheet cutting and conveying mechanism according to the first embodiment. 
         FIG. 15  is a flowchart of the operation of Nth conveying roller pair R(N) following the fourth conveying roller pair of the sheet cutting and conveying mechanism according to the first embodiment. 
         FIG. 16  is a schematic diagram illustrating the configuration of a sheet cutting and conveying mechanism according to a second embodiment. 
         FIG. 17  illustrates an example of images formed on a yet-to-be-cut continuous sheet corresponding to the sheet cutting and conveying mechanism of the second embodiment. 
         FIGS. 18A to 18D  are schematic diagrams illustrating the operation of the sheet cutting and conveying mechanism of the second embodiment. 
         FIGS. 19A to 19D  are schematic diagrams illustrating the operation of the sheet cutting and conveying mechanism of the second embodiment. 
         FIG. 20  is a chart illustrating a diagram when a sheet is cut and conveyed by the sheet cutting and conveying mechanism of the second embodiment. 
         FIG. 21  is a flowchart of the operation of a conveying roller pair R 3  of the sheet cutting and conveying mechanism according to the second embodiment. 
         FIG. 22  is a flowchart of the operation of a conveying roller pair R 4  of the sheet cutting and conveying mechanism according to the second embodiment. 
         FIG. 23  is a flowchart of the operation of a conveying roller pair R 5  of the sheet cutting and conveying mechanism according to the second embodiment. 
         FIG. 24  is a flowchart of the operation of conveying roller pairs R(N) following the fifth conveying roller pair of the sheet cutting and conveying mechanism according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Inkjet printers according to various example embodiments of the present invention will be described hereinbelow. The printers of the embodiments are high-speed line printers that use a continuous roll sheet. The printers are suitable for the field of apparatuses that print a large quantity of sheets used in, for example, printing companies. 
     First Embodiment 
       FIG. 1  is a schematic sectional view illustrating the internal configuration of a printer that accommodates a sheet cutting and conveying mechanism according to an embodiment of the present invention. The printer roughly accommodates a sheet supplying unit  1 , a decurling unit  2 , a skew straightening unit  3 , a printing unit  4 , a checking unit  5 , a cutting unit  6 , an information recording unit  7 , a drying unit  8 , a discharge conveying unit  10 , a sorting unit  11 , a discharging unit  12 , and a control unit  13 . A sheet is conveyed by a conveying mechanism including roller pairs and a belt along the sheet conveying path indicated by the solid line in the drawing and is processed by the individual units. 
     The sheet supplying unit  1  is a unit that accommodates and supplies a continuous roll sheet. The sheet supplying unit  1  can accommodate two rolls P 1  and P 2  and is configured to selectively draw and supply a sheet. The number of rolls accommodated is not limited to two; it may be one or three or more. 
     The decurling unit  2  is a unit that reduces the curl (warping) of a sheet supplied from the sheet supplying unit  1 . The decurling unit  2  reduces the curl by curving the sheet so as to give warping opposite the curl using two pinch rollers per one driving roller. 
     The skew straightening unit  3  is a unit that straightens the skew (inclination relative to an original advancing direction) of the sheet that passed through the decurling unit  2 . The skew of the sheet is straightened by pushing a reference end of the sheet against a guide member. 
     The printing unit  4  is a unit that forms an image on the conveyed sheet using a print head  14 . The printing unit  4  further includes a plurality of conveying rollers that convey the sheet. The print head  14  has line print heads in which an inkjet nozzle array is formed in a range that covers the supposed maximum width of the sheet. The print head  14  is configured such that the plurality of print heads are arranged in parallel along the conveying direction. The inkjet system can employ a system that uses heating elements, a system that uses piezoelectric elements, a system that uses electrostatic elements, a system that uses MEMS elements, etc. Color inks are supplied from ink tanks to the print head  14  through respective ink tubes. 
     The checking unit  5  is a unit that checks the state of the nozzles of the print heads, the sheet conveying state, image positions, etc. by optically reading a check pattern or image printed on the sheet by the printing unit  4 . 
     The cutting unit  6  is a unit equipped with a mechanical cutter that cuts the printed sheet into a predetermined length. The cutting unit  6  also has a plurality of conveying rollers for forwarding the sheet to the next process and a space for storing waste generated by cutting. 
     The drying unit  8  is a unit that heats the sheet printed by the printing unit  4  to dry applied ink in a short time. The drying unit  8  is also equipped with a heater, a conveying belt for forwarding the sheet to the next process, and a conveying roller. 
     The discharge conveying unit  10  is a unit that conveys the sheets that are cut by the cutting unit  6  and dried by the drying unit  8  to the sorting unit  11 . The sorting unit  11  is a unit that divides the printed sheets into groups and may discharge them into different trays of the discharging unit  12 . 
     The control unit  13  is a unit that controls the components of the entire printer. The control unit  13  includes a CPU  601 , a memory, a controller  15  equipped with various I/O interfaces, and a power source. The operation of the printer is controlled on the basis of an instruction from the controller  15  or an external unit  16 , such as a host computer, connected to the controller  15  via an I/O interface. 
       FIG. 2  is a block diagram illustrating the control unit  13 . The controller  15  (enclosed by the broken line) included in the control unit  13  is constituted by a CPU  201 , a ROM  202 , a RAM  203 , a HDD  204 , an image processing portion  207 , an engine control portion  208 , and an individual-unit control portion  209 . The CPU  201  (central processing unit) integrally controls the operations of the individual units of the printer. The ROM  202  stores programs for the CPU  201  to execute and fixed data for the various operations of the image forming apparatus. The RAM  203  is used as a work area of the CPU  201  or a temporary storage of various received data or is used to store various set data. The HDD  204  (hard disk) can store and read programs for the CPU  201  to execute, print data, and set information for the various operations of the image forming apparatus. The operating unit  206  is an input/output interface with a user and includes an input unit, such as a hard key and a touch panel, and an output unit, such as a display that presents information and a voice generator. 
     Units that require high-speed data processing are provided with dedicated processing units. The image processing portion  207  performs image processing of print data handled by the image forming apparatus. The image processing portion  207  converts the color space (for example, YCbCr) of input image data to a standard RGB color space (for example, sRGB). The image data is subjected to various image processings, such as resolution conversion, image analysis, and image correction. Print data obtained through those image processings is stored in the RAM  203  or the HDD  204 . The engine control portion  208  controls driving of the print head  14  of the printing unit  4  in accordance with print data on the basis of a control command received from the CPU  201  or the like. The engine control portion  208  further controls the conveying mechanisms for the components in the image forming apparatus  200 . The individual-unit control portion  209  is a subcontroller for individually controlling the sheet supplying unit  1 , the decurling unit  2 , the skew straightening unit  3 , the checking unit  5 , the cutting unit  6 , the information recording unit  7 , the drying unit  8 , the reversing unit  9 , the discharge conveying unit  10 , the sorting unit  11 , and the discharging unit  12 . The operations of the individual units are controlled by the individual-unit control portion  209  on the basis of an instruction of the CPU  201 . The external interface  205  is an interface (I/F) for connecting the controller  15  to the external unit  16 , which is a local I/F or a network I/F. The above components are connected by a system bus  210 . 
     The external unit  16  is a unit that serves as the source of image data for the image forming apparatus to perform printing. The external unit  16  may be either a general-purpose or dedicated computer or a dedicated imaging device, such as an image capture, a digital camera, and a photostorage having an image reader. If the external unit  16  is a computer, an OS, application software for generating image data, and a print driver for the image forming apparatus are installed in a storage included in the computer. It is not essential to implement the foregoing processes using software; part or all of the processes may be implemented using hardware. 
       FIG. 3  is a schematic diagram illustrating the operation of the printer accommodating the sheet cutting and converting mechanism according to an embodiment of the present invention. A conveying path through which a sheet supplied from the sheet supplying unit  1  is printed and is then discharged to the discharging unit  12  is indicated by a bold line. The sheet supplied from the sheet supplying unit  1  is processed by the decurling unit  2  and the skew straightening unit  3 . The front surface (first surface) of the sheet is printed by the printing unit  4 . Images of a predetermined unit length (unit images) in the conveying direction are printed on a long continuous sheet in sequence to form a plurality of images in a line. The printed sheet passes through the checking unit  5  and is cut into unit images of a predetermined length by the cutting unit  6 . Print information may be recorded on the back of the cut sheets that are cut every image by the information recording unit  7 . The cut sheets are conveyed to the drying unit  8  one by one and are dried. Thereafter, the cut sheets are sequentially discharged to the discharging unit  12  of the sorting unit  11  through the discharge conveying unit  10  and are stacked. On the other hand, a sheet left at the printing unit  4  side after the cutting of the last unit image is fed back to the sheet supplying unit  1  and is rolled back by the roll P 1  or P 2 . 
     The cutting unit  6  that is the sheet cutting and conveying mechanism of the printer with the above configuration according to an embodiment of the present invention will be described in more detail. 
     In the first embodiment, an example in which only one cutter is used will be described. 
       FIG. 4  is a schematic diagram illustrating the configuration of a cutter that is a cutting unit included in the sheet cutting and conveying mechanism according to an embodiment of the present invention. The cutter is of generally called a sliding type and is constituted by a fixed blade  401  and a movable blade  402 . The movable blade  402  is driven by a cutter motor  403  serving as a driving source via a cam  404 , a driving-side link  405 , and a driven-side link  406  to move vertically in contact with the fixed blade  401  at an angle. Since a load during cutting is large, a DC motor is used as the cutter motor  403 . A cutter sensor  407  detects the top dead center of the movable blade  402  and stops the movable blade  402  by means of a short-circuit brake that directly couples both terminals of the DC motor in accordance with detection timing, achieving a high-speed vertical reciprocating movement. 
       FIG. 5  is a schematic diagram illustrating the configuration of the sheet cutting and conveying mechanism of the first embodiment. In  FIG. 5 , the sheet is conveyed from the right to the left in  FIG. 5  as indicated by arrow A. A cutter C 1  that is a sheet cutting unit is a sliding-type cutter described using  FIG. 4 . A sheet conveying unit is a conveying roller pair constituted by a driving roller that rotates by obtaining motive power from a motor (not shown) and a driven roller that rotates freely in pressure contact with the driving roller. A sheet guide member is disposed as a supplemental conveying unit between the rollers, which is not shown in  FIG. 5  because it is not necessary for describing the present invention. 
     A conveying roller pair RC that is the extreme upstream conveying unit feeds a continuous sheet to the cutter C 1  at an upstream constant speed Vp (first conveying speed). The conveying roller pair RC does not change in speed for the cutting motion of the cutter C 1  and may be included, for example, in the sheet cutting and conveying mechanism or alternatively in the checking unit that is an upstream process. A conveying roller pair R 1  that is a first conveying unit is disposed upstream in the conveying direction of the cutter C 1 , and a conveying roller pair R 2  that is a second conveying unit is disposed downstream in the conveying direction of the cutter C 1 . Furthermore, a roller pair R 3  is disposed downstream of the conveying roller pair R 2 , and a conveying roller pair R 4  that is a third conveying unit is disposed downstream thereof. Furthermore, roller pairs R 5  to RN that are a plurality of conveying units are disposed downstream of the conveying roller pair R 4  at a pitch shorter than the shortest cut length that can be achieved by the apparatus. Edge sensors SE 2 , SE 3 , SE 4 , SE 5  to SEN that are detecting units capable of detecting the leading edge or the trailing edge of the conveyed sheet are disposed upstream of the conveying roller pairs R 2 , R 3 , R 4 , R 5  to RN, respectively. The conveying roller pairs R(N) each have a dedicated driving source, which allows changes in speed and halting to be independently controlled. Examples of the driving sources of the conveying roller pairs R(N) include a stepping motor and a motor that employs an encoder to allow measurement of the conveying length. In the case where the cut sheet product is long, edge sensors SE(N) and conveying roller pairs R(N) are added to the downstream side. Since a control method, to be described below, uses positional information on the individual conveying roller pairs R(N) and edge sensors SE(N),  FIG. 5  illustrates the positions of the individual conveying roller pairs R(N) and edge sensors SE(N) with reference to the cutting position of the cutter C 1 . 
       FIG. 6  is a block diagram illustrating the control configuration of the sheet cutting and conveying mechanism according to an embodiment of the present invention. The outputs of the edge sensors SE 2 , SE 3  to SEN, etc. are input to the CPU  601 . The CPU  601  controls driving of motors M 1 , M 2 , M 3  to MN, etc. that are dedicated driving sources for driving the conveying roller pairs R 1 , R 2 , R 3  to RN, etc., respectively, via individual drivers. The cutter motor  403  and the cutter sensor  407  included in the configuration of the cutter C 1  are also connected to the CPU  601 , so that the motion of the cutter C 1  can be controlled. Control programs to be executed by the CPU  601  are stored in a ROM  603 , and data for use in the control of the CPU  601  is stored in a RAM  602 . Of the control data, data on the length of a downstream cut sheet that is a cut product and its cutting position is input from the external unit  16  to the controller  15 , is processed by an image-information processing portion  604  in the controller  15 , and is input to the CPU  601 . 
       FIG. 7  illustrates an example of images formed on a yet-to-be-cut continuous sheet SHr corresponding to the sheet cutting and conveying mechanism of the first embodiment. Image products SHc are continuously printed on the yet-to-be-cut continuous sheet SHr, and no waste is generated by the cutting of the cutter. 
       FIGS. 8A to 8E  are schematic diagrams illustrating a process, in stages, in which a sheet is cut and conveyed by the sheet cutting and conveying mechanism of the first embodiment. 
       FIG. 8A  illustrates a state until a printed sheet reaches a cutting position. The yet-to-be-cut continuous sheet SHr that is continuously conveyed from the upstream side at the conveying speed Vp that is the first conveying speed passes through the conveying roller pairs R 1 , R 2 , and R 3  ahead and behind the cutter C 1  that moves at the same conveying speed Vp to reach the cutting position. The cutting position can be determined, for example, by detecting the leading edge of the yet-to-be-cut continuous sheet SHr after passing through the conveying roller pair R 1  by the edge sensor SE 2  and determining the length, that is, the cutting position, after the sheet SHr passes between the blades of the cutter C 1  at a conveying amount of the conveying roller pair R 1  after the detection. The cutting position can also be determined by detecting the formed image using an image sensor not by the edge sensor SE 2 . 
       FIG. 8B  illustrates a state during cutting. The conveying roller pairs R 1 , R 2 , and R 3  that nip the yet-to-be-cut continuous sheet SHr halt and hold the yet-to-be-cut continuous sheet SHr during the motion of the cutter C 1 . Since the yet-to-be-cut continuous sheet SHr on which images are printed is conveyed from the upstream side also while the continuous sheet SHr is halted at the cutter C 1 , the yet-to-be-cut continuous sheet SHr slacks and is accumulated like a loop at the upstream side of the conveying roller pair R 1 . Although cutting time Tc during which the cutter C 1  is moving varies due to such factors as the width and thickness of the sheet, the haling time Tw of the conveying roller pairs R 1  to R 3  is set to a constant (Tw&gt;Tc) because the constant halting time Tw makes it easy to control the timing at which the subsequent conveying speed is changed. 
       FIG. 8C  illustrates a state directly after completion of the cutting, that is, after a lapse of the halting time Tw of the conveying roller pairs R 1  to R 3 . After completion of the cutting, the image product SHc after cutting is conveyed at a third conveying speed Vh (Vh&gt;Vp) to reduce the slack of the sheet formed during the halting and to prevent the yet-to-be-cut continuous sheet SHr and the product SHc from overlapping. After the sheet is cut, the conveying roller pairs R 2 , R 3 , and R 4  are driven at the conveying speed Vh to convey the cut image product SHc by a specified distance La from the cutter C 1 , with the continuous-sheet-side conveying roller pair R 1  halted. At that time, the cut sheet and the subsequent continuous sheet are spaced from each other. At that time, the control timing for the downstream roller pairs, to be described later, can be accurately managed by setting the length Lse 3  from the cutter C 1  to the edge sensor SE 3  smaller than La (Lse 3 &lt;La). 
       FIG. 8D  illustrates a state after a minute time Td has passes directly after the product SHc is conveyed at the conveying speed Vh. The continuous sheet SHr is conveyed by a specified length Lc from the cutter C 1  at the speed Vl (Vl&gt;Vp) in cooperation of the conveying roller pairs R 1  and R 2  to eliminate the loop accumulated during the time (Tw+Td) during which the conveying roller pair R 1  halts. The speed Vl is a second conveying speed. At that time, since the product SHc is moved prior to the leading edge of the continuous sheet SHr, setting the conveying length to La&gt;Lc prevents overlapping. 
       FIG. 8E  illustrates a state after completion of the conveyance at the conveying speeds Vh and Vl. The product SHc is conveyed by the conveying roller pairs R 3  and R 4  at a forth conveying speed Vd that is used for the drying unit  8 . To provide an interval between the products SHc, it is highly beneficial if Vd&gt;Vp is satisfied. The continuous-sheet side conveying roller pairs R 1  and R 2  convey the continuous sheet to the next cutting position at the conveying speed Vp, the conveying speed of the conveying roller pair R 3  changes from Vd to Vp, and the mechanism returns to the state in  FIG. 8A  and repeats the conveyance. 
       FIG. 9  is a chart illustrating a diagram when a sheet is cut and conveyed by the sheet cutting and conveying mechanism of the first embodiment which is described using  FIGS. 8A to 8E . The vertical axis indicates distance from the cutter C 1 , and the horizontal axis indicates time, which are marked in correspondence with the individual states shown in  FIGS. 8A to 8E . The chart represents the distance between the trailing edge SH 1  of the product SHc directly after cutting and the leading edge SH 2  of the subsequent yet-to-be-cut continuous sheet SHr. Although the distance between the trailing edge SH 1  and the leading edge SH 2  is increased once because the subsequent yet-to-be-cut continuous sheet SHr is also halted for cutting, the distance becomes constant after the subsequent product SHc is completed, and the sheet is conveyed at the downstream conveying speed Vd. 
     By changing the sheet conveying speed as shown in  FIG. 9  irrespective of the cutting length Ln of the product SHc, the sheet cutting and conveying mechanism with the configuration of the first embodiment can cut and convey the sheet without overlapping. 
     To cope with changes in the cutting length Ln of the product SHc, information on the product cutting length Ln is obtained in advance and individual conveying roller pairs R(N) are independently controlled while assigning speed-switching conditions thereto. 
       FIG. 10  is a flowchart of the operation of the sheet cutting and conveying mechanism according to an embodiment of the present invention. Upon starting a printing operation, in step S 1001 , the individual conveying roller pairs R(N) are processed in parallel according to independent subroutines. Since the cutting motion of the cutter C 1  is synchronized with the halting of the conveying roller pair R 1 , the cutting motion is included in the subroutine of the conveying roller pair R 1 . 
     The operation subroutines of the individual conveying roller pairs R(N) will be described. 
       FIG. 11  is a flowchart of the operation of the conveying roller pair R 1  that is the first conveying unit of the sheet cutting and conveying mechanism according to the first embodiment. The conveying roller pair R 1  conveys a sheet at the same conveying speed Vp as the upstream conveying speed in step S 1101 . When the sheet reaches a cutting position in step S 1102 , the conveying roller pair R 1  halts for a specified time (Tw+Td) in steps S 1104  and S 1106  on the start of a cutting operation. Next, the conveying roller pair R 1  conveys a specified quantity of sheet at the high speed Vl in step S 1107  to reduce a loop formed during the halting, and repeats the above process. 
       FIG. 12  is a flowchart of the operation of the conveying roller pair R 2  that is the second conveying unit of the sheet cutting and conveying mechanism according to the first embodiment. The conveying roller pair R 2  also halts at the timing of step S 1202 , like the upstream conveying roller pair R 1 , in synchronization with the conveying roller pair R 1 . After halting for a specified time Tw in step S 1203 , the conveying roller pair R 2  starts moving Td earlier than the conveying roller pair R 1 . In step S 1204 , the conveying roller pair R 2  first conveys a specified feed of product SHc at the speed Vh. The feed at the speed Vh is set by adding a margin corresponding to the apparatus to the distance Lr 2  from the cutter C 1  to the nip of the conveying roller pair R 2  to reliably convey the product SHc until the product SHc is separated from the nip of the conveying roller pair R 2 . 
     Next, in step S 1205 , the leading edge of the continuous sheet SHr fed from the upstream conveying roller pair R 1  is conveyed at the conveying speed Vl in cooperation with the conveying roller pair R 1 . After the loop is eliminated by a specified feed of conveyance, the conveying roller pair R 2  returns to step S 1201 , where the speed Vl shifts to the same conveying speed Vp as the upstream conveying speed at the same timing as the upstream conveying roller pair R 1 . 
     Setting Vl=Vh allows the step S 1204  and step S 1205  to be integrated. 
       FIG. 13  is a flowchart of the operation of the conveying roller pair R 3  of the sheet cutting and conveying mechanism according to the first embodiment. The conveying roller pair R 3  also halts at the cutting position in step S 1302  in synchronization with the conveying roller pairs R 1  and R 2 . 
     Since the distance Lr 3  from the cutter C 1  to the nip of the conveying roller pair R 3  is larger than La, after the specified time Tw has passed in step S 1303 , the product SHc is conveyed by the specified distance La at the conveying speed Vh in step S 1304 . In the next step S 1305 , the product SHc is conveyed by a specified feed at the downstream conveying speed Vd. The feed at the speed Vd is set to (Lr 3 −La+x) that is obtained by subtracting the distance La from the distance Lr 3  from the cutter C 1  to the nip of the conveying roller pair R 3  and adding a margin x corresponding to the apparatus thereto to reliably convey the product SHc until the product SHc is separated from the conveying roller pair R 3 . 
     After the conveyance, the conveying speed Vd shifts again to the same conveying speed Vp as the upstream conveying speed. 
       FIG. 14  is a flowchart of the operation of the conveying roller pair R 4  that is a third conveying unit of the sheet cutting and conveying mechanism according to the first embodiment. The conveying roller pair R 4  changes in speed depending on the length Ln of the cut product SHc. First, in step S 1401 , the leading edge of a sheet conveyed next is detected by an edge sensor SE 4  that is disposed upstream of the conveying roller pair R 4 . The length Ln of the cut product SHc is obtained at the detection timing, and the length Ln is compared with a constant to determine the conveying speed. 
     In step S 1403 , it is determined whether the conveying roller pair R 4  nips the sheet during cutting. If the length Ln is larger than the distance Lr 4  from the cutter C 1  to the nip of the conveying roller pair R 4  in step S 1403 , the conveying roller pair R 4  moves to step S 1405  and is driven at the upstream conveying speed Vp to convey the yet-to-be-cut continuous sheet SHr. If the sheet is conveyed to the cutting position in step S 1406 , the conveying roller pair R 4  halts for the specified time Tw in step S 1407 . At that time, the sheet is cut, while the conveying roller pair R 4  nips the sheet during the cutting of the sheet. Upon completion of the cutting after a lapse of time Tw, the conveying roller pair R 4  moves to step S 1408 , in which the sheet is conveyed at the high running speed Vh. 
     If the length Ln is smaller than (Lr 4 −La), the conveying roller pair R 4  moves to step S 1404  and then to step S 1411  to convey the cut product SHc at the downstream conveying speed Vd that is a fourth conveying speed. 
     If the length Ln lies therebetween (Lr 4 ≧Ln≧(Lr 4 −La), the conveying roller pair R 4  conveys the cut product SHc at the high running speed Vh in step S 1408 . The cut product SHc is conveyed by (La−Lse 3 ) at the conveying speed Vh in step S 1410  after the timing at which the edge sensor SE 3  has detected the trailing edge SH 1  of the cut product SHc in step S 1409 , and then the conveying speed Vh shifts to the downstream constant speed Vd in step S 1411 . At that time, the leading-edge detection timing of the sensor SE 4  is set to always precede the trailing-edge detection timing of the sensor SE 3 . This requires the condition that the feed at the speed Vh after the trailing edge has passed through the edge sensor SE 3  is smaller than the distance between the conveying roller pair R 4  and the edge sensor SE, that is, (La−Lse 3 )&lt;(Lr 4 −Lse 4 ). 
       FIG. 15  is a flowchart of the operation of the conveying roller pairs R(N) of the sheet cutting and conveying mechanism according to the first embodiment. If the length Ln of the cut product SHc is large, edge sensors SE(N) and conveying roller pairs R(N) are added to the downstream side. When edge sensors SE(N) and conveying roller pairs R(N) are disposed at a constant pitch, and a condition, (La−Lse 3 )&lt;(Lr(n)−Lse(n)), is satisfied, a sequence similar to that in  FIG. 14  can be applied only by changing steps S 1501 , S 1503 , S 1504 , and S 1513 . 
     In a printer that forms images on a continuous sheet using an inkjet recording unit, and after forming the images, that cuts the continuous sheet into simple image products, and that conveys the cut image products to a drying process, this embodiment offers the advantages of enhancing the speed, reducing the size, and coping with a sheet cutting length. 
     Second Embodiment 
     In a second embodiment, an example in which two cutting units are used will be described. 
       FIG. 16  is a schematic diagram illustrating the configuration of a sheet cutting and conveying mechanism of the second embodiment. In  FIG. 16 , the sheet is conveyed from the right to the left in  FIG. 6  as indicated by arrow A. The cutting unit includes two sets of sliding-type cutters constituted by a pair of fixed blade and movable blade, as in the first embodiment, that is, a first cutter C 1  and a second cutter C 2  that is a second cutting unit. A sheet conveying unit includes conveying roller pairs that can be independently driven, as in the first embodiment, and the functions, such as a driving source, are also the same as in the first embodiment. A sheet guide member is disposed as a supplemental conveying unit between the rollers, which is not shown also in  FIG. 16  because it is not necessary for describing the present invention. 
     A conveying roller pair RC that is the extreme upstream conveying unit feeds a continuous sheet to the first cutter C 1  at a constant speed Vp that is a first conveying speed and does not change the speed for the cutting motion of the first cutter C 1 . The conveying roller pair RC is not necessarily be included in the sheet cutting and conveying mechanism but may be included in the checking unit that is an upstream process. A conveying roller pair R 1  that is a first conveying unit is disposed upstream of the first cutter C 1 ; conveying roller pairs R 2  and R 3  are disposed between the first cutter C 1  and the second cutter C 2 ; and conveying roller pairs R 4 , R 5 , R 6 , and R 7  are disposed downstream of the second cutter C 2 . Edge sensors SE 2 , SE 3 , SE 4 , SE 5 , SE 6 , and SE 7  that can detect the leading edge or the trailing edge of the conveyed sheet are disposed upstream of the conveying roller pairs R 2 , R 3 , R 4 , R 5 , R 6 , and R 7 , respectively. If the length of the cut product SHc is large, edge sensors SE(N) and conveying roller pairs R(N are added to the downstream side. Since a control method, to be described below, uses positional information on the individual conveying roller pairs R(N) and edge sensors SE(N),  FIG. 16  illustrates the positions of the individual conveying roller pairs R(N) and edge sensors SE(N) with reference to the cutting position of the first cutter C 1 . 
       FIG. 17  illustrates an example of images formed on a yet-to-be-cut continuous sheet SHr corresponding to the sheet cutting and conveying mechanism of the second embodiment. Images are printed on the yet-to-be-cut continuous sheet SHr in such a manner that image products SHc that are cut into prints and nonproducts SHw to be discarded continue alternately. The first cutter C 1  and the second cutter C 2  separate prints and wastes from each other. The nonproducts SHw to be discarded are used to obtain the cut products SHc and are also used for marking for accurate detection of cutting positions, for overprinting for obtaining marginless image products, for maintaining print heads, etc. A position at which the sheet is cut by the first cutter C 1  is a boundary SH 1  at the leading edge of the second image from the leading edge of the sheet. A semiproduct SWc having an image at the leading edge and the subsequent nonproduct SHw is formed by cutting. The downstream portion of the sheet cut by the first cutter C 1  is conveyed, with the nonproduct SHw left at the upstream side, and is then cut by the second cutter C 2 . A position at which the sheet is cut by the second cutter C 2  is a boundary SH 2  at the trailing edge of the image, at which the nonproduct SHw is cut off from the image portion to form a product. 
       FIGS. 18A to 18D and 19A to 19D  are schematic diagrams illustrating a process in which a sheet is cut and conveyed by the sheet cutting and conveying mechanism of the second embodiment. 
       FIG. 18A  illustrates a state until a printed sheet reaches a cutting position, which is the same as in first embodiment. The yet-to-be-cut continuous sheet SHr that is continuously conveyed from the upstream side at the conveying speed Vp passes through the conveying roller pairs R 1 , R 2 , and R 3  ahead and behind the first cutter C 1  cutting that moves at the same conveying speed Vp to reach the cutting position. The cutting position can be determined, for example, by detecting the leading edge of the yet-to-be-cut continuous sheet SHr after passing through the conveying roller pair R 1  by the edge sensor SE 2  and determining the length, that is, the cutting position, after the sheet SHr passes between the blades of the cutter C 1  at the conveying amount of the conveying roller pair R 1  after the detection. The cutting position can be determined also by detecting the formed image using an image sensor not by the edge sensor SE 2 . 
       FIG. 18B  illustrates a state in which the yet-to-be-cut continuous sheet SHr is cut by the first cutter C 1 , which is the same operation as in the first embodiment. The conveying roller pairs R 1 , R 2 , and R 3  that nip the continuous sheet SHr halt and hold the continuous sheet SHr during the motion of the first cutter C 1 . Since the continuous sheet SHr on which images are printed is conveyed from the upstream side also while the continuous sheet SHr is halted at the first cutter C 1 , the continuous sheet SHr slacks and is accumulated like a loop at the upstream side of the conveying roller pair R 1 . Although cutting time Tc during which the first cutter C 1  is moving varies due to such factors as the width and thickness of the sheet, the haling time Tw of the conveying roller pairs R 1  to R 3  is set to a constant (Tw&gt;Tc) because the constant halting time Tw makes it easy to control the timing at which the subsequent conveying speed is changed. 
       FIG. 18C  illustrates a state directly after completion of the cutting of the first cutter C 1 . After completion of the cutting, the cut product SHc that is a downstream side sheet is conveyed at the third conveying speed Vh higher than the continuous-sheet conveying speed Vp to prevent the yet-to-be-cut continuous sheet SHr and the product SHc from overlapping. The conveying roller pairs R 2 , R 3 , and R 4  are driven at the conveying speed Vh to convey the semiproduct SWc including the nonproduct SHw to be discarded after cutting to the cutting position of the second cutter C 2  while the conveying roller pair R 1  at the continuous sheet side halted. 
       FIG. 18D  illustrates a state after a minute time Td has passed directly after the semiproduct SWc including the nonproduct SHw is conveyed at the speed Vh. The continuous sheet SHr is conveyed by a specified length Lc from the first cutter C 1  at the second conveying speed Vl (Vl&gt;Vp) in cooperation of the conveying roller pairs R 1  and R 2  to eliminate the loop accumulated during the time (Tw+Td) during which the conveying roller pair R 1  halts. At that time, a condition is set for preventing the semiproduct SWc and the leading edge of the continuous sheet SHr from overlapping. 
       FIG. 19A  illustrates a state in which the semiproduct SWc including the nonproduct SHw, cut by the first cutter C 1 , has reached the cutting position of the second cutter C 2 . The cutting position can be determined by detecting the leading edge of the continuous sheet SHr that is cut by the first cutter C 1  and is conveyed at the conveying speed Vh with the edge sensor SE 4  and determining the length, that is, the cutting position, after the continuous sheet SHr passes between the blades of the second cutter C 2  at the rotational speed of the conveying roller pair R 4  after the detection. The cutting position can be determined by detecting the formed image using an image sensor not by the edge sensor SE 4 , as in the first cutter C 1 . 
       FIG. 19B  illustrates the cutting of the second cutter C 2 . The semiproduct SWc including the nonproduct SHw, cut by the first cutter C 1 , is nipped and halted by the conveying roller pairs R 4  and R 5  downstream of the second cutter C 2 . The roller pairs R 4  and R 5  halt during the motion of the second cutter C 2 . The nonproduct SHw upstream of the second cutter C 2  is cut off and may be discharged from the sheet conveying path by free fall or the like. 
       FIG. 19C  illustrates a state directly after completion of the cutting of the second cutter C 2 . The product SHc produced by the cutting of the second cutter C 2  is conveyed from the upstream side at the high running speed Vh. To prevent the product SHc from overlapping with a new semiproduct SWc after completion of cutting of the first cutter C 1 , the product SHc is conveyed by a specified distance La 2  by the conveying roller pairs R 4 , R 5 , and R 6  at the speed Vh higher than the continuous-sheet conveying speed Vp. At that time, the control timing for the downstream roller pairs, described hereinbelow, can be accurately managed by setting the length (Lse 5 −Lc 2 ) from the second cutter C 2  to the edge sensor SE 5  smaller than La 2  ((Lse 5 −Lc 2 )&lt;La 2 ). 
       FIG. 19D  illustrates a state subsequent to  FIG. 19C . The product SHc is conveyed by the conveying roller pairs R 5  and R 6  at the speed Vd that is used for the drying unit  8 . The conveying roller pair R 4  from which the product SHc is separated returns to the state in  FIG. 19A  and repeats the conveyance. At that time, to provide an interval between the products SHc, Vd&gt;Vp or Vd=Vp should be satisfied. 
       FIG. 20  is a chart illustrating a diagram when a sheet is cut and conveyed by the sheet cutting and conveying mechanism of the second embodiment. The vertical axis indicates distances from the first cutter C 1 , and the horizontal axis indicates time. The chart represents the distance between the trailing edge SH 2  of the product SHc directly after the sheet is cut and the leading edge SH 1  of the subsequent continuous sheet SHr. Although the distance between the trailing edge SH 2  and the leading edge SH 1  is increased once because the subsequent continuous sheet SHr is also halted for cutting, the distance becomes constant after the trailing edge SH 2  of the subsequent sheet SHr is cut, and the sheet is conveyed at the downstream conveying speed Vd. 
     By changing the sheet conveying speed as shown in  FIG. 20  irrespective of the cutting length Ln of the product SHc, the sheet cutting and conveying mechanism with the configuration of the second embodiment can cut and convey the sheet without overlapping, as in the first embodiment. To cope with changes in the cutting length Ln of the products SHc, information on the product cutting length Ln is obtained in advance and the individual conveying roller pairs R(N) are independently controlled while assigning speed-switching conditions thereto, as in the first embodiment. 
     Flowcharts for the operation of the sheet cutting and conveying mechanism according to the second embodiment will be described hereinbelow. A flowchart of the entire mechanism is the same as that of the first embodiment in  FIG. 10 , in which, upon starting a printing operation, the individual conveying roller pairs R(N) are processed in parallel according to independent subroutines. Since the cutting motions of the first and second cutters C 1  and C 2  are synchronized with the halting of the conveying roller pair R 1 , the cutting motion of the first cutter C 1  is included in the subroutine of the conveying roller pair R 1 , and the cutting motion of the second cutter C 2  is included in the subroutine of the conveying roller pair R 4 . 
     Since the subroutines of the conveying roller pair R 1  that is the first conveying unit and the conveying roller pair R 2  that is the second conveying unit are omitted because they are the same as in the first embodiment, the subroutine of the operation of the conveying roller pair R 3  will now be described. 
       FIG. 21  is a flowchart of the operation of the conveying roller pair R 3  of the sheet cutting and conveying mechanism according to the second embodiment. The conveying roller pair R 3  also halts in step S 1902  at the cutting-position halt timing, like the upstream conveying roller pairs R 1  and R 2 , in synchronization therewith. After halting for a specified time Tw in step S 1903 , the conveying roller pair R 2  conveys a specified feed of product SHc at the speed Vh in step S 1904  and returns to step S 1901 . The feed at the speed Vh is set to (Lr 3 +x) obtained by adding a margin x corresponding to the apparatus to the distance Lr 3  from the first cutter C 1  to the nip of the conveying roller pair R 3  to reliably convey the cut product SHc until the product SHc is separated from the nip of the conveying roller pair R 3 . 
       FIG. 22  is a flowchart of the operation of the conveying roller pair R 4  that is a third conveying unit of the sheet cutting and conveying mechanism according to the second embodiment. The conveying roller pair R 4  changes in speed depending on the length Ln of the cut product SHc. First, in step S 2001 , the leading edge SH 1  of a sheet conveyed next is detected by the edge sensor SE 4  disposed upstream of the conveying roller pair R 4 . The length Ln of the cut product SHc is obtained at the detection timing, and the length Ln is compared with a constant to determine the conveying speed. 
     If the length Ln is larger than the distance Lr 4  from the first cutter C 1  to the nip of the conveying roller pair R 4  in step S 2003 , the conveying roller pair R 4  moves to step S 2004  and is driven at the upstream conveying speed Vp to convey the yet-to-be-cut continuous sheet SHr. 
     If the length Ln is smaller than or equal to Lr 4 , the conveying roller pair R 4  nips the sheet during cutting, and after the cutting, moves to step S 2007 , in which it conveys the cut product SHc from the state at the high running speed Vh. In step S 2009 , the conveying roller pair R 4  halts at the cutting position of the second cutter C 2 , and in step S 2010 , conveys a specified feed at the high running speed Vh. The feed at the speed Vh in step S 2010  is set to (Lr 4 −Lc 2 +x) that is obtained by adding a margin x corresponding to the apparatus to the distance (Lr 4 −Lc 2 ) from the second cutter C 2  to the nip of the conveying roller pair R 4  to reliably convey the cut product SHc until the product SHc is separated from the nip of the conveying roller pair R 4 . 
     After the conveyance at the speed Vh, the conveying roller pair R 4  returns to detection step S 2001  of the edge sensor SE 4 . 
       FIG. 23  is a flowchart of the operation of the conveying roller pair R 5  of the sheet cutting and conveying mechanism according to the second embodiment. The conveying roller pair R 5  also changes in speed depending on the length Ln of the cut product SHc. First, in step S 2101 , the leading edge SH 1  of a sheet conveyed next and disposed upstream of the conveying roller pair R 5  is detected by an edge sensor SE 5 . The length Ln of the cut product SHc is obtained in step S 2102  next to the detection timing, and the length Ln is compared with a constant to determine the conveying speed. 
     In step S 2103 , the length Ln and the distance Lr 5  from the first cutter C 1  to the nip of the conveying roller pair R 5  are compared. If the length Ln is larger than the distance Lr 5  from the first cutter C 1  to the nip of the conveying roller pair R 5 , the conveying roller pair R 5  moves to step S 2106  and is driven at the upstream conveying speed Vp to convey the yet-to-be-cut continuous sheet SHr. 
     If the length Ln is smaller than a value obtained by subtracting the feed (La 2 ) at the high running speed Vh from the distance (Lr 5 −Lc 2 ) from the second cutter C 2  to the nip of the conveying roller pair R 5 , that is, {(Lr 5 −Lc 2 )−La 2 }, the conveying roller pair R 5  moves to step S 2115 . In this case, the conveying roller pair R 5  conveys the cut product SHc at the downstream conveying speed Vd. 
     If the length Ln is smaller than or equal to the distance Lr 5  from the first cutter C 1  to the nip of the conveying roller pair R 5  and larger than the distance (Lr 5 −Lc 2 ) from the second cutter C 2  to the nip of the conveying roller pair R 5 , the conveying roller pair R 5  moves to step S 2109 . In step S 2109 , the conveying roller pair R 5  conveys the cut product SHc that is cut by the first cutter C 1  to the second cutter C 2  at the high running speed Vh. 
     If the length Ln is larger than or equal to {(Lr 5 −Lc 2 )−(La 2 )} and smaller than or equal to the distance (Lr 5 −Lc 2 ) from the second cutter C 2  to the nip of the conveying roller pair R 5 , the conveying roller pair R 5  moves to step S 2112 . In step S 2112 , the conveying roller pair R 5  conveys the cut product SHc that is cut by the second cutter C 2  at the high running speed Vh. The cut product SHc is conveyed by (La 2 −Lse 5 ) at the conveying speed Vh after the timing at which the edge sensor SE 5  has detected the trailing edge SH 2  of the cut product SHc, and then the conveying speed Vh shifts to the downstream constant speed Vd. 
       FIG. 24  is a flowchart of the operation of the conveying roller pair R(N) of the sheet cutting and conveying mechanism according to the second embodiment. If the length Ln of the cut product SHc is large, edge sensors SE(N) and conveying roller pairs R(N) are added to the downstream side at a constant pitch. A sequence similar to that in  FIG. 23  can be applied only by changing steps S 2201 , S 2203 , S 2204 , S 2205 , and S 2214 . 
     Both the first and second embodiments may also be provided with a sheet guide member at the sheet conveying path. Although the conveying path is straight in the drawings, it may be curved, and the number of independently driven conveying roller pairs may be increased depending on the cutting length of the product. 
     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 and encompass, among other things, all modifications and equivalent structures and functions.