JP2020131682A - Ink jet image formation apparatus and image quality adjustment method - Google Patents

Abstract

To provide an ink jet image formation apparatus and image quality adjustment method which can maintain the stable image quality even when various factors, errors and the like that may affect the image quality of the final image occur.SOLUTION: An ink jet image formation apparatus comprises: a transfer unit which makes an intermediate transfer body carry ink discharged from an ink jet head to transfer it to a recording medium; an ink state detection unit which detects the crushed state of the ink that is carried and pressed by the intermediate transfer body; and a control unit which controls a control parameter that affects the gradation characteristics of the ink when being transferred to the recording medium in association with the detection result of the ink state detection unit.SELECTED DRAWING: Figure 7

Description

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

In recent years, as an apparatus for recording a high-definition image on various recording media such as paper and cloth, an image forming apparatus by an inkjet method in which ink is ejected from an inkjet head and printed on the recording medium (hereinafter, an inkjet image forming apparatus). ) Is widespread.

In an inkjet image forming apparatus, an intermediate in which ink discharged from an inkjet head is supported on an image carrier (intermediate transfer body) such as an intermediate transfer belt, and the supported ink is transferred to a recording medium as an ink image at a transfer nip portion. A transfer method has been proposed (see, for example, Patent Document 1).

JP-A-2018-122503

By the way, in the intermediate transfer type inkjet image forming apparatus, the ink dots formed (supported) on the intermediate transfer body are sandwiched between the transfer portions (transfer nip) after the pre-curing process for adjusting the viscosity. While being pressed, the ink image is transferred to the recording medium. Therefore, the final ink image transferred to the recording medium is various, such as the state of the intermediate transfer body, the state of hardening (thickening) of the ink, the pressing force (transfer pressure) by the transfer nip, the type of the recording medium, and the like. It is affected by conditions (eg temperature and humidity).

In addition, the image quality of the final ink image transferred to the recording medium, for example, the fine line reproducibility of the ink image changes due to an error of the apparatus or the like. In general, in an inkjet image forming apparatus, various factors and errors that may affect the image quality of the final image transferred to a recording medium and output may occur.

With respect to this problem, the technique of Patent Document 1 employs a configuration in which the amount of the transferability improving liquid applied is adjusted according to the amount of transfer residual ink on the transfer belt. However, the technique described in Patent Document 1 is mainly aimed at improving transferability, and does not control the degree of enlargement of ink dots after pressing from the viewpoint of image quality, so that fine line reproducibility and the like are not controlled. The image quality of the final ink image cannot be sufficiently ensured.

An object of the present invention is to provide an inkjet image forming apparatus and an image quality adjusting method capable of maintaining a stable image quality even if various factors and errors that may affect the image quality of the final image occur. Is.

The inkjet image forming apparatus according to the present invention is
A transfer unit that supports the ink ejected from the inkjet head on an intermediate transfer body and transfers it to a recording medium.
An ink state detecting unit that detects a crushed state of ink supported and pressed on the intermediate transfer body, and an ink state detecting unit.
A control unit that controls control parameters that affect the gradation characteristics of the ink when it is transferred to the recording medium according to the detection result of the ink state detection unit.
To be equipped.

The image quality adjustment method according to the present invention
This is an image quality adjustment method in an inkjet image forming apparatus in which ink ejected from an inkjet head is supported on an intermediate transfer body and transferred to a recording medium by a transfer unit.
The crushed state of the ink supported and pressed on the intermediate transfer body is detected, and the ink is crushed.
The control parameters that affect the gradation characteristics of the ink when it is transferred to the recording medium are controlled according to the crushed state of the ink.

According to the present invention, stable image quality can be maintained even if various factors and errors that may affect the image quality of the final image occur.

It is a schematic block diagram of the inkjet image forming apparatus in this embodiment. It is a block diagram which shows the main functional structure of the inkjet image forming apparatus of FIG. It is a flowchart explaining the process at the time of execution of a normal print job. 4A and 4B are diagrams showing states after the ink dots land on the intermediate transfer body, before being pressed, and after being pressed, respectively. 5A and 5B are diagrams for explaining the fine line reproducibility, and are partially cutaway drawings showing the shape of the input image and the shape of the actual output image, respectively. It is a figure which compares and shows the reference gradation curve and the gradation curve detected based on the detection result of an image sensor. It is a flowchart which shows an example of the process which concerns on image quality adjustment in this embodiment. It is a figure which shows the other arrangement example of an image sensor. It is a figure which shows the other arrangement example of an image sensor.

Hereinafter, the inkjet image forming apparatus according to the present embodiment will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of an inkjet image forming apparatus 1 according to the present embodiment. Further, FIG. 2 is a block diagram showing a main functional configuration of the inkjet image forming apparatus 1.

The inkjet recording device 1 includes a head unit 10 on which an inkjet head 102 (see FIG. 2) is mounted, a transfer belt 20 as an image carrier or an intermediate transfer body, and a driven roller that rotatably stretches the transfer belt 20. It includes 21 and 22, a transfer roller 23 as a drive roller, a transfer drum 24 for conveying the recording medium P, and a control unit 40 (see FIG. 2) for controlling the entire apparatus.

Further, the inkjet recording device 1 includes a first UV irradiation unit 25 that adjusts the viscosity of the ink ejected to the transfer belt 20, a second UV irradiation unit 26 that cures the ink transferred to the recording medium P, and a transfer belt 20. It includes a cleaning unit 27 for cleaning, an image sensor 30, and a transfer drive unit 51 for driving a transfer roller 23 and a transfer drum 24. Of the above, the first UV irradiation unit 25 corresponds to the "viscosity adjusting unit" of the present invention. Further, the image sensor 30 and the control unit 40 correspond to the "ink state detection unit" of the present invention.

Although not shown, the inkjet recording device 1 discharges the feeding unit that loads the recording medium P and feeds it to the transport drum 24, and the recording medium P on which the image is transferred to the downstream side in the transport direction of the transport drum 24. It is provided with a discharge unit, a display unit for displaying the status of the device, and the like. Since these have known configurations, illustration and description thereof will be omitted. Further, as the recording medium P, in addition to paper such as plain paper and coated paper, various media such as cloth or sheet-like resin capable of fixing the ink landed on the surface can be used.

The transfer belt 20 is bridged by the driven rollers 21 and 22 arranged above and the transfer roller 23 arranged below, and the driving force of the transfer motor (not shown) of the transport drive unit 51 is applied to the transfer roller 23. By being transmitted to, it rotates clockwise in FIG. As a specific example, in the transfer belt 20, an elastic layer of silicon rubber, a reflective layer on which aluminum (Al) is vapor-deposited, and a surface layer of polypropylene (PP) are laminated on a polyimide (PI) base material. An endless belt is used. Further, as a specific example of the transfer roller 23, a rubber roller having a diameter of 100 mm and a surface layer rubber thickness of 10 mm is used.

In the inkjet image forming apparatus 1, the transfer belt 20 is rotationally driven in the clockwise direction by driving the transfer motor described above based on the control signal of the control unit 40 and rotating the transfer roller 23 in the clockwise direction in FIG. (Refer to each arrow in the figure). In one specific example, the rotation speed of the transfer roller 23 is controlled so that the transfer belt 20 rotates at a speed of 600 mm / sec (printing speed) under the control of the control unit 40.

The transport drum 24 is around a rotation axis extending in a direction perpendicular to the drawing of FIG. 1 (hereinafter, referred to as “orthogonal direction”) while holding the recording medium P on a cylindrical outer peripheral curved surface (convey surface). By rotating with, the recording medium P is conveyed in the conveying direction along the conveying surface. Specifically, the transport drum 24 includes a transport drum motor (not shown), and the motor is driven by the control of the control unit 40 to rotate in the counterclockwise direction in FIG. In one specific example, as the transport drum 24, a large metal drum (for example, a triple cylinder for a printing machine) is used.

In one specific example, the transfer belt 20 and the transfer drum 24 described above have a width of 800 mm, that is, an axial length.

The transfer roller 23 is arranged so as to face the upper part of the transfer drum 24, and pressurizes the transfer drum 24 via the transfer belt 20. Further, the transfer drum 24 is pressed against the transfer roller 23 with the transfer belt 20 sandwiched therein, so that a transfer nip NP for transferring an ink image from the transfer belt 20 to the recording medium P is formed. Such a transfer nip NP corresponds to the "transfer portion" of the present invention.

In one specific example, the initial value of the weighted or pressure contact force (hereinafter referred to as “transfer pressure”) in the transfer nip NP is set to 80N. Further, the transfer pressure can be changed by slightly moving the transfer roller 23 in the vertical direction in FIG. 1 under the control of the control unit 40. In one specific example, the shaft of the transfer roller 23 is connected to a solenoid or the like (not shown) of the transfer drive unit 51, and the solenoid or the like is driven under the control of the control unit 40 to drive the shaft of the transfer roller 23. Can move slightly downward or upward in FIG. 1 to change the transfer pressure to a value higher or lower than 80N. Such a solenoid or the like corresponds to the "transfer pressure adjusting unit" of the present invention.

The head unit 10 ejects ink to the transfer belt 20 from a nozzle opening provided on the ink ejection surface facing the transfer belt 20 to support the image on the transfer belt 20. The transport drum 24 transports the recording medium P so that the image carried on the transfer belt 20 is transferred to a predetermined position on the recording medium P by the transfer nip NP.

In the inkjet recording device 1 according to the present embodiment, four head units 10 corresponding to four colors of ink of yellow (Y), magenta (M), cyan (C), and black (K) rotate the transfer belt 20. They are arranged so as to be arranged at predetermined intervals in the order of Y, M, C, and K colors from the upstream side in the direction.

Each head unit 10 includes an inkjet head 102 (see FIG. 2). The inkjet head 102 is provided with a plurality of recording elements each having a pressure chamber for storing ink, a piezoelectric element provided on the wall surface of the pressure chamber, and a nozzle. When a drive signal for deforming the piezoelectric element is input to this recording element, the pressure chamber is deformed due to the deformation of the piezoelectric element, the pressure in the pressure chamber changes, and ink is discharged from a nozzle communicating with the pressure chamber.

The arrangement range of the nozzles included in the inkjet head 102 in the orthogonal direction covers the width of the recording medium P conveyed by the transfer drum 24 in the orthogonal direction of the region where the image is recorded. The head unit 10 is used with its position fixed with respect to the rotation axis of the transport drum 24 when recording an image. That is, the inkjet recording device 1 is a single-pass type inkjet recording device.

In the present embodiment, the amount of active energy rays supplied to the transfer belt 20 as the ink discharged from the inkjet head 102 to the transfer belt 20 (in this example, ultraviolet rays (UV: ultra) output from the first UV irradiation unit 25). An ink whose viscosity changes according to the amount of light of ultraviolet) is used. Specifically, an ink having a property that the viscosity increases as the amount of ultraviolet rays emitted from the first UV irradiation unit 25 increases is used. That is, the image forming portion of the inkjet image forming apparatus 1 adopts a UV curable inkjet method.

The first UV irradiation unit 25 is arranged so as to irradiate the transfer belt 20 on the upstream side of the transfer nip NP with ultraviolet rays (hereinafter referred to as “UV light”), and the transfer belt 20 is controlled by the control unit 40. It plays a role of pre-curing the ink adhering to the belt. In one specific example, the first UV irradiation unit 25 includes a UV light source that outputs UV light having a wavelength of 395 nm, and the default value of irradiation intensity in a normal printing job is 5 mW / cm ² .

The second UV irradiation unit 26 is arranged so as to irradiate the recording medium P conveyed to the downstream side of the transfer nip NP with UV light, and is pre-cured by the first UV irradiation unit 25 under the control of the control unit 40. It plays a role of main curing the ink (ink image) transferred by the transfer nip NP. In one specific example, the second UV irradiation unit 26 includes a UV light source that outputs UV light having a wavelength of 395 nm, and the default value of the irradiation intensity in a normal printing job is 10 mW / cm ² .

The cleaning unit 27 is arranged to face the surface of the transfer belt 20 between the driven roller 21 and the transfer roller 23, and includes a dry web that cleans the surface of the transfer belt 20. The dry web of the cleaning unit 27 is configured to be detachable from the transfer belt 20, and by contacting the transfer belt 20 under the control of the control unit 40, residual ink or the like on the surface of the transfer belt 20 is removed. Remove.

In the present embodiment, the image sensor 30 photographs an ink image after transfer on the recording medium P that has passed through the transfer nip NP, and outputs such imaging data to the control unit 40. In this example, the image sensor 30 is a CMOS (Complementary MOS) sensor.

Next, other main functional configurations in the inkjet image forming apparatus 1 will be described mainly with reference to FIG. The inkjet image forming apparatus 1 includes a head drive unit 101 and an inkjet head 102 included in the head unit 10, a control unit 40, a transport drive unit 51, and an input / output interface 52.

The head drive unit 101 outputs a drive signal for deforming the piezoelectric element according to the image data to the recording element of the inkjet head 102 at an appropriate timing based on the control of the control unit 40. An amount of ink corresponding to the pixel value of the image data is ejected from the nozzle.

The control unit 40 includes a CPU 41 (Central Processing Unit), a RAM 42 (Random Access Memory), a ROM 43 (Read Only Memory), and a storage unit 44.

The CPU 41 reads various control programs and setting data stored in the ROM 43, stores them in the RAM 42, executes the programs, and performs various arithmetic processes. In addition, the CPU 41 controls the overall operation of the inkjet image forming apparatus 1.

The RAM 42 provides the CPU 41 with a working memory space and stores temporary data. The RAM 42 may include a non-volatile memory.

The ROM 43 stores various control programs, setting data, and the like executed by the CPU 41. Instead of the ROM 43, a rewritable non-volatile memory such as an EEPROM (Electrically Erasable Programmable Read Only Memory) or a flash memory may be used.

The storage unit 44 stores a print job (image formation instruction including various user setting information such as the number of prints) input from the external device 2 via the input / output interface 52 and image data related to the print job. .. As the storage unit 44, for example, an HDD (Hard Disk Drive) may be used, or a DRAM (Dynamic Random Access Memory) or the like may be used in combination.

The transfer drive unit 51 supplies a drive signal to the transfer drum motor of the transfer drum 24 based on the control signal supplied from the control unit 40 to rotate the transfer drum 24 at a predetermined speed and timing. Further, the transport drive unit 51 supplies a drive signal to the motor of the transfer roller 23 based on the control signal supplied from the control unit 40, and rotates the transfer belt 20 at a predetermined speed and timing.

The input / output interface 52 mediates the transmission and reception of data between the external device 2 and the control unit 40. The input / output interface 52 is composed of, for example, any of various serial interfaces, various parallel interfaces, or a combination thereof.

The external device 2 is, for example, a personal computer, and supplies print jobs, image data, and the like to the control unit 40 via the input / output interface 52.

Next, with reference to the flowchart of FIG. 3, processing related to a normal print job executed by the control unit 40 when a print job, image data, or the like is received from the external device 2 will be described.

In step S10 after the start of the print job, the control unit 40 drives the transfer drum 24 and the transfer roller 23 to control the transfer drive unit 51 so as to start the transfer of the recording medium P.

In step S20, the control unit 40 ejects ink onto the transfer belt 20 from the inkjet head 102 of the color head unit 10 used for printing (image formation) based on the received image data and user setting information. The head drive unit 101 is controlled in this way. By such an operation, an image based on the image data is formed (supported) on the transfer belt 20.

In step S30, the control unit 40 controls the output (UV light amount) of the first UV irradiation unit 25 to the above-mentioned intensity at the timing when the image on the transfer belt 20 comes to the position of the first UV irradiation unit 25, thereby controlling the ink image. Perform the pre-curing step of.

In step S40, the control unit 40 controls the transport drive unit 51 so as to transport the recording medium P to the transfer nip NP at a predetermined timing, and uses the pre-cured image (ink) as an ink image on the recording medium P. Transfer. At this time, the ink is pressed by the preset transfer pressure (sandwiched by the transfer nip NP), so that the ink pre-cured on the transfer belt 20 has the ink density after pre-curing, the transfer pressure, and the like. It is transferred to the recording medium P as an ink image that spreads as a whole.

In step S50, the control unit 40 controls the output of the second UV irradiation unit 26 at the timing when the recording medium P on which the ink image is transferred comes to the position of the second UV irradiation unit 26, thereby performing the main curing step of the ink image. carry out. After that, the recording medium P is discharged to a discharge unit (not shown).

In step S60, the control unit 40 executes a process of cleaning the residual ink and the like on the transfer belt 20 by outputting a drive signal to the cleaning unit 27.

In step S70, the control unit 40 determines whether or not the print job has been completed. Here, when it is determined that the print job has not been completed (step S70, NO), the control unit 40 returns to step S10 and repeatedly executes the processes of steps S10 to S70 described above. On the other hand, when the control unit 40 determines that the print job has ended (step S70, YES), the control unit 40 ends a series of processes.

Thus, in the intermediate transfer type inkjet image forming apparatus, the amount of ink ejected from the inkjet head 102 can be suppressed and the ink can be spread smoothly on the recording medium P without bleeding, thus saving ink. There is a merit that can be done.

On the other hand, in the intermediate transfer type inkjet image forming apparatus, the transfer belt 20 is operated by the viscosity adjusting unit (first UV irradiation unit 25 in this example) before the recording medium P passes through the transfer nip NP, depending on the type of ink used and the like. A pre-curing step is required to increase the viscosity of the image (ink) above.

That is, if an attempt is made to transfer the ink as an ink image to the recording medium P by the transfer nip NP without increasing the viscosity of the ink on the transfer belt 20, the ink is crushed by the pressing force of the transfer nip NP depending on the type of ink and the like ( There is a problem (it spreads too much on the recording medium P). Therefore, even in the inkjet image forming apparatus 1 of the present embodiment, the viscosity of the ink on the transfer belt 20 is increased by irradiating the UV light from the first UV irradiation unit 25 before the ink image is transferred by the transfer nip NP. It is increasing to some extent.

On the other hand, in the pre-curing step, the transfer state changes depending on the printing environment (transfer conditions such as the state of the transfer belt 20, the curing state of the ink, and the transfer pressure), and the ink image finally printed on the recording medium P. Image quality is affected. Further, the ease of ink bleeding (permeability) at the time of transfer differs depending on the type of recording medium P used.

Due to the above factors, when the ink (image) carried on the transfer belt 20 is transferred to the recording medium P by the transfer nip NP, the enlargement ratio of the transferred image or the ink dots constituting the image The shape and roundness become unstable, which causes variations in the image quality of the final ink image, especially the fine line reproducibility.

This problem will be described more specifically with reference to FIGS. 4 and 5. 4A and 4B schematically show a state before pressing and a state after pressing the ink dots landed on the transfer belt 20, respectively. 5A and 5B are diagrams for explaining the fine line reproducibility, FIG. 5A shows the shape of the input image, and FIG. 5B shows the shape of the corresponding output image (after pressing).

In the inkjet image forming apparatus 1 provided with the transfer belt 20, as can be seen by comparing FIGS. 4A and 4B, the ink dots are expanded and enlarged by the pressing force during transfer to the recording medium P, thereby causing the image to be enlarged. The concealment rate goes up. Therefore, for example, in the case of forming a solid image, while suppressing the amount of ink ejected from the inkjet head 102, the ink is spread to a larger diameter by pressing the transfer nip NP at the time of transfer to the recording medium P. There are merits such as easy formation of a desired solid image on the recording medium P.

On the other hand, when trying to form various images other than solid images on the recording medium P, for example, if the viscosity of the ink is excessively low, the ink dots are excessively enlarged by pressing the transfer nip NP, and as a result, the image quality is improved. There is a problem that gets worse. In this respect, even if the viscosity of the ink landed on the transfer belt 20 is appropriate, if the pressing force at the transfer nip NP is too high, the same problem occurs.

In particular, in an image in which a plurality of thin lines are lined up as shown in FIG. 5A, if the image transferred to the recording medium P by pressing the transfer nip NP spreads too much in the width direction, the lines are drawn together as shown in FIG. 5B. There is a problem that the fine line reproducibility deteriorates because the gap becomes excessively narrow and the lines are connected and visually recognized.

In order to deal with such a problem, how to control the output of the first UV irradiation unit 25 and the transfer pressure of the transfer nip NP becomes an important issue. That is, in order to ensure fine line reproducibility (visibility between fine lines), the output of the first UV irradiation unit 25 is set so that the crushed state of the ink dots at the time of transfer is kept in an appropriate state (which is the target diameter). And it is necessary to control the transfer pressure of the transfer nip NP.

Regarding such a problem, as described above, a technique of adjusting the amount of the transfer improving liquid applied according to the amount of transfer residual ink on the transfer belt 20 is known, but in such a technique, the ink dots after pressing are known. Since control is not performed from the viewpoint of image quality such as the degree of enlargement of the ink, it is not possible to sufficiently secure the image quality of the final ink image such as fine line reproducibility.

On the other hand, in the present embodiment, the gradation characteristic is detected through the detection of the crushed state of the dots of the ink supported and pressed on the transfer belt 20, and the ink is transferred to the recording medium P according to the detection result. By controlling the control parameters that affect the gradation characteristics of the ink image at the time of operation with the control unit 40, the final image quality of the ink image such as fine line reproducibility is stably secured even if there is an error in the device. I made it possible. Hereinafter, the findings of the present inventors and the principle of the solution means in the present invention will be described.

In the inkjet image forming apparatus 1 of the present embodiment, the image sensor 30 captures an ink or ink image after being ejected (supported) by the transfer belt 20 and pressed (spread out so as to be crushed), and the captured image is taken. Is analyzed by the control unit 40 to detect the gradation characteristics of the ink.

Here, the detected "ink gradation characteristic" means "relationship (ratio, etc.) between the density of the input image and the output image density of the pressed ink" (see FIGS. 5A and 5B). ..

In the example shown in FIG. 1, the object photographed by the image sensor 30 is an ink image (on the second UV irradiation unit 26) on the recording medium P (first recording medium) on which the image is transferred by being pressed by the transfer nip NP. This is an image before the main curing is performed). Examples of different objects captured by the image sensor 30 will be described later in FIGS. 8 and 9.

In the present embodiment, in order to detect the image quality of the output image, particularly the fine line reproducibility, the image sensor 30 and the control unit 40 detect the “ink crushed state”.

Here, the "crushed state" of the ink means the degree of enlargement of the ink per amount of liquid drops after the pressing step is performed (see FIGS. 4A and 4B). In order to detect such a "crushed state", in the present embodiment, a reference curve as an ideal value regarding the relationship of the gradation (density) detected by the image sensor 30 with respect to each gradation (density) of the input image. (Reference gradation curve) is set in advance, and a method of detecting a difference from the gradation characteristic (image gradation curve) of the ink or the ink image described later is used.

In this embodiment, only the "ink crushed state" is detected on the premise that the transferability is ensured, and the transferability such as ink loss and dot loss is not detected.

Ideally, the state of each ink dot (that is, the ink state from a micro viewpoint) such as the shape and roundness of the ink taken by the image sensor 30 in dot units is detected as a "crushed state". Good.

However, in reality, it is not easy to detect the shape of a single ink dot or the like from the viewpoint of cost or the like. In addition, by actually detecting the gradation characteristics (image gradation curve) described later and controlling the ink density and transfer conditions according to the detection result, the fine line reproducibility of the ink image printed on the recording medium can be improved. It turned out that it could be secured sufficiently.

Therefore, in the present embodiment, the recording medium P (second recording medium) to be printed next is printed according to the detection result of the ink crushed state (that is, the gradation characteristic) by the image sensor 30 and the control unit 40. Control to adjust the viscosity or transfer conditions of the ink transferred to the recording medium P (second recording medium) to be printed next so that the crushed state (gradation characteristic) of the transferred ink approaches the specified state. I do.

More specifically, the image sensor 30 and the control unit 40 detect the gradation characteristic of the ink after the pressing process is executed as a crushed state, and the control unit 40 detects the detected gradation characteristic in a preset reference scale. At least one of the viscosity of the ink and the transfer condition of the transfer nip NP is controlled as control parameters so that the curve approaches the tonal characteristics (curve).

Here, the control unit 40 controls the output of the first UV irradiation unit 25 (viscosity adjustment unit) in order to adjust the viscosity of the ink. Further, the control unit 40 controls to change the transfer pressure to the transfer nip NP by the transfer roller 23 in order to adjust the transfer conditions of the transfer nip NP. In other words, the control unit 40 uses at least one of the adjustment amount of the first UV irradiation unit 25 (viscosity adjustment unit) and the adjustment amount of the solenoid or the like (transfer pressure adjustment unit) connected to the transfer roller 23 as control parameters. Control.

Hereinafter, the content of control by the control unit 40 will be described with reference to FIG. FIG. 6 is a graph showing a comparison between a predetermined reference gradation curve STC and ink gradation characteristics (image gradation curves TC ₁ and TC ₂ ) detected by the image sensor 30 after performing the pressing process. is there. In this graph, the gradation component (for example, 256 image gradations) included in the input image data is shown on the horizontal axis, and the output image gradation (gradation) detected by the image sensor 30 corresponding to the gradation component. Detected value) is shown on the vertical axis.

It can be seen that the image gradation curve TC ₁ shown in FIG. 6 has a higher detection value (in other words, the image is “dark” or “dark”) at all gradations as compared with the reference gradation curve STC. In this case, the control unit 40 presumes that the dot diameter of each of the inks is too wide, and describes the output (UV light irradiation intensity, the same applies hereinafter) of the first UV irradiation unit 25 (viscosity adjustment unit). Control to raise above the default value. As a result, the viscosity of the ink on the transfer belt 20 when reaching the transfer nip NP becomes high, so that the gradation detection value at each image gradation detected by the image sensor 30 at the time of printing on the next recording medium P Approaches the reference gradation curve STC. Alternatively, the control unit 40 may perform control to lower the transfer pressure of the transfer nip NP in order to bring the image gradation curve TC ₁ closer to the reference gradation curve STC.

On the other hand, as shown in FIG. 6, the image gradation curve TC ₂ detected by the image sensor 30 has a lower detection value at all gradations than the reference gradation curve STC (in other words, the image is " It can be seen that it is "thin" or "bright"). In this case, the control unit 40 estimates that the dot diameter of each ink has not expanded to the reference diameter, and controls the output of the first UV irradiation unit 25 (viscosity adjustment unit) to be lower than the default value. .. As a result, the viscosity of the ink on the transfer belt 20 when reaching the transfer nip NP becomes low, so that when printing the next recording medium P, the gradation detection value at each image gradation detected by the image sensor 30 Approaches the reference gradation curve STC. Alternatively, the control unit 40 may control to increase the transfer pressure of the transfer nip NP in order to bring the image gradation curve TC ₂ closer to the reference gradation curve STC.

Next, an example of the process related to image quality adjustment in the present embodiment will be described with reference to the flowchart shown in FIG. The processing of the flowchart shown in FIG. 7 is executed when printing an image for inspection, and is executed, for example, after the end of a normal print job, when the device is started, or when maintenance is executed. Alternatively, in the process shown in FIG. 7, the ink state and the ink curing state change, such as when the number of printed sheets reaches a predetermined number (for example, 1000 sheets) or when the temperature and humidity environment around the apparatus suddenly changes. It can be done at a timing that is easy to do.

In step S110, the control unit 40 performs a process of generating a test patch for inspection. In one specific example, the test patches are different, for example, 25%, 50%, 75%, 100%, in which the density or gradation changes stepwise (that is, the dot density gradually increases). It is an image having a density region of (hereinafter referred to as a patch image). The control unit 40 reads out the data (input images) of the plurality of patch images having different densities from the RAM 42 or the like, and generates them for each color of YMCK.

In step S120, the control unit 40 prints each patch image of the generated test patch from each head unit 10 (injection head 102) on the transfer belt 23. After that, the control unit 40 controls the output parameters of the first UV irradiation unit 25 at the timing when the patch image on the transfer belt 20 comes to the position of the first UV irradiation unit 25, and conveys the recording medium P to the transfer nip NP. And transfer the patch image. The ink constituting the patch image is pressed by the transfer nip NP at a predetermined transfer pressure and transferred to the recording medium P as a patch image of four colors (YMCK). The patch image transferred to the recording medium P is imaged by the image sensor 30, and the main curing process is performed by the second UV irradiation unit 26 (see step S50).

In step S130, the control unit 40 calculates a curve of gradation characteristics of the patch image printed on the recording medium P (curve of output density with respect to density of input image) based on the detection signal output from the image sensor 30. It is detected, an image gradation curve TC (see FIG. 4) is generated, and the generated image gradation curve TC is saved in the memory. The control unit 40 may appropriately perform processing such as interpolation when generating the image gradation curve TC.

In step S140, the control unit 40 compares the generated image gradation curve (TC) with the reference gradation curve STC, and the difference (deviation, etc.) between the image gradation curve (TC) and the reference gradation curve STC. ) Is out of the permissible range.

Here, when the control unit 40 determines that the difference (deviation, etc.) from the reference gradation curve STC is within the permissible range (step S140, NO), the output of the first UV irradiation unit 25 and the transfer of the transfer nip NP are transferred. The process is terminated assuming that the pressure parameter value can guarantee the image quality with the current setting.

On the other hand, when the control unit 40 determines that the difference (deviation, etc.) from the reference gradation curve STC is out of the permissible range (steps S140, YES), the control unit 40 proceeds to step S150.

In step S150, in step S150, the gradation value of the image gradation curve (TC) of the patch image identified in step S130 (value detected by the image sensor 30, see the vertical axis of FIG. 4) is the reference gradation curve. Determine if it is greater than the corresponding value of STC.

Here, the control unit 40 determines that the gradation value of the image gradation curve (TC) of the patch image is larger than the corresponding value of the reference gradation curve STC (see image gradation curve TC ₁ in FIG. 4). If so (step S150, YES), the process proceeds to step S160. On the other hand, when the control unit 40 determines that the gradation of the image gradation curve (TC) of the patch image is smaller than the corresponding value of the reference gradation curve STC (see the image gradation curve TC ₂ in FIG. 4). (Step S150, NO), the process proceeds to step S170.

In step S160, the control unit 40 changes the control parameters of each of these units so as to increase the output (the amount of UV light emitted) of the first UV irradiation unit 25 or decrease the transfer pressure of the transfer nip NP. On the other hand, in step S170, the control unit 40 changes the control parameters of each of these units so as to reduce the output of the first UV irradiation unit 25 or increase the transfer pressure of the transfer nip NP. These changed control parameters are stored in the RAM 42 or the storage unit 44.

When the processing of step S160 or step S170 is executed, the control unit 40 returns to the above-mentioned step S110 and executes the above-mentioned processing again, and the difference (difference, etc.) from the reference gradation curve STC is within the permissible range. Each process is repeated until it is determined (step S140, NO).

Although the control unit 40 has executed the adjustment (change of control parameter) in steps S160 or S170 N times (N is a user-set value), the image gradation of the patch image printed and detected later. When it is not determined that the deviation of the curve (TC) from the reference gradation curve STC is within the permissible range (step S140, NO), the following processing is performed. In this case, the control unit 40 considers that the device has a life or an abnormality, and it is expected that the image quality printed by the print job will deteriorate. Therefore, the control unit 40 urges the user to perform maintenance work by a service person or the like. Display.

According to the present embodiment in which such control is performed, stable image quality can be maintained even if various factors and errors that may affect the image quality of the final image occur. Further, according to the present embodiment, the image quality of the final ink image transferred to the recording medium P by the printing job, particularly the fine line reproducibility of the ink image is maintained or improved, while the processing is low cost and simple. be able to. Therefore, according to the present embodiment, it is possible to stably obtain a high-quality image in which linear reproducibility is ensured.

In the configuration example described above, the image sensor 30 is configured to photograph the ink on the recording medium P on which the ink image is transferred through the transfer nip NP. The arrangement or shooting target of the image sensor 30 is not limited to this. Hereinafter, other examples such as the arrangement of the image sensor 30 will be described with reference to FIGS. 8 and 9.

FIG. 8 is a schematic view showing a second configuration example of the inkjet image forming apparatus. In FIG. 8, the same reference numerals are given to the configurations equivalent to those shown in FIG. 1, and the description thereof will be omitted as appropriate. In this respect, FIG. 9 described later is also the same.

As can be seen in comparison with FIG. 1, in the inkjet image forming apparatus 1A shown in FIG. 8, the arrangement of the image sensor 30 is different from the example of FIG. 1, and the other configurations are the same. Specifically, the image sensor 30 of the inkjet image forming apparatus 1A is arranged in a direction for photographing the remaining ink on the transfer belt 20 after transferring the ink image (patch image) from the transfer belt 20 to the recording medium P. There is.

Then, in the inkjet image forming apparatus 1A, in step S130, the control unit 40 changes the state of the remaining ink of the patch image on the transfer belt 20 on the downstream side of the transfer nip NP photographed by the image sensor 30 (image gradation). From the above, the gradation characteristics of the ink image transferred to the recording medium P are estimated. In this case, as the recording medium P used for transferring the above-mentioned patch image, it is preferable to use a medium having a property of being difficult to transfer (for example, paper having a rough surface).

FIG. 9 is a schematic view showing a third configuration example of the inkjet image forming apparatus.

As can be seen in comparison with FIG. 1, in the inkjet image forming apparatus 1B shown in FIG. 9, the arrangement of the image sensor 30 is different from the example of FIG. 1, and the transfer nip NP is upstream and the first UV irradiation unit 25 A pressing roller 29 is added as a pressing member for pressing the transfer belt 20 on the downstream side of the above. Here, the pressing roller 29 is made of a material in which the ink of the transfer belt 20 does not easily adhere (transfer). Further, the pressing roller 29 can be separated from and detached from the transfer belt 20, and the pressing pressure on the transfer belt 20 can be changed by the control of the control unit 40.

In the inkjet image forming apparatus 1B having such a configuration, the ink adhering to the transfer belt 20 and pre-cured by the first UV irradiation unit 25 can be pressed and spread by the pressing roller 29 and the transfer nip NP. Therefore, according to the inkjet image forming apparatus 1B, even if the transfer pressure of the transfer nip NP cannot be increased too much due to, for example, the type of recording medium P, the pressing force of the insufficient amount is applied by the pressing force of the pressing roller 29. Can be supplemented.

Further, in the inkjet image forming apparatus 1B, the image sensor 30 uses the image sensor 30 after the test patch image attached on the transfer belt 20 is pressed by the pressing roller 29 and before the image reaches the transfer nip NP. Arranged to photograph the state.

In the inkjet image forming apparatus 1B having such a configuration, in step S160 (or step S170) described above, the control unit 40 replaces the process of lowering (or increasing) the transfer pressure of the transfer nip NP with the pressing force of the pressing roller 29. Control (change of control parameters) may be performed so as to lower (or raise).

In the above-described embodiment, the case where the image sensor 30 is a CMOS (Complementary MOS) sensor has been described. As another example, the image sensor 30 may be a sensor such as a CCD (Charge Coupled Device). Further, when it is difficult for these image sensors 30 to capture the ink image due to the case where the transfer belt 20 is colored or the like, the difference in gloss between the surface of the transfer belt 20 and the ink surface is measured instead of the image sensor 30. It may be configured using a gloss meter. In this case, the control unit 40 estimates the gradation characteristic (and thus the dot state) of the ink after pressing by using the glossiness data measured by the glossiness meter.

In the above-described embodiment, the ink ejected from the inkjet head 102 has a property that the viscosity changes according to the ultraviolet rays (UV light output from the first UV irradiation unit 25 and the second UV irradiation unit 26) applied to the ink. The case where the above ink is used has been described.

As another example, an ink having a property of increasing the viscosity by being irradiated with an active energy ray (for example, an electron beam) other than UV light may be used. In this case, the first UV irradiation unit 25 and the second UV irradiation unit 26 may be configured to include an electron beam generation source for irradiating the electron beam.

As another example, an ink having a property that the viscosity changes by changing the heating amount to the transfer belt 20 may be used. For example, in the case of an ink that is gel-like at room temperature and becomes sol-like when heated, its viscosity decreases as it is heated. In this case, the first UV irradiation unit 25 and the second UV irradiation unit 26 may be configured to include a heating source such as a heater or a halogen lamp.

On the other hand, when an ink containing water is used as a solvent, the water evaporates and the viscosity increases as the ink is heated. In this case, the first UV irradiation unit 25 and the second UV irradiation unit 26 adjust the blowing temperature of the above heating source or an air knife that generates an air flow (wind) for evaporating the water in the ink. Those capable of controlling the transfer characteristics of the ink image can be used. Further, as long as the solvent containing water in the ink can be removed, the present invention is not limited to the above configuration, and for example, a member in which a porous body is brought into contact with the ink image to absorb the solvent containing water, or water. You may use a squeegee blade roller or the like that squeezes out a solvent containing.

In addition, the above-described embodiments are merely examples of implementation of the present invention, and the technical scope of the present invention should not be construed in a limited manner by these. That is, the present invention can be implemented in various forms without departing from its gist or its main features.

1,1A, 1B Inkjet image forming apparatus 10 Head unit 20 Transfer belt (intermediate transfer body)
21,22 Driven roller 23 Transfer roller 24 Conveyance drum 25 1st UV irradiation part (viscosity adjustment part)
26 2nd UV irradiation unit 27 Cleaning unit 29 Pressing roller 30 Image sensor (ink status detection unit)
40 Control unit (ink status detection unit)
51 Transport drive unit 102 Inkjet head NP transfer nip (transfer unit)
P Recording medium STC reference gradation curve TC ₁ , TC ₂ Detected image gradation curve

Claims (17)

A transfer unit that supports the ink ejected from the inkjet head on an intermediate transfer body and transfers it to a recording medium.
An ink state detecting unit that detects a crushed state of ink supported and pressed on the intermediate transfer body, and an ink state detecting unit.
A control unit that controls control parameters that affect the gradation characteristics of the ink when it is transferred to the recording medium according to the detection result of the ink state detection unit.
An inkjet image forming apparatus comprising. The ink state detection unit detects the crushed state of the ink by detecting the gradation characteristic indicating the relationship between the pressed ink and the output image density of the pressed ink for each gradation of the input image.
The inkjet image forming apparatus according to claim 1. The control unit controls the inkjet head so that the intermediate transfer body carries a plurality of ink images having different gradations.
The ink state detection unit detects the gradation characteristic based on the plurality of ink images.
The inkjet image forming apparatus according to claim 2. The control unit controls the control parameters so that the gradation characteristic detected by the ink state detection unit approaches the reference gradation characteristic.
The inkjet image forming apparatus according to claim 2 or 3. The ink state detection unit detects the gradation characteristic of the ink on the recording medium.
The inkjet image forming apparatus according to any one of claims 2 to 4. The ink state detection unit estimates the gradation characteristics of the ink on the recording medium based on the remaining ink on the intermediate transfer body after the ink is transferred to the recording medium.
The inkjet image forming apparatus according to any one of claims 2 to 4. A pressing member for pressing the intermediate transfer body is provided.
The ink state detecting unit detects the gradation characteristic of the ink on the intermediate transfer body before being pressed by the pressing member and transferred to the recording medium.
The inkjet image forming apparatus according to any one of claims 2 to 4. The control unit controls at least one of the viscosity of the ink supported on the intermediate transfer body and the transfer conditions of the transfer unit as the control parameters.
The inkjet image forming apparatus according to any one of claims 1 to 7. A viscosity adjusting unit that adjusts the viscosity of the ink supported by the intermediate transfer body on the upstream side of the transfer unit,
A transfer pressure adjusting unit for adjusting the transfer pressure of the transfer unit is provided.
The control unit controls at least one of the adjustment amount of the viscosity adjustment unit and the adjustment amount of the transfer pressure adjustment unit as the control parameters.
The inkjet image forming apparatus according to any one of claims 1 to 8. The control unit controls the control parameters during a period during which the print job is not being executed.
The inkjet image forming apparatus according to any one of claims 1 to 9. The ink is an ink whose viscosity is increased by supplying active energy rays.
The inkjet image forming apparatus according to claim 9. The viscosity adjusting unit adjusts the viscosity of the ink by changing the amount of active energy rays supplied to the intermediate transfer body.
The inkjet image forming apparatus according to claim 11. The active energy ray is ultraviolet light,
The inkjet image forming apparatus according to claim 12. The active energy ray is an electron beam,
The inkjet image forming apparatus according to claim 12. The ink is an ink containing water as a solvent.
The inkjet image forming apparatus according to any one of claims 1 to 10. The viscosity adjusting unit adjusts the viscosity of the ink by changing the amount of heat supplied to the intermediate transfer body.
The inkjet image forming apparatus according to claim 9. This is an image quality adjustment method in an inkjet image forming apparatus in which ink ejected from an inkjet head is supported on an intermediate transfer body and transferred to a recording medium by a transfer unit.
The crushed state of the ink supported and pressed on the intermediate transfer body is detected, and the ink is crushed.
Control parameters that affect the gradation characteristics of the ink when it is transferred to the recording medium according to the crushed state of the ink.
Image quality adjustment method.

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