Inkjet-type image forming apparatus, control method for image forming apparatus, and control program for image forming apparatus

Abstract

A control unit (21c) of an image forming apparatus (1) is configured to execute a first process for acquiring image data to be printed, a second process for acquiring distance data on the distance from a nozzle surface of a discharge head (20) to a surface to be coated of an image-formation object (P), and a third process for determining, on the basis of the image data, a droplet size of ink droplets discharged from the discharge head (20) and determining, on the basis of the distance data, a discharge condition for discharging the ink droplets from the discharge head (20) so that the ink droplets land at a target position on image-formation object (P) on which an image is to be formed. In the third process, an ink coating layer is formed on the image-formation object (P), and the discharge condition is changed according to changes in the distance.

Description

Inkjet image forming apparatus, control method for said image forming apparatus, and control program for said image forming apparatus

[0001] The present disclosure relates to an inkjet image forming apparatus, a control method for the image forming apparatus, and a control program for the image forming apparatus.

[0002] 2. Description of the Related Art Conventionally, inkjet image forming apparatuses have been known that use a discharge head that discharges ink using a piezoelectric element to discharge ink droplets onto a target, thereby forming an image on the target.

[0003] This type of image forming apparatus can easily and inexpensively create images, and so it has been applied to a variety of printing fields, including photography, various printing, marking, and special printing such as color filters.In recent years, it has also been increasingly applied to printing on various materials such as glass, resin, and metal.

[0004] Against this background, image forming technologies have evolved in various ways in recent image forming devices, making it possible to form images with high precision even on objects whose coating surfaces have uneven or curved shapes.

[0005] For example, Patent Document 1 discloses a technology that calculates the distance between the nozzle and the object to be coated with ink, adjusts the timing of ink ejection from the nozzle according to that distance, and corrects the landing position.

[0006] Furthermore, Patent Document 2 discloses a technique for changing the droplet volume by comparing the length of a first curved surface at a certain reference position with the length of the curved surface at the position of the curved surface to be ejected, in order to perform accurate patterning on a curved surface shape.

[0007] Furthermore, Patent Document 3 discloses a technology that has a means for changing the ejection speed of droplets ejected from the head depending on the ink ejection distance between the head and the object to be drawn, and that makes the ejection speed faster when the ink ejection distance is long than when it is short.

[0008] JP 2016-068290 A JP 2016-123942 A JP 2015-136821 A

[0009] Incidentally, in recent years, as inkjet technology has been increasingly introduced into the industrial field, there has been a demand for this type of image forming apparatus to be able to handle thicker coating films.

[0010] In this regard, UV-curable ink and heat-curable ink can be instantly cured and dried, and therefore, for this type of image forming apparatus, a method is being considered in which an ink coating layer is formed on the object to be printed using UV-curable ink or the like, thereby increasing the thickness of the coating film.

[0011] FIG. 1 is a diagram showing an example of a method for thickening a coating film using an inkjet image forming apparatus. In this method, the image forming apparatus first ejects ink from an ejection head H onto a target object P to form a first ink coating layer L1 on the target object P. Next, the image forming apparatus hardens the first ink coating layer L1. Next, the image forming apparatus ejects ink from the ejection head H onto the first ink coating layer L1 on the target object P to form a second ink coating layer L2 on the first ink coating layer L1. Next, the image forming apparatus hardens the second ink coating layer L2. Next, the image forming apparatus ejects ink from the ejection head H onto the second ink coating layer L2 on the target object P to form a third ink coating layer L3 on the second ink coating layer L2. Next, the image forming apparatus hardens the third ink coating layer L3.

[0012] In this way, by repeatedly ejecting ink from the ejection head H onto the object to be drawn, multiple ink coating layers L1, L2, L3 are formed on the object to be drawn P, thereby achieving a thick coating film. The number of ink coating layers stacked depends on the target film thickness of the ink coating layers, but for example, five or more layers may be stacked. Furthermore, the thickness of the ink coating layers formed by stacking them may be 0.5 mm or more.

[0013] Here, an embodiment has been shown in which each ink coating layer is cured by UV irradiation or the like after it has been formed. However, in reality, it is possible to stack ink coating layers by natural drying alone, and there are also cases in which multiple ink coating layers are formed and then cured all at once by UV irradiation or the like.

[0014] However, when the coating film formed on the drawing object is thickened using this method, the distance between the nozzle surface of the ejection head and the surface of the drawing object to be coated gradually decreases as the ink coating layers are stacked. This changes the time it takes for ink droplets ejected from the ejection head to land on the drawing object. As a result, as the ink coating layers are stacked, the landing position of the ink droplets on the drawing object gradually shifts from the target position set based on the state before the ink coating layers are formed. In other words, the formation position of the ink coating layer on the drawing object changes for each layer of the ink coating layer, resulting in a deterioration in image quality.

[0015] Furthermore, in recent image forming devices, due to the demand for high-speed printing, ink is ejected while the ejection head H is moved relative to the object to be drawn, and it can be said that this configuration makes it more likely that the above-mentioned deviation in the landing position of ink droplets will occur.

[0016] In the following, the surface of the object on which the ink coating layers are formed (typically the upper surface) will also be referred to as the “coating target surface.” Also, the distance between the nozzle surface of the ejection head and the coating target surface of the object will also be referred to as the “ink ejection distance.”

[0017] The prior art such as Patent Documents 1 to 3 does not recognize this problem at all and is not capable of solving this problem.

[0018] The present disclosure has been made in consideration of these problems. That is, an object of the present disclosure is to provide an inkjet image forming apparatus that can form high-quality images even when forming a thick coating film on an object to be imaged. In another aspect, an object of the present disclosure is to provide a control method for the image forming apparatus. In another aspect, an object of the present disclosure is to provide a control program for the image forming apparatus.

[0019] The present disclosure primarily solves the above-mentioned problems and is an inkjet type image forming apparatus having a discharge head and a control unit that controls the discharge head, and that repeatedly discharges ink from the discharge head onto a drawing object to form a laminated ink coating layer on the drawing object, wherein the control unit is configured to execute: a first process that acquires image data of a printing object; a second process that acquires distance data from a nozzle face of the discharge head to a coating target surface of the drawing object; and a third process that determines the droplet size of ink droplets to be discharged from the discharge head based on the image data, and determines the discharge conditions for discharging the ink droplets from the discharge head based on the distance data so that the ink droplets land at target positions on the drawing object, wherein in the third process, the ink coating layer is formed on the drawing object, and the discharge conditions are changed as the distance changes.

[0020] In another aspect, there is provided a control method for an inkjet image forming apparatus having a discharge head and a control unit that controls the discharge head, and which repeatedly discharges ink from the discharge head onto a drawing object to form a laminated ink coating layer on the drawing object, the control method including: a first process of acquiring image data of a printing object; a second process of acquiring distance data from a nozzle face of the discharge head to a coating target surface of the drawing object; and a third process of determining a droplet size of ink droplets to be discharged from the discharge head based on the image data, and determining discharge conditions for discharging the ink droplets from the discharge head based on the distance data so that the ink droplets will land at target positions on the drawing object, wherein in the third process, the ink coating layer is formed on the drawing object, and the discharge conditions are changed as the distance changes.

[0021] In another aspect, there is provided a control program for an inkjet image forming apparatus having a discharge head and a control unit for controlling the discharge head, and which repeatedly discharges ink from the discharge head onto an object to be drawn, thereby forming a laminated ink coating layer on the object to be drawn, the control program causing a computer to execute: a first process for acquiring image data of an object to be printed; a second process for acquiring distance data from a nozzle face of the discharge head to a surface to be coated of the object to be drawn; and a third process for determining a droplet size of ink droplets to be discharged from the discharge head based on the image data, and determining, based on the distance data, discharge conditions for discharging the ink droplets from the discharge head so that the ink droplets land at target positions on the object to be drawn; and in the third process, the ink coating layer is formed on the object to be drawn, and the discharge conditions are changed as the distance changes.

[0022] According to the image forming apparatus according to the present disclosure, it is possible to form a high-quality image even when forming a thick coating film on an object to be imaged.

[0023] FIG. 1 is a diagram illustrating an example of a method for thickening a coating film using an inkjet image forming apparatus. FIG. 2 is a block diagram illustrating an example of the functional configuration of the image forming apparatus. FIG. 3 is a diagram illustrating an example of the device configuration of the image forming apparatus. FIG. 4A is a diagram illustrating an example of the configuration of an ejection head (normal state). FIG. 4B is a diagram illustrating an example of the configuration of an ejection head (pressure chamber contracted state). FIG. 5 is a diagram illustrating an example of a waveform pattern of a drive waveform supplied from a head IC. FIG. 6 is a diagram illustrating an example of control processing of the head IC. FIG. 7 is a diagram illustrating an example of the configuration of the head IC. FIG. 8 is a diagram illustrating an example of a grayscale setting table. FIG. 9 is a diagram illustrating a drive waveform pattern defined in the grayscale setting table. FIG. 10 shows measurement results of the degree of deceleration of the ejection speed due to air resistance for each droplet size. FIG. 11 is a diagram illustrating a modified grayscale setting table. FIG. 12 is a diagram illustrating a modified grayscale setting table. FIG. 13 is a diagram illustrating another modified grayscale setting table. FIG. 14 is a diagram illustrating another modified grayscale setting table. FIG. 15 is a diagram illustrating a modified drive waveform pattern for adjusting the impact position of ink droplets.

[0024] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functions are designated by the same reference numerals, and redundant description will be omitted.

[0025] [Overall Configuration of Image Forming Apparatus] An example of the overall configuration of an inkjet type image forming apparatus 1 according to one embodiment of the present invention will be described below, which will be abbreviated as "image forming apparatus 1" hereinafter.

[0026] Fig. 2 is a block diagram showing an example of the functional configuration of the image forming apparatus 1. Fig. 3 is a diagram showing an example of the device configuration of the image forming apparatus 1. Here, three mutually orthogonal axes (X-axis, Y-axis, and Z-axis) are defined as shown in Fig. 3. The Z-axis direction corresponds to the vertically upward direction, and the X-axis and Y-axis directions correspond to horizontal directions perpendicular to the vertically upward direction.

[0027] The image forming apparatus 1 includes an apparatus main body 10 and ejection heads 20Y, 20M, 20C, and 20K corresponding to the colors yellow, magenta, cyan, and black, respectively.

[0028] The device main body 10 includes a control unit 11, an interface 12, a page memory 13, a line memory 14, a drive waveform generating unit 15, a UV irradiation unit 16, a drive unit 17, and the like.

[0029] The ejection heads 20Y, 20M, 20C, and 20K are respectively equipped with head ICs 21B, 21M, 21C, and 21K, head main bodies 22Y, 22M, 22C, and 22K, and distance sensors 23Y, 23M, 23C, and 23K. Note that the ejection heads 20Y, 20M, 20C, and 20K are assumed to have the same configuration, and will hereinafter be simply referred to as the "ejection head 20," the "head IC 21," the "head main body 22," and the "distance sensor 23."

[0030] The control unit 11 is, for example, a microcomputer configured with a CPU, a ROM, a RAM, etc. The control unit 11 controls the overall operation of each unit of the image forming apparatus 1 in accordance with various data and programs stored in the ROM, RAM, etc.

[0031] The control unit 11 controls the ink ejection from the ejection head 20, the drive waveform generated by the drive waveform generation unit 15, the drive operation of the drive unit 17, and the irradiation operation of the UV irradiation unit 16. The control unit 11 also supplies a clock signal to the head IC 21 to control the timing of various data transfers.

[0032] The interface 12 is a means for transmitting and receiving data to and from an external device M such as a personal computer. The interface 12 sends data (for example, image data) received from the external device M to the control unit 11.

[0033] The page memory 13 stores image data to be printed that is transmitted from the external device M. The line memory 14 temporarily stores pixel data to be ejected from each nozzle of the head main body 22 from the image data stored in the page memory 13 when printing an image on the drawing target P. The line memory 14 then sequentially transfers the stored pixel data to the head IC 21. Note that pixel data refers to the pixel value of each pixel position in the image data.

[0034] The drive waveform generation unit 15 generates a drive waveform and supplies the drive waveform to the ejection head 20. The head IC 21 selects an output destination for the drive waveform supplied to the ejection head 20, and the drive waveform is supplied to the piezoelectric element of the channel in the head body 22 from which ink is to be ejected. The drive waveform generation unit 15 is configured, for example, by a DA conversion circuit. Under the control of the control unit 11, the drive waveform generation unit 15 generates a drive waveform that depicts a predetermined voltage waveform.

[0035] The drive waveform generated by the drive waveform generating unit 15 is, for example, a pulse waveform, in which the rise time, fall time, pulse width, and peak value are appropriately set according to the characteristics of the ejection head 20.

[0036] The drive waveform generating unit 15 generates a voltage waveform for non-ejection of ink in addition to the drive waveform for ejection of ink, and supplies this to the head IC 21. The voltage waveform for non-ejection of ink is, for example, a low-voltage pulse oscillation waveform. This non-ejection waveform is a waveform that oscillates the meniscus of the channel of the ejection head 20 and subsequently facilitates the ink ejection operation during ink ejection.

[0037] The ejection head 20 receives ink of the corresponding color from ink storage units for each color that store ink of that color, and selectively ejects the ink onto the object to be drawn P. In this way, printing is performed on the surface of the object to be drawn P.

[0038] The ejection head 20 is disposed so that the nozzle surface, on which the nozzles for ejecting ink droplets are formed, faces downward. In the image forming apparatus 1, the ejection head 20 is disposed so that the nozzle surface faces the upper surface of the object P to be drawn in the ±Z direction.

[0039] The positional relationship between the discharge head 20 and the object P to be drawn is controlled by the driving unit 17 of the device main body 10 .

[0040] 3, the drive unit 17 has an X-direction drive unit 17a and a Y-direction drive unit 17b. In the image forming apparatus 1, the Y-direction drive unit 17b is built into the table TK, and the drawing object P is fixed on a turntable TT attached to the Y-direction drive unit 17b. In other words, the turntable TT serves as a stage on which the drawing object P is placed. Note that, taking into consideration the formation of ink coating layers on the drawing object P, the turntable TT is set so that the ink ejection distance before the formation of the ink coating layer is, for example, 5 mm or more.

[0041] The image forming apparatus 1 also includes a table TK that is positioned horizontally relative to the floor, and two stands TS that extend vertically. One end of each of the two stands TS is attached to the table TK, and the other end supports an X-direction drive unit 17a. The X-direction drive unit 17a holds the ejection head 20 and moves the ejection head 20 linearly in the X direction. Note that only one of the ejection heads 20Y, 20M, 20C, and 20K is depicted in FIG. 3.

[0042] In this way, the image forming apparatus 1 has drive units 17 for each direction (X direction, Y direction), namely, X-direction drive unit 17a and Y-direction drive unit 17b, and by combining the operations of the drive units 17 for each of these directions, it becomes possible to move the ejection head 20 relative to the drawing object P. Note that the drive unit 17 may also have a Z-direction drive unit that moves the ejection head 20 along the Z direction relative to the drawing object P.

[0043] The drive unit 17 operates under the control of the control unit 11 to cooperate with ink ejection from the ejection head 20. For example, the control unit 11 operates the X-direction drive unit 17a to move the ejection head 20 from left (-X) to right (+X) along the X-axis direction, causing the ejection head 20 to eject ink at each position. Then, when the control unit 11 has completed drawing one line, it operates the Y-direction drive unit 17b to move the ejection head 20 a predetermined distance in the +Y direction to a position where it will draw the next line. Then, the control unit 11 again operates the X-direction drive unit 17a to move the ejection head 20 from left (-X) to right (+X) along the X-axis direction, causing the ejection head 20 to eject ink at each position. The control unit 11 repeats this operation to draw a predetermined area on the drawing target P.

[0044] The UV irradiation unit 16 irradiates the ink-applied area on the drawing object P with UV light, heats and dries the ink on the drawing object P, and fixes it on the drawing object P. The UV light emitted by the UV irradiation unit 16 is, for example, UV light with a wavelength (e.g., 400 nm) that corresponds to the light absorption wavelength of the ink. Under the control of the control unit 11, the UV irradiation unit 16 irradiates the drawing object P with UV light at a predetermined timing.

[0045] Note that various objects capable of fixing ink that has landed on the surface can be used as the drawing object P. In particular, the image forming apparatus 1 according to this embodiment can be suitably used for drawing objects P whose application target surface has an uneven or curved shape.

[0046] The ink ejected from the ejection head 20 may be, for example, a heat-curable or photo-curable ink (here, photo-curable ink). The ink ejected from the ejection head 20 is ejected as sol-state ink droplets. After landing on the object P to be drawn, the ink droplets are irradiated with UV light by the UV irradiation unit 16, whereby they quickly gel and solidify on the object P to be drawn.

[0047] [Configuration of ejection head 20 (head main body 22)] Figures 4A and 4B are diagrams showing an example of the configuration of the head main body 22 of the ejection head 20. Here, Figures 4A and 4B are cross-sectional views of the head main body 22 viewed from the top side. Note that Figure 4A shows the state of the pressure chamber Td when no drive waveform is applied to the piezoelectric element T. Also, Figure 4B shows the state of the pressure chamber Td when a drive waveform is applied to the piezoelectric element T.

[0048] The head main body 22 has, for example, a plurality of channels Tc each of which ejects ink. 256 channels Tc, Tc_ch1 to Tc_ch256, are shown in Figures 4A and 4B.

[0049] Each channel Tc is formed, for example, by a nozzle Te, a pressure chamber Td communicating with the nozzle Te and containing ink, and a piezoelectric element T arranged to form part of the wall of the pressure chamber Td. The piezoelectric element T is formed by a piezoelectric member Tf and a first electrode Tg and a second electrode Th sandwiching the piezoelectric member Tf from the left and right. When the potential difference between the first electrode Tg and the second electrode Th changes, the piezoelectric element T changes state to, for example, an elongated state, as shown in FIG. 4B . The first electrode Tg is an electrode separated for each piezoelectric member Tf and serves as a drive electrode for each piezoelectric member Tf. The second electrode Th serves as a common electrode shared by all piezoelectric members Tf. The common electrode is, for example, grounded.

[0050] Each channel Tc changes the state of the piezoelectric element T, for example, to an expansion state, in response to the drive waveform input from the head IC 21. When a ground voltage is supplied to each channel Tc, for example, the channel Tc contracts the piezoelectric element T and supplies ink from the supply path into the pressure chamber Td ( FIG. 4A ). When a positive drive waveform is supplied to each channel Tc, for example, the channel Tc expands the piezoelectric element T and contracts the pressure chamber Td, thereby ejecting ink from the nozzle Te ( FIG. 4B ).

[0051] Each channel Tc ejects ink droplets from the nozzle Te at different droplet sizes and ejection speeds according to the waveform pattern of the drive waveform supplied from the head IC 21 .

[0052] FIG. 5 is a diagram showing an example of a waveform pattern of a drive waveform supplied from the head IC 21. As shown in FIG.

[0053] 5 shows a state in which two positive voltage pulse drive waveforms are successively supplied to channel Tc in synchronization with clock signal GSCLK. Typically, one positive voltage pulse drive waveform causes one ink ejection operation in channel Tc. Therefore, when the waveform pattern of the drive waveform shown in FIG. 5 is supplied, two ink ejection operations are performed in channel Tc. Note that hereinafter, the positive voltage pulse drive waveform is also referred to as an "ejection pulse."

[0054] The ejection head 20 according to this embodiment is configured to change the size of ink droplets ejected from each channel Tc by adjusting the number of ink droplets ejected continuously. This type of control is also called a multi-drop method. That is, by supplying multiple ejection pulses to the channel Tc at predetermined intervals, multiple ink droplets are continuously extruded from the nozzle Te and ejected as a continuous ink mass without separating from each other. The ink mass then coalesces during flight and lands on the drawing target P as a single ink droplet. That is, the ink droplet that lands on the drawing target P has a volume that corresponds to the number of ink ejection operations.

[0055] Therefore, the head IC 21 can adjust the size of droplets ejected from the channel Tc by adjusting the number of consecutive ejection pulses supplied to the channel Tc. For example, when the number of consecutive ejection pulses supplied to the channel Tc is one, the ink droplets ejected from the channel Tc are small-sized ink droplets. When the number of consecutive ejection pulses supplied to the channel Tc is two, the ink droplets ejected from the channel Tc are medium-sized ink droplets. When the number of consecutive ejection pulses supplied to the channel Tc is three, the ink droplets ejected from the channel Tc are large-sized ink droplets.

[0056] The period of the ejection pulses continuously supplied to the channel Tc is set to, for example, approximately the natural vibration period (1 AL: Acoustic Length) of the ink in the pressure chamber Td. In this embodiment, the period of the clock signal GSCLK is set to a period (for example, 1 AL) corresponding to the natural vibration period of the ink in the pressure chamber Td.

[0057] Furthermore, the ejection head 20 according to this embodiment is configured to change the ejection speed of ink droplets ejected from the channel Tc by changing the voltage amplitude of the ejection pulse. That is, the head IC 21 can increase the speed at which the pressure chamber Td changes from an expanded state to a contracted state by increasing the voltage amplitude of the ejection pulse supplied to the channel Tc. This can increase the ejection speed of ink droplets ejected from the channel Tc.

[0058] [Configuration of distance sensor 23] The distance sensor 23 is configured, for example, by a known optical displacement sensor having a light-projecting unit and a light-receiving unit. The light-projecting unit emits laser light downward in the Z-axis direction. The light-receiving unit receives light diffused and reflected from the surface of the object P to be drawn using a line sensor, lens, or the like. The distance sensor 23 then determines the distance from the distance sensor 23 to the surface of the object P to be drawn using, for example, a triangulation method. That is, the distance sensor 23 thereby measures the distance from the nozzle surface of the ejection head 20 to the surface of the object P to be drawn (ink ejection distance).

[0059] The distance sensor 23 is, for example, disposed integrally with the head main body 22. The distance sensor 23 operates under the control of the control unit 11 and measures the ink ejection distance at a predetermined timing. The distance sensor 23 measures the ink ejection distance, for example, before an ink coating layer is formed on the drawing object P. The distance sensor 23 also measures the ink ejection distance, for example, every time one ink coating layer or a predetermined number of ink coating layers are stacked on the drawing object P.

[0060] If the surface of the object to be drawn P to be coated has an uneven or curved shape, the control unit 11 may measure the three-dimensional shape of the surface to be coated of the object to be drawn P. In this case, the control unit 11 controls the drive unit 17 to, for example, control the movement of the distance sensor 23 so as to scan within the XY plane of the surface to be coated of the object to be drawn P. The control unit 11 then measures the ink discharge distance at multiple points (X, Y) within the XY plane, thereby measuring the three-dimensional shape of the surface to be coated of the object to be drawn P.

[0061] Although the distance sensor 23 is configured as a reflective optical sensor in the above example, the distance sensor 23 is not limited to this sensor. For example, other optical sensors, contact sensors, ultrasonic sensors, etc. may also be used.

[0062] [Configuration of head IC 21] As described above, the ink ejection distance changes as the ink coating layers are laminated. In this case, if no changes are made to the ink ejection conditions, the landing position of the ink droplets on the imaged object will gradually deviate from the target position set based on the state in which the ink coating layers have not yet been formed.

[0063] Therefore, in the image forming apparatus 1 according to this embodiment, the formation position of the ink coating layer formed on the object P is adjusted for each layer by controlling the head IC 21. This adjustment is realized by changing the waveform pattern of the drive waveform supplied to the channel Tc of the ejection head 20 under the control of the head IC 21 in accordance with the ink ejection distance.

[0064] FIG. 6 is a diagram showing an example of a control process of the head IC 21.

[0065] In step S1, the head IC 21 determines whether it is time to measure the ink ejection distance. If it is time to measure (S1: YES), the head IC 21 proceeds to step S2. On the other hand, if it is not time to measure (S1: NO), the head IC 21 proceeds to step S3.

[0066] Here, the timing for measuring the ink ejection distance can be, first, the timing before an ink coating layer is formed on the drawing object P. Second, the timing for measuring can be the timing after an ink coating layer is formed on the drawing object P. In this case, it is preferable to measure the ink ejection distance every time one ink coating layer is laminated on the drawing object P.

[0067] In step S2, the head IC 21 uses the distance sensor 23 to measure the ink ejection distance.

[0068] Incidentally, when the surface of the object to be drawn P to be coated is flat, it is sufficient to measure the ink ejection distance at only one point. However, when the surface of the object to be drawn P to be coated is uneven or curved, it is preferable to measure the ink ejection distance at each position of the area to be drawn on the surface of the object to be drawn P. In this case, the head IC 21 may measure the three-dimensional shape of the surface of the object to be drawn P to be coated.

[0069] In step S3, the head IC 21 acquires image data of the printing target. Here, the head IC 21 acquires, for example, pixel data of the target area from which ink will be ejected next. The pixel value indicated by the pixel data represents the amount of ink required to be ejected at that pixel position. That is, on the drawing target P, shades of light and dark are expressed according to the area covered by ink in each pixel.

[0070] In step S4, the head IC 21 acquires the distance data of the ink ejection distance obtained in step S2.

[0071] The ink ejection distance indicates the timing at which ink droplets ejected from the nozzles Te of the ejection head 20 will land on the drawing object P. In order to ensure that ink droplets ejected from the nozzles Te land at the same target position on the drawing object P regardless of the ink ejection distance, it is effective to adjust the ejection speed of the ink droplets, for example. Specifically, the longer the ink ejection distance, the more effective it is to increase the ejection speed of the ink droplets. Therefore, in step S5, the head IC 21 determines the ejection speed of the ink droplets based on the ink ejection distance.

[0072] In step S5, the head IC 21 selects a waveform pattern of a drive waveform to be supplied to the piezoelectric elements T of the ejection head 20 based on the image data of the print target and distance data of the ink ejection distance. That is, the head IC 21 determines the droplet size of the next ink droplet to be ejected based on the image data of the print target. Then, the head IC 21 determines the ejection speed of the next ink droplet to be ejected based on the ink ejection distance. Then, the head IC 21 supplies the waveform pattern of a drive waveform to achieve the droplet size and ejection speed to the piezoelectric elements T of the ejection head 20. An example of the waveform pattern of the drive waveform used at this time will be described later with reference to FIGS. 8 and 9.

[0073] If the surface of the drawing object P to be coated has an uneven or curved shape, the ink ejection distance will differ for each pixel position to which ink is ejected. In this case, the head IC 21 may refer to the ink ejection distance data for each pixel position to which ink is ejected and determine the ejection speed for that pixel position.

[0074] In this way, the head IC 21 performs the processes of S1 to S5 for each pixel position on the drawing object P, selects an appropriate waveform pattern of the drive waveform, and drives the piezoelectric elements T of the ejection head 20. As a result, the ejection head 20 drives each piezoelectric element T with the selected waveform pattern of the drive waveform for each pixel position of the image data, thereby forming an image on the drawing object P. The head IC 21 repeats this process, adjusts the ejection speed when ejecting ink onto the drawing object P in accordance with the ink ejection distance, and adjusts the formation position of the ink coating layer formed on the drawing object P for each layer.

[0075] Although a description of the ink curing process has been omitted here, it is preferable that the image forming apparatus 1 carries out an ink curing step each time one ink coating layer is formed on the drawing object P. In the ink curing step, for example, under the control of the control unit 11, the UV irradiation unit 16 irradiates the ink coating layer formed on the drawing object P with UV light, thereby curing the ink coating layer. However, it is also possible to stack ink coating layers by natural drying alone, and multiple ink coating layers may be formed and then cured all at once.

[0076] Next, an example of the configuration of the head IC 21 will be shown.

[0077] Fig. 7 is a diagram showing an example of the configuration of the head IC 21. In the configuration of the head IC 21 shown in Fig. 7, the function of adjusting the drive waveform pattern described above is realized by a grayscale controller 21c. The grayscale controller 21c is, for example, a microcomputer including a CPU, ROM, RAM, input ports, output ports, etc.

[0078] Fig. 8 is a diagram showing an example of the grayscale setting table D1 referred to by the grayscale controller 21c. Fig. 9 is a diagram explaining the drive waveform patterns defined in the grayscale setting table D1. Hereinafter, the grayscale controller 21c will be referred to as the "GS controller 21c." Also, the grayscale setting table D1 will be referred to as the "GS setting table D1." The GS setting table D1 is, for example, a data table stored in the ROM of the GS controller 21c.

[0079] The head IC 21 includes a shift register 21 a, a latch circuit 21 b, a GS controller 21 c, and a buffer amplifier 21 d. In this example, the number of channels Tc of the head main body 22 is 256, and 3-bit pixel data for 256 channels Tc is processed in parallel.

[0080] The shift register 21a is, for example, a FIFO-type memory that stores pixel data for 256 channels Tc. The shift register 21a stores each pixel data input in parallel from the line memory 14 in synchronization with a transfer clock signal CLK. The 256 pixel data stored in the shift register 21a are then output in parallel to the latch circuit 21b all at once at a predetermined timing.

[0081] The latch circuit 21b holds the pixel data for 256 channels Tc output from the shift register 21a until the timing specified by the latch signal LAT, and then outputs the pixel data to the GS controller 21c in response to the input of the latch signal LAT.

[0082] The GS controller 21c receives a plurality of types of voltage waveforms from the drive waveform generating unit 15. The GS controller 21c then selects one of the plurality of drive waveform pattern data registered in the GS setting table D1 based on the pixel data and ink ejection distance data. The GS controller 21c then sequentially outputs voltage waveforms corresponding to the selected drive waveform pattern data to the buffer amplifier 21d.

[0083] The voltage waveforms input from the drive waveform generation unit 15 to the GS controller 21c include a drive waveform for ink ejection and a non-ejection waveform for non-ejection of ink. The GS controller 21c selectively supplies one of these to each channel Tc of the ejection head 20 in accordance with the pixel data and ink ejection distance data. In this embodiment, the drive waveform for ink ejection input from the drive waveform generation unit 15 is one type for each timing, but multiple types may be input.

[0084] The GS controller 21c causes the distance sensor 23 to measure the ink discharge distance at a predetermined timing, and acquires ink discharge distance data from the distance sensor 23. The timing at which the GS controller 21c acquires the ink discharge distance data is as described in step S2 of FIG.

[0085] The GS controller 21c also receives a clock signal GSCLK from the control unit 11, control commands (commands indicating operation details) from the control unit 11, and the like.

[0086] The buffer amplifier 21d amplifies the power of the drive waveform output from the GS controller 21c. The buffer amplifier 21d then supplies the amplified drive waveform to each channel Tc of the ejection head 20. The buffer amplifier 21d is supplied with a power supply voltage VH for amplifying the drive waveform.

[0087] Here, the drive waveform patterns registered in the GS setting table D1 will be described with reference to FIGS.

[0088] The drive waveform pattern defines a selection pattern of the drive waveform within, for example, eight counts of the clock signal GSCLK. That is, the drive waveform pattern is set so that the ejection head 20 ejects ink for one pixel every eight counts of the clock signal GSCLK.

[0089] 8, 16 types of drive waveform patterns are registered. The GS controller 21c selects one of these 16 drive waveform patterns every 8 counts based on the pixel data and ink ejection distance data. The GS controller 21c then sequentially outputs the drive waveforms corresponding to the selected drive waveform pattern to the buffer amplifier 21d.

[0090] Here, the 16 types of drive waveform patterns each have a different drive waveform pattern so that ink can be ejected from the channel Tc with a desired droplet size and at a desired ejection speed.

[0091] In this embodiment, eight types of ejection pulses with different voltage amplitudes are input from the drive waveform generating unit 15 in accordance with eight counts of the clock signal GSCLK. The eight types of ejection pulses have pulse waveforms whose voltage amplitudes increase sequentially from the first count to the eighth count. More specifically, the ejection pulse at the first count of GSCLK is the ejection pulse with the smallest voltage amplitude (hereinafter referred to as the "first ejection pulse"). The ejection pulse at the second count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the first ejection pulse (hereinafter referred to as the "second ejection pulse"). The ejection pulse at the third count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the second ejection pulse (hereinafter referred to as the "third ejection pulse"). The ejection pulse at the fourth count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the third ejection pulse (hereinafter referred to as the "fourth ejection pulse"). The ejection pulse at the fifth count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the fourth ejection pulse (hereinafter referred to as the "fourth ejection pulse"). The ejection pulse at the sixth count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the fifth ejection pulse (hereinafter referred to as the "sixth ejection pulse"). The ejection pulse at the seventh count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the sixth ejection pulse (hereinafter referred to as the "seventh ejection pulse"). The ejection pulse at the eighth count of GSCLK is the ejection pulse with the largest voltage amplitude (hereinafter referred to as the "eighth ejection pulse").

[0092] The GS controller 21c changes the ink ejection speed by using any one of the first to eighth ejection pulses. That is, the GS controller 21c uses the first ejection pulse when the ink ejection speed is to be minimized. On the other hand, the GS controller 21c uses the eighth ejection pulse when the ink ejection speed is to be maximized.

[0093] The GS controller 21c adjusts the droplet size by the number of ejection pulses continuously supplied to the piezoelectric element T. That is, when ejecting small-sized ink droplets, the GS controller 21c supplies only one ejection pulse of any of the first to eighth ejection pulses to the piezoelectric element T. When ejecting medium-sized ink droplets, the GS controller 21c supplies two consecutive ejection pulses of the first to eighth ejection pulses to the piezoelectric element T. When ejecting large-sized ink droplets, the GS controller 21c supplies three consecutive ejection pulses of the first to eighth ejection pulses to the piezoelectric element T.

[0094] The 16 types of drive waveform patterns registered in the GS setting table D1 are patterns with different selection modes for the first to eighth ejection pulses described above. In the GS setting table D1 shown in Fig. 8, "1" indicates the timing at which an ejection pulse is supplied. Also, "0" indicates the timing at which an ejection pulse is not supplied. When it is "0", for example, a non-ejection waveform is supplied to channel Tc.

[0095] In the GS setting table D1 shown in FIG. 8, the ejection speed for small size ink ejection can be adjusted in eight stages. Here, the ejection speed of the ink increases in the order of "Small 1" to "Small 8" as listed in the remarks column of FIG. 8. Furthermore, the ejection speed for medium size ink ejection can be adjusted in five stages. Here, the ejection speed of the ink increases in the order of "Medium 1" to "Medium 5" as listed in the remarks column of FIG. 8. Furthermore, the ejection speed for large size ink ejection can be adjusted in three stages. Here, the ejection speed of the ink increases in the order of "Large 1" to "Large 3" as listed in the remarks column of FIG. 8.

[0096] In this way, pulse-like drive waveforms with different voltage amplitudes are input to the GS controller 21c in ascending or descending order of voltage amplitude in synchronization with an 8-count clock signal, which constitutes one cycle. The GS controller 21c supplies only one ejection pulse to the piezoelectric element T for small-sized ink droplets within one cycle. For medium-sized ink droplets, the GS controller 21c supplies two consecutive ejection pulses to the piezoelectric element T within one cycle. For large-sized ink droplets, the GS controller 21c supplies three consecutive ejection pulses to the piezoelectric element T within one cycle.

[0097] This configuration maximizes the number of drive waveform patterns that can be output from the GS controller 21c. This allows for fine adjustment of the ink ejection speed. In particular, this configuration allows for fine adjustment of the ejection speed of small ink droplets.

[0098] Figure 10 shows the results of measuring the degree of deceleration of the ejection velocity due to air resistance for each droplet size. As can be seen from Figure 10, the ejection velocity of small-sized ink droplets decreases more rapidly after ejection than that of large-sized ink droplets due to the influence of air resistance. In other words, small-sized ink droplets are more susceptible to the influence of air resistance than large-sized ink droplets, and the change in landing position due to changes in the ink ejection distance is greater. This shows that it is preferable to be able to change the ejection velocity of small-sized ink droplets gradually in response to changes in the ink ejection distance.

[0099] Of the first to eighth ejection pulses, it is preferable to set the pulse width of the ejection pulse having a large voltage amplitude shorter than that of the ejection pulse having a small voltage amplitude. Alternatively, for the ejection pulse having a large voltage amplitude, a post-pulse may be set, or the voltage amplitude of the post-pulse may be increased.

[0100] This is because if the voltage amplitude of the ejection pulse is increased, the size of the droplets ejected by one ejection pulse may be slightly larger than that of an ejection pulse with a smaller voltage amplitude. In this regard, by adjusting the pulse width of the ejection pulse and the post-pulse, it is possible to change the ink droplet ejection speed while maintaining the same ink droplet size. The post-pulse is a short pulse that is input to channel Tc following the ejection pulse within one count of GSCLK. The post-pulse serves to suppress ink ejection from channel Tc.

[0101] [Effects] As described above, the image forming apparatus 1 according to this embodiment is configured such that the control unit (here, the GS controller 21c) executes the following processes: a first process of acquiring image data of the object to be printed; a second process of acquiring distance data from the nozzle surface of the ejection head to the surface of the object to be drawn on which the ink is to be applied; and a third process of determining the droplet size of the ink droplets to be ejected from the ejection head based on the image data, and determining the ejection conditions for ejecting the ink droplets from the ejection head based on the distance data so that the ink droplets land at target positions on the object to be drawn on. In the third process, the ink coating layer is formed on the object to be drawn on, and the ejection conditions are changed as the distance changes.

[0102] This makes it possible to eject ink onto the object P so that the landing positions of ink droplets on the object P do not deviate from the target positions when forming any of the multiple ink coating layers. In other words, this makes it possible to form a high-quality image even when thickening the coating film by forming multiple ink coating layers.

[0103] In particular, in the image forming apparatus 1 according to this embodiment, the landing position of the ink droplets is adjusted by changing the ejection speed of the ink droplets, which makes it possible to easily suppress deviation of the landing position of the ink droplets on the drawing target P.

[0104] The control mode of the image forming apparatus 1 according to this embodiment is particularly suitable when the target film thickness of the ink coating layers formed on the drawing object P is 0.5 mm or more. In such a case, the number of ink coating layers to be laminated increases. This is because, in such a case, the deviation of the landing position of the ink droplets on the drawing object P increases in the process of forming the ink coating layers on the drawing object P.

[0105] <Modification 1> The image forming apparatus 1 according to this modification differs from the image forming apparatus 1 according to the above embodiment in the control mode of the GS controller 21c.

[0106] 11 and 12 are diagrams showing modified examples of the GS setting table D1, with Fig. 12 illustrating the drive waveform patterns defined in the GS setting table D1 according to this modified example.

[0107] In the above embodiment, one pixel's worth of ink is ejected every eight counts of the clock signal GSCLK. However, in order to form images at higher speeds, it is preferable to shorten the unit time for ejecting one pixel's worth of ink. From this perspective, the GS controller 21c in this modified example ejects one pixel's worth of ink every four counts of the clock signal GSCLK. The GS setting table D1 in this modified example is configured so that the ink droplet ejection speed can be variously changed with four counts of the clock signal GSCLK.

[0108] The GS setting table D1 according to this modification defines the drive waveform selection pattern within four counts of the clock signal GSCLK. Eight drive waveform patterns are registered in the GS setting table D1. The GS controller 21c selects one of these eight drive waveform patterns every four counts based on the pixel data and ink ejection distance data. The GS controller 21c then sequentially outputs the drive waveforms corresponding to the selected drive waveform pattern to the buffer amplifier 21d.

[0109] Here, each of the eight types of drive waveform patterns has a different drive waveform pattern so that ink can be ejected from channel Tc at a desired droplet size and a desired ejection speed. Specifically, as shown in Fig. 12, four types of ejection pulses with different voltage amplitudes are input from the drive waveform generation unit 15 in accordance with four counts of the clock signal GSCLK. The four types of ejection pulses have pulse waveforms whose voltage amplitudes increase sequentially from the first count to the fourth count.

[0110] More specifically, the ejection pulse at the first count of GSCLK is the ejection pulse with the smallest voltage amplitude (hereinafter referred to as the "first ejection pulse"). The ejection pulse at the second count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the first ejection pulse (hereinafter referred to as the "second ejection pulse"). The ejection pulse at the third count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the second ejection pulse (hereinafter referred to as the "third ejection pulse"). The ejection pulse at the fourth count of GSCLK is the ejection pulse with the next smallest voltage amplitude after the third ejection pulse (hereinafter referred to as the "fourth ejection pulse").

[0111] The GS controller 21c changes the ink ejection speed by using any one of the first to fourth ejection pulses. That is, the GS controller 21c uses the first ejection pulse when the ink ejection speed is to be minimized. On the other hand, the GS controller 21c uses the fourth ejection pulse when the ink ejection speed is to be maximized.

[0112] The GS controller 21c adjusts the droplet size by the number of ejection pulses successively supplied to the piezoelectric element T. That is, when ejecting small-sized ink droplets, the GS controller 21c supplies only one ejection pulse of any of the first to fourth ejection pulses to the piezoelectric element T. When ejecting medium-sized ink droplets, the GS controller 21c supplies two consecutive ejection pulses of the first to fourth ejection pulses to the piezoelectric element T. When ejecting large-sized ink droplets, the GS controller 21c supplies three consecutive ejection pulses of the first to fourth ejection pulses to the piezoelectric element T.

[0113] The eight types of drive waveform patterns registered in the GS setting table D1 shown in Fig. 11 are patterns with different selection modes for the first to fourth ejection pulses described above. In the GS setting table D1 shown in Fig. 11, "1" indicates the timing at which an ejection pulse is supplied, and "0" indicates the timing at which an ejection pulse is not supplied.

[0114] 13 and 14 are diagrams showing other modified examples of the GS setting table D1. In Fig. 13 and Fig. 14, the GS setting table D1 shows an aspect that defines a selection pattern of a drive waveform within five counts of the clock signal GSCLK.

[0115] As described above, the image forming apparatus 1 according to this modified example can also eject ink onto an object so that the landing position of ink droplets on the object does not deviate from the target position, even when the ink ejection distance changes, as in the above embodiment. In other words, this makes it possible to form a high-quality image even when the coating film is thickened by laminating ink coating layers.

[0116] The control mode of the image forming apparatus 1 according to this modification is advantageous in that it allows the unit time for ejecting ink for one pixel to be set shorter than the control mode of the image forming apparatus 1 according to the above embodiment. However, the control mode of the image forming apparatus 1 according to this modification makes it difficult to fine-tune the ejection speed of ink ejection.

[0117] From this viewpoint, it is preferable to have a configuration in which the user can select which control mode to use depending on the image formation speed desired by the user.

[0118] In this case, the image forming apparatus 1 is configured to change the number of counts of the clock signal for one cycle. The GS controller 21c can then use a GS setting table D1 corresponding to the number of counts of the clock signal for one cycle to set the pattern of the drive waveform to be supplied to the piezoelectric element T. It is preferable to prepare multiple types of GS setting tables D1 in advance corresponding to the number of counts of the clock signal for one cycle, as shown in Figures 8, 11, and 13.

[0119] <Modification 2> The image forming apparatus 1 according to this modification differs from the image forming apparatus 1 according to the above embodiment in the control mode of the GS controller 21c.

[0120] 15 is a diagram illustrating a modified example of a drive waveform pattern for adjusting the landing position of ink droplets. The control of the GS controller 21c using the drive waveform pattern shown in FIG. 15 may be performed using a setting table similar to the GS setting table D1 shown in FIG.

[0121] In the above embodiment, the deviation of the ink droplet landing position is adjusted by changing the ink droplet ejection speed according to the ink ejection distance. In this modified example, instead of such an adjustment, the deviation of the ink droplet landing position is adjusted by changing the ink droplet ejection timing according to the ink ejection distance. Specifically, the GS controller 21c adjusts the ink droplet ejection timing to be earlier than the reference timing as the ink ejection distance becomes longer.

[0122] That is, the GS controller 21c according to this modification is capable of supplying to the channel Tc ejection pulses with different ejection timings for ink droplets using the clock signal GSCLK as the reference timing. Specifically, eight types of ejection pulses with different ejection timings are input from the drive waveform generating unit 15 in accordance with eight counts of the clock signal GSCLK. The eight types of ejection pulses have pulse waveforms in which the ejection timings with respect to the reference timing become earlier in order from the first count to the eighth count. Note that the voltage amplitudes of the eight types of ejection pulses are set to be the same.

[0123] More specifically, the ejection pulse at the first count of GSCLK is the ejection pulse with the latest ejection timing relative to the reference timing (hereinafter referred to as the "first ejection pulse"). The ejection pulse at the second count of GSCLK is the ejection pulse with the next latest ejection timing relative to the reference timing after the first ejection pulse (hereinafter referred to as the "second ejection pulse"). The ejection pulse at the third count of GSCLK is the ejection pulse with the next latest ejection timing relative to the reference timing after the second ejection pulse (hereinafter referred to as the "third ejection pulse"). The ejection pulse at the fourth count of GSCLK is the ejection pulse with the next latest ejection timing relative to the reference timing after the third ejection pulse (hereinafter referred to as the "fourth ejection pulse"). The ejection pulse at the fifth count of GSCLK is the ejection pulse with the next latest ejection timing relative to the reference timing after the fourth ejection pulse (hereinafter referred to as the "fifth ejection pulse"). The ejection pulse at the sixth count of GSCLK is the ejection pulse with the next latest ejection timing relative to the reference timing after the fifth ejection pulse (hereinafter referred to as the "sixth ejection pulse"). The ejection pulse at the seventh count of GSCLK is the ejection pulse (hereinafter referred to as the "seventh ejection pulse") whose ejection timing is the next latest after the sixth ejection pulse relative to the reference timing. The ejection pulse at the eighth count of GSCLK is the ejection pulse (hereinafter referred to as the "eighth ejection pulse") whose ejection timing is the earliest relative to the reference timing.

[0124] The GS controller 21c changes the ejection timing of ink ejection depending on which of these first to eighth ejection pulses is used. That is, the GS controller 21c uses the first ejection pulse when the ejection timing relative to the reference timing is to be the latest. On the other hand, the GS controller 21c uses the eighth ejection pulse when the ejection timing relative to the reference timing is to be the latest.

[0125] The GS controller 21c adjusts the droplet size by the number of ejection pulses successively supplied to the piezoelectric element T. That is, when ejecting small-sized ink droplets, the GS controller 21c supplies only one ejection pulse of any of the first to eighth ejection pulses to the piezoelectric element T. When ejecting medium-sized ink droplets, the GS controller 21c supplies two consecutive ejection pulses of the first to eighth ejection pulses to the piezoelectric element T. When ejecting large-sized ink droplets, the GS controller 21c supplies three consecutive ejection pulses of the first to eighth ejection pulses to the piezoelectric element T.

[0126] The 16 types of drive waveform patterns registered in the GS setting table D1 according to this modification are patterns with different selection modes for the first to eighth ejection pulses described above. Note that the 16 types of drive waveform pattern data stored in the GS setting table D1 according to this modification are the same as those shown in FIG.

[0127] That is, with the GS setting table D1 according to this modification, the ejection timing for small ink droplets can be adjusted in eight stages, the ejection timing for medium ink droplets can be adjusted in five stages, and the ejection timing for large ink droplets can be adjusted in three stages.

[0128] As described above, the image forming apparatus 1 according to this modified example can also eject ink without causing deviation in the landing position of ink droplets on the object to be drawn, similar to the embodiment described above, even when the ink ejection distance changes. In other words, this makes it possible to form high-quality images even when the coating film is thickened by laminating ink coating layers.

[0129] However, since the adjustable range of the ejection timing relative to the reference timing is limited to the interval of the clock signal GSCLK, the adjustable range of the deviation of the ink droplet landing position is also limited in the image forming apparatus 1 according to this modified example. From this perspective, it can be said that a more preferable method is to adjust the deviation of the ink droplet landing position by changing the ejection speed of the ink droplets, as in the image forming apparatus 1 according to the above embodiment.

[0130] In order to control the landing position of ink droplets on the object, the control of the ink ejection speed described in the above embodiment and the control of the ink ejection timing described in this modification may be used together. In other words, the ejection pulses output from the drive waveform generation unit 15 may be changed in terms of both voltage amplitude and pulse output timing.

[0131] Other Embodiments The present invention is not limited to the above-described embodiment, and various modifications are possible.

[0132] For example, in the above embodiment, an aspect has been shown in which ink coating layers are formed on the object to be drawn using a single ejection head 20. However, a plurality of ejection heads 20 may be used when forming ink coating layers on the object to be drawn P. For example, the image forming apparatus 1 may be provided with a separate ejection head 20 for forming a first ink coating layer and a separate ejection head 20 for forming a second ink coating layer.

[0133] In the above embodiment, the ejection pulse is rectangular. However, it is preferable that the ejection pulse be a slope-shaped pulse with a predetermined rise time and fall time. This can prevent satellites and stabilize the ejection pattern.

[0134] In the above embodiment, the GS controller 21c controls the ink droplet ejection speed depending on the ink ejection distance, thereby adjusting the deviation of the ink droplet landing position. However, in the present invention, the main controller of this control does not necessarily have to be the GS controller 21c, and the control unit 11 may perform this control, or the control unit 11 and the GS controller 21c may work together to perform this control.

[0135] In the above embodiment, the control unit 11 and the GS controller 21c are configured by a microcomputer. However, instead of a microcomputer, they may be configured by hardware circuits such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field-Programmable Gate Array). Furthermore, each of these functions is not limited to software processing, and can of course also be realized by processing by dedicated hardware circuits or a combination of these.

[0136] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0137] The disclosures of the specification, drawings and abstract contained in Japanese Patent Application No. 2024-005322, filed on January 17, 2024, are incorporated herein by reference in their entirety.

[0138] According to the image forming apparatus according to the present disclosure, it is possible to form a high-quality image even when forming a thick coating film on an object to be imaged.

[0139] REFERENCE SIGNS LIST 1 Image forming apparatus 10 Apparatus main body 11 Control unit 12 Interface 13 Page memory 14 Line memory 15 Drive waveform generating unit 16 UV irradiation unit 17 Drive unit 20 Discharge head 21 Head IC 22 Head main body 23 Distance sensor M External device T Piezoelectric element Tc Channel Td Pressure chamber Te Nozzle Tf Piezoelectric member Tg First electrode Th Second electrode P Printed object

Claims

1. An inkjet image forming apparatus having a discharge head and a control unit for controlling the discharge head, wherein an ink coating layer is laminated and formed on a workpiece by repeatedly discharging ink from the discharge head onto the workpiece, and the control unit includes: a first process of acquiring image data of a print target; a second process of acquiring distance data from a nozzle surface of the discharge head to a coating target surface of the workpiece; and a third process of determining a droplet size of an ink droplet discharged from the discharge head based on the image data, and determining discharge conditions for discharging the ink droplet from the discharge head so that the ink droplet lands on a target position on the workpiece based on the distance data. In the third process, the discharge conditions are changed as the ink coating layer is formed on the workpiece and the distance changes.

2. The image forming apparatus according to claim 1, wherein in the third process, the control unit changes the discharge conditions related to the discharge speed of the ink droplet as the distance changes.

3. The image forming apparatus according to claim 2, wherein the control unit changes the discharge speed of the ink droplet by changing a voltage amplitude of a discharge pulse supplied to the discharge head.

4. The image forming apparatus according to claim 3, wherein the control unit changes the voltage amplitude of the discharge pulse, and changes a pulse width of the discharge pulse or a voltage amplitude of a post-pulse of the discharge pulse, so as to change the discharge speed of the ink droplet while keeping the droplet size of the ink droplet discharged from the discharge head the same.

5. The image forming apparatus according to claim 1, wherein in the third process, the control unit changes the droplet size of the ink droplet by adjusting the number of discharge pulses continuously supplied to the discharge head.

6. The image forming apparatus according to claim 1, wherein the control unit is constituted by a grayscale controller, and in the third process, the control unit selects drive waveform pattern data corresponding to the droplet size and the discharge conditions of the ink droplet from among a plurality of types of drive waveform pattern data registered in a grayscale setting table, and sequentially supplies a drive waveform set in the drive waveform pattern data to the discharge head.

7. In the gray scale controller, taking a clock signal of a predetermined count number as one cycle, ejection pulses having different voltage amplitudes are sequentially input in ascending or descending order of the voltage amplitude in synchronization with the clock signal. The gray scale controller supplies only one ejection pulse to the ejection head within the one cycle for the small-sized ink droplets, supplies two consecutive ejection pulses to the ejection head within the one cycle for the medium-sized ink droplets, and supplies three consecutive ejection pulses to the ejection head within the one cycle for the large-sized ink droplets. The image forming apparatus according to claim 6.

8. The count number of the clock signal for the one cycle is changeable, and the gray scale controller sets the pattern of the drive waveform using the gray scale setting table corresponding to the count number of the clock signal for the one cycle. The image forming apparatus according to claim 6.

9. In the gray scale setting table, more patterns are registered for the drive waveform pattern related to the small-sized ink droplets than for the drive waveform pattern related to the large-sized ink droplets. The image forming apparatus according to claim 6.

10. The control unit executes a fourth process of measuring the distance before the ink coating layer is formed on the coating target surface of the object to be drawn, and a fifth process of measuring the distance after the ink coating layer is formed on the coating target surface of the object to be drawn. The image forming apparatus according to claim 1.

11. The control unit measures the distance each time one layer or a predetermined number of layers of the ink coating layer is formed on the object to be drawn. The image forming apparatus according to claim 1.

12. The ink droplets are thermosetting ink or photocuring ink. The image forming apparatus according to claim 1.

13. The coating target surface of the object to be drawn has an uneven shape or a curved shape. The image forming apparatus according to claim 1.

14. In the third process, the control unit changes the ejection conditions related to the ejection timing of the ink droplets according to the change in the distance. The image forming apparatus according to claim 1.

15. The thickness of the ink coating layer laminated and formed on the object to be drawn is 0.5 mm or more. The image forming apparatus according to claim 1.

16. A control method for an inkjet image forming apparatus having a discharge head and a control unit for controlling the discharge head, the method comprising repeatedly discharging ink from the discharge head onto a drawing object to form a laminated ink coating layer on the drawing object, the method including: a first process of acquiring image data of a print target; a second process of acquiring distance data from a nozzle surface of the discharge head to a coating target surface of the drawing object; and a third process of determining a droplet size of an ink droplet discharged from the discharge head based on the image data and determining discharge conditions for discharging the ink droplet from the discharge head so that the ink droplet lands on a target position on the drawing object based on the distance data, wherein in the third process, the discharge conditions are changed as the distance changes when the ink coating layer is formed on the drawing object.

17. A control program for an inkjet image forming apparatus having a discharge head and a control unit for controlling the discharge head, the method comprising repeatedly discharging ink from the discharge head onto a drawing object to form a laminated ink coating layer on the drawing object, the program causing a computer to perform: a first process of acquiring image data of a print target; a second process of acquiring distance data from a nozzle surface of the discharge head to a coating target surface of the drawing object; and a third process of determining a droplet size of an ink droplet discharged from the discharge head based on the image data and determining discharge conditions for discharging the ink droplet from the discharge head so that the ink droplet lands on a target position on the drawing object based on the distance data, wherein in the third process, the discharge conditions are changed as the distance changes when the ink coating layer is formed on the drawing object.

Images