Ink viscosity change detection method and ink jet printing device

Abstract

To provide a method which does not cause enlargement of a device and an increase of costs.SOLUTION: A method is executed in a printing device 1 which includes: a fixing section 2 of a workpiece 11; an ink jet head 4 which has a nozzle for discharging ink; a movement mechanism 3 which relatively moves the ink jet head 4 with respect to the fixing section 2; a camera 6 which generates an image capturing the workpiece 11; and a control section 7 which controls the ink jet head 4, the movement mechanism 3 and the camera 6. Printing processing is performed so that a prescribed printing pattern 25 is printed at a target printing coordinate for the workpiece 11 in two states in which relatively specific physical conditions between the workpiece 11 and the nozzle are made different from each other. Each offset of an actual printing coordinate with respect to the target printing coordinate in each state is acquired. Injection speed of the ink is calculated using the offsets, and a viscosity change of the ink is detected by comparing the injection speed with a prescribed reference injection speed.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for detecting a change in viscosity of ink and an inkjet printing apparatus. [Background technology]

[0002] 2. Description of the Related Art Inkjet printing devices are widely known as a method for printing characters on printed wiring boards (see Patent Document 1).

[0003] Inkjet inks used on printed wiring boards and the like usually have a high viscosity at room temperature, as shown in Figure 12, and the ink cannot be ejected. Therefore, it is necessary to lower the viscosity of the ink by heating it, and print at an appropriate viscosity. However, the ink becomes more viscous when it is heated or when it changes over time. When the ink viscosity changes, the ink ejection speed also changes, causing problems such as a change in the ink landing position (misalignment). For this reason, it is necessary to adjust the ink landing position according to the ink viscosity.

[0004] As a method for detecting changes in ink viscosity, a method has been proposed in which the viscosity of the ink is calculated by calculating the ejection speed of ejected ink droplets using a light-emitting element and a light-receiving element provided at specific positions (see Patent Document 2).

[0005] Also, a method has been proposed in which the viscosity of the ink is measured by using a viscosity sensor that measures the viscosity in a holder (tank) that holds the ink (see Patent Document 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 9-232701 [Patent Document 2] JP 2006-212868 A [Patent Document 3] JP 2015-193112 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the method of Patent Document 2 requires providing a light emitting element and a light receiving element at specific positions, which poses problems such as an increase in the size of the device and an increase in costs.

[0008] Furthermore, the method of Patent Document 3 has the problem that, in addition to increasing costs, it cannot be applied to cases where there is no holder to begin with (for example, an ink cartridge type that has no ink tank, etc.). [Means for solving the problem]

[0009] In order to solve the above problems, the method for detecting a change in viscosity of ink of the present invention comprises: A fixing part for fixing a workpiece to be printed; A nozzle for ejecting ink for printing; a moving mechanism that moves the nozzle relative to the fixed portion; A camera that generates an image of a printing surface of the workpiece fixed to the fixing portion; a control unit configured to perform a printing process for printing on the workpiece by ejecting ink from the nozzle while moving the nozzle relative to the workpiece fixed to the fixed unit using the moving mechanism, and to perform an image acquisition process for acquiring an image of the workpiece from the camera; A method for detecting a change in viscosity of ink using an inkjet printing device comprising: a two-state offset acquisition step in which the printing process is performed so as to print a predetermined print pattern on the work at target print coordinates in two states in which relative specific physical conditions between the work and the nozzle are made different from each other, and an offset of an actual print coordinate with respect to the target print coordinate in each state is acquired by using an image of the work acquired by the image acquisition process; an ink ejection velocity calculation step of calculating an ink ejection velocity from the nozzle using the physical conditions in the two states and offsets of the two states; and an ink viscosity change determination step of determining whether or not there is a change in the viscosity of the ink by comparing the ink ejection speed with a predetermined reference ejection speed based on a predetermined determination criterion.

[0010] The relatively specific physical conditions in the method for detecting a change in the viscosity of ink are not particularly limited, but the following aspects are exemplified. (1) The speed of the relative movement between the nozzle and the workpiece. (2) The distance between the ink ejection port of the nozzle and the workpiece (printing height)

[0011] The manner in which the actual print coordinates are acquired is not particularly limited, but the following manner is exemplified. (a) An image of the workpiece after printing is displayed on the screen of a display device provided in the control unit, and the actual printing coordinates are obtained by the user visually identifying the reference points of the printing pattern contained in the image. (b) An image of the workpiece after printing is input from the camera to the control unit, and the actual print coordinates (the coordinates actually printed on the workpiece) of the reference point of the print pattern contained in the image are measured by automatically detecting them through image analysis processing. An example of an automatic detection through image analysis processing is a mode in which the actual print coordinates are obtained by searching for the position of the reference point of the print pattern through pattern matching from an image of the workpiece after printing.

[0012] The specified reference ejection speed is not particularly limited, but an example is the ejection speed of ink obtained using ink in a specified reference state (for example, ink immediately after being poured into an ink tank, or ink in an unopened ink cartridge, etc., in a state where there is no thickening).

[0013] The predetermined judgment criterion is not particularly limited, but examples thereof include a mode in which the judgment is made when the injection speed has decreased by a predetermined rate or more, or a mode in which the judgment is made when the injection speed has decreased by a predetermined value or more.

[0014] According to this method, it can be easily implemented as long as the camera that generates an image of the printing surface of the work fixed to the fixed part is provided. In this way, there is no need to provide a dedicated hardware configuration for acquiring the ink viscosity (for example, a light-emitting element and a light-receiving element for obtaining the ink droplet ejection speed, or a viscosity sensor for measuring the viscosity in the ink tank), so there is no problem of the device becoming larger and the cost increasing. In addition, the viscosity sensor cannot be applied to an ink cartridge type that does not have an ink tank, but the present invention does not have such a restriction. And, when the viscosity of the ink changes, the ink landing position shifts (the printing position accuracy deteriorates), so it is possible to use it to recover the printing position accuracy by readjusting the timing of ejecting the ink.

[0015] In addition, the inkjet printing apparatus of the present invention is A fixing part for fixing a workpiece to be printed; A nozzle for ejecting ink for printing; a moving mechanism that moves the nozzle relative to the fixed portion; A camera that generates an image of a printing surface of the workpiece fixed to the fixing portion; a control unit configured to perform a printing process for printing on the workpiece by discharging ink from the nozzle while moving the nozzle relative to the workpiece fixed to the fixed unit by the moving mechanism, and to perform an image acquisition process for acquiring an image of the workpiece from the camera; a two-state offset acquisition process for performing the printing process so as to print a predetermined print pattern on the work at target print coordinates in two states in which relative specific physical conditions between the work and the nozzle are different from each other, and acquiring an offset of an actual print coordinate with respect to the target print coordinate in each state by using an image of the work acquired by the image acquisition process; an ink ejection velocity calculation process for calculating an ink ejection velocity from the nozzle using the physical conditions in the two states and offsets between the two states; The ink ejection speed is compared with a predetermined reference ejection speed based on a predetermined determination criterion, thereby determining whether or not there is a change in the viscosity of the ink.

[0016] The relative specific physical conditions in the inkjet printing device, the manner in which the offset of the actual printing coordinates relative to the target printing coordinates is obtained, the specified reference ejection speed, and the specified judgment criteria are the same as those in the ink viscosity change detection method of the present invention.

[0017] This configuration also provides the same effects as the ink viscosity change detection method described above. Effect of the Invention

[0018] The ink viscosity change detection method and ink jet printing apparatus according to the present invention have the excellent effect of preventing the apparatus from becoming larger and increasing costs. [Brief description of the drawings]

[0019] [Figure 1] 1 is a block diagram of a main part of a control system of an inkjet printing apparatus according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a front view of a main part of the overall configuration of the printing apparatus. [Diagram 3] FIG. 2 is a plan view of the main parts of the overall configuration of the printing apparatus. [Figure 4]FIG. 2 is a front view of a nozzle surface of an inkjet head of the printing device. [Diagram 5] 6 is a flowchart showing the flow of an ink viscosity change detection process executed by a control unit of the printing device. [Figure 6] 3A and 3B are diagrams showing examples of patterns printed by the printing device. [Figure 7] FIG. 13 is a diagram in which the center of the printed pattern is aligned with the center of the image captured by the camera. [Figure 8] FIG. 2 is a front view showing the relative physical state between the workpiece and the nozzle of the printing apparatus. [Figure 9] 11 is a table showing offset values ​​measured at five different printing speeds for each of two inkjet heads included in an inkjet printing apparatus according to an embodiment. [Figure 10] This is a table that uses the measurement results of Figure 9 to summarize the ink ejection speed values ​​when the ink droplet motion is calculated as uniform velocity motion and when it is calculated as falling motion. [Figure 11] This is a summary of the average ejection speed, minimum ejection speed, and maximum ejection speed of the ink. (a) is a summary of the measurement results of the ejection speed of droplets equipped in an inkjet printing device, and (b) is a summary of the calculation results when the movement of the ink droplets is calculated as uniform velocity movement. [Figure 12] 1 is a graph showing the relationship between temperature and viscosity of inkjet ink. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, a first embodiment of an inkjet printing device 1 embodying the present invention will be described together with a method for detecting a change in viscosity of ink carried out by the device with reference to Figures 1 to 11. As shown in Figures 1 to 4, the inkjet printing device 1 includes a fixing unit 2, a moving mechanism 3, an inkjet head 4, a curing unit 5, a camera 6, and a control unit 7 in a device body 1a.

[0021] The fixing unit 2 is equipped with a mounting table 8 on which a workpiece 11 (a substrate such as a printed wiring board) to be printed is placed, and a fixing means (not shown) for fixing the workpiece 11 at a required position on the mounting table 8.

[0022] The moving mechanism 3 includes an X-axis moving mechanism 3a, a Y-axis moving mechanism 3b, and a Z-axis moving mechanism 3c, and is configured to move the inkjet head 4, the curing unit 5, and the camera 6 relative to the fixed unit 2 in the X-direction, Y-direction, and Z-direction.

[0023] The X-axis movement mechanism 3a is configured to move the X-axis movable body 12 in the X direction relative to the device body 1a, and includes an X-axis ball screw 16 that is rotatably supported on the device body 1a and extends in the X direction, the X-axis movable body 12 having a female screw hole (not shown) that screws into the X-axis ball screw 16, an X-axis linear guide 18 that guides the movement of the X-axis movable body 12 in the X direction, and an X-axis motor 20 that drives the X-axis ball screw 16.

[0024] The Y-axis moving mechanism 3b is configured to move the fixed part 2 in the Y direction relative to the device main body 1a, and includes a Y-axis ball screw 17 that is rotatably supported relative to the device main body 1a and extends in the Y direction, the fixed part 2 having a female threaded hole (not shown) that screws into the Y-axis ball screw 17, a Y-axis linear guide 19 that guides the movement of the fixed part 2 in the Y direction, and a Y-axis motor 21 that drives the Y-axis ball screw 17.

[0025] The Z-axis moving mechanism 3c is configured to move the Z-axis moving body 14 in the Z direction relative to the X-axis moving body 12, and includes a Z-axis ball screw (not shown) rotatably supported on the X-axis moving body 12 and extending in the Z direction, the Z-axis moving body 14 having a female screw hole (not shown) screwed into the Z-axis ball screw, a Z-axis linear guide (not shown) that guides the movement of the Z-axis moving body 14 in the Z direction, and a Z-axis motor (not shown) that drives the Z-axis ball screw. The inkjet head 4, the curing unit 5, and the camera 6 are attached to the Z-axis moving body 14.

[0026] The inkjet head 4 has a plurality of nozzles 4a formed therein as shown in FIG. 4, and is configured to eject droplets 27 of ink for printing from each nozzle 4a to apply the ink to the workpiece 11.

[0027] The curing units 5 are configured to cure the ink attached to the workpiece 11, and are attached to both sides in the movement direction (X direction in this example) of the inkjet head 4 so as to move in conjunction with the inkjet head 4. The curing unit 5 is not particularly limited, but for example, when a UV-curable ink is used, a UV curing lamp is used.

[0028] The camera 6 is disposed behind one of the curing sections 5 and is configured to move integrally with the inkjet head 4 and the curing section 5 so as to generate an image of the printing surface of the workpiece 11 fixed to the fixed section 2.

[0029] The control unit 7 is configured to be able to perform a printing process for printing on the workpiece 11 by ejecting ink from the nozzle 4a while moving the inkjet head 4 having a nozzle 4a relative to the workpiece 11 fixed to the fixed part 2 using the moving mechanism 3, an image acquisition process for acquiring an image of the workpiece 11 from the camera 6, an ink ejection speed calculation process for calculating the ink ejection speed, and an ink viscosity detection process for detecting changes in the viscosity of the ink.

[0030] Next, an outline of the operation of the printing process of the inkjet printer 1 of this embodiment will be described. Fig. 8 shows the positional relationship of the inkjet head 4, nozzle 4a, workpiece 11, and ink droplets 27 of the inkjet printer 1 during printing. As preparation before the printing process, printing data is created and the workpiece 11 is fixed to the fixing part 2. The contents of the printing data are not particularly limited, but an example will be one in which information on characters, figures, or patterns to be printed, alignment marks, etc. is included. (1) First, the camera 6 is used to capture an image of an alignment mark (not shown) on the surface of the workpiece 11, and position information of the workpiece 11 is obtained. (2) Based on the position information of the workpiece 11, the expansion / contraction and mounting angle of the workpiece 11 are calculated, and expansion / contraction correction and rotation correction are performed on the printing data. After that, the data is rasterized (complex data such as paths and large linked images / embedded images are converted into bitmap images) to obtain rasterized data. (3) The inkjet head 4 is moved (Z direction) to a height where the ink is to be ejected (for example, 1 mm from the work surface). (4) After the inkjet head 4 is moved in the X and Y directions relative to the fixed part 2, the ink ejection from each nozzle 4a is controlled while the inkjet head 4 is moved in the X direction to deposit ink droplets 27 at predetermined positions. At this time, the ink is cured in the curing part 5 almost simultaneously with the ejection of the ink, if necessary. (5) After printing is completed over the nozzle width of the inkjet head 4 (the vertical width of all nozzles 4a shown in Figure 4, i.e., the printing width), the fixed part 2 is moved in the Y direction relative to the inkjet head 4 by the nozzle width. (6) Repeat steps (4) and (5) until printing of all the areas to be printed is complete, and then printing will be completed.

[0031] Next, an overview of the operation of the image acquisition process of the inkjet printing device 1 in this example will be described. The moving mechanism 3 controlled by the control unit 7 moves the camera 6 relative to the fixed part 2 in the X and Y directions, thereby moving the camera 6 above a required portion of the printing surface of the workpiece 11 fixed to the fixed part 2. An image of the required portion is captured by the camera 6 controlled by the control unit 7, and the control unit 7 acquires the image from the camera 6.

[0032] Next, the ink viscosity detection process (see FIG. 5) executed by the control unit 7 will be described. As a preparation before carrying out this process, an adjustment board (a board without copper foil on the entire board) is fixed to the fixing unit 2 as the work 11. This ink viscosity detection process includes a two-state offset acquisition process (step S101) in which the print process is performed so as to print a predetermined print pattern 25 on the work 11 at target print coordinates in two states in which relative specific physical conditions between the work 11 and the nozzle 4a are made different from each other, and the offsets x1 and x2 of the actual print coordinates relative to the target print coordinates in each state are acquired by using the image of the work 11 acquired by the image acquisition process, an ink ejection speed calculation process (step S102) in which the ejection speed of the ink from the nozzle is calculated using the offsets x1 and x2 of the two states together with the physical conditions in the two states, and an ink viscosity change determination process (step S103) in which the presence or absence of a change in the viscosity of the ink is detected by comparing the ejection speed of the ink with a predetermined reference ejection speed based on a predetermined determination criterion.

[0033] In addition, in this ink viscosity detection process, the printing process is performed to print a predetermined printing pattern 25 on the work 11 at target printing coordinates in two states in which the relative specific physical conditions between the work 11 and the nozzle 4a are different from each other, and an ink viscosity detection method is implemented that includes a two-state offset acquisition stage (implemented by the two-state offset acquisition process) in which offsets x1 and x2 of the actual printing coordinates relative to the target printing coordinates in each of the states are acquired by using the image of the work 11 acquired by the image acquisition process, an ink ejection velocity calculation stage (implemented by the ink ejection velocity calculation process) in which the ink ejection velocity from the nozzle is calculated using the offsets x1 and x2 of the two states together with the physical conditions in the two states, and an ink viscosity change determination stage (implemented by the ink viscosity change determination process) in which the presence or absence of a change in the viscosity of the ink is detected by comparing the ink ejection velocity with a predetermined reference ejection velocity based on a predetermined determination criterion.

[0034] The two-state offset acquisition process (step S101) will be described. In this process, an image of the workpiece 11 (an image resulting from printing a predetermined print pattern 25 at target print coordinates) acquired under specific relative physical conditions between the workpiece 11 and the nozzle 4a is used to acquire an offset x (the deviation of the ink landing position from the target coordinates) of the actual print coordinates relative to the target print coordinates in that state, and a single-state offset acquisition step (step S101a) is executed for each of two states with different specific physical conditions. In this example, the print speed S (m / sec) is used as the specific physical condition, and offsets x1 and x2 are acquired for the different print speeds S1 and S2, respectively.

[0035] The single state offset acquisition process (step S101a) will be described. (1) First, a print pattern 25 is printed with the aim of positioning a reference point 25a of a given print pattern 25 shown in Fig. 6 at a given target print coordinate on the workpiece 11. This printing is specifically performed by carrying out the print processing described above. (2) An image 24 of the workpiece 11 after printing is acquired by the image acquisition process, and actual print coordinates (coordinates actually printed on the workpiece 11) of the reference point 25a of the printed print pattern 25 relative to the target print coordinates in the image 24 are acquired. This acquisition mode is not particularly limited, but the following modes are exemplified. (a) As shown in Fig. 7, an image of the workpiece 11 after printing is displayed on the screen 26 of a display device provided in the control unit 7, and the user visually identifies the reference point 25a of the print pattern contained in the image and specifies the position with a pointing device provided in cooperation with the display device, thereby acquiring the actual print coordinates. Note that in Fig. 7, the guideline 26a for reading the coordinates of the image 24 of the camera 6 is shown by a dashed line, the position of the print pattern 25 when it is printed as intended at the target print coordinates is shown by a two-dot chain line, and the position where the print pattern 25 is actually printed is shown by a solid line. (b) An image 24 of the workpiece 11 after printing is input from the camera 6 to the control unit 7, and the actual printing coordinates (the coordinates actually printed on the workpiece 11) of the reference point 25a of the print pattern 25 contained in the image 24 are automatically detected and acquired by image analysis processing. As an example of this automatic detection by image analysis processing, a mode in which the actual printing coordinates are acquired by searching for the position of the reference point 25a of the print pattern 25 by pattern matching from the image 24 of the workpiece 11 after printing is shown. (3) Calculate the offset x (see FIG. 7) in the X direction of the actual print coordinates relative to the target print coordinates.

[0036] The ink ejection speed calculation process (step S102) will be described below. First, the method of calculating the ink ejection speed will be described. In order to accurately calculate the ink ejection speed, it is necessary to take into account air resistance, etc. However, since the printing height during actual printing is a very small distance of about several millimeters, and 1.3 mm in the experiment, air resistance, etc. will be ignored to simplify the calculation.

[0037] As shown in Fig. 8, if the distance (=print height) between the nozzle surface of the inkjet head 4 and the workpiece 11 is H (mm), the ink ejection speed is V0 (m / sec), and the time it takes for the ink droplet 27 to land on the workpiece 11 is t (sec), the following formula (1) approximately holds. Note that the Z direction component is actually a vertically thrown down falling motion, which differs from formula (1) (formula (1) is a uniform velocity motion), but when H is small, formula (1) approximately holds. Calculations for the vertically thrown down falling motion will be described later, but the values ​​of the formula (1) and the formula for the vertically thrown down falling motion are almost the same (same up to four decimal places).

[0038]

number

[0039] In addition, the offset x, which is the distance in the XY directions until the ink lands on the workpiece at a printing speed S (m / sec), is expressed by the following equation (2).

[0040]

number

[0041] From the above, if (a) the offset, which is the distance moved at printing speed S1, is x1, and (b) the offset, which is the distance moved at printing speed S2, is x2, then the difference D between (a) and (b) is expressed by the following equation (3).

[0042]

number

[0043] From equations (1) and (3), V0 is expressed as follows:

[0044]

number

[0045] In this formula, H, S1, and S2 are known (specified in the print settings), so the ink ejection speed V0 can be calculated by using x1 and x2 obtained by measurement. By implementing this ink ejection speed calculation method, the ink ejection speed calculation process (step S102) is configured to calculate the ink ejection speed V0.

[0046] Next, the calculation of the ink ejection velocity assuming that the ink motion is a vertically thrown downward falling motion will be described.

[0047] In order to make an accurate calculation, it is necessary to take into account air resistance, etc., but since the printing height during actual printing is about a few millimeters, and in the experiment it was 1.3 mm, air resistance, etc. will be ignored to simplify the calculation.

[0048] If the distance between the nozzle surface of the inkjet head and the workpiece (=printing height) is H (mm), the ink ejection speed is V0 (m / sec), the time it takes for the ink droplet 27 to land on the workpiece is t (sec), the speed in the Z direction when the ink droplet 27 lands on the workpiece is V (m / sec), and the gravitational acceleration is g, the following equation (4) is established.

[0049]

number

[0050] V can also be expressed as in the following equation (5).

[0051]

number

[0052] From equations (4) and (5), the following equation (6) can be obtained.

[0053]

number

[0054] In addition, the offset x, which is the distance in the XY directions until the ink lands on the workpiece at a printing speed S (m / sec), can be expressed as follows:

[0055]

number

[0056] From the above, if (a) the offset, which is the distance moved at printing speed S1, is x1, and (b) the offset, which is the distance moved at printing speed S2, is x2, then the difference D between (a) and (b) is expressed by the following equation (7).

[0057]

number

[0058] From equations (6) and (7), V0 is expressed as follows:

[0059]

number

[0060] H, g, S1 and S2 are known (H, S1 and S2 are specified in the print settings, g is 9.8 m / s 2 ), the ink ejection velocity V0 can be calculated by using x1 and x2 obtained by measurement. By implementing this ink ejection velocity calculation method, the ink ejection velocity calculation process (step S102) can be configured to calculate the ink ejection velocity V0.

[0061] Here, an embodiment will be described in which the movement of the ink droplet 27 is compared between a case where it is calculated as a uniform velocity movement and a case where it is calculated as a falling movement.

[0062] In this embodiment, the offset x [μm] was measured when the print pattern was printed under the following conditions. (Condition 1) Measured at five different print speeds (50.3936 / 123.5202 / 251.9934 / 370.5606 / 484.5812) [mm / sec] (Condition 2) Print height H: Fixed at 1.3 mm In addition, since the inkjet printing device 1 used in this example is a model equipped with two inkjet heads 4, measurements were performed using both inkjet heads (head 1 and head 2). In addition, the inkjet printing device 1 used in this example has a function for measuring the ejection speed of droplets 27.

[0063] FIG. 9 shows the offset x values ​​measured at five different printing speeds S for each of head 1 and head 2. FIG. 10 shows the measurement results of the ejection speed of the droplet 27 of the inkjet printing device for every combination of two printing speeds selected from the five printing speeds S for each of head 1 and head 2, and the values ​​of the ink ejection speed V0 when the movement of the ink droplet 27 is calculated as a uniform speed movement and when it is calculated as a falling movement. FIG. 11 shows the average ejection speed, minimum ejection speed, and maximum ejection speed of the ink ejection speed V0, where (a) shows the measurement results of the ejection speed of the droplet 27 of the inkjet printing device, and (b) shows the calculation results when the movement of the ink droplet 27 is calculated as a uniform speed movement. In FIG. 11(b), the maximum and minimum values ​​are excluded from the ejection speed obtained by calculation.

[0064] As shown in Figure 10, the ink ejection velocity V0 is the same when the movement of the ink droplet 27 is calculated as a uniform velocity movement and when it is calculated as a falling movement. Although only up to the third decimal place is shown in Figure 10, the values ​​match up to the fourth decimal place. Also, as shown in Figures 10 and 11, it can be seen that the measurement results of the ejection velocity using the inkjet printing device and the ejection velocity obtained by calculation match with a high degree of accuracy.

[0065] The ink viscosity change determination process (step S103) will be described. The ink ejection speed calculated in the ink ejection speed calculation process (step S102) is compared with a predetermined reference ejection speed (in this example, the ink ejection speed obtained using ink in a state where there is no increase in viscosity immediately after the ink is put into the ink tank is used). It is determined whether the ink has increased in viscosity. For example, if the ink ejection speed meets a predetermined determination criterion (for example, the ejection speed has decreased by a predetermined percentage or more, or the ejection speed has decreased by a predetermined value or more) with respect to the predetermined reference ejection speed, it is determined that the ink has increased in viscosity, and if not, it is determined that the ink has not increased in viscosity. This determination may be automatically performed by the control unit 7 based on the calculated ink ejection speed, the predetermined reference ejection speed, and the predetermined determination criterion, or determination information including these pieces of information may be presented to the user so that the user can make the determination.

[0066] According to the ink viscosity change detection method and inkjet printing device 1 of this embodiment configured as described above, it can be easily implemented if a camera 6 that generates an image of the printing target surface of the work 11 fixed to the fixed part 2 is provided. In this way, there is no need to provide a dedicated hardware configuration for acquiring the ink viscosity (for example, a light-emitting element and a light-receiving element for obtaining the ejection speed of the ink droplets 27, or a viscosity sensor for measuring the viscosity in the ink tank), so there is no problem of the device becoming larger and the cost increasing. In addition, the viscosity sensor cannot be applied to an ink cartridge type that does not have an ink tank, but the present invention does not have such a restriction. And, when the viscosity of the ink changes, the ink landing position is shifted (the printing position accuracy is deteriorated), so it is possible to use it to recover the printing position accuracy by readjusting the timing of ejecting the ink.

[0067] Next, a second embodiment of the present invention will be described. This embodiment differs from the first embodiment mainly in the following points. Therefore, the same reference numerals are used to designate the same parts as the first embodiment, and the duplicated description will be omitted.

[0068] In this example, the printing height H is used as a relative specific physical condition between the workpiece 11 and the nozzle 4a, and offsets x1 and x2 are obtained for heights H1 and H2 which are different from each other, respectively.

[0069] A method for calculating the ink ejection speed in the ink ejection speed calculation process (step S102) of this example will be described. In order to precisely calculate the ink ejection speed, it is necessary to take into account air resistance, etc. However, since the print height during actual printing is a few millimeters, and even in experiments it is a very small distance of 1.3 mm, air resistance, etc. will be ignored to simplify the calculation. In this embodiment, only a method for calculating the motion of the ink droplets 27 as uniform velocity motion will be described, and a method for calculating as falling motion will be omitted.

[0070] As shown in Figure 8, if the distance between the nozzle surface of the inkjet head and the work (=printing height) is H (mm), the ink ejection speed is V0 (m / sec), and the time it takes for an ink droplet 27 to land on the work is t (sec), the following equation (8) approximately holds.

[0071]

number

[0072] In addition, the offset x, which is the distance in the XY directions until the ink lands on the workpiece at a printing speed S (m / sec), is expressed by the following equation (9).

[0073]

number

[0074] By substituting equation (8) for t in equation (9), we obtain the following equation.

[0075]

number

[0076] From the above, if (a) the offset, which is the distance moved at print height H1, is x1, and (b) the offset, which is the distance moved at print height H2, is x2, then the difference D between (a) and (b) is expressed by the following equation (10).

[0077]

number

[0078] From equation (10), V0 is expressed as follows:

[0079]

number

[0080] In this formula, S, H1, and H2 are known (specified in the print settings), so by using x1 and x2 obtained by measurement, the ink ejection speed calculation process (step S102) calculates the ink ejection speed V0 from the above formula.

[0081] The ink viscosity change detection method and inkjet printing apparatus 1 of this embodiment can also provide the same effects as those of the first embodiment.

[0082] The present invention is not limited to the above-described embodiment, and can be embodied by making appropriate modifications without departing from the spirit of the invention, for example, as described below. (1) An object other than a substrate such as a printed wiring board is used as the workpiece 11 to be printed. (2) The nozzles 4a in the inkjet head 4 may be arranged in a pattern different from that in the above embodiment. (3) The camera 6 is supported by the device body 1 a so as to be fixedly provided at a predetermined position above the fixed part 2 . [Explanation of symbols]

[0083] 1 Inkjet printing device 1a Device body 2 Fixed part 3 Moving mechanism 3a X-axis movement mechanism 3b Y-axis movement mechanism 3c Z-axis movement mechanism 4 Inkjet head 4a Nozzle 5 Hardened part 6. Camera 7 Control section 8. Placement table 11 Work 12 X-axis moving body 14 Z-axis moving body 16 X-axis ball screw 17 Y-axis ball screw 18 X-axis linear guide 19 Y-axis linear guide 20 X-axis motor 21 Y-axis motor 24 images 25 Printing Pattern 25a Reference point of print pattern 26 screens 26a Guidelines 27 Ink Droplets

Claims

1. A fixing part for fixing a workpiece to be printed; A nozzle for ejecting ink for printing; a moving mechanism that moves the nozzle relative to the fixed portion; A camera that generates an image of a printing surface of the workpiece fixed to the fixing portion; a control unit configured to perform a printing process for printing on the workpiece by ejecting ink from the nozzle while moving the nozzle relative to the workpiece fixed to the fixed unit using the moving mechanism, and to perform an image acquisition process for acquiring an image of the workpiece from the camera; A method for detecting a change in viscosity of ink using an inkjet printing device comprising: a two-state offset acquisition step in which the printing process is performed so as to print a predetermined print pattern on the work at target print coordinates in two states in which relative specific physical conditions between the work and the nozzle are made different from each other, and an offset of an actual print coordinate with respect to the target print coordinate in each state is acquired by using an image of the work acquired by the image acquisition process; an ink ejection velocity calculation step of calculating an ink ejection velocity from the nozzle using the physical conditions in the two states and offsets of the two states; and an ink viscosity change determination step for determining whether or not there is a change in the viscosity of the ink by comparing the ink ejection speed with a predetermined reference ejection speed based on a predetermined determination criterion. A method for detecting changes in ink viscosity.

2. 2. The method for detecting a change in viscosity of ink according to claim 1, wherein the specific relative physical condition is a speed of the relative movement between the nozzle and the workpiece.

3. 2. The method for detecting a change in viscosity of ink according to claim 1, wherein the relatively specific physical condition is a distance between the ink ejection opening of the nozzle and the workpiece.

4. A fixing part for fixing a workpiece to be printed; A nozzle for ejecting ink for printing; a moving mechanism that moves the nozzle relative to the fixed portion; A camera that generates an image of a printing surface of the workpiece fixed to the fixing portion; a control unit configured to perform a printing process for printing on the workpiece by discharging ink from the nozzle while moving the nozzle relative to the workpiece fixed to the fixed unit by the moving mechanism, and to perform an image acquisition process for acquiring an image of the workpiece from the camera; a two-state offset acquisition process for performing the printing process so as to print a predetermined print pattern on the work at target print coordinates in two states in which relative specific physical conditions between the work and the nozzle are different from each other, and acquiring an offset of an actual print coordinate with respect to the target print coordinate in each state by using an image of the work acquired by the image acquisition process; an ink ejection velocity calculation process for calculating an ink ejection velocity from the nozzle using the physical conditions in the two states and offsets between the two states; and performing an ink viscosity change determination process for determining whether or not there is a change in the viscosity of the ink by comparing the ink ejection speed with a predetermined reference ejection speed based on a predetermined determination criterion. Inkjet printing device.

5. 5. The inkjet printing apparatus according to claim 4, wherein the specific relative physical condition is a speed of the relative movement between the nozzle and the workpiece.

6. 5. The inkjet printing apparatus according to claim 4, wherein the relative specific physical condition is a distance between the ink ejection opening of the nozzle and the workpiece.

Images