Image forming apparatus and image forming method

Abstract

Provided are an image forming apparatus and an image forming method, the image forming apparatus being capable of forming images on both sides of a recording medium and including: a drying unit for drying the recording medium on one side of which an image has been formed, the drying unit including a heating part for heating the recording medium and a conveyance part for conveying the recording medium in a state in which the whole area of the part of the recording medium heated by the heating part is restrained; and a processor for controlling the drying unit. The processor controls the drying unit under a condition that the difference in water content in a non-image section of the recording medium before and after the recording medium is heated and dried by the drying unit is 6% or less.

Description

Image forming apparatus and image forming method

[0001] The present disclosure relates to an image forming apparatus and an image forming method.

[0002] Some image forming devices, such as inkjet printing devices that use aqueous ink, are equipped with a drying unit for fixing and drying the ink formed on the recording surface of the paper (for example, JP 2020-015272 A, JP 2020-146993 A, etc.).

[0003] When ink and paper are heated in the drying unit, the moisture in the paper evaporates, which can cause the paper to shrink. This is particularly true when using water-based ink, where a large amount of thermal energy is applied to the paper to evaporate the moisture and solvent in the ink, making the paper more susceptible to shrinkage. In image forming devices capable of double-sided printing, shrinkage of the paper due to drying after image formation on the front side can result in misregistration when printing on the back side.

[0004] To address this issue, there are known techniques for front-to-back registration that predict the amount of paper shrinkage in advance or measure the paper size after printing on the front side. However, because the amount of shrinkage varies depending on the paper type and thickness, and the distribution of shrinkage within the paper surface also varies depending on the image pattern of the image to be printed, it is often not possible to fully improve front-to-back registration.

[0005] The present disclosure has been made in consideration of the above circumstances, and aims to provide an image forming apparatus and an image forming method that can suppress misregistration between the front and back sides during double-sided printing.

[0006] The image forming apparatus of the present disclosure is an image forming apparatus capable of forming images on both sides of a recording medium, and is equipped with a drying unit that dries the recording medium after image formation on one side, the drying unit including a heating section that heats the recording medium and a transport section that transports the recording medium while constraining the entire area of ​​the portion of the recording medium that is heated by the heating section, and a processor that controls the drying unit, and the processor controls the drying unit under the condition that the difference in moisture content of the non-image portion of the recording medium before and after heating of the recording medium by the drying unit is 6% or less.

[0007] Preferably, the processor is configured to control the drying unit to achieve a moisture content difference of 3.5% or less.

[0008] The processor may be configured to control the drying unit under conditions of a moisture content difference of 5% or less and a confining pressure of 4 kPa or more.

[0009] The heating unit preferably heats the recording medium from both the front and back sides of one surface.

[0010] The processor may be configured to change settings of the drying unit or issue an alert to change settings based on the information about the recording medium.

[0011] The drying unit may have a plurality of heating sections, including a first heating area having one of the plurality of heating sections, and a second heating area having another heating section, the second heating area being arranged downstream of the first heating area in the transport direction of the recording medium.In this case, it is preferable that the recording medium is transported in a state where its entire surface is restrained while being transported from the first heating area to the second heating area in the drying unit.

[0012] The drying unit may have a plurality of heating sections, including a first heating area having one of the plurality of heating sections, and a second heating area having another heating section, the second heating area being arranged downstream of the first heating area in the transport direction of the recording medium.In this case, the processor may be configured to control the amount of heat applied to the recording medium in the first heating area to be smaller than the amount of heat applied to the recording medium in the second heating area, and to control the restraining pressure that restrains the recording medium in the first heating area to be smaller than the restraining pressure in the second heating area.

[0013] The image forming method disclosed herein is an image forming method in an image forming apparatus capable of forming images on both sides of a recording medium, and includes a drying process in which a recording medium having an image formed on one side is heated and transported, in which the recording medium is transported in a state in which at least the entire heated area of ​​the recording medium is constrained, and the recording medium is heated under conditions in which the difference in moisture content of the non-image area of ​​the recording medium before and after the drying process is 6% or less.

[0014] According to the image forming apparatus and image forming method of the present disclosure, misregistration between the front and back sides during double-sided printing can be suppressed.

[0015] 1 is an overall configuration diagram of an inkjet printing apparatus according to an embodiment; FIG. 2 is an enlarged view of a portion of the inkjet printing apparatus shown in FIG. 1; (a) a side view showing a drying unit, and (b) a plan view of a transport surface; (b) a functional block diagram showing a schematic configuration of a control system of the inkjet printing apparatus; (c) a diagram showing a table showing combinations of paper type, drying conditions, suction pressure, and moisture content differences; (d) a side view showing a drying unit of modified example 1; (e) a side view showing a drying unit of modified example 2; (f) an overall configuration diagram of an inkjet printing apparatus of a modified example;

[0016] Hereinafter, an embodiment of an image forming apparatus and an image forming method according to the present disclosure will be described with reference to the drawings. In each drawing, the same elements are denoted by the same reference numerals.

[0017] "Configuration of Inkjet Printing Apparatus" Fig. 1 is a diagram showing the overall configuration of an inkjet printing apparatus 1 according to an embodiment of the image forming apparatus of the present disclosure. Fig. 2 is an enlarged view of the left half of the inkjet printing apparatus 1 shown in Fig. 1.

[0018] The inkjet printing device 1 is an inkjet color digital printing device that forms a desired image on a sheet of paper P. The inkjet printing device 1 is capable of single-sided printing, in which an image is formed on only one side of the paper P, and double-sided printing, in which an image is formed on both sides of the paper P. The paper P is an example of a recording medium of the technology disclosed herein.

[0019] 1, the inkjet printing apparatus 1 includes a transport mechanism 10, a paper feeding device 20, a pretreatment liquid application unit 30, a pretreatment liquid drying unit 35, an image forming unit 40, a drying unit 50, a cooling unit 60, and an accumulation device 70. Although not shown in FIG. 1, the inkjet printing apparatus 1 also includes a processor 100 (see FIG. 4) as a control device. The cooling unit 60 includes a first cooling unit 61 and a second cooling unit 62.

[0020] The transport mechanism 10 has a transport path 12 along which the paper P is transported. In Fig. 1, the transport path 12 along which the paper P is transported is indicated by a two-dot chain line. The pretreatment liquid application unit 30, the image forming unit 40, the drying unit 50, and the cooling unit 60 are arranged on the transport path 12, and the paper P is transported along the transport path 12 to various units, where it is subjected to various processes.

[0021] The transport path 12 includes a main transport path 13, a supply path 14 that supplies paper to the main transport path 13, a discharge path 15 that discharges paper from the main transport path 13, and a return transport path 16 that branches off from the main transport path 13 and the discharge path 15. The return transport path 16 forms a path that returns paper P that has passed through the main transport path 13 back to the main transport path 13.

[0022] The supply path 14 is connected to the main conveying path 13 at a first connection portion 21. The discharge path 15 is connected to the main conveying path 13 at a second connection portion 22. The starting end of the return conveying path 16 is connected to the main conveying path 13 at the second connection portion 22. The terminal end of the return conveying path 16 is connected to the main conveying path 13 at the first connection portion 21. In this way, the starting end of the return conveying path 16 is connected to the terminal end of the main conveying path 13, and the terminal end is connected to the starting end of the main conveying path 13, forming a circular path together with the main conveying path 13.

[0023] The supply path 14 supplies paper P from the paper feed device 20 to the main transport path 13. One end of the supply path 14 is disposed on the paper feed device 20 side, and the other end is connected to the main transport path 13 by a first connection portion 21. The paper P is supplied from the paper feed device 20 to one end of the supply path 14, transported along the supply path 14, and supplied from the other end of the supply path 14 to the main transport path 13.

[0024] The discharge path 15 transports the paper P from the main transport path 13 to the stacking device 70. One end of the discharge path 15 is connected to the main transport path 13 at the second connection portion 22, and the other end is connected to the stacking device 70. The paper P is discharged from the main transport path 13 to one end of the discharge path 15, transported along the discharge path 15, and discharged from the other end of the discharge path 15 to the stacking device 70.

[0025] The return transport path 16 forms a circular path together with the main transport path 13, and is a path for returning the paper P that has passed through the main transport path 13 back to the main transport path 13. In other words, the return transport path 16 returns the paper P that has passed through the main transport path 13 and has an image formed on its first side by the image forming unit 40 to the image forming unit 40, allowing an image to be formed on its second side.

[0026] The first connection portion 21 is configured to be able to receive the paper P transported from the supply path 14 and the paper P transported from the return transport path 16 into the main transport path 13. The paper P transported from the return transport path 16 is transported to the main transport path 13 in a state where the front and back of the paper P transported from the supply path 14 are reversed.

[0027] The second connection part 22 is provided with a switching mechanism that switches between a state in which the main transport path 13 and the discharge path 15 are connected and a state in which the main transport path 13 and the return transport path 16 are connected. In this example, the switching mechanism provided in the second connection part 22 includes a rotatable branch guide 18 (see FIG. 2 ) and an actuator (not shown) such as a solenoid that rotates the branch guide 18.

[0028] The switching mechanism rotates the branch guide 18 to switch the path of the paper P at the second connection portion 22 between a path from the main transport path 13 toward the discharge path 15 and a path toward the return transport path 16. In other words, the switching mechanism switches between a state in which the paper transported from the main transport path 13 is discharged to the discharge path 15 and a state in which the paper is transported to the return transport path 16 and circulated from the return transport path 16 to the main transport path 13.

[0029] The return transport path 16 is provided with a switchback unit 17 that reverses the traveling direction of the paper P (see FIG. 2). The switchback unit 17 temporarily pulls out the paper P from the return transport path 16 and reverses the traveling direction of the paper P. That is, the leading edge of the paper P in the traveling direction when the paper P was transported on the return transport path 16 before being pulled into the switchback unit 17 becomes the trailing edge in the traveling direction after the paper P is returned from the switchback unit 17 to the return transport path 16. By passing through this switchback unit 17, the front and back of the paper P that is supplied again from the return transport path 16 to the main transport path 13 is reversed.

[0030] A switching mechanism is provided at the connection between the switchback unit 17 and the return transport path 16, which switches between a path for pulling the paper P from the return transport path 16 into the switchback unit 17 and a path for returning the paper from the switchback unit 17 to the return transport path 16. In this example, the switching mechanism provided in the switchback unit 17 includes a rotatable branch guide 19 (see FIG. 2) and an actuator (not shown), such as a solenoid, that rotates the branch guide 19.

[0031] The pretreatment liquid application unit 30, the pretreatment liquid drying unit 35, the image forming unit 40, the drying unit 50, and the first cooling unit 61 are arranged on the main transport path 13, and the second cooling unit 62 is arranged on the return transport path 16.

[0032] The transport mechanism 10 includes a plurality of transport members arranged along the transport path 12. Examples of the plurality of transport members include a transport drum, a belt conveyor, transport roller pairs, chain grippers, and transport guides. The paper feed drum 24, pretreatment liquid application drum 32, pretreatment liquid drying drum 36, imaging drum 42, and belt conveyor 54, which will be described later, are also transport members and constitute part of the transport mechanism 10. The plurality of transport members of the transport mechanism 10 include a roller transport unit 90 including a plurality of belt conveyors 81 to 85 and a plurality of transport roller pairs 91, which are arranged along the transport path 12. The transport mechanism 10 also includes a motor (not shown) as a power source and a drive unit such as a motor drive circuit (not shown). The paper P is transported along the transport path 12 by these elements that constitute the transport mechanism 10. Details of the transport mechanism 10 will be described later.

[0033] The paper feed device 20 includes a paper feed tray on which a stack of multiple sheets of paper P is placed. The type of paper P is not particularly limited, but printing paper primarily made of cellulose, such as high-quality paper, coated paper, and art paper, can be used. The maximum paper size that can be used in the inkjet printing device 1 is, for example, A0 size (841 mm x 1189 mm).

[0034] The paper feeder 20 takes out the sheets P one by one from the stack set therein, starting from the top, and supplies them to the supply path 14 of the transport path 12 .

[0035] The pretreatment liquid application unit 30 applies a pretreatment liquid to the paper P. The pretreatment liquid may be called a "precoat," "preconditioner," "undercoat liquid," or "treatment agent." The pretreatment liquid is a liquid that has the function of aggregating, insolubilizing, or thickening colorant components in ink. The pretreatment liquid application unit 30 includes a pretreatment liquid application drum 32 and a pretreatment liquid application device 33. The pretreatment liquid application drum 32 receives the paper P from the paper feed drum 24 and transports the received paper P to the pretreatment liquid drying unit 35. The pretreatment liquid application drum 32 includes a gripper (not shown) on its circumferential surface. The gripper grips the leading edge of the paper P and rotates, thereby wrapping the paper P around the drum circumferential surface and transporting it.

[0036] The pretreatment liquid application device 33 includes an application roller 34, and applies pretreatment liquid to the paper P transported by the pretreatment liquid application drum 32. The application roller 34 is supported by a movement mechanism (not shown) that is movable between an application position where the application roller 34 comes into contact with the paper P to apply pretreatment liquid to the paper P and a retracted position where the application roller 34 is separated from the paper P and does not apply pretreatment liquid.

[0037] The area where the pretreatment liquid is applied to the paper P may be a full application where the pretreatment liquid is applied to the entire paper P, or a partial application where the pretreatment liquid is applied to a portion of the area where ink is applied in the image forming unit 40. From the viewpoints of uniformly adjusting the amount of pretreatment liquid applied, uniformly recording thin lines and fine image portions, and suppressing density unevenness such as image irregularities, a full application where the pretreatment liquid is applied to the entire image forming surface of the paper P by application using an application roller or the like is preferred.

[0038] The method for applying the pretreatment liquid is not limited to the roller application method, and other methods may be applied to the pretreatment liquid application device 33. Examples of other methods for the pretreatment liquid application device 33 include application using a blade, ejection using an inkjet method, and spraying using a spray method.

[0039] The pretreatment liquid drying unit 35 dries the paper P on which the pretreatment liquid has been applied. The pretreatment liquid drying unit 35 includes a pretreatment liquid drying drum 36. The pretreatment liquid drying drum 36 receives the paper P from the pretreatment liquid application drum 32 and transports the received paper P to the image forming unit 40. The pretreatment liquid drying drum 36 includes a gripper (not shown) on its circumferential surface. The pretreatment liquid drying drum 36 transports the paper P by rotating while gripping the leading edge of the paper P with the gripper. The circumferential surface of the pretreatment liquid drying drum 36 is made of a material with high thermal conductivity, such as metal. The circumferential surface is heated by a heat source, such as a heater, provided inside the circumferential surface, thereby drying the pretreatment liquid while the paper P is transported by the pretreatment liquid drying drum 36.

[0040] The image forming unit 40 includes a printing drum 42 and a head unit 44. The printing drum 42 receives the paper P from the pretreatment liquid drying drum 36 and transports the received paper P to the drying unit 50 via a chain gripper 27 (see FIG. 2 ). The printing drum 42 includes a gripper (not shown) on its circumferential surface, and by gripping the leading edge of the paper P with the gripper and rotating, the paper P is wound around the drum circumferential surface and transported. The printing drum 42 also includes a suction mechanism (not shown), which adsorbs the paper P wound around the drum circumferential surface and transports it. Negative pressure is used for adsorption. The printing drum 42 includes multiple suction holes on its circumferential surface, and by applying suction from the inside of the printing drum 42 through these suction holes, the paper P is adsorbed to the circumferential surface of the printing drum 42.

[0041] The head unit 44 includes inkjet heads 46C, 46M, 46Y, and 46K. The inkjet head 46C is a recording head that ejects droplets of cyan ink. The inkjet head 46M is a recording head that ejects droplets of magenta ink. The inkjet head 46Y is a recording head that ejects droplets of yellow ink. The inkjet head 46K is a recording head that ejects droplets of black ink. Each of the inkjet heads 46C, 46M, 46Y, and 46K is supplied with ink from an ink tank (not shown), which is an ink supply source of the corresponding color, via a piping path (not shown). For example, a water-based ink is used as the ink for drawing. Water-based ink refers to ink in which a coloring material such as a pigment or dye is dissolved or dispersed in water and / or a water-soluble solvent.

[0042] Ink droplets are ejected from at least one of the inkjet heads 46C, 46M, 46Y, and 46K toward the paper P being transported by the drawing drum 42, and the ejected droplets adhere to the paper P, thereby forming an image on the paper P.

[0043] In this example, a configuration using four ink colors, CMYK, is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment, and light ink, dark ink, special color ink, etc. may be added as needed. For example, a configuration is also possible in which inkjet heads that eject light-colored inks such as light cyan and light magenta are added, and / or inkjet heads that eject special color inks such as green, orange, or white are added. Furthermore, the arrangement order of the inkjet heads of each color is not particularly limited.

[0044] The drying unit 50 performs a drying process by applying heat to the paper P on which an image has been formed by the image forming unit 40 to dry the ink while transporting the paper P. The drying unit 50, for example, includes a belt conveyor 54 equipped with a heating belt 51 and a heater 57 positioned opposite the transport surface of the heating belt 51. In addition to the heating belt 51, the belt conveyor 54 includes a drive roller 52 and a driven roller 53. The belt conveyor 54, which transports the paper P, constitutes part of the transport mechanism 10. The heating belt 51 is an endless transport belt having a transport surface made of a highly thermally conductive material such as metal. The back side of the transport surface is heated by a heat source such as a heater via a suction box 55 (described below). The paper P is heated by the heating belt 51 and the heater 57 while being transported along the heating belt 51. The temperature of the transport surface of the heating belt 51 is set to a desired temperature, for example, in the range of 80°C to 150°C, and can be changed as needed. Similarly, by changing the output of the heater 57, the heating temperature can be changed and the amount of heat applied to the paper P can be adjusted. The heater 57 and the heating belt 51 are an example of a heating unit in the present disclosure, and in this example, the heating unit heats the paper P from both the front side and the back side. The belt conveyor 54 is an example of a transport unit in the present disclosure. As the heater 57, convection heating means such as a hot air blower, radiant heating means such as an infrared (IR) lamp, an ultraviolet (UV) lamp, or a microwave generator, a superheated steam generator, etc. can be used.

[0045] FIG. 3A shows a side view of the drying unit 50, and FIG. 3B shows a plan view of the conveying surface of the heating belt 51. As shown in FIG. 3B, the heating belt 51 has a plurality of suction holes 51a for suctioning and conveying the paper P. A suction box 55 is disposed in the space between the drive roller 52 and the driven roller 53 on the back side of the conveying surface of the heating belt 51. The suction box 55 is connected to an exhaust pump (not shown). A vacuum blower such as a ring blower can be used as the exhaust pump. The suction box 55 generates suction pressure in the suction holes of the heating belt 51, thereby adsorbing the paper P to the conveying surface. The suction box 55 is made of a highly thermally conductive material such as metal, and transfers heat from a heater (not shown) to the heating belt 51.

[0046] As shown in Figure 3(b), the conveying surface of the heating belt 51 is provided with a plurality of suction holes 51a across the entire paper support area that supports the paper P. In other words, the conveying surface is composed of suction holes 51a and non-suction hole portions 51b that are not suction holes 51a. The diameter and arrangement of the suction holes 51a are determined from the perspective of achieving good suction of the paper P. The diameter of the suction holes 51a is, for example, 0.1 to 1.0 mm, and more preferably 0.2 to 0.5 mm. In this example, the suction holes 51a are arranged in a staggered pattern.

[0047] In Figure 3, paper P is transported in the transport direction A indicated by the arrow. The back side of paper P placed on heating belt 51 is adsorbed by suction pressure generated in suction holes 51a via suction box 55 located below the transport surface. In transport direction A, paper P is constrained by suction from position X1 where it is placed on heating belt 51. Suction box 55 transfers heat from a heater to heating belt 51. Here, heating of paper P begins at position X2, the upstream end of suction box 55, and ends at position X3 on the downstream side. The constraint of paper P due to suction is released at position X4 where it is discharged from heating belt 51. In other words, belt conveyor 54, which is the transport unit, transports paper P while constraining the entire heated portion of paper P.

[0048] It is sufficient that the heated portion of the transported paper P is restrained, and in the example shown in Figure 3, it is sufficient that the portion located between position X2 and position X3 is restrained.

[0049] The paper P is transferred from the imaging drum 42 to the chain gripper 27 (see FIG. 2), not shown in FIG. 1, and placed on the heating belt 51 with the leading edge of the paper P gripped by the gripper 27a, and is adsorbed to the heating belt 51. The leading edge of the paper P gripped by the gripper 27a is a non-image forming portion where no image is formed. When the paper P is adsorbed to the heating belt 51, the gripper 27a releases the paper P, and the paper P is transported only by the heating belt 51. As a result, the paper P is heated and dried by the heating belt 51 and heater 57 while being transported by the heating belt 51.

[0050] As described above, the cooling unit 60 includes a first cooling unit 61 and a second cooling unit 62. The first cooling unit 61 is disposed on the main transport path 13 after the drying unit 50. The first cooling unit 61 includes, for example, a blower. The second cooling unit 62 is disposed on the return transport path 16, and during double-sided printing, further cools the paper P cooled by the first cooling unit 61. In this example, the second cooling unit 62 is disposed on the switchback section 17 after the switchback section 17. The second cooling unit 62 includes, for example, a blower.

[0051] The first cooling unit 61 and the second cooling unit 62, for example, blow room temperature air (about 25°C) onto the paper P. By cooling the paper P after it has been dried in the drying unit 50, the ink film is solidified and the evaporation of moisture is promoted. Either or both of the first cooling unit 61 and the second cooling unit 62 may blow cooling air (about 0 to 24°C) that is lower in temperature than room temperature air onto the paper P.

[0052] The stacking device 70 stacks the image-formed sheets P. The stacking device 70 receives the sheets P discharged from the discharge path 15 of the conveyance path 12, and stacks the sheets P in a bundle on a stacking tray (not shown).

[0053] Here, the transport mechanism 10 will be further described. As described above, the transport mechanism 10 includes a plurality of transport members, which transport the paper P along the transport path 12. As shown in FIG. 1 , a plurality of transport roller pairs 25 and a transport guide 26 are arranged on the most upstream side of the supply path 14 and the main transport path 13, and the paper P is transported by the transport roller pairs 25. The configuration of the transport roller pairs 25 is substantially the same as the configuration of a transport roller pair 91, which will be described later. Furthermore, the paper P is transported along the main transport path 13 by the paper feed drum 24, the pretreatment liquid application drum 32, the pretreatment liquid drying drum 36, and the imaging drum 42.

[0054] As described above, the paper P on which the image has been formed by the image forming drum 42 is transported along the transport guide 28 gripped by the chain gripper 27 and delivered to the heating belt 51 of the drying unit 50 .

[0055] The transport mechanism 10 includes a roller transport section 90 including a plurality of belt conveyors 81 to 85 and a plurality of transport roller pairs 91 along the transport path 12 as transport members for transporting the paper P discharged from the drying unit 50 .

[0056] As shown in FIG. 2, the belt conveyors 81 to 85 each include endless conveyor belts 81a to 85a, drive rollers 81b to 85b, and driven rollers 81c to 85c.

[0057] Each of the conveyor belts 81a to 85a has a plurality of suction holes for suctioning and conveying the paper P. The belt conveyors 81 to 85 are equipped with suction boxes (not shown) in the spaces between the drive rollers 81b to 85b and the driven rollers 81c to 85c on the backside of the conveyor belts 81a to 85a. The suction boxes are connected to exhaust pumps (not shown). A vacuum blower such as a ring blower can be used as the exhaust pump. The suction boxes generate suction pressure in the suction holes of the conveyor belts 81a to 85a, thereby adsorbing the paper P to the conveyor surface. The belt conveyors 81 to 85 transport the paper P by adsorbing it to the conveyor surface. While the paper P is being transported by the belt conveyors 81 to 85, the conveyor members do not come into contact with the image forming surface on which the image is formed.

[0058] The belt conveyor 81 is disposed from the main conveying path 13 to the return conveying path 16, and is provided with a guide roller 81d for changing the direction of travel at a second connecting portion 22 connecting the main conveying path 13 and the return conveying path 16, and an auxiliary roller 81e for assisting the conveyance of the conveyor belt 81a. The belt conveyor 81 has a conveying surface that is disposed horizontally along the main conveying path 13, and a conveying surface that is disposed inclined relative to the horizontal along the return conveying path 16.

[0059] The belt conveyor 82 is disposed on the discharge path 15. The conveying surface of the belt conveyor 82 is disposed horizontally. The conveying surface of the belt conveyor 82 is substantially flush with the horizontal conveying surface of the belt conveyor 81 along the main conveying path 13. The belt conveyor 82 receives the paper P traveling in a straight line from the horizontal conveying surface of the belt conveyor 81, and conveys it to the stacking device 70.

[0060] The belt conveyor 83 constitutes the switchback section 17. The conveying surface of the belt conveyor 83 is arranged horizontally. The belt conveyor 83 draws the paper P from the return conveying path 16 into the switchback section 17, and discharges the paper P into the return conveying path 16 by rotating the drive roller 83b in the reverse direction.

[0061] The belt conveyor 84 is disposed at a position where it receives the paper P discharged from the belt conveyor 83 of the switchback section 17, with its conveying surface inclined relative to the horizontal.

[0062] The belt conveyor 85 is disposed at a position where it receives the paper P transported by the belt conveyor 84, with its transport surface being horizontal.

[0063] In this way, multiple belt conveyors 81 to 85 are provided following the belt conveyor 54 in the drying unit 50. In other words, the transport mechanism 10 is configured to transport the paper P, which has passed through the image forming unit 40 and has an image formed on one side, without contacting the transport member while transporting the paper P to a position where it passes through the second cooling unit 62. Here, "one side" refers to the first side of the two sides of the paper P that is printed first, and this first side that is printed first may also be referred to as the front side of the paper P. The side of the paper P opposite to the first side may also be referred to as the second side or the back side of the paper P.

[0064] Here, the belt conveyors 54, 81 to 85 all use suction to attract the paper P to the transport surface and transport it, but the method of attracting the paper P to the transport surface may also be electrostatic attraction or the like.

[0065] In addition, while the conveying mechanism 10 conveys the paper P that has passed through the image forming unit 40 and has an image formed on one side to a position where it passes through the second cooling unit 62, the conveying means for conveying the paper P without it coming into contact with the conveying member is not limited to a belt conveyor.

[0066] As shown in Figure 1, a roller conveyance unit 90 is provided downstream of the belt conveyor 85 on the return conveyance path 16. The roller conveyance unit 90 includes a plurality of conveyance roller pairs 91 and a conveyance guide 92 arranged along the return conveyance path 16. The conveyance roller pair 91 is made up of two rollers that press against each other, an upper roller 91a with a relatively small diameter and a lower roller 91b with a relatively large diameter (see Figure 2). The conveyance roller pair 91 sandwiches the paper P between the upper roller 91a and the lower roller 91b and sends it downstream in the conveyance direction.

[0067] The transport mechanism 10 further includes, as transport members, a transport drum 94 and guide rollers 95 and 96 at the end of the roller transport section 90. The transport drum 94 has a gripper (not shown) on its circumferential surface, and by gripping the leading end of the paper P with the gripper and rotating, the paper P is wrapped around the drum circumferential surface and transported. The transport drum 94 re-supplies the paper P to the main transport path 13 at the first connection section 21.

[0068] 4 is a functional block diagram showing a schematic configuration of a control system of the inkjet printing apparatus 1. In addition to the processor 100, the inkjet printing apparatus 1 includes a storage device 102, a communication unit 104, an input device 106, and a display device 108.

[0069] The processor 100 includes a CPU (Central Processing Unit). The processor 100 functions as a processing unit and / or a control unit that performs various processes by executing instructions of a program stored in the storage device 102. The processor 100 comprehensively controls the conveyance mechanism 10, the paper feed device 20, the pretreatment liquid application unit 30, the pretreatment liquid drying unit 35, the image forming unit 40, the drying unit 50, the cooling unit 60, and the stacking device 70.

[0070] The storage device 102 is a non-transitory storage medium and a tangible computer-readable medium. The storage device 102 includes a memory serving as a main storage device and a storage serving as an auxiliary storage device. The storage device 102 may be, for example, a semiconductor memory, a hard disk drive (HDD), a solid state drive (SSD), or a combination of these. A part or all of the storage area of ​​the storage device 102 may be included in the processor 100.

[0071] The storage device 102 stores various parameters used in the inkjet printing apparatus 1 and programs used in each section of the inkjet printing apparatus 1. The storage device 102 also functions as a temporary storage section for various data including image data.

[0072] Various parameters stored in the storage device 102 are read out via the processor 100 and set in each part of the device. Various programs stored in the storage device 102 are read out via the processor 100 and executed in each part of the device.

[0073] The communication unit 104 has a required communication interface. The inkjet printing apparatus 1 is connected to the host computer 110 via the communication unit 104, and can send and receive data to and from the host computer 110. Here, "connection" includes a wired connection, a wireless connection, or a combination of these. The communication unit 104 may be equipped with a buffer memory for speeding up communication processing. The communication unit 104 serves as an image input interface unit for acquiring image data representing an image to be printed. The image data acquired from the host computer 110 via the communication unit 104 is stored in the storage device 102.

[0074] The input device 106 is configured by, for example, operation buttons, a keyboard, a mouse, a touch panel, a multi-touch screen, other pointing devices, a voice input device, or an appropriate combination of these. The input device 106 accepts various inputs from an operator.

[0075] The display device 108 is configured by, for example, a liquid crystal display, an organic electro-luminescence (OEL) display, a projector, or an appropriate combination of these.

[0076] Information input via the input device 106 is sent to the processor 100. The processor 100 causes each unit to execute various processes in accordance with the information input from the input device 106. Information input from the input device 106 includes the print mode (single-sided printing or double-sided printing), the type of paper P, and the like.

[0077] The display device 108 can display various information such as various setting information of the device or abnormality information in response to commands from the processor 100. A user (operator) can set various parameters and input and edit various information using the input device 106 while viewing the content displayed on the display device 108.

[0078] The inkjet printing device 1 is capable of single-sided printing and double-sided printing, and is configured to be able to selectively switch between a single-sided printing mode and a double-sided printing mode. Depending on which mode is selected, the transport path is switched, and the paper P is transported along the transport path appropriate for each mode.

[0079] In the single-sided printing mode, the paper P is transported along a path that passes through the supply path 14, the main transport path 13, and the discharge path 15. More specifically, the paper P fed from the paper feed device 20 to the supply path 14 is transported to the main transport path 13, and along the main transport path 13, the pretreatment liquid is applied to the first side by the pretreatment liquid application unit 30, the pretreatment liquid is dried by the pretreatment liquid drying unit 35, an image is formed on the paper by the image forming unit 40, the drying unit 50 drys the paper, and the first cooling unit 61 cools the paper, in this order. Thereafter, the paper P with the image printed on its first side is transported to the discharge path 15 and discharged to the stacking device 70.

[0080] In the double-sided printing mode, the paper P is transported along a path that passes through the supply path 14, the main transport path 13, the return transport path 16, the main transport path 13, and the discharge path 15 in this order. Specifically, the paper P fed from the paper feed device 20 to the supply path 14 is transported to the main transport path 13, where the application of pretreatment liquid to the first side of the paper P, drying of the pretreatment liquid, image formation on the first side, drying, and cooling are performed in this order. The paper P is then transported from the main transport path 13 to the return transport path 16 and is turned over to its front and rear ends by passing through the switchback unit 17. The paper P with its front and rear ends turned over is then transported along the return transport path 16. In the return transport path 16, the paper P is cooled by the second cooling unit 62. The paper P cooled by the second cooling unit 62 is returned from the return transport path 16 to the main transport path 13. When the paper P is returned to the main transport path 13, it is turned over so that the second side of the paper P becomes the image formation side. The application of the pretreatment liquid to the second side, drying of the pretreatment liquid, image formation on the second side, drying, and cooling are carried out in this order on the main transport path 13. Then, the paper P with images printed on both sides is transported from the main transport path 13 to the discharge path 15 and discharged to the stacking device 70.

[0081] In the inkjet printing device 1, in double-sided printing mode, after forming an image on one side (first side) of the paper P, the drying unit 50 conveys the paper P while constraining at least the entire heated area of ​​the paper P, and heats the paper P under conditions where the difference in moisture content of the non-image portion of the paper P before and after the drying process is 6% or less. The drying unit 50 is controlled by the processor 100, and operates under conditions where the difference in moisture content of the non-image portion of the paper P before and after the drying process is 6% or less. Control of the drying unit 50 by the processor 100 will be described later.

[0082] "Processing by Processor 100" The processor 100 causes each unit to execute various processes in accordance with information input from the input device 106.

[0083] For example, the processor 100 receives a designation of single-sided printing or double-sided printing from the input device 106 and sets the mode to single-sided printing or double-sided printing. That is, the processor 100 controls the conveying mechanism 10 to switch the conveying path to a path for single-sided printing or a path for double-sided printing. The path switching is achieved by a path switching using the branch guide 18 of the switching mechanism of the second connection unit 22.

[0084] The transport mechanism 10 includes elements such as transport members and a power source involved in transporting the paper P from the paper feed device 20 to the stacking device 70 described in Fig. 1. The processor 100 controls each element of the transport mechanism 10 so as to transport the paper P from the paper feed device 20 to the stacking device 70 according to the set transport path. The processor 100 also controls the paper feed device 20 to start feeding the paper P and stop feeding the paper P.

[0085] The processor 100 performs various types of image processing, such as conversion processing, correction processing, and halftone processing, on image data to be printed.

[0086] The processor 100 operates the pretreatment liquid application unit 30 and the pretreatment liquid drying unit 35. The processor 100 controls the application operation of the pretreatment liquid application device 33, such as the amount and timing of application of the pretreatment liquid. The processor 100 controls the pretreatment liquid drying unit 35 to control the drying output and / or drying time. The drying output is the output of the heat source, and, for example, if the heat source is a heater, it is the heater output, and if the heat source is a hot air blower, it is the temperature and flow rate of the hot air blown out from the hot air blower.

[0087] The processor 100 operates the image forming unit 40 to form an image on the paper P based on the image data stored in the storage device 102. The processor 100 controls the ejection operations of the inkjet heads 46C, 46M, 46Y, and 46K based on the dot data of each ink color generated through image processing, so as to record an image on the paper P being transported by the image recording drum 42.

[0088] The processor 100 also controls and operates the drying unit 50. The processor 100 changes the drying conditions of the drying unit 50 and the confining pressure of the paper P during heated transport. The drying conditions are, for example, the heating output of the heating unit and the transport speed of the transport unit. The heating output of the heating unit is, for example, the IR heater output if the heat source is an IR heater, and the temperature and flow rate of the hot air blown out from the hot air blower if the heat source is a hot air blower. In this example, the confining pressure of the paper P is the suction pressure that adsorbs the transported paper P to the transport surface of the heating belt 51, and can be controlled by adjusting the exhaust power of the exhaust pump.

[0089] At least in double-sided printing mode, processor 100 controls drying unit 50 under conditions such that the difference in moisture content of the non-image portion of paper P before and after heating by drying unit 50 after image formation on one side (front side) of paper P by image forming unit 40 is 6% or less. Specifically, the drying conditions by drying unit 50 are adjusted so that the difference in moisture content of the non-image portion of paper P before and after heating is 6% or less. It is more preferable that processor 100 controls drying unit 50 under conditions such that the difference in moisture content is 3.5% or less.

[0090] The processor 100 may be configured to control the drying unit 50 under the conditions that the moisture content difference is 5% or less and the confining pressure is 4 kPa or more. The processor 100 may be configured to control the drying unit 50 under the conditions that the moisture content difference is 3.5% or less and the confining pressure is 1 kPa or more and 10 kPa or less.

[0091] "Non-image area" refers to an area on the paper P where no image is formed, i.e., an area on which no ink is applied. The area on the paper P on which ink is applied is evaluated based on the difference in moisture content before and after heating, as it is difficult to evaluate the difference in moisture content in the non-image area.

[0092] Here, before and after heating refers to before and after the drying unit 50 heats the paper P on which an image has been formed on the first side during double-sided printing, meaning before and after the drying process by the drying unit 50. If the moisture content difference is ΔW, the moisture content of the non-image area of ​​the paper P before heating in the drying unit 50 is W1, and the moisture content of the non-image area of ​​the paper P after heating is W2, then ΔW is expressed as ΔW=W1-W2. The moisture content can be calculated by measuring the amount of moisture using, for example, the Karl Fischer method, but the measurement method is not limited to this.

[0093] In the present embodiment, when processing such as applying a pretreatment liquid to a sheet of paper P and drying the pretreatment liquid before image formation is performed, the moisture content W1 of the non-image portion of the sheet of paper P before heating by the drying unit 50 is the moisture content W1 of the non-image portion of the sheet of paper P after drying the pretreatment liquid and before heating by the drying unit 50. "After heating" refers to the period after passing through the drying unit 50 and before other processing such as cooling or image formation on the other side of the sheet of paper P is performed. In this example, the "after heating" period refers to the period after passing through the drying unit 50 and before cooling by the first cooling unit 61. Therefore, the moisture content W2 of the non-image portion of the sheet of paper P after heating is the moisture content W2 of the non-image portion of the sheet of paper P after passing through the drying unit 50 and before cooling by the first cooling unit 61.

[0094] Note that if no processing that affects the moisture content of the paper P, such as applying a pretreatment liquid or drying the pretreatment liquid, is performed on the paper P between paper feeding and image formation, the moisture content of the paper P will remain substantially unchanged during this period. Furthermore, even after an image is formed on one side of the paper P, the moisture content of the non-image portion will remain substantially unchanged. Therefore, if the pretreatment liquid is not applied to the paper P and the pretreatment liquid is not dried before image formation, the moisture content of the paper P before loading into the paper feeding device 20, i.e., before printing, can be considered to be the moisture content W1 of the non-image portion of the paper P before heating in the drying unit 50.

[0095] An example of a configuration for setting the difference in moisture content of the non-image portion of the paper P before and after heating by the drying unit 50 after an image is formed on one side of the paper P to a predetermined value of 6% or less in the inkjet printing device 1 will be described.

[0096] For example, a table T such as that shown in FIG. 5 is created and stored in advance in the storage device 102, which contains information about the recording medium, such as the type of paper P used (specifically, the brand, type, and thickness of the paper P), as well as the drying conditions, suction pressure, and moisture content difference ΔW. The processor 100 then references the table T and sets the drying conditions, such as the heating temperature and conveyance speed, and the suction pressure, based on the input information about the paper P (here, the brand, type, and thickness). Table T shown in FIG. 5 indicates, for example, that for paper of brand A, glossy type, and thickness X, ΔW is 1% to 5% under drying conditions A1 to A5. Table T specifies the suction pressure α as a value sufficient to suppress shrinkage of paper with a moisture content difference ΔW of 5% or less before and after heating for paper of brand A, glossy type, and thickness X. Furthermore, even for the same paper type, different adsorption pressures may be specified for each moisture content difference ΔW, such as setting a higher adsorption pressure for a larger moisture content difference ΔW and a lower adsorption pressure for a smaller moisture content difference ΔW.

[0097] The processor 100 may be configured to refer to Table T and, if it determines from the input information about the recording medium that the moisture content difference ΔW would exceed a threshold value if dried under the current drying conditions, change the drying conditions to ones that will make the moisture content difference ΔW equal to or less than the threshold value. Alternatively, the processor 100 may be configured to refer to Table T and, if it determines from the input information about the recording medium that the moisture content difference ΔW would exceed a threshold value if dried under the current drying conditions, prompt the user to change the drying conditions, for example, by displaying an alert on the display device 108. Here, the threshold value is a preset value such as 6%, 5%, 4%, or 3.5%. Note that the "current drying conditions" here refer to the drying conditions set before the print settings for the paper P to be printed on are made, such as the default drying conditions or the drying conditions used in the most recent print job.

[0098] When an unknown recording medium is used, the processor 100 may be configured to reference conditions for similar paper types in a table stored in the storage device 102. For example, if the unknown recording medium is paper of an unknown brand but with information on paper type and thickness, the processor 100 determines, based on the input paper type and thickness information, the moisture content difference ΔW of the paper as determined by a paper type having a paper type and thickness similar to the input paper type and thickness in the stored table and the current drying conditions (e.g., default drying conditions). If the moisture content difference ΔW exceeds a threshold, the processor 100 may extract conditions for that paper type that result in a moisture content difference ΔW within the threshold, and automatically change the drying conditions or prompt the user to change the drying conditions by, for example, displaying an alert on the display device 108. Note that the "paper types" shown in Table T are merely examples. Even if the same gloss paper has different characteristics, it may be distinguished as "Gloss 1" and "Gloss 2."

[0099] Instead of setting the drying conditions and / or suction pressure by referencing Table T as described above, the processor 100 may perform a test print of the same paper type prior to the actual printing run and set the drying conditions and suction pressure in the drying unit 50. For example, as shown in FIG. 3 , moisture content meters 58 and 59 that measure moisture content non-contact are installed upstream and downstream of the drying unit 50 on the conveyance path. Then, during the test print run, the processor 100 calculates the moisture content difference ΔW between the moisture content W1 obtained from the upstream moisture content meter 58 and the moisture content W2 obtained from the downstream moisture content meter 59. If ΔW exceeds a threshold value (e.g., 3.5%), the processor 100 may change the drying conditions to reduce the amount of heat during heating and / or increase the suction pressure. Alternatively, the processor 100 may prompt the user to change the drying conditions or suction pressure by displaying an alert on the display device 108, for example.

[0100] The processor 100 operates the cooling unit 60. The processor 100 controls the cooling output and / or cooling time of the first cooling unit 61 and the second cooling unit 62. If the cooling source is a fan, the cooling output refers to the temperature and volume of air blown out from the fan.

[0101] The hardware structure of the processor 100 may be any of the various processors listed below. The various processors include a CPU, which is a general-purpose processor that executes software (programs) and functions as various processing units, as well as dedicated electrical circuits such as a PLD (Programmable Logic Device) that can change its circuit configuration after manufacture, such as an FPGA (Field-Programmable Gate Array), and an ASIC (Application Specific Integrated Circuit), which is a processor with a circuit configuration designed specifically for executing specific processing.

[0102] The above-described processing may be performed by one of these various processors, or by a combination of two or more processors of the same or different types (e.g., a plurality of FPGAs, or a combination of a CPU and an FPGA). Furthermore, a plurality of processing units may be configured by a single processor. An example of configuring a plurality of processing units by a single processor is a system-on-chip (SOC), in which a processor is used to realize the functions of an entire system including a plurality of processing units on a single IC (Integrated Circuit) chip.

[0103] Furthermore, more specifically, the hardware structure of these processors can be an electric circuit that combines circuit elements such as semiconductor elements.

[0104] As described above, the inkjet printing apparatus 1 according to this embodiment includes a drying unit 50 that dries a recording medium (paper P in this example) on one side of which an image has been formed. The drying unit 50 includes a heating section (heating belt 51 and heater 57 in this example) that heats the recording medium and a transport section (belt conveyor 54 in this example) that transports the recording medium while constraining the entire area of ​​the recording medium heated by the heating section. The inkjet printing apparatus 1 also includes a processor 100 that controls the drying unit 50. The processor 100 controls the drying unit 50 under the condition that the difference in moisture content of the non-image area of ​​the recording medium before and after heating by the drying unit 50 is 6% or less. By transporting the recording medium while constraining the entire heated area and by maintaining a moisture content difference of 6% or less before and after heating, shrinkage of the recording medium, such as paper, due to moisture loss during heating can be suppressed. As a result, size changes during front-side printing can be suppressed, thereby suppressing misregistration between the front and back sides after back-side printing. Furthermore, for example, if the recording medium is paper, if there is even a partially unconstrained portion of the heated area, the paper will shrink in the unconstrained portion, causing uneven shrinkage across the paper surface, resulting in misregistration between the front and back of the paper in some areas, or the paper may wrinkle when absorbed during the transport process after heating, or paper jamming or other paper feed problems. However, in the inkjet printing device 1 of this embodiment, the entire heated area is constrained, preventing uneven shrinkage of the paper and paper feed problems. This configuration allows for both high-speed drying and reduced misregistration between the front and back of the paper.

[0105] One possible way to prevent misregistration between front and back sheets is to define a range for the moisture content W2 of the paper after heating. However, the moisture content W2 depends on the moisture content W1 before heating, which varies depending on, for example, the paper brand, paper type, paper thickness, and storage environment. Therefore, the amount of paper shrinkage cannot be uniquely determined by the moisture content W2 after heating. On the other hand, the difference in moisture content ΔW before and after heating can be pre-matched to the amount of paper shrinkage, allowing for back-side registration to be performed according to the amount of shrinkage, thereby preventing misregistration between front and back sheets.

[0106] By appropriately setting the suction pressure under drying conditions where the moisture content difference ΔW before and after heating is 6% or less, it is possible to suppress shrinkage of the paper after drying and prevent misregistration between the front and back of the paper.

[0107] If the processor 100 is configured to control the drying unit 50 under the condition that the moisture content difference is 3.5% or less, the shrinkage rate can be sufficiently suppressed even if the restraining pressure required to restrain the recording medium is reduced, and misregistration between the front and back sides can be suppressed. From the perspective of sufficiently drying the ink, it is more preferable that the moisture content difference ΔW be 2% or more.

[0108] In the inkjet printing apparatus 1 of this embodiment, the drying unit 50 includes a heating belt 51 and a heater 57 as a heating section, and the paper P transported by the heating belt 51 is heated from both the front and back sides of the image-formed surface. However, the drying unit 50 is not limited to this configuration. It may also include only the heating belt 51, which heats the paper P from the back side, or only the heater 57, which heats the paper P from the front side. However, as in this embodiment, a heating section that heats the paper P from both the front and back sides of the image-formed surface allows for faster drying than heating from only one of the front and back sides. Compared to heating from only one of the front and back sides, the amount of heat applied to the paper per unit time can be increased, thereby achieving the heating required to dry the ink even when transported at high speed. Furthermore, if the amount of heat applied per unit time increases, the shrinkage rate of the paper P increases if the paper P is not constrained. However, because the entire heated area of ​​the paper P is constrained, shrinkage of the paper P can be suppressed even when the amount of heat applied per unit time increases. Therefore, in the drying unit 50, the heating section is configured to heat the paper P from both the front and back sides, and if the paper P is transported with the heated area of ​​the paper P constrained, misalignment between the front and back can be suppressed even during high-speed printing.

[0109] As a heating unit that heats the recording medium from both sides, it is particularly preferable to combine heat transfer heating from a restraining surface such as a heating belt with convection heating or radiation heating from the front surface, as in this embodiment. In particular, as a heating method to be combined with heat transfer heating, a method that does not excessively heat the paper and reduces the moisture contained in the paper as much as possible, such as convection heating by blowing warm air, is desirable.

[0110] The ink used for image formation is not limited to aqueous ink, but aqueous ink is particularly effective because a larger amount of heat is applied to the paper as a post-treatment to volatilize the water after application of the ink, compared to other ink types such as UV (ultraviolet) curable ink.

[0111] In the above embodiment, the method of restraining the paper P in the drying unit 50 has been described as a vacuum suction method in which the paper P is attracted to the suction holes in the heating belt, but the restraining method is not limited to this. Electrostatic suction may also be used, or tension may be applied to the paper by pulling both ends of the paper with a tensioning jig, thereby restraining the paper on the support surface of the paper back surface support member.

[0112] The back surface support member that supports and restrains the paper from the back surface is not limited to a conveying belt such as a heating belt, but may be a plate-like member such as a platen, a conveying drum, or the like.

[0113] Furthermore, instead of the drying unit 50, a drying unit 150 according to a first modification shown in Fig. 6 or a drying unit 150A according to a second modification shown in Fig. 7 may be provided. In Fig. 6 and Fig. 7, components equivalent to those shown in Fig. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

[0114] The drying unit 150 of Modification 1 shown in FIG. 6 includes multiple (two in this example) heating sections 151, 152 arranged at different positions in the conveying direction A. The drying unit 150 includes a first heating area HA1 including one of the heating sections 151 and a second heating area HA2 including the other heating section 152. The second heating area HA2 is arranged downstream of the first heating area HA1 in the conveying direction A of the paper P. The heating section 151 includes a first belt conveyor 54a and a first heater 57a. The heating section 152 includes a second belt conveyor 54b and a second heater 57b. The first belt conveyor 54a and the second belt conveyor 54b have the same configuration as the belt conveyor 54 shown in FIG. 1. That is, each includes a heating belt 51, a drive roller 52, and a driven roller 53. In each of the heating units 151 and 152, a suction box 55 connected to an exhaust pump (not shown) is disposed in the space between the drive roller 52 and the driven roller 53 below the conveying surface of the heating belt 51.

[0115] In this way, when the drying unit 150 has multiple heating zones HA1 and HA2 in the conveying direction A, it is preferable that the processor 100 controls the amount of heat applied to the paper P in the first heating zone HA1 located upstream to be smaller than the amount of heat applied to the paper P in the second heating zone HA2, and controls the confining pressure (here, suction pressure) that constrains the paper P in the first heating zone HA1 to be smaller than the confining pressure in the second heating zone HA2. In this case, the confining pressure in the first heating zone HA1 may be set to zero, so that the paper P is not constrained. Note that the first heater 57a of the heating section 151 may be a heater with a smaller output than the second heater 57b of the heating section 152.

[0116] When forming an image using aqueous ink, particularly when using a single-component aggregation-type ink without the application of a pretreatment liquid, the ink may have high fluidity after image formation. When the ink has high fluidity after image formation, on a conveyance surface having suction holes 51a, such as the conveyance surface of the heating belt 51 shown in FIG. 1, density unevenness may occur when the ink dries due to heating due to the temperature difference between the suction holes 51a and the non-suction hole portions 51b where the suction holes 51a are provided. Density unevenness is particularly likely to occur on paper types that are difficult for ink to penetrate. However, as described above, by performing a stepwise heating process in which the ink fluidity is eliminated in the first heating region HA1, which dries the ink with a low heat amount and low restraining pressure, and then the ink is thoroughly dried in the second heating region HA2, it is possible to prevent density unevenness caused by the temperature difference between the suction holes 51a and the non-suction hole portions 51b while suppressing misregistration.

[0117] In addition, if there are multiple heating areas (here, the first heating area HA1 and the second heating area HA2), the restraint of the paper P may be released while it is transported from the first heating area HA1 to the second heating area HA2, or the paper P may be kept restrained on its entire surface.

[0118] In the drying unit 150 shown in Figure 6, while the paper P is being transferred between the first belt conveyor 54a and the second belt conveyor 54b, no suction force is applied to the paper P and the paper P is in a released state.

[0119] The drying unit 150A of Modification 2 shown in FIG. 7 includes a first heating area HA1 and a second heating area HA2, but only one belt conveyor 154 as a conveying section. The belt conveyor 154 includes a conveying belt 156, a driving roller 157, and a driven roller 158. A suction box 155 connected to an exhaust pump (not shown) is disposed in the space between the driving roller 157 and the driven roller 158 below the conveying surface of the conveying belt 156. The conveying belt 156 has multiple suction holes on its conveying surface, similar to the heating belt 51 shown in FIG. 3, and conveys the paper P by suction from the rear side. However, in the example shown in FIG. 7, no heater is provided below the conveying belt 156, and the conveying belt 156 is not used as a heating belt for heat transfer. That is, the heating section 151 includes only the first heater 57a, and the heating section 152 includes only the second heater 57b as a heating means. In this example, the sheet P is conveyed by the belt conveyor 154 from the first heating area HA1 to the second heating area HA2. At this time, the sheet P is conveyed with its entire surface adsorbed to the conveyance surface from the first heating area HA1 to the second heating area HA2. That is, in this example, the sheet P is conveyed with its entire surface restrained, even in the unheated area between the first heating area HA1 and the second heating area HA2. In this way, when multiple heating areas are provided, shrinkage of the sheet P can be more effectively suppressed by conveying the sheet P between the heating areas in a restrained state.

[0120] The image forming apparatus according to the present disclosure may be configured without the pretreatment liquid application unit 30 and the pretreatment liquid drying unit 35, as in the modified inkjet printing apparatus 2 shown in Fig. 8. In Fig. 8, the same components as those in the inkjet printing apparatus 1 shown in Fig. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted. The inkjet printing apparatus 2 does not include the pretreatment liquid application unit 30 and the pretreatment liquid drying unit 35, but the other components are similar to those of the inkjet printing apparatus 1 described above.

[0121] Furthermore, although the inkjet printing apparatuses 1 and 2 shown in Figures 1 and 8 are provided with a cooling unit 60, they may be configured without the cooling unit 60, or may be configured with only the first cooling unit 61 arranged on the main transport path 13 or only the second cooling unit 62 arranged on the return transport path 16.

[0122] The term "recording medium" is a general term for various terms such as paper, recording paper, printing paper, printing medium, print medium, print-receiving medium, image-forming medium, image-receiving medium, and ejection-receiving medium. The assumed medium material is one that shrinks when heated.

[0123] The configurations described in the above embodiments and the features described in the modified examples can be used in appropriate combinations, and some features can also be replaced.

[0124] The embodiments of the present disclosure described above may have their constituent elements modified, added, or deleted as appropriate without departing from the spirit of the present disclosure. The present disclosure is not limited to the embodiments described above, and many modifications may be made by those having ordinary skill in the relevant art within the technical concept of the present disclosure.

[0125] In the inkjet printing device 2 shown in FIG. 8 , which uses a water-based ink and does not apply a pretreatment liquid, the difference in moisture content ΔW before and after heating when the paper P after image formation on one side is dried by the drying unit 50, and the front-to-back registration performance when the confining pressure (here, the adsorption pressure) during drying is changed were evaluated.

[0126] For the evaluation, OK Topcoat 127 gsm paper (manufactured by Oji Paper Co., Ltd.), 530 x 750 mm, T-grain paper was used. The moisture content of the paper before printing was designated as the moisture content of the paper before heating (W1). In the drying unit 50, the heating belt 51 had suction holes with a diameter of 0.2 mm formed over an area equal to or larger than the paper size, allowing suction to be applied across at least the entire heated portion of the paper. The paper was restrained from the backside by vacuum suction through the suction holes from the backside of the paper placed on the heating belt 51. The suction pressure as the restraining pressure was variable by changing the suction force. In the drying unit 50, a hot air blower was used as the heater 57. The moisture content after heating (W2) was varied by changing at least one of the heating output and heating time of the heating belt 51 and heater 57.

[0127] The moisture content was measured using the Karl Fischer method. The moisture content in the non-image areas of the paper was measured using the Karl Fischer method, and the moisture content was calculated. The moisture content of the unprinted paper loaded into the paper feeder 20 was measured as the moisture content W1 of the paper before heating. The moisture content W2 of the paper after heating was measured for each condition using paper that had an image formed on its front side (i.e., one side), passed through the drying unit 50, and was removed before being cooled by the cooling unit 60. For each moisture content difference and suction pressure condition, the paper used to evaluate front-to-back misregistration and the paper used to measure moisture content W2 were different, but the paper type and printing conditions were the same.

[0128] Table 1 shows the results of evaluating front-to-back register misalignment by changing the moisture content difference ΔW and the suction pressure. Images for front-to-back register adjustment were printed on the front and back of the paper, and the front-to-back register misalignment was measured and evaluated according to the following criteria: A: No problem: Misalignment 0.3 mm or less B: Within tolerance: Misalignment more than 0.3 mm, 0.5 mm or less C: Out of tolerance: Misalignment more than 0.5 mm, 1.0 mm or less D: Out of tolerance: Misalignment more than 1.0 mm

[0129] As shown in Table 1, when the moisture content difference ΔW was 6% or less, the misregistration between the front and back registers was within the allowable range. When the moisture content difference ΔW was 4% or less, the misregistration between the front and back registers was suppressed to 0.3 mm or less. More specifically, when the moisture content difference ΔW was 6%, the misregistration between the front and back registers was suppressed to within the allowable range by setting the suction pressure to 6 kPa or more. Table 1 clearly shows that the smaller the moisture content difference ΔW, the lower the suction pressure. When the moisture content difference ΔW was 5% or less and the suction pressure was 4 kPa or more, the misregistration between the front and back registers was suppressed to within the allowable range. It was also found that when the moisture content difference ΔW was 3.5% or less, a suction pressure as low as 1 kPa was sufficient to suppress the misregistration between the front and back registers, which is particularly preferable. Suppressing the misregistration between the front and back registers with a low suction pressure allows the output of the suction exhaust pump to be reduced, leading to reduced power consumption.

[0130] The following supplementary notes are further disclosed in relation to the above embodiments. <Supplementary Note 1> An image forming apparatus capable of forming images on both sides of a recording medium, comprising: a drying unit for drying a recording medium after an image has been formed on one side thereof, the drying unit including a heating section for heating the recording medium and a transport section for transporting the recording medium while constraining the entire area of ​​the portion of the recording medium heated by the heating section; and a processor for controlling the drying unit, wherein the processor controls the drying unit under the condition that the difference in moisture content in a non-image portion of the recording medium before and after heating of the recording medium by the drying section is 6% or less. <Supplementary Note 2> The image forming apparatus according to Supplementary Note 1, wherein the processor controls the drying unit under the condition that the difference in moisture content is 3.5% or less. <Supplementary Note 3> The image forming apparatus according to Supplementary Note 1 or Supplementary Note 2, wherein the processor controls the drying unit under the condition that the difference in moisture content is 5% or less and the constraining pressure is 4 kPa or more. <Supplementary Note 4> The image forming apparatus according to any one of Supplementary Notes 1 to 3, wherein the heating section heats the recording medium from both the front side and the back side of one side. <Supplementary Note 5> The image forming apparatus according to any one of Supplementary Notes 1 to 4, wherein the processor changes the settings of the drying unit or issues an alert to prompt a change of the settings based on information about the recording medium. <Supplementary Note 6> The image forming apparatus according to any one of Supplementary Notes 1 to 5, wherein the drying unit includes a plurality of heating sections, a first heating area including one of the plurality of heating sections, and a second heating area including another heating section, the second heating area being disposed downstream of the first heating area in the recording medium transport direction, and wherein the recording medium is transported in a state where its entire surface is constrained while being transported from the first heating area to the second heating area in the drying unit.<Supplementary Note 7> The image forming apparatus according to any one of Supplementary Note 1 to Supplementary Note 6, wherein the drying unit has a plurality of heating sections, including a first heating section having one of the plurality of heating sections, and a second heating section having another heating section, the second heating section being disposed downstream of the first heating section in the conveying direction of the recording medium, and the processor performs control to make the amount of heat applied to the recording medium in the first heating section smaller than the amount of heat applied to the recording medium in the second heating section, and also performs control to make the confining pressure that constrains the recording medium in the first heating section smaller than the confining pressure in the second heating section. <Supplementary Note 8> An image forming method in an image forming apparatus capable of forming images on both sides of a recording medium, comprising a drying step of conveying a recording medium having an image formed on one side while heating it, in the drying step, the recording medium is conveyed in a state where at least the entire area of ​​the heated portion of the recording medium is constrained, and the recording medium is heated under conditions such that the difference in moisture content of a non-image portion of the recording medium before and after the drying step is 6% or less.

[0131] The disclosure of Japanese Patent Application No. 2024-027471, filed on February 27, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually indicated to be incorporated by reference.

Claims

1. An image forming apparatus capable of forming images on both sides of a recording medium, comprising: a drying unit for drying the recording medium after an image has been formed on one side, the drying unit comprising a heating section for heating the recording medium and a transport section for transporting the recording medium while constraining the entire area of ​​the portion of the recording medium heated by the heating section; and a processor for controlling the drying unit, wherein the processor controls the drying unit under the condition that the difference in moisture content of the non-image portion of the recording medium before and after heating of the recording medium by the drying unit is 6% or less.

2. The image forming apparatus according to claim 1, wherein the processor controls the drying unit under the condition that the moisture content difference is 3.5% or less.

3. The image forming apparatus according to claim 1, wherein the processor controls the drying unit under conditions such that the moisture content difference is 5% or less and the confining pressure is 4 kPa or more.

4. The image forming apparatus according to claim 1, wherein the heating section heats the recording medium from the front side and the back side of the one surface.

5. The image forming apparatus according to any one of claims 1 to 4, wherein the processor changes the settings of the drying unit or issues an alert to prompt a user to change the settings based on the information about the recording medium.

6. An image forming apparatus according to any one of claims 1 to 4, wherein the drying unit is provided with a plurality of the heating sections, and includes a first heating area having one of the plurality of heating sections and a second heating area having another of the heating sections, the second heating area being arranged downstream of the first heating area in the transport direction of the recording medium, and wherein the recording medium is transported in a state where its entire surface is constrained while being transported from the first heating area to the second heating area in the drying unit.

7. The image forming apparatus of claim 5, wherein the drying unit comprises a plurality of the heating sections, a first heating area having one of the plurality of heating sections, and a second heating area having another of the heating sections, the second heating area being located downstream of the first heating area in the transport direction of the recording medium, and wherein the recording medium is transported in a state where its entire surface is constrained while being transported from the first heating area to the second heating area in the drying unit.

8. The image forming apparatus of any one of claims 1 to 4, wherein the drying unit has a plurality of the heating sections, including a first heating area having one of the plurality of heating sections and a second heating area having another of the heating sections, the second heating area being arranged downstream of the first heating area in the transport direction of the recording medium, and the processor controls the amount of heat applied to the recording medium in the first heating area to be smaller than the amount of heat applied to the recording medium in the second heating area, and controls the confinement pressure that constrains the recording medium in the first heating area to be smaller than the confinement pressure in the second heating area.

9. The image forming apparatus of claim 5, wherein the drying unit is provided with a plurality of the heating sections, including a first heating area having one of the plurality of heating sections and a second heating area having another of the heating sections, the second heating area being located downstream of the first heating area in the transport direction of the recording medium, and the processor controls the amount of heat applied to the recording medium in the first heating area to be smaller than the amount of heat applied to the recording medium in the second heating area, and controls the confining pressure that confines the recording medium in the first heating area to be smaller than the confining pressure in the second heating area.

10. The image forming apparatus of claim 6, wherein the drying unit has a plurality of the heating sections, including a first heating area having one of the plurality of heating sections and a second heating area having another of the heating sections, the second heating area being arranged downstream of the first heating area in the transport direction of the recording medium, and the processor controls the amount of heat applied to the recording medium in the first heating area to be smaller than the amount of heat applied to the recording medium in the second heating area, and controls the confinement pressure that confines the recording medium in the first heating area to be smaller than the confinement pressure in the second heating area.

11. The image forming apparatus of claim 7, wherein the drying unit has a plurality of the heating sections, including a first heating area having one of the plurality of heating sections and a second heating area having another of the heating sections, the second heating area being arranged downstream of the first heating area in the transport direction of the recording medium, and the processor controls the amount of heat applied to the recording medium in the first heating area to be smaller than the amount of heat applied to the recording medium in the second heating area, and controls the confinement pressure that confines the recording medium in the first heating area to be smaller than the confinement pressure in the second heating area.

12. An image forming method for an image forming device capable of forming images on both sides of a recording medium, comprising a drying process in which the recording medium having an image formed on one side is heated while being transported, in which the recording medium is transported in a state in which at least the entire heated area of ​​the recording medium is constrained in the drying process, and the recording medium is heated under conditions in which the difference in moisture content of the non-image area of ​​the recording medium before and after the drying process is 6% or less.

Images