JP7147483B2 - image forming device - Google Patents

Description

The present invention relates to an image forming apparatus that includes a line head and performs image formation (printing, drawing) using ink.

Some image forming apparatuses print using ink. 2. Description of the Related Art In an image forming apparatus that uses ink for printing, ink sprayed onto a sheet of paper may soak into the sheet of paper, causing show-through. Also, the portion of the paper to which the ink adheres swells. Bulging can cause curling and cockling. Japanese Patent Application Laid-Open No. 2002-300003 describes an example of an inkjet recording apparatus that takes into consideration show-through, curling, and waviness.

Patent Document 1 describes an inkjet recording apparatus capable of double-sided printing, in which the number of passes for printing on the same area differs between single-sided printing and double-sided printing, and the number of passes for double-sided printing is greater than the number of passes for single-sided printing. . This technique focuses on the point that, as the number of passes increases, the show-through decreases, the amount of curling decreases, and the amount of cockling decreases. The number of passes is changed between single-sided printing and double-sided printing, and the optimal number of passes is set for each printing (Patent Document 1: Claims 1 and 2, paragraphs [0052] and [0053]).

Japanese Patent Application Laid-Open No. 2001-071479

The paper absorbs the ink that is sprayed on it. The part that absorbs the ink swells. As a result, the edges of the paper in the sub-scanning direction (paper transport direction) may curl (roll up). It curls in the direction opposite to the expanded surface (the surface on which ink is ejected). For example, when the surface onto which ink is ejected is the upper surface of the paper, the edges of the paper curl downward. Also, in general, the more ink ejected to the edge of the paper, the greater the amount of curl.

When performing double-sided printing, the head first ejects ink onto one side of the paper. After single-sided printing, the front and back sides of the paper are reversed. The reversed paper is sent back upstream of the head. Then, the single-sided printed paper is transported to the head. The head ejects ink onto the other side of the paper. As a result, printing is performed on both sides of the paper.

The wider the distance between the nozzle (head) and the paper, the more the ink landing position shifts. Therefore, the distance between the nozzles and the transport path for transporting the paper is short. For example, the interval is several millimeters or less. Since the gap is narrow, in double-sided printing, if the amount of curl at the end of the paper printed on one side is large, there is a problem that the paper jams in the gap between the head and the transport path. When the paper is jammed, it is necessary to remove the paper. Productivity of the image forming apparatus is lowered. Also, jammed paper may damage the head.

In the ink jet recording apparatus described in Patent Document 1, the number of passes is increased in order to reduce show-through and curl. For example, about 4 to 8 passes are increased per surface (Patent Document 1: see FIGS. 20 and 21). In the technique described in Patent Document 1, the number of passes required for printing one sheet increases in double-sided printing, and there is a risk that productivity (printing speed) will drop considerably. The technique described in Patent Document 1 still has a problem in terms of productivity.

SUMMARY OF THE INVENTION In view of the above problems, the present invention prevents paper from clogging the head due to curl while maintaining productivity during double-sided printing.

An image forming apparatus according to the present invention includes a line head, a paper transport section, a switchback section, a first transport section, a second transport section, and a control section. The line head includes nozzles that eject ink onto the paper being conveyed. The line head is fixed. The paper transport section includes transport rollers. The paper transport section transports the paper toward the line head. The paper conveying unit conveys the paper with the nozzles facing the paper while providing an interval. The switchback section includes a pair of switchback rollers. Since the switchback unit performs double-sided printing, it reverses the front and back of the paper and the front and back of the paper. The first transport section transports the paper printed by the line head toward the switchback section. The second transport section transports the sheet switched back by the switchback section. The second transport section returns the paper to the paper transport section on the upstream side of the line head. A plurality of line heads are provided for each ink color. The ink colors include at least cyan, magenta, yellow, and black. In the case of double-sided printing, the control unit sets one of the front side and the back side as the first side and sets the other side as the second side. The control unit starts printing from the first surface and causes the second surface to be printed after printing the first surface. Based on the image data of the first surface used for the printing of the first surface, the control section confirms the specific area ink amount, which is the amount of ink to be ejected to the specific area of the first surface. The control section divides the specific area ink amount by a predetermined solid printing ink amount to obtain an ink amount rate. When the determined ink amount rate exceeds a predetermined reference ink amount rate, the control unit performs black conversion processing on pixels included in the conversion target range in the image data of the first surface. The specific area is a strip-shaped area having a width of a predetermined distance in the sub-scanning direction and including an upstream side in the sheet conveying direction of the first surface among the sides facing each other in the sub-scanning direction. The conversion target range is a range corresponding to the specific area in the image data of the first surface. The black conversion process is a process of converting pixels from which three colors of cyan, magenta, and yellow are ejected into pixels from which only black ink is ejected.

According to the present invention, it is possible to prevent paper from clogging the head due to curling without lowering productivity during double-sided printing.

1 is a diagram illustrating an example of a printer according to an embodiment; FIG. 1 is a diagram illustrating an example of a printer according to an embodiment; FIG. FIG. 7 is a diagram showing an example of the flow of processing during single-sided printing of the printer according to the embodiment; FIG. 7 is a diagram showing an example of the flow of setting the first side and the second side during double-sided printing of the printer according to the embodiment; It is a figure which shows an example of the surface edge area|region and back surface edge area|region which concern on embodiment. FIG. 7 is a diagram showing an example of the flow of processing during double-sided printing of the printer according to the embodiment; FIG. 7 is a diagram showing an example of the flow of processing during double-sided printing of the printer according to the embodiment; FIG. 4 is a diagram showing an example of curling of paper; FIG. 5 is a diagram showing an example of curl amount with respect to ink amount rate; FIG. 10 is a diagram showing an example of changes in curl amount from ink ejection;

An embodiment of the present invention will be described below with reference to FIGS. 1 to 10. FIG. A printer 100 will be described as an example of an image forming apparatus. The printer 100 is of the inkjet type. Note that the printer 100 may be, for example, a multifunction device including a scanner.

(Overview of Printer 100)
First, the outline of the printer 100 according to the embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 and 2 are diagrams showing an example of a printer 100 according to an embodiment.

Printer 100 includes control unit 1 , storage unit 2 , operation panel 3 , printing unit 4 and communication unit 10 . A control unit 1 controls each unit of the printer 100 . The control section 1 includes a control circuit 11 and an image processing circuit 12 . The control circuit 11 is, for example, a CPU. The control circuit 11 performs calculations and processes based on control programs and control data stored in the storage unit 2 . The storage unit 2 includes a non-volatile storage device such as ROM and storage (HDD, flash ROM) and a volatile storage device such as RAM.

The operation panel 3 includes a display panel 31 and a touch panel 32 . The display panel 31 displays setting screens and information. The control unit 1 causes the display panel 31 to display operation images such as keys, buttons, and tabs. The touch panel 32 detects touch operations on the display panel 31 . Based on the output of the touch panel 32, the control unit 1 recognizes the operated image for operation. The control unit 1 recognizes the setting operation performed by the user.

Printer 100 includes a printing unit 4 . A control unit 1 controls the operation of the printing unit 4 . The printing section 4 includes a paper feeding section 4 a , a paper conveying section 5 , an image forming section 6 , a first conveying section 7 , a switchback section 8 and a second conveying section 9 . The paper feed section 4a accommodates a stack of sheets. The paper feed unit 4 a includes a paper feed roller 41 . The paper feed roller 41 is in contact with the uppermost sheet of the paper stack set in the paper feed section 4a. A paper feed motor (not shown) is provided for rotating the paper feed roller 41 . During a print job, the control unit 1 rotates the paper feed motor to rotate the paper feed roller 41 . As a result, the paper is fed from the paper feeding section 4a to the paper conveying section 5. As shown in FIG.

The paper transport section 5 transports the paper. The control section 1 causes the sheet conveying section 5 to convey the sheet supplied from the sheet feeding section 4 a toward the image forming section 6 . In FIG. 2, the paper transport path is indicated by a thick line. A transport roller 50 and a plurality of paper transport roller pairs 51 are provided along the transport path. A paper transport motor (not shown) is provided for rotating these rotating bodies. During a print job, the controller 1 rotates the paper transport motor to rotate the transport roller 50 and the paper transport roller pair 51 .

The paper transport section 5 includes a transport unit 53 . The transport unit 53 includes a drive roller 54 , a plurality of driven rollers 55 and a transport belt 56 . A transport belt 56 is looped around the drive roller 54 and each driven roller 55 . During a print job, the control section 1 rotates the belt motor. A belt motor rotates the drive roller 54 . As a result, the paper supplied from the paper feeder 4a is transported on the transport belt 56. FIG. Note that the transport unit 53 includes a suction section. The adsorption unit adsorbs the sheet to the conveying belt 56 by sucking air or by electrostatic force. The suction fixes the position of the paper and the distance between each nozzle and the paper. During printing, the control unit 1 operates the suction unit.

The image forming section 6 records an image by ejecting ink onto a sheet of paper to be conveyed. The image forming section 6 includes multiple line heads 60 . Specifically, the printer 100 includes four line heads 60 (6Bk, 6C, 6M, 6Y). The line head 6Bk ejects black ink. The line head 6C ejects cyan ink. The line head 6M ejects magenta ink. The line head 6Y ejects yellow ink.

Each line head 60 is provided above the transport belt 56 . Each line head 60 is fixed. Each line head 60 includes multiple nozzles. The nozzles are arranged in the main scanning direction (perpendicular to the transport direction). Each nozzle ejects ink onto the paper being transported. The opening of each nozzle faces the conveyor belt 56 . During printing, the paper transport section 5 transports the paper toward the line head 60 and also transports the paper facing the nozzles.

A gap (interval) is provided between each line head 60 and the conveyor belt 56 . The length of the interval is predetermined. The interval is, for example, about 1 mm to several mm. Ink ejected from the nozzles lands on the conveyed paper. Thus, an image is recorded (formed). An ink tank for supplying ink is provided for each line head 60 .

The control unit 1 causes the first conveying unit 7 to convey the sheet that has passed the line head 60 . The first transport section 7 transports the paper toward the discharge tray 73 or the switchback section 8 . The first conveying section 7 includes a plurality of first conveying roller pairs 70 , first switching guides 71 and second switching guides 72 . A first transport motor (not shown) is provided to rotate each first transport roller pair 70 . The first switching guide 71 and the second switching guide 72 are guide plates for switching the paper transport path. The first switching guide 71 guides the paper to the discharge tray 73 or the switchback section 8 . A first switching motor (not shown) is provided to rotate the first switching guide 71 . The second switching guide 72 guides the sheet to the switchback section 8 or the sheet conveying section 5 (upstream of the image forming section 6). A second switching motor (not shown) is provided to rotate the second switching guide 72 .

The switchback section 8 includes a switchback roller pair 80 . For double-sided printing, the control section 1 causes the switchback section 8 to reverse the front and back of the paper and the front and back of the paper.

The second conveying section 9 conveys the sheet switched back by the switchback section 8 . The second transport section 9 includes a second transport roller pair 90 . A second transport motor (not shown) is provided to rotate the second transport roller pair 90 . The control unit 1 causes the second conveying unit 9 to convey the paper that has been switched back. The second transport unit 9 returns the paper that has been switched back to the upstream side of the line head 60 . The second conveying section 9 merges the paper that has been switched back to the paper conveying section 5 .

Control unit 1 is connected to communication unit 10 . The communication unit 10 includes communication hardware (connector, communication circuit) and software. The communication section 10 communicates with the computer 200 . Computer 200 is, for example, a PC or a server. The control unit 1 receives print data from the computer 200 . The print data includes, for example, data described in a page description language and print setting information. The print setting information is setting information of a print job. The print setting information includes the print settings made by the user on the computer 200 . For example, when the computer 200 is set to perform double-sided printing, print setting information for performing double-sided printing is included in the print data.

The control unit 1 (image processing circuit 12) analyzes the received (input) print data (data described in a page description language). Based on the data described in the page description language, the control section 1 generates print image data (raster data). The control unit 1 performs image processing based on the image data for printing. Finally, the control unit 1 performs image processing such as dot processing to generate image data for ink ejection. The image data for ejecting ink is data indicating the positions of nozzles for ejecting ink and the ejection timing. Based on the image data for ink ejection, the controller 1 causes the line head 60 to eject ink for each line.

(Processing for single-sided printing)
Next, with reference to FIG. 3, an example of the flow of processing during single-sided printing of the printer 100 according to the embodiment will be described. FIG. 3 is a diagram showing an example of the flow of processing during single-sided printing of the printer 100 according to the embodiment. Note that when printing on a plurality of sheets continuously, the control unit 1 may perform the processing in FIG. 3 in parallel.

First, based on the received print data, the control section 1 generates ink ejection image data for a page to be printed (step #11). Next, the control section 1 causes the paper feeding section 4a to supply paper (step #12). As a result, the paper is sent out from the paper feed section 4a. Next, the control section 1 causes the sheet conveying section 5 to convey the sheet toward the image forming section 6 (step #13). When the paper reaches the image forming section 6, the control section 1 causes each line head 60 to perform ink ejection based on the generated ink ejection image data (step #14). The ejection of ink from each line head 60 completes printing (drawing) on one side of the paper.

Then, the control section 1 causes the first conveying section 7 to convey the sheet toward the discharge tray 73 (step #15). At this time, the controller 1 rotates the plurality of first transport roller pairs 70 . Further, the control unit 1 rotates the first switching guide 71 to the position where the paper is discharged to the discharge tray 73 . In other words, the control unit 1 causes the first switching guide 71 to close the passage to the switchback unit 8 . Finally, the first conveying unit 7 discharges the printed paper to the discharge tray 73 (step #16, discharging in direction A in FIG. 2). This completes single-sided printing of one page of paper (end).

(Setting of the 1st side and the 2nd side)
Next, with reference to FIGS. 4 and 5, an example of the flow of setting the first side and the second side during double-sided printing of the printer 100 according to the embodiment will be described. FIG. 4 is a diagram showing an example of the flow of setting the first side and the second side during double-sided printing of the printer 100 according to the embodiment. FIG. 5 is a diagram showing an example of the front end region F1 and the back end region F2 according to the embodiment.

The start in FIG. 4 is the time when double-sided printing is started based on the received print data. In this case, the print data includes print setting information defining double-sided printing. The control unit 1 analyzes the print setting information and recognizes that double-sided printing should be performed.

First, based on the received print data, the control section 1 generates ink ejection image data for each of the front and back pages (step #21). For double-sided printing, the data for printing includes data described in the page description language for the front page and data described in the page description language for the back page. The control unit 1 (image processing circuit 12) generates print image data (raster data) for each of the front and back pages. Then, the control unit 1 generates ink ejection image data for each of the front and back pages. In the case of color printing, the control unit 1 generates image data for ejecting C, M, Y, and K inks for the front side. The control unit 1 also generates C, M, Y, and K ink ejection image data for the back side.

Next, based on the ink ejection image data for the surface of the first page, the control unit 1 obtains the amount of surface ink to be ejected onto the surface end region F1 of the surface (step #22). Further, based on the ink ejection image data for the back side of the first page, the control section 1 obtains the amount of back side ink to be ejected to the back side end region F2 of the back side (step #23).

As shown in FIG. 5, the front surface edge region F1 includes an edge side on the upstream side (rear side) in the paper transport direction among the sides facing each other in the sub-scanning direction. The upstream side (trailing side) in the paper transport direction is the upstream side when printing the front side. The front surface edge region F1 has a width of a predetermined distance L1 in the sub-scanning direction. The surface edge region F1 is a strip-shaped region. On the other hand, as shown in FIG. 6, the rear end region F2 is the rear region of the front end region F1. The front end region F1 and the back end region F2 have the same size.

The predetermined distance L1 is the paper transport distance from the transport roller 50 closest to the line head 60 on the most upstream side in the paper transport direction to the line head 60 on the most upstream side in the paper transport direction. In FIG. 2 , the conveying roller closest to the line head 60 is denoted by 50 . The predetermined distance L1 is the distance from the transport roller 50 to the side surface parallel to the main scanning direction of the most upstream line head 60 in the paper transport direction. Thus, predetermined distance L1 is the distance from the nip of transport roller 50 and transport belt 56 to where the spacing begins.

Calculation of the surface ink amount and the back surface ink amount will be described. First, the amount of ink ejected to one pixel is determined for each color. Further, in the printer 100 of the present embodiment, the amount of ink ejected to one pixel is the same (or may be different) for each color. For example, the amount of ink ejected to one pixel is about 5 to 7.5 pL per color.

The control unit 1 counts the number of pixels to which ink is ejected for each color in the area corresponding to the surface edge area F1 of the image data for ejecting ink on the surface. The control unit 1 obtains the number of pixels for which ink is to be ejected for each of cyan, yellow, magenta, and black. The control unit 1 obtains the total number of pixels of each color. The control unit 1 multiplies the amount of ink ejected for one color and one pixel by the calculated total value to obtain the surface ink amount (step #22).

Further, the control unit 1 counts the number of pixels to which ink is ejected for each color in the area corresponding to the back surface end area F2 of the image data for ink ejection of the back surface. The control unit 1 obtains the number of pixels for which ink is to be ejected for each of cyan, yellow, magenta, and black. The control unit 1 obtains the total number of pixels of each color. The control section 1 multiplies the amount of ink ejected for one color and one pixel by the obtained total value to obtain the amount of ink on the back surface (step #23).

Next, the controller 1 confirms whether or not the surface ink amount is equal to or less than the back surface ink amount (step #24). In other words, the control unit 1 checks which of the front face end region F1 and the back face end region F2 has a larger ink discharge amount.

When the amount of ink on the front surface is equal to or less than the amount of ink on the back surface (Yes in step #24), the control unit 1 sets the front surface as the first surface and the back surface as the second surface (step #25). On the other hand, when the amount of ink on the front surface exceeds the amount of ink on the back surface (No in step #24), the controller 1 sets the front surface as the first surface and the back surface as the second surface (step #26). This flow ends (end) at step #25 or step #26. The control unit 1 sets one of the front surface and the back surface as the first surface, and sets the other as the second surface. Thereafter, the control unit 1 starts printing each sheet from the first side, and after printing the first side, prints the second side. Thereby, the control unit 1 determines which side to print from, the front side or the back side.

Note that when a print job is to perform double-sided printing on a plurality of sheets in succession, when the front side of the first sheet is set to be the first side, the control section 1 sets the front side of all pages to be the first side. When the back side is set as the first side for the first sheet, the control unit 1 sets the back side as the first side for all pages. As a result, the front and back sides of all pages of the ejected paper are aligned.

(Flow of double-sided printing)
Next, an example of the flow of double-sided printing in the printer 100 according to the embodiment will be described with reference to FIGS. 6 to 10. FIG. 6 and 7 are diagrams showing an example of the flow of processing during double-sided printing of the printer 100 according to the embodiment. FIG. 8 is a diagram showing an example of paper curl. FIG. 9 is a diagram showing an example of the curl amount with respect to the ink amount rate. FIG. 10 is a diagram showing an example of changes in curl amount from ink ejection.

After setting the first side and the second side, the controller 1 checks the amount of ink in the specific area, which is the amount of ink in the specific area (step #31). Here, the specific area includes the upstream side (trailing side) in the paper transport direction among the sides of the paper facing each other in the sub-scanning direction. The upstream side (trailing side) in the paper transport direction is the upstream side when printing the first surface. The specific area has a width of a predetermined distance L1 in the sub-scanning direction. The specific area is a strip-shaped area.

When the surface is the first surface, the surface end region F1 corresponds to the specific region. When the back surface is the first surface, the specific area includes the upstream side (trailing side) in the paper transport direction at the time of back side printing, among the sides facing each other in the sub-scanning direction. The upstream side (trailing side) in the paper transport direction is the upstream side when printing the back side.

When the surface is the first surface, the controller 1 sets the surface ink amount as the specific area ink amount. When the back surface is the first surface, the control unit 1 counts the number of pixels for each color in the area corresponding to the specific area in the ink ejection image data for the back surface. The control unit 1 obtains the number of pixels for which ink is to be ejected for each of cyan, yellow, magenta, and black. The control unit 1 obtains the total number of pixels of each color. The controller 1 multiplies the amount of ink to be ejected for one color and one pixel by the calculated total value to obtain the amount of ink for the specific region (step #31).

Then, the control section 1 obtains the ink amount rate (step #32). The control unit 1 obtains a value obtained by dividing the specific area ink amount by the solid printing ink amount 21 as the ink amount ratio. The solid printing ink amount 21 is determined in advance. The storage unit 2 nonvolatilely stores the solid printing ink amount 21 (see FIG. 1). The control unit 1 performs calculation using the solid printing ink amount 21 stored in the storage unit 2 .

The amount of ink for solid printing 21 is the amount of ink when ink of all colors is ejected to all pixels in a specific area. In other words, the solid printing ink amount 21 is the amount of ink consumed when the C, M, Y, and K inks are ejected onto all the pixels in the specific area. By multiplying the total number of pixels in the specific area by the ink ejection amount per pixel for one color by 4 (4 colors), the solid printing ink amount 21 can be obtained (ink amount 400%). The ink amount ratio indicates the ratio of the amount of ink in a specific area to the amount of ink in four-color solid printing.

Then, the control section 1 confirms whether or not the obtained ink amount rate exceeds the reference ink amount rate 22 (step #33). The reference ink amount rate 22 is the ink amount rate when the curl amount of the paper becomes the gap (interval) between the line head 60 and the paper (conveyor belt 56). A reference ink amount rate 22 is predetermined. The storage unit 2 stores the reference ink amount rate 22 in a non-volatile manner. The control section 1 refers to the reference ink amount rate 22 stored in the storage section 2 .

First, paper curl will be described with reference to FIG. Ink is ejected onto the paper. The paper absorbs the ejected ink. The part that absorbs the ink swells. If one side of the paper and the other have different coefficients of expansion, the paper will curl. The example of FIG. 8 shows an example in which ink is ejected onto the upper surface. For example, when ink is ejected onto the upper surface on the upstream side in the paper transport direction, the upstream end of the paper curls (curves) downward.

As shown in FIG. 8, the curl amount is the amount (height) by which the edge of the paper is lifted. The amount of curl can also be said to be the distance between the horizontal surface and the edge of the paper in the vertical direction when the paper is placed on the horizontal surface. Then, as shown in FIG. 9, the amount of curl increases as the amount of ink ejected to the specific region increases. This is because more liquid is absorbed by the paper and the degree of expansion is increased.

Here, an example of the reference ink amount rate 22 is indicated by a dashed line in FIG. By repeating the experiment, it is possible to confirm the ink amount rate at which the amount of curl becomes equal to the interval. If double-sided printing is performed when the ink amount rate is the reference ink amount rate 22, the probability that the paper will clog the line head 60 increases. The reference ink amount rate 22 serves as a reference for determining whether or not the line head 60 is clogged with paper.

When the determined ink amount rate exceeds the reference ink amount rate 22 (Yes in step #33), the control unit 1 converts the first-side image data (ink ejection image data for the first side) into Pixels included in the target range are subjected to black conversion processing (step #34). The range to be converted is the range corresponding to the specific area in the image data for ink ejection on the first surface.

On the other hand, when the calculated ink amount rate is equal to or less than the reference ink amount rate of 22 (No in step #33), the control section 1 skips steps #34 and #35. That is, the control unit 1 does not perform black conversion processing on the pixels in the conversion target range.

The black conversion process is a process of converting pixels from which three colors of cyan, magenta, and yellow are ejected into pixels from which only black ink is ejected. When reproducing black, one pixel may be overlaid with cyan, magenta, and yellow inks. Only black ink is ejected to pixels where cyan, magenta and yellow inks are superimposed. Specifically, the control unit 1 converts pixels in which the three colors of cyan, magenta, and yellow overlap among the image data for ink ejection of cyan, magenta, and yellow into non-ejection (white) pixels. Further, the control unit 1 sets the pixels at the same positions as the pixels converted to non-ejection among the black ink ejection image data to be ejection (black). This reduces the amount of ink used for that pixel to 1/3. By performing black conversion processing, the amount of curl in the specific area can be reduced.

Here, the control unit 1 (image processing circuit 12) may convert all pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range into pixels ejecting only black ink ( first method). In this case, the control unit 1 performs black conversion processing so as to reduce the amount of ink ejected to the specific area as much as possible.

In addition, the control unit 1 (image processing circuit 12) converts the pixels ejecting the three colors of cyan, magenta, and yellow within the conversion target range to black ink so that the ink amount rate is equal to or less than the reference ink amount rate of 22. It may be converted into a pixel that only ejects (second method). In the second black conversion method, a target ink amount rate is determined in advance. The target ink amount rate may be the reference ink amount rate 22 . Also, the target ink amount rate is a value smaller than the reference ink amount rate 22, and may be a predetermined value.

For example, the control unit 1 obtains the absolute value of the difference between the obtained ink amount rate and the reference ink amount rate 22 . The control unit 1 multiplies the absolute value by the solid printing ink amount 21 to obtain the reduction target value. The reduction target value indicates the amount of ink to be reduced in order to bring the ink amount rate to the target ink amount rate. By dividing the target reduction value by the reduction amount of ink used per pixel when the black conversion process is performed, the control unit 1 can obtain the number of pixels to be subjected to the black conversion process. The control unit 1 performs black conversion processing for the obtained number of pixels to be subjected to black conversion processing. When the number of pixels capable of black conversion processing is smaller than the number of pixels to be subjected to black conversion processing, the control unit 1 performs black conversion processing on all pixels capable of black conversion processing.

Further, the control unit 1 (image processing circuit 12) selects a predetermined conversion number (upper limit number) of the pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range with only black ink. may be converted into pixels for ejecting (third method). Black conversion processing may change the tone and atmosphere of an image. By setting the upper limit number of conversions in advance, it is possible to reduce changes in the color tone and atmosphere of the image.

The operation panel 3 may accept selection of the black conversion process to be used among the first to third methods. In this case, the control unit 1 (image processing circuit 12) executes the selected black conversion process.

If the black conversion process has been performed, the control section 1 obtains the post-conversion ink volume rate based on the post-conversion image data for the first surface (ink ejection image data for the first surface) (step #35). Specifically, the control unit 1 obtains the post-conversion ink amount, which is the amount of ink to be ejected onto the specific region. The control unit 1 counts, for each color, the number of pixels for which ink is to be ejected in an area corresponding to the specific area in the converted ink ejection image data for the first surface. The control unit 1 obtains the number of pixels for which ink is to be ejected for each of cyan, yellow, magenta, and black. The control unit 1 obtains the total number of pixels of each color. The control unit 1 multiplies the amount of ink to be ejected for one color and one pixel by the calculated total value to obtain the ink amount after conversion. Then, the control unit 1 divides the obtained ink amount after conversion by the solid printing ink amount 21 to obtain the ink amount ratio after conversion.

Next, the control section 1 causes the paper feeding section 4a to supply paper (step #36). As a result, the paper is sent out from the paper feed section 4a. Next, the control section 1 causes the sheet conveying section 5 to convey the sheet toward the image forming section 6 (step #37). When the paper reaches the image forming section 6, the control section 1 causes each line head 60 to perform ink ejection based on the ink ejection image data for the first surface (step #38). The ejection of ink from each line head 60 completes printing (drawing) on one side of the paper.

Then, the control section 1 causes the first conveying section 7 to convey the sheet toward the switchback section 8 (step #39). The control unit 1 causes the paper to enter the switchback unit 8 . The controller 1 rotates the plurality of first transport roller pairs 70 . Further, the control section 1 rotates the first switching guide 71 to the position where the paper is sent to the switchback section 8 . In other words, the control unit 1 causes the first switching guide 71 to open the passage to the switchback unit 8 . Further, the control section 1 rotates the second switching guide 72 so that the sheet enters the switchback section 8 . As a result, the paper is transported in the B direction in FIG.

Then, the control section 1 causes the switchback section 8 to perform a switchback operation (step #310). First, the control unit 1 rotates the switchback roller pair 80 so as to enter the switchback unit 8 on the far side. Then, the control unit 1 stops the switchback roller pair 80 before the paper passes through the nip of the switchback roller pair 80 . The controller 1 stops the switchback roller pair 80 when the trailing edge of the paper crosses the branch point of the first conveying section 7 , the switchback section 8 , and the second conveying section 9 . As a result, in the subsequent transport, the end of the sheet having the specific area becomes the downstream side (leading side) in the sheet transport direction. A certain end of the specific area enters the line head 60 (the gap between the nozzles and the conveyor belt 56) first.

Then, the control section 1 confirms whether or not the black conversion process has been performed (step #311). If the black conversion process has been performed (Yes in step #311), the control section 1 confirms whether or not the post-conversion ink amount rate exceeds the reference ink amount rate 22 (step #312). When the post-conversion ink amount rate exceeds the reference ink amount rate 22 (Yes in step #312), the control unit 1 waits for a predetermined waiting time 23 and the switchback unit 8 for the single-sided printed paper ( step #313).

As shown in FIG. 10, the amount of curl at the edge of the paper tends to decrease as the time elapses after ink ejection. Paper jams in the line head 60 may be avoided by delaying entry of the paper that has been switched back into the line head 60 after ejection of ink to the first surface. The storage unit 2 nonvolatilely stores the waiting time 23 . The control unit 1 refers to the waiting time 23 stored in the storage unit 2 . The waiting time 23 can be determined as appropriate. A plurality of types of waiting time 23 may be defined. For example, a longer waiting time 23 may be defined as the post-conversion ink volume rate is greater. Further, when the solid printing ink amount 21 is ejected to the specific area, the waiting time 23 may be defined as the time when the curl amount of the specific area is equal to or less than the gap between the nozzle and the conveyor belt 56 .

After waiting for the sheet printed on one side (step #313), the control unit 1 returns the sheet (printed on one side) to the upstream side of the line head 60 (step #314). If the black conversion process is not performed (No in step #311), or if the post-conversion ink amount rate does not exceed the reference ink amount rate 22 (No in step #312), the controller 1 waits. (without providing the waiting time 23), the paper (printed on one side) is returned to the upstream side of the line head 60 (step #314).

The control unit 1 rotates the second switching guide 72 to an angle leading to the sheet conveying unit 5 (upstream of the image forming unit 6). Further, the control unit 1 rotates the switchback roller pair 80 in the direction of discharging from the switchback unit 8 . The control unit 1 also rotates the second transport roller pair 90 (second transport motor) to join the paper to the transport path of the paper transport unit 5 .

In order to start printing on the second surface, control section 1 causes sheet conveying section 5 to convey the sheet toward image forming section 6 (step #315). When printing on the first surface, the paper is transported with the front and back of the paper turned upside down. When the sheet that has been switched back reaches the image forming section 6, the control section 1 causes each line head 60 to perform ink ejection based on the ink ejection image data for the second surface (step #316). Ink ejection from each line head 60 completes printing (drawing) on the second surface of the paper.

Then, the control section 1 causes the first conveying section 7 to convey the sheet toward the discharge tray 73 (step #317). Before long, the double-sided printed paper is discharged to the discharge tray 73 . Next, control unit 1 confirms whether or not the print job has been completed (step #318). In other words, the control unit 1 confirms whether double-sided printing to be printed in the print job has been completed. When the print job is completed (Yes in step #318), this flow ends (END). When the print job is incomplete (No at step #318), the flow returns to step #31.

Thus, the image forming apparatus (printer 100 ) according to the embodiment includes the line head 60 , paper transport section 5 , switchback section 8 , first transport section 7 , second transport section 9 and control section 1 . The line head 60 includes nozzles that eject ink onto the conveyed paper. The line head 60 is fixed. The paper transport section 5 includes transport rollers 50 . The paper conveying section 5 conveys the paper toward the line head 60 . The paper conveying unit 5 conveys the paper with the nozzles and the paper facing each other while providing an interval. The switchback section 8 includes a switchback roller pair 80 . Since the switchback unit 8 performs double-sided printing, it reverses the front and back of the paper and the front and back of the paper. The first transport section 7 transports the paper printed by the line head 60 toward the switchback section 8 . The second conveying section 9 conveys the sheet switched back by the switchback section 8 . The second transport section 9 returns the paper to the paper transport section 5 on the upstream side of the line head 60 . A plurality of line heads 60 are provided for each ink color. Ink colors include at least cyan, magenta, yellow, and black. In the case of double-sided printing, the control section 1 sets one of the front side and the back side as the first side, and sets the other side as the second side. The control unit 1 starts printing from the first side, and after printing the first side, prints the second side. Based on the image data of the first side used for printing on the first side, the controller 1 confirms the specific area ink amount, which is the amount of ink to be ejected to the specific area of the first side. The control unit 1 divides the specific area ink amount by a predetermined solid printing ink amount 21 to obtain the ink amount rate. When the calculated ink amount rate exceeds the predetermined reference ink amount rate 22, the control section 1 performs black conversion processing on the pixels included in the conversion target range in the image data of the first surface. The specific area is a strip-shaped area having a width of a predetermined distance L1 in the sub-scanning direction and including the upstream side in the conveying direction of the paper on the first side among the sides facing each other in the sub-scanning direction. The conversion target range is a range corresponding to a specific area within the image data of the first surface. The black conversion process is a process of converting pixels from which three colors of cyan, magenta, and yellow are ejected into pixels from which only black ink is ejected.

According to this configuration, the ink amount rate can be obtained based on the amount of ink ejected to the specific area. The specific area is an area on the upstream side in the paper transport direction (the trailing side in the sub-scanning direction) when printing the first surface. When switched back for double-sided printing (when printing on the second side), the specific area is the downstream side in the paper transport direction (the leading side in the sub-scanning direction). That is, the specific area is the portion that reaches the line head 60 first when printing the second side. Then, by comparing the obtained ink amount rate with the reference ink amount rate 22, it is possible to determine whether or not the curl in the specific area is large during printing on the second surface. In other words, at the time of the second pass (paper passing under the line head 60), it is possible to determine whether or not the leading edge (downstream side in the transport direction) of the paper on which the second surface is to be printed has a large amount of curl.

When printing the second side, black conversion processing can be performed when curl in a particular area is expected to be high. The black conversion process can reduce the amount of curl in the specific area when printing the second side (during the second pass). Since the amount of curl can be suppressed, jamming of paper in the line head 60 can be prevented. Also, there is no need to increase the number of passes to suppress curl. During double-sided printing, it is possible to prevent paper jams in the head while maintaining productivity.

The reference ink amount rate 22 is the ink amount rate when the curl amount of the paper becomes the interval. When the calculated ink amount rate is equal to or less than the reference ink amount rate 22, the control section 1 does not perform the black conversion process on the pixels included in the conversion target range. When the amount of curl is such that the paper does not clog the line head 60, black conversion processing can be avoided. During printing of the second side of duplex printing (during the second pass), black conversion processing can be performed only when curl occurs enough to cause a paper jam.

The control unit 1 obtains the amount of surface ink to be ejected onto the surface end region F1 of the surface. The control unit 1 obtains the amount of back ink to be ejected onto the back surface edge region F2, which is the area behind the front surface edge region F1 of the back surface. When the amount of ink on the front side is equal to or less than the amount of ink on the back side, the controller 1 sets the front side as the first side and the back side as the second side. When the amount of ink on the front side exceeds the amount of ink on the back side, the controller 1 sets the back side as the first side and the front side as the second side. The front surface edge area F1 is a strip-shaped area having a width of a predetermined distance L1 in the sub-scanning direction and including the edge on the upstream side in the paper transport direction when the front surface is printed, among the sides facing each other in the sub-scanning direction. . Of the front side and the back side, the side with less ink used in the area corresponding to the specific area can be set as the first side. When printing on the second side of double-sided printing (during the second pass), printing can be started from the side with less curl at the end on the downstream side in the paper transport direction (the leading side in the sub-scanning direction).

The predetermined distance L1 is the paper transport distance from the transport roller 50 closest to the line head 60 on the most upstream side in the paper transport direction to the line head 60 on the most upstream side in the paper transport direction. The transport roller 50 is in contact with the paper. After passing the transport roller 50, the paper is no longer pressed. As a result, after passing through the nip of the transport rollers 50, the edge curl of the paper is curled again. Determining the width in the sub-scanning direction of the specific area, the front surface edge area F1, and the back surface edge area F2 based on the curled portion after passing the conveying roller 50 closest to the line head 60 until reaching the line head 60. can be done.

The control unit 1 may convert all of the pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range into pixels ejecting only black ink. It is possible to minimize the amount of ink ejected to the specific region of the first surface. Curling in a specific area can be suppressed as much as possible.

The control unit 1 converts the pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range into pixels ejecting only black ink so that the ink amount rate is equal to or less than the reference ink amount rate of 22. may When printing on the second side of duplex printing (during the second pass), curling at the downstream side in the paper transport direction (the leading side in the sub-scanning direction) can be suppressed to the extent that paper jams do not occur. can. Moreover, the number of pixels subjected to black conversion processing can be reduced as much as possible.

The control unit 1 may convert a predetermined conversion number of pixels among the pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range into pixels ejecting only black ink. When printing the second side of double-sided printing (during the second pass), curling can be suppressed to the extent that paper jams do not occur in the line head 60 . A certain number of pixels can be converted to black.

When the black conversion process is performed, the control unit 1 obtains the amount of ink after conversion, which is the amount of ink to be ejected on the specific area, based on the image data of the first surface after conversion. The control unit 1 divides the obtained ink amount after conversion by the solid printing ink amount 21 to obtain the ink amount ratio after conversion. When the calculated post-conversion ink amount rate exceeds the reference ink amount rate 22 , the control section 1 causes the one-sided printed paper to wait in the switchback section 8 for a predetermined waiting time 23 . The amount of curl generally decreases as time elapses after the ink is ejected. According to this configuration, even if the black conversion process is performed, when ink is ejected to the specific area of the first surface to such an extent that the line head 60 is jammed with paper, the switchback section 8 can wait for the paper. When printing the second side of duplex printing (during the second pass), after reducing the amount of curl at the edge on the downstream side in the paper transport direction (the leading side in the sub-scanning direction), the paper is placed below the line head 60. can enter.

When printing a plurality of sheets continuously, when the front side is set as the first side for the first sheet, the control section 1 sets the front side as the first side for all pages. When the back side is set as the first side for the first sheet, the control unit 1 sets the back side as the first side for all pages. The relationship between the front and back sides of printed paper can be unified for all pages.

Although the embodiments of the present invention have been described, the scope of the present invention is not limited thereto, and various modifications can be made without departing from the gist of the invention.

INDUSTRIAL APPLICABILITY The present invention can be used for an image forming apparatus that performs double-sided printing using ink.

100 printer (image forming apparatus) 1 control unit 21 ink amount for solid printing 22 reference ink amount rate 23 waiting time 5 paper conveying unit 50 conveying roller 60 line head 7 first conveying unit 8 switchback unit 80 switchback roller pair 9 second 2 Conveying portion F1 Front end region F2 Back end region L1 Predetermined distance

Claims (9)

a fixed line head including nozzles for ejecting ink onto a conveyed paper;
a paper transport unit that includes a transport roller and transports the paper toward the line head and transports the paper with the nozzles and the paper facing each other while providing a gap;
a switchback unit that includes a pair of switchback rollers and reverses the front and back of the paper and the front and back of the paper for double-sided printing;
a first transport unit that transports the paper printed by the line head toward the switchback unit;
a second conveying unit that conveys the sheet switched back by the switchback unit and returns the sheet to the sheet conveying unit on the upstream side of the line head;
a controller;
A plurality of the line heads are provided for each ink color,
the ink colors include at least cyan, magenta, yellow, and black;
For double-sided printing,
The control unit
One of the front surface and the back surface is set as the first surface, and the other is set as the second surface,
starting printing from the first side, printing the second side after printing the first side,
confirming a specific area ink amount, which is the amount of ink ejected to a specific area of the first surface, based on the image data of the first surface used for printing on the first surface;
Dividing the specific area ink amount by a predetermined solid printing ink amount to obtain an ink amount rate,
when the determined ink amount rate exceeds a predetermined reference ink amount rate, black conversion processing is performed on pixels included in a conversion target range in the image data of the first surface;
The specific region is a strip-shaped region including an upstream side in the paper transport direction of the first surface among the sides facing each other in the sub-scanning direction and having a width of a predetermined distance in the sub-scanning direction,
the conversion target range is a range corresponding to the specific region in the image data of the first surface;
The image forming apparatus according to claim 1, wherein the black conversion process is a process of converting pixels for which three colors of cyan, magenta, and yellow are ejected into pixels for which only black ink is ejected. The reference ink amount rate is the ink amount rate when the curl amount of the paper becomes the interval,
2. The image forming method according to claim 1, wherein the control unit does not perform the black conversion process for pixels included in the conversion target range when the determined ink amount rate is equal to or less than the reference ink amount rate. Device. The control unit
obtaining an amount of surface ink to be ejected onto a surface end region of the surface;
determining an amount of back surface ink to be ejected to a back surface end region that is a region behind the front surface end region of the back surface;
when the amount of ink on the front surface is equal to or less than the amount of ink on the back surface, setting the front surface as the first surface and the back surface as the second surface;
when the amount of ink on the front surface exceeds the amount of ink on the back surface, setting the back surface as the first surface and the front surface as the second surface;
The front surface end region is a strip-shaped region having a width in the sub-scanning direction that is the predetermined distance and includes an upstream side in the paper transport direction when the front surface is printed, among the sides facing each other in the sub-scanning direction. 3. The image forming apparatus according to claim 1, wherein: 3. The predetermined distance is a paper transport distance from the transport roller closest to the line head on the most upstream side in the paper transport direction to the line head on the most upstream side in the paper transport direction. 4. The image forming apparatus according to any one of 3. 5. The control unit according to any one of claims 1 to 4, wherein the control unit converts all pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range into pixels ejecting only black ink. 1. The image forming apparatus according to item 1 or 1. The control unit changes the pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range to the pixels ejecting only black ink so that the ink amount rate is equal to or less than the reference ink amount rate. 5. The image forming apparatus according to any one of claims 1 to 4, wherein the conversion is performed. The control unit converts a predetermined conversion number of pixels among pixels ejecting three colors of cyan, magenta, and yellow within the conversion target range into pixels ejecting only black ink. The image forming apparatus according to any one of claims 1 to 4. When the black conversion process is performed,
The control unit
obtaining a post-conversion ink amount, which is the amount of ink to be ejected in the specific area, based on the converted image data of the first surface;
Dividing the obtained converted ink amount by the solid printing ink amount to obtain the converted ink amount ratio,
When the calculated post-conversion ink amount rate exceeds the reference ink amount rate,
8. The image forming apparatus according to any one of claims 1 to 7, wherein the one-sided printed sheet is kept waiting in the switchback section for a predetermined waiting time. When printing multiple sheets continuously,
The control unit
When the front side is set as the first side for the first sheet, the front side is set as the first side for all pages,
9. The image forming apparatus according to claim 1, wherein when the back side is set as the first side for the first sheet, the back side is set as the first side for all pages. .

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