Abstract:
A medium holding apparatus holds a medium to be transferred in a printer. A pair of holding members are mounted to undergo movement and are configured to continuously hold respective end sections of the medium in a width direction of the medium during transfer of the medium in the printer. A support structure supports the pair of holding members so that the pair of holding members are permitted to advance and retract in the width direction of the medium in accordance with positions of the end sections of the medium in the width direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage application of International Application No. PCT/JP2009/050043 filed Jan. 6, 2009, claiming a priority date of Feb. 4, 2008, and published in a non-English language. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a medium press device for holding a medium conveyed in an ink jet printer and to an ink jet printer provided with the medium press device. 
     2. Background Information 
     It is known that in ink jet printers, a distance between an ink jet head ejecting ink and a medium largely influences on a print quality. The ink jet head is arranged to a printer chassis with good precision. Contrary to this, it is not a rare case that the medium to be carried may not keep a desired distance between the ink jet head due to occurrence of curling, existence of remaining cutting trash, attachment of foreign matter, and other factors. When conducting printing under such a state, it becomes difficult to control positions of ink droplets to be adhered onto the medium and the diameters of the droplets. As a result, there is a fear in that the print quality becomes unstable. 
     Contrary to this, in the conventional ink jet printer, there is proposed a mechanism for pressing a medium to be carried. For example, Patent Document 1 discloses a paper pressing mechanism including a pair of paper pressers for pressing lateral end portions of the paper to the platen. In the paper pressing mechanism, the paper pressers may be fixed to positions in accordance with a width of the paper by using setting screws. With this, the paper is pressed during printing to secure the distance between the ink jet head. 
     On the other hand, Patent Document 2 discloses a paper pressing mechanism including a pair of press clamps slidably provided on the platen. In the paper pressing mechanism, by depressing a press knob, a plate spring floats to thereby facilitate the insertion of end portions in a width direction of the printing paper, and by releasing the pressing force of the pressing knob, it is possible to press the end portions in the width direction of the paper.
     [Patent Document 1] Japanese. Patent Application Laid-open No. Hei 4-69264   [Patent Document 2] Japanese Patent Application Laid-open No. 2004-90538   

     However, in the paper presser of Patent Document 1, the pressing mechanism is fixed during the paper conveyance, and hence in the case of abnormal conveyances such as the paper is slantly conveyed with respect to an intrinsic conveying direction, a zigzag conveyance case, or the other cases, the end portions of the paper may dislodge from the pressing mechanism, the end portions may bump to curl up, or may interfere with the ink jet head. In those cases, the distance with the ink jet head may not be kept, or there is a fear of becoming difficult to continue the conveyance. 
     On the other hand, in the press cramp of Patent Document 2, there is employed the structure in which after the insertion of printing paper, the pressing pressure of the pressing knob is released to press the end portions in the width direction of the paper against the platen side. As a result, similar to the paper presser of Patent Document 1, in the case where the paper is conveyed slantly, or the zigzag movement, the end portions of the paper may dislodge from the pressing mechanism, the end portions may bump to curl up, or may interfere with the ink jet head. In addition, in the press cramps of Patent Document 2, in the case where, along with the slant movement of the paper, etc., the press cramps are moved outside the paper in the width direction, the press cramps do not return to their original positions, and hence it becomes difficult to press the end portions of the paper to be conveyed later. 
     Contrary to this, a holding force by the pressing mechanism is enhanced, thereby being capable of forcedly pressing the paper at the time of the abnormal conveyance. However, in this case, frictional resistance is applied to the paper under conveyance, there is a risk of breaking the paper. This becomes markedly if the paper is conveyed slantly or in zigzag. 
     Contrary to this, a holding force by the pressing mechanism is enhanced, thereby being capable of forcedly pressing the paper at the time of the abnormal conveyance. However, in this case, a frictional resistance is applied to the paper under conveyance, and thus there is a risk of breaking the paper. This risk increases if the paper is conveyed slantly or in a zigzag manner. 
     On the other hand, in the case where the gap between the pressing mechanism and the paper is increased and the holding force by the pressing mechanism is weakened, the paper may be conveyed smoothly even if the paper is conveyed slantly or in a zigzag manner. However, as the holding force is weak, the gap between the ink jet head and the paper is not constant, resulting in degradation of the printing quality due to possible displacements of adhering positions of the ink ejected from the ink jet head. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a medium press device capable of positively holding both ends of a recording medium (hereafter referred to as “medium”), such as printing paper or the like, in the width direction thereof, irrespective of a conveying state of the medium, and to an ink jet printer provided with such medium press device. In other words, an object of the present invention is to provide a medium press device capable of continuously pressing side end portions (hereafter referred to as “end portions”) of a recording medium even if the medium is conveyed slantly or in zigzag, and, with this, capable of maintaining a gap between the ink jet head and the medium into a desired gap to keep a printing quality uniform, and an to ink jet printer provided with such medium press device. 
     MEANS FOR SOLVING THE PROBLEMS 
     In order to solve the above-mentioned problems, according to the present invention, a medium press device for holding a medium conveyed in an ink jet printer, is characterized by including: a pair of press members for holding both ends of a width direction of the medium, respectively; and support tools for supporting the pair of press members, and is characterized in that the support tools support the press members so as to be capable of advancing and retreating in the width direction of the medium in alignment with positions of end portions of the width direction of the medium. 
     In the medium press device having the above-mentioned structure, it is preferred that each of the support tools include: an elastically deformable elastic support member; and a hold tool for holding the elastic support member, and that each of the support tools support the press member so as to be capable of advancing and retreating in the width direction of the medium through an elastic force provided to the elastic support member. 
     It is preferred that the hold tool maintain its position when a force received from the medium is within a predetermined range, and is slidable in the width direction of the medium when the force received from the medium exceeds the predetermined range, and that the hold tool include a hold member which is juxtaposed with the press member, in the width direction of the medium, through intermediation of the elastic support member. 
     Further, it is preferred that the hold tool include a gap restriction member for regulating a gap between the press member and the hold member, and further include the elastic restriction member which is elastically deformable in the width direction of the medium, and the elastic restriction member regulate a sliding range of the gap restriction member through the elastic force. 
     The pair of press members provided to the medium press device of the present invention is regulated in its sliding in the width direction of the medium through a resultant force of the elastic force of the elastic restriction member and a frictional force between the elastic restriction member and a movement restriction member which is provided to the ink jet printer, and when the force received from the medium exceeds the resultant force, the press member may slide in the width direction of the medium. 
     In the medium press device thus constructed, it is preferred that the elastic restriction member is received within a groove portion provided to the movement restriction member in a state abutting against an inner wall so that the frictional force is produced between the inner wall. In this case, the movement restriction member may be a platen. 
     As the elastic support member, a spring having elasticity in the width direction of the medium may be chosen, and the spring processed into a linear shape is preferred. 
     Further, an ink jet printer according to the present invention includes: a medium conveying device for conveying a medium; an ink jet head for discharging ink to the medium; and a medium press device for holding the medium, and is characterized in that: the medium press device includes: a pair of press members for holding both ends of the width direction of the medium, respectively; and support tools for supporting the pair of press members, and that the support tools support the press members so as to be capable of advancing and retreating in the width direction of the medium in alignment with positions of end portions of the width direction of the medium. 
     EFFECT OF THE INVENTION 
     According to the present invention, the pair of press members for holding the both ends of the width direction of the medium and the support tools for supporting the pair of press members are provided, and the support tools support the press members so as to be capable of advancing and retreating in the width direction of the medium in alignment with positions of end portions of the width direction of the medium. As a result, even in a case where a medium is conveyed diagonally or in a zigzag line, it is possible to keep pressing the ends of the medium, whereby it is possible to maintain a gap between the ink jet head and the medium to a desired gap to keep a printing quality constant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A perspective view illustrating the structure of an ink jet printer according to an embodiment of the present invention. 
         FIG. 2  A perspective view illustrating the structures of a platen and a medium press device according to an embodiment of the present invention. 
         FIG. 3  A perspective view illustrating the structure of the medium press device according to the embodiment of the Present invention. 
         FIG. 4  A side view illustrating the structure of the medium press device according to the embodiment of the present invention, which is viewed from a IV direction of  FIG. 3 . 
         FIG. 5  An enlarged view of a V portion of  FIG. 4 , illustrating an engagement state of a plate spring according to an embodiment of the present invention. 
         FIG. 6  A perspective view illustrating the structure of the medium press device according to the embodiment of the present invention. 
         FIG. 7  A perspective view illustrating a state in which the medium press device according to the embodiment of the resent invention is movably arranged with respect to the platen. 
         FIG. 8  A side view illustrating a state in which the medium press device according to the embodiment of the present invention is movably arranged with respect to the platen which is viewed from a VIII direction of  FIG. 7 . 
         FIG. 9  A sectional view taken along the line IX-IX of  FIG. 8 , illustrating an abutting state between an inner wall of groove portion according to an embodiment of the present invention and a plate spring. 
         FIG. 10  An enlarged sectional view taken along the line X-X of F  9  illustrating the abutting state between the inner wall of the groove portion according to the embodiment of the present invention and the plate spring. 
         FIG. 11  A plan view illustrating the structure of the medium press device in a state in which a first plate and second plate are spaced apart from each other when a wire working spring according to an embodiment of the resent invention is free from an outside force. 
         FIG. 12  A plan view illustrating the structure of the medium press device in a state in which the first plate and the second plate are closed with each other when the outside force causing the first plate to a roach to the second plate is applied. 
         FIG. 13  A plan view illustrating the structure of the medium press device according to a modification example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, detailed descriptions are made of a medium press device and an ink jet printer according to embodiments of the present invention with reference to drawings. 
     First, while referring to  FIG. 1 , an overall structure of an ink jet printer  10  according to this embodiment is described. Here,  FIG. 1  is a perspective view illustrating the structure of the ink jet printer  10 , which is viewed from a front upper direction. 
     The ink jet printer  10  includes at least, within a main body  12  disposed on a movable base  11 , a medium conveying mechanism  13 , an ink jet head  14 , a platen  15 , and a medium press device (medium holding apparatus)  30 . Note that, in the following description, a width direction of the medium refers to a direction B, which is orthogonal to a medium conveying direction A of  FIG. 1 . 
     The medium conveying mechanism  13  feeds the medium on which printing is performed to a predetermined printing position between the ink jet head  14  and the platen  15 , and discharges the medium which has been printed by the ink jet head  14 . Here, the medium on which printing by the ink jet is performed includes, for example, paper, a cloth, and a plastic sheet. 
     The ink jet head  14  includes a predetermined number of nozzles for ejecting ink downward with respect to the medium fed by the medium conveying mechanism  13 . The ink is ejected from one or two or more of the nozzles in accordance with an image to be printed. On the medium, by repeating the ink ejection and the conveying of the medium, a desired image is formed. Note that, the structures of the base  11 , the main body  12 , the medium conveying mechanism  13 , and the ink jet head  14  are well known, and hence detailed description thereof is omitted. 
     The platen  15  has a long plate shape, and is provided below the ink jet head  14  so that a longitudinal direction L thereof is in parallel with a width direction B of the medium. As illustrated in  FIG. 2 , the suction holes  16   a  are formed in the platen  15 . Here,  FIG. 2  is a perspective view viewed from front upper, and illustrates the structures of the platen  15  and the medium press device  30 . The suction holes  16   a  are formed so as to penetrate the platen  15  in a thickness direction, and pressures within the holes are made negative through an operation of a suction device (not shown) provided in the main body  12 , whereby the medium being conveyed on the platen  15  may be sucked. The suction force by the suction holes  16   a  is set so that the medium is not prevented from being conveyed, and the medium and the ink jet head  14  are kept at a predetermined gap. The suction holes  16   a  may have an arbitrary shape, and may be arbitrarily arranged, but the width of the medium is generally standardized. Accordingly, it is preferred to provide congest-arranged holes  16   b , in which the suction holes  16   a  are congested and more closely arranged than in other places, at positions in the vicinity of widths of a plurality of media that may be assumed in advance. This is to bring the end portions of the medium into close contact with the platen  15  so that the end portions of the medium are hard to curl up. 
     On both end sides of the longitudinal direction L or the platen  15 , the medium press devices (medium holding apparatuses)  20  and  30  are arranged, respectively. In the platen  15 , two groove portions  17  and  18  are formed so as to extend in the longitudinal direction L. The medium press devices  20  and  30  are movable in the longitudinal direction L in sliding contact with inner walls of the groove portions  17  and  18 . Opening portions of the groove portions  17  and  18  are narrower than bottom portions thereof so that the medium press devices  20  and  30  are free from being dislodged from the opening portions. The medium press devices  20  and  30  have the same structure, and are provided symmetric with respect to a center line  15   z  in the longitudinal direction L of the platen  15  to hold the both ends of the width direction of the medium. In the following description, only the medium press device  30  is described, and the detailed description of the medium press device  20  is omitted. 
     The medium press device  30  includes, as illustrated in  FIG. 3 , a first plate  31 , a second plate  32 , slider blocks  34 ,  35 ,  36 , and  37 , wire working springs  41  and  42 , stopper blades  51  and  52 , and plate springs  61  and  62 . Here,  FIG. 3  is a perspective view viewed from upper, for illustrating the structure of the medium press device  30 . The second plate  32 , the slider blocks  34 ,  35 ,  36 , and  37 , the wire working springs  41  and  42 , the stopper blades  51  and  52 , and the plate springs  61  and  62  constitute a support or supporting tool. Further, the second plate  32 , the slider blocks  34 ,  35 ,  36 , and  37 , the stopper blades  51  and  52 , and the plate springs  61  and  62  constitute a hold tool. The medium press device  30  holds the medium conveyed in the ink jet printer  10 , and the support tool supports the first plate  31  and a medium press portion  33  as one press member of the pair of the press members or holding members so as to be capable of advancing and retreating in the width direction of the medium in alignment with a position of the end portion. 
     Hereinafter, description is made of respective members constituting the medium press device  30 . 
     The first plate  31  and the second plate  32  as the hold member are each long plate shape members made of metals, such as stainless plate, for example. As illustrated in  FIG. 2 , from an inner side to an outer side in the longitudinal direction L of the platen  15 , the first plate  31  and the second plate  32  are arranged in parallel with each other in the stated order. Note that, as described hereinbelow, it is preferred that the members constituting the medium press device  30  be all constructed of metal. In this manner, when all the members are constructed of metal, compared with a case of constructing the members using a resin, influences due to temperature rise of the platen  15  or influences due to solvents constituting ink are less likely to occur thereby metal as the material being preferred. 
     As illustrated in  FIG. 3 , on an upper surface of the first plate  31 , engagement portions  31   a ,  31   b ,  31   c , and  31   d  are provided in the stated order in the longitudinal direction. The engagement portions  31   a  and the engagement portions  31   b  and the engagement portions  31   c  and the engagement portion  31   d  are arranged symmetric with respect to a center line  31   z  in the longitudinal direction of the first plate  31 . On an upper surface of the second plate  32 , engagement portions  32   a ,  32   b ,  32   c , and  32   d  are provided in the longitudinal direction in the stated order so as to correspond to the engagement portions  31   a ,  31   b ,  31   c , and  31   d , respectively. 
     Wire working springs  41  and  42  as elastic support members are linearly-processed metal members having the same shape, and are constructed of, for example, a steel wire for springs, which being one kind of a piano wire. As illustrated in  FIG. 3 , by bending the wire working springs into a U-shape, there is obtained an elastic force with which the spring tends to return to a straight line. As for the wire working spring  41 , one end portion  41   a  is inserted and held into/by the engagement portions  31   a  of the first plate  31 , and another end portion  41   b  is inserted and held into/by the engagement portions  32   a  of the second plate  32 . In addition, midways between those end portions are held by the engagement portions  31   b  and  32   b , whereby the wire working spring  41  is bent into the U-shape and held on the first plate  31  and the second plate  32 . 
     As for the wire working spring  42 , as well as the wire working spring  41 , one end portion  42   a  is inserted and held into/by the engagement portion  31   d  of the first plate  31 , and also another end portion  42   b  is inserted and held into/by the engagement portion  32   d  of the second plate  32 . In addition, midways between those end portions are held by the engagement portions  31   c  and  32   c , whereby the wire working spring  42  is bent into the U-shape on the first plate  31  and the second plate  32 . 
     Both the wire working spring  41  and the wire working spring  42  have the U-shape in which the end portion sides in the longitudinal direction of the first plate  31  and the second plate  32  are opened, thereby being expandable in the width directions of the first plate  31  and the second plate  2 , namely, in the width direction B of the medium. The wire working spring  41  and the wire working spring  42  are caused to have the U-shape described above, and the both ends thereof are held by the first plate  31  and the second plate  32 . As a result, in the width direction B of the medium, an elastic force caused by the wire working spring  41  and the wire working spring  42  may be applied between the first plate  31  and the second plate  32 . Further, if the wire working spring is used as the elastic support member, the elastic force may easily be set finely, and even if the wire working spring is arranged on the first plate  31  and the second plate  32 , the sizes in the thickness direction thereof may not be changed. In addition, the wire working springs  41  and  42  may only be mounted on the first plate  31  and the second plate  32 , and hence by forming groove portions similar to the groove portions  17  and  18  of this embodiment, the platen of a type in which the medium press member is held, may be used without modification. 
     On the first plate  31 , there is formed the medium press portion (medium press member)  33  along a long side which is far from the first plate  31 . The medium press portion  33  is formed through bending the second plate  32 . Note that, the medium press portion  33  may be formed by welding or bonding the medium press member which is a separate body to the first plate  31 . As illustrated in  FIG. 4  and  FIG. 5 , the medium press portion  33  is formed so as to extend in a direction separating from the second plate  32  at a position higher than the top surface of the first plate  31 . The medium press portion  33  presses downward, namely, on the platen  15  side the end portion of the width direction B of the medium to be conveyed by sandwiching it between, with a gap G as illustrated in  FIG. 5 , a lower surface  33   c  of the medium press portion  33  and an upper surface of the platen  15 , receives a side surface within a space extending  31   a  of the first plate  31 . Here,  FIG. 4  is a side view illustrating the structure of the medium press device  30 , which is viewed from a IV direction of  FIG. 3 .  FIG. 5  is an enlarged side view of a V portion of  FIG. 4  illustrating an engagement state of the plate spring  62  with respect to the stopper blades  52 . In  FIG. 5 , the platen  15  is illustrated by a dotted line for illustrating a relation with the platen  15  when the medium press device  30  is mounted to the ink jet printer  10 . 
     Further, medium guide portions  33   a  and  33   b  which are slanted upwardly as approaching to tips thereof are formed at the both ends in the longitudinal direction of the medium press portion  33 , respectively. The medium guide portion  33   a  facilitates the introduction of the medium conveyed to the platen  15  into a lower side of the medium press portion  33 , and the medium guide portion  33   b  facilitates the discharge of the medium after printing. 
     Further, as illustrated in  FIG. 3 , tilting pieces  31   e  and  31   f  are formed at a long side on the second plate  32  side of the first plate  31 . The tilting pieces  31   e  and  31   f  are formed so as to protrude toward the second plate  32  side at symmetrical positions with respect to a center line  31   z  of the first plate  31 . The tilting pieces  31   e  and  31   f  are provided so as to extend over the upper surface of the second plate  32  so that the first plate  31  is prevented from colliding with the second plate  32  when the first plate  31  and the second plate  32  are caused to approach with each other against an elastic force of the wire working spring  41  and the wire working spring  42  ( FIG. 12 ). When a user of the ink jet printer  10  depresses the tilting pieces  31   e  and  31   f  downwardly, the medium press portion  33  moves upward. As a result, the insertion of the medium into the under the medium press portion  33  is facilitated. 
     As illustrated in  FIG. 3 , the slider blocks  34  and  36  are fixed to the both ends of the longitudinal direction of the first plate  31  at the lower surface thereof, respectively. The slider blocks  34  and  36  are metal members having a same shape with each other and formed of an aluminium material, for example, and respective slider blocks  34  and  36  are fixed at the both ends of the longitudinal direction of the first plate  31  with screws  34   a  and  36   a , so as to extend toward the width direction of the first plate  31 . Further, the slider blocks  35  and  37  each are fixed to the both ends of the longitudinal direction at the lower surface of the second plate  32 . The slider blocks  35  and  37  are metal members having the same shape with the slider blocks  34  and  36  fixed to the first plate  31  and formed of an aluminium material, for example, and respective slider blocks  35  and  37  are fixed at the both ends of the longitudinal direction of the second plate  32  with screws  35   a  and  37   a , so as to extend toward the width direction of the second plate  32 . Note that, the slider blocks  34 ,  35 ,  36 , and  37  may be formed of plastic material. 
     As illustrated in  FIG. 3 , the slider blocks  34  and  35  are mounted on the stopper blades  51 , and the slider blocks  36  and  37  are mounted on the stopper blades  52 , respectively. The stopper blades  51  and  52  as the gap restriction members are members obtained by processing the both end portions of the longitudinal direction of the long plate shape member made of metal so as to bend upwardly, are formed of a stainless plate, for example, and have a same shape. The stopper blade  51  has a substantially same width with the slider blocks  34  and  35 . The distance between the movement restriction portions  51   a  and  51   b  (refer to  FIG. 6 ), which are formed by bending the both ends of the stopper blades  51 , is larger than the length of the slider block  34  and the slider block  35  which are aligned in the longitudinal direction. With this, the slider block  34  and the slider block  35  are movable on the stopper blades  51  until being brought into contact with the movement restriction portion  51   a  or the movement restriction portion  51   b . Here,  FIG. 6  is a perspective view illustrating the structure of the medium press device  30 , which is viewed from below. The length of each of the stopper blades  51  and  52  corresponds to a slidable amount of the first plate  31 , and may be determined based on a movable tolerance of the end portion of the width direction of the medium. 
     Similar to the stopper blades  51 , the stopper blades  52  has a substantially same width with the slider blocks  36  and  37 , on the other hand, the distance between the movement restriction portions  52   a  and  52   b , which are formed by bending the both ends of the stopper blades  52 , is larger than the length of the slider blocks  36  and the slider block  37  which are aligned in the longitudinal direction. With this, the slider block  36  and the slider block  37  are movable on the stopper blades  52  until being brought into contact with the movement restriction portion  52   a  or the movement restriction portion  52   b.    
     The slider blocks  34 ,  35 ,  36 , and  37  and the stopper blades  51  and  52  are constructed as described above. As a result, the first plate  31  and the second plate  32  become a state of separating from each other, in a natural state of being not applied with an outer force, due to an elastic force of the wire working springs  41  and  42 , under a state in which the slider block  34  is brought into contact with the movement restriction portion  51   a , the slider block  35  is brought into contact with the movement restriction portion  51   b , the slider block  36  is brought into contact with the movement restriction portion  52   a , and the slider block  37  is brought into contact with the movement restriction portion  52   b , respectively. Contrary to this, when the first plate  31  is caused to move toward the second plate  32  side against the elastic force of the wire working springs  41  and  42 , together with the first plate  31 , the slider block  34  moves on the stopper blades  51 , and the slider block  36  moves on the stopper blades  52 , respectively. Therefore, for example, the medium, which is normally conveyed, may be pressed with the first plate  31  and the second plate  32  of the natural state, whereas in the case where the medium is conveyed slantly to cause an end portion of the medium in the width direction is moved outside, the first plate  31  is caused to move toward the second plate  32  side by a force generated by the movement of the medium toward outside, thereby being capable of continuously pressing the medium. 
     As illustrated in  FIG. 6 , the plate springs  61  and  62  as the elastic restriction member are engaged to the movement restriction portion  51   a  of the stopper blades  51  and the movement restriction portion  52   a  of the stopper blades  52 . The plate springs  61  and  62  are metal members having the same shape and formed of a stainless plate, for example, and engagement methods to the stopper blades  51  and  52  are the same. Accordingly, in the following description, description is made of the plate spring  62 , and description of the plate spring  61  is omitted. 
     As illustrated in  FIG. 4 , the plate spring  62  is a long plate shape member having a substantially same width with the slider blocks  36 ,  37 , and the stopper blades  52 , and the both ends of the longitudinal direction thereof is bent toward the lower side to form the engagement portion  62   a  and the bending portion  62   b , respectively. The engagement portion  62   a  of the plate spring  62  is engaged with the movement restriction portion  52   a  of the stopper blades  52 . 
     Note that, the stopper blades  51  and  52 , and the plate springs  61  and  62  may be formed as an integral member, respectively. However, as in the embodiment described above, if they are formed as a separate member, deformations of the plate springs  61  and  62  do not hinder the slides of the slider blocks  34 ,  35 ,  36 , and  37  on the stopper blades  51  and  52 , thereby being preferred. 
     As illustrated in  FIG. 7  and  FIG. 8 , the stopper blades  51 , the slider blocks  34  and  35  mounted on the stopper blades  51 , and the plate spring  61  engaged to the stopper blades  51  are slidably received within the groove portions  17  recessed along the width direction B of the medium in the platen  15  as the movement restriction member. Similarly, the stopper blades  52 , the slider blocks  36  and  37  mounted on the stopper blades  52 , and the plate spring  62  engaged with the stopper blades  52  are received within the groove portions  18  recessed in parallel with the groove portions  17  in the platen  15 . Here,  FIG. 7  is a perspective view illustrating a state in which the medium press device  30  is movably arranged with respect to the platen  15 , which is viewed from above.  FIG. 8  is a side view illustrating a state in which the medium press device  30  is movably arranged with respect to the platen  15 , which is viewed from a VIII direction of  FIG. 7 . 
     As illustrated in  FIG. 9  and  FIG. 10 , a bending portion  62  forming the engagement portion  62   a  of the plate spring  62 , and the bending portion  62   b  on the end portion side of the other side of the plate spring  62  abut against an upper wall  18   a  of the groove portions  18 . Further, a flat plate portion  62   d  of a center of the longitudinal direction of the groove portions  18  abuts against the bottom wall  18   b  of the groove portions  18 . Here,  FIG. 9  is a sectional view taken along the line IX-IX of  FIG. 8 , illustrating an abutting state between an inner wall of the groove portion  18  and the plate spring  62 .  FIG. 10  is an enlarged sectional view taken along the line X-X of  FIG. 9 , illustrating the abutting state between the inner wall of the groove portion  18  and the bending portion  62   b  side of the plate spring  62 . 
     As stated above, the bending portions  62   b  and  62   c  of the plate spring  62  are caused to abut against the upper wall  18   a  of the groove portions  18 , the flat plate portion  62   d  is caused to abut against the bottom wall  18   b  of the groove portions  18 , respectively. As a result, a frictional force is generated between the plate spring  62  and the inner wall of the groove portions  18 , and hence by the frictional force and the elastic force of the plate spring  62 , the movement of the stopper blades  52  engaged to the plate spring  62  within the groove portions  18  may be restricted. This is the same with the plate spring  61  received within the groove portions  17 . 
     In the medium press device  30  thus constructed, first, in a state in which the medium is normally conveyed, the first plate  31  is separated from the second plate  32  by an elastic force of the wire working spring  41  and the wire working spring  42  which tend to expand in the width direction B of the medium, ( FIG. 11 ). Contrary to this, when the position of the end portion in the width direction of the medium expands outside due to reasons such as being conveyed slantly, the first plate  31  moves toward the second plate  32  side against the elastic force of the wire working spring  41  and the wire working spring  42  ( FIG. 12 ). Here, if the resultant force as a sum of the elastic force of the plate spring  62  and the frictional force between the plate spring  62  and the inner wall of the groove portion  18  is made larger than the elastic force of the wire working springs  41  and  42 , during the movement of the first plate  31  toward the second plate  32  side, the second plate  32  applied with an elastic force that tends to expand outside, which is received from the wire working spring  41  and the wire working spring  42 , may be latched to its original position. This is the same with the plate spring  61 . Thus, even in the case where the position in the width direction of the medium is moved outside and then returned to a normal convey, the first plate  31  may follow the end portion of the width direction of the medium, whereby the medium press device  30  may always press the end portion in the width direction of the medium. 
     In addition, in the case where the position in the width direction of the medium is so extremely displaced outside as a force exceeding the sum of the elastic force of the plate spring  62  and the frictional force between the plate spring  62  and the inner wall of the groove portions  18  is generated, the entire medium press device  30  may be moved. With this, the breakage of the medium may be prevented. In other words, when the force from the medium and received by the medium press device  30  falls within a predetermined range which is determined by the sum of the elastic force of the plate spring  62  and the frictional force between the plate spring  62  and the inner wall of the groove portion  18 , the position of the second plate  32  is maintained, whereas when the force received from the medium exceeds the above-mentioned predetermined range, the entire medium press device  30  is allowed to slide in the width direction of the medium. Here,  FIG. 11  is a plan view illustrating the structure of the medium press device  30  in a state in which the first plate  31  and the second plate  32  are spaced apart from each other when the wire working springs  41  and  42  are free from an outside force.  FIG. 12  is a plan view illustrating the structure of the medium press device  30  in a state in which the first plate  31  and the second plate  32  are closed with each other when the outside force causing the first plate  31  to approach to the second plate  32  is applied 
     When printing, the ink jet head  14  passes above the medium press device  30 . In the medium press device  30 , the medium press portion  33  is disposed at the highest position, namely, at a place close to the ink jet head  14 , and the wire working springs  41  and  42 , the engagement portions  31   a ,  31   b ,  31   c , and  31   d  of the first plate  31 , and the engagement portions  32   a ,  32   b ,  32   c , and  32   d  of the second plate  32  are arranged at the lower positions than the medium press portion  33 , namely, at the positions far from the ink jet head  14 . 
     Note that, in the medium press portion  33 , the medium guide portions  33   a  and  33   b  are provided at the positions where the ink jet head  14  does not pass, thereby being capable of employing a shape in which the medium guide portions  33   a  and  33   b  come to higher positions as approaching to the tips thereof. As a result, it is possible to facilitate the medium to be introduced and discharged. Further, in the first plate  31  and the second plate  32 , the screws  34   a ,  35   a ,  36   a , and  37   a  are arranged at regions where the ink jet head  14  does not pass. 
     The region at which ink is ejected from the ink jet head  14  corresponds to a region which is on the platen  15  and is sandwiched by the medium press device  20  and the medium press device  30 . Therefore, in the ink ejection region, the medium may positively be restricted at least by the medium press devices  20  and  30 , thereby being capable of the zigzag movement of the medium. 
     Hereinafter, description is made of a modification example thereof. 
     The medium press devices  20  and  30  may be arranged at arbitrary positions in the longitudinal direction of the platen  15 . For example, in a state where outside force is not applied to the wire working springs  41  and  42 , the first plate  31  may be brought into contact with the end portion of the width direction of the medium which is normally conveyed, or the end portion of the width direction of the medium may be covered with the medium press portion  33 , though being not brought into contact with the first plate  31 . In the case where the first plate  31  is disposed so as to contact with the end portion of the width direction of the medium, even if the medium slides in a direction being away from the first plate  31 , the medium may be maintained to be held if being covered with the medium press portion  33 . 
     Depending on the kinds of the medium or the other conditions, there may a case in which slant movement of the medium is liable to largely cause. In this case, however, if the first plate  31  is disposed so as to apply a force to the end portion of the medium, the holding of the medium is easily continued. Further, the positions of the medium press devices  20  and  30  are set so that the medium may be positively pressed at least a region at which the ink is ejected from the ink jet head. 
     In the embodiment described above, by using the elastic force of the wire working springs  41  and  42  having a U-shape, the gap between the first plate  31  and the second plate  32  are made adjustable. In place of this, as illustrated in  FIG. 13 , the coil springs  141  and  142  may be disposed between the first plate  131  and the second plate  132 . Here,  FIG. 13  is a plan view illustrating the structure of the medium press device  130  according to a modification example. The first plate  131  and the second plate  132  correspond to the first plate  31  and the second plate  32 , respectively, and the outer shapes when viewed planarly are the same with the first plate  31  and the second plate  32 , and are formed with the same material. The tilting pieces  131   e  and  131   f  of the first plate  131  are constructed similar to the tilting pieces  31   e  and  31   f  of the first plate  31 . In the medium press device  130 , the same effects with that of the above-mentioned embodiment may be obtained. In addition, the engagement portions  31   a ,  31   b ,  31   c ,  31   d ,  32   a ,  32   b ,  32   c , and  32   d  are not necessary to be formed on the first plate  31  and the second plate  32 , and hence the processing cost thereof becomes unnecessary, resulting in enhancing the strength of the first plate  131  and the second plate  132 . Further, the coil springs  141  and  142  are arranged at regions at which the ink jet head  14  does not pass as well as the screws  34   a ,  35   a ,  36   a , and  37   a  and the medium guide portions  33   a  and  33   b , the coil springs  141  and  142  are not brought into contact with the ink jet head  14 , thereby being capable of increasing the diameters thereof. 
     Further, in place of the coil springs  141  and  142 , a material equipped with elasticity such as sponge, rubber, felt, or the like may be used to adjust the gap between the first plate  31  and the second plate  32 . 
     Further, in place of the plate springs  61  and  62 , a material equipped with elasticity such as a coil spring, sponge, rubber, felt, or the like, or a member having a given frictional force may also be used. If the coil spring is adopted, for example, it is preferred that one end portion of the coil spring be abutted against the movement restriction portion  51   a  of the stopper blades  51  and the movement restriction portion  52   a  of the stopper blades  52 , and another end portion of the coil spring be abutted against the wall separately provided, respectively. 
     Still further, in the above-mentioned embodiment, two U-shape springs as the wire working springs  41  and  42  are used. However, the shape of the spring and the number of the springs may arbitrary be set. For example, only one U-shape spring may be disposed at the center in the longitudinal direction of the first plate  31 . Further, as the shape of the spring, for example, a V-shape or a rectangle shape may be employed other than the U-shape. In addition, even in the case where two U-shape springs are used, the two U-shape springs may be arranged so that the center side in the longitudinal direction of the first plate  31  is opened. 
     Further, between the first plate  31  and the movement restriction portion  51   b  of the stopper blades  51 , and between the first plate  31  and the movement restriction portion  52   b  of the stopper blades  52 , an elastic member such as a linear shape spring or a coil spring may be arranged. According to this structure, when the medium is normally conveyed, the elastic member is in the natural state. However, when the end portion in the width direction of the medium moves outside, the elastic member is compressed. Even in this case, too, in the case where the position in the width direction of the medium is displaced so extremely outside as exceeding the frictional force between the plate spring  62  and the inner wall of the groove portion  18 , the entire medium press device  30  may de moved outside. With this, the breakage of the medium may be prevent. 
     Further, the medium press devices  20  and  30  were supported by the platen  15 , but the medium press devices  20  and  30  may be supported other than the platen  15  by causing the plate springs  61  and  62  to be received in the groove portions  17  and  18 , respectively. 
     Still further, there may also be provided a lock mechanism for switching on and off of the sliding of the medium press devices  20  and  30 . 
     As constructed as described above, according to the above-mentioned embodiments, the following effects may be obtained. 
     (1) Irrespective of the conveying state of printing paper or other medium, it is possible to securely hold the both ends of the width direction thereof. 
     (2) Even in a case where a medium is conveyed diagonally or in a zigzag line, it is possible to keep pressing the ends of the medium, whereby it is possible to maintain a gap between an ink jet head  14  and the medium to a desired gap to keep a printing quality constant.
 
(3) The medium press devices  20  and  30  always cover the end portions of the width direction of the medium, whereby it is possible to prevent the printing quality from degrading due to floating of the end portions of the medium, curling, and influences caused by cutting trash and other foreign matter remained in the end portions of the medium.
 
(4) As the medium is not pressed strongly, it is possible to suppress breakage of the medium.
 
     While the present invention was described with reference to the above-mentioned embodiments, but is not limited to the above-mentioned embodiments, and improvements and modifications may be possible for the purpose of improvements or within a scope of sprits of the present invention. 
     As described above, the medium press device according to the present invention is useful for an ink jet printer in which ink is ejected from an ink jet head to the medium, thereby forming an image, and is suited, in particular, to an inkjet printer in which the size of the medium is large, thereby being likely to cause the fluctuation of the conveying state thereof.