Sheet conveying apparatus

Abstract: In the present invention, a U-turn path G is formed by facing an outer guide surface 2a of a guide member 2 to an outer peripheral surface 1a of a convey roller 1. A terminal edge 2A of the outer guide surface 2a forming a part of an outlet Gb of the U-turn path G is configured not to coincide with a straight line but is tapered to be convex at a central portion. When a thick sheet is conveyed through the U-turn path G, a trail end of the thick sheet is shifted to the terminal edge 2A while rubbing the outer guide surface 2a. Then, the trail end of the sheet leaves the terminal edge 2A. Since the terminal edge 2A has the tapered configuration, the trail end of the sheet is gradually disengaged from the terminal edge, thereby preventing the snapping restoring action of the trail end to weaken shock.

Description: The present relates to an image forming apparatus comprising a sheet conveying device having an arcuate convey path. Related Background Art Fig. 6 is a perspective view of a sheet conveying device of a generic image forming apparatus as disclosed in EP-A-0 737 589 (state of the art pursuant to Art. 54(3) EPC). The sheet conveying device has a so-called U-turn path for greatly changing a conveying direction of a sheet on which an image is to be formed. This sheet conveying device comprises a convey roller 100 rotated in a direction shown by the arrow R100, a convey guide 101 having an outer guide surface 101a facing to an outer peripheral surface 100a of the convey roller 100, and a pinch roller (convey sub-roller) 102 disposed downstream of the convey guide 101 in a rotating direction of the convey roller 100. Between the outer peripheral surface 100a of the convey roller 100 and the outer guide surface l01a of the guide 101, there is disposed an arc-shaped U-turn path G through which the sheet is passed. The pinch roller 102 is rotatably mounted on a front end of a pressure plate 103 rocked around a shaft 103a, and a rear end of the pressure plate 103 is biased upwardly by a pair of tension springs 104 so that the pinch roller is urged against the outer peripheral surface 100a of the convey roller 100 with moderate pressure from the above. Incidentally, a downstream outlet Gb of the U-turn path G is defined as a straight gap formed between the generatrix of the convey roller 100 and a terminal edge 101b of the outer guide surface 101a of the guide 101. The sheet fed to an inlet Ga of the U-turn guide G from a direction shown by the arrow A is conveyed through the U-turn path G by the convey roller 100 rotating in the direction R100. When a tip end of the sheet leaves the outlet Gb, the tip end of the sheet reaches the pinch roller 102 while being guided by a lower surface of the pressure plate 103. When the tip end of the sheet reaches the pinch roller 102, the sheet is conveyed by the convey roller 100 and the pinch roller 102. As the sheet is further conveyed, a trail end of the sheet passes through the outlet Gb of the U-turn path G and then leaves through the pinch roller 102. Then, the sheet is conveyed to a downstream image forming portion. However, in the above-mentioned conventional image forming apparatus, when a sheet (for example, a thick sheet) having great resiliency is conveyed and image formation is effected regarding the thick sheet at the downstream image forming portion, there arose a problem that a good image could not often be formed. That is to say, as shown in Fig. 7, since the sheet P having great resiliency is hard to be bent or curved, when the sheet is being conveyed through the U-turn path G, it is forcibly curved in accordance with a curvature of the U-turn path G. As a result, the tip and trail ends of the sheet rub the outer guide surface 101a of the guide 101. And, when the trail end Pb of the sheet P leaves the outlet Gb of the U-turn path G, i.e., when the trail end Pb is disengaged from the terminal edge 101b of the outer guide surface 101a of the guide 101, since the trail end Pb of the sheet P is abruptly returned from a posture shown by the solid line to a posture shown by the broken line by the resiliency (storing force) of the sheet P, a convey amount of the sheet P becomes unstable due to the shock, thereby worsening the image quality. For example, as shown in Fig. 7, when an image forming apparatus 105 includes a recording head 106 disposed immediately downstream of the pinch roller 102, since the sheet P is intermittently conveyed by the convey roller 100 by a predetermined amount and one-line recording is effected by the recording head 106, if the convey amount of the sheet P does not coincide with the recording width (in the conveying direction), a recording area will be deviated from a correct one, so that the desired image cannot be obtained. If the convey roller 100 is stopped in a condition (as shown by the solid line) that the trail end Pb of the sheet P is slightly caught by the terminal edge 101b of the outer guide surface 101a, the trail end Pb of the sheet is snappingly disengaged from the terminal edge 101b to reach the posture shown by the broken line. In this case, due to the presence of any play in a drive transmitting system for the convey roller 100 and any plays in bearings, the desired convey amount cannot be obtained. To eliminate such inconvenience, there is proposed a technique in which a braking means is incorporated into the drive transmitting system to minimize influence of the play. However, in this case, since load torque is increased and greater motor torque is required, a desired result cannot be achieved. As another proposal, there has been provided a technique in which a flexible sheet member (for example, PET sheet member) is adhered to the guide 101 so that a buffer (provided by the flexible sheet member) is formed between the terminal edge 101b of the outer guide surface 101a and the pressure plate 103, thereby eliminate the above drawback. However, in this case, if the sheet member is not adhered to the guide at a correct or accurate position, the sheet jam may frequently be caused. Thus, the accuracy of adhesion must be maintained, which in turn worsen the operability. Further, it is referred to EP-A-514155 which discloses a serial impact printer including an arcuate convey path formed between an outer cylindrical surface of a platen and a guide member having a guide surface curved along the outer cylindrical surface of the platen. The guide member moves with an impact dot head for guiding a printing sheet over the platen after the sheet passed the impact dot head. The guide member has two finger-shaped end portions guiding the front end of the printing sheet to a pair of downstream tension rollers. SUMMARY OF THE INVENTION An object of the invention is to provide an image forming apparatus comprising a sheet conveying device including an arcuate convey path and an image forming portion disposed downstream of the sheet conveying device which can prevent dispersion of a convey amount of a sheet caused by a restoring force of the sheet generated when a trail end of the sheet is disengaged from a terminal edge of an outer guide surface. The image forming apparatus should have a simple construction without using additional means such as the above-mentioned braking means or flexible member. According to the present invention, the above object is achieved by an image forming apparatus as defined in claim 1. The dependent claims set forth developments of the invention. With the arrangement as defined in claim 1, since the terminal edge of the first guide (outer guide surface) is configured not to coincide with the straight line defined by the longitudinal axis of the convey roller, for example, even when a trail end of the sheet (such as a thick sheet) having great resiliency is disengaged from the terminal edge, the entire trail end of the sheet is not disengaged at once. Thus, the trail end of the sheet is gradually disengaged from the terminal edge, so that the restoring force can be weakened. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an elevational sectional view of a sheet conveying device in a first embodiment of the present invention; Fig. 2 is a perspective view of the sheet conveying device in the first embodiment; Fig. 3 is a perspective view of a sheet conveying device in a second embodiment of the present invention; Fig. 4 is a perspective view of a sheet conveying device in a third embodiment of the present invention; Fig. 5 is a perspective view of a sheet conveying device in a fourth embodiment of the present invention; and Figs. 6 and 7 are perspective views of a conventional sheet conveying apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. <First Embodiment> A sheet conveying device shown in Figs. 1 and 2 comprises a convey roller 1, a guide member 2 and a convey sub-roller 5. The convey roller 1 having an outer peripheral surface 1a is rotatably supported by a frame (not shown) of the sheet conveying device and is rotated in a direction shown by the arrow R1 by a drive means (not shown). The guide member 2 is disposed to face to the convey roller 1 at the right side. The guide member 2 has an arc-shaped outer guide surface 2a facing to the outer peripheral surface 1a of the convey roller 1 and substantially coaxial with the outer peripheral surface 1a of the convey roller 1, so that a U-turn path G for conveying a sheet P is formed between the outer guide surface 2a and the outer peripheral surface 1a of the convey roller 1. The U-turn path G has a lower inlet Ga for the sheet P and an upper outlet Gb to change a conveying direction of the sheet from a direction shown by the arrow A to a direction shown by the arrow B. The outlet Gb is defined between a terminal edge 2A (disposed at a downstream end in the sheet conveying direction) of the outer guide surface 2a and the generatrix of the outer peripheral surface 1a opposed to the terminal edge. The terminal edge 2A is formed along the generatrix of the outer peripheral surface 1a (transverse to the sheet conveying direction), but is configured not to coincide with a straight line parallel to a longitudinal axis of the convey roller 1. For example, as shown in Fig. 2, the terminal edge 2A is tapered so that a central portion (in the axial direction) of the terminal edge is convex outwardly. A pinch sub-roller 3 urged against the outer peripheral surface 1a of the convey roller 1 is disposed within the U-turn path G. The convey sub-roller 5 is urged against the outer peripheral surface 1a of the convey roller 1 immediately at a downstream side of the outlet Gb of the U-turn path G in the sheet conveying direction and is rotatably mounted on a distal end 4b of a pressure plate (pressurizing member) 4. The pressure plate 4 can be rocked around a shaft 4a (parallel to the generatrix of the convey roller) and a proximal end 4c of the pressure plate is biased upwardly by tension springs 4d. With this arrangement, the convey sub-roller 5 mounted on the distal end 4b is urged against the outer peripheral surface of the convey roller 1 from the above with moderate pressure. In the illustrated embodiment, a portion (left side portion) of a lower surface of the pressure plate 4 which faces to the outer peripheral surface 1a between the outlet Gb and the convey sub-roller 5 acts as an auxiliary guide surface for guiding the sheet P from the outlet Gb to the convey sub-roller 5. Further, a portion (right side portion) of the lower surface of the pressure plate 4 which is contiguous to the downstream auxiliary guide surface acts as a second guide surface, and a straight path G1 for conveying the sheet P in a direction shown by the arrow B is defined between the second guide surface (and the auxiliary guide surface) and an upper surface of the guide member 2. An image forming portion is disposed downstream of the convey sub-roller 5, and includes a platen 6 for supporting a lower surface of the sheet P during the image formation, a recording head 9 for effecting the recording (for example, by discharging ink onto the sheet P), and a carriage 10 on which the recording head 9 is mounted and which is reciprocally shifted in a direction transverse to the sheet conveying direction. At a downstream side of the image forming portion, there are disposed a discharge roller 7 and spur wheels 8 for pinching and discharging the sheet P on which the image was recorded. With the arrangement as mentioned above, thick sheet P conveyed from the direction A is pinched between the convey roller 1 and the pinch sub-roller 3 to be conveyed through the U-turn path G. When a tip end of the sheet leaves the outlet Gb of the U-turn path G, the tip end of the sheet is guided by the pressure plate 4 and then is pinched by the convey sub-roller 5. Thereafter, the sheet P is further conveyed to pass through the platen 6. When the tip end of the sheet is pinched between the discharge roller 7 and the spur wheels 8, the sheet is temporarily stopped. In this condition, the recording is started. That is to say, first of all, one-line recording is performed by reciprocally shifting the recording head 9 by the carriage 10 while discharging the ink toward the surface of the sheet P, and then, the sheet P is conveyed by the convey roller 1 by a predetermined amount. One-line recording and the predetermined amount conveyance of the sheet are repeated until one-page recording is finished. In the above sheet conveying device, since there is the straight path G1, it is necessary to form a gap (for the straight path G1) between the upper surface of the guide member 2 and the lower surface of the pressure plate 4. With this arrangement, as mentioned above, when the trail end Pb of the sheet P having great resiliency leaves the terminal edge 2A of the outer guide surface 2a, the trail end Pb is snappingly returned to its straight condition, thereby worsening the conveying accuracy. However, in the illustrated embodiment, as mentioned above, since the terminal edge 2A of the guide member 2 is configured not to coincide with a straight line but to be convex at the central portion, the trail end Pb of the sheet P is gradually disengaged from the terminal edge 2A from its both ends to its central portion. Accordingly, the conveying accuracy is maintained, thereby providing the good image. Further, since any additional braking means or an additional flexible sheet member is not required, the apparatus can be made cheaper and the operability can be improved. <Second Embodiment> Fig. 3 is a perspective view showing a sheet conveying device in to a second embodiment of the present invention. In this second embodiment, a terminal edge 2B of the outer guide surface 2a is not curved to be convex outwardly (as is in the first embodiment), but is steppingly protruded outwardly (in the sheet conveying direction) from its both ends to its central portion as steps, as shown in Fig. 3. With this arrangement, the trail end Pb of the sheet P is gradually disengaged from the terminal edge 2B from its both ends to its central portion, thereby achieving the same advantage as the first embodiment. <Third Embodiment> Fig. 4 is a perspective view showing a sheet conveying device in a third embodiment of the present invention. In this third embodiment, a terminal edge 2C of the outer guide surface 2a is tapered to be concave inwardly at its central portion. With this arrangement, the trail end Pb of the sheet P is gradually disengaged from the terminal edge 2C from its central portion to its both ends, thereby achieving the same advantage as those in the first and second embodiments. <Fourth Embodiment> Fig. 5 is a perspective view showing a sheet conveying device in a fourth embodiment of the present invention. In this fourth embodiment, a terminal edge 2D of the outer guide surface 2a is linearly tapered so that one end 2D1 of the terminal edge is protruded from the other end 2D2 in the sheet conveying direction by an amount of X. With this arrangement, the trail end Pb of the sheet P is gradually disengaged from the terminal edge 2D from the left end (end 2D2) to the right end (end 2D1). Also in this embodiment, substantially the same advantage as those in the first to third embodiments can be expected. Incidentally, in the arrangement as shown in Fig. 5, the tapered amount X may be greater than 2 mm. The reason is that, even if the sheet is skew-fed more or less or the dimension of the sheet itself has slight error, the desired advantage can be obtained. In the above-mentioned first to fourth embodiments, while examples that the terminal edges 2A, 2B, 2C and 2D are tapered or stepped were explained, the terminal edge can be formed by combination of tapers or tapers and steps or may be curved to have an arcuate curvature. That is to say, the terminal edge is not limited to a special configuration, but may be appropriately configured so long as the trail end Pb of the sheet P can be gradually disengaged from the terminal edge. The configuration of the terminal edge may be determined by tests for providing the optimum advantage for various sheets having different size-and resiliency or may be selected on the basis of design and/or manufacturing limitations. As mentioned above, according to the present invention, by using the downstream terminal edge of the outer guide surface of the U-turn path having the configuration other than the straight line, the configuration of the terminal edge can be differentiated from the edge (generally, straight line) of the trail end of the sheet to be disengaged from the terminal edge so that the trail end of the sheet is gradually disengaged from the terminal end. Thus, the shock of the disengagement is reduced to prevent the conveying accuracy from being worsened. As a result, when the image forming portion is disposed immediately at a downstream side of the U-turn path, the poor image due to the reduction in conveying accuracy can be avoided. In the present invention, the above-mentioned effective advantage can be obtained with a simple construction in which the terminal edge of the outer guide surface is merely altered and without using any additional braking means or additional flexible sheet member. Thus, the apparatus can be made cheaper and the operability can be improved. Incidentally, the configuration of the terminal edge from which the trail end of the sheet is gradually disengaged may be taper(s) of steps. The recording head used in the above-mentioned embodiments may be an ink jet head which includes heat generating elements disposed in nozzles and in which an ink droplet is discharged from the corresponding nozzle by growth of a bubble generated in the ink by thermal energy from the selected heat generating element.

Claims: An image forming apparatus for forming an image on a sheet by using a recording head (9), said image forming apparatus comprising: a sheet conveying device including an arcuate convey path (G) formed between an outer peripheral surface (1a) of a convey roller (1) for conveying the sheet and a first guide (2) having a guide surface (2a) curved along the outer peripheral surface (1a) of said convey roller (1) to guide the sheet along the arcuate convey path (G), and further including a second guide (4) disposed downstream of said first guide (2) in a sheet conveying direction (B) to guide the sheet passed through said first guide; and an image forming portion disposed downstream of said second guide (4) for forming the image by using the recording head (9) on the sheet guided by the second guide, wherein the downstream terminal edge (2A; 2B; 2C; 2D) of said first guide (2) in the sheet conveying direction (B) is configured not to be parallel with a straight line defined by a longitudinal axis of said convey roller (1). An image forming apparatus according to claim 1, wherein said terminal edge (2A; 2C) of said first guide (2) is formed by tapers to be convex or concave at its central portion in the sheet conveying direction (B). An image forming apparatus according to claim 1, wherein said terminal edge (2B) of said first guide (2) is formed by steps to protrude in the sheet conveying direction (B) from its both ends to its central portion. An image forming apparatus according to claim 1, wherein said terminal edge (2D) of said first guide (2) is linearly tapered so that one end thereof is protruded from the other end in the sheet conveying direction (B). An image forming apparatus according to any preceding claim, further comprising a second convey path (G 1 ) capable of entering a sheet through a gap formed between the terminal edge (2A) of said first guide (2) and said second guide (4) so that the sheet is guided by said second guide without passing through said first guide. An image forming apparatus according to claim 5, wherein said second guide (4) extends toward the downstream side beyond said terminal edge (2A) to thereby form said second convey path (G 1 ). An image forming apparatus according to any preceding claim, further comprising a pressurizing member (4) for urging a convey sub-roller (5) against the outer peripheral surface (1a) of said convey roller (1), the pressurizing member (4) having an guide surface facing to said outer peripheral surface between the outlet (Gb) of said arcuate convey path (G) and the convey sub-roller (5) so as to form said second guide. An image forming apparatus according to any preceding claim, wherein said recording head (9) is an ink jet head for forming the image on the sheet by discharging ink. An image forming apparatus according to claim 8, wherein said recording head (9) forms the image by using an ink droplet discharged by thermal energy.

Inventors: MIYAUCHI YASUO, MIYAUCHI, YASUO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: B41J13
IPC codes: B41J13, B65H5, B65H15, B41J11