PAPER FEEDING DEVICE FOR A PRINTER

Abstract: The present invention relates to a printer which is capable of eliminating the slack of paper, wherein, at the time of printing, the paper is fed by a pinch roller (4), while, at the time of paper feeding, the paper is caused to slide relatively at least on a platen roller (6a).

Description: The present invention relates to a printer. Fig. 1 is a perspective view of a known heat transfer colour printer. The heat transfer colour printer includes: a lower cabinet 1; a circuit section 2; a power source panel 3; a pinch roller 4; a capstan roller 5 with which the pinch roller 4 makes contact to be driven by it; and a platen roller 6. The platen roller 6, the pinch roller 4, and the capstan roller 5 together form a paper feed mechanism. The capstan roller 5 and the platen roller 6 are driven by a main motor 7. The printer also includes: a pinch roller lever 8 for contacting the pinch roller 4 with the capstan roller 5 and separating the pinch roller 4 from the capstan roller 5 in synchronization with the movement of a cam; a peel-off roller 9 for separating an ink film from a sheet of paper; a paper feed roller 10 for contacting the sheet of paper with the platen roller 6; a paper guide 11 for guiding the paper; a paper support 12 for housing a roll of paper; a cover frame 13 which is opened and closed by the disengagement and engagement respectively of a locking lever 14 with a locking lever shaft 15, the cover frame 13 having a ribbon feed gear (not shown) for positioning one end of a ribbon feed which comprises a roll of ink film and a ribbon feed presser 17 for pressing the opposing end of the ribbon feed, a printing head holder 18; a printing head 19 fixed to the printing head holder 18; a head arm 20 which is moved in synchronization with the cam and to which the printing head holder is fixed; a cooling fan 21 for radiating heat from the main motor 7 and the circuit section 2; an operation panel 22; a top cover 23; and an upper cabinet 24. In this known heat transfer colour printer, paper feeding and printing are carried out in the manner illustrated in Figs. 2a and 2b. At the time of printing, as shown in Fig. 2b, paper 25 is gripped between the capstan roller 5 and the pinch roller 4 and is fed in the direction of arrow B. At this time, the printing head 19 is pressed against the platen roller with the paper 25 passing therebetween. The platen roller 6, in contact with the paper feed roller 10, is rotatively driven clockwise by means of a one-way clutch (not shown in this figure) and is rotated by an amount equivalent to paper 25 fed by the capstan roller 5 and pinch roller 4. Meanwhile, at the time of paper feeding, as shown in Fig. 2a, the capstan roller 5 and the pinch roller 4, while gripping the paper, rotate in the direction of the arrows a, b. At this time, the printing head 19 is located at a position spaced apart from the platen roller 6, and the paper 25 is gripped between the platen roller 6 and the paper feed roller 10 and is fed in direction of arrow A by the platen roller 6 which is engaged with the capstan roller 5 by means of the one way clutch via gears. Incidentally, yellow, magenta, and cyan are arranged on an ink film 27 sequentially for each image plane, and the above-described operation is repeated three times to effect colour printing. However, with the arrangement shown in Figs. 2a and 2b, at the time when the paper 25 is set between the platen roller 6 and the pinch roller 4, if the paper 25 is set oblique with respect to the direction A or B, or if it is set with the paper 25 slackened between the pinch and capstan rollers 4, 5 on the one hand, and the platen roller 6 on the other, the state of the oblique setting and slackening of the paper 25 remain as they are since both the platen roller 6 and the capstanroller 5 are rotatively driven at the time of the paper feeding and printing. For this reason, when printing is effected, there may be a drawback in so far that oblique printing and misalignment of colour printing can occur. Patent Abstracts of Japan Volume 10, No. 231 of 12 August, 1986 discloses a printer which includes a platen roller against which a printing head abuts, in use, pinch rollers for pressing paper against the platen roller and feed rollers, located on the exit side of the platen roller, which are driven at a greater peripheral speed than said platen roller in order to remove slack from paper being fed between said platen roller and feed rollers. The end portions of the platen roller, which, in use, contact with a thermal transfer paper, are formed with a low coefficient of friction. Whilst this printer includes means to remove paper slack, these means only remove slackening which occurs in the paper whilst the paper is being fed between the platen roller and feed rollers. United Kingdom patent specification No. GB 2 141 385 A also discloses a printer which includes conveying rollers on the exit side of a platen roller. The conveying rollers comprise a pair, one of which has a surface with a high coefficient of friction. However, the conveyor rollers in this printer are provided to remove slack, in the form of bends, from paper during feeding and printing where such slack is a known consequence of the operation of said printer and said conveying rollers are deliberately initiated for short periods during operation of the printer to remove said slack. It is an object of the present invention to provide a heat transfer colour printer having a paper advancing mechanism which is capable of positively eliminating slackness of paper between a capstan roller and a platen roller at the time of paper feed. Accordingly, the present invention provides a printer comprising: a capstan roller (5); a pinch roller (4) for pressing paper (25) against said capstan roller (5); a platen roller (6a); a printing head (19) adapted to abut, in use, against said platen roller (6a), with paper (25) and an ink film (27) being interposed therebetween, for applying heat to said ink film (27) to effect printing on the paper; a paper feed roller (10) disposed adjacent the platen roller (6a) to cause the paper (25) to pass around said platen roller (6a); a paper feed means (26); and a paper drive means for driving the capstan roller (5) and platen roller (6a) during paper feeding and at least the capstan roller (5) during printing, wherein the platen roller (6a) has a low coefficient of friction over its entire outer surface to allow slipping of the paper (25) over said surface, the platen roller (6a), during paper feeding, is driven with a peripheral speed greater than that of the capstan roller (5) in order to take up any slack or correct misalignment in the paper (25). The foregoing and further features of the present invention will be more readily understood from the following description of the preferred embodiments, by way of examples thereof with reference to the accompanying drawings of which:- Fig. 1 is an exploded perspective view of a known heat transfer colour printer; Figs. 2a and 2b, are diagrams schematically illustrating paper feeding and printing mechanisms of a known heat transfer colour printer; Figs 3a and 3b are diagrams schematically illustrating paper feeding and printing mechanisms according to one embodiment of the invention; Figs 4a and 4b are top plan views illustrating a paper slackening correcting mechanism; Figs. 5a and 5b and 5c are diagrams schematically illustrating the operation of paper-feeding and printing mechanisms according to another embodiment of the invention; and Fig. 6a is a diagram schematically illustrating an engagement and disengagement mechanism for a printing head realizing the operation shown in Figs. 5a, 5b and 5c and Fig. 6 is a diagram schematically illustrating a pinch roller opening and closing mechanism. The present invention is arranged such that, during printing, paper is fed by a capstan roller, and during paper feeding paper is fed at least by a platen roller, and the paper is caused to slide relatively over the platen roller. Therefore, when an uneven slackening has occurred between right- and left-hand sides of paper spanning between the capstan roller and the platen roller, such a slackening of the paper is eliminated by causing the paper to slide over the platen roller during paper feeding. Referring now to the accompanying drawings, Figs. 3a and 3b are diagrams schematically illustrating paper feeding and printing sections of a printer in accordance with a first embodiment of the present invention. First, at the time of paper feeding, as shown in Fig. 3a, paper 25 is fed by an amount corresponding to one image plane from a paper holder 26 in the direction of arrow A by the clockwise rotational driving forces of the capstan roller 5 and a platen roller 6a. At this time, a printing head 19 is located at a position distant from the platen roller 6a, and an ink film 27 remains stopped, and printing is not effected. In order to prevent a slackening of the paper between the capstan roller 5 and the platen roller 6a, the outside diameter of the platen roller 6a is made slightly larger than a theoretical value governed by it having the same peripheral speed as the capstan roller 5, and the platen roller 6a is thereby adapted to feed the paper 25 at a greater rate than that of the capstan roller 5. In addition, in this embodiment, the surface of the platen roller 6a has a lower coefficient of sliding friction than that of a conventional platen roller 6, and there is provided a paper feed roller for bringing the paper 25 into contact with the platen roller 6a at a fixed winding angle. To achieve a lower coefficient of sliding friction, the surface of the platen roller 6a is coated with a Teflon (Trade name for polytetrafluoroethene)-based resin material. Incidentally, in Fig. 3a, reference numeral 28 denotes an ink film holder, while numeral 29 denotes an ink film take up holder. At the time of printing, as shown in Fig. 3b, the platen roller 6a and the pinch roller 4 rotate clockwise, and the paper 25 is unwound by the amount of one image plane in the direction of the arrow B. At this time, the head 19 is in contact with the platen roller 6a, the ink film holder 28 and the ink film 27 is also unwound in the direction of the arrow B. Subsequently, heat is applied from the head 19 to the ink film 27, and predetermined printing is effected on the paper 25. Yellow, magenta, and cyan are arranged sequentially within each image plane, and colour printing is carried out by repeating the above-described operation three times. Figs. 4a and 4b are top plan views schematically illustrating paper feeding and printing mechanism for a heat transfer colour printer in accordance with an embodiment of the present invention. First, the paper 25 is fed forward while being clamped by the pinch roller 4 and the capstan roller 5. At this time, the platen roller 6a is rotated by a one-way clutch 35 in synchronization with the capstan roller 5 via gears 32, 33 and 34, and the paper feed rate of the platen roller 6a is greater than that of the pinch roller 4 and the capstan roller 5 by an amount proportional to the increase in diameter of the platen roller 6a over its theoretical value. A pressing force of the paper feed roller 10 acting on the paper 25 assists feeding of the paper 25 further by applying tension thereon. Slackening of the paper 25 due to slippage of the paper 25 over the platen roller 6a is thereby eliminated. Subsequently, the head 19 is brought into contact with the platen roller 6a, and as the motor 36 is reversibly rotated, the paper 25 is fed backwards while being pinched between the capstan roller 5 and the pinch roller 4, and printing is then carried out. In this embodiment, by virtue of the above described arrangement, as shown in Fig. 4a, if a greater amount of slackening has occurred to the right-hand side of the paper 25, as viewed in the figure, between the capstan roller 5 and the platen roller 6a, the tension on the left-hand side becomes greater than that on the right-hand side. Accordingly, a component force A is generated in the direction of the left-hand side by the tension acting on the left-hand side of the paper 25, and the paper 25 is thereby pulled leftwardly until the slackening of the paper 25 is eliminated. In addition, if a greater amount of slackening has occurred on the left-hand side, as shown in Fig. 4b, the paper 25 is pulled rightward by a component force B until this slackening of the paper is eliminated, and the slacking is thus corrected. Hence, correction of the path of the paper 25 is also effected by allowing the paper 25 to slide on the platen roller 6a. As has been described above, in accordance with this embodiment, since it is possible to correct the slackening of the paper resulting from a degree of non-parallelism of the paper between the rollers, printing is always effected in a favourable state, so that it is possible to provide a high-quality printer. In the case of the above-described embodiment, it is impossible to correct a large degree skew (meandering) which occurs when the paper 25 is mounted. In another embodiment of the invention, an arrangement is provided to allow a large degree skew to be firstly corrected followed by correcting a small degree skew. Figs. 5a, 5b and 5c are diagrams schematically illustrating the operation of paper feeding and printing mechanisms of a heat transfer colour printer in accordance with this embodiment. First, at the time of the primary paper feeding for correcting a large degree skew, as shown in Fig. 5a, the pinch roller 4 is located distant from the capstan roller 5, and no driving force is imparted to the paper 25 by the capstan roller 5. Consequently, the paper 25 is fed by about an amount corresponding to one fourth of the image plane in the direction of arrow A by the counter-clockwise rotation of the platen roller 6a alone. If the paper 25 slants toward, for instance, the right-hand side with respect to the direction of the arrow A, the tension on the paper 25 spanning between the platen roller 6a and the paper holder 26 in the direction of the arrow B is greater on the left-hand side thereof with respect to the direction of the arrow A than on the right-hand side. Accordingly, as the platen roller 6a rotates, the paper 25 slidingly moves toward the left-hand side from the right-hand side with respect to the direction of the arrow A in the axial direction of the platen roller 26. The correction of the path of the paper 25 is thus carried out. In addition, even if the paper 25 is slackly set, it is possible to also eliminate the slack in the paper 25 by the above-described principle. During the secondary paper feeding for correcting a small degree skew, as shown in Fig. 5B, the pinch roller 4 moves to a position at which it is brought into contact with the capstan roller 5, and the paper 25 is then fed about by three fourths of the image plane in the direction of arrow A, and the feeding operation of the paper 25 is thereby completed. At this time, the correction of paper slackening as described with reference to the first embodiment is carried out. During printing, in the same way as the conventional example, the paper 25 and the ink film 27 are unwound by one image plane in the direction of the arrow B, and printing on one image plane is effected in the meantime (see Fig. 5C). Yellow, magenta, and cyan are arranged on the ink film 27 sequentially on each image plane, and colour printing is effected by repeating the above-described operation three times. Accordingly, since, upon starting of each paper feeding sequence, the operation shown in Fig. 5A is carried out, the feed path of the paper 25 is corrected on each such occasion. Therefore, printing is always effected in a favourable condition. In particular, since the slackening of the paper 25 can be caused by the non-uniformity of the diameter of the capstan roller 5, non-uniform pressing, uneven urging forces of springs disposed at opposite ends of the paper holder 26, shrinkage of the paper, or the like, the fact that the paper feed path is corrected on the occasion of each printing sequence has a significant effect. Referring now to Figs. 6a and 6b, description will now be given of the operation of the mechanisms depicted therein. Fig. 6a is a diagram schematically illustrating a printing head engagement and disengagement mechanism. As is apparent from the drawing, by virtue of the counter-clockwise rotation of a cam 30 and the action of a printing head holding spring 20c, the head arm 20 which traces the cam 30 by means of a roller 20b moves vertically. Namely, the printing head 19 is brought into contact with the platen roller 6a when the roller 20b traces each point 30a, 30b and 30c of the cam 30, the point 30a corresponding to yellow printing, the point 30b to magenta printing, and the point 30c to cyan printing, respectively. Fig. 6b is a diagram schematically illustrating a pinch roller opening and closing mechanism. As is apparent from the drawing, by virtue of the counter-clockwise rotation of a cam 31 and the action of a spring 8c, the pinch lever 8 which traces the cam 31 by means of a roller 8b moves vertically about a fulcrum 8a, and the gap between the pinch roller 4 and the capstan roller 5 is thereby opened and closed. Namely, the gap between the pinch roller 4 and the capstan roller 5 is opened at a point 31a on the cam 31 but is closed at all other points. The cams 30 and 31 are adapted to rotate concentrically, and the operations shown in Figs. 6a and 6b are effected simultaneously in conjunction with the rotation of the cams 30, 31 and the operations described with reference to Figs. 5a, 5b and 5c. Although, in the above-described embodiment, the operation is effected by means of a cam mechanism, it goes without saying that it is possible to realize the operations shown in Figs. 5a to 5c even if, for instance, a solenoid or the like is used. As has been described above, in accordance with the present invention, since the correction of the feed path of the paper can be performed in advance of printing, printing is always carried out in a favourable condition, so that it is possible to provide a high-quality heat transfer printer.

Claims: A printer comprising: a capstan roller (5); a pinch roller (4) for pressing paper (25) against said capstan roller (5); a platen roller (6a); a printing head (19) adapted to abut, in use, against said platen roller (6a), with paper (25) and an ink film (27) being interposed therebetween, for applying heat to said ink film (27) to effect printing on the paper; a paper feed roller (10) disposed adjacent the platen roller (6a) to cause the paper (25) to pass around said platen roller (6a); a paper feed means (26); and a paper drive means for driving the capstan roller (5) and platen roller (6a) during paper feeding and at least the capstan roller (5) during printing, wherein the platen roller (6a) has a low coefficient of friction over its entire outer surface to allow slipping of the paper (25) over said surface, the platen roller (6a), during paper feeding, is driven with a peripheral speed greater than that of the capstan roller (5) in order to take up any slack or correct misalignment in the paper (25). A printer according to claim 1, which includes an opening and closing means (31) for opening and closing a gap between the pinch roller (4) and capstan roller (5) wherein, for the duration of an initial period of paper feeding, the opening and closing means (31) maintains the pinch roller (4) distant from the capstan roller (5) and paper (25), whereby paper (25) is fed during this initial period by rotation of the platen roller (6a) only and a back tension exerted on the paper (25) by the paper feed means (26) causes slipping of the paper (25) over the surface of the platen roller (6a) in order to correct large degree misalignment of the paper (25). A printer according to claim 2, wherein the duration of the initial period of paper feeding corresponds to the paper (25) being fed by an amount approximately equal to a quarter image plane. A printer according to any preceding claim, wherein the surface of the platen roller (6a) is coated with a teflon (Trade name for polytetrafluoroethene) resin.

Inventors: KITAGAWA, TOSHIHARU, NOMURA, YOSHIKAZU, TAKITA, MASAAKI, TSURU, YOSHIKAZU
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: B41J15
IPC codes: B41J15