Abstract:
Disclosed is an inkjet printer comprises: an ink jet head having a first array of a plurality of nozzles from which ink drops are discharged; a scanning unit for scanning said ink jet head in a first direction so that the ink drops are discharged within a predetermined extent with respect to the first direction; and a transporter for reciprocally transporting a recording medium in a second direction orthogonal to the first direction, said transporter comprising at least one member brought into contacting with a recording surface of the recording medium, said at least one member being provided at a position that is outside of the predetermined extent with respect to the first direction. Since the transporter is brought into contact with the recording surface of the recording medium only at the position outside of the predetermined extent, the recorded ink that have not been dry is prevented from contacting with the member of the transporter, e.g., a roller. Therefore, a low cost printer that is capable of high speed recording without image quality deterioration can be obtained.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is based on Japanese Patent Application No. 2000-223723 filed in Japan on Jul. 25, 2000, the entire content of which is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an inkjet printer that performs printing by discharging ink toward a recording medium.  
           [0004]    2. Description of the Related Art  
           [0005]    Conventionally known inkjet printers form images by repeating the operation in which an inkjet head having multiple nozzles (hereinafter the ‘head’) is moved along the recording medium (hereinafter the ‘sheet’) in a direction perpendicular to the alignment of the nozzles, and the sheet is forwarded at a prescribed pitch in a direction parallel to the alignment of the nozzles. In order to perform high-speed printing using this construction, the number of nozzles may be increased, but because this entails an increase in size of the head to the same degree, the space required for the movement of the head also increases, which leads to an increase in the size of the apparatus.  
           [0006]    In order to avoid this problem, a hypothetical inkjet printer shown in FIG. 11 is possible. This printer  2  has a head  4  and a platen (support member)  6  located a prescribed distance from the ink discharge surface (bottom surface) of the head  4 . Upstream of the head  4  in terms of the direction in which the sheet  8  is conveyed is located a pair of paper supply rollers  10  by which to convey the sheet  8 , which was conveyed from the paper supply tray not shown in the drawing, onto the platen  6 . Further downstream of the head  4  in terms of the direction in which the sheet  8  is conveyed is located a pair of paper eject rollers  12  by which to eject the sheet  8  onto the paper eject tray not shown in the drawing. Here, the direction of sheet conveyance means the direction in which the sheet is supplied to the area that faces the head  4  or the direction in which the sheet is ejected from this area. The multiple nozzles  14  of the head  4  are aligned perpendicular to the path of conveyance of the sheet  8  (i.e., in the direction that is perpendicular to the sheet in FIG. 11( a ) and the top-down direction in FIG. 11( b ), such direction being hereinafter referred to as the ‘head forwarding direction’) at a prescribed pitch.  
           [0007]    Using this construction, an image is formed by repeating the operation in which the head is first placed at a prescribed position, the sheet is moved in either scanning direction, which is perpendicular to the head forwarding direction, the head is forwarded by a prescribed pitch in the head forwarding direction, and the sheet is moved in the opposite scanning direction. In +this printer  2 , the range of movement of the head  4  does not change even when the number of nozzles  14  is increased and the head  4  is elongated. However, because the ink is discharged onto the sheet  8  while the sheet  8  is moved in the scanning directions back and forth, it is possible that the conveyance rollers (i.e., the paper supply roller  10  and paper eject roller  12  on the side of the head  4  in the paper conveyance path in the drawing) that convey the sheet  8  may come into contact with the ink before the ink is dry, resulting in smudged images. Therefore, a certain length of latency is required before the sheet  8  is moved, and therefore it is difficult to increase the printing speed.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is accordingly to reduce the size of the apparatus and to provide an inkjet printer that can perform printing at a relatively high speed through the use of a head having nozzles aligned perpendicular to the sheet conveyance path.  
           [0009]    To achieve at least one of the above mentioned objects, an ink jet printer reflecting one aspect of the present invention comprises:  
           [0010]    an ink jet head having a first array of a plurality of nozzles from which ink drops are discharged, said nozzles being aligned at a prescribed pitch;  
           [0011]    a scanning unit for scanning said ink jet head in a first direction so that the ink drops are discharged within a predetermined extent with respect to the first direction; and  
           [0012]    a transporter for reciprocally transporting a recording medium in a second direction orthogonal to the first direction, said transporter being brought into contacting with a surface of the recording medium on which the ink drops are to be adhered, i.e., a recording surface, only at a position that is an outside of the predetermined extent with respect to the first direction.  
           [0013]    In this ink jet printer, a part of intended image can be obtained by discharging ink drops from the nozzles with moving the recording medium in the second direction by transporter and with fixing the ink jet head at a position with respect to the first direction. After that, the ink jet head is moved to and fixed at a different position with respect to the first direction by the scanning unit, and then, other part of the intended image is formed by discharging ink drops from the nozzles with moving the recording medium in the second direction by transporter. By repeating the above process, the entire of the intended image can be finally formed on the recording medium. Since the transporter is brought into contact with the recording surface of the recording medium only at the position outside of the predetermined extent, the recorded ink that have not been dry is prevented from contacting with the member of the transporter, e.g., a roller. Therefore, a low cost printer that is capable of high speed recording without image quality deterioration can be obtained.  
           [0014]    The nozzles of the first array may be aligned on a line parallel to or substantially parallel to the first direction, or be staggeringly aligned along with a line parallel to or substantially parallel to the first direction. The ink jet head may further have a second array of a plurality of nozzles aligned on a line parallel to or substantially parallel to the first direction, or be staggeringly aligned along with a line parallel to or substantially parallel to the first direction. The first and second arrays may be provided for mutually different colors.  
         BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]    These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings in which:  
           [0016]    FIGS.  1 ( a ) through  1 ( c ) are drawings showing a first embodiment of the inkjet printer pertaining to the present invention;  
           [0017]    [0017]FIG. 2 is drawings schematically showing the process of image formation using the inkjet printer shown in FIGS.  1 ( a ) through  1 ( c );  
           [0018]    FIGS.  3 ( a ) through  3 ( c ) are drawings showing a second embodiment of the inkjet printer pertaining to the present invention;  
           [0019]    FIGS.  4 ( a ) and  4 ( b ) are a drawing showing the recording area (the area in which images may be formed) when the inkjet printer shown in FIGS.  1 ( a ) and  1 ( b ) is used, and a drawing showing the recording area when the inkjet printer shown in FIGS.  3 ( a ) and  3 ( b ) is used, respectively;  
           [0020]    FIGS.  5 ( a ) through  5 ( c ) are drawings showing a third embodiment of the inkjet printer pertaining to the present invention;  
           [0021]    FIGS.  6 ( a ) through  6 ( c ) are drawings showing a fourth embodiment of the inkjet printer pertaining to the present invention;  
           [0022]    [0022]FIG. 7 is a drawing showing the recording area when the inkjet printer shown in FIGS.  6 ( a ) and  6 ( b ) is used;  
           [0023]    [0023]FIG. 8 is a side elevation showing a fifth embodiment of the inkjet printer pertaining to the present invention;  
           [0024]    [0024]FIG. 9 comprises side elevations showing modifications of the inkjet printer shown in FIG. 8;  
           [0025]    FIGS.  10 ( a ) and  10 ( b ) show other examples of the alignment of the nozzles of the inkjet head; and  
           [0026]    FIGS.  11 ( a ) and  11 ( b ) are drawings showing one example of the inkjet printer that forms images by repeating the operation in which the sheet is moved in either scanning direction and the head is forwarded by a prescribed pitch in the head forwarding direction. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    The preferred embodiments of the present invention are explained below with reference to the accompanying drawings. In the explanation below, an array-type inkjet head is used that has four nozzle arrays for yellow (Y), cyan (C), magenta (M) and black (K), which extend along the entire width of the recording area along the head forwarding path. However, an inkjet printer that incorporates an inkjet head having at least one nozzle array or a serial-type inkjet head having nozzles that extend in an area smaller than the entire width of the recording area along the head forwarding path is also included in the scope of the present invention.  
         [0028]    (First Embodiment)  
         [0029]    FIGS.  1 ( a ) through  1 ( c ) schematically show a first embodiment of the inkjet printer pertaining to the present invention. This printer  20  has an inkjet head  22  and a plate-like support member  24  that is located under the inkjet head  22 . This support member  24  supports the sheet  26  such that the distance between the sheet  26  and the ink discharge surface is approximately maintained at constant. The head  22  is an array-type head having four nozzle arrays, and each nozzle array has nozzles  28  ( 28 K,  28 M,  28 C,  28 Y) that are aligned at a prescribed pitch (a 120 dpi printing density, or nozzle density, for example) along the head forwarding path perpendicular to the direction in which the sheet  26  is forwarded. In the example shown in the drawing, the nozzle arrays are located in the order of black, magenta, cyan and yellow along the sheet conveyance path.  
         [0030]    The head  22  can move at a prescribed forwarding pitch in the head forwarding direction via the scanning mechanism S.  
         [0031]    The support member  24  has at one end of its length, which extends along the length of the head  22  (or along the head forwarding path), a vertical wall  24   a  that extends along the sheet conveyance path, such that the sheet  26  comes into contact with this wall  24   a , and moves on the support member  24  while being positioned at a certain position in the head forwarding path. The support member  24  has a pierced hole  24   b , which extends along the head forwarding path, between the vertical wall  24   a  and the head  22 . Conveyance rollers  30  and  32  are located above and below the support member  24 , respectively, such that they are in contact with each other and sandwich the pierced hole  24   b . To be more specific, the nipping areas (contact areas) of the conveyance rollers  30  and  32  are located on the same plane as the top surface of the support member  24 . The lower conveyance roller  32  is connected to a motor M that can rotate forward and backward, while the upper conveyance roller  30  rotates as the lower conveyance roller  32  rotates. At the other end of the length of the support member  24 , which extends along the length of the head  22  (or along the head forwarding path or the first path) is another conveyance roller  34 , such that it is in contact with the top surface of the support member  24 . This roller  34  rotates as the sheet  26  held between the nipping area formed by the support member  24  and the conveyance roller  34  moves. The conveyance roller  34  is also located such that the line connecting the nipping area formed by the support member  24  and the conveyance roller  34  and the nipping area formed by the conveyance rollers  30  and  32  is parallel to the head forwarding path.  
         [0032]    Using this construction, the sheet  26  can move forward or backward in either sheet scanning direction (the second path), which is perpendicular to the head forwarding path, based on the forward or backward rotation of the motor connected to the conveyance roller  32 , while being held at either edge thereof between the nipping areas of the conveyance rollers  30  and  32  and between the nipping areas of the conveyance roller  34  and the support member  24 .  
         [0033]    FIGS.  2 ( a ) through  2 ( c ) show a printing method in which a solid image having a 720 dpi resolution is printed using the head  22  having nozzles aligned at a 120 dpi pitch (in this drawing, the head has one nozzle array comprising four nozzles  28  for purposes of simplification, and each small circle represents an ink dot formed on the sheet  26 ). As shown in FIG. 1( c ), the leading edge of the sheet  26  is made to come into contact with the nipping areas of the conveyance rollers  30  and  32  and the nipping areas of the conveyance roller  34  and the support member  24 , the sheet  26  is moved upward in the drawing along the sheet scanning path, and one line of dots is printed by each nozzle  28  (FIG. 2( a )). The head  22  is then moved in the head forwarding direction in FIG. 1( c ) by a distance equivalent to one pixel (approximately 35 μm). The sheet  26  is now moved downward along the sheet scanning path, and a second line is printed by each nozzle  28  (FIG. 2( b )). By repeating this operation in which the head  22  is moved by a distance equivalent to one pixel in the head forwarding direction and the sheet  26  is moved, a solid image is formed (FIG. 2( c )).  
         [0034]    In this embodiment, because the conveyance members that move the sheet  26  forward and backward are located near the edges of the sheet  26 , which are outside the recording area (the area in which images may be formed), the ink does not come into contact with the conveyance members, and high-quality images may be obtained even during high-speed printing.  
         [0035]    (Second Embodiment)  
         [0036]    FIGS.  3 ( a ) through  3 ( c ) show a second embodiment of the inkjet printer pertaining to the present invention. In this printer  38 , the conveyance mechanisms that move the sheet  26  forward and backward and that are located at either end of the head  22  along the head forwarding path comprise conveyance rollers  30 A,  32 A and  34 A and conveyance rollers  30 B,  32 B and  34 B, which are located downstream and upstream of the head  22 , respectively, in terms of the direction in which the sheet  26  is conveyed. Either the lower conveyance roller  32 A or  32 B is connected to a motor (not shown in the drawings) that can rotate forward and backward. A belt  40  is wrapped around the lower conveyance rollers  32 A and  32 B, and the conveyance rollers  30 A and  30 B come into contact with the belt  40  via a pierced hole  24   b ′ formed on the support member  24 ′, so that the same sheet conveyance speed is maintained between the upstream and downstream sides of the head  22 .  
         [0037]    Using this construction, the sheet  26  may be moved forward or backward along the sheet scanning path based on the forward or backward rotation of the motor connected to the conveyance roller  32 A or  32 B while being held at either edge thereof between the nipping area formed by the conveyance rollers  30 A,  30 B and the belt  40  and the nipping area formed by the conveyance rollers  34 A,  34 B and the support member  24 ′.  
         [0038]    Incidentally, in the printer  20  shown in FIGS.  1 ( a ) through  1 ( c ), printing is performed after the leading edge of the sheet  26  comes into contact with the nipping area formed by the conveyance rollers  30  and  32  and the nipping area formed by the conveyance roller  34  and the support member  24 . Therefore, as shown in FIG. 4( a ), it is necessary to have a blank area in the leading edge area of the sheet  26  (similarly, a blank area needs to exist in the tail edge area as well). In contrast, in this embodiment, printing is performed after the leading edge of the sheet  26  comes into contact with the nipping area formed by the conveyance rollers  30 B and  32 B, and the nipping area formed by the conveyance roller  34 B and the support member  24 ′, which are all upstream of the head  22 . Therefore, as shown in FIG. 4( b ), it is possible to keep the sheet  26  away from the area under the nozzles  28  while the leading edge of the sheet  26  is in contact with the above nipping areas. In this case, it is not necessary to have a blank area at the leading edge area of the sheet  26  (similarly, it is not necessary to have a blank area at the tailing edge area of the sheet  26  either).  
         [0039]    (Third Embodiment)  
         [0040]    FIGS.  5 ( a ) through  5 ( c ) show a third-embodiment of the inkjet printer pertaining to the present invention. In this printer  42 , the lower conveyance rollers  32 A and  32 B are driven and connected by one or more intermediate rollers  44 , which are in contact with the outer surfaces of the conveyance rollers  32 A and  32 B (i.e., motive power is transmitted via the frictional force at the nipping area formed by the intermediate roller  44  and the conveyance roller  32 A and at the nipping area formed by the intermediate roller  44  and the conveyance roller  32 B), such that the same sheet conveyance speed is maintained between the upstream and downstream sides of the head  22 .  
         [0041]    (Fourth Embodiment)  
         [0042]    FIGS.  6 ( a ) through  6 ( c ) show a fourth embodiment of the inkjet printer pertaining to the present invention. In this printer  46 , the conveyance rollers  30 A,  32 A and  34 A and the conveyance rollers  30 B,  32 B and  34 B, which are located downstream and upstream of the head  22  in terms of the sheet conveyance direction, respectively, are located such that the head  22  does not interfere with the conveyance rollers  30 A,  30 B,  34 A and  34 B when the head  22  moves in the head forwarding direction. To be more specific, a belt  48  is wrapped around the lower conveyance rollers  32 A and  32 B, and this belt  48  is bent downward via a pair of intermediate rollers  50  and  52 , which are located essentially in the center between the lower conveyance rollers  32 A and  32 B, such that the belt  48  does not come into contact with the support member  24 ″. In addition, the belt  48  is in contact with the upper conveyance rollers  30 A and  30 B via pierced holes  24   b ″ and  24   b ′″ formed in the support member  24 ″.  
         [0043]    In this embodiment, because the conveyance rollers  30 A,  30 B,  34 A and  34 B are located such that they do not hinder the movement of the head  22  in the head forwarding direction, the side blank areas of the sheet  26  (the blank areas on the edges along the head forwarding path) may be made smaller than the side blank areas in the first embodiment shown in FIG. 4( a ) or the side blank areas of the second embodiment shown in FIG. 4( b ), as shown in FIG. 7.  
         [0044]    In this embodiment, it is preferred that the construction be such that the conveyance rollers  34 A and  34 B, which function to press down the sheet, are movable laterally (in either direction parallel to the head forwarding path) as shown in FIG. 6( c ), so that the printer  46  can handle sheets having a small width.  
         [0045]    (Fifth Embodiment)  
         [0046]    [0046]FIG. 8 shows a fifth embodiment of the inkjet printer pertaining to the present invention. In this printer  54 , a convex member that curves away from the head  22  with a prescribed radius of curvature is used as the support member  24 ′″, and the upper conveyance rollers  30 A and  30 B are located such that the distance between the centers of the rollers  30 A and  30 B is smaller than the distance between the centers of the lower conveyance rollers  32 A and  32 B (i.e., as shown in the drawing, the angle θ between the line connecting the centers of the upper conveyance roller  30 B and the lower conveyance roller  32 B and the tangent line of the support member  24 ′″ at the crossing point between the above connecting line and the support member  24 ′″ is less than 90 degrees).  
         [0047]    Using this construction, the area of the sheet that faces the head  22  may be placed in closer contact with the support member  24 ′″ than in the other embodiments described above, and as a result, the distance between the ink discharge surface of the head  22  and the sheet  26  may be maintained at an appropriate amount.  
         [0048]    [0048]FIG. 9( a ) and FIG. 9( b ) show modifications of the printer shown in FIG. 8. In the printer  54 ′ of FIG. 9( a ), the height of the nozzle is varied from one color to another along the sheet scanning path, such that the distance between the nozzle and the sheet  26  is essentially the same for all colors. In the printer  54 ″ of FIG. 9( b ), the part of the support member  24 ″″ that faces the head  22  is made flat, such that the distance between the nozzle and the sheet  26  is essentially the same for all colors.  
         [0049]    By making the distance between the support member and the nozzle of each color the same in this way, the variations in the positions of the dots of different color inks on the sheet may be reduced to the same degree.  
         [0050]    The above explanation relates to some embodiments of the present invention, and the present invention may be implemented in various other forms. For example, the sheet conveyance path (the path that incorporates the path in which the sheet is supplied to the area that faces the head  22  and the path in which the sheet is ejected from said area) does not need to be parallel to the sheet scanning path, and these paths may be perpendicular to each other, for example.  
         [0051]    In addition, the nozzles  28  need not be aligned straight along the head forwarding path, as shown in FIG. 1( c ). For example, as shown in FIG. 10( a ), the nozzles  28  may be aligned in straight lines which are angled a prescribed amount relative to the head forwarding path, or as shown in FIG. 10( b ), they may be divided into multiple blocks, with the nozzles  28  aligned straight in each block in a direction that has a prescribed angle to the head forwarding path.  
         [0052]    The construction of the present invention in which a pair of conveyance members are located near either edge of the recording medium may be applied in an inkjet printer that has (i) an inkjet head that is located a prescribed distance from the recording medium and that has a multiple ink discharge outlets aligned along the sheet conveyance path, and (ii) a pair of conveyance members that (a) are in contact with either edge area of the recording medium in terms of the head scanning path, which is perpendicular to the sheet conveyance path and that (b) move the recording medium along the sheet conveyance path, wherein images are formed by repeating the operation in which the inkjet head is moved forward and backward as to the recording medium in the head scanning path and the recording medium is moved in the sheet conveyance direction by the pair of conveyance members.  
         [0053]    Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.