JP7769866B2 - Image forming method and image forming apparatus - Google Patents

Description

The present invention relates to an image forming method and an image forming apparatus.

Image forming devices comprised of one of a copier, facsimile, printer, printing press, and inkjet recording device, or a multifunction device that combines at least two of these, are equipped with a correction mechanism that corrects the posture of paper (the recording medium) immediately before it is fed into the image forming unit. This correction mechanism nonstop detects the amount of skew and lateral misalignment of the paper while it is being transported, and based on the detection results, performs skew correction and lateral misalignment correction while the paper is being transported by the transport rollers (paper clamping rollers) (see Patent Document 1: JP 2014-088263 A).

However, even with this type of correction, it is difficult to completely eliminate skew or horizontal misalignment. If there is any correction residual for skew or horizontal misalignment, problems will occur in print quality, especially in double-sided printing. In other words, if there is correction residual, the images on the front and back of the sheet will not be aligned properly when double-sided printing is performed, and the finished product will have a significant misalignment on one side when 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.

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

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. 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, the object of this invention is to make it possible to easily reduce front-to-back misregistration in double-sided printing.

To solve the above problem, the image forming apparatus of the present invention, in which an image forming unit sequentially forms images on both sides of a sheet, 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 disposed upstream of the conveying means for detecting the amount of skew of the sheet fed into the conveying means, and a second skew amount detection means disposed downstream of the conveying means for detecting the amount of skew of the sheet sent out from the conveying means, wherein the control means controls the conveying means to correct the skew of the sheet based on the amount of skew 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 is not fully corrected by the correction, and controls the amount of skew of the sheet based on the detected skew correction residual when forming an image on the back side of the sheet to reduce misregistration between the front and back sides of the sheet using the conveying means.

This invention makes it easy to reduce misregistration between the front and back of a sheet during double-sided printing.

1 is a schematic diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic diagram of an image forming unit. 1A and 1B are a plan view and a side view, respectively, of a skew and lateral registration correction mechanism. FIG. 2 is a block diagram of a skew and lateral registration correction mechanism. 10 is a flowchart of a skew and lateral registration correction mechanism. 10 is a timing chart of the skew and lateral registration correction mechanism. 10(I) to 10(VI) are plan views showing the steps of skew and lateral registration correction. 10A and 10B are explanatory diagrams illustrating the operation of a skew and lateral registration correction mechanism. 10A and 10B are explanatory diagrams illustrating the operation of a skew and lateral registration correction mechanism. FIG. 2 is a block diagram of a front-to-back registration mechanism. 1A is an explanatory diagram showing front-to-back registration during printing on the front side, and FIG. 1B is an explanatory diagram showing back-to-back registration during printing on the back side. 1A is an explanatory diagram showing misregistration on the front and back sides when printing on the front side, and FIG. 1B is an explanatory diagram showing misregistration on the back side when printing on the back side. FIG. 10 is a diagram illustrating the relationship between paper skew and print image coordinates. 10A to 10B are diagrams illustrating coordinate transformation when performing upside-down reversal processing from (a) to (b). 10A and 10B are flowcharts of front-to-back registration, in which (a) the sheet is switched back and (b) the sheet is not switched back.

(● 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 to which liquid is to be applied (a recording medium or sheet), 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.

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

The carry-in unit 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 unit 10 to the pre-processing unit 20. The pre-processing unit 20 includes an application unit 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 ink colorant and prevent show-through.

The image forming unit 30 includes a transport drum 31, which is a support member (rotating body) that rotates while supporting 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 paper P sent from the pre-processing unit 20 and transfers paper P between it and the transport drum 31, and a transfer drum 35 that receives paper P transported by the transport drum 31 and transfers it to the drying unit 40.

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

The paper 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 P transported by the transfer drum 34 is handed over to the transport drum 31 at a position opposite the transport drum 31.

A gripping means (paper gripper) is also provided on the surface of the transport drum 31, and the leading edge of the paper P is gripped by the gripping means (paper gripper). Multiple suction holes are formed in a distributed pattern on the surface of the transport drum 31, and the suction means generates a suction airflow that flows inward from the required suction holes in the transport drum 31. The paper P transferred from the transfer drum 34 to the transport drum 31 has its leading edge gripped by the paper gripper, and is adsorbed and held on the transport drum 31 by the suction airflow from the suction means, and is transported as the transport drum 31 rotates.

The droplet discharge section 32 is equipped with four droplet discharge units 33 (33A to 33D) that discharge droplets. These droplet discharge units 33 (33A to 33D) are arranged radially, equidistantly, and symmetrically in FIG. 1 along the upper outer periphery of the transport drum 31.

The droplet ejection unit 33 is a functional component that ejects and sprays liquid from the nozzles. The ejected liquid is not particularly limited as long as it has a viscosity and surface tension that allows it to be ejected from the head, but it is preferable that the viscosity be 30 mPa·s or less at room temperature and pressure, or when heated or cooled.

Droplet ejection unit 33A can eject cyan (C) liquid, droplet ejection unit 33B can eject magenta (M) liquid, droplet ejection unit 33C can eject yellow (Y) liquid, and droplet ejection unit 33D can eject black (K) liquid. In addition, droplet ejection units that eject special liquids such as white and gold (silver) can also be used.

Each droplet ejection unit 33 of the droplet ejection section 32 has its ejection operation controlled by a drive signal corresponding to the printing information. When the paper P carried on the transport drum 31 passes through the 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.

The paper P to which liquid has been applied by the droplet ejection unit 32 is passed from the transport drum 31 to the transfer drum 35, and the paper P received by the transfer drum 35 is passed to the transport mechanism unit 41 and transferred to the drying unit (heating unit) 40. The drying unit 40 dries the liquid that has been applied 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 fix on the paper P, and curling of the paper P is suppressed.

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

The discharge section 50 includes an output tray 51 on which multiple sheets of paper P are stacked, and a paper transport device 502. The sheets of paper P transported through the reversing mechanism section 60 are stacked and held in order on the stack section 501.

(● Skew and horizontal 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 the transfer drum 34 disposed upstream of the droplet ejection unit 33. The nip rollers 22 constitute a skew/lateral registration correction mechanism as a paper posture correction mechanism that corrects the amount of skew and lateral registration deviation of the paper P.

Here, the amount of skew refers to the angle of inclination that the side edge of the paper P makes with the paper transport direction after it has been de-skewed by the registration rollers located upstream of the clamping rollers 22. The amount of lateral registration deviation refers to the length by which the paper is de-skewed in a direction perpendicular to the paper transport direction.

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

The droplet ejection units 33A to 33D, arranged around the upper periphery of the transport drum 31, each have line heads arranged in the X-axis direction (main scanning direction). As the paper P is transported by the transport drum 31 beneath the heads of each droplet ejection unit 33A to 33D, ink is ejected from each droplet ejection unit 33A to 33D to form an image on the paper P. The timing of ink ejection is determined based on the point in time when the paper P passes by the print timing sensor.

As shown in Figure 2A, a second edge sensor CIS2 and a third edge sensor CIS3 are arranged before and after the clamping roller 22 to detect the amount of skew and lateral misalignment of the paper P. Furthermore, a first edge sensor CIS1 is arranged further upstream of the upstream second edge sensor CIS2.

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 paper being fed into the nip 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 paper being sent out from the nip rollers 22 as a conveying means.

The first edge sensor CIS1 and second edge sensor CIS2 detect the amount of skew and lateral misalignment of the paper before it enters the clamping rollers 22. 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, edge sensors CIS1 to CIS3 are connected to the rotation motor 23 and shift motor 24 of the clamping rollers 22 via an encoder, controller, and motor driver.

The rotation motor 23 and shift motor 24 are configured so that the rotation amount and shift amount from the initial state (home position) of the clamping roller 22 can be measured using an encoder. The rotation motor 23 and shift motor 24 in Figure 2A may be arranged together at one end of the clamping roller 22, as shown in Figures 3A and 3B. This allows for a compact drive system for the rotation motor 23 and shift motor 24. The edge sensors CIS1 to CIS3 may also be arranged on the opposite side, as shown in Figures 3A and 3B.

(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.

Section 1 in Figure 2D is the time during which the first edge sensor CIS1 and the second edge sensor CIS2 calculate the amount of paper positional misalignment. Paper P fed from the feeding device 12 in Figure 1A is transported into the downstream transport path.

As shown in Figure 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 Figure 2E(I)). That is, when paper P attempts to pass through the transport rollers 28 as shown in Figure 2E(I), the first edge sensor CIS1 and the second edge sensor CIS2 detect the amount of misalignment of paper P. From the detected results (amount of skew and amount of lateral misalignment), the rotational movement amount and shift movement amount of the clamp rollers 22 that will reduce the amount of skew and amount of lateral misalignment to zero are determined in encoder pulse units.

Section 2 in Figure 2D is the pick-up time for the clamping roller 22. That is, in steps S3 to S7, the rotation motor 23 and the clamping roller 22 perform pick-up and return operations. In addition, in steps S8 to S12, the shift motor 24 and the clamping roller 22 perform pick-up and return operations (see Figure 2E (II) to (IIIB)).

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

Section 3 in Figure 2D is the return time for correcting the paper position. The leading edge of the transported paper P is brought into the nipping rollers 22. At this time, the nipping rollers 22 are in contact with the paper P, nipping the paper P between them and separating the transport roller pair 28 (Figure 2E (IIIA)).

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 and diagonal directions (Figure 2E (IIIB)). This operation corrects the skew and lateral registration misalignment of the paper P to zero. This drive operation is completed before the paper P reaches the third edge sensor CIS3.

In step S13, the third edge sensor CIS3 detects the paper P (see Figure 2E (IV)). Based on the detection result of the third edge sensor CIS3, steps S14 to S17 calculate the amount of skew and skew correction of the paper, calculate the encoder count number, and rotate the rotation motor 23 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, calculate the encoder count number, and rotate the shift motor 24 to re-correct the amount of lateral registration deviation.

Section 4 in Figure 2D represents 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 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))

Interval 5 in Figure 2D is the feedback re-correction time. The rotation motor 23 and shift motor 24 are operated to correct the positional deviation calculated in interval 4 (Figure 2E (V)-(IV)).

Section 6 in Figure 2D is the time it takes for the clamping rollers 22 to return to their home position. When the paper reaches the transport section for the next process, the clamping rollers 22 are released and return to their home position in preparation for transporting the next paper (Figure 2E (IV)).

In the timing chart of Figure 2D, as an example, the motor rotation motor 23 and shift motor 24 are rotated forward as feedback re-correction operations in the skew direction and lateral registration direction. After feedback re-correction is completed, the clamping rollers 22 are separated (step S22).

After the clamping 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 clamping rollers 22 are closed in step S25, preparing for the transport of the next sheet P.

In Figures 3A and 3B, the center of rotation of the clamping roller 22 is set at one end in the longitudinal direction of the clamping roller 22. By setting the center of rotation at one end in the longitudinal direction in this way, it becomes easier to position the rotation motor.

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 CIS1-3.

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 transport direction L23: Distance between edge sensors CIS2 and CIS3 in the transport direction L1: Length from the original position of edge sensor CIS1 to the paper edge L2: Length from the original position of edge sensor CIS2 to the paper edge L3: Length from the original position of edge sensor CIS3 to the paper edge a: Length from the rotation center position to the transport center d: Length from the original position of edge sensor CIS to the transport center L': Length from the paper edge specified position to the transport center.

After the paper is sandwiched between the nip rollers 22, the detected "skew amount" is corrected by rotating the nip rollers 22 using a rotation motor 23, and the "main scanning deviation amount" is corrected by shifting the nip 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, just like the edge sensors CIS1 and CIS2. In Figure 3B, the skew amount θ ₂₃ is exaggerated to make the correction residuals easier to understand. After the correction residual is detected, the nip rollers 22 re-correct the paper attitude when printing on the back side, and the image print position correction means in Figure 3C corrects the print position.

(● Block diagram of front and back registration mechanism)
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.

Furthermore, when forming images continuously on multiple sheets of paper, the paper transport section can be provided with a storage and calculation means that stores multiple skew correction residuals detected by the second skew amount detection means and calculates an average residual from the multiple skew correction residuals. Then, when forming images on the surface of the paper, the control means can correct the skew of the paper 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 storage and calculation means.

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.

The paper posture correction amount determination means of the paper transport section determines the amount of correction for the paper posture from the paper posture. This paper posture correction amount determination means can also determine the amount of correction so that the correction residual for the front side is zero, based on the correction residual for the front side statistically stored in the correction residual storage means. In addition, the correction amount for the back side can be determined from the correction residual for the front side.

When performing a switchback to invert the paper for backside printing, the paper transport means can be configured so that the correction residuals for the front and back sides are in opposite directions. When performing a switchback to invert the paper for backside printing without performing a switchback, the paper transport means can be configured so that the correction residuals for the front and back sides are in the same direction.

The paper posture correction means corrects the paper posture based on the correction amount determined by the paper posture correction amount determination means. The correction residual storage means stores the correction residual for the front side. The paper transport means transports the paper for double-sided printing.

(● 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 the front and back register in this embodiment, and Figure 4B shows the front and back register in the conventional method.

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 clamping 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 and 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.

In other words, 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.

Before printing on the back side of the paper, the paper is switched back by the inversion mechanism 60 in Figure 1A. As a result, the paper coordinates are inverted upside down, as shown in Figure 4A (b).

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

However, as can be seen from Figures 4A (a) and (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 are reduced as a result.

In contrast, if the print is flipped over without using the switchback method, and residual skew remains in opposite directions on the front and back sides, the front and back register errors will be increased by +θ1° and -θ1°.

On the other hand, in the conventional front-back registration 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 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 sides. Here, the skew residual on the front side is θ1, and the skew residual on the back side is θ2.

As shown in Figure 4B (a), 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.

Before printing on the back side of the paper, the paper is switched back by the reversing mechanism 60 in Figure 1A. As a result, the paper coordinates are reversed upside down, as shown in Figure 4B (b).

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), if skew residuals remain in the same direction on the front and back sides, the front-to-back register errors will be increased by the sum of θ1° and θ2°.

In contrast, when flipping the print without using the switchback method, if residual skew remains in the same direction on the front and back sides, the front-to-back register error will be reduced as θ1 and θ2 will subtract (cancel out) each other.

(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 when light ink is printed on the surface of paper skewed at _-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 and transformed by _θ0 with the origin as the reference point.

First, the parameters of the rotation angle −θ ₀ are expressed as follows using the coordinates of points A and C:

5A, the upper left edge of the paper is set as the origin (0,0), and the paper indicated by the dashed line is rotated by −θ ₀ as indicated by the solid line using an affine transformation. The coordinate transformation formula is as follows:

Therefore, the coordinates are as follows:
<Printing coordinates>

<Paper edge>

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

In the following, "marks" refer to the black circles attached to the four corners of the image in FIG.
<Mark>

<Paper edge>

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

The paper edge coordinates are

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

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

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

The paper edge coordinates are

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

The printing coordinates on the front side are as follows:

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

Therefore, in the case of a paper skew of -θ° on the front/back side, the difference in print coordinates between the front and back sides is

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

It can be seen that the error components are the same for all print coordinates when the paper skew is -θ° for the front side and +θ° for the back side. Also, for the Y coordinate, the error factors of +2Xa sinθ ₀ for coordinates a and c and +2Xb sinθ ₀ for coordinates b and d disappear.

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. When reviewing the equation again, in the case of a front/back paper skew of -θ°, the difference in print coordinates between the front and back is

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

Therefore, it can be seen that the front-to-back registration error is smaller when the front side is skewed by -θ° and the back side is skewed by +θ°. Additionally, in order to align the front and back registration using paper transport alone, it is desirable to perform this embodiment after minimizing the skew correction residual. Therefore, based on the statistics of the correction residual for the front side, improvements 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 if performed 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.

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 shown by the common terms *1 and *2 in equation 17 above, so the front and back register error can be reduced to zero by printing the printed 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.

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.

(Comparison of flowcharts with and without switchback)
7A and 7B show a comparison of the flow charts for when switchback is performed (transport with switchback) and when it is not performed (transport without switchback) during backside printing. Fig. 7A shows the flow chart for when switchback is performed (transport with switchback) and Fig. 7B shows the flow chart for when switchback is not performed.

In the switchback conveying mode, the paper is turned over and fed to the clamping rollers 22 from the rear end for back-side printing, but in the non-switchback conveying mode, the paper is turned over and fed to the clamping rollers 22 from the front end for back-side printing. The differences between Figures 7(a) and (b) are enclosed in rectangular frames to make them easier to understand.

First step: Set the target for the front paper orientation. Set it so that the tilt is 0 with respect to the printing standard. Second step: Correct the front paper orientation. Correct it so that the tilt is 0 with respect to the printing standard. Third step: Detect the correction residual (tilt) with respect to the corrected target.
Fourth step: The image is printed on the front side of the paper. The image is printed with a deviation from the reference axis of the paper by the amount of the correction residual.

Fifth step:
(With switchback): The sheet is switched back for printing on the back side. This reverses the printing direction on the front and back sides.
(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.

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. (Without 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.

Seventh 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 a residual in the same direction as the surface correction residual remains.

Eighth step:
(With switchback): The image and paper reference axis are misaligned 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 on the front side. The front and back register positions are aligned without switchback transport.

The present invention has been specifically described above based on an embodiment, but it goes without saying that the present invention is not limited to the above embodiment 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 transport means, two rollers whose rotational speeds can be independently changed may be used, and the skew and lateral registration of the paper may be corrected by varying the rotational speed of the two rollers or by moving them laterally in the paper width direction.

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: Coating section 22: Nipping roller 23: Rotating motor 24: Shift motor 28: Conveying roller 30: Printing section (image forming section)
31: Conveyor drum 32: Droplet discharge section 33 (33A to 33D): Droplet discharge unit 34: Transfer drum 35: Transfer drum 40: Drying section (heating section)
41: Conveying mechanism section 50: Discharge section 51: Discharge tray 60: Reversing mechanism section 61: Conveying path 501: Stacking section 502: Paper conveying device P: Paper (sheet)
CIS1-CIS3: Edge sensor

JP 2014-088263 A Japanese Patent Application Laid-Open No. 2005-263463

Claims (9)

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. The image forming apparatus of claim 1, 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 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 but in the opposite direction. The image forming apparatus according to claim 1, characterized in that 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 the skew correction residual of the sheet on which the image is formed on the front side remains equal in magnitude. An image forming apparatus according to any one of claims 1 to 3, characterized in that, when forming images continuously on a plurality of sheets, a storage and calculation means is provided that stores a plurality of skew correction residuals detected by the second skew amount detection means and calculates an average residual from the plurality of skew correction residuals, and when forming images on the front 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 storage and calculation means. An image forming apparatus according to any one of claims 1 to 4, characterized in that the image forming unit forms an image on the back surface of the sheet in a manner that reduces misregistration between the front and back surfaces. An image forming apparatus according to any one of claims 1 to 5, characterized in that, when forming an image on the front 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. 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. The image forming method of 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 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 magnitude but in the opposite direction. An image forming method according to claim 7, characterized in that the sheet on which the 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.