Curl correction apparatus

Abstract: Apparatus comprising holder means (4) for holding a rolled sheet material (9); transport means (5) for transporting the sheet material housed in said holder means; a curl correction member (22) positioned between said holder means and said transport means and adapted to guide said sheet material in a curved state for correction of the curl thereof; a guide member (21) contacting a face of said sheet material opposite to the face guided by said curl correction member and serving to guide said sheet material to said curl correction member; and moving means (27) for moving at least one of said curl correction member and said guide member according to the amount of said sheet material remaining in said holder means.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a curl correcting apparatus for correcting curling of a rolled sheet material. Related Background Art The recording system of various office equipment such as facsimile or printer generally employs a continuous web-shaped recording sheet wound in a roll form. Because of the rolled form, the sheet material tends to show curling. For this reason there have been proposed technologies for removing or reducing said curling. As an example of said technologies, Fig. 23 illustrates a curl eliminating apparatus, in which a main body 1 and a cover 2 are mutually articulated movably by a pin 3. In the main body 1 there are provided a roll holder 4, a platen roller 5, a cutter 6 and a guide member 7a constituting the curl eliminating device, while the cover 2 holds a recording head 8 and a guide member 7b constituting the curl eliminating device in cooperation with the guide member 7a. Said roll holder 4 supports a roll 9a of the sheet material 9. The sheet material 9 fed from said roll 9a is transported by the platen roller 5, and the curl is removed in passing an acute angled path formed by the guide members 7a, 7b. Then the sheet is subjected to image formation by the recording head 8, cut by the cutter 6 and discharged from the apparatus. In the conventional structure shown in Fig. 23, the sheet material 9 remains bent between the guide members 7a, 7b in the stand-by state of the apparatus. Thus the sheet material 9 between said guide members 7a, 7b generates curling in a direction opposite to that generated by winding, and such curling may give rise to troubles such as sheet jamming. Also the guide members 7a, 7b of fixed angle may be unable to adequately remove the curling which varies according to the diameter of the roll 9a. Fig. 24 illustrates another example of the already proposed curl removing apparatus. The clockwise rotation of the platen roller 5 is transmitted, through a frictional clutch, to a guide shaft 10 and arms 11 provided on both ends of said shaft 10. At the outer end of the arms 11, there is rotatably supported a curl correction shaft 12. In predetermined positions of the main body 1, there are fixed stoppers 13, 14 for limiting the movement of the arms 11, which is constantly biased toward the stopper 13 by means of a spring. The sheet material 9 is transported by the platen roller 5, of which rotation is transmitted to the guide shaft 10 thereby rotating the arms 11 clockwise. When the arms 11 impinge on the stopper 14, the rotation of the arms 11 is stopped by the slippage of the frictional clutch. The sheet material 9 is bent in acute angle in a path from the roll 9a, through the guide shaft 10, to the curl correction shaft 12 for curl correction, and is then transported to the platen roller 5. When the rotation of the platen roller 5 is terminated, the arms 11 rotate anticlockwise by the biasing force of the spring and return to the position in contact with the stopper 13. In this conventional structure, the curling on the sheet material 9 is removed by the guide shaft 10 and the curl correction shaft 12 mounted on the arms 11 rotated by the transportation of the sheet material 9. In the conventional structure shown in Fig. 24, in loading a new roll 9a in the roll holder 4, the leading end of the sheet material 9 has to be threaded between the guide shaft 10 and the curl correction shaft 12, so that a cumbersome operation is required. Also the sheet material 9 subjected to curl correction by the shaft 12 shows a large wrapping angle on the platen roller 5, thus eventually developing curl again. Also in the standby state of the apparatus, the sheet material 9 hangs down between the platen roller 5 and the roll 9a, thereby generating curl opposite to the curl of rolling. Also the main body 1 has to be made larger in order to accommodate the rotation of the arms 11 about the guide shaft 10 for curl correction of the sheet material 9. Also the curl correction apparatus disclosed in the Japanese Patent Laid-open No. 55-15381 is associated with a drawback of curl generation opposite to the curl by rolling, since the sheet material remains bent for curl correction in the stand-by state of the apparatus. Furthermore, a curl correction apparatus according to the preamble of the main claim is known from the document JP-A-62 175377. This apparatus is provided with a curl correction member fixed to a leaf spring and urges the sheet material in a curved state so as to avoid curling of the paper. Due to the decreasing rotational inertia load of a roll of sheet material from which the sheet material is withdrawn, the bending of the sheet material is reduced with a decreasing diameter of the roll. Also, document JP-A-63 74862 shows a curl correction apparatus in which a large range for correcting the curl is obtained by providing a movable additional movable roller which cooperates with the curl correction roller such that the angle of winding onto the correction roller is arbitrarily changed. In view of this prior art, it is the object of the invention to provide a curl correction apparatus which surely avoids curling of a sheet material in any transporting state of the material. The object is solved with a curl correction apparatus having the features of claim 1. Advantageously there is provided a retracting means composed of a rotatable member supporting the curl correction member or a moving member for moving said curl correction member in relation to the opening motion of the cover. If according to an advantageous modification the curl correction member is made retractable from the working position by the opening of the cover, even when the guide member and the curl correction member are involved in the sheet jamming developed in the course of curl correction, the disposal of such sheet jamming can be easily made by retracting the curl correction member by opening the cover. Also the loading of the roll is facilitated since the curl correction member is retractable from the working position by the opening of the cover and the threading of the leading end of the sheet material through the curl correction member is not necessary. Furthermore it is rendered possible to correct the curling of the sheet material in a recording apparatus by incorporating said curl correction mechanism in the apparatus, and to reduce the possibility of troubles such as sheet jamming, resulting from the curling of the sheet material. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view of a recording apparatus; Figs. 2A and 2B are views showing the principle of curl correction; Figs. 3A and 3B are magnified views of a part of Figs. 2A and 2B; Figs. 4A and 4B are schematic views of curl in a sheet material; Fig. 5 is a chart showing experimental results; Figs. 6A and 6B are views showing state when the sheet material is pulled back; Fig. 7 is a view of a state of opening the cover; Figs. 8 and 9 are views showing the loading of a roll; Fig. 10 is a schematic view of another embodiment; Figs. 11, 12 and 13 are schematic views of still another embodiment; Figs. 14 and 15 are perspective view of an embodiment in which the present invention is applied to the recording system of a facsimile apparatus; Fig. 16 is a cross-sectional view of the entire facsimile apparatus; Fig. 17 is a schematic view of recording state with a loaded roll of a large diameter; Fig. 18 is a schematic view of recording state with a loaded roll of a small diameter; Figs. 19 and 20 are schematic views of a curl correction mechanism; Figs. 21A to 21C are views showing the relations of a spring clutch and a stopper; Fig. 22 is a schematic view of another embodiment; and Figs. 23 and 24 are schematic views of prior arts. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, the present invention will be clarified in detail by embodiments thereof applied in a recording apparatus. Fig. 1 is a schematic perspective view of a recording apparatus; Figs. 2A and 2B are views showing the principle of curl correction; Figs. 3A and 3B are partial magnified views thereof; Fig. 4 is a schematic view of curling of a sheet material; and Fig. 5 is a chart showing experimental results. At first there will be briefly explained the structure of the recording apparatus. A cover 2 is rotatably articulated by a shaft 3 on a main body 1, and is made engageable with the main body 1 by a click mechanism (not shown). In a rear part of the main body 1 there is provided a roll holder 4, in which a roll 9a of sheet material 9 such as plain paper or thermal recording paper is loaded. The sheet material 9 extracted from the roll 9a goes around a guide shaft 21 constituting the guide member and a curl correction shaft 22 constituting the curl correction member, and is transported by a platen roller 5. A recording head 8, mounted on the cover 2 by means of a compression spring 8a, is maintained in pressure contact with said platen roller 5. An image is formed on the sheet material 9 by driving the recording head 8 according to image signals supplied from a control unit (not shown) while the sheet material 9 is transported by the platen roller 5. After said image formation, the sheet material 9 is cut by a cutter 6 and discharged from the apparatus. In the above-explained structure, the curl of the sheet material 9 is corrected by the wrapping thereof around the curl correction shaft 22. The amount of movement of the guide shaft 21 varies according to the weight of the roll 9a loaded in the roll holder 4, thereby varying the angle  of the sheet 9 around the curl correction shaft 22. If said angle  is larger, or the wrapping amount of the sheet 9 on the curl correction shaft 22 is larger, the curl correcting effect becomes smaller, and vice versa. In the following there will be explained various parts of the apparatus. The roll holder 4 is shaped as a polygon open on the top, and is positioned in the rear part of the main body 1. A part of the wall of said roll holder 4 at the side of platen roller 5 is folded to constitute a stopper 4a for an arm 27 to be explained later. The roll 9a is loaded in the roll holder 4, and a frictional resistance is generated by the contact between the external periphery of said roll 9a and the internal wall of said roll holder 4. Said frictional resistance is large when the roll 9a is large in diameter and heavy, but decreases as the roll 9a becomes lighter. Said stopper 4a is constructed in such a manner that, when in contact with a contactor 27b of the arm 27, the angle  of the sheet material 9 around the curl correction shaft 22 becomes about 30° by the function of a guide shaft 21 mounted on said arm 27. The platen roller 5 is driven by a motor 23 such as a stepping motor. The rotation of said motor 23 is transmitted, from a gear 23a mounted thereon, to a gear 25 mounted on the platen roller 5 through an intermediate gear 24. The recording head 8 is maintained in pressure contact with the platen roller 5 across the sheet material 9, which is transported by said platen roller 5. Said motor 23 is controlled by a control unit to rotate the platen roller in a direction a or b. In the image formation on the sheet material 9, the platen roller 5 is rotated in the direction a to advance the sheet material 9 in a direction c. After the sheet cutting by the cutter 6 subsequent to the image formation, the platen roller 5 is rotated in a direction b to return the leading end of the sheet material 9 to a predetermined stand-by position. The cutter 6 is composed of a pair of blades 6a, 6b. The blade 6a is fixed to the main body, while the blade 6b is rotatable about a shaft 6b1 and is driven by drive means (not shown). In response to a cutting signal from the control unit, the blade 6b rotates in shearing motion with respect to the blade 6a thereby cutting the sheet material 9. Said cutter 6 may be so constructed as to be driven by the motor 23, or by other drive means. Between said cutter 6 and platen roller 5, there are provided paired cutter guide members 6c, 6d for guiding the sheet material 9. The recording head 8, to be driven by image signals from the control unit for forming an image on the sheet material 9, is supported on the cover 2 through a compression spring 8a and is made rotatable about a shaft 8b at an end. In the present embodiment, the recording head 8 is composed of so-called thermal head having a plurality of heat-generating elements arranged on a face in contact with the sheet material, along the transversal direction thereof. In the image formation, the heat-generating elements 8c are selectively energized according to the image signals, and the image is formed on the sheet material 9 by the heat thus generated. The sheet material 9 is composed for example of plain paper or thermal recording paper, of the appropriate characteristics matching the property of the recording means. In the present embodiment employing a thermal head 8, the sheet material 9 is composed of thermal recording paper wound on a core 9b as a roll 9a. Because of said rolled form, the sheet material 9 develops curling, of which level depends on the diameter of the roll 9a. The curl developed at a large roll diameter has a small height H as shown in Fig. 4A, but the curl height H increases as the roll diameter decreases, eventually reaching a rounded curl state as shown in Fig. 4B. For this reason, there is required a curl correction mechanism functioning depending on the diameter of the roll 9a, for achieving effective curl correction. The guide shaft 21, constituting the guide member, serves to correct the curl developed in the sheet material 9, in cooperation with the curl correction shaft 22 to be explained later. Said guide shaft is rotatably mounted on an arm 27 which is rotatable about a shaft 26. Said shaft 26 is rotatably supported by the main body 1, and receives the power of the motor 23, through a slip clutch (frictional joint means) 28. Thus the power of the motor 23 is transmitted, through the gear 23a fixed to the motor 23, intermediate gear 24, gear 25 fixed to the platen roller 25 and an intermediate gear 29, to a gear 28a constituting the slip-clutch 28. The transmission system from the motor 23 to the platen roller 5 and the transmission system from the motor 23 to the arm 27 are preferably so constructed that the rotating speed of the arm 27 is equal to or slightly faster than the rotating speed of the platen roller 5 or the advancing speed of the sheet material 9. The power transmitted from said motor 23 to the shaft 26 rotates said shaft 26 in the same direction as that of the platen roller 5. The arm 27 need not necessarily be driven by the motor 23 for driving the platen roller 5, but can be driven by another motor. The slip clutch 28 is composed of a clutch plate 28b fixed to the shaft 26, a friction member 28c in face contact with said clutch plate 28b, and a compression spring 28d mounted on an end of the shaft 26. The gear 28a is pressed to the clutch plate 28b across the friction member 28c by means of the compression spring 28d, and the power of the motor 23 is transmitted to the shaft 26 by the friction of contact. Consequently the power transmitted to the shaft 26 is determined by the frictional coefficient between the clutch plate 28b and the friction member 28c, and the biasing force of the compression spring 28d. Therefore, the biasing force of the spring 28d is preferably made adjustable, by a nut engaging on a screw thread formed on the end portion of the shaft 26. A pair of arms 27 is fixed, at the approximate center thereof, to end portions of the shaft 26. The guide shaft 21 is rotatably supported on an end 27a of said arms 27 close to the roll 9a. The other end 27b of the arms 27 constitutes a contactor 27b for contacting the stopper 4a formed in a part of the roll holder 4 or a stopper 32 provided on a mounting member 30 to be explained later. Said arms 27 are so shaped that, when rotated in a direction e, the guide shaft 21 is accommodated in a space defined above a wall of the roll holder 4 opposed to the platen roller 5 and below a guide plate 34. The curl correction shaft 22 is rotatably mounted on a rotatable member (retracting means) 31, which is rotatably mounted on a mounting member 30 fixed to the cover 2. Said mounting member 30 is composed of a guide portion 30a serving as a guide member for the sheet material 9 and as a stopper for limiting the rotary motion of the recording head 8 when the cover 2 is opened, brackets 30b extending downwards from both sides of said guide portion 30a, a stopper 30c for the rotatable member 31, formed at the upstream side of the guide portion 30a with respect to the advancing direction of the sheet material 9, a stopper 30d for the rotatable member 31, formed at the downstream side, and brackets 30e extending upwards from both sides of the guide portion 30a. At the end of said brackets 30b there are provided stoppers 32 for contacting the contactors 27b of the arms 27 thereby limiting the rotatable range thereof. The brackets 30e rotatably support the rotatable member 31 by means of a shaft 33. Said rotatable member 31 is composed of axe-shaped plates rotatably mounted, in a direction f or g, on both ends portions of the shaft 33. The curl correction shaft 22 is rotatably mounted in the axe-shaped protruding part of said rotatable member 31. Consequently the rotatable member 31 always suspends vertically, by the weight thereof and the weight of the shaft 22, regardless of the opened angle of the cover 2. The rotatable range of the member 31 is limited as an upper face 31a impinges on the stopper 30c of the mounting member 30 and a lateral face 31b impinges on the stopper 30d. Also a lower face 31c of the rotatable member 31 impinges on the shaft 26 when the cover 2 is closed. When the cover 2 is closed, the rotatable member 31 can assume, as shown in Figs. 3A and 3B, a position in which the upper face 31a is in contact with the stopper 30c but the lower face 31c is separated from the shaft 26, or a position in which the upper face 31a is separated from the stopper 30c but the lower face 31c is in contact with the shaft 26. When the cover 2 is opened, the upper face 31a of the rotatable member 31 is separated from the stopper 30c of the mounting member 30 by said opening, or is already separated from the stopper 30c even prior to said opening, and the lower face 31c of the rotatable member 31 moves in a direction g along the shaft 26. When the cover 2 is closed, said lower face 31c moves in a direction f along the shaft 26, whereby the position of the rotatable member 31 is limited by the stopper 30c and the shaft 26. Consequently, when the cover 2 is closed as shown in Fig. 2A, 2B or 9, the rotatable member 31 is either in a position in which the upper face 31a is in contact with the stopper 30c and the lower face 31c is separated from the shaft 26, or in a position in which the upper face 31a is separated from the stopper 30c and the lower face 31c is in contact with the shaft 26. The curl correction shaft 22 is provided at a position at the upstream side of the platen roller 5 with respect to the advancing direction of the sheet material 9 and not providing an excessively large introduction angle of the sheet material 9 into the platen roller 5. The guide plate opposed to the shaft 22 extends at an end close to the platen roller 5 and at the other end close to the upright wall of the roller holder 4, and serves to guide the sheet material 9 from the roll 9a to the platen roller 5. In the following there wil be explained the principle of curl correction in the above-explained recording apparatus. Fig. 2A shows the curl correction in case the roll diameter is large, and Fig. 2B shows that in case of small roll diameter. The motor 23 rotates the platen roller 5 in a direction a, thereby advancing the sheet material 9 in a direction c. At the same time the power of the motor 23 is transmitted to the slip clutch 28, thereby rotating the arms 27, fixed on the shaft 26, in a direction d and bringing the guide shaft 21 into contact with the sheet material 9. Thus the transport path of the sheet material 9 is defined by the roll 9a, guide shaft 21, curl correction shaft 22 and platen roller 5, wherein the sheet material 9 wrappes the shaft 21 in the same direction as the winding direction of the roll 9a but the shaft 22 in the opposite direction. The curl of the sheet material 9 is corrected by the wrapping on the curl correction shaft 22. By the contact friction between the roll holder 4 and the roll 9a loaded therein, the sheet material 9 is subjected to a tension F, proportional to the weight of the roll 9a. In Fig. 2A, the sheet material 9 is subjected to a tension F 1 , corresponding to the weight G 1 of the roll 9a, before and after the guide shaft 21 and the curl correction shaft 22. When the shaft 21 moves in a direction d, the upper face 31a of the rotatable member 31 is brought into contact with the stopper 30c by the synthesized tensions F 1 before and after the curl correction shaft 22, thereby defining the position of said shaft 22. At the same time the wrapping amount of the sheet material 9 on the guide shaft 21 increases, whereby the resistance by said synthesized tension F 1 to the rotating force of the arms 27 (torque transmitted by the slip clutch 28) increases. Thus the rotation of said arms 27 is stopped at a position where the rotating force thereof balances with said synthesized force. In this state slippage is generated between the clutch plate 28b and the friction member 28c of the slip clutch 28. When the rotating force on the arms 27 balances with the tension F 1 on the sheet material 9, said sheet material 9 moves from the shaft 21 to 22 with an angle 1, and then proceeds to the platen roller 5. When the roll 9a is reduced in diameter, with a weight G 2 smaller than G1, as shown in Fig. 2B, the tension F 2 on the sheet becomes smaller than F 1 . Consequently, the resistance on the guide shaft 21 by said tension is reduced, and the arms 27 further rotate in the direction d, from the position under the tension F 1 . The arms 27 are stopped either at a position where the rotating force therefor balances with said resistance, or at a position where the contactors 27b engages with the stopper 4a if the rotating force is larger. The curl correction shaft 22 is lifted by the tension of the sheet material 9, and is stopped where the upper face 31a of the rotatable member 31 engages with the stopper 30c. In this state the angle 2 of the sheet material 9 before and after the shaft 22 is smaller than 1. Consequently, the wrapping amount of the sheet 9 on the curl correction shaft 22 is small for a large roll diameter, and increases as the roll diameter decreases. Fig. 4 shows the curl developed in the sheet material 9, wherein H indicates the curl height. Fig. 5 shows the curl height H of the sheet material 9 of a predetermined length, cut from the roll 9a with and without curl correction. In the present experiment, the sheet material 9 was composed of thermal recording paper of a width of 210 mm (corresponding to A4 size) and a length of 100 m, wound as a roll 9a on a core 9b of a diameter of 1 inch (25.4 mm). The initial roll diameter was 96 mm. The rotating force of the arms 27 was selected as 1 kg, and the diameter of the curl correction shaft 22 was 4 mm. The angle  of the sheet material 9 before and after the shaft 22 was selected as 130 - 30°. The sheet material 9 was cut at a length of 297 mm corresponding to A4 size. In Fig. 5, white triangles indicate sheets without curl correction, showing curl as indicated in Fig. 4A. White circles indicate those with curl correction, showing curl as indicated in Fig. 4A. Black triangles indicate sheets without curl correction, showing curl as illustrated in Fig. 4B. As will be apparent from Fig. 5, the height H of the curl developed in the sheet material 9 becomes smaller as the diameter of the roll 9a increases, and vice versa. A fully rounded curl is observed in the sheet material 9 when the diameter of the roll 9a becomes smaller than about 40 mm. On the other hand, the sheet material 9 corrected by the curl correction mechanism of the present embodiment is almost free from curl development, and the measured curl height H is almost constant. This fact indicates that appropriate correction was applied to the sheet materials 9 of different curl heights H depending on the roll diameter. As explained in the foregoing, the curl correction mechanism of the present embodiment defines the transport path of the sheet material 9 by the guide shaft 21 mounted the arms 27, depending on the balance between the tension F on the sheet 9 and the rotating force on said arms 27, thereby varying the angle of the sheet material 9 before and after the curl correction shaft 22 and correcting the curl of said sheet material 9 according to the tension F applied thereon. After the image recording of a page and the cutting of the sheet material 9 with the cutter 6 in the above-explained mechanism, the motor 23 is activated to rotate the platen roller 5 in a direction b in order to bring back the leading end of the remaining sheet material 9 toward the recording position of the recording head 8, whereby the power of the motor 23 is at the same time transmitted, through the slip clutch 28, to the shaft 26, thereby rotating the arms 27 in a direction e, until the contactor 27b impinge on the stopper 32. In this state, the guide shaft 21 is positioned on the extention of the guide plate 34 as shown in Figs. 6A and 6B, closing a space 35 defined by the roll holder 4 and the roll 9a. Also the slip clutch 28 shows slippage between the clutch plate 28b and the friction member 28c. Also since the curl correction shaft 22 is disengaged from the sheet material 9, the rotatable member 31 descends by the weight thereof to a position where the lower face 31c of the rotatable member 31 is contact with the shaft 26. The rotation of the arms 27 in the direction e releases the guide shaft 21 from the contact with the sheet material, whereby the guide shaft 21 no longer defines the transport path of the sheet material 9, thus generating a slack in the sheet material 9 between the roll 9a and the platen roller 5. Said slack portion of the sheet material 9 extends along the guide plate 34 positioned opposite to the shaft 22, and is supported by the shaft 21 positioned on the extention of the guide plate 34. Consequently, the sheet material 9 does not enter the space 35, but is accommodated in a space between the roll 9a and the cover 2, showing a mild curve as illustrated. Thus the sheet material does not develop local folds. When the diameter of the roll 9a decreases as shown in Fig. 2B, the guide shaft 21 moves to above the curl correction shaft 22 by the rotation of the arms 27 in a direction d. The shafts 21, 22 mutually interfere if the cover 2 is opened in this state, for example for removing a sheet jamming developed in the apparatus. In the present embodiment, the cover 2 can be easily opened even when the guide shaft 21 and the curl correction shaft 22 mutually interfere, because the shaft 22 is mounted on the rotatable member 31 which is rotatably mounted on the mounting member 30 and can be retracted at the opening of the cover 2. When the cover 2 is closed, the position of the rotatable member 31 is limited as the upper face 31a thereof is in contact with the stopper 30c as explained before. When the cover 2 is opened, the upper face 31a or the rotatable member 31 is released from the stopper 30c, whereby the lower face 31c rotates along the shaft 26. Then, when the cover 2 is further opened as shown in Fig. 7, the lower face 31c of the rotatable member 31 is released from the shaft 26, whereby the rotatable member 31 is rendered rotatable in a direction f or g about the shaft 33, in a range from a position where the upper face 31a is in contact with the stopper 30c to a position where the lateral face 31b is in contact with the stopper 30c. Consequently, when the cover 2 is opened in a state of mutual interference of the guide shaft 21 and the curl correction shaft 22 as shown in Fig. 2B, the rotatable member 31 rotates, as shown in Fig. 7, in a direction g, keeping the lower face 31c in contact with the shaft 26, by means of the weight of said member 31 and that of the shaft 22. Said rotation of the rotatable member 31 in the direction g eliminates the mutual interference of the shafts 21 and 22, thereby enabling easy opening of the cover 2. On the other hand, when the cover 2 is closed with the arms 27 in a position shown in Fig. 7, the rotatable member 31 rotates with respect to the cover 2 by means of the weight of said member 31 and that of the curl correction shaft 22, but the lower face 31c of the rotatable member 31 is positioned above the shaft 26 while the upper face 31a is positioned below the guide shaft 21, because the lateral face 31b of said member 31 is in contact with the stopper 30d as shown in Fig. 7. When the cover 2 is further closed, the lower face 31c of the rotatable member 31 is brought into contact with the shaft 26 and rotates along said shaft 26, so that the cover 2 can be closed without the interference of the rotatable member 31 and the guide shaft 21. In the following there will be explained the image forming operation on the sheet material 9 by the recording apparatus equipped with the above-explained curl correction mechanism. At first the roll 9a is loaded in the roll holder 4 by opening the cover 2 as shown in Fig. 8. Due to the opening of the cover 2, the contactors 27b of the arms 27 are released from the stopper 32, whereby the arms 27 are no longer limited in rotation in the direction e. In this state the arms 27 maintain a position indicated by a double-dotted chain line. If the roll 9a touches the guide shaft 21 at the loading, said shaft 21 is rotated in the direction e, by the weight of the roll 9a, to a gap between the roll holder 4 and the guide plate 34. Consequently, the guide shaft 21 does not unfavorably affect the loading of the roll 9a, whereby the entire apparatus can be made compact. Then the sheet material 9 is pulled out from the roll 9a, and is threaded through the platen roller 5 and between the cutter guides 6c, 6d. Then the cover 2 is closed as shown in Fig. 9 whereby the stopper 32 is brought into contact with the contactors 27b, thus rotating the arms 27 from said chain-lined position in the direction d to the initial stand-by position. Also the rotatable member 31 is brought to the initial position in which the lower face 31c is in contact with the shaft 26, so that a gap is maintained between the upper face 31a and the stopper 30c. Said gap is provided for ensuring an over-stroke of the cover 2 in engaging the same with the main body 1. As explained in the foregoing, the present embodiment allows to dispense with the threading of the sheet material under the curl correction shaft, whereby the setting of the sheet material is facilitated and the entire apparatus can be compactized. When the recording signal is sent from the control unit (not shown), the motor 23 is activated to rotate the platen roller 5 in the direction a as shown in Figs. 2A and 2B, thereby advancing the sheet material 9 in the direction c. At the same time the arms 27 rotates in the direction d, and stops at a position balanced with the tension F on the sheet material 9. Also the rotatable member 31 rotates in the direction f, and stops at a position where the upper face 31a thereof is in contact with the stopper 30c. In this state, the transport path of the sheet material 9 is defined by the guide shaft 21 and the curl correction shaft 22. Thus the sheet material 9 wrappes the shaft 22 with an angle corresponding to the tension F on said sheet material, whereby the curl is corrected. On the other hand, the recording head 8 is given image signals in synchronization with the transportation of the sheet material 9 to activate the heat-generating elements 8c, thereby forming an image on the sheet material 9. After image formation, the sheet material 9 continues to be advanced in the direction c. When the rear end of the image formed on the sheet material 9 reaches the cutter 6, said cutter 6 is energized to cut the sheet material 9. The cut sheet 9 is then discharged from the main body 1. After said cutting, the platen roller 5 is rotated in the direction b by the motor 23 as shown in Figs. 6A and 6B, and the arms 27 simultaneously rotate in the direction e. Since the rotating speed of the arms 27 is equal to or faster than the transporting speed of the sheet material 9 by the platen roller 5, the guide shaft 21 is released from the sheet material 9 and is retracted to the initial position where the contactors 27b of the arms 27 are in contact with the stopper 32. Also the rotatable member 31 descends by the weight thereof, and stops at the initial position where the lower face 31c is in contact with the shaft 26. The sheet material 9 is pulled back by the rotation of the platen roller 5 in the direction b, and the motor 23 is stopped and enters the stand-by state when the leading end of the sheet material 9 reaches a predetermined stand-by position. When the next recording signal is sent from the control unit in said stand-by state, the motor 23 rotates the platen roller 5 in the direction a as shown in Figs. 2A and 2B, and the arms 27 rotate simultaneously in the direction d. Also the rotatable member 31 rotates and stops at a position where the upper face 31a is in contact with the stopper 30c. In this operation, at first transported is the slack of the sheet material 9 pulled back by the rotation of the platen roller 5 in the direction b after the preceding image recording. While said slack is transported, the arms 27 rotate to limit the transport path of the sheet material 9, whereby the curl thereof can be appropriately corrected. Fig. 10 shows another embodiment of the curl correction mechanism in which the curl correction shaft 22 can be moved by the opening and closing motion of the cover 2. In Fig. 10, same or equivalent components as those in the foregoing first embodiment are represented by same numbers, and will not be explained further. On both sides of the main body 1, there are fixed rail members 40. The curl correction shaft 22 is supported at both ends by said rail members 40, and is made movable along said rail members 40. Tension springs 42 are provided between both ends of the shaft 22 and a pin 41 provided in the main body 1. Also on each end of the shaft 22 there is fixed a wire 43 which goes around pins 44a - 44c provided in the main body 1 and is connected at an end to the cover 2. Consequently, the curl correction shaft 22 is constantly biased, by the tension springs 42, in a direction h along the rail members 40, and is maintained in contact with the end of said rail members 40 when the cover 2 is closed. When the cover 2 is opened, the wires 43 are pulled in a direction j, whereby the shaft 22 moves in a direction i along the rail members 40. In the present embodiment, as explained above, the curl correction shaft 22 is movable from the initial position along the rail members 40, by opening of the cover 2. Consequently, even when the cover 2 is opened in a state of mutual interference of the guide shaft 21 and the curl correction shaft 22 as indicated by solid lines in Fig. 10, the shaft 22 can be retracted from the initial position, so that the cover 2 can be easily opened without interference of said shaft 22 with the guide shaft 21. In the above-explained structure, the curl correction for the sheet material 9 can be achieved as in the first embodiment explained above. In the following there will be explained still another embodiment with reference to Fig. 11 which is a schematic perspective view of a recording apparatus, and Figs. 12 and 13 which are views showing the principle of curl correction, and Figs. 14 and 15 which are views for explaining curl of the sheet and result of the experiment, wherein same components as those in the foregoing embodiments are represented by same numbers and will not be explained further. On the cover 2, a recording head 61 is mounted by means of a compression spring 61a, in a position opposed to the platen roller 5. Also in a predetermined position of the cover 2, there is detachably mounted a cartridge 63 housing an ink sheet 62 constituting a transfer medium, by means of engaging means (not shown). On said cartridge 63 there is rotatably mounted the curl correction shaft 22. Said recording head 61 is maintained in pressure contact with the platen roller 5, across the recording sheet 9 and the ink sheet 62, and an image is formed on the recording sheet 9 by driving the recording head 61 according to image signals from a control unit (not shown), while the recording sheet 9 is advanced by the platen roller 5. After image formation, the recording sheet 9 is cut by the cutter 6 and discharged from the apparatus. In the present embodiment, the recording sheet 9 is advanced by pinching said sheet 9 and the ink sheet 62 between said platen roller and the recording head 61. Said recording head 61 is composed of so-called thermal head having a plurality of heat-generating elements (not shown) arranged on a face in contact with the ink sheet 62, along the transversal direction thereof. At the image formation, the heat-generating elements are selectively activated according to the image signals to generate heat, whereby the ink coated on the ink sheet 62 is transferred, by fusion or sublimation, to the recording sheet 9 to form an image thereon. The recording sheet 9 is composed of continuous plain paper or plastic film of an appropriate quality matching the structure of the recording means. In the present embodiment employing the thermal head 61, the recording sheet 9 is composed of plain paper wound as a roll 9a on a core 9b. Ends of arms 27 constitute contactors 27b for contacting stoppers 65, 66 provided on the main body 1. The curl correction shaft 22 is rotatably mounted on mounting members 64 protruding from the cartridge 63. When the cover 2 is closed, said shaft 22 is maintained in a constant position in the main body 1, since it is mounted on the mounting members 64 protruding from the cartridge 63. The ink sheet 62 constituting the transfer medium is formed by coating a substrate such as a polyethylene terephthalate film with heat-fusible ink. The cartridge 63 is mounted detachably on the cover 2 by engaging means (not shown), and houses the ink sheet 62 extended between a feed roll 63a and a take-up roll 63b. The cartridge 63 is provided, in a position opposed to the platen roller 5, with a window 63c in which the ink sheet 62 is exposed. When the cover 2 is closed, the recording head 61 is maintained in contact with the platen roller 5 through said window 63c. On the lower face of said cartridge 63, and at the upstream side of the platen roller 5 with respect to the transport direction of the recording sheet 9, there are formed protruding mount members 64 rotatably supporting the curl correction shaft 22. Opposed to the shaft 22 there is provided a guide plate 34, of which an end extends close to the platen roller 5 while the other end extends close to a vertical wall of the roll holder 4. Said guide plate 34 serves to guide the recording sheet 9 from the roll 9a to the platen roller 5. In the following there will be explained the image formation on the recording sheet 9 by the above-explained recording apparatus. At first, in loading the roll 9a in the main body 1, the cover 2 is opened and the roll 9a is loaded in the roll holder 4. At the same time the recording sheet 9 is pulled out from the roll 9a and is threaded over the platen roller 5 and between the cutter guides 6c and 6d. The curl correction shaft 22 moves with the cover 2, as it is mounted on the cartridge 63 on the cover 2. Consequently the roll holder 4, guide shaft 21, guide plate 34 and platen roller 5 provided in the main body 1 are exposed, and it is not necessary to thread the recording sheet 9 under the curl correction shaft. Thus the roll 9a can be easily set. When a recording signal is sent from the control unit (not shown), the motor 23 rotates the platen roller 5 in a direction a, thereby advancing the recording sheet 9 and the ink sheet 62 in a direction c. Simultaneously, the arms 27 rotate in a direction d and stop at a position balanced with the tension F on the recording sheet 9. In this state, the transport path of the recording sheet 9 is defined by the guide shaft 21 and the curl correction shaft 22, and the recording sheet wraps the shaft 22 with an angle  corresponding to the tension F on said recording sheet 9, and is subjected to curl correction. The recording head 61 receives the image signals in synchronization with the transportation of the recording sheet 9 to heat the ink sheet 62 by heat generated by the heat-generating elements, whereby the ink coated on the ink sheet 62 is transferred, by fusion or sublimation, to the recording sheet 9 to form an image thereon. After the image formation, the recording sheet 9 continues to be advanced in the direction c, and, when the rear end of the image formed on said sheet 9 reaches the cutter 6, said cutter 6 is energized to cut the recording sheet 9 which is subsequently discharged from the main body 1. After said cutting, the platen roller 5 is rotated in the direction b by the motor, and the arms 27 are simultaneously rotated in the direction e. Since the rotating speed of the arms 27 is equal to or larger than the transport speed of the recording sheet 9 by the platen roller 5, the guide shaft 21 is rapidly released from the recording sheet 9 and is retracted into the initial position in a space 67. When the leading end of the recording sheet 9 pulled back from the cutter 6 by the rotation of the platen roller 5 in the direction b, the motor 23 is stopped and enters the stand-by state. When a next recording signal is sent from the control unit in said stand-by state, the platen roller 5 is rotated in the direction a by the motor 23, and the arms 27 are simultaneously rotated in the direction d. In this operation, at first transported is the slack of the recording sheet 9 pulled back by the rotation of the platen roller 5 in the direction b after the preceding recording. During the transportation of said slack, the arms 27 rotate to define the transport path of the recording sheet 9, whereby the curl of the recording sheet 9 can be appropriately corrected. In the foregoing embodiments, the rolled sheet material is placed on the roll holder, but the present invention is likewise applicable to a rolled sheet material rotatably supported by a shaft inserted into the center of said roll, if the sheet pulling resistance decreases with the decrease of the sheet material. Also in the foregoing embodiment the curl correction is achieved by a curl correction shaft and a guide shaft of both cylindrical shape, but either or both may be replaced by a guide plate or guide plates. In the following there will be explained still another embodiment of the present invention applied to a facsimile apparatus, with reference to Figs. 14 and 15 which are perspective views of a recording system of the facsimile apparatus and Fig. 16 which is a cross-sectional view of the entire facsimile apparatus. As shown in Fig. 16, the facsimile apparatus is composed of a recording system B equipped with a curl correction mechanism, and an original reading system C. Referring to Fig. 16, the recording system B has a main body 100 and a cover 102 articulated thereto by a shaft 103, wherein said cover 102 can engage with the main body 100 by a click mechanism (not shown). In a roll holder 104 formed in the rear part of the recording system, there is loaded a roll 105a of a thermal recording sheet 105. Said sheet material 105 is advanced by the rotation of a platen roller 106a constituting transport means, and is subjected to curl correction by being bent, in passing the curl correction mechanism A, in a direction opposite to the direction of curl. After curl correction, the sheet material 105 is subjected to image formation by recording means 106, and the sheet material 105 after image formation is cut by a cutter 107 and discharged by discharge rollers 108 onto a discharge stacker 109. In the original reading system C, plural original documents 111 are set on an original stacker 110 formed on the upper face of said cover 102. In the reading operation, several lowermost sheets of said plural originals 111 are advanced by a preliminary transport roller 111a and a pressure member 112b, and then are separated and advanced one by one by a separating roller 113a and a pressure member 113b in contact therewith. The separated original 111 is illuminated by a light source 116 while it is transported by paired transport rollers 114a, 114b; 115a, 115b, and the reflected light is introduced, through a mirror 117 and a lens 118, into a photoelectric converting device 119 such as a CCD for conversion into electrical signals. Said signals are transmitted to the recording system B in case of the copy mode, or transmitted to the recording system of another apparatus in the facsimile mode. In the following explained is the structure of the recording system B equipped with the curl correction mechanism A. The roll holder 104 is open on the stop and is positioned in the rear part of the main body 100 of the recording system. The sheet roll 105a is loaded in said roll holder 104, and frictional resistance is generated by the contact of the external periphery of said roll 105a with the internal wall of the holder 104. Said frictional resistance is large when the roll 105a is large in diameter and heavy, but decreases as the roll 105a becomes lighter in weight by the consumption of the sheet 105. The tension on the sheet material 105 varies proportionally with said frictional resistance. Recording means 106, for recording an image on said sheet material 105, is composed of a platen roller 106a and a recording head 106b. The platen roller 106a is composed of a material of a high frictional coefficient, such as hard rubber, formed as a roller, and is rotatably supported in the main body 100 and driven by a motor 120. As shown in Fig. 14, the rotation of the motor 120 is transmitted from a gear 121a fixed to the motor shaft, through an intermediate gear 121b, to a gear 121c fixed on the shaft of the platen roller 106a, thereby rotating the platen roller 106a. Said platen roller 106a serves also as transport means for the sheet material 105. The recording head 106b heats the sheet material 105 according to the image signals to form an image on said sheet material 105, and is pressed to the platen roller 106a across said sheet material 105. More specifically said head is rotatably mounted about a shaft 106c on the cover 102, and is pressed to the platen roller 106a by a compression spring 106d when the cover 102 is closed. Consequently the sheet material 105 is advanced by the rotation of the platen roller 106a while said sheet 105 is sandwiched between the platen roller 106a and the recording head 106b. The recording head 106b in the present embodiment is composed of so-called line thermal head having a plurality of heat-generating elements 106b 1 arranged on a face in contact with the sheet material 105 along the transversal direction of the sheet material 105, and the energization of said heat-generating elements 106b 1 according to the image signals selectively heats the sheet material 105, thereby developing a color on the thermal recording sheet 105. The cutter 107 in the present embodiment is composed of a rotary cutter consisting of a fixed blade 107a and a driven blade 107b. More specifically the fixed blade 107a is fixed on the main body 1, while the driven blade 107b is rotatable about a shaft 107c and, when rotated by drive means, cuts the sheet material 105 in cooperation with the fixed blade 107a. Said driven blade 107b may be driven by the motor 120 for driving the platen roller 106a, or by another independent motor. The sheet material 105 cut by said cutter 107 is discharged to a stacker 109 by discharge rollers 108 driven by drive means (not shown). The sheet material 105 used in the recording is curled when pulled out from the roll 105a, because of winding around the core 105b, the level of said curling depends on the diameter of the roll 105a. As shown in Figs. 14 and 15, a curl correction shaft 122 is rotatably mounted on a mounting member 124 fixed on the cover 102. Said mounting member 124 is composed of a flat plate 124a serving as a guide for the sheet material 105, and brackets 124b standing on both ends of said flat plate 124a, wherein the distance of said brackets 124b is larger than the width of the sheet material 105 but smaller than the distance of arms supporting the both ends of the guide shaft 123 to be explained later. Said brackets 124b rotatably support the curl correction shaft 122. As shown in Figs. 17 and 18, said shaft 122 is always maintained in a constant position in the main body 100 when the cover 102 is closed, said position being at the upstream side of the platen roller 106a in the transport direction of the sheet material 105, so as not to excessively increase the introduction angle of the sheet material 105 to the platen roller 106a. Opposed to said curl correction shaft 122, there is provided a guide plate 104a, for guiding the sheet material 105 pulled out from the roll 105a to the platen roller 106a. Said guide plate 104a is extended at an end thereof to the vicinity of the platen roller 106a, and is integrally united, at the other end, with the roll holder 104. The guide shaft 123 is rendered movable by moving means, with respect to the curl correction shaft 122. As shown in the perspective view in Fig. 19 and the cross-sectional view in Fig. 20, said moving means is composed of a shaft 125 mounted in the main body 100 and two arms 126 fixed to said shaft 125 and rotatably supporting the guide shaft 123, whereby the rotation of the shaft 125 causes integral displacement of the arms 126 thereby moving the guide shaft 123 with respect to the curl correction shaft 122. Said shaft is rotatably supported by bearings 127 mounted on the main body 100, and is provided, at an end thereof, with a clutch flange 128, a clutch gear 129 and a spring clutch 130 for transmitting the one directional rotating force to the shaft 125. Said clutch flange 128 is composed of a cylindrical portion 128a and a flange portion 128b and integrally rotates with said shaft 125 by means of an engaging pin 131. The clutch gear 129 is composed of a cylindrical portion 129a and a gear portion 129b, and has a central hole slightly larger than the diameter of the shaft 125, whereby said gear 129 is rotatably fitted on said shaft 125. The spring clutch 130 is composed of a coil of a steel wire, a steel belt or a plastic wire, wound over the cylindrical portions 128a, 129a of the clutch flange 128 and clutch gear 129, and engages with the flange portion 128b of the clutch flange 128 at an end. The other end is bent outward to constitute an engaging portion 130. Said spring clutch 130 transmits the rotation of the clutch gear 129, only in one direction, to the clutch flange 128. More specifically, when the clutch gear 129 rotates in the direction a shown in Fig. 19, the spring clutch 130 becomes slack and the rotation is not transmitted to the clutch flange 128. On the other hand, when the clutch gear 129 rotates in the opposite direction a' (hereinafter the sign ' indicates opposite direction), the spring clutch 130 is tightened over the cylindrical portions 128a, 129a to maintain these portions in a locked state, whereby the rotation is transmitted to the clutch flange 128 thereby moving the guide shaft 123 in the direction b. Said clutch gear 129 is driven by the motor 120 used for driving the platen roller 106a. As shown in Fig. 14, the power of the motor 120 is transmitted to the platen roller 106a through the gears 121a - 121c as explained before, and the gear 121c on the platen roller shaft meshes with the gear portion 129b of the clutch gear 129 through an intermediate gear 121d. Consequently the forward rotation, in the direction c of the motor 120 as shown in Fig. 14 causes the rotation of the platen roller 106a in the direction d, thereby rotating the clutch gear 129 in the direction a . Thus, when the platen roller 106a rotates for advancing the sheet material 105 in the direction e, the spring clutch 130 is in the non-transmitting free state. When the motor 120 rotates in the opposite direction c', the platen roller 106a rotates so as to reverse the sheet material 105, and the clutch gear 129 rotates in the direction a' to maintain the spring clutch 130 in the locked state, thereby rotating the guide shaft 123 in the direction b shown in Fig. 19. In the following, there will be explained the structure of means for biasing said guide shaft 123 so as to balance with the tension of the sheet material 105. As shown in Figs. 19 and 20, a pulley 133 is fixed, by a pin 132, to the other end of the shaft 125. The periphery of said pulley 133 is provided with a groove 133a and a hook 134a at a predetermined position. A tension spring 135 is provided, under a tension, between said hook 134a and a hook 134b (Fig. 15) provided in the main body 100, whereby the pulley 133 is constantly biased in a direction f in Fig. 19 (direction for increasing the curl correction effect by the guide shaft 123). Also in the main body 100, there are provided stoppers 136 in the rotating range of said arms 126. Said stopper 136 engages with the arm 126 when it rotates in the direction b' in Fig. 19, thereby limiting the rotation in said direction, thus defining the maximum wrapping amount of the sheet material 105 on the curl correction shaft 122. Further as shown in Figs. 19 and 21, a stopper 137 is provided in the main body 100, in the rotating range of the engaging portion 130a of the spring clutch 130. Said stopper 137 serves to limit the rotation of the spring clutch 130 rotating with the gear 129 in the locked state in response to the rotation of the clutch gear 129 in the direction a', thereby limiting the rotation of the arms 126. More specifically, if the clutch gear 129 rotates in said direction a' while the engaging portion 130a is in contact with the stopper 137, the spring clutch 130 is loosened whereby the rotation is not transmitted to the clutch flange 128 and the rotation of the arms 126 in the direction b is limited. Consequently, the arms 126 rotate within the range defined by the stoppers 136, 137. In the following there will be explained the recording operation with the recording system B equipped with the above-explained curl correction mechanism, with emphasis on the curl correcting function for a large roll diameter shown in Fig. 17 and a small roll diameter shown in Fig. 18. At first the cover 102 is opened, then the sheet roll 105a is loaded in the roll holder 104 and the leading end of the sheet is pulled out to the platen roller 106a. Since the curl correction shaft 122 is mounted on the cover 102 while the guide shaft 123 is mounted in the main body 100, said shafts 122, 123 are separated by the opening of the cover 102 as shown in Figs. 14 and 15 to facilitate the setting of the sheet material 105. When a recording start signal is entered after the closing of the cover 102, the motor 120 rotates in the forward direction to rotate the platen roller 106a in the direction d, thereby advancing the sheet material 105 in the direction e. In synchronization the heat-generating elements 106b 1 of the recording head 106b are selectively energized to form a record on the sheet material 105. In the course of transportation of said sheet material 105, the curl thereof is corrected by the function of the curl correction mechanism A. The forward rotation of the motor 120 is transmitted to the clutch gear 129, but not to the clutch flange 128 because of the above-explained free state of the spring clutch 130. On the other hand, the shaft 125 is biased in the direction f by the tension spring 135 as shown in Fig. 19, whereby the guide shaft 123 moves by the rotation of the arms 126 in the direction b' and stops at a position balanced with the tension on the sheet material 105. Thus the sheet material 105, in the transport path thereof, wraps the guide shaft 123 in the direction of curl of the sheet roll 105a, and the curl correction shaft 122 in the opposite direction. The curl correction is achieved by said wrapping of the sheet material 105 on the shaft 122. The tension on the sheet material 105 is determined by the contact friction between the roll holder 104 and the roll 105a loaded therein. As shown in Fig. 17, the sheet 105 before and after the guide shaft 123 is subjected to a tension F1, corresponding to the weight G1 of the roll 105a. When the guide shaft 123 moves in the direction b', the wrapping amount of the sheet material 105 on the shaft 123 increases whereby the rotating power of the arm 126 (torque of the pulley 133 by the tension spring 135) increases against the synthesized tension F1. Thus the arms 126 stop at a position where said rotating force of the arms 126 balances with said synthesized tension. When the roll diameter is large as shown in Fig. 17, the sheet material 105 shows an angle 1 before and after the curl correction shaft 122 when the tension F1 of the sheet 105 balances with the rotating force of the arms 126. When the roll diameter is reduced as shown in Fig. 18, the roll weight is reduced to G2 (< G1) so that the tension on the sheet material 105 is also reduced to F2 (< F1). Thus the arms 126 rotate further in the direction b' than in the case of larger roll diameter, and stop at a position where the rotating force thereof balances with the aforementioned resistance, or in contact with the stopper 136 if the rotating force is larger. In this case the angle 2 of the sheet material 105 before and after the curl correction shaft 122 is smaller than the angle 1 corresponding to the larger roll diameter. Consequently, the wrapping amount of the sheet material on the shaft 122 is small for a large roll diameter, but increases as the curl becomes stronger for a smaller roll diameter. The curl correcting effect becomes stronger as said wrapping amount increases because the sheet material 105 is more strongly bent opposite to the direction of curl. Thus the curl correcting effect appears stronger at a smaller roll diameter where the curling becomes stronger. As explained in the foregoing, the curl correction mechanism A is capable of appropriate curl correction, by varying the correcting effect according to the level of curling, utilizing the balance of the tension on the sheet material 105 and the rotating force on the arms 126. The arms 126 are biased by the tension spring 135, and the rotation of the motor 120 in the direction c is transmitted to the platen roller 106a but not to the arms 126. Consequently, the precision of sheet transportation can be improved to achieve a high quality recording, as the power for moving the arms 126 is not required for the motor 120 at the image recording. The sheet material 105 thus subjected to curl correction is then subjected to image recording, then cut with the cutter 107 and discharged by the discharge rollers 108. On the other hand, the sheet material 105 remaining in the apparatus is transported backward by a distance ℓ corresponding to the distance between the position of the cutter 107 to the recording means 106, in order to avoid formation of an empty space in the leading end part of the sheet 105 at the next recording. For this purpose the motor 120 is reversed by a predetermined amount, whereby the power thereof is transmitted not only to the platen roller 106a but also to the clutch flange 128 through the spring clutch 130 in the locked state as explained before, thus generating a force to rotate the arms 126 in the direction b. In this state the tension spring 135 exerts a force against said rotation, as shown in Figs. 19 and 21. For this reason, the reversing power of the motor 120 in the present embodiment is selected larger than the arm rotating force of the tension spring 135. Therefore, when the motor 120 is reversed for reversing the sheet material 105, the arms 126 are rotated in the direction b and separated from the sheet material 105. At the same time the engaging member 130a of the spring clutch 130 rotates substantially integrally with the arms 126 (Fig. 21 A → B → C), and the spring clutch 130 becomes loose when the clutch gear 129 rotates while the engaging member 130a is in contact with the stopper 137 as shown in Fig. 21C. Thus the clutch gear 129 slips with respect to the clutch flange 128, and the arms 126 returns to the initial position. Said initial position is defined in such a manner that the guide shaft 123 does not close the aperture of the roll holder 104, and the guide shaft 123 does not interfere with the curl correction shaft 122 at the opening of the cover 102. More specifically, the engaging member 130a of the spring clutch 130 is in contact with the stopper 137 as shown in Fig. 21C. In this state, since the guide shaft 123 is separated from the sheet material 105, a curling of opposite direction is not generated even in a prolonged stand-by state. The motor 120 stops after the reversing of the sheet material 105 by said amount ℓ, and the arms 126 are in a biased state by the tension spring 135. As shown in Fig. 19, said tension tends to rotate the pulley 133 in the direction f, whereby the spring clutch 130 is locked and biases the clutch gear 129 in the direction a , thereby giving a torque to the motor 120 through the gears 121a - 121d. In the recording apparatus of the present embodiment, the arms 126 in the stand-by state are maintained in the initial position, represented by double-dotted chain lines in Figs. 17 and 18. Consequently, in opening the cover 102 for example for loading the roll 105a, the guide shaft 123 does not interfere with the curl correction shaft 122, and the cover 102 can be smoothly opened. The apparatus of the present embodiment of the above-explained structure is not only capable of curl correction according to the level of curling, but also precise sheet transportation since the motor 120 is only required to rotate the platen roller 106a at the recording. Furthermore the replacement of the roll 105a can be easily achieved. In the foregoing embodiment, the guide shaft 123 is rendered movable with respect to the curl correction shaft 122. Now, Fig. 22 illustrates another embodiment in which the guide shaft 123 is rendered movable with respect to the curl correction shaft 122. In Fig. 22, same or equivalent components as those in Figs. 14 to 19 are represented by sage numbers and will not be explained further. Referring to Fig. 22, the guide shaft 123 is rotatably supported by support members 138 of the main body 100. The curl correction shaft 122 is rotatably mounted on arms 140 fixed to a shaft 139 which is rotatably mounted on the cover 102, and effects curl correction for the sheet material 105 when the arms 140 are rotated in a direction g, but are released from said curl correction when the arms 126 are rotated in a direction g'. On an end of said shaft 139 there is rotatably mounted a clutch gear 142 fixed to a solenoid clutch 141, which, in response to a signal from a control unit (not shown), transmits or does not transmit the rotation of said clutch gear 142 to the shaft 139. Said clutch gear 142 meshes with a gear 144 mounted on a motor 143, whereby the rotation of said motor 143 is transmitted to said clutch gear 142. On the other end of said shaft 139 there is fixed a pulley 145 as in the foregoing embodiment, and a tension spring 147 is provided between a projection formed on the periphery of said pulley and a hook 146 provided in a predetermined position on the cover 102, thereby biasing the arms 140 constantly in the direction g. Also in the rotating range of the arms 140 on the cover 102 there is provided a microswitch 148 which is actuated by the arms 140 upon arriving at the initial position by rotation in the direction g', whereby the motor 143 is stopped by a signal from said microswitch 148. In the above-explained structure, the solenoid clutch 141 is energized in the stand-by state, whereby the arms 140 are maintained in the double-dotted chain line position by the self-holding force of the motor 143. Consequently, the curl correction shaft 122 and the guide shaft 123 do not interfer each other at the opening of the cover 102, so that the replacement of the roll 105a can be easily achieved. At the image recording, the cover 102 is closed, and the motor 120 is activated to rotate the platen roller 106a, thereby advancing the sheet material 105 in the direction e. At the same time the recording head 106b is activated according to the image signals to form an image on the sheet material 105. Simultaneous with the activation of the motor 120, the solenoid clutch 141 is turned off whereby the arms 140 rotate in the direction g by the tension spring 147, and the curl correction shaft 122 is brought into contact with the sheet material 105 as represented by solid lines in Fig. 22. The position of said shaft 122 is determined by the balance of the biasing force of the tension spring 147 and the tension of the sheet material 105 as in the foregoing embodiment, and the curl correction of the sheet material 105 is achieved by bending with the shaft 122 opposite to the direction of curling. After the recording, the motor 120 is reversed to move the leading end of the sheet material 105 from the cutter 107 to the platen roller 106a. At the same time the solenoid clutch 141 is energized and the motor 143 is reversed by a predetermined amount to rotate the arms 140 in the direction g'. When the arms 140 are retracted to the initial position, the microswitch 148 is actuated to stop the motor 143, whereby the curl correction shaft 122 is retained in the initial position. The returning of the arms 140 to the initial position can be detected not only by the microswitch 148, but also any other means capable of detecting the position of the arms 140, such as a photointerruptor. The movable curl correction shaft 122 explained above provides the same effects as in the foregoing embodiments. As still other embodiments, it is also possible to move the guide shaft 123 or the curl correction shaft 122 in parallel manner along racks or rails, or to move said shaft by a cam. Also the shaft 122 or 123 may be fixed to the mounting members 124 or arms 126, or may be composed of a metal plate or the like suitably formed to have a predetermined radius of curvature. It is furthermore possible to construct both of the shafts 122 and 123 movable. Furthermore, the spring clutch 130 or the solenoid clutch 141 employed for selectively transmitting the power of the motor to the shaft 122 or 123 may be replaced by other devices, such as a needle clutch of roller type. Also the guide shaft 123 is moved by the motor 120 used for driving the platen roller 106a, but they may naturally be driven by separate motors. Also the driving force need not necessarily supplied by a motor but by other means such as a plunger. Also in the foregoing embodiment, a tension spring is employed for biasing the curl correction shaft 122 or the guide shaft 123 in a direction toward stronger curl correcting function, but there maybe employed other means, for example, various springs such as torsion coil spring, compression spring or spiral spring, cylinders such as air cylinder or oil cylinder, or a solenoid.

Claims: A curl correction apparatus comprising a holder means (4, 104) for holding a rolled sheet material (9, 9a, 105, 105a), transport means (5, 106a) for transporting said sheet material (9a, 105a) housed in said holder means (4, 104) in a first direction and a second direction opposite to the first direction, a curl correction member (22, 122) positioned between said holder means (4, 104) and said transport means (5, 106a) and guiding said sheet material (9, 105) in a curved state for correcting the curl thereof, a guide member (21, 123) contacting a face of said sheet material (9, 105) opposite to a face guided by said curl correction member (22, 122) and serving to guide said sheet material (9, 105) to said curl correction member (22, 122), and moving means (26, 27, 125, 126, 39, 40) for moving said curl correction member (22, 122) and/or said guide member (21, 123) in a direction to decrease the curl correcting operation when said transport means (5, 106a) transports said sheet in said second direction. An apparatus according to claim 1, wherein said holder means (4, 104) comprises a supporting face for supporting the rolled sheet material (9, 9a, 105, 105a). An apparatus according to claim 1 or 2, wherein said curl correction member (22, 122) is a cylindrical member having a guide face on the external periphery thereof. An apparatus according to any one of claims 1 to 3, wherein said guide member (21, 123) is a rotatable cylindrical member. An apparatus according to any one of the preceding claims, wherein said moving means (26, 27, 125, 126, 39, 40) comprises a drive source (23, 120), and a plurality of transmission members (23a, 24, 25, 121a-121d, 129b) for transmitting a drive force of said drive source (23, 120) as a rotating force. An apparatus according to claim 5, wherein said moving means (26, 27, 125, 126, 39, 40) comprises a frictional joint (28, 128). An apparatus according to any one of the preceding claims, wherein said guide member (21, 123) is provided in a main body (1, 100) while said curl correction member (22, 122) is provided on a cover (2, 102) which is openably connected to said main body (1, 100). An apparatus according to claim 5, wherein said drive source (23, 120) also serves to drive said transport means (5, 106a). An apparatus according any one of claims 1 to 4, wherein said moving means (26, 27, 125, 126, 39, 40) comprises an elastic member (35, 135, 47) for biasing at least one of said curl correction member (22, 122) and said guide member (21, 123).

Inventors: AWAI TAKASHI, SASAI KEIZO, AWAI, TAKASHI, SASAI, KEIZO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: B65H23
IPC codes: B41J15, B65H23