Patent Publication Number: US-7594656-B2

Title: Paper feed mechanism

Description:
This application claims the benefit of the Japanese Patent Application No. 2005-209491 filed on Jul. 20, 2005 and Japanese Patent Application No. 2005-266933 filed on Sep. 14, 2005 both of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     A paper feed mechanism is provided. 
     2. Related Art 
       FIG. 11  is a view schematically showing a generally employed paper feed mechanism  40  disclosed in Patent Application Publication No. H10-119374, which includes a metal cylindrical paper feed roller  41  on which protrusive portions  43  each having a predetermined width are formed at predetermined intervals. The protrusive portion  43  includes a plurality of projections  42  each having a predetermined height, a predetermined pitch, and an acute-angled top end on an outer peripheral surface of the paper feed roller  41  in the longitudinal direction. 
     Press contact rollers  45  are disposed opposite to the protrusive portions  43  such that a sheet of recording paper  44  is in tight contact with the paper feed roller  41  at a predetermined contact pressure. 
     As the press contact rollers  45  press the recording paper  44 , the projections  42  of the protrusive portions  43  are pushed into the back surface of the recording paper  44 . In this state, the paper feed roller  41  is driven to rotate such that the recording paper  44  formed as a board is fed while printing. 
     Referring to  FIG. 12 , a printer  46  equipped with the paper feed mechanism  40  further includes a platen roller  47  to the right of the paper feed roller  41 . A thermal head  48  is allowed to be in contact with (head-down) or released from (head-up) the platen roller  47 . 
     For the purpose of printing the color image on the recording paper  44  as the board such as a photographic printing paper, the generally employed paper feed mechanism  40  is operated to feed the recording paper  44  from the upstream side at the right side of the drawing to be interposed between the paper feed roller  41  that rotates in the direction of an arrow A and the press contact rollers  45  while holding the thermal head  48  in the head-up state. Different ink colors are printed on the recording paper  44  by performing the head-down and head-up operations of the thermal head  48  repeatedly as well as feeding the recording paper  44  back and forth in directions of A and B repeatedly such that a desired color image is printed. 
     The head-down operation of the thermal head  48  is performed at a starting position where a top edge  44   a  of the recording paper  44  is gripped between the paper feed roller  41  and the press contact roller  45  so as to start printing. 
     With the aforementioned structure, the printing cannot be performed in the area of the recording paper  44  from the top edge  44   a  to at least the distance of C. The resultant margin may waste the recording paper  44 , thus leading to the cost increase. 
     SUMMARY 
     A paper feed mechanism according to a first embodiment includes a printing device capable of printing a desired image on a sheet of recording paper, and paper feed rollers capable of feeding the recording paper to the printing device. In the paper feed mechanism, a plurality of projections arranged at predetermined pitches in an axial direction and a circumferential direction and having a predetermined height are formed on the outer peripheries of the surface of each of the paper feed rollers. The paper feed rollers and press contact rollers in press contact therewith are provided at an upstream side and a downstream side of the printing device in a direction in which the recording paper is fed during printing. The recording paper that is subjected to printing by the printing device is fed while receiving a pushing force from the plurality of projections on the paper feed rollers into a back surface of the recording paper by a predetermined depth. 
     The paper feed rollers include a first paper feed roller provided at the upstream side of the printing device, and a second paper feed roller provided at the downstream side of the printing device. Traces formed by the pushing force from the projections on the first paper feed roller on the recording paper and traces formed by the pushing force from the projections of the second paper feed roller on the recording paper are displaced from each other so as not to overlap. 
     The first and the second paper feed rollers are displaced from each other in the axial direction, and the traces are formed to be displaced from each other on the recording paper so as not to overlap. 
     The first and the second paper feed rollers are displaced from each other by about ½ of a pitch of the projections. 
     The projections of the first and the second paper feed rollers are displaced from each other in the circumferential direction such that the traces formed on the recording paper are displaced so as not to overlap. 
     A paper feed mechanism according to a second embodiment includes a printing device capable of printing a desired image on a sheet of recording paper, and paper feed rollers capable of feeding the recording paper to the printing device. In the paper feed mechanism, the paper feed rollers include a first paper feed roller provided at the upstream side of the printing device, and a second paper feed roller provided at the downstream side of the printing device in a direction in which the recording paper is fed during printing. Protrusive portions each having a predetermined width in an axial direction are formed on the outer peripheries of the surfaces of the first and the second paper feed rollers, including a plurality of projections arranged at a predetermined pitch and having a predetermined height. 
     The projections of the first and the second paper feed rollers are arranged such that the position at which the recording paper receives a pushing force from the projections on the first paper feed roller is different from that at which the recording paper receives a pushing force from the projections on the second paper feed roller except for a group of the projections. 
     The projections formed on the first and the second paper feed rollers are aligned such that virtual lines formed by connecting adjacent projections become diagonal with respect to the axial direction at predetermined angles, and the predetermined angles of the diagonal virtual lines on the first paper feed roller is different from those of the diagonal virtual lines on the second paper feed roller. 
     A paper feed mechanism according to a third embodiment includes a printing device capable of printing a desired image on a sheet of recording paper, and paper feed rollers capable of feeding the recording paper to the printing device. In the paper feed mechanism, the paper feed rollers include a first paper feed roller provided at the upstream side of the printing device, and a second paper feed roller provided at the downstream side of the printing device in a direction in which the recording paper is fed during printing. Protrusive portions each having a predetermined width in an axial direction are formed on the outer peripheries of the surfaces of the first and the second paper feed rollers, including a plurality of projections arranged at a predetermined pitch and having a predetermined height. The end portions of the protrusive portions are diagonal with respect to the axial direction of the first and the second paper feed rollers at predetermined angles. 
     The protrusive portions of the first and the second paper feed rollers are diagonal with respect to the axial direction at the predetermined angles in opposite directions with respect to each other. 
     The protrusive portions of the first and the second paper feed rollers are diagonal at the same angle in opposite directions with respect to each other. 
     The plurality of projections of the protrusive portions on the first and the second paper feed rollers are displaced from each other in a circumferential direction such that a position at which the recording paper receives a pushing force from the projections of the first paper feed roller does not overlap with that at which the recording paper receives a pushing force from the projections of the second paper feed roller. 
     In the paper feed mechanism according to the first embodiment, a plurality of projections arranged at predetermined pitches in an axial direction and a circumferential direction and having a predetermined height are formed on the outer peripheries of the surface of each of the paper feed rollers. The paper feed rollers and press contact rollers in press contact therewith are provided at an upstream side and a downstream side of the printing device in a direction in which the recording paper is fed during printing. The recording paper that is subjected to printing by the printing device is fed while receiving a pushing force from the plurality of projections on the paper feed rollers into a back surface of the recording paper by a predetermined depth. The resultant paper feed mechanism is capable of accurately feeding the recording paper during printing, and of realizing high quality image printing. 
     In the embodiment, the paper feed rollers include a first paper feed roller provided at the upstream side of the printing device, and a second paper feed roller provided at the downstream side of the printing device. Traces formed by the pushing force from the projections on the first paper feed roller on the recording paper and traces formed by the pushing force from the projections of the second paper feed roller on the recording paper are displaced from each other so as not to overlap. As the projections formed on the first and the second paper feed rollers are displaced from each other, the traces formed by a pushing force applied thereby do not overlap, thus performing the accurate paper feeding. 
     In the embodiment, the first and the second paper feed rollers are displaced from each other in the axial direction, and the traces are formed to be displaced from each other on the recording paper so as not to overlap. This makes it possible to feed the recording paper with high accuracy during printing. 
     In the embodiment, as the first and the second paper feed rollers are displaced from each other by about ½ of a pitch of the projections, the traces formed by a pushing force applied by the projections are not overlapped. 
     In the embodiment, as the projections of the first and the second paper feed rollers are displaced from each other in the circumferential direction such that the traces formed on the recording paper are displaced so as not to overlap, the recording paper may be fed with high accuracy during printing. 
     In the paper feed mechanism according to the second embodiment, protrusive portions each having a predetermined width in an axial direction are formed on the outer peripheries of the surfaces of the first and the second paper feed rollers, including a plurality of projections arranged at a predetermined pitch and having a predetermined height. The projections of the first and the second paper feed rollers are arranged such that the position at which the recording paper receives a pushing force from the projections on the first paper feed roller is different from that at which the recording paper receives a pushing force from the projections on the second paper feed roller except for a group of the projections. The paper feed mechanism including the first and the second paper feed rollers makes it possible to perform the full page printing on the recording paper from its top to bottom edges, thus eliminating the waste of the recording paper and reducing costs. It is structured such that a portion of the recording paper into which the projections on the first paper feed roller apply the pushing force is different from the portion of the recording paper into which the projections on the second paper feed roller apply the pushing force except a certain group of projections. The recording paper may be fed with high accuracy during printing, resulting in high quality image printing. 
     In the embodiment, the projections formed on the first and the second paper feed rollers are aligned such that virtual lines formed by connecting adjacent projections become diagonal with respect to the axial direction at predetermined angles, and the predetermined angles of the diagonal virtual lines on the first paper feed roller is different from those of the diagonal virtual lines on the second paper feed roller. The portion of the recording paper into which the projections of the first paper feed roller apply the pushing force is different from that of the recording paper into which the projections of the second paper feed roller apply the pushing force except a certain group of projections. This makes it possible to feed the recording paper with further improved accuracy. 
     In the sheet feeder mechanism according to the third embodiment, protrusive portions each having a predetermined width in an axial direction are formed on the outer peripheries of the surfaces of the first and the second paper feed rollers, including a plurality of projections arranged at a predetermined pitch and having a predetermined height, and the end portions of the protrusive portions are diagonal with respect to the axial direction of the first and the second paper feed rollers at predetermined angles. The paper feed mechanism equipped with a pair of paper feed rollers allows a full page printing of the recording paper from its top to bottom edges, thus eliminating waste of the recording paper as well as reducing the costs. As the protrusive portions are formed on the outer peripheries of the surface of the paper feed rollers diagonally, the traces on the recording paper formed by the pushing force from the projections are arranged into a zigzag shape. This makes it possible to increase the number of projections pushed into the recording paper. 
     The protrusive portions of the first and the second paper feed rollers are diagonal with respect to the axial direction at the predetermined angles in opposite directions with respect to each other. This makes it possible to increase the area of the zigzag traces formed on the recording paper, thus reliably feeding the recording paper. 
     As the protrusive portions of the first and the second paper feed rollers are diagonal at the same angle in opposite directions with respect to each other, the same paper feed rollers may be oppositely combined. The first and the second paper feed rollers may be the same, thus improving the manufacturing efficiency. 
     The plurality of projections of the protrusive portions on the first and the second paper feed rollers are displaced from each other in a circumferential direction such that a position at which the recording paper receives a pushing force from the projections of the first paper feed roller does not overlap with that at which the recording paper receives a pushing force from the projections of the second paper feed roller. The recording paper may be accurately fed, thus printing the image with higher accuracy. 
    
    
     
       DRAWINGS 
         FIG. 1  is a schematic view of a printer that employs a paper feed mechanism according to a first embodiment; 
         FIG. 2  is a schematic view that shows a positional relationship between a first paper feed roller and a second paper feed roller; 
         FIG. 3  is a schematic view that shows traces formed by the pushing force from the projections on the paper feed roller; 
         FIG. 4  is a schematic view that shows traces formed by the pushing force from the projections on the paper feed roller; 
         FIG. 5  is a schematic view that shows a preferred embodiment; 
         FIG. 6  is a schematic view that shows the first and the second paper feed rollers according to a second embodiment; 
         FIG. 7  is a schematic view that shows traces formed by the pushing force from the projections on the first and the second paper feed rollers according to the second embodiment; 
         FIG. 8  is a schematic view that shows traces formed by the pushing force from the projections of the first and the second paper feed rollers according to the second embodiment; 
         FIG. 9  is a schematic view that shows a first paper feed roller and a second paper feed roller according to a third embodiment; 
         FIG. 10  is a schematic view that shows traces formed by the pushing force from the projections on the first and the second paper feed rollers according to the third embodiment; 
         FIG. 11  is a side view of a generally employed paper feed mechanism; and 
         FIG. 12  is a schematic view that shows a printer equipped with the generally employed paper feed mechanism. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A paper feed mechanism according to the preferred embodiments will be described referring to the drawings. 
     A paper feed mechanism  1  according to a first embodiment includes a metal cylindrical first paper feed roller  2  provided upstream of a printing device  8  in a feeding direction of a recording paper  11  to be described later, for example, a direction D so as to be rotatably driven both clockwise and counterclockwise. 
     A plurality of projections  3   a  are equally spaced on the outer peripheral surface of a first paper feed roller  2  at a pitch p in the axial direction and a pitch t in the circumferential direction to form protrusive portions  3 , each having a predetermined width. 
     The projections  3   a  are arranged at the pitch p in the range of about 0.2 mm to about 1.0 mm, each of which has a height from the outer peripheral surface of the first paper feed roller  2  in the range of about 40 μm to about 100 μm. 
     The projections  3   a  are arranged at the pitch t in the circumferential direction in the range of about 0.4 mm to about 1.0 mm. A first press contact roller  4  formed of a hard rubber is disposed above the first paper feed roller  2  so as to be in press contact therewith. 
     The first press contact roller  4  is structured to rotatably follow the rotation of the first paper feed roller  2 . 
     The paper feed mechanism  1  of the present invention is equipped with a second paper feed roller  5  downstream of the printing device  8  to be described later so as to be rotatably driven. 
     A plurality of projections  6   a  are arranged on the outer peripheral surface of the second paper feed roller  5  at the approximately the same pitches p and t as those of the projections  3   a  to form protrusive portions  6 , each having a predetermined width. 
     A second press contact roller  7  formed of a hard rubber is disposed above the second paper feed roller  5  so as to be in press contact therewith. 
     The second press contact roller  7  is structured to rotatably follow the rotation of the second paper feed roller  5 . 
     The printer equipped with the paper feed mechanism  1  of the present invention includes the printing device  8  disposed between the first and the second paper feed rollers  2  and  5 . The printing device  8  includes a cylindrical platen roller  9 , and a thermal head  10  disposed above the platen roller  9  such that a heat generating portion  10   a  may be brought into contact with (head-down operation) or released from (head-up operation) the outer peripheral surface of the platen roller  9 . 
     The recording paper  11  formed as a board, for example, photographic printing paper may be fed in the directions of D and E between the platen roller  9  and the thermal head  10  such that the color images are printed thereon. 
     The upper surface of the printing paper  11  shown in the drawing is a printed surface  11   a  on which the color images are printed. The projections  3   a  and  6   a  on the first and the second paper feed rollers  2  and  5  are pushed into a back surface  11   b  as the lower surface of the recording paper  11  in the drawing by a predetermined depth. 
       FIG. 2  represents the positional relationship between the projections  3   a  and  6   b  formed on the first and the second paper feed rollers  2  and  5  of the paper feed mechanism  1  according to a preferred embodiment. Referring to  FIG. 2 , the first and the second rollers  2  and  5  are displaced from each other in the axial direction. 
     As the recording paper  11  is fed during printing, the projections  3   a  and  6   a  on the first and the second paper feed rollers  2  and  5  are pushed into the back surface  11   b  of the recording paper  11  to leave traces  3   b  and  6   b  as shown in  FIG. 3 . 
     The projections  3   a  and  6   a  are displaced from each other by p/2 as shown in  FIG. 2  such that the resultant traces  3   b  and  6   b  formed by the pushing force from the projections  3   a  and  6   a  on the back surface  11   b  of the recording paper  11  does not overlap. 
     The printer equipped with the above-structured paper feed mechanism  1  will be described taking a thermoelectric printer for example referring to  FIG. 1 . The recording paper  11  is gripped between the first paper feed roller  2  and the press contact roller  4  while holding the thermal head  10  in the head-down state. The recording paper  11  is fed toward the downstream side in the direction of the arrow D by rotating the paper feed roller  2  counterclockwise. As the recording paper  11  is fed by the feeding force generated by the first paper feed roller  2 , the top edge of the recording paper  11  serves to move the thermal head  10  in the head-down state upward. This embodiment makes it possible to have a sublimation printing of the first ink of the ink ribbon on the recording paper  11  from the top edge thereof. 
     As the platen roller  9  has been rotatably driven counterclockwise, the recording paper  11  pressed against the thermal head  8  may be smoothly fed toward the downstream side in the direction of D. This may prevent the recording paper  11  formed as the board from being bent between the first paper feed roller  2  and the platen roller  9 . 
     The recording paper  11  printed from the top edge is further gripped between the second paper feed roller  5  and the second press contact roller  7 . The second paper feed roller  5  is rotatably driven counterclockwise such that the recording paper  11  is fed toward the downstream side after printing. 
     Referring to  FIGS. 3 and 4 , the traces  3   b  formed by the pushing force from the projections  3   a  on the recording paper  11  are displaced from the traces  6   b  formed by the pushing force from the projections  6   a  on the recording paper  11  so as not to overlap. 
     When the full page printing of the first color to the bottom edge of the recording paper  11  has been performed, the thermal head  10  is brought into the head-up state, and the first and the second paper feed rollers  2  and  5  are rotated clockwise. The recording paper subjected to the first color printing is fed back to the upstream side in the direction of an arrow E. 
     The projections  3   a  and  6   a  on the first and the second paper feed rollers  2  and  5  are pushed into the traces  3   b  and  6   b  that have been formed thereby respectively on the recording paper  11  to be fed back to the upstream side in the direction of the arrow E. This makes it possible to feed the recording paper  11  backward with high accuracy without causing the displacement. 
     This allows the projections  3   a  and  6   a  on the first and the second paper feed rollers  2  and  5  to be pushed into the same traces  3   b  and  6   b  repeatedly irrespective of the displacement between the respective pitches p and t of the projections  3   a  and  6   a.    
     When the recording paper  11  is fed backward until the top edge of the recording paper  11  reaches the heat generating portion  10   a  of the thermal head  10 , each clockwise rotation of the first and the second paper feed rollers  2  and  5  is stopped, and the thermal head  10  is brought into the head-down state. 
     The first and the second paper feed rollers  2  and  5  are rotatably driven counterclockwise, and the platen roller  9  is rotatably driven such that the second ink color is printed over the first printed image on the recording paper  11  from the top to bottom edges for the full page printing. The desired color image may be printed on the recording paper from its top edge to the bottom edge by repeating the aforementioned operations. 
     In the first embodiment, the projections  3   a  and  6   a  of the first and the second paper feed rollers  2  and  5  are displaced from each other by P/2. The first and the second protrusive portions  3  and  6  may be arranged to be displaced by h as shown in  FIG. 5  so as not to overlap. 
     In the preferred embodiment embodiment, the first and the second protrusive portions  3  and  6  are formed at two positions on the first and the second paper feed rollers  2  and  5 . However, the first and the second protrusive portions  3  and  6  may be formed at three positions on the respective paper feed rollers. 
     Arbitrary numbers of projections  3   a  and  6   a  may be provided to form the first and the second protrusive portions  3  and  6 . Those numbers do not have to be constant, but may be variable in accordance with the size of the recording paper  11 . 
     A paper feed mechanism  21  according to a second embodiment is substantially the same as that of the first embodiment except that the first and the second paper feed rollers  2  and  5  are replaced by the first and the second paper feed rollers  22  and  24  as shown in  FIG. 6 . The first and the second paper feed rollers  22  and  24  will be described hereinafter. 
     First protrusive portions  23  each having a width F in the axial direction are formed on the outer peripheries of the surfaces of the first paper feed roller  22 . The first protrusive portion  23  includes a plurality of first projections  23   a  arranged at a predetermined pitch and has a predetermined height. The first projections  23   a  are aligned at the predetermined pitch such that each virtual line J formed by connecting adjacent first projections  23   a  is diagonal at an angle α with respect to the axis of the first paper feed roller  22 . 
     The adjacent first projections  23   a  on the virtual line J diagonal at the angle α are arranged at a pitch p in the range of about 0.2 mm to about 1.0 mm, and project at the height from the outer peripheral surface of the first paper feed roller  22  in the range of about 40 μm to about 100 μm. 
     The first projections  23   a  are arranged at the pitch t in the circumferential direction in the range of about 0.2 mm to about 1.0 mm. The first press contact roller  4  formed of a hard rubber is provided above the first paper feed roller  22  so as to be in press contact therewith. The first press contact roller  4  is structured to rotatably follow the rotation of the first paper feed roller  22 . 
     Second protrusive portions  25  each having a width F in the axial direction are formed on the outer peripheral surface of the second paper feed roller  24  likewise the first paper feed roller  22 . A plurality of second projections  25   a  likewise the first projections  23   a  are formed to define the second protrusive portions  25 . The second projections  25   a  are aligned at the predetermined pitch such that each virtual line K formed by connecting adjacent second projections  25   a  is diagonal at the angle β with respect to the axis of the second paper feed roller  24 . 
     The diagonal angles α and β of the virtual lines J and K respectively formed by connecting the first and the second projections  23   a  and  25   a  on the first and the second paper feed rollers  22  and  24  have different values. 
     In this preferred embodiment, the first and the second projections  23   a  and  25   a  are formed on the first and the second paper feed rollers  22  and  24  such that the virtual lines J and K formed by connecting the first and the second projections  23   a  and  25   a  are diagonal at different angles α and β with respect to the axis of the paper feed rollers. In this embodiment, the diagonal angle of the virtual line J is different from that of the virtual line K. Both the virtual lines J and K may be diagonal at the same angle. 
     The second press contact roller  7  formed of a hard rubber is disposed above the second paper feed roller  24  so as to be in press contacted therewith. 
     The second press contact roller  7  is structured to rotatably follow the rotation of the second paper feed roller  24 . 
     In the paper feed mechanism  21  according to the second embodiment, the first and the second projections  23   a  and  25   a  on the first and the second paper feed rollers  22  and  24  are pushed into the back surface  11   b  of the recording paper  11  to be fed during printing such that the first and the second traces  23   b  and  25   b  are formed at the positions into which the first and the second projections  23   a  and  25   a  are pushed as shown in  FIG. 7 . 
     The first traces  23   b  shown as filled circles are aligned such that the virtual lines J are diagonal at the angle α with respect to the line G in parallel with the axis of the first paper feed roller  22 . The second traces  25   b  shown as unfilled circles are aligned such that the virtual lines K are diagonal at the angle β that is different from the angle α. 
     With the paper feed mechanism  21  according to the second embodiment, traces  23   c  shown as unfilled triangles are defined by the first and the second traces  23   b  and  25   b  that partially overlap. Most of the first and the second traces  23   b  and  25   b  are arranged so as not to overlap. 
     The projections  23   a  and  25   a  on the first and the second paper feed rollers  22  and  24  are arranged such that positions on the recording paper  11  that receive the pushing force from the first projections  23   a  on the first paper feed roller  22  are different from those that receive the pushing force from the second projections  25   a  on the second paper feed roller  24  except a certain group of projections (that form the overlapped traces  23   c ). This makes it possible to feed the recording paper  11  accurately both in the directions D and E. 
     The first and the second paper feed rollers  22  and  24  may be structured to displace the first and the second projections  23   a  and  25   a  by the pitch such that the first and the second traces  23   b  and  25   b  at the left and right sides of the width F are arranged to be equally spaced along the circumferential direction. 
     In the aforementioned embodiment where the first and the second traces  23   b  and  25   b  at both sides of the width F are equally spaced along the circumferential direction, the number of the overlapped traces  23   c  may be reduced. 
     In the paper feed mechanism  21  according to the second embodiment, the projections that form the protrusive portions each with a predetermined width are arranged to be diagonal with respect to the axial direction. This increases the number of the projections formed in the circumferential direction, and accordingly, the number of the protrusions that are pushed into the back surface  11   b  of the recording paper  11 . 
     In the aforementioned embodiment, the recording paper may be reliably fed by the first and the second paper feed rollers during printing or after printing, resulting in the high quality image printing. 
     The paper feed mechanism  31  according to a third embodiment is substantially the same as that of the second embodiment except that the first and the second paper feed rollers  22  and  24  are replaced by the first and the second paper feed rollers  32  and  34  as shown in  FIG. 9 . The explanation with respect to the first and the second paper feed rollers  32  and  34  will be described hereinafter. 
     First protrusive portions  33  each having a width H in the axial direction are formed on the outer peripheries of the surfaces of the first paper feed rollers  32 . The first protrusive portions  33  include a plurality of first projections  33   a  arranged at a predetermined pitch and have predetermined heights. 
     The side edge of the first protrusive portion  33  including the plurality of the first projections  33   a  is diagonal at the angle α with respect to the axial direction of the first paper feed roller  32 . 
     Second protrusive portions  35  each having a width H in the axial direction are formed on the outer peripheries of the surfaces of the second paper feed rollers  34 . The second protrusive portions  35  include a plurality of second projections  35   a  each arranged at a predetermined pitch having a predetermined height. 
     The side edge of the second protrusive portion  35  including the plurality of the second projections  35   a  is diagonal at the angle β with respect to the axial direction of the second paper feed roller  34 . The second protrusive portions  35  are diagonal at the angle β in the opposite direction of the first protrusive portions  33  diagonal at the angle α. The first and the second protrusive portions  33  and  35  are diagonal in the opposite direction at the same angle. 
     The first and the second projections  33   a  and  35   a  are arranged at the same pitch and have heights on the first and the second protrusive portions  33  and  35  as in the first embodiment. 
     The paper feed mechanism  31  according to the third embodiment allows the first and the second projections  33   a  and  35   a  on the first and the second paper feed rollers  32  and  34  to be pushed into the back surface  11   b  of the recording paper  11  fed during printing. The back surface  11   b  of the recording paper  11  has the first traces  33   b  shown as filled circles and second traces  35   b  shown as unfilled circles, which are displaced from each other so as not to overlap. Those traces form a zigzag arrangement with the same width H as that of the first and the second protrusive portions  33  and  35 , as shown in  FIG. 10 . 
     The first and the second projections  33   a  and  35   a  on the first and the second paper feed rollers  32  and  34  are displaced by the P/2 so as not to overlap. The second traces  35   b  may be formed between the adjacent first traces  33   b  and  33   b  in the axial direction (not shown). 
     In the third embodiment, the first protrusive portion  33  is diagonal at the angle α opposite to the second protrusive portion  35  diagonal at the angle β. The first and the second protrusive portions  33  and  35  may be diagonal in the same direction, and the first and the second paper feed rollers  32  and  34  are displaced from each other in the axial direction so as not to overlap. 
     In the embodiment, the first and the second protrusive portions  33  and  35  are diagonal at the same angle in the opposite direction. They may be diagonal at different angles. 
     In the paper feed mechanism  31  according to the third embodiment, the protrusive portions each having a predetermined width and including the respective projections are diagonal with respect to the axial direction. This may increase the number of the projections in the circumferential direction, and accordingly the number of the projections pushed into the back surface  11   b  of the recording paper  11 . This allows the first and the second paper feed rollers to reliably feed the recording paper during or after printing, resulting in the high quality image printing.