Curl correcting device

Abstract: Device comprising a sheet transport path, an opening member (2) openable from a main body (1) of the device, for opening said sheet transport path, transport means (6a) for transporting a sheet along said sheet transport path, a first guide member (23) for guiding the sheet transported by said sheet transport means, a second guide member (22) for guiding the sheet transported by said sheet transport means and correcting the curl of said sheet in cooperation with said first guide member, and moving means (26) for moving at least one of said first and second guide members to a position not correcting the curl tendency, in a state in which said opening member is closed.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a curl correcting or decurling device for correcting the curl of rolled sheet member. Related Background Art Presently popular office equipment such as facsimile apparatus and printers generally employ a long recording sheet rolled on a core in their recording system. Because of the rolled state, such sheet tends to show curling, and such curled sheet often results in jamming in the transportation. Therefore, for the purpose of eliminating or reducing such curl, a curl correcting mechanism is often employed in the recording apparatus utilizing such rolled sheet. Fig. 22 shows an example of such curl correcting mechanism, in which, in a main body 51 and a cover member 52 openably linked with a pin 53, a roll holder 54, a platen roller 55 and a cutter 55 are provided in the main body 51 while a curl correcting shaft 57 constituting the curl correcting mechanism and a recording head 58 are fixed on the cover member 52. In the above-explained mechanism, a roll 59a of a sheet material 59 is loaded in the roll holder 54, and said sheet material 59 is advanced through the shaft 57 by the rotation of said platen roller 56, subjected to image formation by the recording head 58 and discharged from the mechanism after cutting by the cutter 56. In this mechanism, the curl is reduced by bending the sheet material 59 by the curl correcting shaft 57 in a direction opposite to that of curling. In such mechanism, the curl correcting effect is known to become larger as the wrapping angle α, shown in Fig. 23, of the sheet material 59 on the curl correcting shaft 57 becomes larger. In the curl correcting mechanism shown in Fig. 22, the diameter of the sheet roll 59a decreases as the sheet material 59 is used, whereby the wrapping angle of the sheet material 59 on the shaft 57 becomes gradually smaller from α 1 to α 2 . Consequently, the curl correcting effect on the sheet material 59 decreases gradually, and satisfactory curl correction is often not achieved when diameter of the sheet roll 59a is small where the curling tendency is stronger. Fig. 24 shows another curl correcting mechanism further having a guide shaft 60 for maintaining a constant wrapping angle α of the sheet material 59 on the curl correcting shaft 57. The platen roller 55 and the guide shaft 60 are mounted in the main body 51 while the shaft 57 and the recording head 58 are mounted on the cover member 52. In this mechanism, the shaft 57 and the recording head 58 are rotated upwards together with the cover member 52 about the pin 53 to open the upper face of the main body 51 as shown by chain lines, whereby the loading of the sheet material 59 into the main body 51 is facilitated. In the curl correcting mechanism shown in Fig. 24, the guide shaft 60 of the main body cannot be positioned above the shaft 57, as the shaft 57 rotates together with the cover member 52. For this reason, the wrapping angle α of the sheet material 59 on the shaft 57 can only be increased up to about 90°, so that satisfactory curl correcting effect cannot be obtained. For increasing the curl correcting effect, the wrapping angle α has to be further increased for example by positioning the guide shaft 60 in the rotating direction of the curl correcting shaft 57 as shown in Fig. 25A, but, in such arrangement, the cover member 52 cannot be rotated upwards at the loading of the sheet material 59 into the main body 51 as the shaft 57 interferes with the guide shaft 60. For this reason the shaft 57 and the guide shaft 60 have to be mounted in the main body 51. However, at the loading of the sheet material 59, it then becomes necessary to thread the leading end of the sheet material 59 between the shaft 57 and 60 and pass it around the shaft 57, so that the loading operation becomes cumbersome. Also for mounting the shaft 57 on the cover member 52 and still obtaining a large wrapping angle α of the sheet material 59 on the shaft 57, it is necessary to position the shaft 57 considerably separate from the platen roller 55 and the guide shaft 60 as shown in Fig. 25B, but such arrangement requires a significantly extended sheet path, resulting in frequent sheet jamming. Fig. 26 shows another curl correcting mechanism, in which, in a main body 81 and a cover member 82 openably linked with a pin 83, a roller holder 84, a platen roller 85, a cutter 86 and a guide member 87a constituting the curl correcting mechanism are mounted in the main body 81 while a recording head 88, and a guide member 87b constituting the curl correcting mechanism in cooperation with said guide member 87a are mounted on the cover member 82. In this mechanism, a sheet material 89, formed as a roll 89a, is advanced by the rotation of the platen roller 85, subjected to the recording head 88 and discharged from the mechanism after cutting by the cutter 86. The curl of the sheet material 89 is reduced when it passes a bent path formed by the guide members 87a, 87b and is bent in a direction opposite to that of curling. Furthermore there has been proposed another curl correcting mechanism as shown in Figs. 27. In this mechanism, arms 91, rotatably mounted on both ends of a guide shaft 90, rotatably supports a curl correcting shaft 92 at the free ends. Said arms 91 are constantly biased anticlockwise by a spring but is driven clockwise through a friction clutch (not shown) when the platen roller 85 is rotated in a direction indicated by an arrow. Stoppers 93, 94 are provided in predetermined positions of the main body 81 for limiting the rotation of the arms 91. In the above-explained mechanism, the sheet material 89 is advanced by the rotation of the platen roller 85 in the direction indicated by the arrow. At the same time, the arms 91 rotate clockwise but are stopped in contact with the stopper 94 whereupon the friction clutch starts to slip. Thus the advancing sheet material 89 is guided through the guide shaft 90 from the roll 89a and is bent by the shaft 92 in a direction opposite to that of the curl, whereby the curl is reduced. When the rotation of the platen roller 85 is terminated, the arms 91 return to the stand-by position in contact with the stopper 93, by the biasing force of the spring. The curling of the sheet material 89 depends not only on the roller diameter but also on other factors such as the thickness and kind of the sheet material 89, and the elapsed time since the sheet is rolled. For example, a thermal paper has a thickness of about 65 µm while a plain paper used for ink transfer recording has a larger thickness of about 85 µm, so that the plain paper tends to show stronger curl for a same roll diameter. However, in the curl correcting mechanism shown in Fig. 26, the bending angle defined by the curl correcting guide members 87a, 87b is constant. Also in the curl correcting mechanism shown in Fig. 27, the curl correcting effect determined by the position of the arms 91 or curl correcting shaft 92 is defined constant by the stopper 94. Consequently, if the bending angle defined by the guide members 87a, 87b or the position of the stopper 94 is determined for example for the thermal recording sheet, the curl correcting effect becomes insufficient for the plain recording sheet. On the other hand, if the curl correcting conditions are determined for the plain recording paper, an excessive correcting effect will appear for the thermal recording sheet, which will therefore be curled in the opposite direction. In either case there will result defective sheet transportation, leading eventually to sheet jamming. Particularly, the recording unit of a facsimile apparatus is designed to use both the thermal recording sheet and plain recording sheet, and appropriate curl correction cannot be attained depending on the thickness or the kind of the sheet material 89, if the curl correcting effect is fixed as shown in Figs. 26 or 27. From the JP-A-61/75 764 a curl correcting device is known comprising a sheet transport path, an opening member openable from a main body of the device for opening said sheet transport path, transport means for transporting a sheet along said sheet transport path and a guide member for guiding and correcting the curl of said sheet. An uncurling lever is fixed to an opening door so that it can swing about an axis spaced apart from the uncurling lever via a lever; this uncurling lever cooperates with a residual amount meter for easy display of the residual amount of sheets. When opening or closing the door, the uncurling lever changes from an operation position to a stand-by position. This is achieved by a guide being provided on the uncurling lever which is guided by the motion of a post part fixed on a rotatable stay which is provided to open and close the door. SUMMARY OF THE INVENTION An object of the present invention is to provide a curl correcting mechanism free from the above-mentioned drawbacks of prior art, capable of effective curl correction, providing a simple structure in the sheet path and enabling an easy loading operation of the sheet material, and a recording apparatus provided with said curl correcting mechanism. Another object of the present invention is to provide a curl correcting mechanism in which one of guide members or curl correcting members is mounted on a cover member, and a support member is provided for forcedly moving the guide member or the curl correcting member mounted in a main body of the mechanism at the opening or closing operation of the cover member, and a recording apparatus equipped with said curl correcting mechanism. Still another object of the present invention is to provide a curl correcting mechanism capable of effective curl correction without unnecessary load on the transport means for sheet material, and enabling compactization of the mechanism, and a recording apparatus equipped with said curl correcting mechanism. These objects are achieved by a curl correcting device according claim 1. Further improvements of the present invention are subject-matter of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a first embodiment of the present invention applied to the recording system of a facsimile apparatus; Fig. 2 is a cross-sectional view of a facsimile apparatus; Fig. 3 is a partial cross-sectional view of a rotation transmission system consisting of a clutch gear, a clutch spring and a crank; Figs. 4A and 4B are views showing the curl of a sheet; Fig. 5A is a view showing the state of recording with a sheet roll of a large diameter, Fig. 5B is a view showing the wrapping of the sheet material on a curl correcting shaft in said state; Fig. 6A is a view showing the state of recording with a sheet roll of a small diameter, Fig. 6B is a view showing the wrapping of the sheet material on the curl correcting shaft in said state; Figs. 7A to 7C are views showing the operation of sheet roll loading; Fig. 8 is a cross-sectional view of a second embodiment; Fig. 9 is a perspective view of another embodiment of the curl correcting device; Fig. 10 is an elevation view thereof; Figs. 11 and 12 are perspective views of a facsimile apparatus; Figs. 13A to 13C are views showing the relation between a spring clutch and a stopper; Figs. 14A to 14D and 15 are views of a facsimile apparatus constituting a further embodiment; Fig. 16 is a block diagram of a control system; Fig. 17A and 17B are flow charts of a control sequence; Figs. 18 and 19 are respectively a perspective view and an elevation view of a facsimile apparatus constituting a further embodiment; Fig. 20 is a block diagram of the control system thereof; Fig. 21 is a flow chart of the control sequence thereof; Figs. 22, 23, 24, 25A, 25B, 26 and 27 are views of prior arts. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following there will be explained a first embodiment of the present invention, with reference to Figs. 1 which is a perspective view of a recording system of a facsimile apparatus and Fig. 2 which is a cross-sectional view of said facsimile apparatus. As shown in Fig. 2, said facsimile apparatus is composed of a recording unit B including a curl correcting device A, and an original reading unit C. At first the entire structure will be briefly explained with reference to Fig. 2. The recording unit B has a cover member 2 which is hinged by a shaft 3 to the main body 1 of the apparatus and can be engaged with said main body 1 by an engaging mechanism to be explained later. In a predetermined position of the main body 1 there is provided a roll holder 4 in which loaded is a roll 5a of a thermal recording sheet 5. Said sheet material 5 is transported by the rotation of a platen roller 6a constituting transport means, and is subjected to curl correction by being bent in a direction opposite to that of curl in passing the curl correcting device A. After said curl correction, the sheet 5 is subjected to image formation by recording means 6, then cut by a cutter 7 and discharged by discharge rollers 8 onto a discharged sheet stacker 9. In the original reading unit C, plural original sheets 11 are sent on an original stacker 10 formed on the upper face of said cover member 2. In the original reading operation, several originals at the bottom in the stack are advanced by a preliminary transport roller 12a and a pressure member 12b, and the lowermost one is separated and advanced by a separating roller 13a and a pressure member 13b cooperating therewith. The separated original 11 is illuminated by a light source 16 while being transported by paired transport rollers 14a, 14b and 15a, 15b, and the reflected light is guided through mirrors 17 and a lens 18 to a photoelectric converting element 19 such as a CCD for conversion into an electrical signal. Said signal is transmitted to the recording unit B of the same apparatus in case of copy mode, or of another apparatus in case of facsimile mode. Below the original reading unit C there is provided a power supply unit D. In the following, there will be explained the structure of the recording unit B equipped with the curl correcting mechanism A. The roll holder 4, open at the upper side, is positioned at the far side in the main body 1 of the recording unit. The sheet roll 5a is loaded in said roll holder 4, and a friction resistance is generated by the contact between the external periphery of said roll 5a and the internal face of the roll holder 4. Said friction resistance to the dragging of the sheet 5 is large when the roll 5a is large in diameter and heavy, but decreases as the roll 5a decreases in weight by the use of the sheet 5. Said friction resistance gives a tension to the dragged sheet 5, and said tension varies in proportion to said friction resistance. The recording means 6 for image recording on said sheet 5 is composed of a platen roller 6a and a recording head 6b. The platen roller 6a is formed by a roller-shaped member of a high friction coefficient such as hard rubber and is rotatably mounted in the main body 1 for driving by a motor 20. As shown in Fig. 1, the motor 20 rotates the platen roller 6a through a gear 21a fixed on the motor shaft, an intermediate gear 21b and a gear 21c fixed on the shaft of the platen roller 6a. The platen roller 6a functions also as transport means for advancing the sheet 5. The recording head 6b forms an image on the sheet 5 by heating said sheet 5 according to an image signal, and is pressed to said platen roller 6a across the sheet 5. More specifically, it is rotatably mounted on the cover member 2 by means of a shaft 6c (Fig. 2), and is pressed to the platen roller 6a by a compression spring 6d when the cover 2 is closed. Thus, the sheet 5 pinched between the platen roller 6a and the recording head 6b can be transported by the rotation of said platen roller 6a. The recording head 6b in the present embodiment is composed of so-called line thermal head having, on a face contacting the sheet 5, plural electric heat-generating elements 6b 1 arranged in the transversal direction of the sheet 5, and selectively heats the sheet 5 by current supply to said elements 6b 1 according to the image signal, thereby forming a color on said thermal recording sheet 5. The cutter 7 is composed of a fixed blade 7a fixed in the main body 1 and a rotary blade 7b rotatable about a shaft 7c. Said rotary blade 7b, when rotated by drive means, slides on the fixed blade 7a, thereby cutting the sheet 5 (Fig. 2). Said rotary blade 7b may be driven by the motor 20 for driving the platen roller 6a, or by another independent motor. The sheet 5, cut by said cutter 7, is discharged onto the stacker 9 by the discharge rollers 8 driven by drive means (not shown). Said sheet 5, when pulled out from the roll 5a, shows curl because it has been wound on the core 5b. Said curl depends on the diameter of the roll 5a. The curl height h is smaller for a larger roll diameter as shown in Fig. 4A, but becomes larger and the sheet may eventually be rounded for a smaller roll diameter as shown in Fig. 4B. In the present embodiment, said curl is corrected when the sheet 5 passes through the curl correcting mechanism A. More specifically, a guide shaft 23 constituting a guide member is movably provided with respect to a curl correcting shaft 22 constituting the curl correcting member (decurling member), and the curl correction is achieved by bending in a direction opposite to that of said curl when the sheet 5 is guided by said shafts 22, 23. In the present embodiment, the guide shaft 23 is rendered movable with respect to the curl correcting shaft 22 as explained before, so that the wrapping angle of the sheet 5 on the curl correcting shaft 22 can be increased to 150 - 180°. Also the guide shaft 23 is biased toward the sheet 5 by the biasing means to be explained later, thereby varying the curl correcting effect for a large roll as shown in Fig. 5A or a small roll as shown in Fig. 6B. In the following, the curl correcting mechanism A will be explained in greater detail. As shown in Fig. 1, the curl correcting shaft 22 is rotatably supported by a pair of mounting members 24 fixed on the cover member 2. Said mounting members 24 stand from the cover member 2, with a distance larger than the width of the sheet 5 but smaller than the distance of arms supporting the guide shaft 23 as will be explained later. Said curl correcting shaft 22 is composed for example of a metal shaft of a diameter of about 4 mm, and is at a constant position in the main body 1 when the cover member 2 is closed, as shown in Figs. 5A and 6A. Said position is selected between the roll holder 4 and the platen roller 6a in such a manner that the angle of entry of the sheet 5 into the platen roller 6a is not excessively large. In a position opposite to said curl correcting shaft 22 in the main body 1, there is provided a guide member 36 for guiding sheet 5, pulled out from the roll 5a, to the platen roller 6a. An end of said guide is extended to the vicinity of the platen roller 6a, while the other end is integrated with the roll holder 4. On the other hand, the guide shaft 23 is rendered movable by moving means, in the vicinity of the curl correcting shaft 22, and is composed for example of a metal shaft of a diameter of about 4 mm. The structure of the moving means is shown in Fig. 1. Two arms 26 are fixed on a shaft 25 mounted on the main body 1, and rotatably support the guide shaft 23, whereby said arms 26 integrally rotate with the shaft 25 and the guide shaft 23 is rendered movable with respect to the curl correcting shaft 22. Said shaft 25 is rotatably mounted by bearings 27 on the main body 1, and is provided, at an end, with a clutch gear 28, a spring clutch 29 and a crank 30 whereby the shaft 25 receives the rotating force of a direction only. As shown in Fig. 3, said clutch gear 28 is composed of a cylindrical part 28a and a gear part 28b with an internal diameter slightly larger than the diameter of the shaft 25, so that it can rotate on the shaft 25. Also said crank 30 is composed of a cylindrical part 30 a fitted on the shaft 25 and a crank arm 30b, and rotates integrally with the shaft 25 by a fixing pin (not shown). Said spring clutch 29 is composed of a steel wire, a steel belt or a plastic wire wound as a coil over the cylindrical parts 28a, 30a of the clutch gear 28 and the crank 30, and is fixed at an end on the crank 30. Said spring clutch 29 selectively transmits the rotation of the clutch gear 28, through the clutch 30, to the shaft 25, arms 26 and guide shaft 23. When the clutch gear 28 is rotated in a direction a shown, in Fig. 1, the spring clutch 29 is loosened whereby the rotation is not transmitted to the crank 30. On the other hand, when the clutch gear 28 rotates in the opposite direction -a (hereinafter the minus sign indicates a direction opposite to the arrow), the spring clutch 29 is tightened on the cylindrical parts 28a, 30a to realize a locked state, whereby the rotation is transmitted to the shaft 25 through the crank 30, thus moving the guide shaft 23 in a direction b. Said clutch gear 28 is rotated by the motor 20 for driving the platen roller 6a. The rotation of said motor 20 is transmitted to the platen roller 6a through the gears 21a to 21c as shown in Fig. 1, and said gear 21c on the platen roller shaft meshes with the gear part 28b of the clutch gear 28 through the intermediate gear 21d. Thus, when the motor 20 rotates in a direction c as shown in Fig. 1, the platen roller 6a rotates in a direction d, while the clutch gear 28 rotates in a direction a. Stated otherwise, the spring clutch 23 is in the free state when the platen roller 6a rotates in a direction for pulling out the sheet 5 in a direction e. When the motor 20 rotates inversely in a direction -c, the platen roller 6a rotates in a direction for reversing the sheet 5, and the clutch gear 28 rotates in a direction -a to lock the spring clutch, thereby rotating the guide shaft 23 in a direction b shown in Fig. 1. On said crank 30 there is provided a tension spring 31 for biasing the moving means so as to balance the guide shaft 23 with the tension of the sheet 5. More specifically, a pin 30c, provided on an end of the external face of the crank arm 30b, supports the tension spring 31 in extended state, in cooperation with a pin 32 formed in the main body 1, so that said crank 30 is constantly given a torque in a direction a , by the tension of said spring 31. Thus the shaft 25 is constantly biased, by the tension of the spring 31 transmitted through the crank 30, in a direction f shown in Fig. 1, namely in a direction to increase the curl correcting effect of the guide shaft 23. In a predetermined position in the main body 1 there is provided a stopper 33 for limiting the rotation, in the direction -b, of the arms 26. Said stopper 33 comes into contact with the arms 26 in its rotation in the direction -b shown in Fig. 1, thereby limiting the amount of rotation of the arms 26 and defining the maximum value of the wrapping angle of the sheet 5 on the curl correcting shaft 22. Though not illustrated, there is provided a stopper in the main body 1 for contacting the other end of the clutch spring 29 when the arms 26 rotate in the direction b shown in Fig. 1, thereby loosening said clutch spring 29 and preventing the transmission of rotation from the clutch gear 28 to the arms 26. At the free end of the cover member 2, there are rotatably provided, by means of pins 34c, engaging members 34 composed of connecting parts 34b and a pair of engaging parts 34a formed by bending both ends of said connecting part substantially perpendicularly. On lateral walls of the main body 1 in the vicinity of the discharge rollers 8, there are provided locking pins 35. When the cover member 2 is closed, said engaging parts 34a engage with said locking pins 35, whereby the cover member 2 is positioned relative to the main body 1 and is prevented from being opened therefrom. Said engaging members 34 are provided with springs (not shown) for biasing for engaging with the locking pins 35. In the present embodiment, as explained before, the curl currecting shaft 22 is mounted on the cover member 2 while the guide shaft 23 is mounted on the main body 1 and is rendered movable with respect to the curl correcting shaft 22 in order to increase the wrapping angle of the sheet 5 on the curl correcting shaft 22. However, if the cover member 2 is opened while the guide shaft 23 is positioned substantially above the curl correcting shaft 22, there may result mutual interference of said shafts. Therefore, in the present embodiment, opening member 36 for disengaging the engaging members 34a from the locking pins 35 for opening the cover member 2 is slidably provided on the cover member 2 and is provided with a projection 37 for preventing the interference of the guide shaft 23 and the curl correcting shaft 22. The opening member 36 is formed as a flat plate, rendered slidable in directions g and -g shown in Fig. 7A, and is provided with a pin 36a closer to an end in the longitudinal direction, for rotating the engaging member 34 and disengaging it from the locking pins 35 when moved in the direction g. The substantially L-shaped projection 37, positioned close to an end of the opening member 36 in the longitudinal direction thereof moves in the direction g when said opening member 36 is moved in said direction, thereby rotating the arms 26 in the direction b, thus separating the guide shaft 23 from the curl correcting shaft 22 and preventing the mutual interference thereof. The opening member 36 is provided with a spring (not shown) for biasing in the direction -g. In the following there will be explained the recording operation with the recording unit B with the above-explained curl correcting mechanism A, with emphasis on the curl correcting function for a large roll diameter as shown in Figs. 5A and 5B and a small roll diameter as shown in Figs. 6A and 6B. In Figs. 5A and 6A, the opening member 36 and the projection 37 are omitted. At first the cover member 2 is opened, then the sheet roll 5a is loaded in the roll holder 4 and the leading end of the sheet 5 is pulled out to the platen roller 6a. Since the curl correcting shaft 22 is mounted on the cover member 2 while the guide shaft 23 is mounted in the main body as shown in Fig. 1, said shafts 22, 23 are mutually separated by the opening of the cover member 2, whereby the loading of the sheet 5 is facilitated. When the cover member 2 is closed and a record start signal is entered, the motor 20 is activated to rotate the platen roller 6a in the direction d, thereby advancing the sheet in the direction e. In synchronization the heat-generating elements 6b 1 of the recording head 6b are activated according to an image signal read from an original in another facsimile apparatus and transmitted therefrom, thereby recording an image on the sheet 5. In the transportation of the sheet 5, curl correction is made by the function of the curl correcting mechanism A. The forward rotation of the motor 20 is also transmitted to the clutch gear 28, but the spring clutch 29 reaches the free state as explained before, so that the clutch gear 28 slips with respect to the crank 30. On the other hand, because the shaft 25 receives the biasing force in the direction f by the tension spring 31, the guide shaft 23 moves by the rotation of the arms 26 in the direction -b, and stops at a position where said biasing force is balanced with the tension of the sheet 5. The sheet 5 pulled out from the sheet roll 5a wraps the guide shaft 23 in the direction of curl, and wraps the curl correcting shaft in the opposite direction. The curl of the sheet 5 is corrected by said wrapping on the curl correcting shaft 22. The tension on the sheet 5 is determined by the friction resistance between the roll holder 4 and the roll 5a loaded therein. As shown in Fig. 5, the sheet 5 is subjected, before and after the guide shaft 23, to a tension F1, corresponding to the weight G1 of the roll 5a. Movement of the guide shaft 23 in the direction -b increases the wrapping angle of the sheet 5 on the shaft 23, whereby the resistance to the rotation of the arms 26 corresponding to said tension F1 (torque on the shaft 25 caused by the spring 31) increases. Thus the arms 26 stop at a position where the tension F1 is balanced with the rotating force of the arms 26. In case the roll diameter is large as shown in Fig. 5A, the wrapping angle of the sheet 5 on the curl correcting shaft 22 is α 1 as shown in Fig. 5B when the tension F1 of the sheet 5 is balanced with the rotating force on the arms 26. When the roll 5a is reduced in diameter as shown in Fig. 6A, the weight thereof also decreases to G 2 (< G 1 ) whereby the tension on the sheet 5 is also reduced to F2 (< F1). Consequently the arms 26 rotate further in the direction -b in comparison with the case of larger roll diameter. Thus the arms 26 stop at a position where the rotating force thereof is balanced with said resistance, or at a position in contact with the stopper 36 if said rotating force is larger. In this state said wrapping angle assumes a value α 2 as shown in Fig. 6B, larger than α 1 for the larger roll diameter. Consequently, the wrapping angle of the sheet 5 on the curl correcting shaft 22 is smaller or larger respectively for a larger or smaller roll diameter, and a larger wrapping angle provides a higher curl correcting effect as the sheet 5 is bent more strongly in the direction opposite to that of the curl. In this manner the curl correcting effect becomes stronger as the curl gets stronger as smaller roll diameter. An optimum curl correcting effect can be obtained by regulating the tension of said tension spring 31, so as to realize an optimum wrapping angle α corresponding to the diameter of the roll 5a. For example, for a roll 5a of thermal recording sheet of a width of 210 mm (A4 size), and a thickness of 65 µm wound on a core of 1 inch (25.4 mm), the tension of the spring 31 is preferably so adjusted that the wrapping angle α 1 is about 60 to 90° for a large roll diameter as shown in Fig. 5A, but said angle α 2 is about 150 to 180° for a small roll diameter as shown in Fig. 6A. As explained above, the curl correcting mechanism A varies the curl correcting effect, according to the level of the curl, by the balance between the tension on the sheet 5 and the rotating force on the arms 26, thereby achieving appropriate curl correction. The arms 26 are biased by the tension spring 31, and the rotation of the motor 20 in the direction c is transmitted to the platen roller 6a but not to the arms 26. Consequently, the motor 20 is not subjected to the load for moving the arms 26 at the recording operation, so that the precision of sheet transportation can be improved and image recording of high quality can be achieved. The sheet 5 subjected to curl correction proceeds to the image recording of a page, then is cut with the cutter 7 and is discharged by the discharge rollers 8. On the other hand, the sheet 5 remaining in the main body 1 is retracted by a distance ℓ corresponding to the distance between the cutter 7 and the recording means 6 (cf. Fig. 6A), in order to prevent blank at the leading end of the sheet in the next recording operation. For this operation the motor is rotated by a predetermined amount in reverse direction. The rotation of the motor is transmitted not only to the platen roller 6a but also to the crank 30 through the spring clutch 29 in the locked state, thereby rotating the arms 26 in the direction b. In this state, the arms 26 are subjected to a tension, exerted by the spring 31, in a direction to hinder said rotation, as shown in Fig. 1. Consequently, the motor 20 in the present embodiment has a driving force at said reverse rotation, larger than the rotating force of the tension of the spring 31 on the arms 26. Thus, when the motor 20 is reversed for reversing the sheet 5, the arms 26 rotate in the direction b, thereby being separated from the sheet 5, and returns to the initial position (lower limit position of the guide shaft 23) indicated by chain lines in Fig. 5A. In said initial position, the guide shaft 23 does not block the aperture of the roll holder 4, nor interferes with the curl correcting shaft 22 at the opening of the cover member 2. In this state, since the guide shaft 23 is separated from the sheet 5, an inverse curl is not given thereto even if the standby state is prolonged. Also the cutting and reversing of the sheet explained above may be conducted after the image recording of a communication, instead of image recording of each page. In the following there will be explained the loading operation of the sheet roll 5a, with reference to Figs. 7A to 7C. At first, for opening the cover member 2, the opening member 36 is moved in the direction g shown in Fig. 7A, whereby the pin 36a engages with the engaging part 34a to rotate the engaging member 34 in a direction i about the pin 34c (cf. Fig. 7C) and disengaging the engaging parts 34a from the locking pins 35. Then the cover member 2 is lifted upwards about the pin 3, whereby the recording head 6b, mounting members 24 and curl correcting shaft 22 move upwards. The opening member 36 moves in the direction -g to the initial position by the function of the spring (not shown), and the engaging member 34 also moves in a direction opposite to the direction i to the initial position, by the function of the spring (not shown). In the normal state, as the arms 26 return to the chain-lined initial position (lower limit position of the guide shaft 23) by the reverse rotation of the motor 20, the curl correcting shaft 22 can be moved upwards without interference with the guide shaft 23. However, the guide shaft 23 may be stopped substantially above the curl correcting shaft 22 (solid-lined position in Fig. 7A), for example in case of power breakage in the course of sheet transportation. In such case the curl correcting shaft 22 interferes with the guide shaft 23. However, in the movement of the opening member 36 in the direction g, before the pin 36a engages with the engaging member 34a as shown in Fig. 7B, the projection 37 comes into contact with the arms 26 and rotates said arms 26 in the direction b, thereby retracting the guide shaft 23 from the position above the curl correcting shaft 22. In succession the pin 36a engages with the engaging member 34a as shown in Fig. 7C, thereby rotating said member 34 in the direction i about the pin 34c, thus disengaging the members 34a from the locking pins 35. Therefore, in the upward rotation of the cover member 2 about the pin 3, the guide shaft 23 is retracted from the position substantially above the curl correcting shaft 22, so that the curl correcting shaft 22 can be moved upwards without any interference with the guide shaft 23. Subsequently the sheet roll 5a is replaced with a new one, and the leading end of the sheet is passed over the guide shaft 23 and the platen roller 6a and threaded between the cutter members 7a, 7b. The loading of the sheet 5 is completed by closing the cover member 2. The amount of rotation of the arms 26 at the reverse rotation of the motor 20 for reversing the sheet 5 by the distance ℓ is selected equal to the angle from the solid-lined upper limit position of the guide shaft 23 in Fig. 6A to the broken-lined lower limit position. Stated differently, the distance ℓ between the platen roller 6a and the cutter 7, ratio of rotation of the platen roller 6a and the shaft 25, and the position of the stopper 33 are so determined that the amount of rotation of the motor 20 for reversing the sheet 5 by the length ℓ becomes equal to the amount of rotation of the motor 20 for rotating the arms 26 in the direction b. In the following, there will be explained the driving force of the motor required for driving various components in the forward and reverse rotation of the motor 20. The power P 1 of the motor 20 in the forward rotation (for advancing the sheet 5 in the recording direction) is only required to rotate the platen roller 6a, since the spring clutch 29 is freed in this state. On the other hand, the power P 2 of the motor 20 in the reverse rotation (for reversing the sheet 5) is required to rotate the platen roller 6a and to rotate the arms 26 against the force of the tension spring 31 because the spring clutch 29 is locked, and is therefore larger than the power P 1 in the forward rotation. However, if the nominal power of the motor 20 is selected at said power P 2 , the excessive power (P 2 - P 1 ) of the motor 20 in the forward transportation of the sheet 5 may result in vibration, eventually leading to uneven transportation and noises. For this reason, in the present embodiment, the motor 20 is driven with a power P 1 in the forward rotation and a larger power P 2 in the reverse rotation. Such drive is easily achievable by increasing the driving current at the reverse rotation than in the forward rotation, or by reducing the revolution at the reverse rotation. Even if the driving current is increased or the revolution is reduced, the temperature rise of the motor 20 at the reverse rotation or the delay in reversing of the sheet 5 is negligibly small since the amount ℓ of reversing of the sheet 5 (for example about 20 mm) is sufficiently smaller than the amount of forward transportation for recording (for example 210 mm in case of A4 size). The rotating speed of the arms 26 driven by the motor 20 is preferably selected equal to or slightly higher than the peripheral speed of the platen roller 6a (transport speed of the sheet 5), for example by the selection of the gear ratio. In this manner the guide shaft 23 returns rapidly to the lower limit position at the reversing of the sheet 5. The motor 20 is stopped after reversing the sheet 5 by the predetermined amount ℓ, and the arms 26 are biased by the tension spring 31 in this state. As shown in Fig. 1, said tension tends to rotate the shaft 25 in the direction f, thereby locking the spring clutch 29 and biasing the clutch gear 28 in the direction a, and functions as a torque for rotating the motor 20 through the gears 21a to 21d. For example, for a torque T1 (for example 1 kg·cm) for rotating the shaft 25 in the direction f and for a revolution ratio of 10:1 between the motor 20 and the clutch gear 28, the torque T2 transmitted to the motor 20 is equal to T1/10 (0.1 kg·cm). In the present embodiment, therefore, in order to prevent rotation of the arms 26 in the stopped state of the motor 20, the self-retaining torque of the motor 20 (torque for maintaining the rotor consisting of permanent magnets in the stopped state against an externally applied torque) is selected at a level (for example 0.2 kg·cm) higher than the above-mentioned torque T2 transmitted to the motor 20. Thus, in the recording apparatus of the present embodiment, the arms 26 are maintained at the chain-lined initial position shown in Figs. 6A and 6B in the standby state for recording. Consequently, at the loading of the sheet roll 5a, the cover member 2 can be smoothly opened without the interference between the guide shaft 23 and the curl correcting shaft 22. Also even if the guide shaft is stopped substantially above the curl correcting shaft 22 for example by power breakage, the cover member 2 can be smoothly opened without said interference, since the opening member 36 forcedly retracts the guide shaft 23 from above the curl correcting shaft 22 as explained above. In the following there will be explained variations of the curl correcting shaft 22 and the guide shaft 23. In the foregoing first embodiment, the guide shaft is rendered movable with respect to the curl correcting shaft, but it is possible to render the curl correcting shaft 22 movable. Also in the foregoing embodiment, the guide shaft 23 is mounted on arms and is moved by the rotation of said arms, but the moving means is not limited to such structure. For example, the guide shaft 23 or the curl correcting shaft 22 may be moved in parallel manner along racks or rails, or may be moved by an upward movement caused for example by a cam. Furthermore both the curl correcting shaft 22 and the guide shaft 23 may be rendered movable. Also in the foregoing embodiment the curl correcting shaft 22 and the guide shaft 23 are composed of metal shafts, but they may also be composed for example of a metal plate formed with a predetermined curvature to constitute a surface coming into contact with the sheet 5. Furthermore, the spring clutch 29 for selectively transmitting the power of the motor to said guide shaft 23 may be replaced, for example, by a needle clutch of roller type. Also in the foregoing embodiment the movement of the guide shaft 23 and the rotation of the platen roller 6a are achieved by the motor 20, but there may naturally be employed separate motors. Furthermore such driving power need not necessarily be supplied by motors but may be provided for example by plungers. Furthermore, the tension spring 31, used for biasing the curl correcting shaft 22 in a direction for increasing the curl correcting effect, may be replaced for example by a spring such as torsion coil spring, compression or spiral spring, or a cylinder such as air cylinder or oil cylinder, or a magnet. In the following there will be explained variations of the opening member 36 and the interference preventing member 37. In the foregoing embodiment, the opening member 36 is integrated with the pin 36a and the projection 37 for actuating the engaging part 34a and the arms 26, but such actuation may be achieved by link members or the like separate from the opening member 36. It is also possible to detect the sliding motion of the opening member 36 for example with a microswitch, and to reverse the motor thereby moving the arms 26 to the chain-lined position shown in Fig. 5A. In such case, the arms 26 may be moved by a driving source separate from the motor for the platen roller 6a. The recording system explained above has been applied to a thermal recording apparatus employing a thermal recording sheet, but it is likewise applicable to a thermal transfer recording apparatus for transferring ink from an ink sheet to a plain paper sheet. Also in the above-explained first embodiment, the curl correcting mechanism A is accommodated in a space E 4 in four dead spaces E 1 , E 2 , E 3 and E 4 resulting from housing the circular sheet roll 5a in the rectangular main body 1 and the moving range of the guide shaft 23 is limited within said space E 4 , so that the rate of space utilization in the main body 1 is improved, and the apparatus can be easily compactized. In the following there will be explained another embodiment employing another structure as moving means for the guide shaft 23. As shown in a perspective view in Fig. 9 and a cross-sectional view in Fig. 10, said moving means has a shaft 25 mounted in the main body 1 and provided with two arms 26. Said arms 26 support the guide shaft 23 in rotatable manner. Thus the rotation of the shaft 25 moves said arms 26 whereby the guide shaft 23 is rendered movable with respect to the curl correcting shaft 22. Said shaft 25 is rotatably mounted in the main body 1 by means of bearings 27, and, on an end, there are mounted a clutch flange 28, a clutch gear 29 and a spring clutch 130 to transmit the rotation to the shaft 25 only in one direction. Said clutch flange 28 is composed of a cylindrical part 28a and a flange part 28b, which integrally rotate with the shaft 25 by means of a fixing pin 131. The clutch gear 29 is composed of a cylindrical part 29a and a gear part 29b, and has an internal hole slightly larger than the diameter of the shaft 25, thereby being rotatable with respect thereto. The spring clutch 130 is composed of a steel wire, a steel belt or a plastic wire which is wound in a coil over the cylindrical parts 28a, 29a of said clutch flange 28 and clutch gear 29, and of which an end is fixed on the flange part 28b of the clutch flange 28 while the other end is bent to form an engaging part 30a. Said spring clutch 130 transmits the rotation of the clutch gear 29 only in one direction but not in the other direction. When the clutch gear 29 rotates in the direction a shown in Fig. 9, the spring clutch 130 is in a loosened free state, whereby the rotation is not transmitted to the clutch flange 28. On the other hand, when the clutch gear 29 rotates in the opposite direction - a , the spring clutch 130 is tightened around the cylindrical parts 28a, 29a and locks said parts, whereby the rotation is transmitted to the clutch flange 28 for moving the guide shaft 22 in a direction b. Said clutch gear 29 is driven by the motor 20 used for rotating the platen roller 6a. As shown in Fig. 11, the rotation of the motor 20 is transmitted to the platen roller 6a through gears 121a to 121c, and the gear 121c mounted on the platen roller shaft meshes with the gear part 23b of the clutch gear 29 through an intermediate gear 121d. Thus the forward rotation of the motor 20 in the direction c shown in Fig. 11 rotates the platen roller 6a in the direction d, and the clutch gear 29 in the direction a . Consequently, the spring clutch 130 is in the free state when the platen roller 6a rotates in a direction to advance the sheet 5 in the direction e. On the other hand, when the motor 20 rotates in the opposite direction -c, the platen roller 6a rotates in a direction to retract the sheet 5, and the clutch gear 29 rotates in the direction - a to lock the spring clutch 130, whereby the rotation is transmitted to move the guide shaft 23 in the direction b shown in Fig. 9. In the following there will be explained biasing means for biasing the guide shaft 23 so as to be balanced with the tension of the sheet 5. On the other end of the shaft 25, as shown in Figs. 9 and 10, there is fixed a pulley 133 by means of a fixing pin 132. Said pulley 133 is provided with a groove 133a on the periphery thereof and a hook 134e at a predetermined position. A tension spring 135 is provided under a tension between said hook 134a and another hook 134b provided in a predetermined position of the main body 1 (cf. Fig. 12), whereby the pulley 133 is constantly biased in a direction f in Fig. 9, namely in a direction for increasing the curl correcting effect by the guide shaft 23. Also a stopper 136 is provided within the moving range of said arms 26 in the main body 1. Said stopper 136 constitutes limiting means which comes into contact with the arms 26 in the rotation thereof in the direction -b shown in Fig. 9, thereby limiting the amount of rotation in said direction -b, and thus defining the maximum wrapping amount of the sheet 5 on the curl correcting shaft 22. There is further provided a stopper 137 within the rotation range of the engaging part 130a of the spring clutch 130 in the main body 1, as shown in Figs. 9 and 13. Said stopper 137 constitutes limiting means for limiting the rotation of the spring clutch 130, rotating in the locked state with the clutch gear 29 in its rotation in the direction - a , thereby limiting the rotation of the arms 26. When the clutch gear 29 rotates in the direction - a while said engaging part 130a is in contact with the stopper 137, said spring clutch 130 is loosened whereby the rotation is not transmitted to the clutch flange 28 and the rotation of the arms 26 in the direction b is limited. Consequently the arms 26 rotate within a range defined by the stoppers 136, 137. Also in the present embodiment, the arms 26 stop at a position where the tension of the spring 135 is balanced with the tension F1 of the sheet 5, caused by the friction between the roll holder 4 and the sheet roll 5a. In case the roll diameter is large as shown in Fig. 5A, the wrapping angle of the sheet 5 on the curl correcting shaft 22 is  1 as shown in Fig. 5B when the tension F1 of the sheet 5 is balanced with the rotating force on the arms 26. When the roll 5a is reduced in diameter as shown in Fig. 6A, the weight thereof also decreases to G 2 (< G 1 ) whereby the tension on the sheet 5 is also reduced to F2 (< F1). Consequently the arms 26 rotate further in the direction -b in comparison with the case of larger roll diameter. Thus the arms 26 stop at a position where the rotating force thereof is balanced with said resistance, or at a position in contact with the stopper 136 if said rotating force is larger. In this said wrapping angle assumes a value  2 larger than  1 for the larger roll diameter. Consequently, the wrapping angle of the sheet 5 on the curl correcting shaft 22 is smaller or larger respectively for a larger or smaller roll diameter, and a larger wrapping angle provides a higher curl correcting effect as the sheet 5 is more strongly bent in the direction opposite to that of the curl. In this manner the curl correcting effect becomes stronger as the curl gets stronger at smaller roll diameter. The sheet 5 subjected to curl correction proceeds to the image recording, then is cut with the cutter 7 and is discharged by the discharge rollers 8. On the other hand, the sheet 5 remaining in the main body 1 is retracted by a distance ℓ corresponding to the distance between the cutter 7 and the recording means 6, in order to avoid blank area at the leading end of the sheet in the next recording operation. For this purpose the motor is rotated in reverse direction by a predetermined amount. The rotation of the motor is transmitted not only to the platen roller 6a but also to the clutch flange 28 through the spring clutch 130 in the aforementioned locked state, thereby rotating the arms 26 in the direction b. In this state, as shown in Figs. 9 and 13, the arms 26 are subjected to a tension, exerted by the spring 135, in a direction to hinder said rotation. Consequently, the motor 20 in this embodiment has a driving force at said reverse rotation, larger than the rotating force of the tension of the spring 135 on the arms 26. Thus, when the motor 20 is reversed for retracting the sheet 5, the arms 26 rotate in the direction b, thereby being separated from the sheet 5. In this state, th engaging part 30a of the spring clutch 30 rotates substantially integrally with the arms 26 (Figs. 13A → 13B → 13C), and the spring clutch 130 is loosened as the clutch gear 29 rotates while the engaging part 130a is in contact with the stopper 137. Consequently the clutch gear 29 rotates idly with respect to the clutch flange 28, and the arms 26 returns to and stops at the initial position. In said initial position, the guide shaft 23 does not block the aperture of the roll holder 4, nor interferes with the curl correcting shaft 22 at the opening of the cover member 2. More specifically, in this position, the engaging part 30a of the spring clutch 30 is in contact with the stopper 137. In this state, since the guide shaft 23 is separated from the sheet 5, an inverse curl is not given thereto even if the standby state is prolonged. In the following there will be explained a further embodiment in which the tension of the spring 135 of the third embodiemnt is made variable. As shown in Figs. 14 and 15, a switch lever 240 is rotatably provided by a pin 241 on a lateral face of the main body 1, on which the pulley 133 is positioned. Stoppers 243, 244 are provided for limiting the rotating range of said lever. Now reference is made to Fig. 14 for explaining the entire structure. The recording unit B has a main body 1 and a cover member 2 openably linked thereto by a shaft 3, and said cover member 2 can be engaged with the main body 1 by a click mechanism (not shown). On the bottom side of said cover member 2 there are mounted a feed roll 250a of an ink sheet 250 consisting of a substrate film such as polyester film coated with thermotransferrable ink, and a takeup roller 250b. In the far side of the main body there is provided a roll holder 4, in which loaded is a roll 5a of a sheet 5 consisting of plain paper. Said ink sheet 250 and the sheet 5 are advanced by the rotation of a platen roller 6a. The curl of the sheet 5 is corrected in a curl correcting mechanism A, by bending in a direction opposite to that of the curl. Image is recorded on the sheet 5 by fusing the ink of the ink sheet 250 and transferring it onto the sheet 5 in the recording means 5. The sheet 5 after image recording is cut by the cutter 7 and discharged onto a stacker 9 by discharge rollers 8. Said ink sheet 250 is separated from the sheet 5 by a separating shaft 25a before reaching the cutter 7, and is wound by the takeup roller 250b. The feed roller 250a and the takeup roller 250b are driven with tension on the ink sheet, in order to avoid slack therein, by a driving system 250c (cf. Fig. 16). The recording head 6b heats the ink sheet 250 according to the image signal, thereby fusing ink of said ink sheet and transferring said ink onto the sheet 5, thus forming an image thereon, and is pressed to the platen roller 6a across the sheet 5 and the ink sheet 250. More specifically the recording head 6b is rotatably mounted, by a shaft 6c, on the cover member 2, and presses the platen roller 6a by a compression spring 6d when the cover member is closed. Thus the sheet 5 and the ink sheet 250 are advanced by the rotation of the platen roller 6a while said sheets are pinched between the platen roller 6a and the recording head 6b. The recording head 6b in the present embodiment is so-called line thermal head, having, on a face in contact with the ink sheet 250, plural heat-generating elements arranged in the transversal direction of the ink sheet 250. Electric currents corresponding to image signal are supplied to the heat-generating elements 6b 1 to selectively heat the ink sheet 250, thereby fusing the ink in the heated portions and transferring said ink onto the sheet 5, thus recording an image. On the rotating end of the switch lever 240 there is provided a hook 242, on which engaged is the other end of said tension spring 135. Thus said spring 135 is provided under tension between said hooks 134a and 142, whereby the pulley 133 is constantly biased in a direction f shown in Fig. 9, namely a direction for increasing the curl correcting effect by the guide shaft 23. Said switch lever 240 is provided for switching the tension of the tension spring 135. In the solid-lined position in contact with the stopper 243 as shown in Fig. 20, the tension of the spring 135 is small as the distance between the hooks 134a, 242 is short. Consequently the torque of the pulley 133 in the direction f is weak, and the curl correcting effect is weak. On the other hand, in the chain-lined position in contact with the stopper 244, as shown in Fig. 15, the spring 135 is more extended and has a stronger tension, as the distance between the hooks 134a and 242 is longer. Consequently the pulley 133 receives a stronger torque in the direction f, and the curl correcting effect becomes stronger. In this manner the rotation of the switch lever 240 allows to change the tension of the spring 135, thereby increasing or decreasing the curl correcting effect. In the movement k of the switch lever 240 from the solid-lined position in Fig. 15 to the chain-lined position, the spring 135 is at first extended to increase the resistance to said movement. However, when the lever 240 moves from a substantially vertical position to the side of the stopper 244 (when the central line of the spring 135 passes through the pin 241 toward the stopper 244), the force of the spring 135 tends to rotate the lever 240 in the direction k, so that the lever 240 rotates spontaneously to the position of the stopper 244. In the present embodiment, the lever 240 is placed in the chain-lined position, since the sheet 5 is composed of plain paper having strong curling tendency. Thus the tension of the spring 135 (or torque on the arms 26) is increased, so that, even for a large diameter of the roll 5a as shown in Fig. 14C, the amount of rotation of the arms 26 increases and the angle of the sheet 5 over the curl correcting shaft 22 becomes smaller than  1 , thus intensifying the curl correcting effect. When the tension of the spring 135 is selected stronger by the switch lever 240, the curl correcting effect increases with the decrease of the roll 5a in diameter as shown in Fig. 14D, but the minimum angle of the sheet 5 over the curl correcting shaft 22 is determined by the position of the stopper 136 and cannot become smaller than  2 . The position of said stopper 136 has therefore to be changed for obtaining a further stronger curl correcting effect. When the ink sheet 250 is removed and the plain recording sheet is replaced by thermal recording sheet of less curling tendency, the switch lever 240 is moved to the solid-lined position in Fig. 15. Since the torque on the arms 26 is reduced in this state, the arms 26 rotates less in the direction -b even for a small roll diameter, whereby the angle of the sheet 5 over the curl correcting shaft 22 becomes larger than  2 and the curl correcting effect is weakened. As explained in the foregoing, the rotating operation of the switch lever 240 allows to regulate the level of curl correcting effect, thereby achieving appropriate curl correction according to the kind, thickness etc. of the sheet 5. In the present embodiment, a microswitch 45 to be actuated by the switch lever 240 is provided in the vicinity thereof, as shown in Figs. 14 and 15. Said microswitch detects the position of said lever 240, and the output P2 of the motor 20 at the reverse rotation can be switched according to said detection. More specifically, when the microswitch 45 is on, the spring 135 has the lower tension to exert the lower torque on the arms 26, so that the motor power P 2 at the reverse rotation is switched to a smaller output P 2a (> P 1 ). When said microswitch 45 is off, the spring 135 has the higher tension to exert the higher torque on the arms 26, so that the motor power P 2 is switched to a larger output P 2b (< P 2a ). Fig. 16 is a block diagram of the control system of the entire recording unit B, including the output control for the platen motor 20 at the reverse rotation, wherein provided are a CPU 260 for controlling the entire system, a ROM 261, a RAM 262, and an interface 263. Said interface 263 is connected to a motor actuating circuit 264 for the platen motor 20, ink sheet motor 250c and discharge motor 8a,; a recording head actuating circuit 265 for the recording head 6b; and the microswitch 245 explained above. The ROM 261 stores a control program corresponding to a flow chart shown in Fig. 17, and the CPU 260 control the motor actuating circuit 264 and the recording head actuating circuit 265 according to said program. Also in response to a signal supplied from the microswitch 245 through the interface 263, the CPU 260 switches the output of the motor 20 to P 2a or P 2b through the motor actuating circuit 264. In the recording operation of the recording unit B, at a step S10 rotates the platen motor 20 in the forward direction, and a step S11 activates the ink sheet motor 250c. Then a step S12 energizes the recording 6c. Then a step S13 discriminates whether the recording of a page has been completed, and, if completed, a step S14 deactivates the platen motor 20 and the ink sheet motor 250c when the rear end of the recorded image reaches the position of the cutter 7. Then a step S15 activates the cutter 7 to cut the sheet 5. A next step S16 discriminates the state of the microswitch 245, and, if it is on, a step S17 reverses the platen motor 20 with the smaller power P 2a , thereby retracting the leading end of the sheet 5 from the position of the cutter 7 to the position of the recording meand 6. If the microswitch 245 is off, a step S18 reverses the platen motor 20 with the larger power P 2b for retracting the sheet in the same manner. In this state the arms 26 are returned to the position shown in Fig. 13C. Consequently the cover member 2 can be opened smoothly, without interference of the guide shaft 23 with the curl correcting shaft 22, for example for the loading of the roll 5a. Then a step S19 discriminates the command for the recording of next page, and if the command is present, the sequence returns to the step S10 to repeat the above-explained sequence. If the command is absent, the operation of the recording system B is terminated. As explained in the foregoing, the present embodiment is capable of appropriate curl correction according to the level of curling and the kind, thickness etc. of the sheet, and is also capable of precise sheet transportation since the motor 20 is only required to rotate the platen roller 6a at the recording operation. Also it enables easy replacement of the sheet roll 5a. In the following there will be explained a further embodiment in which the switch lever in the foregoing embodiment is automatically switched. As shown in Figs. 18 and 19, a switch lever 240 is rotatably provided by a pin 24a on a lateral face of the main body 1, on which the pulley 133 is positioned. Also provided are drive means 345 for actuating said switch lever 240, and stoppers 243, 244 for limiting the rotating range of said lever 240. On the rotating end of the switch lever 240 there is provided a hook 242, on which engaged is the other end of said tension spring 135. Thus said spring 135 is provided under a tension between said hooks 242 and 134a, whereby the pulley 133 is constantly biased in a direction f in Fig. 19, namely a direction to increase the curl correcting effect by the guide shaft 23. The other end of the switch lever 240 is connected, through a pin 345b, to a plunger 345a of the drive means 345. Thus the switch lever 240 can be placed at the solid-lined position or the chain-lined position in Fig. 19, by advancing or retracting the plunger 345a by the drive means 345. Said switch lever 240 changes the tension of the spring 135. In the solid-lined position in contact with the stopper 243, as shown in Fig. 19, the spring 135 is extended less because of the shorter distance between the hooks 134a and 242, whereby the tension is lower and the torque on the pulley 133 in the direction f is weaker. Consequently the curl correcting effect is weaker. For intensifying the curl correcting effect, the drive means 345 is activated to advance the plunger 345a, thereby rotating the switch lever 240 in the direction k shown in Fig. 19. The lever 240 rotates to the position of the stopper 244 and stops in contact therewith (chain-lined position in Fig. 19). In this position the spring 135 is extended more because of the longer distance between the hooks 134a and 242, whereby the tension is higher and the torque on the pulley 133 in the direction f is stronger. Consequently the curl correcting effect becomes stronger. For reducing the curl correcting effect from this state, the drive means is activated to retract the plunger 245a, thereby rotating the switch lever 240 in the direction -k. In this manner the rotation of the switch lever 240 by the drive means 245 allows to regulate the tension of the spring 135, thereby intensifying or reducing the curl correcting effect. In the following there will be explained detecting means for detecting the presence of the ink sheet 250, and control means for controlling said switch means in repsonse to the signal from said detecting means. As shown in Fig. 14C, an ink sheet sensor 346 is provided as detecting means, in the vicinity of the path of the ink sheet 350. Said ink sheet sensor is composed for example of a photosensor, which projects light toward the ink sheet 350 and receives reflected light to detect the presence or absence of the ink sheet 350. The output of the ink sheet sensor 346 is supplied to control means 360 shown in Fig. 20. Said control means 360 is composed of a CPU 361 for controlling the entire system, a ROM 362 storing the control program, a RAM 363 and an interface 364, and serves to control the platen motor 20, recording head 6b etc. in addition to the drive means 345 for rotating the switch lever 240 in response to the detection signal from the ink sheet sensor 346. Said interface is connected to a motor actuating circuit 365 for said platen motor 20, ink sheet motor 250c and discharge motor 8a; a heat actuating circuit 366 for said recording head 6b; drive means 345; and said ink sheet sensor 346. The ROM 362 stores a control program corresponding to the flow chart shown in Fig. 21, and the CPU 361 controls the motor actuating circuit 364 and the recording head actuating circuit 365 according to said program. Also in response to a signal indicating the presence or absence of the ink sheet, supplied from the ink sheet sensor 346 through the interface 364, the CPU 361 controls the drive means 345 to shift the switch lever 240 to the solid-lined position or the chain-lined position shown in Fig. 19. More specifically, at the recording operation of the recording unit B, at first a step S10 discriminates the presence or absence of the ink sheet 350, and if present, a step S11 discriminates whether the switch lever 240 is at the stopper 243. If it is at the stopper 243, the drive means 345 is activated to advance the plunger 345a thereby shifting the switch lever 240 to the side of the stopper 244, for stronger curl correcting effect. Then the sequence proceeds to a step S15. On the other hand, if the switch lever 240 is not at the stopper 243, the sequence proceeds to the step S15 without activation of the drive means 345. Also if the step S10 identifies the absence of the ink sheet 350, a step S13 discriminates whether the switch lever 240 is at the side of the stopper 244. If it is at the stopper 244, a step S14 activates the drive means 345 to retract the plunger 346a thereby shifting the lever 240 to the stopper 243 for weaker curl correcting effect, and the sequence proceeds to the step S15. If the lever 240 is not at the stopper 244, the sequence proceeds to the step S15 without actuvation of the drive means 345. Then the step S15 rotates the platen motor 20 in the forward direction, and activates the ink sheet motor 50c, and a step S16 energizes the recording head 6c. Then a step S17 discriminates if the recording of a page has been completed, and if completed, a step S18 deactivates the platen motor 20 and the ink sheet motor 50c when the rear end of the recorded image reaches the position of the cutter 7. A next step S19 activates the cutter 7 to cut the sheet 5. Then a step S20 reverses the platen motor 20 with a power corresponding to the presence or absence of the ink sheet 350 as will be explained later, thereby retracting the leading end of the sheet 5 from the position of the cutter 7 to the position of the recording means 6. In this state the arms 26 return to the position shown in Fig. 13C. Then a step S21 discriminates the command for the recording of next page, and if said command is present, the sequence returns to the step S15 to repeat the above-explained procedure. In the absence of said command, the operation of the recording unit B is terminated. When the sheet 5 is changed to the thermal recording sheet, the ink sheet 350 is removed. Consequently the ink sheet sensor sends a signal, indicating the absence of the ink sheet, to the CPU 36a through the interface 364. In response, the drive means 345 is controlled to shift the switch lever 240 to the solid-lined position shown in Fig. 19. Thus the torque on the arms 26 is reduced, so that the amount of rotation of the arms 26 in the direction -b is reduced even for a smaller roll diameter as shown in Fig. 6A. Therefore, the angle of the sheet 5 over the curl correcting shaft 22 becomes larger than  2 , and the curl correcting effect is weakened. As explained in the foregoing, the rotation of the switch lever 240 by the drive means 345 allows to regulate the curl correcting effect, and appropriate curl correction can be achieved according to the kind, thickness etc. of the sheet 5. In the foregoing embodiments, the radius of curvature of the guide shaft 23 is selected larger than that of the curl correcting shaft 22, but these radii are not limited to the figures mentioned above. In fact the curl correcting effect can be intensified if the difference of said radii is larger. Also a larger radius of curvature of the guide shaft 23 provides less bending of the sheet 5, thereby enabling stable transportation without creases or skewed feeding. This invention relates to a curl correcting device comprising a sheet transport path, an opening member openable from a main body of the device, for opening said sheet transport path, transport means for transporting a sheet along said sheet transport path, a first guide member for guiding the sheet transported by said sheet transport means, a second guide member for guiding the sheet transported by said sheet transport means and correcting the curl of said sheet in cooperation with said first guide member, and moving means for moving at least one of said first and second guide members to a position not correcting the curl tendency, in a state in which said opening member is closed.

Claims: A curl correcting device comprising: a sheet convey path; a cover member (2) openable relative to a main body (1) of said curl correcting device, for opening said sheet convey path; a curl correcting member (22) for correcting a curl of said sheet, a guide member (23) for adjusting the wrapping angle of a sheet (5) on said curl correcting member (22) in order to correct the curl of said sheet (5) conveyed along said sheet convey path, said curl correcting member (22) and/or said guide member (23) being shiftable relative to each other, engaging means (34, 35) for holding said cover member (2) in a closed position to said main body (1), opening means (36, 36a) for releasing a hold operation of said engaging means (34, 35) to said main body (1), and position changing means (37, 37a) for changing a relative position between said guide member (23) and said curl correcting member (22), wherein, when said opening means (36, 36a) releases the hold operation of said engaging means (34, 35) for opening said cover member (2), in a state where said cover member (2) is closed, and said curl correcting member (22) and said guide member (23) are in a position interfering with each other to correct the curl of said sheet material, said position changing means (37, 37a) are controlled by said opening means (36, 36a) to change the relative position between said curl correcting member (22) and said guide member (23) for avoiding the interference therebetween and weakening the curl correcting operation. A device according to claim 1, wherein said sheet convey path is adapted to convey a sheet wound as a roll. A device according to claim 2, wherein said sheet is wound in such a direction that a face of the sheet opposite to said cover member (2) is curled inwards. A device according to claim 3, wherein said guide member (23) is adapted to guide the sheet in contact with a face of the sheet facing said cover member (2). A device according to claim 4, wherein said curl correcting member (22) is adapted to guide the sheet in contact with a face of the sheet opposite to the face thereof contacting said guide member (23). A device according to claim 5, wherein said guide member (23) has a first guide face of cylindrical shape contacting the sheet. A device according to claim 6, wherein said curl correcting member (22) has a second guide face of cylindrical shape contacting the sheet. A device according to claim 2, further comprising storage means (4) for storing said rolled sheet. A device according to claim 8, further comprising moving means (25, 26) for moving said guide member (23) or said curl correcting member (22) according to the diameter of the rolled sheet stored in said storage means (4). A device according to claim 9, wherein said moving means (25, 26) is adapted to move said guide member (23) or said curl correcting member (22) in a direction for bending the sheet in a direction opposite to the direction of curl formed in said sheet, as the diameter of the rolled sheet stored in said storage means (4) decreases. A device according to claim 10, wherein said moving means (25, 26) comprises biasing means (31, 47, 135) for biasing said guide member (23) or said curl correcting member (22) in a direction for bending the sheet in a direction opposite to the direction of curl formed in said sheet, as the diameter of the rolled sheet stored in said storage means (4) decreases. A device according to claim 1, wherein said position change means (37, 37a) is adapted to move said guide member (23) or said curl correcting member (22) to a position which does not hinder the opening of said cover member (2) in linkage with the releasing operation of said opening means (36, 36a). A device according to claim 9, further comprising drive means (20) for driving said moving means (25, 26) to move said guide member (23) or said curl correcting member (22) in the releasing operation of said opening means (36, 36a). A device according to claim 1, further comprising recording means (6) for recording an image on the sheet passing through said sheet convey path.

Inventors: SASAI KEIZO, SASAI, KEIZO, SASAI, KEIZO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: B65H23
IPC codes: B65H23, B41J15