WO2025182191A1 - Image forming apparatus and image forming method - Google Patents

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

This disclosure relates to an image forming apparatus and an image forming method.

Some image forming devices, such as inkjet printers that use aqueous ink, are equipped with a drying unit to fix and dry the ink formed on the recording surface of paper (see, for example, JP 2020-015272 A, JP 2020-146993 A, etc.).

When the 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 lot of heat energy is applied to the paper to evaporate the moisture and solvents in the ink, making the paper more likely to shrink. In image forming devices capable of double-sided printing, if the paper shrinks due to drying after forming an image on the front side, this can cause misregistration when printing on the back side.

In response to this, there are known technologies that predict the amount of paper shrinkage in advance or measure the paper size after printing on the front side and then perform front-to-back registration. 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 being printed, it is often not possible to fully improve front-to-back registration.

This disclosure has been made in light of the above circumstances, and aims to provide an image forming apparatus and image forming method that can reduce misregistration between the front and back sides during double-sided printing.

The image forming apparatus of the present disclosure is an image forming apparatus capable of forming images on both sides of a recording medium,
a drying unit for drying a recording medium on one side of which an image has been formed, the drying unit including a heating section for heating the recording medium and a conveying section for conveying the recording medium while constraining the entire area of the portion of the recording medium that is heated by the heating section;
a processor for controlling the drying unit;
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.

It is preferable that the processor be configured to control the drying unit under conditions that keep the moisture content difference to 3.5% or less.

The processor may be configured to control the drying unit under conditions where the moisture content difference is 5% or less and the confining pressure is 4 kPa or more.

It is preferable that the heating unit heat the recording medium from both the front and back sides of one surface.

The processor may be configured to change the settings of the drying unit based on the information about the recording medium, or may be configured to issue an alert prompting a change of settings.

The drying unit may have multiple heating sections, including a first heating area having one of the multiple heating sections, and a second heating area having another heating section, the second heating area being located downstream of the first heating area in the recording medium transport direction. In this case, it is preferable that 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.

The drying unit may have multiple heating sections, including a first heating area having one of the multiple heating sections, and a second heating area having another heating section, the second heating area being located downstream of the first heating area in the recording medium transport direction. 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 confinement pressure that confines the recording medium in the first heating area to be smaller than the confinement pressure in the second heating area.

The image forming method of the present disclosure is an image forming method in an image forming apparatus capable of forming images on both sides of a recording medium,
A drying process is performed in which a recording medium having an image formed on one surface thereof is heated and conveyed,
In the drying step, the recording medium is conveyed while at least the entire area of the heated portion of the recording medium is constrained;
The recording medium is heated under conditions such that the difference in moisture content of the non-image portion of the recording medium before and after the drying step is 6% or less.

The image forming apparatus and image forming method disclosed herein can reduce misregistration between the front and back sides during double-sided printing.

1 is a diagram illustrating the overall configuration 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 . FIG. 2A is a side view showing the drying unit, and FIG. 2B is a plan view of the conveying surface. FIG. 2 is a functional block diagram showing a schematic configuration of a control system of the inkjet printing apparatus. FIG. 10 is a diagram showing a table showing combinations of paper types, drying conditions, and suction pressures, and moisture content differences. FIG. 10 is a side view showing a drying unit of a first modified example. FIG. 10 is a side view showing a drying unit of a second modified example. FIG. 10 is a diagram illustrating the overall configuration of a modified inkjet printing apparatus.

Embodiments of the image forming apparatus and image forming method disclosed herein will be described below with reference to the drawings. In each drawing, the same elements are designated by the same reference numerals.

"Configuration of inkjet printing device"
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.

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 according to the technology disclosed herein.

As shown in FIG. 1, the inkjet printing apparatus 1 includes a transport mechanism 10, a paper feeder 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.

The transport mechanism 10 has a transport path 12 along which the paper P is transported. In Figure 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, image forming unit 40, drying unit 50, and 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 undergoes various processes.

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.

The supply path 14 is connected to the main conveying path 13 at the first connection 21. The discharge path 15 is connected to the main conveying path 13 at the second connection 22. The starting end of the return conveying path 16 is connected to the main conveying path 13 at the second connection 22. The terminal end of the return conveying path 16 is connected to the main conveying path 13 at the first connection 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.

The supply path 14 supplies paper P from the paper feeder 20 to the main transport path 13. One end of the supply path 14 is located on the paper feeder 20 side, and the other end is connected to the main transport path 13 at a first connection section 21. Paper P is supplied from the paper feeder 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.

The discharge path 15 transports 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 by the second connection part 22, and the other end is connected to the stacking device 70. 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.

The return transport path 16 forms a circular path together with the main transport path 13, and is a path for returning 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 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.

The first connection section 21 is configured to be able to accept paper P transported from the supply path 14 and paper P transported from the return transport path 16 into the main transport path 13. Paper P transported from the return transport path 16 is transported to the main transport path 13 upside down relative to paper P transported from the supply path 14.

The second connection section 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 section 22 includes a rotatable branch guide 18 (see Figure 2) and an actuator (not shown), such as a solenoid, that rotates the branch guide 18.

By rotating the branch guide 18, the switching mechanism switches the path of the paper P at the second connection section 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, it 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 back to the main transport path 13.

The return transport path 16 is equipped with a switchback section 17 that reverses the direction of travel of the paper P (see Figure 2). The switchback section 17 temporarily pulls the paper P out of the return transport path 16 and reverses the direction of travel of the paper P. In other words, the leading edge of the paper P in the direction of travel when it was being transported along the return transport path 16 before being pulled into the switchback section 17 becomes the trailing edge in the direction of travel after it is returned from the switchback section 17 to the return transport path 16. By passing through this switchback section 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.

The connection between the switchback unit 17 and the return transport path 16 is equipped with a switching mechanism that switches between a path that pulls paper P from the return transport path 16 into the switchback unit 17 and a path that returns paper from the switchback unit 17 to the return transport path 16. In this example, the switching mechanism equipped in the switchback unit 17 includes a rotatable branch guide 19 (see Figure 2) and an actuator (not shown), such as a solenoid, that rotates the branch guide 19.

The pretreatment liquid application unit 30, pretreatment liquid drying unit 35, image forming unit 40, drying unit 50, and 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.

The transport mechanism 10 is equipped with multiple transport members along the transport path 12. Examples of the multiple 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 multiple transport members of the transport mechanism 10 include multiple belt conveyors 81-85 and a roller transport unit 90 including multiple 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 make up the transport mechanism 10. Details of the transport mechanism 10 will be described later.

The paper feed device 20 has a paper feed tray on which a large number of sheets of paper P are placed in a stacked bundle. There are no particular restrictions on the type of paper P, but for example, cellulose-based printing paper 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).

The paper feeder 20 takes out the paper P from the stack set inside one sheet at a time, starting from the top, and supplies it to the supply path 14 of the transport path 12.

The pretreatment liquid application unit 30 applies 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 the colorant components in the 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 is equipped with a gripper (not shown) on its circumferential surface, and by rotating while gripping the leading edge of the paper P with the gripper, the paper P is wrapped around the drum circumferential surface and transported.

The pretreatment liquid application device 33 includes an application roller 34, which 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 can move between an application position where it comes into contact with the paper P and applies pretreatment liquid to the paper P, and a retracted position where it is separated from the paper P and does not apply pretreatment liquid.

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 perspective of uniformly adjusting the amount of pretreatment liquid applied, uniformly recording fine lines and fine image areas, 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.

Note that the method for applying the pretreatment liquid is not limited to the roller application method. 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, or spraying using a spray method.

The pretreatment liquid drying unit 35 dries the paper P coated with the pretreatment liquid. 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 highly thermally conductive material such as metal, and 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.

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 Figure 2). The printing drum 42 has 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 wrapped around the drum circumferential surface and transported. The printing drum 42 also has a suction mechanism (not shown), which adsorbs the paper P wrapped around the drum circumferential surface and transports it. Negative pressure is used for adsorption. The printing drum 42 has multiple suction holes on its circumferential surface, and by suctioning from inside the printing drum 42 through these suction holes, the paper P is adsorbed to the circumferential surface of the printing drum 42.

Head unit 44 includes inkjet heads 46C, 46M, 46Y, and 46K. Inkjet head 46C is a recording head that ejects droplets of cyan ink. Inkjet head 46M is a recording head that ejects droplets of magenta ink. Inkjet head 46Y is a recording head that ejects droplets of yellow ink. Inkjet head 46K is a recording head that ejects droplets of black ink. Ink is supplied to each of inkjet heads 46C, 46M, 46Y, and 46K from an ink tank (not shown), which is an ink supply source for the corresponding color, via a piping path (not shown). For example, water-based ink is used as the ink for drawing. Water-based ink is ink in which coloring materials such as pigments and dyes are dissolved or dispersed in water and/or a water-soluble solvent.

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 image drum 42, and the ejected droplets adhere to the paper P, forming an image on the paper P.

In this example, a configuration using four colors of ink, 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 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. There are also no particular limitations on the arrangement order of the inkjet heads of each color.

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. The belt conveyor 54 includes a drive roller 52 and a driven roller 53 in addition to the heating belt 51. 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 with a transport surface made of a highly thermally conductive material such as metal, and is heated from the back side of the transport surface 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 heating belt 51 are an example of a heating unit of the present disclosure, and in this example, the heating unit heats the paper P from both the front and back sides. The belt conveyor 54 is an example of a transport unit of the present disclosure. The heater 57 can be a convection heating unit such as a hot air blower, a radiant heating unit such as an infrared (IR) lamp, an ultraviolet (UV) lamp, or a microwave generator, or a superheated steam generator.

Figure 3 shows (a) a side view of the drying unit 50 and (b) a plan view of the conveying surface of the heating belt 51. As shown in Figure 3(b), the heating belt 51 has multiple suction holes 51a for adsorbing and conveying paper P. A suction box 55 is disposed in the space between the drive roller 52 and 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. This allows paper P to be adsorbed 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.

As shown in Figure 3(b), the conveying surface of the heating belt 51 has multiple suction holes 51a formed throughout 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 sections 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.

In Figure 3, paper P is transported in transport direction A, indicated by the arrow. Paper P placed on heating belt 51 has its backside sucked by suction pressure generated in suction holes 51a via suction box 55, which is located below the transport surface. In transport direction A, paper P is restrained by suction from position X1, where it is placed on heating belt 51. Suction box 55 transfers heat from the 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 suction restraint of paper P 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 with the entire heated portion of paper P restrained.

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

The paper P is handed over from the imaging drum 42 to the chain gripper 27 (see Figure 2), not shown in Figure 1, and with the leading edge of the paper P gripped by the gripper 27a, it is placed on the heating belt 51 and adsorbed to the heating belt 51. Note that the leading edge of the paper P held by the gripper 27a is a non-image forming area where no image is formed. Once 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.

As already mentioned, the cooling unit 60 comprises a first cooling unit 61 and a second cooling unit 62. The first cooling unit 61 is arranged on the main transport path 13, downstream of the drying unit 50. The first cooling unit 61 comprises, for example, a blower. The second cooling unit 62 is arranged 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 arranged downstream of the switchback section 17. The second cooling unit 62 comprises, for example, a blower.

The first cooling unit 61 and the second cooling unit 62, for example, blow room temperature air (around 25°C) onto the paper P. By cooling the paper P after it has been dried in the drying unit 50, the ink film solidifies and promotes evaporation of moisture. Either or both of the first cooling unit 61 and the second cooling unit 62 may blow cooling air (around 0-24°C) that is cooler than room temperature air onto the paper P.

The stacking device 70 stacks the paper sheets P on which images have been formed. The stacking device 70 receives the paper sheets P discharged from the discharge path 15 of the conveying path 12 and stacks them in a bundle on a stacking tray (not shown).

Here, the transport mechanism 10 will be further described. As previously mentioned, the transport mechanism 10 includes multiple transport members, which transport the paper P along the transport path 12. As shown in FIG. 1, multiple transport roller pairs 25 and transport guides 26 are arranged on the 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 the transport roller pairs 91, which will be described later. Furthermore, the paper P is transported along the main transport path 13 by the paper feed drum 24, pretreatment liquid application drum 32, pretreatment liquid drying drum 36, and imaging drum 42.

As mentioned above, the paper P on which the image is formed on the drawing drum 42 is transported along the transport guide 28 gripped by the chain gripper 27 and handed over to the heating belt 51 of the drying unit 50.

The transport mechanism 10 is equipped with a roller transport unit 90 including multiple belt conveyors 81-85 and multiple transport roller pairs 91 along the transport path 12 as transport members for transporting the paper P discharged from the drying unit 50.

As shown in Figure 2, the belt conveyors 81-85 each include endless conveyor belts 81a-85a, drive rollers 81b-85b, and driven rollers 81c-85c.

Each of the conveyor belts 81a-85a has multiple suction holes for adsorbing and transporting the paper P. The belt conveyors 81-85 are equipped with a suction box (not shown) in the space between the drive rollers 81b-85b and driven rollers 81c-85c on the back side of the conveyor surface of the conveyor belts 81a-85a. The suction box 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 generates suction pressure in the suction holes of the conveyor belts 81a-85a, thereby adsorbing the paper P to the conveyor surface. The belt conveyors 81-85 transport the paper P by adsorbing it to the conveyor surface. While the paper P is being transported by the belt conveyors 81-85, the conveyor members do not come into contact with the image forming surface on which the image is formed on the paper P.

The belt conveyor 81 is arranged from the main conveying path 13 to the return conveying path 16, and is equipped with a guide roller 81d for changing the direction of travel at the second connection 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 arranged horizontally along the main conveying path 13, and a conveying surface that is arranged inclined relative to the horizontal along the return conveying path 16.

The belt conveyor 82 is arranged in the discharge path 15. The conveying surface of the belt conveyor 82 is arranged horizontally. The conveying surface of the belt conveyor 82 is approximately flush with the horizontal conveying surface of the belt conveyor 81 along the main conveying path 13. The belt conveyor 82 receives 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.

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 then discharges the paper P onto the return conveying path 16 by rotating the drive roller 83b in the reverse direction.

The belt conveyor 84 is positioned to receive paper P discharged from the belt conveyor 83 in the switchback section 17, with its conveying surface inclined relative to the horizontal.

The belt conveyor 85 is positioned so that its conveying surface is horizontal and receives the paper P transported by the belt conveyor 84.

In this way, multiple belt conveyors 81-85 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 it coming into contact with any transport members while transporting it to a position where it passes through the second cooling unit 62. Here, "one side" refers to the first of the two sides of the paper P, which is the first side to be printed, and this first side to be printed first is sometimes referred to as the front side of the paper P. The side opposite the first side of the paper P is sometimes referred to as the second side or the back side of the paper P.

Here, all of the belt conveyors 54, 81-85 use suction to attract paper P to the transport surface and transport it, but electrostatic attraction or other methods may also be used to attract paper P to the transport surface.

Note that the conveying mechanism 10 uses a conveying means other than a belt conveyor to convey the paper P, which 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 without contacting the conveying member.

As shown in Figure 1, a roller conveying section 90 is provided downstream of the belt conveyor 85 on the return conveying path 16. The roller conveying section 90 includes multiple conveying roller pairs 91 and a conveying guide 92 arranged along the return conveying path 16. The conveying 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 conveying roller pair 91 sandwiches the paper P between the upper roller 91a and the lower roller 91b and sends it downstream in the conveying direction.

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

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

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

The storage device 102 is a non-transitory storage medium and a tangible computer-readable medium. The storage device 102 includes memory, which is a primary storage device, and storage, which is 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. Some or all of the storage area of the storage device 102 may be included in the processor 100.

The storage device 102 stores various parameters used by the inkjet printing device 1 and programs used by each part of the inkjet printing device 1. The storage device 102 also functions as a temporary storage unit for various data, including image data.

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.

The communication unit 104 has the required communication interface. The inkjet printing device 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 to speed up communication processing. The communication unit 104 serves as an image input interface unit for acquiring image data representing the image to be printed. Image data acquired from the host computer 110 via the communication unit 104 is stored in the storage device 102.

The input device 106 may be configured, for example, with operation buttons, a keyboard, a mouse, a touch panel, a multi-touch screen, other pointing devices, or a voice input device, or an appropriate combination of these. The input device 106 accepts various inputs from the operator.

The display device 108 may be, for example, a liquid crystal display, an organic electroluminescence (OEL) display, a projector, or an appropriate combination of these.

Information input via the input device 106 is sent to the processor 100. The processor 100 causes each component 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 or double-sided printing), the type of paper P, etc.

In response to commands from the processor 100, the display device 108 can display various information, such as various device setting information or abnormality information. The user (operator) can use the input device 106 to set various parameters and input and edit various information while viewing the content displayed on the display device 108.

This inkjet printing device 1 is capable of single-sided and double-sided printing, and is configured to be able to selectively switch between single-sided printing mode and 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.

In single-sided printing mode, paper P is transported along a path that passes through the supply path 14, main transport path 13, and discharge path 15. In detail, paper P fed from the paper feeder 20 to the supply path 14 is transported to the main transport path 13, where the pretreatment liquid application unit 30 applies pretreatment liquid to the first side, the pretreatment liquid drying unit 35 dries the pretreatment liquid, the image formation unit 40 forms an image, the drying unit 50 drys the paper, and the first cooling unit 61 cools the paper, in that order. The paper P with the image printed on its first side is then transported to the discharge path 15 and discharged to the stacking device 70.

In double-sided printing mode, paper P is transported along a path that passes through the supply path 14, main transport path 13, return transport path 16, main transport path 13, and discharge path 15 in that order. Specifically, paper P fed from the paper feed device 20 to the supply path 14 is transported to the main transport path 13, where pretreatment liquid is applied to the first side of the paper P, the pretreatment liquid dries, an image is formed on the first side, drying, and cooling are performed in that order. Paper P is then transported from the main transport path 13 to the return transport path 16, where it is inverted to its front and rear ends by passing through the switchback section 17, and the inverted paper P is transported along the return transport path 16. In the return transport path 16, paper P is cooled by the second cooling unit 62. Paper P cooled by the second cooling unit 62 is returned from the return transport path 16 to the main transport path 13. When returned to the main transport path 13, paper P is inverted so that its second side becomes the image formation side. In the main transport path 13, the application of pretreatment liquid to the second side, drying of the pretreatment liquid, image formation on the second side, drying, and cooling are carried out in that order. 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.

In the inkjet printing device 1, in double-sided printing mode, after forming an image on one side (first side) of paper P, the drying unit 50 conveys paper P while constraining at least the entire heated area of the paper P, and heats the paper P under conditions such that the difference in moisture content of the non-image areas 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 such that the difference in moisture content of the non-image areas 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.

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

For example, the processor 100 accepts a specification of single-sided or double-sided printing from the input device 106 and sets the mode to single-sided 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 change using the branch guide 18 of the switching mechanism of the second connection unit 22.

The transport mechanism 10 includes elements such as transport members and power sources involved in transporting paper P from the paper feeder 20 to the stacker 70, as described in Figure 1. The processor 100 controls each element of the transport mechanism 10 so that paper P is transported from the paper feeder 20 to the stacker 70 according to the set transport path. The processor 100 also controls the paper feeder 20's operations to start and stop feeding paper P.

The processor 100 performs various conversion processes, correction processes, and image processing such as halftone processing on the image data to be printed.

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; 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.

Processor 100 operates image forming unit 40 to form an image on paper P based on image data stored in storage device 102. Based on the dot data for each ink color generated through image processing, processor 100 controls the ejection operation of each of inkjet heads 46C, 46M, 46Y, and 46K to record an image on paper P transported by image drum 42.

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. Drying conditions include, 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.

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

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

"Non-image area" refers to the area of the paper P where no image is formed, i.e., the area where no ink is applied. Since it is difficult to evaluate the difference in moisture content before and after heating in the ink-applied areas of the paper P due to the influence of the ink, the evaluation is based on the difference in moisture content in the non-image area.

Here, before and after heating refers to before and after the paper P on which the first side has had an image formed in double-sided printing is heated in the drying unit 50, 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 = W1 - W2. The moisture content can be calculated by measuring the moisture content using, for example, the Karl Fischer method, but the measurement method is not limited to this.

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

Note that if no processes that affect the moisture content of the paper P, such as applying pretreatment liquid or drying the pretreatment liquid, are performed on the paper P between paper feeding and image formation, the moisture content of the paper P will remain almost 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 area will remain almost unchanged. Therefore, if 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 feed device 20, i.e., before printing, can be considered to be the moisture content W1 of the non-image area of the paper P before heating in the drying unit 50.

This section describes an example configuration for the inkjet printing device 1, which sets 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.

For example, a table T such as that shown in FIG. 5 is created and stored in advance in the storage device 102, containing information about the recording medium, such as the type of paper P used, specifically the paper type, such as the brand, paper type, and paper thickness of the paper P, as well as drying conditions, suction pressure, and moisture content difference ΔW. The processor 100 is then configured to reference table T and set drying conditions, such as heating temperature and conveying speed, and suction pressure, based on the input information about the paper P (here, brand, paper type, and paper thickness). Table T shown in FIG. 5 indicates, for example, that for paper of brand A, paper type gloss, and paper thickness X, ΔW is 1 to 5% for drying conditions A1 to A5. Table T defines α as the suction pressure value that is 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, paper type gloss, and paper thickness X. Furthermore, even for the same paper type, different suction pressures may be specified for different moisture content differences ΔW, such as setting a higher suction pressure for larger moisture content differences ΔW and a lower suction pressure for smaller moisture content differences ΔW.

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 that were set before the print settings were made for the paper P to be printed on, such as the default drying conditions or the drying conditions used in the most recent print.

When an unknown recording medium is used, the processor 100 may be configured to reference the conditions for a similar paper type in a table stored in the storage device 102. For example, if the unknown recording medium is a paper of unknown brand but with information on paper type and thickness, the processor 100, based on the input paper type and thickness information, determines the moisture content difference ΔW of the paper to be the moisture content difference ΔW of the paper, based on the input paper type and thickness information 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 will keep the moisture content difference ΔW within the threshold, and automatically change the drying conditions, or prompt the user to change the drying conditions by displaying an alert on the display device 108, for example. Note that the "paper type" shown in Table T is an example; even if the same gloss paper has different characteristics, it may be distinguished as "gloss 1" and "gloss 2."

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 using the same paper type prior to actual printing and set the drying conditions and suction pressure in the drying unit 50. For example, as shown in FIG. 3, moisture content meters 58, 59 that measure moisture content non-contact are installed upstream and downstream of the drying unit 50 on the conveying path. Then, during the test print, the processor 100 calculates the moisture content difference ΔW from 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 change the settings, such as increasing the suction pressure. Alternatively, the processor 100 may prompt the user to change the drying conditions or suction pressure settings by, for example, displaying an alert on the display device 108.

Processor 100 operates cooling unit 60. Processor 100 controls the cooling output and/or cooling time of first cooling unit 61 and second cooling unit 62. If the cooling source is a blower, cooling output refers to the temperature and volume of air blown out from the blower.

The hardware structure of the processor 100 can be any of the various processors listed below. These include CPUs, which are general-purpose processors that execute software (programs) and function as various processing units, as well as PLDs (Programmable Logic Devices) such as FPGAs (Field-Programmable Gate Arrays) whose circuit configuration can be changed after manufacture, and dedicated electrical circuits such as ASICs (Application Specific Integrated Circuits), which are processors with a circuit configuration specifically designed to perform specific processing.

Furthermore, the above-mentioned processing may be executed by one of these various processors, or by a combination of two or more processors of the same or different types (for example, multiple FPGAs, or a combination of a CPU and an FPGA). Furthermore, multiple processing units may be configured with a single processor. An example of configuring multiple processing units with a single processor is a system-on-chip (SOC), which uses a processor that realizes the functions of an entire system including multiple processing units with a single IC (Integrated Circuit) chip.

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

As described above, the inkjet printing device 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 portion of the recording medium heated by the heating section. The inkjet printing device 1 also includes a processor 100 that controls the drying unit 50, which controls the drying unit 50 under the condition that the difference in moisture content of the non-image portion 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 area of the heated portion and by maintaining a moisture content difference of 6% or less before and after heating, it is possible to suppress shrinkage of recording media such as paper due to moisture loss during heating. 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 the amount of shrinkage to be uneven across the surface of the paper, which can lead to 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 paper shrinkage and paper feed problems. This configuration enables high-speed drying while preventing misregistration between the front and back of the paper.

As a rule for preventing misregistration between the front and back of the paper, it is possible to set a range for the moisture content W2 of the paper after heating, but the moisture content W2 depends on the moisture content W1 before heating, which in turn 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 based on the moisture content W2 after heating. On the other hand, the difference in moisture content ΔW before and after heating can be matched in advance to the amount of paper shrinkage, allowing for back-side registration to be adjusted according to the amount of shrinkage, thereby preventing misregistration between the front and back of the paper.

Under drying conditions where the moisture content difference ΔW before and after heating is 6% or less, by appropriately setting the suction pressure, it is possible to suppress shrinkage of the paper after drying and reduce misregistration between the front and back of the paper.

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 can be suppressed. From the perspective of sufficiently drying the ink, it is more preferable to set the moisture content difference ΔW to 2% or more.

In the inkjet printing device 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 side on which an image is formed. However, the configuration of the drying unit 50 is not limited to this. It may also be configured with 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, if the heating section heats the paper P from both the front and back sides of the side on which an image is formed, it is possible to dry the paper P faster than when heating from only the front or back side. Compared to when heating from only the front or back side, the amount of heat applied to the paper per hour can be increased, so the heating required to dry the ink can be achieved even when the paper P is transported at high speed. Furthermore, if the amount of heat applied per hour increases, the shrinkage rate of the paper P increases if the paper P is not restrained. However, because the entire heated area of the paper P is restrained, shrinkage of the paper P can be suppressed even when the amount of heat applied per hour increases. Therefore, in the drying unit 50, if the heating section is configured to heat the paper P from both the front and back sides, and the paper P is transported while the heated area of the paper P is constrained, misregistration between the front and back sides can be suppressed even during high-speed printing.

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 radiant heating from the surface, as in this embodiment. In particular, a heating method to combine with heat transfer heating is desirable that does not excessively heat the paper and minimizes the reduction of moisture contained in the paper, such as convection heating using warm air blowing.

The ink used for image formation is not limited to water-based ink, but water-based ink is particularly effective because a greater amount of heat is applied to the paper as a post-processing step to volatilize the water after application, compared to other ink types such as UV (ultraviolet) curable ink.

In the above embodiment, the method of restraining the paper P in the drying unit 50 was described as a vacuum suction method in which the paper P is adsorbed through suction holes in the heating belt, but the restraint 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 against the support surface of the paper back surface support member.

The back support member that supports and restrains the paper from the back side is not limited to a conveyor belt such as a heating belt, but may also be a plate-like member such as a platen or a conveyor drum.

Furthermore, instead of the drying unit 50, a drying unit 150 according to modified example 1 shown in FIG. 6 or a drying unit 150A according to modified example 2 shown in FIG. 7 may be provided. In FIGS. 6 and 7, components equivalent to those shown in FIG. 1 are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

The drying unit 150 of variant 1 shown in Figure 6 has multiple (two in this example) heating sections 151, 152 arranged at different positions in the conveying direction A. The drying unit 150 has a first heating area HA1 equipped with one of the heating sections 151, and a second heating area HA2 equipped with 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 has a first belt conveyor 54a and a first heater 57a. The heating section 152 has 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 Figure 1. That is, they include a heating belt 51, a drive roller 52, and a driven roller 53. In addition, 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 driven roller 53 below the conveying surface of the heating belt 51.

In this way, when the drying unit 150 has multiple heating areas HA1 and HA2 in the transport direction A, it is preferable that the processor 100 controls the amount of heat applied to the paper P in the first heating area HA1 located upstream to be smaller than the amount of heat applied to the paper P in the second heating area HA2, and controls the confining pressure (here, suction pressure) that constrains the paper P in the first heating area HA1 to be smaller than the confining pressure in the second heating area HA2. In this case, the confining pressure in the first heating area 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.

When forming an image using aqueous ink, especially when using a single-component aggregation-type ink that does not require the application of a pretreatment liquid, the ink may have high fluidity after image formation. If the ink has high fluidity after image formation, on a transport surface having suction holes 51a, such as the transport surface of the heating belt 51 shown in Figure 1, the temperature difference between the suction holes 51a and the non-suction hole portions 51b where the suction holes 51a are provided can cause density unevenness when the ink dries due to heating. Density unevenness is particularly likely to occur on paper types that are difficult for ink to penetrate. However, as described above, by using a stepwise heating process in which the ink's fluidity is eliminated in the first heating area HA1, which dries the ink with a low amount of heat and low restraining pressure, and then the ink is thoroughly dried in the second heating area 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 between the front and back of the paper.

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

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.

Furthermore, the drying unit 150A of variant 2 shown in Figure 7 has a first heating area HA1 and a second heating area HA2, but only one belt conveyor 154 as a transport section. The belt conveyor 154 includes a transport belt 156, a drive roller 157, and a driven roller 158. A suction box 155 connected to an exhaust pump (not shown) is located in the space between the drive roller 157 and the driven roller 158 below the transport surface of the transport belt 156. The transport belt 156 has multiple suction holes on its transport surface, similar to the heating belt 51 shown in Figure 3, and transports the paper P by suction from the back side. However, in the example shown in Figure 7, no heater is provided below the transport belt 156, and the transport belt 156 is not used as a heating belt for heat transfer. In other words, the heating section 151 has only the first heater 57a as a heating means, and the heating section 152 has only the second heater 57b as a heating means. In this example, the sheet P is transported by the belt conveyor 154 from the first heating area HA1 to the second heating area HA2. During this transport, the entire sheet P is adhered to the transport surface until it passes through the first heating area HA1 and the second heating area HA2. In other words, in this example, the entire sheet P is also transported in a constrained state through 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 transporting the sheet P in a constrained state between the heating areas.

The image forming apparatus of the present disclosure may be configured without the pretreatment liquid application unit 30 and pretreatment liquid drying unit 35, as in the modified inkjet printing apparatus 2 shown in Figure 8. In Figure 8, components that are the same as those in the inkjet printing apparatus 1 shown in Figure 1 are given the same reference numerals, and detailed descriptions will be omitted. The inkjet printing apparatus 2 does not include the pretreatment liquid application unit 30 and pretreatment liquid drying unit 35, but the other components are the same as those of the inkjet printing apparatus 1 described above.

Furthermore, while the inkjet printing devices 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.

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, image-receiving medium, and ejection-receiving medium. The assumed medium material is one that shrinks when heated.

The configurations described in the above embodiments and the features described in the variations can be used in appropriate combinations, and some features can be substituted.

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 within the technical spirit of the present disclosure by those with ordinary skill in the relevant field.

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

For the evaluation, OK Topcoat 127 gsm (manufactured by Oji Paper Co., Ltd.), 530 x 750 mm, T-grain paper was used. The moisture content of the paper before printing was defined as the moisture content of the paper before heating, W1. In the drying unit 50, the heating belt 51 was equipped with suction holes with a diameter of 0.2 mm over an area equal to or larger than the paper size, allowing suction to be applied to at least the entire heated area 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 could be changed by varying 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 changed by varying at least one of the heating output and heating time of the heating belt 51 and heater 57.

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 paper before printing, loaded into the paper feeder 20, was measured as the moisture content W1 of the paper before heating. For each condition, the moisture content W2 of the paper after heating was measured 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 for front-to-back misregistration evaluation and the paper used for moisture content W2 measurement were different, but the paper type and printing conditions were the same.

Table 1 shows the results of evaluating front-to-back misregistration 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 misregistration was measured and evaluated according to the following criteria.
A: No problem: Misalignment 0.3 mm or less B: Within tolerance: Misalignment over 0.3 mm, 0.5 mm or less C: Unacceptable: Misalignment over 0.5 mm, 1.0 mm or less D: Unacceptable: Misalignment over 1.0 mm

As shown in Table 1, when the moisture content difference ΔW was 6% or less, it was possible to keep front-to-back register misalignment within the allowable range, and when the moisture content difference ΔW was 4% or less, it was possible to suppress front-to-back register misalignment to 0.3 mm or less. More specifically, when the moisture content difference ΔW was 6%, a suction pressure of 6 kPa or more enabled front-to-back register misalignment to be kept within the allowable range. Table 1 clearly shows that the smaller the moisture content difference ΔW, the lower the suction pressure that can be used. When the moisture content difference ΔW was 5% or less and the suction pressure was 4 kPa or more, it was possible to keep front-to-back register misalignment 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 front-to-back register misalignment, which is particularly preferable. Suppressing front-to-back register misalignment with a low suction pressure allows the output of the suction exhaust pump to be reduced, leading to reduced power consumption.

The following additional notes are further disclosed regarding the above embodiment.
<Appendix 1>
An image forming apparatus capable of forming images on both sides of a recording medium,
a drying unit for drying a recording medium on one side of which an image has been formed, the drying unit including a heating section for heating the recording medium and a conveying section for conveying the recording medium while constraining the entire area of the portion of the recording medium that is heated by the heating section;
a processor for controlling the drying unit;
The processor controls the drying unit under conditions such 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.
<Appendix 2>
The processor controls the drying unit under the condition that the moisture content difference is 3.5% or less.
The image forming apparatus according to claim 1 <Supplementary Note 3>
The processor controls the drying unit under conditions of a moisture content difference of 5% or less and a confining pressure of 4 kPa or more.
3. The image forming apparatus according to claim 1 or 2.
<Appendix 4>
The heating unit heats the recording medium from the front side and the back side of one surface.
4. The image forming apparatus according to claim 1, wherein the image forming apparatus is a
<Appendix 5>
The processor changes the settings of the drying unit or issues an alert to change the settings based on the information about the recording medium.
5. The image forming apparatus according to claim 1,
<Appendix 6>
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 conveying direction of the recording medium;
In the drying unit, the recording medium is conveyed in a state where the entire surface of the recording medium is constrained while being conveyed from the first heating area to the second heating area.
6. The image forming apparatus according to claim 1,
<Appendix 7>
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 conveying direction of the recording medium;
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
performing control so that the confining pressure for confining the recording medium in the first heating area is smaller than the confining pressure in the second heating area;
7. The image forming apparatus according to claim 1,
<Appendix 8>
An image forming method in an image forming apparatus capable of forming images on both sides of a recording medium, comprising:
A drying process is performed in which a recording medium having an image formed on one surface thereof is heated and conveyed,
In the drying step, the recording medium is conveyed while at least the entire area of the heated portion of the recording medium is constrained;
An image forming method, comprising heating a recording medium under conditions in which the difference in moisture content of a non-image area of the recording medium before and after a drying step is 6% or less.

The disclosure of Japanese Patent Application No. 2024-027471, filed on February 27, 2024, is incorporated herein by reference in its entirety.
All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims (12)

An image forming apparatus capable of forming images on both sides of a recording medium,
a drying unit for drying the recording medium on one side of which an image has been formed, the drying unit including a heating section for heating the recording medium and a conveying section for conveying the recording medium while constraining the entire area of the portion of the recording medium that is heated by the heating section;
a processor for controlling the drying unit;
The image forming apparatus, wherein the processor controls the drying unit under conditions in which a difference in moisture content of a non-image portion of the recording medium before and after heating of the recording medium by the drying unit is 6% or less. the processor controls the drying unit under conditions in which the moisture content difference is 3.5% or less.
The image forming apparatus according to claim 1 . 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.
The image forming apparatus according to claim 1 .
the heating unit heats the recording medium from the front side and the back side of the one surface;
The image forming apparatus according to claim 1 . The processor changes settings of the drying unit or issues an alert to prompt a change of settings based on the information about the recording medium.
The image forming apparatus according to claim 1 . the drying unit includes a plurality of the heating sections, a first heating area including one of the plurality of heating sections, and a second heating area including another of the heating sections, the second heating area being disposed downstream of the first heating area in the transport direction of the recording medium;
In the drying unit, the recording medium is conveyed in a state where the entire surface of the recording medium is constrained while being conveyed from the first heating area to the second heating area.
The image forming apparatus according to claim 1 . the drying unit includes a plurality of the heating sections, a first heating area including one of the plurality of heating sections, and a second heating area including another of the heating sections, the second heating area being disposed downstream of the first heating area in the transport direction of the recording medium;
In the drying unit, the recording medium is conveyed in a state where the entire surface of the recording medium is constrained while being conveyed from the first heating area to the second heating area.
The image forming apparatus according to claim 5 . the drying unit includes a plurality of the heating sections, a first heating area including one of the plurality of heating sections, and a second heating area including another of the heating sections, the second heating area being disposed downstream of the first heating area in the transport direction of the recording medium;
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
performing control so that a confining pressure that constrains the recording medium in the first heating area is smaller than a confining pressure in the second heating area;
The image forming apparatus according to claim 1 . the drying unit includes a plurality of the heating sections, a first heating area including one of the plurality of heating sections, and a second heating area including another of the heating sections, the second heating area being disposed downstream of the first heating area in the transport direction of the recording medium;
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
performing control so that a confining pressure that constrains the recording medium in the first heating area is smaller than a confining pressure in the second heating area;
The image forming apparatus according to claim 5 . the drying unit includes a plurality of the heating sections, a first heating area including one of the plurality of heating sections, and a second heating area including another of the heating sections, the second heating area being disposed downstream of the first heating area in the transport direction of the recording medium;
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
performing control so that a confining pressure that constrains the recording medium in the first heating area is smaller than a confining pressure in the second heating area;
The image forming apparatus according to claim 6 . the drying unit includes a plurality of the heating sections, a first heating area including one of the plurality of heating sections, and a second heating area including another of the heating sections, the second heating area being disposed downstream of the first heating area in the transport direction of the recording medium;
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
performing control so that a confining pressure that constrains the recording medium in the first heating area is smaller than a confining pressure in the second heating area;
The image forming apparatus according to claim 7 . 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 heating and transporting the recording medium having an image formed on one surface thereof,
In the drying step, the recording medium is conveyed while the entire area of at least the heated portion of the recording medium is constrained;
The image forming method further comprises heating the recording medium under conditions in which the difference in moisture content of the non-image portion of the recording medium before and after the drying step is 6% or less.