Sheet conveying apparatus and printing apparatus

A sheet conveying apparatus coveys a sheet while nipping the sheet between a conveyance roller and a pinch roller. The pinch roller includes a first roller portion and a second roller portion adjacent to each other in a rotational axial direction thereof. The pinch roller further includes a mechanism configured to change a difference between a pressing force that the first roller portion applies to the conveyance roller and a pressing force that the second roller portion applies to the conveyance roller.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for a sheet conveying apparatus usable with a printing apparatus.

2. Description of the Related Art

Japanese Patent Application Laid-Open No. 11-208923 discusses a printing apparatus that conveys a sheet by using a conveying mechanism including rollers. A sheet is nipped by a roller pair constituted by a conveyance roller and a driven roller, and is conveyed according to a rotation of the roller pair. The driven roller is divided into a plurality (three) of small rollers along the direction of the rotational axis thereof. The plurality of divided rollers is collectively pressed by using a single pressing unit, and the nip pressures thereof are changed as the sheet is transported forward.

The printing apparatus discussed in Japanese Patent Application Laid-Open No. 11-208923 is configured in such a manner that forces provided to change the nip pressures act on the plurality of divided driven rollers in a uniform way for all of them, and these forces cannot be adjusted individually.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a sheet conveying apparatus and a printing apparatus capable of conveying a sheet at high accuracy regardless of a sheet that the printing apparatus uses.

According to an aspect of the present invention, a sheet conveying apparatus includes a conveyance roller, and a pinch roller configured to nip a sheet between the conveyance roller and the pinch roller. The pinch roller includes a first roller portion and a second roller portion adjacent to each other in a rotational axial direction thereof, and further includes a mechanism configured to change a difference between a pressing force that the first roller portion applies to the conveyance roller and a pressing force that the second roller portion applies to the conveyance roller.

According to an exemplary embodiment of the present invention, the difference between the respective pressing forces of the first roller portion and the second roller portion included in the pinch roller can be changed, so that a sheet conveying apparatus and a printing apparatus capable of conveying a sheet at high accuracy regardless of a sheet that the printing apparatus uses can be implemented.

DESCRIPTION OF THE EMBODIMENTS

A printing apparatus based on the inkjet printing method according to an exemplary embodiment of the present invention is a high-speed line printer using a long continuous sheet (a continuous sheet longer than a repeated print unit referred to as “one page” or “unit image” in a conveyance direction) and capable of performing both one-sided printing and two-sided printing. For example, this printing apparatus is suitable to the field of printing of a large number of sheets, for example, in a print lab.

The present exemplary embodiment can be widely applied to a printing apparatus that uses ink and has to dry the ink thereafter, such as a printer, a multifunction peripheral, a copying machine, a facsimile apparatus, and manufacturing apparatuses of various kinds of devices. Further, the present exemplary embodiment can be also applied to a printing apparatus that forms a latent image on a sheet coated with a photosensitive material by, for example, a laser, and performs printing by the liquid development method. Further, the present exemplary embodiment can be also applied to not only a sheet processing apparatus that performs printing processing, but also sheet processing apparatuses that perform various kinds of processing (for example, recording, processing, coating, irradiating, reading, and inspecting) on a sheet.

FIG. 1is a cross-sectional view schematically illustrating the internal configuration of the printing apparatus according to the present exemplary embodiment. The printing apparatus according to the present exemplary embodiment uses a sheet wound into a rolled shape, and can perform two-sided printing on a first surface of the sheet and a second surface of the sheet, which is the back surface side of the first surface. As the internal configuration, the printing apparatus generally includes a sheet feeding unit1, a decurling unit2, a skew correction unit3, a printing unit4, an inspection unit5, a cutter unit6, an information recording unit7, a drying unit8, a reversing unit9, a discharge conveyance unit10, a sorter unit11, a sheet discharge unit12, and a control unit13. The sheet discharge unit12is a unit including the sorter unit11and in charge of sheet discharge processing. A sheet is conveyed along a sheet conveyance path indicated by the solid line inFIG. 1by a conveying mechanism including roller pairs and belts, and undergoes processing at the respective units. The terms “upstream” and “downstream” will be used herein to describe an arbitrary position along the sheet conveyance path from a position where a sheet is fed to a position where the sheet is discharged, in such a manner that a position closer to the sheet feeding unit1is referred to as “upstream” and a position farther away from the sheet feeding unit1is referred to as “downstream”.

The sheet feeding unit1is a unit for holding and feeding a continuous sheet wound into a rolled shape. The sheet feeding unit1can contain two rolls R1and R2, and is configured to selectively pull out one of them to feed it. The number of rolls that the sheet feeding unit1can contain is not limited to two, and the sheet feeding unit1can contain less than or greater than two rolls. Further, the present exemplary embodiment may use any continuous sheet which is not limited to a sheet wound into a rolled shape. For example, the present exemplary embodiment may use a continuous sheet which is perforated at each unit length and folded at each line of the perforation, and is then stacked in this state to be contained in the sheet feeding unit1.

The decurling unit2is a unit for reducing a curl (warpage) of a sheet fed from the sheet feeding unit1. The decurling unit2conveys a sheet while curving the sheet so as to provide a warpage in the opposite direction from a curl with use of two pinch rollers for one conveyance roller to thereby exert a decurling force to reduce the curl.

The skew correction unit3is a unit for correcting a skew state (inclination relative to an originally set forward direction) of a sheet transported from the decurling unit2. The skew correction unit3corrects a skew state of a sheet by pressing the sheet edge of the side that is used as a basis of the correction against a guide member. A loop is formed at the sheet being conveyed at the skew correction unit3.

The printing unit4is a sheet processing unit for forming an image by applying print processing onto a sheet being conveyed from above the sheet by print heads14. In other words, the printing unit4is a processing unit for performing predetermined processing on a sheet. The printing unit4also includes a plurality of conveyance rollers for conveying a sheet. As the print heads14, a plurality of print heads is arranged in parallel with one another along the conveyance direction. In the present exemplary embodiment, the printing apparatus includes seven line print heads corresponding to seven colors of cyan (C), magenta (M), yellow (Y), light cyan (LC), light magenta (LM), gray (G), and black (K). The number of colors and the number of print heads are not limited to seven. Further, as the inkjet printing method, the printing apparatus according to the present exemplary embodiment may employ, for example, the method using a heating element, a piezoelectric element, an electrostatic element, and a micro electric mechanical systems (MEMS) element. Ink of each color is supplied from an ink tank to the print head14through a corresponding ink tube.

The inspection unit5is a unit for optically reading an inspection example or image, which is printed onto a sheet at the printing unit4, by a scanner to inspect, for example, a nozzle state of the print heads14, a sheet conveyance state, and an image position, thereby determining whether an image is correctly printed. The scanner includes a charge coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor.

The cutter unit6is a unit including a cutter for cutting a printed sheet into pieces each having a predetermined length. The cutter cuts a sheet at a margin area between images formed on the sheet and at the rear of an image printed last.

The information recording unit7is a unit for recording print information (unique information) such as a serial number and/or a print date of a print output on an unprinted area of a cut sheet. This recording is performed by printing a character and a code by, for example, the inkjet printing method or the thermal transfer method.

The drying unit8is a unit for heating a sheet printed at the printing unit4to dry the provided ink in a short time. In the drying unit8, hot air is applied to at least the bottom surface of a sheet passing through, thereby drying the surface with ink provided thereon. The drying method here is not limited to applying hot air. The drying unit8may dry a sheet surface by emitting electromagnetic waves (for example, ultraviolet ray or infrared ray) onto the sheet surface.

The sheet conveyance path from the sheet feeding unit1to the drying unit8described above is referred to as a “first path”. The first path is shaped to have a U-turn between the printing unit4and the drying unit8, and the cutter unit6is located at some position along the U-turn shape.

The reversing unit9is a unit for temporarily taking up a continuous sheet with an image printed on the front surface thereof to turn over the sheet during two-sided printing. The reversing unit9is disposed at some position along a path (loop path referred to as a “second path”) from the drying unit8to the printing unit4via the decurling unit2so as to supply a sheet transferred from the drying unit8to the printing unit4again. The reversing unit9includes a take-up rotator (drum) configured to rotate to take up a sheet. An uncut continuous sheet, after printing of the front surface, is temporarily taken up by the take-up rotator. After the take-up is finished, the take-up rotator is rotated in the reverse direction so that the taken up sheet is sent in reverse order of the order at the time of the take-up to be supplied to the decurling unit2, and is then sent to the printing unit4. Since this sheet is turned upside down, the printing unit4can print an image on the back surface of the sheet. Assuming that the sheet feeding unit1is a first sheet feeding unit, the reversing unit9can be considered as a second sheet feeding unit. More specific details of the two-sided printing operation will be described below.

The discharge conveyance unit10is a unit for conveying a sheet cut by the cutter unit6and dried by the drying unit8to transfer the sheet to the sorter unit11. The discharge conveyance unit10is disposed at a path (referred to as a “third path”) different from the second path where the reversing unit9is disposed. A path switching mechanism having a movable flapper is disposed at a path branching position (referred to as a “discharge branching position”) so as to selectively guide a sheet conveyed along the first path to any one of the second path and the third path.

The sheet discharge unit12including the sorter unit11is disposed at a position at the side of the sheet feeding unit1and at the terminal of the third path. The sorter unit11is a unit for sorting printed sheets into groups as necessary. The sorted sheets are discharged onto a plurality of trays included in the sheet discharge unit12. In this way, the third path is laid out so as to extend below the sheet feeding unit1to discharge a sheet to the opposite side of the sheet feeding unit1from the printing unit4and the drying unit8.

In this way, the units from the sheet feeding unit1to the drying unit8are disposed along the first path in this order. The path beyond the drying unit8is branched into the second path and the third path. The second path includes the reversing unit9at a position along it, and is merged with the first path at a position beyond the reversing unit9. The third path includes the sheet discharge unit12at the terminal thereof.

The control unit13is a unit in charge of control of the respective units of the entire printing apparatus. The control unit13includes a central processing unit (CPU), a storage device, a controller (control unit) including various kinds of control units, an external interface, and an operation unit15where a user provides an input and receives an output. The operation of the printing apparatus is controlled based on an instruction from the controller or a host apparatus16such as a host computer connected to the controller via the external interface.

FIG. 2is a block diagram illustrating the configuration of the control unit13. The controller (the range surrounded by the broken line) included in the control unit13is constituted by a central processing unit (CPU)201, a read only memory (ROM)202, a random access memory (RAM)203, a hard disk drive (HDD)204, an image processing unit207, an engine control unit208, and an individual unit control unit209. The CPU201centrally controls the operations of the respective units of the printing apparatus. The ROM202stores programs to be executed by the CPU201, and fixed data required for various kinds of operations of the printing apparatus. The RAM203is used as a work area for the CPU201, a temporary storage area of various kinds of received data, and an area for storing various kinds of setting data. The HDD204can store programs to be executed by the CPU201, print data, and setting information required for various kinds of operations of the printing apparatus, and allows those data pieces to be read out from the HDD204. An operation unit206is an input/output interface with a user, and includes an input unit such as hard keys and a touch panel, and an output unit such as a display, which shows information, and an audio generator.

A dedicated processing unit is provided to a unit that is required to perform data processing at a high speed. The image processing unit207applies image processing to print data handled by the printing apparatus. The image processing unit207converts the color space (for example, Luminance/Chroma Blue/Chroma Red (YCbCr)) of input image data into a commonly-used Red/Green/Blue (RGB) color space (for example, the standard RGB (sRGB) color space). Further, the image processing unit207applies various kinds of image processing such as a resolution conversion, an image analysis, and an image correction to image data as necessary. The print data acquired by these kinds of image processing is stored in the RAM203or the HDD204. The engine control unit208drives and controls the print heads14of the printing unit4according to print data based on a control command received from, for example, the CPU201. Further, the engine control unit208also controls the conveying mechanisms of the respective units in the printing apparatus. The individual unit control unit209is a sub controller for individually controlling the respective units of the sheet feeding unit1, the decurling unit2, the skew correction unit3, the inspection unit5, the cutter unit6, the information recording unit7, the drying unit8, the reversing unit9, the discharge conveyance unit10, the sorter unit11, and the sheet discharge unit12. The individual control unit209controls the operations of the respective units based on an instruction from the CPU201. The external interface205is an interface (I/F) for enabling a connection of the controller to a host apparatus211, and is a local I/F or a network I/F. The constituent elements mentioned above are connected to one another through a system bus210.

The host apparatus16is an apparatus that serves as a supply source of image data that the printing apparatus is ordered to print. The host apparatus16may be either a general-purpose computer or a dedicated computer. Alternatively, the host apparatus16may be a dedicated image device such as an image capture device including an image reader unit, a digital camera, and a photo storage medium. In a case where the host apparatus16is a computer, an operating system (OS), application software for generating image data, and a printer driver for the printing apparatus are installed in a storage apparatus of the computer.

Next, a basic operation performed during printing will be described. The printing apparatus operates in different manners in the one-sided printing mode and the two-sided printing mode, and, therefore, both the one-sided printing mode and the two-sided printing mode will be described, respectively.

First, the one-sided printing mode will be described. The thick solid line inFIG. 1indicates the conveyance path from a supply of a sheet from the sheet feeding unit1to a discharge of the sheet to the sheet discharge unit12after printing of the sheet. The sheet feeding unit1supplies a sheet, and the decurling unit2and the skew correction unit3apply respective processing to the sheet. Then, the printing unit4prints an image onto the front surface (first surface) of the sheet. The printing unit4sequentially prints images (unit images), each of which has a predetermined unit length in the conveyance direction, on the long continuous sheet, and forms a plurality of images while lining up them. After that, the inspection unit5inspects the printed sheet, and the cutter unit6cuts the printed sheet for each unit image. The information recording unit7records the print information onto the back surfaces of the divided cut sheets, when necessary. Then, the cut sheets are conveyed to the drying unit8one-by-one to be dried there. After that, the sheets are sequentially discharged and stacked on the sheet discharge unit12of the sorter unit11through the discharge conveyance unit10. The sheet left at the printing unit4after the cutting of the last unit image is transported back to the sheet feeding unit1, and is wound up around the roll R1or R2. As will be described below, when the remaining portion of the continuous sheet is transported back in this way, the decurling unit2is adjusted to have a reduced decurling force, and the print heads14are controlled to be retracted from the sheet. In this way, in the one-sided printing mode, a sheet is processed while being transported through the first and third paths, but is not transported through the second path.

Next, the two-sided printing mode will be described. The printing apparatus executes a front (first)-surface print sequence, and consecutively executes a back (second)-surface print sequence. In the front-surface print sequence performed first, the respective units from the sheet feeding unit1to the inspection unit5operate in the same manner as the operations for the above-described one-sided printing. The cutter unit6does not cut the sheet at this time, and, therefore, the sheet is conveyed to the drying unit8as a continuous sheet. After the ink on the surface is dried at the drying unit8, the sheet is guided to the path (second path) leading to the reversing unit9, not to the path (third path) leading to the discharge conveyance unit10. In the second path, the sheet is taken up by the take-up drum of the reversing unit9, which rotates in the forward direction (the counterclockwise direction as viewed inFIG. 1). After the printing unit4completes printing all of data supposed to be printed onto the front surface, the cutter unit6cuts the continuous sheet at the trailing edge of the printed area of the continuous sheet. Based on this cut position, the portion (printed portion) of the continuous sheet at the downstream side in the conveyance direction is transported through the drying unit8, and is all taken up at the reversing unit9until even the trailing edge (the cut position) of the portion is wound around the take-up drum. On the other hand, at the same as the take-up of the reversing unit9, the portion of the continuous sheet left at the upstream side (the side including the printing unit4) relative to the cut position in the conveyance direction is returned to the sheet feeding unit1and is wound around the roll R1or R2so as to prevent even the leading edge of the sheet (the cut position) from remaining at the decurling unit2. This returning of the remaining sheet (feedback) prevents the sheet from bumping into the sheet supplied again during the back-surface printing sequence, which will be described below. As will be described below, during this returning of the remaining sheet, the decurling unit2is adjusted to have a reduced decurling force, and the print heads14are controlled to be retracted from the sheet.

After the above-described front-surface printing sequence, the operation is switched to the back-surface printing sequence. The take-up drum of the reversing unit9starts to rotate in the opposite direction (the clockwise direction as viewed inFIG. 1) from the direction at the time of taking up the sheet. The edge of the wound sheet (the trailing end of the sheet when the sheet is taken up becomes the leading edge of the sheet when the sheet is sent out) is sent into the decurling unit2along the path indicated by the broken line inFIG. 1. The decurling unit2corrects a curl added at the take-up rotator. In other words, the decurling unit2is located between the sheet feeding unit1and the printing unit4in the first path, and is also located between the reversing unit9and the printing unit4in the second path, so as to serve as a common unit for providing the decurling function in both paths. The sheet, which is turned upside down, is transported to the printing unit4through the skew correction unit3, and then the printing unit4prints an image onto the back surface of the sheet. After that, the printed sheet is transported to the cutter unit6through the inspection unit5, and then the cutter unit6cuts the printed sheet per predetermined unit length which is set in advance. Since the cut sheets each have images printed on both the front surface and the back surface, the information recording unit7does not record any information onto the sheets at this time. The cut sheets are conveyed to the drying unit8one by one, and are sequentially discharged and stacked onto the sheet discharge unit12of the sorter unit11through the discharge conveyance unit10. In this way, in the two-sided printing mode, the sheet is processed while being transported through the first path, the second path, the first path, and the third path in this order.

FIG. 3is a cross-sectional view illustrating the positional relationship among the print head14of the printing unit4and two roller pairs located upstream and downstream. A first roller pair and a second roller pair are respectively disposed at the upstream side and the downstream side of the print head14in such a direction (the direction indicated by the arrow) that a sheet S is conveyed while being printed. The sheet S is conveyed at the printing unit4by these roller pairs.

The first roller pair includes a conveyance roller101provided with a rotation driving force, and a pinch roller102driven to rotate. Further, an adjustment mechanism110is provided so as to individually variably adjust a nip pressure which the pinch roller102applies to the conveyance roller101. The second roller pair includes a conveyance roller103provided with a rotation driving force, and a pinch roller104driven to rotate. The conveyance forces with which the first roller pair and the second roller pair convey a sheet are set so as to satisfy the relationship expressed by the following mathematical expression (1).
first roller pair>second roller pair  (1)
The conveyance force of a roller pair is determined based on the nip pressure of the pinch roller. This is because, as a nip pressure is increased, this increase makes it difficult to generate a slip between a sheet and the surface of the roller. A nip pressure is determined based on the spring pressure of a spring that presses a pinch roller against a conveyance roller. When this relationship is satisfied, the first roller pair has maximum dominance over the sheet conveyance accuracy.

The conveyance speeds of the respective roller pairs (the circumferential speeds of the conveyance rollers101and103) are set so as to satisfy the relationship expressed by the following mathematical expression (2).
second roller pair>first roller pair  (2)
According to the relationship between the conveyance forces (mathematical expression (1)) and the relationship between the conveyance speeds (mathematical expression (2)), almost no slip is generated at the nip position of the first roller pair (between the conveyance roller101and the sheet S), which is a main conveyance unit. On the other hand, a slip is generated at the nip position of the second roller pair (between the conveyance roller103and the sheet S) due to a difference between the speeds.

In the configuration satisfying the above-described relationships, the first roller pair controls the conveyance accuracy as a whole of the printing unit4. Between the first roller pair and the second roller pair, the sheet S is conveyed while being pulled toward the downstream side by the second roller pair operating at a higher conveyance speed. Therefore, a tension is applied to the sheet S to prevent the sheet S from locally floating, whereby a constant distance is kept between the print head14and the sheet S to maintain the high printing accuracy.

The pinch roller102of the first roller pair located upstream is divided into a plurality of (four) small rollers along the rotational axial direction (the vertical direction on the paper ofFIG. 3) of the pinch roller102. The respective small rollers can be driven to rotate independently of one another. The pinch roller102is divided into the plurality of rollers for the following reason. Since the first roller pair located upstream controls the conveyance, the first roller pair is more strictly required to support a sheet with a nip pressure unchanged throughout the whole area of the sheet in the sheet width direction than the second roller pair.

If the pinch roller102is constituted by a single roller body without being divided, even a slight inclination of the rotational axis, if any, results in an uneven distribution of the nip pressure applied to a sheet. This uneven distribution may cause a deviation of the sheet traveling direction from the originally set direction, i.e., a so-called skew state. Dividing the pinch roller102into a plurality of rollers allows the divided rollers to independently apply a nip pressure, thereby reducing the possibility that the nip pressure may be unevenly applied in the sheet width direction.

Further, a pinch roller constituted by a single body is subject to a deflection, and tends to intensively apply nip pressures at the respective edges of a sheet, whereby a difference is generated between the nip pressures at the respective edges to thereby destabilize force application to the sheet, facilitating generation of, for example, wrinkles, slacks, and a skew state of the sheet. The printing apparatus according to the present exemplary embodiment can use various sizes of sheets. Use of different sizes of sheets in the sheet width direction may cause the rollers located at the ends, among the plurality of divided rollers constituting the pinch roller102included in the first roller pair, to be put in the following three states in the rotational axial direction of the rollers: (1) a state that the roller is in contact with a sheet throughout the whole length of the roller; (2) a state that the roller is in contact with a sheet at only a part of the length of the roller; and (3) a state that the roller is totally out of contact with a sheet. It should be noted that the term “whole length” and “a part of length” here are used to mean the whole or a part of a generally linear narrow area at which a rotating roller contacts a sheet, but not to mean the whole surface of a roller throughout the entire circumference of the roller.

FIGS. 4A,4B, and4C schematically illustrate what the pinch rollers are like in the states (1) to (3), and indicate postural changes of the pinch rollers when they are applied to different sizes of sheets. In particular,FIGS. 4A,4B, and4C illustrate the states (3), (2), and (1), respectively. The pinch roller102is divided into four rollers102a,102b,102c, and102din this order from one end to the other end. A sheet is conveyed in the vertical direction on the paper ofFIGS. 4A,4B, and4C during a printing operation. A sheet is supplied by the method of causing the center of the sheet to pass through a same position in the sheet width direction regardless of the size of the sheet, i.e., the so-called center alignment method. In the examples illustrated inFIGS. 4A,4B, and4C, a sheet, no matter which size the sheet has, is conveyed in such a manner that the center of the sheet passes through the middle position between the roller102band102c.

As will be used herein, the term “first roller portion” is used to refer to the outer rollers102aand102dfarther away from the center of a sheet in the rotational axial direction, while the term “second roller portion” is used to refer to the inner rollers102band102clocated adjacent to the first roller portion and closer to the center of the sheet.

FIG. 4Aillustrates what the rollers102ato102dare like when the printing apparatus uses a sheet S1having a minimum size among sizes of sheets that are expected to be used as a recording medium. The sheet S1has a sheet width approximately equal to the sum of the lengths of the inner two rollers102band102c, and the outer rollers102aand102dadjacent thereto are out of contact with the sheet S1. Therefore, both the outer rollers102aand102dare postured like the state (3).FIG. 4Billustrates what the pinch rollers102ato102are like when the printing apparatus uses a sheet S2having a sheet width of a middle size larger than the minimum size of the sheet S1but smaller than a maximum size, among sheets that are expected to be used as a recording medium. The sheet S2has a sheet width wider than the sum of the lengths of the inner two rollers102band102c, so that the outer rollers102aand102dare located in such a manner that the inner side of each of them is partially in contact with the sheet S2while the remaining outer side thereof is partially out of contact with the sheet S2. Therefore, both the outer rollers102aand102dare postured like the state (2).FIG. 4Cillustrates what the pinch rollers102ato102dare like when the printing apparatus uses a sheet S3having the maximum size among sizes of sheets that are expected to be used as a recording medium. The sheet S3has a sheet width approximately equal to or wider than the sum of all of the lengths of the four rollers102ato102d, so that the whole lengths of the outer rollers102aand102dare in contact with the sheet S3. Therefore, in this case, both the outer rollers102aand102dare postured like the state (1).

In the state (1) or (3), the outer rollers102aand102drotate while keeping the same postures as the inner rollers102band102cwithout being inclined. On the other hand, in the state (2), since the outer rollers102aand102dare partially in contact with the sheet S2, slight postural changes are made at the rollers102aand102d, leading to inclinations of the rollers102aand102(refer toFIG. 4B). Therefore, comparing the state (1) and the state (2), the outer rollers102aand102dapply forces to the sheet in different directions and with different strengths. The inner rollers102band102chave unchanged postures in any of the states (1) to (3).

In the state (2), the inclinations of the rollers102aand102doccur mainly in two directions. A first roller inclination occurs in such a manner that a portion of the roller rides on an edge of a sheet and is lifted up in the diametrical direction of the roller, thereby generating the inclination.FIG. 4Billustrates this inclination. The occurrence of the first inclination causes the roller to apply a force for moving the sheet from the outer side to the inner side to the edge of the sheet in the sheet width direction. More specifically, as illustrated inFIG. 4B, the respective edges of the sheet S2receive the forces f in the direction indicated by the arrows toward the inner side of the sheet S2from the rollers102aand102din contact with them. Therefore, the edges of the sheet S2may be moved inward, and a part of the sheet may be lifted up to generate wrinkles and corrugations. The force f is a component force of the nip pressure between the roller and the sheet, so that an increase in the nip pressure results in an increase in the force f, thereby further facilitating generation of wrinkles and corrugations.

A second roller inclination occurs in the sheet conveyance direction. This inclination occurs as if a part of the roller in contact with a sheet is pulled by the moving sheet from the downstream side.FIG. 5illustrates this inclination. As mentioned above, since the first roller pair located upstream operates at a slower roller circumferential speed (conveyance speed) than the second roller pair located downstream, the sheet S2is put in a state pulled from the downstream side at the position of the pinch roller102. Therefore, the inner rollers102band102care slightly displaced toward the downstream side by being pulled by the sheet S2. However, this displacement does not cause a change in the orientation of the rotational axis thereof. On the other hand, the outer rollers102aand102dare pulled at only the portions thereof in contact with the sheet S2from the downstream side, and this causes the rotational axes thereof to be obliquely inclined relative to the originally set direction. This is the second roller inclination. When the outer rollers102aand102dare inclined in this way, the portions of the sheet in contact with the rollers102aand102dare locally contorted, which may also lead to generation of wrinkles. Further, the first inclination and the second inclination may cause a deviation of the sheet traveling direction from the originally set direction, i.e., a skew state.

Generation of wrinkles and corrugations at a sheet, or generation of a skew state of a sheet during sheet conveyance leads to a deterioration of the quality of the image printed on the sheet. In view of this technical problem, the present exemplary embodiment provides a solution enabling sheet conveyance at high accuracy, no matter which sheet the printing apparatus uses. The present exemplary embodiment changes the difference or the ratio between the pressing force that the first roller portion applies to the conveyance roller101and the pressing force that the second roller portion applies to the conveyance roller101via the adjustment mechanism110according to a sheet that the printing apparatus uses. The concrete structure and operation for carrying out this operation now be described.

FIG. 6is a perspective view illustrating the structural details of the first roller pair. The nip pressures between the respective four rollers102ato102d, which constitute the pinch roller102, and the conveyance roller101are individually adjusted by the adjustment mechanism110disposed above these roller pairs. The structure of the adjustment mechanism110will now be described.

The four rollers102ato102dare held by four holders154ato154drespectively corresponding to the rollers102ato102d, and are arranged to be rotatable around a rotational shaft112. Four plate members113are fixed to a common reference fixation portion123so as to respectively face the holders154ato154d. A rod115and springs, serving as elastic members, are disposed between each of the holders154ato154dand the corresponding plate member113. The springs are constituted by three springs in total, a primary spring114adisposed around the rod115as a helicoidal spring, and auxiliary springs (two springs) disposed at the both sides of the primary spring114a. The three springs are arranged along the axial direction of the rotational shaft112.

Cam mechanisms150are disposed at four positions as driving mechanisms for vertically moving the holders154ato154d, respectively. The cam mechanisms150each include a cam and a cam lever, and convert a displacement of the cam into a vertical movement of the cam lever. The rods115each have one end fixed to the tip of the cam lever, and the other end fixedly inserted through a hole formed at the holder154a,154b,154c, or154dso as to be prevented from being pulled out therefrom. The primary springs114aeach are compressed between the cam lever and the holder154a,154b,154c, or154dwhile being supported by the rod115. Further, the auxiliary springs114beach have one end fixed to the back surface of the plate member113, and the other end fixed to the top surface of the holder154a,154b,154c, or154d, thereby being compressed there between. Since the auxiliary springs114bare symmetrically disposed at the both sides of the primary spring114a, even when the roller102a,102b,102c, or102dis about to be inclined, the auxiliary springs114bcan apply a force for preventing the inclination.

Vertically moving the cam levers by the cam mechanisms150causes vertical movements of the respective holders154ato154dthrough the rods115. Each of the nip pressures of the rollers102ato102dis individually adjusted according to the position of the cam lever in the vertical movement direction and the sum of the spring pressures of the three springs114aand114binterposed in a compressed state.

FIGS. 7A and 7Bare cross-sectional views illustrating the structure of one of the four cam mechanisms150.FIG. 7Aillustrates a nip state in which the rod115is pressed down, whileFIG. 7Billustrates a release state in which the rod115is lifted up. The cam120is fixed eccentrically relative to a rotating cam shaft121. A common single shaft is prepared as the cam shaft121for the cams120disposed at the four positions. The cam lever117is supported rotatably around a pivot118. One end of the cam lever117is in abutment with the surface of the cam120. The upper end of the rod115is rotatably fixed to the tip of the other end of the cam lever117by a pin116. The upper end side of the rod115extends through the hole formed at the plate member113, and is supported by the hole so as to prevent the position of the rod115from being displaced. The lower end side of the rod115extends through the hole formed at the holder154, and is supported by the hole so as to prevent the position of the rod115from being displaced. A clasp152, which is larger in diameter than the hole, is provided at the lower end of the rod115. The clasp152prevents the rod115from being pulled out from the hole of the holder154. The holder154is fixed rotatably around a pivot119. The pinch roller102is held rotatably around the rotational shaft112which is pivotally supported at two positions on the side surfaces of the holders154.

In this structure, a rotation of the cam shaft121changes the phase of the cam120, thereby changing the height of the cam lever117. According thereto, the rod115is vertically moved, and thereby the holder154is vertically moved as well. As a result, the height of the pinch roller102is changed relative to the conveyance roller101having a fixed height, thereby allowing switching between the nip state and the release state. There is a difference between the phase of the cams120corresponding to the pinch rollers102aand102d, and the phase of the cams120corresponding to the pinch rollers102band102c. Therefore, a rotation of the cam shaft121results in different vertical movements according to the pinch rollers.

The control unit13issues an instruction to drive the cam mechanisms150to change the distances of pressing the respective holders154ato154dvia the primary springs114aand the auxiliary springs114b, which are elastic members, thereby determining the nip pressures. The pressing forces (nip pressures) with which the respective rollers102ato102dpress the conveyance roller101can be changed according to the amounts of pressing the rods115(positions of the cam levers117) by the cam mechanisms150.FIG. 8illustrates an example of this setting. In this example, forces of 1000 gf, 1500 gf, 1500 gf, and 1000 gf are applied to the four rollers102ato102din this order, respectively. The total force of a biasing force (a) of the primary spring114aand biasing forces (2c) of the two auxiliary springs114b, i.e., a force of (a+2c) is applied to the each of the rollers102aand102dwhich constitute the first roller portion. For example, if (a) is 600 gf and (c) is 200 gf, the total pressure is 1000 gf. The total force of a biasing force (b) of the primary spring114aand biasing forces (2d) of the two auxiliary springs114b, i.e., a force of (b+2d) is applied to each of the rollers102band102cwhich constitute the second roller portion. For example, if (b) is 500 gf and (d) is 500 gf, the total pressure is 1500 gf. As the whole pinch roller102, the total of these four forces is applied to the conveyance roller101as the nip pressure. In this example, the total pressure is 5000 gf.

The printing apparatus according to the present exemplary embodiment switches the nip pressure to a suitable one to the condition (for example, the size and sheet stiffness) of a sheet that the printing apparatus uses.FIG. 9illustrates five examples (example 1 to example 5) about the set values of the nip pressures. The control unit13controls a motor, which is prepared for rotating the cam shaft121, to rotate and stop at a predetermined position, whereby it is possible to acquire a desired pressure among the five examples 1 to 5.

The example 1 is the setting suitable to a sheet having a large size in the sheet width direction, and relatively high sheet stiffness. The term “large size” here means a size causing the outer two rollers102aand102dto be in contact with the both edges of the sheet throughout the whole lengths of the rollers102aand102d, as illustrate inFIG. 4C. In this example, the adjustment mechanism110is set in such a manner that the uniform and equal forces, 1500 gf, 1500 gf, 1500 gf, and 1500 gf are applied to the four rollers102ato102din this order, respectively. The total pressure of the nip pressures is 6000 gf. Increasing the nip pressures reduces a slip between the sheet surface and the roller surface, thereby realizing sheet conveyance at further higher accuracy. When a sheet to be used has sufficiently high sheet stiffness, even increasing the nip pressures generates only a slight deformation, so that the maximum pressure forces are provided to all four of the four pinch rollers102ato102din the example 1. All of the four rollers102ato102devenly contact the sheet throughout the entire length of the rollers102ato102din the rotational axial direction of the rollers102ato102d, so that the same force is set as the four pressing forces.

The example 2 is the setting suitable to a sheet having a middle size in the sheet width direction, and relatively high sheet stiffness. The term “middle size” means a size causing the outer two rollers102aand102dto be only partially in contact with the both edges of the sheet, as illustrated inFIG. 4B. The adjustment mechanism110is set in such a manner that different forces, 1000 gf, 1500 gf, 1500 gf, and 1000 gf are applied to the four rollers102ato102din this order, respectively. In this example, the total pressure of the nip pressures is 5000 gf, and is the same as the example indicated inFIG. 8. As described above, the outer two rollers102aand102donly partially contact the sheet, whereby the rollers102aand102are inclined, as a result of which wrinkles and corrugations may be formed on the sheet or the sheet may be skewed while being conveyed. As the nip pressures are higher, the generation of wrinkles and corrugations increases. As illustrated inFIG. 4B, since the force f is a component force of the nip pressure between the roller and the sheet, a higher nip pressure leads to a stronger force f. Therefore, the example 2 sets weaker forces as the pressing forces of the outer two rollers102aand102dthan those in the example 1. For the two rollers102band102c, the same maximum pressing forces as the example 1 are set according to the degree of sheet stiffness so as to reduce a slip as much as possible. In this way, the difference or the ratio between the respective pressing forces of the first roller portion and the second roller portion is changed by the adjustment mechanism110between the examples 1 and 2. In other words, comparing the state 1 in which the first roller portion nips a sheet with the entire length thereof, and the second state in which the first roller portion nips the sheet with a part of the length thereof, the adjustment mechanism110is set so as to reduce the pressing force of the first roller portion in the state 2 compared to the pressing force in the state 1.

The example 2 is also the setting suitable to a sheet having a large size in the sheet width direction and relatively low sheet stiffness. Assume that the sheet size in this example is such a size that the both edges of the sheet extend further beyond the outer rollers102aand102d. If the sheet stiffness is lower than the example 1, the maximum nip pressures of the rollers102aand102dmay cause the portions of the sheet extending beyond the rollers102aand102dto curve upwards. These portions, if they are large, may contact the print head14. The example 2 sets the nip pressures of the outer rollers102aand102dto be lower than the inner rollers102band102c, which is effective to prevent the edges of the sheet, which the rollers102aand102dcannot hold, from curving up.

The example 3 is the setting suitable to a sheet having a middle size in the sheet width direction and relatively low sheet stiffness. The adjustment mechanism110is set so that different forces, 200 gf, 1000 gf, 1000 gf, and 200 gf are applied to the four rollers102ato102din this order. The total pressure of the nip pressures is 2400 gf. Since the sheet in the example 3 has lower sheet stiffness than the sheet in the example 2, although the sheet size is the same middle size, the total nip pressure is reduced compared to the example 2 to prevent the nip pressure from deforming the sheet. Different pressing forces are set for the inner rollers102band102c, and the outer rollers102aand102dof the pinch roller102, thereby preventing generation of wrinkles and corrugations at the sheet, and occurrence of a skew state of the sheet.

The example 4 is the setting suitable to a sheet having a small size in the sheet width direction. The term “small size” means such a size that the outer two rollers102aand102dare completely out of contact with the sheet, as illustrated inFIG. 4A. The adjustment mechanism110is set so that a release, a force of 1500 gf, a force of 1500 gf, and a release are applied to the four rollers102ato102din this order, respectively. The total pressure of the nip pressures is 3000 gf. Since the sheet is conveyed by the inner two rollers102band102c, a desired effect can be acquired by setting values suitable to the sheet stiffness as the pressing forces of these rollers. In this example, the maximum pressing forces are applied, assuming that the sheet has high sheet stiffness. Since the outer two rollers102aand102ddo not involve the sheet conveyance at this time, any pressing force can be set thereto. In this example, the rollers102aand102dare released with the pressing forces thereof set to zero.

The example 5 is the setting suitable at the time of rewinding of a sheet after a printing operation, or at the time of maintenance in response to occurrence of a sheet conveyance jam. All of the four rollers102ato102dare set so as to be floated from the conveyance roller101in a release state. All of the forces applied to the four rollers102ato102dare 0 gf. In other words, the total pressure of the nip pressures is 0 gf.

FIG. 10is a cross-sectional view illustrating the structure of an adjustment mechanism according to another exemplary embodiment of the present invention. In the present exemplary embodiment, the adjustment mechanism is configured using a lead screw, instead of the above-described cam mechanism. A rotational driving force of a motor125is transmitted to a lead screw131supported by the reference fixation unit123via a gear array130. The four divided rollers102ato102d, which constitute the pinch roller102, are rotatably supported by holders111, respectively. A spring133is disposed between a spring stopper132and the holder111in a compressed state. In this structure, a rotation of the motor125causes a rotation of the lead screw131, and this rotation is converted into a vertical movement of the spring stopper132. The vertical movement of the spring stopper132allows the nip pressure of the roller102a,102b,102c, or102dto be individually changed via the spring133.

The above-described exemplary embodiments individually adjust the nip pressures according to a condition of a sheet that the printing apparatus uses, such as a sheet size and sheet stiffness. However, the nip pressures may be adjusted according to another condition of a sheet. For example, when the printing apparatus continuously prints data on both surfaces of a sheet as described above, at least one of the first roller portion and the second roller portion may apply different pressing forces to the conveyance roller101between printing on a first surface and printing on a second surface. Sometimes, the printing apparatus may have to print data under different sheet conditions between printing on a first surface and printing on a second surface. For example, a sheet with data printed on the first surface thereof is swollen by absorbing ink, and therefore the sheet stiffness of the sheet may be reduced at the time of printing on the second surface compared to the time of printing on the first surface. In this case, the adjustment mechanism110can be set in such a manner that the first roller portion and the second roller portion apply reduced nip pressures at the time of printing on the second surface compared to the time of printing on the first surface. Further, a sheet with an image printed on the first surface thereof may have a changed friction coefficient by absorbing ink. In this case, the nip pressures can be changed between printing on the first surface and printing on the second surface in consideration of the possibility of a slip. As a result, the printing apparatus can operate during printing on the first surface and printing on the second surface for two-sided printing under a same unchanged sheet conveyance condition, so that it is possible to reduce a positional misalignment between images printed on the front surface and the back surface of the sheet.

As described above, the exemplary embodiments of the present invention include the pinch roller102divided into a plurality of rollers including the first roller portion and the second roller portion adjacent to each other. Further, the exemplary embodiments include the adjustment mechanism110capable of changing the difference or ratio between the respective pressing forces which the first roller portion and the second roller portion apply to the conveyance roller101. The pressing force of the first roller portion is changed according to the size of a sheet, which the printing apparatus uses, in the sheet width direction. Further, as a sheet to be used has lower sheet stiffness, the exemplary embodiments set the pressing force of the first roller portion to a weaker force. As a result, the exemplary embodiments can realize a sheet conveying apparatus and a printing apparatus capable of conveying a sheet at high accuracy regardless of a sheet that the printing apparatus uses.

As illustrated inFIG. 3, in the printing unit4, the relationship between the first roller pair and the second roller pair is such that the second roller pair has a higher circumferential speed of the roller outer circumference (the speed for conveying a sheet) and a lower total nip pressure than those of the first roller pair. In this system, the first roller pair has a greater influence on sheet conveyance (conveyance speed and conveyance accuracy) than the second roller pair. Therefore, the nip roller of the first roller pair, which can be expected to bring a more significant effect than the second roller pair, is divided into a plurality of rollers, and the nip pressures thereof are individually adjusted according to a sheet that the printing apparatus uses. As a result, it is possible to realize a printing apparatus capable of conveying a sheet at higher accuracy and thereby acquiring an excellent print quality.

This application claims priority from Japanese Patent Application No. 2011-028715 filed Feb. 14, 2011, which is hereby incorporated by reference herein in its entirety.