Patent Publication Number: US-11376841-B2

Title: Inkjet image forming apparatus and image quality adjustment method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2019-032575, filed on Feb. 26, 2019, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present invention relates to an inkjet image forming apparatus and an image quality adjustment method. 
     Description of the Related Art 
     In recent years, as apparatuses for recording high-definition images on various recording media such as paper and cloth, image forming apparatuses using an inkjet method (hereinafter referred to as inkjet image forming apparatuses) have been widely used. 
     In recent years, to reduce cost, it has been required of the inkjet image forming apparatuses to reduce the amount of ink ejected from inkjet heads and thus the amount of ink adhering to a recording medium as much as possible. However, reducing the amount of adhesion of ink causes a problem that the contrast ratio decreases in solid image printing, causing inconsistencies in solidness. 
     For this problem, if spherical droplets ejected from the inkjet heads can be flattened (spread) on the recording medium, the decrease in the contrast ratio can be eliminated. On the other hand, using such a method may cause ink bleed on the recording medium, depending on the ink viscosity (particularly when the ink viscosity is low). 
     In order to address the above problems, there has been proposed an intermediate transfer-type inkjet image forming apparatus in which an image formed by ink ejected from inkjet heads is temporarily carried on an intermediate transfer body such as an intermediate transfer belt, and the image is transferred to a recording medium (see JP 2018-122503 A, for example). 
     In general, the intermediate transfer-type inkjet image forming apparatus forms ink dots (an image) on the intermediate transfer body, and irradiates the intermediate transfer body with active energy rays such as UV to perform an ink pre-curing (thickening) process, and then performs a process to transfer the image to a recording medium (such as paper) by a transfer nip. Thus, the intermediate transfer-type apparatus varies in transfer state, depending on the state of the intermediate transfer body, the cured state of the ink, the type of recording medium, and transfer conditions (such as a transfer pressure). In particular, a change in transfer efficiency causes a problem such as an increase in loss due to the occurrence of residual ink after transfer or variations in image density. 
     For this problem, the technique described in JP 2018-122503 A employs a configuration to adjust the amount of application of a transferability improvement liquid to the intermediate transfer body, according to the amount of residual ink after transfer. However, the technique described in JP 2018-122503 A has a problem of increased cost because it applies the transferability improvement liquid. 
     SUMMARY 
     It is an object of the present invention to provide an inkjet image forming apparatus and an image quality adjustment method that can reduce an increase in cost while maintaining high transfer efficiency. 
     To achieve the abovementioned object, according to an aspect of the present invention, an inkjet image forming apparatus reflecting one aspect of the present invention comprises: a transfer part that transfers ink ejected from an inkjet head and carried on an intermediate transfer body to a recording medium; and a hardware processor that: detects a transfer rate when the ink is transferred by the transfer part; and controls a control parameter that influences how the ink spreads when being transferred to the recording medium, according to the transfer rate detected by the hardware processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention: 
         FIG. 1  is a schematic configuration diagram of an inkjet image forming apparatus in the present embodiment; 
         FIG. 2  is a block diagram showing the major functional configuration of the inkjet image forming apparatus in  FIG. 1 ; 
         FIG. 3  is a flowchart for explaining processing when a normal print job is executed; 
         FIG. 4  is a characteristic graph showing the relationship between the amount of UV irradiation light before transfer and the density of an ink image detected after the transfer; 
         FIG. 5  is a diagram for explaining an example of patch images printed on a sheet of paper and a UV irradiation condition; and 
         FIG. 6  is a flowchart showing an example of processing related to image quality adjustment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an inkjet image forming apparatus according to one or more embodiments of the present invention will be described in detail with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments.  FIG. 1  is a diagram showing the schematic configuration of an inkjet image forming apparatus  1  according to the present embodiment.  FIG. 2  is a block diagram showing the major functional configuration of the inkjet image forming apparatus  1 . 
     The inkjet image forming apparatus  1  includes head units  10  each equipped with an inkjet head  102  (see  FIG. 2 ), a transfer belt  20  as an image carrier or an intermediate transfer body, driven rollers  21  and  22  and a transfer roller  23  as a drive roller between which the transfer belt  20  is rotatably stretched, a transport drum  24  that transports a recording medium P, and a control unit  40  that controls the entire apparatus (see  FIG. 2 ). 
     The inkjet image forming apparatus  1  further includes a first UV irradiation unit  25  that adjusts the viscosity of ink ejected to the transfer belt  20 , a second UV irradiation unit  26  that cures the ink transferred to the recording medium P, a cleaning unit  27  that cleans the transfer belt  20 , a density measurement sensor  30 , and a transport drive unit  51  that drives parts such as the transfer roller  23  and the transport drum  24  (see  FIG. 2 ). Among those described above, the first UV irradiation unit  25  corresponds to a “viscosity adjuster” of the present invention. The density measurement sensor  30  and the control unit  40  correspond to a “transfer rate detection unit” of the present invention. 
     Although not shown, the inkjet image forming apparatus  1  includes a feed unit that carries and feeds the recording medium P to the transport drum  24 , an output unit that outputs the recording medium P to which an image has been transferred downstream in a transport direction of the transport drum  24 , a display unit that displays the status of the apparatus, etc. These have known configurations, and thus are not shown and will not be described. As the recording medium P, in addition to paper such as plain paper and coated paper, various media that allow ink landed on a surface to be fixed into place, including cloth and sheet-shaped resin, can be used. 
     The transfer belt  20  is stretched between the driven rollers  21  and  22  disposed above and the transfer roller  23  disposed below, and rotates in a clockwise direction in  FIG. 1  by a driving force of a transfer motor (not shown) of the transport drive unit  51  being transmitted to the transfer roller  23 . As one specific example, for the transfer belt  20 , an endless belt is used in which an elastic layer of silicon rubber, a reflective layer on which aluminum (Al) is vapor-deposited, and a surface layer of polypropylene (PP) are stacked on a substrate of polyimide (PI). As one 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 rotated in the clockwise direction in  FIG. 1  by the transfer motor being driven based on a control signal of the control unit  40  and the transfer roller  23  rotating in the clockwise direction (see arrows in the figure). In one specific example, under the control of the control unit  40 , the rotation speed of the transfer roller  23  is controlled such that the transfer belt  20  rotates at a speed of 600 mm/second (printing speed). 
     The transport drum  24  rotates about a rotation axis extending in a direction perpendicular to the drawing in  FIG. 1  (hereinafter referred to as an “orthogonal direction”) while holding the recording medium P on its outer peripheral curved surface in a cylindrical surface shape (transport surface), thereby transporting the recording medium P in the transport direction along the transport surface. Specifically, the transport drum  24  includes a transport drum motor (not shown), and rotates in a counterclockwise direction in  FIG. 1  by the motor being driven under the control of the control unit  40 . In one specific example, for the transport drum  24 , a large (e.g., a triple size cylinder for a printing press) metal drum is used. 
     In one specific example, the transfer belt  20  and the transport drum  24  described above have a width or an axial length of 800 mm. 
     The transfer roller  23  is disposed opposite the top of the transport drum  24 , and pressurizes the transport drum  24  with the transfer belt  20  therebetween. By the transport drum  24  being pressed against the transfer roller  23  with the transfer belt  20  therebetween, a transfer nip NP that transfers an ink image from the transfer belt  20  to the recording medium P is formed. The transfer nip NP corresponds to a “transfer part” of the present invention. 
     In one specific example, the initial value of weight or pressure contact force (hereinafter referred to as “transfer pressure”) at the transfer nip NP is set to 80 N. The transfer pressure can be changed by the transfer roller  23  slightly moving in a vertical direction in  FIG. 1  under the control of the control unit  40 . In one specific example, a shaft of the transfer roller  23  is connected to a solenoid or the like (not shown) of the transport drive unit  51 , and the solenoid or the like is driven under the control of the control unit  40 , so that the shaft of the transfer roller  23  slightly moves in a downward or upward direction in  FIG. 1  and can change the transfer pressure to a value higher or lower than 80 N. The solenoid or the like corresponds to a “transfer pressure adjuster” of the present invention. 
     The head units  10  each eject ink from nozzle openings provided in an ink ejection surface facing the transfer belt  20  to the transfer belt  20  to cause the transfer belt  20  to carry an image. The transport drum  24  transports the recording medium P such that the image carried on the transfer belt  20  is transferred to a predetermined position of the recording medium P by the transfer nip NP. 
     In the inkjet image forming apparatus  1  in the present embodiment, four head units  10  corresponding one-to-one to ink of four colors of yellow (Y), magenta (M), cyan (C), and black (K) are aligned at predetermined intervals in the order of the colors of Y, M, C, and K from the upstream side in a rotation direction of the transfer belt  20 . 
     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 including a pressure chamber for storing ink, a piezoelectric element provided at a wall surface of the pressure chamber, and a nozzle. When a drive signal for deforming the piezoelectric element is input to the recording element, the pressure chamber is deformed by the deformation of the piezoelectric element, changing the pressure in the pressure chamber, so that the ink is ejected from the nozzle communicating with the pressure chamber. 
     The disposed range of the nozzles included in the inkjet head  102  in the orthogonal direction covers the width in the orthogonal direction of an area on which an image is recorded of the recording medium P transported by the transport drum  24 . The head units  10  are used in positions fixed with respect to the rotation axis of the transport drum  24  at the time of image recording. That is, the inkjet image forming apparatus  1  is a single-pass inkjet image forming apparatus. 
     In the present embodiment, as ink to be ejected from the inkjet heads  102  to the transfer belt  20 , ink whose viscosity changes according to the amount of energy supplied to the transfer belt  20  (in this example, the amount of ultraviolet (UV) light output from the first UV irradiation unit  25 ) is used. Specifically, ink whose viscosity increases as the amount of ultraviolet light applied from the first UV irradiation unit  25  increases is used. That is, an image forming section of the inkjet image forming apparatus  1  employs a UV cure inkjet method. 
     The first UV irradiation unit  25  is disposed 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 plays a role in pre-curing ink that has adhered to the transfer belt  20  under the control of the control unit  40 . 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 print job is set to 1.5 mW/cm 2 . 
     The second UV irradiation unit  26  is disposed to irradiate the recording medium P transported downstream of the transfer nip NP with UV light, and plays a role in fully curing the ink that has been pre-cured by the first UV irradiation unit  25  and transferred by the transfer nip NP, under the control of the control unit  40 . 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 irradiation intensity in a normal print job is set to 5 mW/cm 2 . 
     The cleaning unit  27  is disposed opposite 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  can be brought into contact with and separated from the transfer belt  20 , and is brought into contact with the transfer belt  20  under the control of the control unit  40  to remove residual ink and others on the surface of the transfer belt  20 . 
     The density measurement sensor  30  measures the density of a transferred ink image on the recording medium P that has passed through the transfer nip NP, and outputs the value of the measured density to the control unit  40 . As an example of the density measurement sensor  30 , a filter-type densitometer can be used which irradiates the recording medium P with light and measures reflected light (reflected light intensity) through red, green, and blue filters. As another example of the density measurement sensor  30 , a spectral-type densitometer can be used which irradiates the recording medium P with light, divides the wavelength of reflected light finely every 1 to 10 nm or so, and measures the intensity of the reflected light of each wavelength. 
     Next, the other major functional configuration of the inkjet image forming apparatus  1  will be described mainly with reference to  FIG. 2 . The inkjet image forming apparatus  1  includes the head drive unit  101  and the inkjet head  102  included in each head unit  10 , the control unit  40 , the transport drive unit  51 , and an input-output interface  52 . 
     The head drive unit  101  outputs drive signals for deforming the piezoelectric elements according to image data at proper timings to the recording elements of the inkjet head  102 , based on the control of the control unit  40 , thereby causing amounts of ink corresponding to the pixel values of the image data to be ejected from the nozzles of the inkjet head  102 . 
     The control unit  40  includes a central processing unit (CPU)  41 , random-access memory (RAM)  42 , read-only memory (ROM)  43 , 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 , and executes the programs for various kinds of arithmetic processing. The CPU  41  performs centralized control of the entire operation of the inkjet image forming apparatus  1 . 
     The RAM  42  provides a memory space for work to the CPU  41 , and stores temporary data. The RAM  42  may include nonvolatile memory. 
     The ROM  43  stores the various control programs executed by the CPU  41 , the setting data, etc. Instead of the ROM  43 , a rewritable nonvolatile memory such as an electrically erasable programmable read-only memory (EEPROM) or a flash memory may be used. 
     The storage unit  44  stores a print job (an image formation instruction including various kinds of user setting information such as the number of sheets to be printed) input from an external device  2  via the input-output interface  52  and image data related to the print job. As the storage unit  44 , for example, a hard disk drive (HDD) is used, and dynamic random access memory (DRAM) or the like may be used in combination. 
     The transport drive unit  51  provides a drive signal to the transport drum motor of the transport drum  24  based on a control signal provided from the control unit  40 , to rotate the transport drum  24  at a predetermined speed and timing. The transport drive unit  51  provides a drive signal to the motor of the transfer roller  23  based on a control signal provided from the control unit  40 , to rotate the transfer belt  20  at a predetermined speed and timing. 
     The input-output interface  52  mediates data transmission and reception between the external device  2  and the control unit  40 . The input-output interface  52  is formed, for example, by one of various serial interfaces and various parallel interfaces, or a combination thereof. 
     The external device  2  is, for example, a personal computer, and provides a print job, image data, etc. to the control unit  40  via the input-output interface  52 . 
     Next, with reference to the flowchart in  FIG. 3 , processing executed by the control unit  40  when a normal print job is executed will be described. 
     In step S 10  after receiving a print job, image data, etc., the control unit  40  controls the transport drive unit  51  to drive the transport drum  24  and the transfer roller  23  to start to transport the recording medium P. 
     In step S 20 , the control unit  40  controls the head drive unit(s)  101  based on the received image data and the user setting information, to eject ink from the inkjet head(s)  102  of the head unit(s)  10  of a color(s) used in the printing (image formation) onto the transfer belt  20  (intermediate transfer body). By this operation, an image (ink image) based on the input image data is attached to or formed (carried) on the transfer belt  20 . 
     In step S 30 , the control unit  40  controls the output (UV light amount) of the first UV irradiation unit  25  to the above-described intensity at the timing when the ink image on the transfer belt  20  comes to the position of the first UV irradiation unit  25 , to perform an ink image pre-curing process. 
     In step S 40 , the control unit  40  controls the transport drive unit  51  to transport the recording medium P to the transfer nip NP at a predetermined timing, to transfer the ink image pre-cured on the transfer belt  20  to the recording medium P. At this time, the ink image is pressed at a preset transfer pressure (pinched by the transfer nip NP), so that the ink image pre-cured on the transfer belt  20  is transferred to the recording medium P, spreading in whole according to the ink density after the pre-curing, the transfer pressure, etc. 
     In step S 50 , the control unit  40  controls the output of the second UV irradiation unit  26  at the timing when the recording medium P to which the ink image has been transferred comes to the position of the second UV irradiation unit  26 , to perform an ink image full-curing process. Thereafter, the recording medium P is output to an output unit (not shown). 
     In step S 60 , the control unit  40  outputs a drive signal to the cleaning unit  27 , to perform a process of cleaning residual ink and others on the transfer belt  20 . 
     In step S 70 , the control unit  40  determines whether the print job has been finished. Here, if the control unit  40  determines that the print job has not yet been finished (step S 70 , NO), the control unit  40  returns to step S 10  to repeatedly execute the processing in steps S 10  to S 70  described above. On the other hand, if the control unit  40  determines that the print job has been finished (step S 70 , YES), the series of steps is completed. 
     Thus, the intermediate transfer-type inkjet image forming apparatus can spread ink smoothly on the recording medium P without bleeding while reducing the amount of ink liquid ejected from the inkjet heads  102 , and thus has the advantage of being able to save ink. 
     On the other hand, the intermediate transfer-type inkjet image forming apparatus needs the pre-curing process for increasing the viscosity of an ink image on the transfer belt  20  by the viscosity adjuster (the 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 or the like. 
     That is, an attempt to transfer an ink image on the transfer belt  20  to the recording medium P by the transfer nip NP without increasing its viscosity has a problem that the ink may be crushed by the pressing force of the transfer nip NP (spread too much on the recording medium P), depending on the type of ink or the like. Therefore, the inkjet image forming apparatus  1  of the present embodiment also increases (thickens) to some extent the viscosity of an ink image on the transfer belt  20  by irradiation with UV light from the first UV irradiation unit  25  before the ink image is transferred by the transfer nip NP. 
     On the other hand, in the pre-curing process, the transfer state varies, depending on printing environment (such as the state of the transfer belt  20 , the cured state of ink, the type of recording medium P, and transfer conditions such as the transfer pressure). In particular, a decrease in transfer efficiency (transfer rate) at the transfer nip NP causes problems that the loss of ink increases due to the occurrence of residual ink after transfer, and further the image quality deteriorates due to variations in image density, and so on. 
     For these problems, as described above, there is a known technique of adjusting the amount of application of a transferability improvement liquid to the transfer belt  20  according to the amount of residual ink after transfer. However, this technique has a problem of increased cost because it applies the transferability improvement liquid. 
     By contrast, in the present embodiment, the transfer rate when ink is transferred by the transfer nip NP (transfer part) is detected, and the control unit  40  controls a control parameter that influences how the ink spreads when being transferred to the recording medium P, according to the detected transfer rate, to reduce an increase in cost while maintaining high transfer efficiency. The following describes the findings of the present inventors, the principle of the solution in the present invention, etc. 
     As described above, the intermediate transfer-type inkjet image forming apparatus  1  including the transfer belt  20  performs the pre-curing process to irradiate an ink image on the transfer belt  20  with UV light from the first UV irradiation unit  25  before transfer to increase the ink viscosity. It has been found that if the viscosity of ink dots is too low (i.e., if the degree of ink thickening is insufficient) in the pre-curing process, the internal cohesive force of the ink is weak, and the ink dots may break when transferred by the transfer nip NP, resulting in a decrease in transfer rate. On the other hand, it has been found that if the viscosity of ink dots is too high (if the ink is thickened excessively) in the pre-curing process, adhesion to the recording medium P (such as paper) decreases, so that the transfer rate also decreases. 
     On the other hand, as for the transfer pressure of the transfer nip NP when the irradiation intensity of UV light output from the first UV irradiation unit  25  is set to the above-described default value, the following findings have been obtained. It has been found that if the viscosity of ink dots increased by UV light applied from the first UV irradiation unit  25  is slightly high due to various changes in printing environment or the like, setting the transfer pressure of the transfer nip NP higher may be able to prevent or reduce a decrease in the transfer rate. It has also been found that if the viscosity of ink dots increased by UV light applied from the first UV irradiation unit  25  is slightly low, setting the transfer pressure of the transfer nip NP lower may be able to prevent or reduce a decrease in the transfer rate. However, it has been found as described above that if the viscosity of ink dots increased by UV light applied is too high or too low, it is not possible to reduce a decrease in the transfer rate even if the transfer pressure of the transfer nip NP is adjusted. 
     Therefore, to improve the rate of transfer to the recording medium P at the transfer nip NP, it is important to make the viscosity of an ink image when transferred by the transfer nip NP appropriate mainly by controlling the amount of UV light applied from the first UV irradiation unit  25 . In particular, considering the occurrence of various errors in the inkjet image forming apparatus  1  (such as deterioration of the UV light source of the first UV irradiation unit  25  and changes in ink temperature at the time of ejection), differences in the ink absorbency of recording media P, etc., we have come to obtain the findings that it is very important to optimize the ink viscosity (thickened state) at the time of transfer by appropriately controlling the amount of UV light applied from the first UV irradiation unit  25 . It has also been found that the suitability of the transfer rate of an ink image at the transfer nip NP, the appropriate range of ink viscosity, etc. can be determined by measuring the density of the ink image transferred by the transfer nip NP. 
     In view of these findings, in the present embodiment, as shown in  FIG. 1 , the density measurement sensor  30  is provided which measures the density of an ink image transferred by the transfer nip NP. The control unit  40  estimates (detects) the transfer rate at the transfer nip NP from a measurement value of the density measurement sensor  30  (i.e., the density of an ink image), and in accordance with the detection result, performs control to adjust the viscosity or dot state (the transfer pressure as a transfer condition) of ink to be transferred to a recording medium P (second recording medium) to be printed next so that the density of an ink image transferred thereafter and thus the transfer rate meet a predetermined standard (i.e., exceed a threshold value). 
     In the present embodiment, the control unit  40  estimates (detects) the transfer rate of an ink image after being ejected (carried) on the transfer belt  20  and pressed by the transfer nip NP (that is, spread on the recording medium P), based on a detection result of the density measurement sensor  30 . In the example shown in  FIG. 1 , an object detected by the density measurement sensor  30  is the density of ink on the recording medium P (first recording medium) to which an image has been transferred by being pressed by the transfer nip NP. A configuration example in which the density measurement sensor  30  detects a different object will be described later. 
     In the present embodiment, the control unit  40  performs control to adjust the viscosity or dot state of ink to be transferred to the recording medium P (second recording medium) to be printed next according to the result of detection by the density measurement sensor  30  (the ink density) so that the transfer rate of the ink to be transferred to the recording medium P (second recording medium) to be printed next exceeds the threshold value. That is, the control unit  40  controls the output (UV light amount) of the first UV irradiation unit  25  or the transfer pressure of the transfer nip NP as a control parameter that influences how ink spreads when being transferred to the recording medium P such that the transfer rate of ink to be transferred to the recording medium P (second recording medium) to be printed next meets the predetermined standard. 
       FIG. 4  shows in graph form the relationship between the mount of UV light applied by the first UV irradiation unit  25  before transfer and the density of an ink image after passing through the transfer nip NP measured (actually measured) by the density measurement sensor  30 . In the characteristic graph of  FIG. 4 , the horizontal axis represents the amount of UV light applied from the first UV irradiation unit  25  to an ink image (a solid image in this example) on the transfer belt  20 , and the vertical axis represents the density of the solid image on the recording medium P (paper) after passing through the transfer nip NP, actually measured by the density measurement sensor  30 . A dotted line indicates a density value as a boundary value or a threshold value as to whether the transfer rate satisfies a standard value at the density of the solid image actually measured by the density measurement sensor  30 . 
     As shown in  FIG. 4 , in order for the transfer rate to satisfy the standard value (exceed the threshold value), the amount of UV light applied from the first UV irradiation unit  25  needs to be adjusted so that the value of the ink image density actually measured by the density measurement sensor  30  increases to near a peak point. Here, it has been found that the actually measured value of the ink image density and the transfer rate are in a substantially proportional relationship, and when the former reaches the peak point, the transfer rate is the highest (best). On the other hand, it can be seen that the amount (appropriate range) of UV light from the first UV irradiation unit  25  at which the transfer rate satisfies the standard value has some width or margin. 
     Therefore, the present embodiment appropriately combines the adjustment of the UV light amount and the adjustment of the transfer pressure so that the value (density) actually measured by the density measurement sensor  30  becomes high, to be able to improve image quality and achieve image quality suitable for a user&#39;s purpose or the like while maintaining a high transfer rate. 
     Although not shown, it is considered that by changing the transfer pressure of the transfer nip NP, the dot state of an ink image (how it spreads) is changed, and the transfer rate is changed also by the change in the dot state, and further, the transfer pressure also has an optimum value or an appropriate range depending on conditions of the apparatus. Thus, if the UV light amount of the first UV irradiation unit  25  and the transfer pressure of the transfer nip NP are not accurately controlled, an ink image formed on the transfer belt  20  cannot be transferred to the recording medium P with high efficiency by the transfer nip NP, and a decrease in the transfer rate may deteriorate the image quality. Generally, in order for the intermediate transfer-type inkjet image forming apparatus  1  to ensure the transfer rate at the transfer nip NP and thus image reproducibility, both the adjustment of the transfer pressure when an ink image is transferred to the recording medium P by the transfer nip NP, and the adjustment of the ink viscosity in the previous stage are important. 
     On the other hand, in the inkjet image forming apparatus  1 , various factors, errors, etc. that may affect the transfer of an ink image may occur as described above, and it is difficult to detect or obtain all information on the factors, errors, etc. by the control unit  40 . 
     Therefore, in the present embodiment, the control unit  40  executes “transfer efficiency measurement mode” processing described below at a timing different from that when a normal print job is executed described above in  FIG. 3 . If the setting of the output (UV light amount) of the first UV irradiation unit  25  or the transfer pressure of the transfer nip NP is changed in this processing, a setting value after the change is applied to subsequent print jobs for execution. The outline of the processing in the transfer efficiency measurement mode is as follows. 
     In the transfer efficiency measurement mode, the control unit  40  causes ink to be ejected from the inkjet heads  102  to form test pattern ink images (hereinafter referred to as patch images) on the transfer belt  20 . The control unit  40  changes the output (UV light amount) of the first UV irradiation unit  25  in stages to set the viscosity of the ink constituting the patch images on the transfer belt  20  to a plurality of states, in other words, to increase (or decrease) the viscosity of the ink of the patch images in stages. 
       FIG. 5  shows one specific example of the patch images formed in the transfer efficiency measurement mode.  FIG. 5  shows a state after they are transferred from the transfer belt  20  onto the recording medium P (paper). In this example, solid images I of the same shape (rectangular shape) formed by monochromatic ink are aligned in the order of Y, M, C, and K from the left side in a width direction orthogonal to the transport direction of the recording medium P (see an arrow in  FIG. 5 ). The solid images I of each color (Y, M, C, and K; hereinafter simply referred to as patch images I) are aligned (in three rows in this example) along the transport direction. In the example shown in  FIG. 5 , for the patch images I in the first stage corresponding to an irradiation area A 1  enclosed by a dotted line, the amount of UV light applied from the first UV irradiation unit  25  is set low, for the patch images I in the second stage corresponding to an irradiation area A 2 , the amount of UV light applied from the first UV irradiation unit  25  is set medium, and for the patch images I in the third stage corresponding to an irradiation area A 3 , the amount of UV light applied from the first UV irradiation unit  25  is set high. This setting can increase the ink viscosity of the patch images I in stages in the order of the irradiation areas A 1 , A 2 , and A 3 . Alternatively, the amount of UV light applied from the first UV irradiation unit  25  may be set conversely, that is, such that the ink viscosity of the patch images I is reduced in stages in the order of the irradiation areas A 1 , A 2 , and A 3 . 
     Thereafter, from the results of measurement of the density measurement sensor  30 , the control unit  40  determines the ink density on the recording medium P to which the patch images I have been transferred and thus the transfer rate, for each of the irradiation areas A 1 , A 2 , and A 3 . Here, if the determined transfer rates in all of the irradiation areas A 1 , A 2 , and A 3  do not meet the predetermined standard (see the dotted line and the “appropriate range” in  FIG. 4 ), the control unit  40  changes the setting of the output (UV light amount) of the first UV irradiation unit  25  or the transfer pressure of the transfer nip NP until the transfer rate of at least one of the irradiation areas A 1 , A 2 , and A 3  meets the standard, and repeats the formation, transfer, and measurement of the patch images I described above using the changed setting. If the determined transfer rates meet the predetermined standard, the control unit  40  sets the setting values of the output (UV light amount) of the first UV irradiation unit  25  and the transfer pressure of the transfer nip NP (parameters) at which the standard is met as printing conditions when subsequent print jobs are executed. 
     In the present embodiment, this control allows printing with high transfer efficiency at the time of print job execution. 
     Next, an example of the processing executed by the control unit  40  in the above-described transfer efficiency measurement mode will be described with reference to the flowchart shown in  FIG. 6 . The flowchart shown in  FIG. 6  is executed by a user selecting a switch (not shown) for shifting to the transfer efficiency measurement mode after the end of a normal print job, at the start of the apparatus, or at the time of performance of maintenance, for example. 
     In step S 110  after shifting to the transfer efficiency measurement mode, the control unit  40  performs setting to change the UV irradiation condition (output) by the first UV irradiation unit  25  for the test patch images I described above in  FIG. 5  from one output area to another (of the UV light irradiation areas A 1  to A 3  in this example), and to keep the transfer pressure of the transfer nip NP constant. In this example, the control unit  40  sets the UV irradiation condition (output parameter) by the first UV irradiation unit  25  in such a manner that the UV output is set low in the irradiation area A 1  corresponding to an upstream part of the patch images I (see  FIG. 5 ), the UV output is set medium in the irradiation area A 2 , and the UV output is set high in the irradiation area A 3 . The setting values are temporarily stored in the RAM  42  or the storage unit  44  (hereinafter referred to as the RAM  42  or the like). 
     In step S 120 , the control unit  40  reads data on the patch images I described above in  FIG. 5  from the RAM  42  or the like, and controls to print (eject) the patch images I from the head units  10  (inkjet heads  102 ) onto the transfer belt  20 , and transport the recording medium P by the transport drum  24 . 
     In step S 130 , the control unit  40  controls the output parameter of the first UV irradiation unit  25  according to the above-described setting condition at the timing when the patch images I on the transfer belt  20  come to the position of the first UV irradiation unit  25 , and transports the recording medium P to the transfer nip NP to transfer the patch images I. By this processing, in the patch images I on the transfer belt  20 , the patch images I (solid ink images in this example) corresponding to the irradiation area A 1 , the irradiation area A 2 , and the irradiation area A 3  have different viscosities. Specifically, the ink images corresponding to the irradiation area A 1  are cured to the lowest viscosity, the ink images corresponding to the irradiation area A 2  are cured to a medium viscosity, and the ink images corresponding to the irradiation area A 3  are cured to the highest viscosity. These ink images are pressed by the transfer nip NP at the transfer pressure according to the above setting condition, and transferred to the recording medium P as the four-color (YMCK) patch images I. The patch images I transferred to the recording medium P pass through a detection area of the density measurement sensor  30 , and are subjected to the full-curing process by the second UV irradiation unit  26  (see step S 50 ). 
     In step S 140 , the control unit  40  detects the ink densities of the patch images I after being transferred (pressed) to the recording medium P for each of the irradiation areas A 1  to A 3  and the colors (YMCK) of the patch images I, based on detection signals from the density measurement sensor  30 . In this example, the control unit  40  detects twelve, that is, four colors×three areas (A 1  to A 3 ) ink density values (see  FIG. 5 ). In one specific example, the control unit  40  determines the mean value of the ink density of one color and one area from the results of the ink density detection by the density measurement sensor  30 , and detects the mean value for each of the twelve areas. 
     In step S 150 , the control unit  40  stores the detected ink density values (the mean value of each area in this example) in the RAM  42  or the like. 
     In step S 160 , the control unit  40  determines whether to perform process setting of the first UV irradiation unit  25  and the transfer pressure, according to whether a condition set by the user is satisfied. 
     Here, as one specific example of the “condition set by the user”, it is set as a condition that it have been detected N times (N is a user setting value) that the ink density values of all the twelve areas of the patch images I (see the “density of ink image” in  FIG. 4 ) stored in the RAM  42  or the like fall within an allowable range (see the “appropriate range” in  FIG. 4 ). 
     Here, if the control unit  40  determines that it has not yet been detected N times (step S 160 , NO), the control unit  40  returns to step S 110  and repeats the processing in steps S 110  to S 160  described above. In this case, in step S 110  again, the control unit  40  estimates the UV irradiation condition (gradually increasing output value) and the transfer pressure of the transfer nip NP (constant value) at which the ink density values of all the twelve areas described above fall within the allowable range (appropriate range), and sets values based on the estimation results. The subsequent processing in steps S 120  to S 160  is the same as that described above. 
     If the ink density values of the twelve areas of the patch images I printed later do not fall within the allowable range (see the dotted line and the “appropriate range” in  FIG. 4 ) despite M (M is a user setting value larger than N) executions of the processing in steps S 110  to S 150 , the control unit  40  performs the following processing. In this case, the control unit  40  considers that the apparatus reaches the end of its life or has abnormality, and displays that fact and performs a display to urge the user to perform maintenance work by a serviceman or the like because deterioration of image quality printed in print jobs is expected. 
     Thus, if the control unit  40  determines that the condition set by the user is satisfied (step S 160 , YES), the control unit  40  proceeds to step S 170 . 
     In step S 170 , the control unit  40  sets actual printing process (printing setting) to the UV irradiation condition (the output value of the first UV irradiation unit  25 ) and the transfer pressure at which the density is the highest among the N detection results stored in the RAM  42  or the like, and completes a series of steps. 
     According to the present embodiment that performs the above-described processing, even if there are various factors and errors that affect the transfer rate, high transfer efficiency can be maintained without using a transferability improvement liquid or the like. Therefore, the inkjet image forming apparatus  1  of the present embodiment can maintain high transfer efficiency at low cost. 
     In the configuration example described above, the density measurement sensor  30  measures ink densities on the recording medium P to which ink images have been transferred through the transfer nip NP, and the control unit  40  estimates or determines transfer rates from the measurement results. As another configuration example, the density measurement sensor  30  may measure residual ink densities on the transfer belt  20  on the downstream side of the transfer nip NP, and the control unit  40  may estimate or determine transfer rates from the measurement results. In this case, the recording medium P used for transferring the patch images I may be of a material difficult to transfer. In this case, in step S 170  described above, the control unit  40  may set the actual printing process (printing setting) to the UV irradiation condition (the output value of the first UV irradiation unit  25 ) and the transfer pressure at which the residual ink densities are the lowest among the N detection results stored in the RAM  42  or the like. 
     In the configuration example described above, for simplification, patch image patterns of one density (that is, only solid images with 100% density) are detected. As another example, patch image patterns of a plurality of densities (e.g., 75% and 50%) may be detected. In this case, more accurate transfer rates and thus output of the first UV irradiation unit  25  and transfer pressure suitable for various kinds of image data can be determined. 
     The above-described embodiment has described the case of using, as ink ejected from the inkjet heads  102 , ink whose viscosity changes according to ultraviolet rays applied to the ink (UV light output from the first UV irradiation unit  25  and the second UV irradiation unit  26 ). 
     As another example, ink whose viscosity increases by irradiation with other active energy rays (e.g., electron beams) than UV light may be used. In this case, the first UV irradiation unit  25  and the second UV irradiation unit  26  may include an electron beam generation source that emits electron beams. 
     As another example, ink whose viscosity changes by change of the amount of heat supplied to the transfer belt  20  may be used. For example, ink that is gel at normal temperature and becomes sol by heating decreases in viscosity as it is heated. In this case, the first UV irradiation unit  25  and the second UV irradiation unit  26  may include a heating unit (heat source) such as a heater or a halogen lamp. 
     On the other hand, if ink containing water as a solvent is used, the water evaporates and the viscosity increases as the ink is heated. In this case, for the first UV irradiation unit  25  and the second UV irradiation unit  26 , the above-described heating unit (heat source) or one that can control the transfer characteristics of an ink image by adjusting air-blowing temperature such as an air knife that produces an air flow (wind) for evaporating the water in the ink may be used. The first UV irradiation unit  25  and the second UV irradiation unit  26  are not limited to the above configurations as long as they can remove a solvent containing water in ink. As other examples, a member that brings a porous body into contact with an ink image to absorb a solvent containing water, a squeegee blade and roller that squeezes a solvent containing water out, etc. may be used. 
     In the configuration example described above, the setting values of the transfer conditions (i.e., the UV light amount from the first UV irradiation unit  25  and the transfer pressure) are changed to maximize the ink density and thus the transfer rate, but they may be set within a range that results in predetermined transfer efficiency or more (see the “appropriate range” in  FIG. 4 ). Within that range, the setting values may be determined in relation to another function, for example. Here, another function may be, for example, the transfer rate of only black (K) in monochrome or gray scale printing, or a setting value focusing on color reproducibility or the like of a photographic image in full color printing. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. That is, the present invention can be implemented in various forms without departing from its scope or its major features. The scope of the present invention should be interpreted by terms of the appended claims.