Self-adjusting paper decurler

Abstract: Lower, arced decurling guide (1) is pivoted on pin (3) and biased upward by spring (7). Upper stop (11) prevents the lower guide from moving closer to upper decurling roller (15). Paper (9) is guided between these upper and lower guides and the lower guide is moved outward in proportion to the rigidity of the paper being decurled. This automatic adjustment provides good decurling for all papers within a broad range of rigidity.

Description: This invention relates to an apparatus to straighten sheet material, e.g. paper, which is curled. Paper can be decurled by bending it in a direction opposite from that of the curl and various apparatus exists to guide paper in a bent path for such a purpose. This invention relates to such apparatus having self-adjusting decurl means based on the rigidity of the paper. Paper takes on a semi-permanent set or curl by being bent, especially under heat. Other factors, such as printing on one side of paper, may also cause curl. Electrophotographic imaging typically involves bonding toner to paper using heat as a final step in imaging, which results in significant curling. Where the paper is to be conveyed further, such as for duplex printing on the opposite side to a first printing, decurling typically is employed to ensure that the paper will feed reliably during the second printing operation. U.S. Patent No. 5,066,984 to Coombs teaches a decurler of the general kind in which this invention is employed. That patent employs a stationary guide in the form of an arc spaced from and partially surrounding a rotating roller. The paper is fed between the guide and the roller, where it is bent around roughly 120 degrees of the roller. The roller is rotated in a direction which assists in paper feeding, but the contact with the roller is light because the space between the guide and the roller is more than the thickness of the paper. Japanese Patent No. 60-97163 to T. Hashimoto, issued May 30, 1985, discloses a flat guide spaced from pinch rollers for decurling. The decurler of the foregoing U.S. Patent No. 5,066,984 does not provide satisfactory results for papers of different rigidity. Configurations of the arc guide and the spaced roller may be satisfactory for one paper but produce under decurling or over decurling (curling in the direction opposite from the original curling) for other papers. U.S. Patent No. 2,531,619 to Gonia discloses a decurler in which decurling is by directing paper around a spring-mounted roller for which the pressure is mechanically adjusted to vary the degree of flexing. This adjustment is done by adjusting screws and is not automatic. This is known from US-A-5,066,984 to provide apparatus for decurling sheet material, comprising a first guide surface in the form of an arc; and a second guide surface located opposite said arc of said first guide surface, the arc of said first guide surface being spaced from said second guide surface a distance greater than the thickness of sheet material to be decurled when the first guide surface is pivoted in the extreme position thereof towards the second guide surface. A curling/decurling mechanism is also known from Xerox Disclosure Journal, volume 8, No. 1, 1983, page 5. According to the present invention, the apparatus is characterised in that said first guide surface is pivotable, and in that the apparatus further comprises means biasing said first guide surface to pivot towards said second guide surface; and means to drive said sheet material to first contact said second guide surface and then to move between said first guide surface and said second guide surface; said biasing means providing resistance to pivoting of said first guide surface away from said second guide surface which, in operation is overcome in proportion to the rigidity of the sheet material being decurled so that such pivoting is greater with more rigid sheet materials. In accordance with this invention, it is recognized that light papers typically require a tighter bend than heavy papers to achieve straightening or sufficient decurl. The invention in its preferred forms employs an internal guide surface (the second guide) and an outer guide (the first guide) spaced from the internal guide and formed in an arc partially around the internal surface. The internal guide may be a roller which is rotated to assist paper feed. The arced guide is pivotally mounted and biased toward the roller and is moved outward by heavier papers being decurled, but not moved or moved less by lighter papers. This movement in proportion to the rigidity of the paper being decurled automatically adjusts the outer guide to the rigidity of the paper being fed to provide good decurling for all papers within a broad range of rigidity. An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:- Fig. 1 is an illustrative side view illustrating the decurler mechanisms in their rest or light paper position; Fig. 2 is the same view as Fig. 1 with a heavy paper pushing the guide downward; and Fig. 3 is a top, perspective view of the primary mechanisms of the decurler. Referring to Fig. 1 an arced, decurl guide 1 is mounted on a pin 3 to pivot around the latter. An arm 5 of guide 1 is connected through a spring 7 to the frame (a stationary point) of the decurler. Spring 7 biases guide 1 to move clockwise around pin 3. Fig. 1 shows guide 1 contacting a stationary, upper stop surface 11, which is the rest position and the position when the rigidity of a paper 9 being decurled is not sufficient to overcome the bias of spring 7. Fig. 2 shows guide 1 contacting a stationary, lower stop surface 13, which is the position of guide 1 after the largest movement of guide 1 permitted by the mechanism. Roller 15 is located opposite the arced surface of guide 13, and stop 11 is located to leave a separation of more than the thickness of paper 9 between roller 15 and guide 13. The moment (torque characteristics) and other mechanical characteristics of guide 1 as pivoted on pin 3, and the resilience and other mechanical characteristics of spring 7, are selected so that paper of number 16 weight (international measure: 60gr./m 2 ) does not move guide 1, and paper of number 24 weight (90gr./m 2 ) is just sufficient to move guide 1 against lower stop 13. Pinch rollers 17 and 19 rotate to drive paper 9 between guide 1 and roller 15. Roller 15 is always spaced from guide 1 by more than the thickness of any paper to be decurled, and pinch rollers 17 and 19 are on the input side and are positioned close enough to guide 1 so that the pinch rollers are a significant moving force on paper 9 while the paper passes between guide 1 and roller 15. Pinch rollers 21 and 23 on the output side are located to grasp any paper 9 of length to be decurled 18cm (seven inches) or longer in this specific embodiment) before it leaves rollers 17 and 19, and to pull the paper between guide 1 and roller 15. In this manner paper 9 being decurled is initially moved by rollers 17 and 19. The paper is guided to contact roller 15 opposite guide 1. Roller 15 is of a urethane, high friction material and is rotated to assist the movement of the paper. Before paper 9 exits rollers 17 and 19, it is in the nip of rollers 21 and 23, which rotate to continue the movement of the paper between guide 1 and roller 15. Fig. 3 shows a perspective view of the decurl guide structure of this embodiment. The upper stop 11 comprises the outer surface of a low-friction bushing for a shaft 30 (shown on the right without the right bushing) which supports decurl roller 15. Lower stop 13 is a pin mounted on a frame 32 which extends a limited distance to contact guide 1. Although shown on only one side in Fig. 3, upper stop 11 and lower stop 13 are substantially identical at each end of decurl roller 15. A lower guide 34 extends across the decurler and is of a width of at least as wide as the widest paper 9 to be decurled. This width is almost 23cm (9 inches) in this specific embodiment; since, when guide 34 is wider than the paper, a skewed paper 9 can pass through without encountering frame 32, which is an advantage. Lower guide 34 is of molded plastics and, as is conventional to reduce electrostatic charging, has a number of raised integral, thin guides 36 on which the paper primarily rests. Referring again to Fig. 1 and Fig. 2, guides 36 face an upper guide 38 which is pivoted on a rod 40. In a clockwise rotated position of guide 38 (not shown) paper 9 from rollers 17 and 19 is directed to bypass the decurling system. In the position shown in Figures 1 and 2 the upper guide 38 directs paper 9 for decurling and is positioned opposite guides 36 and the lower edge of guide 34 to direct paper 9 to contact decurl roller 15. Ideally, this contact is tangential to roller 15, but a more directed contact is acceptable. The force from pinch rollers 17 and 19 is not critical to the self-adjustment of this decurler, since paper which is not stiff enough to overcome the force from spring 7 will be deflected by guide 1 even if the force from rollers 17 and 19 is otherwise large. This stiffness characteristic of paper is sometimes termed beam strength. As previously mentioned, roller 15 has a high friction surface and is driven to rotate in the paper feed direction. This facilitates paper movement. Movement of roller 15 is not considered critical to function since it is not the primary drive force during the decurling, and in fact roller 15 might ideally be replaced with a shaped surface of very low drag to paper 9 and having a surface complementary to the arc of guide 1. Alternatively, such a stationary surface might be used having significant friction but with feed of the paper being assisted by, for example, air jets.

Claims: Apparatus for decurling sheet material (9), comprising a first guide surface (1) in the form of an arc; and a second guide surface (15) located opposite said arc of said first guide surface, the arc of said first guide surface being spaced from said second guide surface a distance greater than the thickness of sheet material to be decurled when the first guide surface is pivoted in the extreme position thereof towards the second guide surface; characterised in that said first guide surface (1) is pivotable, and in that the apparatus further comprises means (7) biasing said first guide surface to pivot towards said second guide surface; and means (17,19) to drive said sheet material to first contact said second guide surface and then to move between said first guide surface and said second guide surface; said biasing means providing resistance to pivoting of said first guide surface away from said second guide surface which, in operation is overcome in proportion to the rigidity of the sheet material being decurled so that such pivoting is greater with more rigid sheet materials. Apparatus as claimed in claim 1, further comprising a first stop surface (11) and a second stop surface (13) positioned apart to limit pivoting of said first guide surface (1) by obstructing said first guide surface and thereby limit the range of movement of said first guide surface. Apparatus as claimed in claim 1 or 2, in which said biasing means comprises a spring (7) connected between an arm (5) of said first guide surface (1) and a frame of the apparatus. Apparatus as claimed in any preceding claim, wherein said first guide surface (1) is mounted on a pivot pin (3). Apparatus as claimed in any preceding claim, in which said second guide surface (15) is a roller having a friction surface and which in operation is turned to assist sheet material movement between said first guide surface (1) and said roller.

Inventors: LEEMHUIS MICHAEL CRAIG, WESTHOFF DANIEL JOSEPH, LEEMHUIS, MICHAEL CRAIG, WESTHOFF, DANIEL JOSEPH
Applicants: LEXMARK INT INC, LEXMARK INTERNATIONAL, INC.
Application year: 1993
Main classification: B65H29
IPC codes: B65H29, B65H23