Patent Publication Number: US-2015077456-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119(a) to Japanese Patent Application No. 2013-190777, tiled on Sep. 13, 2013, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Embodiments of this disclosure relate to an image forming apparatus, and more specifically to an image forming apparatus using a rolled printing medium with an adhesive face. 
     2. Description of the Related Art 
     Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or mu functional devices having, e.g., two or more of the foregoing capabilities. As one type of image forming apparatuses, for example, an image forming apparatus, such as a label printer, is known that prints a rolled printing medium having an adhesive face on which a separation sheet is attached (hereinafter, also referred to as “linerless label sheet”), such as a label sheet having no tape or mount sheet, and cuts the printing medium to a desired length after printing to form a printing medium piece (hereinafter, “label piece”). 
     Such an image forming apparatus using a rolled printing medium has a cutting unit to cut the recording medium in response to a user&#39;s request. 
     To prevent waste of a recording medium caused by cutting the recording medium at a downstream side of an image forming unit, for example, an image forming apparatus returns the recording medium in a direction opposite a discharge direction of the recording medium before the next printing, and place the leading end of the recording medium at a position for the next printing. 
     In such a case, typically, the image forming apparatus returns the recording medium to the position while applying tension to the recording medium. 
     BRIEF SUMMARY 
     In at least one embodiment of this disclosure, there is provided an image forming apparatus including a media roll, an image forming device, a conveyance unit, and a controller. In the media roll, a printing medium having an adhesive face and no separation sheet on the adhesive face is wound in a roll shape. The image forming device forms an image on the recording medium. The conveyance unit includes a conveyance rotator to convey the printing medium. The controller controls the conveyance rotator and the media roll to rotate in reverse and pull back the printing medium to the media roll. In pulling hack the printing medium to the media roll, the controller sets a linear velocity of the media roll in a rotation direction of the media roll to be not lower than a linear velocity of the conveyance rotator in a rotation direction of the conveyance rotator. The controller controls a velocity difference between the linear velocity of the media roll and the linear velocity of the conveyance rotator to be Va from a start of pulling back the printing medium to when a separation point of the printing medium from the media roll reaches a tangent line of the media roll in an area between the media roll and the conveyance rotator. When the separation point of the printing medium reaches the tangent line of the media roll, the controller switches the velocity difference from Va to Vb that is a smaller velocity difference than Va. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a front view of a mechanical section of an image forming apparatus according to an embodiment of this disclosure; 
         FIG. 2  is a partial front view of the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a schematic view of driving systems of a sheet feeding unit and a conveyance unit of the image forming apparatus; 
         FIG. 4  is a schematic view of a media-roll remaining amount detector according to an embodiment of this disclosure; 
         FIG. 5  is a block diagram of a controller according to an embodiment of this disclosure; 
         FIGS. 6A and 6B  is a schematic view of pull-back operation of the controller; 
         FIG. 7  is a flow chart of pull-back control according to an embodiment of this disclosure; and 
         FIG. 8  is a flow chart of a media-roll remaining amount detection process conducted by the controller according to an embodiment of this disclosure. 
     
    
    
     The accompanying drawings are intended to depict 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 OF EMBODIMENTS 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification 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 operate in a similar manner and achieve similar results. 
     For example, in this disclosure, the term “image formation” used herein includes providing not only meaningful images, such as characters and figures, but meaningless images, such as patterns, to e (in other words, the term “image formation” also includes causing liquid droplets to land on printing media). 
     The term “ink” is not limited to “ink” in a narrow sense, unless specified, but is used as a generic term for any types of liquid usable as targets of image formation. For example, the term “ink” includes recording liquid, fixing solution, liquid, and so on. 
     The term “image forming apparatus”, unless specified, also includes both serial-type image forming apparatus and line-type image forming apparatus. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable. 
     When a linerless label sheet is used as a recording medium, the recording medium is conveyed while being peeling off from a media roll. Accordingly, when an image is formed on the recording medium conveyed from the media roll, the recording medium is drawn out from the media roll in a normal direction of the roll media. 
     By contrast, when the media roll and a conveyance unit are rotated in reverse to wind back the recording medium, the recording medium is wound back to the media roll from a tangent direction of the media roll. 
     Here, a rotation distance of the media roll from a first position at which the recording medium is drawn out from the media roll in the normal direction to a second position at which the recording medium is returned to the media roll in the normal direction is greater than a difference in the distance from the media roll to the conveyance unit between in the normal direction and in the tangent direction. 
     Accordingly, when the media roll and the conveyance unit are rotated at substantially the same linear velocity, the recording medium loses tension between the media roll and the conveyance unit. If the recording medium is wound hack in a slacked state, the recording medium is likely to be wrinkled, affecting the next printing operation. 
     To deal with the above-described problem, it is conceivable to rotate the media roll at a higher linear velocity to apply more tension to prevent slack of the recording medium. However, after the recording medium reaches the tangent direction of the media roll at which a feed amount for returning the recording medium becomes equivalent between the conveyance unit and the media roll, the difference in linear velocity between the conveyance unit and the media roll would be too large, thus causing art increased driving load or a shift between the conveyance unit and the recording medium. 
     Hence, according to at least one embodiment of this disclosure, an image forming apparatus can perform stable conveyance without shift between the conveyance unit and the printing medium  2  white reducing occurrence of wrinkles due to pull-back operation of the printing medium  2 . 
     Referring now to the drawings, wherein like reference numerals des mate identical or corresponding parts throughout the several views, embodiments of the present disclosure are described below. 
     First, an image forming apparatus according to an embodiment of this disclosure is described with reference to  FIGS. 1 and 2 . 
       FIG. 1  is a front view of a mechanical section of an image forming apparatus  1000  according to an embodiment of this disclosure.  FIG. 2  is a partial front view of the mechanical section with a printing medium. 
     As illustrated in  FIG. 1 , the image forming apparatus  1000  has a sheet feeding unit  101  serving as a sheet feeder, an image forming unit  102  as an image forming device, a conveyance unit  103  as a conveyor, and a discharge conveyance unit  104  as a discharge conveyor within an apparatus body  100 . The image forming apparatus  1000  also has a discharge port  105  and a guide unit  106 . The discharge port  105  serves as a discharge port part to discharge a printing medium  2  having an image formed thereof to the outside of the apparatus body  100 . The guide unit  106  guides a rolled printing medium  2  during conveyance and pull-back of the printing medium  2 . 
     A media roll  4  formed of the rolled printing medium  2  is loaded on the sheet feeding unit  101 . 
     As illustrated in  FIG. 2 , the printing medium  2  is a continuum having an image-formable medium (hereinafter, also referred to as “printing face”)  2   a  and an adhesive layer (hereinafter, referred to as “adhesive face”)  2   b  fumed on a surface of the image-formable medium  2   a.  The printing medium  2  is a linerless label sheet rolled around in a state in which a mount sheet (separation sheet or separator) is not adhered to the adhesive face  2   b.    
     The media roll  4  is engaged with a spool  5 . Each of opposed ends of the spool  5  is rotatably held at three points, i.e., by a first roller  111 , a second roller  112 , and a third roller  113  serving as a first rotary body, a second rotary body, and a third rotary body, respectively. 
     The term “spool” used herein is not limited to a member that is provided separately from a core member and engaged with the core member for use The spool may be held by roll holder members  6  as an integral part of the core member of the media roll  4 . In a case in which the core member of the media roll  4  is directly held, the term “spool” includes the core member. 
     The image forming unit  102  includes a recording head  11  and a carriage  12 . The recording head  11  is a liquid ejection head mounted on the carriage  12  to eject droplets onto a printing medium  2 . The carriage  12  is supported by guide members  13  and  14  so as to be reciprocally movable along a direction (main scanning direction) perpendicular to a conveyance direction of the printing medium  2  indicated by arrow D in  FIG. 1 . Thus, the recording heads  11  are reciprocally movable in the main scanning direction. 
     For this embodiment, a liquid ejection head having two nozzle rows is used as the recording head  11 , and the carriage  12  mounts two recording heads  11 , The two recording heads II eject ink droplets of, for example, black (K), cyan (C), magenta (M), and yellow (Y) from four nozzle rows. It is to be noted that the configuration of the recording head is not limited to the above-described configuration but may be other type of recording head, for example, a line-type recording head. 
     Ink cartridges  15  storing respective color inks are mounted to the apparatus body  100  in a replaceable manner. The color inks are supplied from the ink cartridges  15  to via supply tubes  16  to head tanks mounted on the carriage  12 , and further supplied from the head tanks to the recording heads  11 . 
     The image forming apparatus  1000  also has a waste liquid tank  17  mounted to the apparatus body  100  in a replaceable manner. For example, in a maintenance operation for maintaining and recovering the performance of the recording heads  11 , waste ink is discharged into and stored in the waste liquid tank  17 . 
     The image forming unit  102  is not limited to the above-described liquid ejection head but may be any other type of image forming unit to form an image in a contact or non-contact manner. 
     The conveyance unit  103  includes a protection belt  21  as belt member which is an endless protection member. The protection belt  21  is dispose below the recording heads  11 . The protection belt  21  is looped around a conveyance roller  22  and a tensioned driven roller  23  so as to circulate. 
     The protection belt  21  preferably has no adhesion to the adhesive face  2   b . However, to prevent the printing medium  21  from floating up from the protection belt  21  during conveyance, the protection belt  21  may have such low adhesion to the adhesive face  2   b  that the protection belt  21  is separatable from the adhesive face  26 . Pressing the protection belt  21  against the adhesive face  2   b  allows protection of the adhesive face  2   b  and prevents the adhesive face  2   h  from contacting components inside the image forming apparatus to allow stable conveyance of the printing medium  2 . In addition, since the protection belt  21  is separatable from the adhesive face  2   b,  the printing medium  2  separated from the protection belt  21  is discharged from the discharge port  105 . 
     In other words, in this embodiment, the protection belt  21  protects the adhesive face  2   b  of the printing medium  2  and is transported together with the printing medium  2 . 
     An opposing roller  24  is disposed opposing the conveyance roller  22 . A conveyance roller pair (in this embodiment, pair of rotary bodies) of the conveyance roller  22  and the opposing roller  24  constitutes a conveyor to sandwich the printing medium  2  and the protection belt  21  together and convey the printing medium  2  to an image forming region of the recording heads  11 . 
     The protection belt  21  has multiple holes. Within a loop formed by the protection belt  21 , a suction fan  27  is disposed opposing the recording heads  11  of the image forming unit  102 . The suction fan  27  sucks the printing medium  2  toward a surface of the protection belt  21  via the suction holes. 
     In the above-described configuration, the printing medium  2  is adhered onto the protection belt  21  by suction of air. It is to be noted that the force to adhere the printing medium to the protection belt is not limited to air suction but, for example, electrostatic force may be used to adhere the printing medium onto the protection belt. Alternatively, for example, the printing medium may be supported and guided with a conveyance guide member (e.g., platen member) without using the protection member. 
     In addition, spur roller units  28   a,    28   b,  and  28   c  are disposed near the driven roller  23 . Each of the spur roller units  28   a,    28   b,  and  28   c  has multiple spur rollers arranged in the direction perpendicular to the conveyance direction D. The upstream spur roller units  28   a  and  28   b  are disposed opposing the protection belt  21 , and the most downstream spur roller unit  28   c  is disposed opposing a receive member  30  of the discharge conveyance unit  104 . 
     A separation unit  71  is disposed downstream from the driven roller  23  to separate the printing medium  2  from the protection belt  21 . 
     The receive member  30  guides the printing medium  2  fed from between the protection belt  21  and the spur roller unit  28   b.  The discharge conveyance unit  104  has a cutter unit  31  serving as a cutting device to cut a printing medium  2  into a desired length to form a printing medium piece (label piece)  200 . The discharge conveyance unit  104  is disposed downstream from the receive member  30  in the conveyance direction D of the printing medium  2 . 
     The cutter unit  31  includes a lower blade  31   b  and a cutting blade (upper blade or cutter)  31   a.  The lower blade  31   b  is formed at a downstream edge surface of the receive member  30 . The cutter  31   a  cuts the printing medium  2  between the lower blade  31   b  and the cutter  31   a.  The cutter  31   a  is moved in the direction perpendicular to the conveyance direction D to cut the printing medium  2 . 
     A discharge roller  32  is disposed downstream from the cutter unit  31 , in other words, downstream from the protection belt  21  serving as the belt member in the conveyance direction D. A spur roller  33  is disposed opposing the discharge roller  32  and presses the printing medium  2  against the discharge roller  32 . The discharge roller  32  and the spur roller  33  hold the label piece  200 , which is cut by the cutter unit  31 , in a state in which a leading end of the label piece  200  is sent out to the discharge port  105  serving as a discharge port part of the apparatus body  100 . 
     In this embodiment, a surface of the discharge roller  32  to hold the label piece  200  is, for example, non-adherence processed (processed so that the adhesive surface  2   b  does not adhere to the surface of the discharge roller  32 ), thus allowing separation of the adhesive surface  2   b  of the label piece  200 . In some embodiments, the discharge roller  32  itself may be made of a material allowing separation of the adhesive surface  2   b  of the label piece  200 . 
     A sheet sensor  34  is disposed to detect presence or absence of a printing medium  2 . The sheet sensor  34  is constituted of, for example, a photosensor or a combination of a mechanical lever and a photosensor. 
     On a downstream side of the opposing roller  24  also serving as the first roller constituting the conveyance unit  103 , the guide unit  106  has a second roller  42  serving as a separation roller disposed upstream from the image forming unit  102  and a third roller  43  disposed opposing the second roller  42  via the opposing roller  24 . An endless guide belt  44  is looped around the opposing roller  24 , the second roller  42 , and the third roller  43 . The guide unit  106  also has a front-end press roller  48  to press the printing medium  2  at a position upstream from the recording heads  11  in the conveyance direction D. 
     The guide belt  44  is a belt member having a base member made of, e.g., polyimide and a release layer (e.g., silicone coating) as a surface layer to enhance the release performance of the guide belt  44 . 
     The opposing roller  24 , the second roller  42 , and the third roller  43  are rotatably held by a holder member  45 . The holder member  45  is rotatable around a shaft  46  and is displaceable between at a position at the opposing roller  24  opposes the conveyance roller  22  and a position at which the opposing roller  24  separates from the conveyance roller  22  to open a space between the opposing roller  24  and the conveyance roller  22 . 
     When the media roll  4  is loaded and the printing medium  2  is set on the protection belt  21 , the space between the opposing roller  24  and the conveyance roller  22  is opened. When the printing medium  2  is conveyed, the opposing roller  24  is pressed toward the conveyance roller  22 . Accordingly, the opposing roller  24  is pressed toward the conveyance roller  22  by a pressing unit, such as a spring. In addition, the second roller  42  is pressed toward the protection belt  21  by a pressing unit, such as a spring. 
     For the image forming apparatus  1000  having such a configuration, the protection belt  21  and the printing medium  2  pulled out from the media roll  4  loaded on the sheet feeding unit  101  are sandwiched together between the conveyance roller  22  and the opposing roller  24 . 
     When the conveyance roller  22  is rotated, the printing medium  2   b  and the protection belt  21  are conveyed together in a state in which the adhesive face  2   b  is protected by the protection belt  21 . A desired image is formed on the printing medium  2  by the recording heads  11  of the image forming unit  102 . 
     When the protection belt  21  is separated from the printing medium  2  having the image formed thereon, the printing medium  2  is conveyed to the discharge conveyance unit  104  and cut at a desired position by the cutter unit  31  to form a label piece  200 . The label piece  200  is held between the discharge roller  32  and the spur rollers  33  in a state in which the label piece  200  is extractable from the discharge port  105  of the apparatus body  100 . 
     The guide unit  106  also prevents the printing medium  2  from being reeled by the opposing roller  24  during conveyance and pull-back of the printing medium  2 , in particular, when an image is formed on the adhesive face  2   b  of the printing medium  2 . 
     In other words, even if the surface of the opposing roller  24  is processed for non-adhesion, the adhesive face  2   b  of the printing medium  2  might adhere to the circumferential surface of the opposing roller  24  and be reeled by the opposing roller  24  due to a small curvature of the opposing roller  24 . In such a case, it is conceivable to use a larger curvature of the opposing roller  24 . However, a larger curvature of the opposing roller  24  reduces the size of a nip area between the opposing roller  24  and the conveyance roller  22 , thus preventing obtainment of stable conveyance force. 
     Hence, during conveyance, the printing medium  2  is conveyed while being pressed by the guide belt  44 , and the guide belt  44  is reliably separated from the printing medium  2  by the second roller  42  serving as a separation roller having a large curvature, thus preventing the printing medium  2  from being reeled by the opposing roller  24  during conveyance. 
     When the printing medium  2  is pulled back, the guide belt  44  receives the adhesive face  2   b  of the printing medium  2 , thus preventing the printing medium  2  from being reeled by the opposing roller  24 . 
     When image formation is finished and the printing medium  2  is cut by the cutter unit  31 , the leading end of the printing medium  2  is at the position of the cutter unit  31  and an area of the printing medium  2  opposite the image forming unit  102  is an unused area. In such a state, if the next image forming operation is resumed, the unused area of the printing medium  2  is wasted. Hence, in this embodiment, the printing medium  2  is pulled back in a pull-back direction (opposite the conveyance direction of the printing medium  2 ) to a position at which the leading end of the printing medium  2  is placed upstream from the image forming unit  102  in the conveyance direction. 
     Next, driving systems of the sheet feeding unit and the conveyance unit are described with reference to  FIG. 3 . 
       FIG. 3  is a schematic view of the driving systems according to this embodiment. 
     The sheet feeding unit  101  has a sheet feeding motor  312 . A gear  80  serving as a rotation transmitter is mounted on the spool  5 , and a driving gear  81  is mounted on the apparatus body  100 . The sheet feeding motor  312  rotates the driving gear  81  via a drive force transmission system including, e.g., a gear train and a timing belt. With the rotation of the driving gear  81 , the spool  5  is rotated and the media roll  4  is rotated. 
     The driving gear  81  has a built-in torque limiter  83 . When the driving gear  81  receives a torque equal to or greater than a threshold value from the media roll  4 , the driving gear  81  stops the drive transmission. 
     The conveyance unit  103  includes a conveyance motor  306 . Rotation of the conveyance motor  306  is transmitted to the conveyance roller  22  serving as a conveyance rotator via a drive force transmission system including, e.g., a gear train and a timing belt. 
     In this embodiment, a driving source (the sheet feeding motor  312 ) to rotate the media roll  4  is separately provided from a driving source (the conveyance motor  306 ) to rotate the conveyance roller  22 . Such a configuration can set the conveyance roller  22  and the media roll  4  at different velocities and change the velocities to rotate be conveyance roller  22  and the media roll  4 . 
     Next, a media-roll remaining amount detector serving as a remaining-amount detection unit to detect a remaining amount of the media roll  4  is described with reference to  FIG. 4 . 
       FIG. 4  is a schematic view of a media-roll remaining mount detector  313  according to an embodiment of this disclosure. 
     The media-roll remaining amount detector  313  serving as the remaining-amount detection unit includes a roller  314  to contact a circumferential surface of the media roll  4 , a belt  315  to transmit rotation of the roller  314 , an encoder wheel  316  to receive the transmission of rotation of the roller  314  via the belt  315 , and an encoder sensor  317  to read the encoder wheel  316 . 
     A holder  319  (indicated by broken lines for convenience of illustration in  FIG. 4 ) holds the roller  314 , the belt  315 , the encoder wheel  316 , and the encoder sensor  317 . The holder  319  is rotatable around a rotation shaft  318 . The holder  319  is pressed toward the media roll  4  so that the roller  314  moves in response to a decrease in outer diameter of the media roll  4 . 
     Here, since the outer diameter of the roller  314  and the rotation speed of the sheet feeding motor  312  are known, the main controller  312  calculates the outer diameter of the media roll  4  from an output value of the encoder sensor  317 . 
     It is to be noted that the remaining-amount detection unit may be any other suitable configuration, for example, a displacement sensor, an optical detection unit including a feeler, or a detection unit including a variable resistance board. 
     Next, an outline of a control unit of the image forming apparatus according to an embodiment of this disclosure is described with reference to  FIG. 5 . 
     The control unit includes, e.g., a main controller  301  also serving as a controller according to this embodiment, a head drive controller  302 , a main scanning driver  303 , a conveyance roller driver  305 , a suction fan driver  307 , a cutter driver  309 , and a sheet feed driver  311 . 
     The main controller  301  includes, e.g., a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), an input-and-output unit (I/O), other micro-computers, a volatile random access memory (VRAM), and an application-specific integrated circuit (ASIC). 
     Print information  300  is input from a host to the main controller  301 . 
     To form an image on a recording medium  2  in accordance with the print information  300 , the main controller  301  controls driving of the conveyance motor  306  via the conveyance roller driver  305  and rotates the conveyance roller  22  to intermittently convey the printing medium  2  while drawing the printing medium  2  from the media roll  4 . The main controller  301  controls driving of the main scanning motor  304  via the main scanning driver  303  and controls driving of the recording heads  11  via the head drive controller  302  while moving the carriage  12  for scanning in the main scanning direction, to eject desired droplets from the recording heads  11 . 
     When the conveyance roller  22  is rotated to feed the printing medium  2 , the main controller  301  controls driving of a suction fan motor  308  via the suction fan driver  307  to rotate the first thermistor  27 , thus adheres the printing medium  2  onto the protection belt  21 . 
     When driving of the conveyance roller  22  is transmitted, the conveyance motor  306  is rotated to rotate the discharge roller  32 . 
     The main controller  301  drives the cutter motor  310  via the cutter driver  309  to move a cutter  31   a  of the cutter unit  31  in the main scanning direction and thus cuts the recording medium  2  having an image formed thereon at a desired length into a printing medium piece (label pieces)  200 . 
     The main controller  301  rotates the sheet feeding motor  312  via the sheet feed driver  311  to rotate the spool  5 , thus rotating the media roll  4 . When the media roll  4  is rotated in reverse to wind (pull) back the printing medium  2 , the main controller  301  controls pull-back operation in accordance with detection results of the media-roll remaining amount detector  313 . 
     The main controller  301  is connected to an operation panel (operation unit)  320 . 
     Next, pull-back operation controlled by the main controller according to an embodiment of this disclosure is described with reference to  FIGS. 6A and 6B . 
       FIGS. 6A and 6B  are schematic views of pull-back operation according to this embodiment,  FIG. 6A  shows a state in which the remaining amount of the media roll  4  is large.  FIG. 6B  shows a state in which the remaining amount of the media roll  4  is small. 
     As described above, after an image is formed on a printing medium  2  and the cutter unit  31  cuts the printing medium  2 , the main controller  301  conducts pull-back operation of pulling the leading end of the printing medium  2  to a position upstream from the recording heads  11  in the medium conveyance direction D a by rotating the conveyance roller  22  in reverse. 
     Here, since the printing medium  2  is wound around with no separator on the adhesive face  2   b,  the printing medium  2  is fed while being peeled off from the developing device  4  during image forming operation. At this time, the direction in which the printing medium  2  is drawn out from the media roll  4  is a normal direction of the media roll  4 , and a separation point (separation position) of the printing medium  2  is a position P 1  illustrated in each of  FIGS. 6A and 6B . 
     By contrast, when the printing medium  2  is pulled back to the media roll  4  while being wound around the media roll  4 , the separation point of the printing medium  2  moves from the position P 1  to a position P 2  (tangent position of the media roll  4 ) with rotation of the media roll  4 . At the position P 2 , winding of the printing medium  2  to the media roll  4  is started. 
     Here, if recording medium  2  is a general sheet of paper, the separation point of which is also the position P 2  when drawn out from the media roll  4 , there is no difference in the separation point of the recording medium  2  between when the printing medium  2  is pulled out from the media roll  4  and when the printing medium  2  is pulled back to the media roll  4 . Accordingly, the printing medium  2  is pulled back so that the linear velocity at which the printing medium  2  is pulled back by the media roll  4  is substantially equivalent to the linear velocity at which the printing medium  2  is pulled back by reverse rotation of the conveyance roller  22 . 
     However, for the printing medium  2  having the adhesive face  2   b,  as described above, the separation point moves from the position P 1  to the position P 2  when the printing medium  2  is pulled back. At this time, when the linear velocity of rotation is the same between the media roll  4  and the conveyance roller  22 , the printing medium  2  is fed from the conveyance roller  22  to the media roll  4  by the same distance as the distance at which the separation point of the printing medium  2  from the media roll  4  moves from the position P 1  to the position P 2 , 
     However, a distance between a distance from the conveyance roller  22  to the position P 2  (indicated by a broken line in each of  FIGS. 6A and 6B ) and a distance from the conveyance roller  22  to the position P 1  (indicated by a solid line in each of  FIGS. 6A and 6B ) is smaller than the distance fed from the conveyance roller  22  to the media roll  4  (a distance on the media roll  4  from the position P 1  and the position P 2 ). 
     As a result, the printing medium  2  is wound back with a slack between the conveyance roller  22  and the media roll  4  and is likely to be wrinkled. 
     Hence, for the control of pull-back operation according to this embodiment, while the separation point of the printing medium  2  from the media roll  4  moves from the position P 1  to the position P 2 , a linear velocity V 2  of the media roll  4  in a pull-back rotation direction is set to be greater than a linear velocity V 1  of the conveyance roller  22  in a roll-back rotation direction (V 2 &gt;V 1 ). 
     Such control reduces a slack arising when the printing medium  2  is pulled back, thus reducing occurrence of wrinkles. 
     However, when the printing medium  2  is conveyed at such a linear velocity difference that the printing medium  2  is not slacked, a relatively large difference between the media roll  4  and the conveyance roller  22  may occur after the printing medium  2  reaches the tangent direction of the media roll, thus causing an increased driving load of the media roll  4  having, a greater linear velocity or a shift between the protection belt  21  and the printing medium  2 . 
     Hence, for the control of pull-back operation according to this embodiment, until the separation point of the printing medium  2  from the media roll  4  reaches the tangent direction of the media roll  4  between the media roll  4  and the conveyance roller  22 , the printing medium  2  is conveyed so that the difference between the linear velocity of the media roll  4  in the rotation direction and the linear velocity of the conveyance rotation (the conveyance roller  22 ) in the rotation direction is Va. When the separation point reaches the tangent direction, the difference between the linear velocity of the media roll  4  in the rotation direction and the linear velocity of the conveyance rotation (the conveyance roller  22 ) in the rotation direction is switched to be Vb (Va&gt;Vb). 
     Here, the meaning of the words “when the separation point reaches the tangent direction” is not limited to a moment at which the printing medium  2  reaches the tangent direction but may be before or after the printing medium  2  reaches the tangent direction if the printing medium  2  is placed near the tangent direction. 
     Such control allows stable conveyance Without shift between the conveyance unit and the printing medium  2  while reducing occurrence of wrinkles due to pull-back operation of the printing medium  2 . 
     Here, for the switching of the linear velocity, the linear velocity of the conveyance roller  22  may be switched. Alternatively, the linear velocity of the media roll  4  may be switched. For the switching time, for example, the linear velocity of the media roll  4  or the conveyance roller  22  may be switched when the position of the printing medium  2  is detected with, e.g., an optical sensor or a contact-type sensor provided in the tangent direction of the media roll  4  or when a detected torque of the media roll  4  rises. 
     Alternatively, the time in which the separation point of the printing medium  2  from the media roll  4  moves front the position P 1  to the position P 2  may be calculated from the distance between the position P 1  and the position P 2  and the linear velocity of the media roll  4  in the rotation direction to switch the linear velocity after the calculated time has passed. 
     Here, the time (hereinafter, separation-point moving time) in which the separation point of the printing medium  2  from the media roll  4  moves from the position P 1  to the position P 2  changes with the diameter of the media roll  4 . The diameter of the media roll  4  changes with the remaining amount of the media roll  4 . Accordingly, based on detection results of the media-roll remaining amount detector  313  described above, the main controller  301  control calculation of the separation-point moving tine from the position P 1  to the position P 2 . 
     Alternatively, instead of such control, the above-described torque limiter may be used to rotate the media roll  4  at the same linear velocity so that the media roll  4  follows the conveyance roller  22  after the printing medium  2  is tensioned in the tangent direction of the media roll  4  and the torque of the driving gear  81  increases. 
     Next, pull-back control performed by the main controller according to an embodiment of this disclosure is described with reference to  FIG. 7 . 
     At S 101 , when pull-back operation (rewinding) is started, the main controller  301  determines the remaining amount of the media roll  4  from detection results of the media-roll remaining amount detector  313  and calculates the separation-pint moving time at which the separation point of the printing, medium  2  from the media roll  4  moves from the position P 1  to the position P 2  (S 102 ). 
     At S 103 , the main controller  301  controls the conveyance motor  306  to rotate in reverse and the conveyance roller  22  to pull back the printing medium  2  at a velocity V 1  (peripheral velocity V 1  or the conveyance roller  22 ). At S 104 , the main controller  301  controls the sheet feeding motor  312  to rotate in reverse and the media roll  4  to pull back the printing medium  2  at a velocity V 2  (peripheral velocity V 2  of the media roll  4 ) (V 2 &gt;V 1 ). 
     As a result, as described above, the moving amount with rotation of the media roll  4  becomes greater than the feed amount of the printing medium  2  per unit time from the conveyance roller  22  to the media roll  4 . Such control can reduce a slack of the printing medium  2  caused by a change in conveyance direction between the media roll  4  and the conveyance roller  22  due to the movement of the separation posing from the position P 1  to the position P 2 . 
     When the separation-point moving time has passed (YES at S 105 ), in other words, the printing medium  2  is placed at a position at which the printing medium  2  is drawn out in a tangent direction of the media roll  4 , at S 106 , the main controller  301  controls the conveyance roller  22  to rotate in reverse and pull back the printing medium  2  at a velocity V 1   b  (peripheral velocity V 1   b  of the conveyance roller  22 ) (V 2 ≧V 1   b &gt;V 1 ). 
     As described above, such control prevents excessive tension from applying from the conveyance roller  22  to the printing medium  2 , reduces driving load, and prevents occurrence of shift between the conveyance roller  22  and the printing medium  2 . 
     When the leading end of the printing medium  2  reaches a return position (YES at S 107 ), at S 108  the main controller  301  stops reverse rotation of the conveyance roller  22  and the media roll  4  and finishes the process. 
     In this control, the velocity of the conveyance roller  22  is changed between in the separation-point moving time and after the separation-point moving time has passed. It is to be noted that the velocity of the media roll  4  may be changed to decrease the rotation speed of the media roll  4  after the separation-point moving time has passed. 
     Next, a media-roll remaining amount detection process according to an embodiment of this disclosure is described with reference to  FIG. 8 , 
     In this embodiment, at S 201  the main controller  301  determines presence or absence of records of load and unload of a media roll. When the record is present (YES at S 201 ), at S 202  the main controller  301  adopts a stored value of the diameter of the media roll, At S 205 , the main controller  301  executes printing. At S 206 , the main controller  301  calculates the diameter of the media roll  4  after printing from the amount of the printing medium  2  used by printing and the thickness of the printing medium  2 , and stores a calculated value of the diameter of the media roll  4  for calculation of the separation-point moving lime in pull-back operation. 
     By contrast, when the records of load and unload of the media roll  4  is absent, at S 203  the main controller  301  prompts a user to input the diameter of the media roll  4  and the thickness of the printing medium  2  and at S 204  adopts an input value of the diameter of the media roll  4 . At S 205 , the main controller  301  executes printing. At S 206 , the main controller  301  calculates the diameter of the media roll  4  after printing from the amount of the printing medium  2  used by printing and the thickness of the printing medium  2 , and stores a calculated value of the diameter of the media roll  4  for calculation of the separation-point moving time in pull-back operation. 
     For the user&#39;s input of the diameter of the media roll, for example, the sheet feeding unit  101  may have a scale so that the user can read and input a value of the scale. 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 above teachings, the present disclosure may be practiced otherwise than as specifically described herein. With some embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the present disclosure and appended claims, and all such modifications are intended to be included within the scope of the present disclosure and appended claims.