JP2007223103A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent degrading of image quality due to fluctuation of a conveyance speed of a sheet caused by impact generated when the sheet is transported from an upstream side to a downstream side of a printing section such as a thermal head in an image forming apparatus such as a thermal printer in which conveyance rollers are respectively provided to the upstream side and the downstream side of the printing section so as not to form a blank portion on which the printing can not be carried. <P>SOLUTION: The first and second conveyance rollers 7, 8 are respectively provided to the upstream side and the downstream side of the thermal head 12 for printing on a sheet 3. A drive roller 9 for driving the second conveyance roller 8 is so constituted that a cut-out portion 9a is provided on a part of the circumference of the drive roller 9 to form a cross section having a D-shape. It is possible to form a gap not smaller than a thickness of a sheet between the cut-out portion 9a and a pressing roller 10, and then the sheet 3 can be freely passed therethrough without being interfered. When the drive roller 9 is rotated to be displaced from an angular position where the cut-out portion 9a is opened, the sheet 3 can be conveyed by being nipped between the cut-out portion 9a and the pressing roller 10 (shown in Fig.2). The straining of the sheet 3 between the first conveyance roller 7 at the upstream side and the second conveyance roller 8 does not occurs during the printing so that the sheet conveying speed can be stabilized and variation in an image can be eliminated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a thermal printer that forms an image on a recording paper such as plain paper (hereinafter referred to as “sheet”) and performs printing.

  In a thermal printer, a thermal head (thermal head) is pressed against a platen roller, and a plurality of heating elements arranged in a line on the thermal head are selectively heated based on image information, and the ink on the ink ribbon is thermally sublimated. And transfer it to the sheet. In the case of such a thermal transfer printer, conventionally, a conveyance roller for feeding a sheet to the thermal head has been arranged either upstream or downstream of the thermal head. However, in the configuration in which the conveying roller is arranged only on either the upstream side or the downstream side, the sheet being printed is always nipped and conveyed by the conveying roller, so there is a blank portion that cannot be printed at the leading edge and the trailing edge of the sheet. Remain.

  In order to solve this problem, there is an apparatus in which conveying rollers are arranged upstream and downstream of the thermal head previously proposed by the present applicant. (For example, refer to Patent Document 1). That is, when the leading edge of the sheet is printed, the downstream conveying roller does not interfere, and the sheet is conveyed by being sandwiched and pushed out only by the upstream conveying roller. On the other hand, when printing the trailing edge of the sheet, the upstream conveying roller does not interfere, and the downstream end of the sheet is nipped and pulled by only the downstream conveying roller. In this way, the conveyance rollers are in charge of the conveyance of the sheet on the upstream side and the downstream side so that printing can be effectively performed over the entire length without leaving margins at the leading edge and the trailing edge of the sheet.

  The transport roller has the same structure on the upstream side and the downstream side, and is composed of a feed roller and a pressure roller that face each other across the transport path. The feed roller is used for driving and the rotation power of the drive source motor is transmitted to automatically rotate, and the pressing roller presses the sheet against the feeding roller with an appropriate pressing force, thereby rotating the frictional force between the sheets sandwiched between the two rollers. Carry in.

  When a color image is formed and printed on a sheet, an image of that color is formed for each color each time the sheet is conveyed once. For each conveyance, the upstream and downstream conveyance rollers are switched between forward rotation and reverse rotation, and a color image is formed by overprinting different color inks on the sheet.

JP 2005-271286 JP

  However, like the device disclosed in the above publication, there is a device in which transport rollers are arranged upstream and downstream of the thermal head so as not to leave unprintable margins at the leading and trailing edges of the sheet. However, there are still the following unresolved problems. That is, when the printing in the first half area of the sheet is completed by the thermal head, the process shifts to printing in the second half area. When printing from the first half area to the second half area is performed, if there is a subtle difference in roller rotation speed when the sheet is transferred from the upstream conveyance roller to the downstream conveyance roller, the impact due to the tension is instantaneous. Join the sheet. As a result, the sheet conveyance speed fluctuates, and there is a problem in that a high-quality print image cannot be output due to density unevenness in the print image.

  It is an object of the present invention to change the sheet conveyance speed due to an impact generated when the sheet is transferred from upstream to downstream in an apparatus in which conveyance rollers are arranged upstream and downstream of the printing unit such as a thermal head so that a blank portion that cannot be printed is not generated. An object of the present invention is to provide an improved image forming apparatus that does not deteriorate image quality.

  In order to achieve the above object, in the image forming apparatus of the present invention, first and second transporting units are disposed on the upstream side and the downstream side of an image forming unit that forms and prints an image on a sheet, respectively. , Each of the second conveying means comprises a feed roller and a pressing roller for nipping and conveying the sheet, and the feed roller of the second conveying means is used for rotational driving for rotating by receiving rotational power. The cross section is formed in a D-cut shape with a cutout part in part, and a gap larger than the sheet thickness can be formed between the cutout part and the pressing roller. .

  According to the image forming apparatus of the present invention, since the driving feed roller has a D-cut shape in the downstream second conveying means, the sheet thickness or more is between the notch portion of the feed roller and the pressing roller. Can be freely passed without interfering with the sheet. Further, when the feed roller is rotated to displace the notch from the opened angular position, the sheet can be conveyed while being sandwiched between the pressing roller. By such a roller operation in the second conveying unit on the downstream side, there is no tension or the like that gives an impact to the sheet with the first conveying unit on the upstream side during printing by image formation in the image forming unit. As a result, the sheet conveyance speed is stabilized, and there is an effect that a high-quality printed image can be output without causing unevenness in the image printed on the sheet.

  Hereinafter, a thermal printer as a preferred embodiment of an image forming apparatus according to the present invention will be described in detail with reference to the drawings.

  Promise first about the terminology in the description. As shown in FIG. 1, the direction in which the sheet 3 fed from the sheet storage tray 2 is transported into the printer main body 1 is transported to the thermal head 12 during printing. The reverse occurs as indicated by the arrow D in FIG. Therefore, the expressions “upstream” and “downstream” in the conveyance path are used by reversing the upstream and downstream when the sheet 3 is fed and when it is printed.

  In FIG. 1, a printer main body 1 of a thermal printer is connected to a host computer, a personal computer, and a digital device such as a digital camera via a connection cable or the like so that image information is transferred. When the printer body 1 starts operating, the sheet 3 is sent out from the storage tray 2 and the movable plate 4 moves up and down around the movable axis in the vertical direction as shown in the figure. As a result, the paper supply / discharge roller 6 is brought into pressure contact with the sheet 3 and fed out, and is fed into the printer main body 1.

(First embodiment)
The sheet 3 fed out by the paper supply / discharge roller 6 is sent from a second conveyance roller (second conveyance unit) 8 inside the printer body 1 to a first conveyance roller (first conveyance unit) 7. The second transport roller 8 includes a feed roller 9 and a pressure roller 10 that are opposed to each other with a transport path interposed therebetween. The second transport roller 8 is for driving to transmit rotational power from a motor of a rotational drive source to the feed roller 9. The feed roller 9 is formed in a half-moon shape or a so-called D-cut cross-sectional shape by cutting a part of the peripheral portion, and is provided with a cut-out portion 9a. The other pressing roller 10 has a perfect circle shape. When the sheet 3 passes, the rotation of the feed roller 9 is stopped and the roller rotation angle is set so that a gap larger than the sheet thickness is opened between the notch 9a and the pressing roller 10 (see FIG. 1). Passes without any interference with the second transport roller 8.

  The leading edge of the sheet 3 that has passed through the second conveyance roller 8 eventually reaches the first conveyance roller 7 and is nipped and conveyed. When the trailing edge of the sheet 3 exits the second conveying roller 8, the trailing edge of the sheet 3 (which becomes the starting edge during printing) is detected by the sheet detection sensor 11 disposed in the vicinity of the first conveying roller 7. Is done. When the trailing edge of the sheet 3 is detected by the sheet detection sensor 11, a control signal for instructing rotation off is output from the printer body 1 and sent to the first conveying roller 7. The first conveying roller 7 stops the rotation of the feed roller by a rotation-off signal. At this stage, in the printing unit (image forming unit), a gap that allows the sheet 3 to pass between the thermal head 12 and the platen roller 13 is secured.

  Next, a rotation-on signal is sent from the printer main body 1 to the first conveying roller 7, and the rotation of the feed roller rotates in the direction opposite to the direction in which the sheet 3 has been taken up (arrow D direction). Reverse feed of the sheet 3 is started by reverse rotation. When the sheet 3 is reversely fed, the sheet 3 is conveyed toward the heating element array 14 on the thermal head 12 with the rear end portion thus far as the printing start end portion. When the printing start end of the sheet 3 reaches the heating element array 14, the reverse feeding of the sheet 3 is stopped.

  In the printing unit, the thermal head 12 that operates like swinging around the head arm shaft 15 starts to move toward the platen roller 13, and the ink ribbon 16 and the sheet 3 are put on the platen roller 13 that can freely rotate. Press contact. Energization is started to the heating element array 14 on the thermal head 12 based on the image information, transfer of the ink on the ink ribbon 16 to the sheet 3 is started, and printing is started in the first half area of the sheet 3.

  In synchronization therewith, the first conveyance roller 7 resumes reverse rotation to rotate the ink ribbon winding unit 17 while feeding the sheet 3 backward, and winds the ink ribbon 16 that has been transferred to the sheet 3. To start. The ink ribbon 16 before transfer is wound around an ink ribbon supply unit 18 that can freely rotate, and the ink ribbon 16 is supplied following the winding operation of the ink ribbon winding unit 17.

  By printing in the first half region, the printing start end portion of the sheet 3 passes through the gap between the feed roller 9 and the pressing roller 10 in the second conveying roller means 8 located on the downstream side in this case, and for a while, the upstream The sheet 3 is fed forward with the assistance of the first conveying roller 7 on the side. At this time, a sufficient space for passing the sheet is provided between the cut roller of the D-cut feeding roller 9 and the presser roller 10. When printing on the sheet 3 is started and a certain printing time (amount) passes, the feed roller 9 of the second transport roller 8 starts to rotate. The D-cut portion of the feed roller 9 is displaced by rotation, and the gap with the presser roller 10 is gradually narrowed. FIG. 2 shows such a displaced form of the paper feed roller 9.

  At the moment when the conveyance of the sheet 3 during printing is transferred to the second conveyance roller 8 in this way, the gap between the rollers of the first conveyance roller 7 on the upstream side that has been held so far is the thickness of the sheet 3. Open more. That is, as shown in FIG. 3, the feed roller 7a of the first conveying roller 7 moves in a direction away from the pressing roller 7b to widen the distance between the axes, and opens the gap between the rollers to release the sheet 3 from the nipping. Then, the rear end portion of the second half region of the sheet 3 is freely passed. Thus, the conveyance of the sheet 3 is completely taken over by the feed roller 9 and the pressing roller 10 of the second conveyance roller 8 on the downstream side.

  When the sheet 3 that has been taken over by the second conveying roller 8 and has the rear end portion of the second half area opened is sent to the thermal head 12 as it is, it passes through the heat generating element array 14 and is printed on the rear end portion of the sheet. It is printed effectively so that no part remains. In this way, at the stage where printing is completed without leaving a blank area for printing to the trailing edge of the latter half of the sheet area, the rotation of the feed roller 9 of the second conveying roller 8 stops. At the same time, the winding operation of the ink ribbon 16 is stopped, and the first color printing operation on the sheet 3 is ended.

  As is apparent from the printing operations so far, when the sheet is transported from the second transport roller 8 to the first transport roller 7, the second transport roller 8 freely passes without interfering with the rear end portion of the sheet. Conversely, when the sheet is transported from the first transport roller 7 to the second transport roller 8 (when printing), the first transport roller 7 is in a stage before the trailing edge of the sheet 3 comes off at a suitable timing. The sheet is released from the pinched state and freely passes through the rear end of the sheet. In this way, particularly when the sheet 3 is conveyed and transferred from the first conveying roller 7 to the second conveying roller 8 during printing, the sheet 3 is pulled by a slight difference between the upstream and downstream roller rotation speeds. Absent. This is important, and the conveyance speed of the sheet 3 does not fluctuate stably during printing, and it is possible to prevent image quality deterioration due to density unevenness and image unevenness, and to realize printing with high quality images.

  In other words, the stabilization of the conveyance speed of the sheet 3 is to set the sheet conveyance amount by the first conveyance roller 7 and the second conveyance roller 8 to be the same amount. However, in actuality, it is inevitable that a slight difference occurs in the transport amount between the first and second transport rollers 7 and 8 due to an error in component accuracy. Further, as the restraint time during conveyance while the sheet 3 is sandwiched between the first and second conveyance rollers 7 and 8 becomes longer, the degree to which the sheet 3 is pulled increases. Variations are likely to occur. In this embodiment, such a concern is effectively wiped out.

  At the time of color printing, the above printing operation is repeated several times so that different color inks are overprinted on the sheet 3 to form a color image. After the color image is formed, the sheet 3 is discharged by the flap 19, nipped and conveyed by the rotating paper supply / discharge roller 6 and the paper discharge pressing roller 20, and discharged onto the storage tray 2.

  In the present embodiment, the first and second transport rollers 7 and 8 are disposed in the upstream and downstream transport paths of the thermal head 12, respectively, so that the front end portion and the rear half region of the sheet 3 are located behind the front end region. Of course, it is possible to print effectively over almost the entire area without leaving a margin at the end.

  Further, by setting the feeding roller 9 of the second conveying roller 8 to have a half-moon cross-sectional shape or a D-cut cross-sectional shape, the sheet 3 can be nipped and conveyed, or the nipping can be released to efficiently perform free conveyance. . The feeding roller 7a (see FIG. 3) of the first conveying roller 7 is engaged with and disengaged from the pressing roller 7b, and the sheet 3 is nipped and conveyed, or the timing of the operation of opening the sheet 3 is achieved. A stable conveyance speed can be obtained and high-quality printing can be performed.

FIG. 4 is a diagram illustrating a configuration at the beginning of conveyance in which a sheet of a recording medium is taken from a tray into an apparatus main body in a thermal transfer printer which is an embodiment of an image forming apparatus according to the present invention. FIG. 4 is a diagram illustrating a state in which a sheet taken in the present embodiment is sent to a printing unit (image forming unit) and printing is performed. The figure which shows the aspect which open | releases the rear-end part of the latter half part area | region of a sheet | seat by the 1st conveyance roller of an upstream at the time of printing.

Explanation of symbols

1 Printer body 3 Sheet (recording paper)
7a Feed roller 7b Pressing roller 9 Feed roller with D-cut cross section 10 Pressing roller 12 Thermal head 13 Platen roller 14 Heating element array 16 Ink ribbon

Claims (3)

The first and second conveying units are arranged on the upstream side and the downstream side of the image forming unit that forms and prints an image on the sheet, respectively, and both the first and second conveying units hold the sheet and convey it. In an image forming apparatus comprising a roller and a pressing roller,
The feed roller of the second conveying means is formed in a cross-sectional D-cut shape in which a notch portion is provided in a part of the roller peripheral portion for rotation driving that receives rotational power, and the notch portion and the pressing roller An image forming apparatus characterized in that a gap larger than the sheet thickness can be formed between them.   The feed roller is movable in a direction away from the pressing roller of the first conveying unit, and the printing end portion in the second half area of the sheet is formed in the first conveyance while the image forming unit forms an image on the sheet. The image forming apparatus according to claim 1, wherein the feeding roller moves and the gap with the pressing roller becomes equal to or greater than the sheet thickness before the unit exits.   After the printing start end portion of the first half area of the sheet that has finished image formation in the image forming section has passed through the first conveying means opened in a gap equal to or greater than the sheet thickness, the feed roller of the first conveying means is cut out. 3. The image forming apparatus according to claim 1, wherein the image forming apparatus is configured such that a sheet can be nipped and conveyed between the pressing roller by being rotationally driven in a direction in which the sheet is displaced.

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