Image forming method and image forming apparatus

Abstract

To enable easily reducing out-of-registering at main and back surfaces in two-sided printing.SOLUTION: An image forming apparatus, for forming images in order on both surfaces of a sheet, comprises first skew-amount detecting means that is arranged upstream of transport means (clamp roller 22) to transport a sheet to an image forming unit (printing unit 30) and for detecting a skew amount of the sheet to be delivered to the transport means, and second skew-amount detecting means that is arranged downstream of the transport means and for detecting a skew amount of the sheet delivered from the transport means. Control means, upon forming an image on the main surface of the sheet, performs control to correct a skew of the sheet using the transport means based on the skew amount detected at the first skew-amount detecting means so that a residual of skew correction uncorrected in the correction is detected by the second skew-amount detecting means and, upon forming an image on a backside of the sheet, carries out control of a skew amount of the sheet based on the detected residual of skew correction so as to reduce out-of-registering at main and back surfaces of the sheet using the transport means.SELECTED DRAWING: Figure 4A

Description

[Technical Field]

[0001] The present invention relates to an image forming method and an image forming apparatus. [Background technology]

[0002] In an image forming apparatus configured as a multifunction machine that combines at least two of a copier, facsimile, printer, printing machine, and inkjet recording device, a correction mechanism is provided that corrects the posture of paper (sheets) as a recording medium immediately before the paper is fed into the image forming unit. This correction mechanism nonstop detects the amount of skew and lateral misregistration of the paper while the paper is being transported, and performs skew correction and lateral misregistration correction based on the detection results while the paper is being transported by transport rollers (paper clamping rollers) (see Patent Document 1: JP 2014-088263 A).

[0003] However, even with such corrections, it is difficult to completely eliminate skew and lateral misregistration. Any residual correction error in skew or lateral misregistration can cause problems with print quality, especially in double-sided printing. In other words, if there is residual correction error, the front and back images will not register properly when double-sided printing is performed, resulting in a large misalignment of one side of the finished product after cutting or folding. Furthermore, when printing a face-up image on both sides, the misalignment of the ruled frames on the front and back of the sheet will be noticeable. Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 2 (JP 2005-263463 A) discloses a configuration in which the amount of skew at the rear end of a sheet after skew correction by a skew correction means and the amount of skew at the front end of the inverted sheet are detected, and the skew correction operation by the skew correction means is controlled according to the difference between the amount of skew at the rear end of the sheet and the amount of skew at the front end of the inverted sheet. This allows images to be formed appropriately on both sides of paper that is not exactly rectangular.

[0005] However, even with the technology of Patent Document 2, if there is a skew correction residual, the image will be skewed relative to the edge of the paper, which serves as the reference axis for posture correction, and in double-sided printing, the sum of the correction residual on the front side and the correction residual on the back side will result in a front-to-back registration error. Therefore, an object of the present invention is to make it possible to easily reduce front-to-back misregistration in double-sided printing. [Means for solving the problem]

[0006] In order to solve the above problem, the image forming apparatus of the present invention is an image forming apparatus in which images are formed sequentially on both sides of a sheet by an image forming unit, and comprises: a conveying means for conveying the sheet to the image forming unit; a control means for controlling the conveying means; a first skew amount detection means arranged upstream of the conveying means for detecting the skew amount of the sheet being fed into the conveying means; and a second skew amount detection means arranged downstream of the conveying means for detecting the skew amount of the sheet being sent out from the conveying means, wherein the control means controls the conveying means to correct the skew of the sheet based on the skew amount detected by the first skew amount detection means when forming an image on the front side of the sheet, and the second skew amount detection means detects a skew correction residual that has not been fully corrected by the correction, and controls the skew amount of the sheet using the conveying means based on the detected skew correction residual when forming an image on the back side of the sheet so as to reduce misregistration between the front and back sides of the sheet. [Effects of the Invention]

[0007] According to the present invention, misregistration between the front and back sides in double-sided printing can be easily reduced. [Brief explanation of the drawings]

[0008] [Figure 1A] 1 is a schematic diagram of an image forming apparatus according to an embodiment of the present invention. [Figure 1B] FIG. 2 is a schematic diagram of an image forming unit. [Figure 2A] (a) Plan view and (b) side view of the skew and lateral registration correction mechanism. [Figure 2B] FIG. 1 is a block diagram of a skew and lateral registration correction mechanism. [Figure 2C] 10 is a flowchart of a skew and lateral registration correction mechanism. [Figure 2D] 10 is a timing chart of the skew and lateral registration correction mechanism. [Figure 2E] 1(I) to 1(VI) are plan views showing the steps of skew and lateral registration correction. [Figure 3A] FIG. 10 is an explanatory diagram of the operation of the skew and lateral registration correction mechanism. [Figure 3B] FIG. 10 is an explanatory diagram of the operation of the skew and lateral registration correction mechanism. [Figure 3C] FIG. 2 is a block diagram of a front-to-back registration mechanism. [Figure 4A] FIG. 10 is an explanatory diagram showing front-to-back registration when printing on the front side (a) and when printing on the back side (b). [Figure 4B] FIG. 10 is an explanatory diagram showing misregistration between the front and back sides when printing on the front side (a) and when printing on the back side (b). [Figure 5] FIG. 10 is a diagram illustrating the relationship between paper skew and print image coordinates. [Figure 6] FIG. 10 is a diagram illustrating coordinate transformation when performing upside-down reversal processing from (a) to (b). [Figure 7] 10A and 10B are flowcharts of front-to-back registration, showing (a) a flowchart when the sheet is switched back and (b) a flowchart when it is not switched back. DETAILED DESCRIPTION OF THE INVENTION

[0009] (● Inkjet printer) Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1A is a schematic diagram of a line head type inkjet printer 1 as a device for ejecting liquid. The inkjet printer 1 includes an input section for inputting paper P as an object (recording medium or sheet) to which liquid is to be applied, a pre-processing section 20, an image forming section 30, a drying section 40, an output section 50, and an inverting mechanism section 60. Fig. 1B is an enlarged view of the image forming section 30.

[0010] In the inkjet printer 1, a pretreatment liquid is applied (coated) as needed to paper P carried in (supplied) from a carry-in section 10 in a pretreatment section 20, which is a pretreatment means. Then, a liquid is applied in an image forming section 30 to perform the required printing, and after the liquid adhering to the paper P is dried in a drying section 40, the paper P is discharged to a carry-out section 50.

[0011] The carry-in section 10 includes an input tray 11 (lower input tray 11A, upper input tray 11B) that stores multiple sheets of paper P, and a feeding device 12 (12A, 12B) that separates and sends out the sheets of paper P one by one from the input tray 11. The sheets of paper P are supplied from the carry-in section 10 to a pre-processing section 20. The pre-processing section 20 includes an application section 21, which is a treatment liquid application means that applies a treatment liquid to the printing surface of the paper P, for example, to agglomerate the coloring material of the ink and prevent show-through.

[0012] The image forming unit 30 includes a transport drum 31, which is a support member (rotating body) that rotates while supporting the paper P on its peripheral surface, and a droplet ejection unit 32 that ejects liquid toward the paper P supported on the transport drum 31. The image forming unit 30 also includes a transfer drum 34 that receives the paper P sent from the pre-processing unit 20 and transfers the paper P between it and the transport drum 31, and a transfer drum 35 that receives the paper P transported by the transport drum 31 and transfers it to the drying unit 40.

[0013] The transport drum 31 and the transfer cylinder 34, and the transport drum 31 and the transfer cylinder 35 are connected to each other by gears. The paper P is transported while being gripped by the grippers attached to the transport drum 31, the transfer cylinder 34, and the transfer cylinder 35.

[0014] The paper sheet P transported from the pre-processing unit 20 to the image forming unit 30 has its leading edge gripped by a gripping means (paper gripper) provided on the transfer drum 34, and is transported as the transfer drum 34 rotates. The paper sheet P transported by the transfer drum 34 is handed over to the transport drum 31 at a position opposite the transport drum 31.

[0015] Gripping means (paper grippers) are also provided on the surface of the transport drum 31, and the leading edge of the paper sheet P is gripped by the gripping means (paper grippers). A plurality of suction holes are formed in a dispersed manner on the surface of the transport drum 31, and the suction means generates a suction airflow that flows inward from the required suction holes of the transport drum 31. The paper sheet P transferred from the transfer drum 34 to the transport drum 31 has its leading edge gripped by the paper grippers, and is adsorbed and held on the transport drum 31 by the suction airflow of the suction means, and is transported as the transport drum 31 rotates.

[0016] The droplet discharge section 32 is provided with four droplet discharge units 33 (33A to 33D) that discharge droplets. These droplet discharge units 33 (33A to 33D) are radially arranged along the upper outer periphery of the transport drum 31 at equal intervals and symmetrically in FIG.

[0017] The droplet discharge unit 33 is a functional component that discharges and sprays liquid from a nozzle. The liquid to be discharged is not particularly limited as long as it has a viscosity and surface tension that allows it to be discharged from the head, but it is preferable that the viscosity of the liquid be 30 mPa s or less at room temperature and normal pressure, or by heating or cooling.

[0018] The droplet discharge unit 33A can discharge cyan (C) liquid, the droplet discharge unit 33B can discharge magenta (M) liquid, the droplet discharge unit 33C can discharge yellow (Y) liquid, and the droplet discharge unit 33D can discharge black (K) liquid. In addition, droplet discharge units that discharge special liquids such as white and gold (silver) liquid can also be used.

[0019] The ejection operation of each droplet ejection unit 33 of the droplet ejection section 32 is controlled by a drive signal corresponding to the printing information. When the paper P carried on the transport drum 31 passes through an area facing the droplet ejection section 32, liquid of each color is ejected from the ejection unit 33, and an image corresponding to the printing information is printed.

[0020] The paper P to which liquid has been applied by the droplet ejection unit 32 is delivered from the transport drum 31 to the delivery drum 35, and the paper P received by the delivery drum 35 is delivered to the transport mechanism unit 41 and transferred to the drying unit (heating unit) 40. The drying unit 40 dries the liquid that has adhered to the paper P in the image forming unit 30. This causes the water content and other liquid components in the liquid to evaporate, the colorant contained in the liquid to be fixed on the paper P, and curling of the paper P is suppressed.

[0021] The reversing mechanism 60 is a mechanism that reverses the paper P by a switchback method when performing double-sided printing on the paper P that has passed through the drying unit 40. The reversed paper P is sent back upstream of the transfer drum 34 through a conveying path 61 of the image forming unit 30.

[0022] The discharge section 50 includes a discharge tray 51 on which a plurality of sheets P are stacked, and a sheet transport device 502. The sheets P transported through the reversing mechanism section 60 are sequentially stacked and held on the stack section 501.

[0023] (● Skew and lateral registration correction mechanism) 1B, a pair of upper and lower rollers, namely, nip rollers 22, which nip and transport paper between them, are disposed further upstream of transfer drum 34, which is disposed upstream of droplet discharge unit 33. Nip rollers 22 constitute a skew and lateral registration correction mechanism, which serves as a paper posture correction mechanism that corrects the amount of skew and lateral registration deviation of paper P.

[0024] Here, the amount of skew refers to the angle of inclination that the side edge of the paper P makes with respect to the paper transport direction after the paper P has been de-skewed by the registration rollers arranged upstream of the pinch rollers 22. The amount of lateral registration deviation refers to the length of lateral deviation of the paper P in a direction perpendicular to the paper transport direction after the paper has been de-skewed.

[0025] After the posture of the paper P is corrected by the clamping roller 22, the leading edge of the paper is gripped by a mechanically opening and closing gripper attached to the transfer drum 34, and then the paper is transported in the following order: transfer drum 34 ⇒ transport drum 31 ⇒ transfer drum 35.

[0026] The droplet discharge units 33A to 33D arranged on the upper outer periphery of the transport drum 31 each have a line head connected in the X-axis direction (main scanning direction). When the paper P is transported by the transport drum 31 under the heads of the droplet discharge units 33A to 33D, ink is discharged from each of the droplet discharge units 33A to 33D to form an image on the paper P. The timing of ink discharge is determined based on the point in time when the paper P passes a print timing sensor.

[0027] 2A, a second edge sensor CIS2 and a third edge sensor CIS3 are disposed before and after the pinch roller 22 to detect the amount of skew and the amount of lateral registration deviation of the sheet P. Furthermore, a first edge sensor CIS1 is disposed further upstream of the second edge sensor CIS2 on the upstream side.

[0028] The first edge sensor CIS1 and the second edge sensor CIS2 constitute a first skew amount detection means that detects the amount of skew of a sheet fed to the nipping rollers 22 as a conveying means. The second edge sensor CIS2 and the third edge sensor CIS3 constitute a second skew amount detection means that detects the amount of skew of a sheet sent out from the nipping rollers 22 as a conveying means.

[0029] The first edge sensor CIS1 and second edge sensor CIS2 detect the amount of skew and lateral misregistration of the paper before it enters the nip rollers 22. The edge sensors CIS1 to CIS3 can be configured as contact image sensors that integrate a sensor (light receiving element), a light source (LED), and a rod lens array (equal magnification imaging lens). As shown in Figure 2B, the edge sensors CIS1 to CIS3 are connected to a rotation motor 23 for the nip rollers 22 and a shift motor 24 via an encoder, a controller, and a motor driver.

[0030] Rotation motor 23 and shift motor 24 are configured so that the rotation amount and shift amount from the initial state (home position) of pinch roller 22 can be measured by encoders. Rotation motor 23 and shift motor 24 in FIG. 2A may be disposed together at one end of pinch roller 22, as shown in FIGS. 3A and 3B. This allows the drive systems for rotation motor 23 and shift motor 24 to be configured compactly. Edge sensors CIS1 to CIS3 may also be disposed on opposite sides, as shown in FIGS. 3A and 3B.

[0031] (Example of skew and horizontal registration correction) Fig. 2C is a flowchart for operating the rotation motor 23 and the shift motor 24. Fig. 2D is a timing chart for the edge sensors CIS1 to CIS3, the pinch roller 22, the rotation motor 23, and the shift motor 24. The numbers (1) to (6) indicating the sections at the top of the timing chart correspond to (I) to (IV) in Fig. 2E.

[0032] Section 1 in Fig. 2D is the time for calculating the amount of paper positional deviation by the first edge sensor CIS1 and the second edge sensor CIS2. The paper P fed from the feeding device 12 in Fig. 1A is transported into the transport path on the downstream side.

[0033] As shown in Fig. 2C, after the first edge sensor CIS1 detects paper as a recording medium in step S1, the second edge sensor CIS2 detects the paper in step S2 (see Fig. 2E(I)). That is, when the paper P is about to pass through the conveyance rollers 28 as shown in Fig. 2E(I), the first edge sensor CIS1 and the second edge sensor CIS2 detect the amount of misalignment of the paper P. From the detected results (the amount of skew and the amount of lateral registration deviation), the rotational movement amount and shift movement amount of the pinch rollers 22 that will reduce the amount of skew and the amount of lateral registration deviation to zero are determined in encoder pulse units.

[0034] Section 2 in Fig. 2D is the pick-up time of the clamping roller 22. That is, in the processes of steps S3 to S7, the rotation motor 23 and the clamping roller 22 perform the pick-up operation and the return operation. In addition, in the processes of steps S8 to S12, the shift motor 24 and the clamping roller 22 perform the pick-up operation and the return operation (see Fig. 2E (II) to (IIIB)).

[0035] In the timing chart of FIG. 2D, as an example, the rotation motor 23 and the shift motor 24 are rotated forward to perform the pick-up operation for skew correction and lateral registration correction. In the pick-up operation, the clamping roller 22 is moved in the skew direction and lateral registration direction by the movement amount calculated in section 1. This movement must be completed before the sheet P reaches the clamping roller 22 (FIG. 2E (II)).

[0036] Section 3 in Fig. 2D is the return time for correcting the paper position. The leading edge of the transported paper P is carried into the nipping rollers 22. At this time, the nipping rollers 22 are in contact with the paper P, so that the paper P is nipped by the nipping rollers 22, and the transport roller pair 28 is separated (Fig. 2E (IIIA)).

[0037] When the leading edge of the paper P is nipped by the clamping rollers 22, the clamping rollers 22 rotate to return to their home positions in the width direction and diagonal direction (FIG. 2E (IIIB)). This operation corrects the skew amount and lateral registration deviation amount of the paper P to zero. This driving operation is completed before the paper P reaches the third edge sensor CIS3.

[0038] In step S13, the third edge sensor CIS3 detects the paper P (see FIG. 2E(IV)). Based on the detection result of the third edge sensor CIS3, in steps S14 to S17, the amount of skew and the amount of skew correction of the paper are calculated, the encoder count number is calculated, and the rotation motor 23 is rotated to re-correct the amount of skew. Also, in steps S18 to S21, the amount of lateral registration deviation of the paper and the amount of lateral registration correction are calculated, the encoder count number is calculated, and the shift motor 24 is rotated to re-correct the amount of lateral registration deviation.

[0039] Section 4 in Figure 2D is the feedback time for the amount of paper misalignment measured by the second edge sensor CIS2 and the third edge sensor CIS3. When the paper P reaches the third edge sensor CIS3, the amount of skew and the amount of lateral misalignment at each measurement point are calculated from the measurement results of the second edge sensor CIS2 and the third edge sensor CIS3 (Figure 2E (IV)).

[0040] Section 5 in Fig. 2D is a feedback re-correction time. Rotation motor 23 and shift motor 24 are operated so as to correct the positional deviation calculated in section 4 (Fig. 2E(V)-(IV)).

[0041] Section 6 in Fig. 2D is the time required for returning the clamping rollers 22 to their home positions. When the paper reaches the transport section of the next process, the clamping rollers 22 are separated and return to their home positions to prepare for transporting the next paper (Fig. 2E(IV)).

[0042] 2D, as an example, feedback re-correction operations in the skew direction and lateral registration direction are performed by rotating the rotation motor 23 and the shift motor 24 in the forward direction. After the feedback re-correction is completed, the pinch rollers 22 are separated (step S22).

[0043] After the nipping rollers 22 are separated in step S22, the rotation motor 23 is returned to its origin in step S23, the shift motor 24 is returned to its origin in step S24, and finally the nipping rollers 22 are closed in step S25, and the process is prepared for the next sheet P to be transported.

[0044] 3A and 3B, the center of rotation of pinch roller 22 is set at one end in the longitudinal direction of pinch roller 22. By setting the center of rotation at one end in the longitudinal direction in this way, it becomes easier to arrange the rotation motor.

[0045] The clamping roller 22 is a roller that can rotate and shift (in the Y direction) while transporting the paper, and has the function of readjusting the skew amount and lateral registration deviation amount (main scanning deviation amount) calculated from the paper edges detected by multiple edge sensors CIS1 to CIS3.

[0046] In the case of Figures 3A and 3B, the edge sensors CIS1 and CIS2 detect the "skew amount" of the paper side edge relative to the paper transport direction and the "main scanning deviation amount" relative to the paper edge specified position from the relationship between L12, L1, and L2, and the shaft of the clamping roller 22 is positioned so as to face the paper. L12: Distance between edge sensors CIS1 and CIS2 in the conveying direction L23: Distance between edge sensors CIS2 and CIS3 in the conveying direction L1: Length from the original position of the edge sensor CIS1 to the edge of the paper L2: Length from the original position of the edge sensor CIS2 to the edge of the paper L3: Length from the original position of the edge sensor CIS3 to the edge of the paper a: Length from the rotation center to the conveyance center d: Length from the original position of the edge sensor CIS to the center of conveyance L': Length from the paper edge position to the center of conveyance is.

[0047] After the paper is sandwiched between the sandwiching rollers 22, the detected "skew amount" is corrected by rotating the sandwiching rollers 22 using a rotary motor 23, and the "main scanning deviation amount" is corrected by shifting the sandwiching rollers 22 using a shift motor 24. After these corrections are made, the two downstream edge sensors CIS2 and CIS3 detect the "skew amount" and "main scanning deviation amount" as correction residuals, in the same way as the edge sensors CIS1 and CIS2. Figure 3B shows the skew amount θ to make the correction residuals easier to understand. 23 After the correction residual is detected, the paper posture is corrected again by the nipping rollers 22 when printing on the back side, and the print position is corrected by the image print position correcting means shown in FIG. 3C.

[0048] (● Block diagram of front and back registration mechanism) Figure 3C is a block diagram of the front-to-back registration mechanism. This block diagram is broadly divided into a paper transport unit and an image forming unit. The paper transport unit has a paper attitude detection unit, a paper attitude correction amount determination unit, a paper attitude correction unit, a correction residual storage unit, and a paper transport unit. The paper transport unit constitutes a control unit that controls the transport unit.

[0049] Furthermore, when forming images continuously on a plurality of sheets of paper, the paper transport section can be provided with a memory / calculation means for storing a plurality of skew correction residuals detected by the second skew amount detection means and calculating an average residual from the plurality of skew correction residuals. Then, when forming images on the surfaces of the sheets of paper, the control means can correct the skew of the sheets to the extent that the average residual is eliminated based on the average residual of the skew correction residuals of the nearest constants calculated by the memory / calculation means.

[0050] The paper orientation detection means detects the amount of skew and shift of the paper relative to the paper transport reference axis. The image forming unit also has an image printing means that prints image data in response to print timing instructions sent from the paper transport unit, and an image printing position correction means that corrects the image printing position on the back side based on the correction residual on the front side.

[0051] The paper posture correction amount determining means of the paper transport section determines the amount of paper posture correction from the paper posture. This paper posture correction amount determining means can determine the amount of correction so that the correction residual for the front side becomes zero, based on the correction residual for the front side statistically stored in the correction residual storage means. Also, the correction amount for the back side can be determined from the correction residual for the front side.

[0052] When the sheet is turned over by switching back for printing on the back side, the sheet transport means can be configured so that the correction residuals on the front and back sides are in opposite directions.When the sheet is turned over without switching back for printing on the back side, the sheet transport means can be configured so that the correction residuals on the front and back sides are in the same direction.

[0053] The paper posture correcting means corrects the paper posture based on the correction amount determined by the paper posture correction amount determining means. The correction residual storage means stores the correction residual of the front side. The paper transport means transports the paper for double-sided printing.

[0054] (● Front and back registration) Next, front and back register when the front and back sides are turned upside down using the switchback method will be described with reference to Figures 4A and 4B. Figure 4A shows front and back register in this embodiment, and Figure 4B shows front and back register in the conventional method.

[0055] As shown in Figure 4A(a), in this embodiment, the skew correction residual is θ1° (paper skew) on the front side and -θ1° (paper skew) on the back side. In this way, one feature of this embodiment is that the nip rollers 22 are controlled so that the skew correction residual θ1° on the front side remains as a skew correction residual on the back side that is equal in magnitude but in the opposite direction. In other words, by utilizing the correction residual on the front side as the correction amount for the back side, front and back register adjustment of the skew component can be performed.

[0056] That is, on the front side, the paper is skewed by θ1° relative to the reference axis (perpendicular to the image forming unit at the left edge of the paper), but the printed image is not skewed. As a result, the printed image is shifted from the intended printing position on the paper.

[0057] Before printing on the back side of the paper, the paper is switched back by the reversing mechanism 60 shown in Fig. 1A, so that the paper coordinates are reversed upside down as shown in Fig. 4A(b).

[0058] The paper is skewed by -θ1° in the opposite direction to when printing on the front side, but the printed image is not skewed. As a result, the printed image is shifted from the intended printing position on the paper.

[0059] However, as can be seen from Figure 4A(a)(b), if skew residuals remain in opposite directions on the front and back sides, the print position will shift so that the front and back sides of the paper are in register. As a result, the front and back registration errors cancel each other out and become smaller.

[0060] In contrast, when the sheet is reversed without using the switchback method, if skew residuals remain in opposite directions on the front and back sides, the front and back register errors are increased by +θ1° and −θ1°.

[0061] On the other hand, in the conventional front-back register shown in Figure 4B, the clamping rollers 22 perform the same skew correction on both the front and back sides, leaving skew residuals in the same direction on both the front and back sides. As a result, the front-back register errors are added together and become larger. In other words, the clamping rollers 22 are controlled in the same way to prevent skew residuals from remaining on both the front and back sides, leaving skew residuals in the same direction on both the front and back sides. Here, the skew residual on the front side is designated as θ1, and the skew residual on the back side is designated as θ2.

[0062] As shown in Figure 4B(a), on the front side, the paper is skewed by θ1 relative to the reference axis (perpendicular to the imaging unit at the left edge of the paper), but the printed image is not skewed. As a result, the printed image is shifted from the target printing position on the paper.

[0063] Before printing on the back side of the paper, the paper is switched back by the reversing mechanism 60 shown in Fig. 1A, so that the paper coordinates are reversed upside down as shown in Fig. 4B(b).

[0064] The paper is skewed by θ2 in the same direction as when printing on the front side, but the printed image is not skewed. As a result, the printed image is shifted from the intended printing position on the paper. As can be seen from Figure 4B(b), when skew residuals remain in the same direction on the front and back sides, the front-to-back register errors are increased by the sum of θ1° and θ2°.

[0065] In contrast, when the sheet is reversed without using the switchback method, if residual skew remains in the same direction on the front and back sides, the front-back register error is reduced because θ1 and θ2 are subtracted (cancelled out).

[0066] (●Relationship between paper skew and print image coordinates) Figure 5 shows the relationship between paper skew and print image coordinates. Figure 5(a) shows the state in which light ink is printed on the surface of paper skewed by -θ0. Figure 5(b) shows the coordinate system when an image is formed on the back of the paper with the print on the front so that the front and back registers match. In this coordinate system, the light ink image coordinates on the front are rotated by θ0 with respect to the origin.

[0067] First, the parameters of the rotation angle -θ0 are expressed as follows using the coordinates of points A and C:

number

[0068] The upper left edge of the paper in Figure 5(a) is set as the origin (0,0), and the paper indicated by the dashed line is rotated by -θ0 as shown by the solid line using an affine transformation. The coordinate transformation formula is as follows:

number

[0069] Therefore, the coordinates are as follows: <Print coordinates>

number

[0070] <Paper edge>

number

[0071] (● Coordinate conversion by switchback) FIG. 6 shows coordinate conversion due to switchback. FIG. 6(a) is the image on the front side, and (b) is the image on the back side after switchback. When forming an image on the back side of a sheet by switchback, the sheet is fed into the clamping roller 22 from the rear edge side. Therefore, by switching back from FIG. 6(a) to (b), the coordinate relationship between the front side and the back side is upside down. In other words, C in FIG. 6(a) and C1 in (b) are the same paper edge, but are upside down.

[0072] In the following, "marks" refer to the black circles at the four corners of the image in Figure 6. <Mark>

number

[0073] <Paper edge>

number

[0074] Front / back: In the case of -θ° paper skew, the coordinates after skew correction on the front and back are paper edge coordinates: XA=0, YA=0, mark coordinates: Xa=Xc, Xb=Xd, Ya=Yb, Yc=Yd, then the print coordinates are

number

[0075] The paper edge coordinates are

number

[0076] If the printing coordinates on the back side are inverted upside down to match the printing surfaces on the front and back sides, the printing coordinates on the back side will be

number

[0077] The difference in printing coordinates between the front and back sides can be expressed as follows:

number

[0078] In the case of a paper skew of -θ° on the front side and +θ° on the back side, the coordinates after skew correction on the front side are the same as the above formula, but the coordinates after skew correction on the back side are as follows: Paper edge coordinates: XA=0, YA=0, Mark coordinates: Xa=Xc, Xb=Xd, Ya=Yb, Yc=Yd. If these are given, the print coordinates are:

number

[0079] The paper edge coordinates are

number

[0080] If the printing coordinates on the back side are inverted upside down to match the printing surfaces on the front and back sides, the printing coordinates on the back side will be

number

[0081] The printing coordinates on the front side are as follows:

number

[0082] The difference in printing coordinates between the front and back sides can be expressed as follows:

number

[0083] Therefore, in the case of a paper skew of -θ° between the front and back sides, the difference in print coordinates between the front and back sides is

number

[0084] In the case of a paper skew of -θ° on the front side and +θ° on the back side, the difference in print coordinates between the front and back sides is

number

[0085] It can be seen that in the case of front side: -θ° paper skew and back side: +θ° paper skew, the error components are the same for all print coordinates. Also, for the Y coordinate, the error factors of +2Xa·sinθ0 for coordinates a and c and +2Xb·sinθ0 for coordinates b and d disappear.

[0086] To further organize the equation, XC, Xa, Xc, Ya, and Yb are coordinates close to the origin, so they are set to 0, and YC and Yc are close values, so YC=Yc is used to reexamine the equation. In the case of a front / back paper skew of -θ°, the difference in print coordinates between the front and back is:

number

[0087] In the case of a paper skew of -θ° on the front side and +θ° on the back side, the difference in print coordinates between the front and back sides is

number

[0088] Therefore, it can be seen that the front-to-back registration error is smaller when the front side is skewed at -θ° and the back side is skewed at +θ°. Additionally, in order to align the front and back sides of the paper using paper transport alone, it is desirable to perform this embodiment after minimizing the skew correction residual. Therefore, based on statistics of the correction residual for the front side, improvement can be expected by shifting the target value for the front side and applying this embodiment with the correction residual for the front side reduced. This can be expected to be more effective by performing this for each paper type. The correction residual can be obtained by checking the skew amount of the front and back printed images using a scanner.

[0089] Furthermore, if it is possible to align the front and back registers using image processing, in the case of front: -θ° paper skew and back: +θ° paper skew, the amount of misalignment will be the same at all four corners as in the common terms *1 and *2 of equation 17 above, so the front and back register error can be reduced to zero by printing the print image on the back side shifted by the amount of misalignment of that common term. In the case of front / back: -θ° paper skew, the error components are not common, so deformation rather than just shifting is required.

[0090] vinegar In the case of double-sided printing without switchback, misregistration between the front and back sides can be reduced to zero by setting the correction residuals on the front and back sides to the same amount, assuming that the edge of the paper on the paper posture correction reference axis does not change.

[0091] (Comparison of flowcharts with and without switchback) Figure 7 shows a comparison of the flow charts for when switchback is performed (transport with switchback) and when it is not performed (transport without switchback) when printing on the back side. Figure 7(a) shows the case with switchback and (b) shows the case without switchback.

[0092] In the case of conveyance with switchback, the paper is turned over and fed from the rear end side to the clamping rollers 22 for back-side printing, but in the case of conveyance without switchback, the paper is turned over and fed from the front end side to the clamping rollers 22 for back-side printing. The differences between Figures 7(a) and (b) are enclosed in rectangular frames to make them easier to understand.

[0093] First step: Set the target orientation of the surface paper. Set the tilt to 0 with respect to the printing standard. Second process: Correct the front paper position. Correct so that the tilt is 0 with respect to the printing standard. Third step: Detect the correction residual (slope) relative to the corrected target. Fourth process: The image is printed on the surface of the paper. The image is printed with a deviation from the reference axis of the paper by the amount of the residual correction.

[0094] Fifth step: (With switchback): The sheet is conveyed in a switchback for printing on the back side. This results in the printing direction on the front and back sides being reversed. (No switchback): No switchback is performed to print on the back side. This ensures that the printing direction on the front and back sides is the same.

[0095] 6th step: (With switchback): Set the target paper posture for the back side. Set the target so that the residual in the opposite direction to the front side correction residual remains. (No switchback): Set the target paper posture for the back side. Set the target so that the residual in the same direction as the front side correction residual remains.

[0096] 7th step: (With switchback): Correction is performed so that the residual in the opposite direction to the surface correction residual remains. (No switchback): Correction is performed so that the residual remains in the same direction as the surface correction residual.

[0097] 8th step: (With switchback): The image and paper reference axis are misaligned and printed in the opposite direction to the front side. With switchback transport, the front and back register positions are aligned. (No switchback): The image and paper reference axis are misaligned in the same direction as the front side. The front and back register positions are aligned without switchback transport.

[0098] While the present invention has been specifically described above based on the embodiments, it goes without saying that the present invention is not limited to the above embodiments and can be modified in various ways within the scope of the technical concept described in the claims. For example, instead of the clamping roller 22 as a conveying means, two rollers whose rotation speeds can be changed independently may be used, and the skew and lateral registration of the paper may be corrected by changing the rotation speeds of the two rollers or by moving them laterally in the paper width direction. [Explanation of symbols]

[0099] 1: Inkjet printer (a device that ejects liquid) 10: Loading section 11: Loading tray 11A: Lower loading tray 1B: Upper loading tray 12 (12A, 12B): Feeding device 20: Pre-processing section 21: Application section 22: Clamping roller 23: Rotation motor 24: Shift motor 28: Conveying roller 30: Printing unit (image forming unit) 31: Conveyor drum 32: Droplet ejection unit 33 (33A to 33D): droplet discharge unit 34: transfer drum 35: Transfer cylinder 40: Drying section (heating section) 41: Transport mechanism section 50: Unloading section 51: Output tray 60: Reversing mechanism 61: Transport path 501: Stack section 502: Paper transport device P: Paper (sheet) CIS1-CIS3: Edge sensors [Prior art documents] [Patent documents]

[0100] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-088263 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-263463

Claims

1. In an image forming apparatus in which images are sequentially formed on both sides of a sheet by an image forming unit, a conveying means for conveying the sheet to the image forming unit; a control means for controlling the conveying means; a first skew amount detecting means disposed upstream of the conveying means for detecting a skew amount, which is an inclination angle formed by a side edge of a sheet fed into the conveying means with respect to a conveying direction of the sheet; a second skew amount detecting means disposed downstream of the conveying means for detecting a skew amount, which is an inclination angle formed by a side edge of the sheet fed from the conveying means with respect to the sheet conveying direction; and The control means controls the conveying means to correct the skew of the sheet based on the skew amount detected by the first skew amount detection means when forming an image on the front side of the sheet, and the second skew amount detection means detects a skew correction residual that has not been fully corrected by the correction, and based on the detected skew correction residual, controls the skew amount of the sheet using the conveying means when forming an image on the back side of the sheet so as to reduce misregistration between the front and back sides of the sheet.

2. The image forming apparatus of claim 1, characterized in that the sheet on which the image has been formed on the front side by the image forming unit is switched back and inverted, and fed into the conveying means from the rear end side of the sheet, and when an image is formed on the back side of the sheet, the control means controls the amount of skew of the sheet using the conveying means so that the skew correction residual of the sheet on which the image has been formed on the front side remains equal in magnitude and in the opposite direction.

3. 2. The image forming apparatus according to claim 1, wherein the sheet on which the image is formed on the front side by the image forming unit is inverted and fed into the conveying means from the front end side of the sheet, and when an image is formed on the back side of the sheet, the control means controls the skew amount of the sheet using the conveying means so that a skew correction residual of the sheet on which the image is formed on the front side remains equal in size.

4. An image forming apparatus according to any one of claims 1 to 3, characterized in that when forming images continuously on multiple sheets, a memory calculation means is provided which stores multiple skew correction residuals detected by the second skew amount detection means and calculates an average residual from the multiple skew correction residuals, and when forming images on the surface of the sheet, the control means corrects the skew of the sheet to the extent that it eliminates the average residual based on the average residual of the skew correction residuals of the nearest constants calculated by the memory calculation means.

5. 5. The image forming apparatus according to claim 1, wherein the image forming unit forms an image on the back surface of the sheet so as to reduce misregistration between the front and back surfaces.

6. An image forming apparatus according to any one of claims 1 to 5, characterized in that, when forming an image on the surface of the sheet, the control means controls the conveying means to correct the skew of the sheet based on a value obtained by multiplying the skew amount detected by the first skew amount detection means by a coefficient set for each type of sheet.

7. 1. An image forming method for sequentially forming images on both sides of a sheet conveyed to an image forming section by a conveying means, An image forming method characterized by controlling the conveying means to correct the skew of the sheet based on the amount of skew of the sheet fed into the conveying means when forming an image on the front side of the sheet, and controlling the amount of skew of the sheet based on a skew correction residual that has not been fully corrected by the correction when forming an image on the back side of the sheet so as to reduce misregistration between the front and back sides of the sheet.

8. 8. An image forming method according to claim 7, wherein the sheet on which an image has been formed on the front side by the image forming unit is switched back and inverted, and fed into the conveying means from the rear end side of the sheet, and when an image is formed on the back side of the sheet, the skew amount of the sheet is controlled using the conveying means so that the skew correction residual of the sheet on which an image has been formed on the front side remains equal in magnitude but in the opposite direction.

9. 8. An image forming method according to claim 7, wherein the sheet on which an image has been formed on the front side by the image forming unit is inverted and fed into the conveying means from the front end side of the sheet, and when an image is formed on the back side of the sheet, the conveying means is used to control the amount of skew of the sheet so that the skew correction residual of the sheet on which the image has been formed on the front side remains equal in size.

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