PRINT HEAD MAINTENANCE

Abstract

A maintenance unit for an inkjet system with a print head assembly having at least one print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the at least one print head towards a substrate includes a wiper to wipe along the surface of the at least one print head; a force actuator to apply a force to the wiper in a direction perpendicular to the surface of the at least one print head; a force measuring unit configured to determine a wiping force with which the wiper is pressed against the surface of the at least one print head; and a controller configured to control the force applied by the force actuator in dependency of an output of the force measuring unit in order to press the wiper against the surface of a print head with a predetermined wiping force.

Claims

1. A maintenance unit for an inkjet system with a print head assembly, said print head assembly comprising at least one print head, the at least one print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the at least one print head towards a substrate, the maintenance unit comprising: a wiper to wipe along the surface of the at least one print head; a force actuator to apply a force to the wiper in a direction perpendicular to the surface of the at least one print head; a force measuring unit configured to determine a wiping force with which the wiper is pressed against the surface of the at least one print head; and a controller configured to control the force applied by the force actuator in dependency of an output of the force measuring unit in order to press the wiper against the surface of a print head with a predetermined wiping force.

2. The maintenance unit for an inkjet system according to claim 1, wherein the maintenance unit further comprises: a position sensor to measure the position of the wiper relative to the surface of the at least one print head; and a set point generator for generating a set point corresponding to a desired position of the wiper relative to the surface of the at least one print head seen in a direction perpendicular to the surface of the at least one print head, wherein the controller is arranged to drive the force actuator in dependency of an output of the position sensor and the set point, wherein, in order to wipe along the surface of the at least one print head, the set point generator is configured to output a set point corresponding to a position in which the wiper is at least partially inside the at least one print head, and wherein the controller is configured to limit the maximum applicable force of the force actuator to the predetermined wiping force.

3. The maintenance unit according to claim 1, further comprising a wiper moving device for moving the wiper, wherein the controller is configured to drive the wiper moving device such that the wiper is moved along the surface of the at least one print head.

4. The maintenance unit according to claim 1, further comprising a frame and a guide to guide movement of the wiper with respect to the frame in a direction parallel to the wiping force.

5. The maintenance unit according to claim 3, wherein the wiper moving device is configured to operate on the frame to move the wiper.

6. The maintenance unit according to claim 1, wherein the force actuator is an electromagnetic actuator.

7. The maintenance unit according to claim 6, wherein the electromagnetic actuator is a Lorentz actuator.

8. The maintenance unit according to claim 3, wherein the guide is configured to guide movement of the wiper without applying significant forces to the wiper, or wherein the guide guides movement of the wiper while applying a constant force to the wiper.

9. The maintenance unit according to claim 3, wherein the wiper moving device is configured to move the wiper in two degrees of freedom in a plane parallel to the surface of the at least one print head.

10. The maintenance unit according to claim 1, wherein multiple wipers are arranged on a common wiper support frame.

11. The maintenance unit according to claim 10, wherein respective wiper moving devices are provided in between the respective wiper and the support frame, such that movement of each wiper can individually be controlled by the controller.

12. The maintenance unit according to claim 10, wherein the wiper support frame is moveable relative to the print head assembly in one direction only, and wherein the wiper moving devices on the wiper support frame are configured to move the respective wiper in a direction different from said one direction of the wiper support frame such that the wiper is moveable in a two dimensional plane parallel to the surface of the at least one print head.

13. The maintenance unit according to claim 1, comprising a heating device to heat the wiper in order to melt ink fluid that has accumulated on the wiper, thereby removing the ink fluid from the wiper.

14. An inkjet system comprising: a print head assembly with at least one print head, the at least one print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the at least one print head towards a substrate; and the maintenance unit according to claim 1 to perform maintenance on the at least one print head.

15. The inkjet system according to claim 14, wherein the wiper of the maintenance unit is moveable between an operational position in which the wiper is able to perform a wiping action with respect to the at least one print head, and a non-operational position in which the wiper is arranged at a distance from the print head assembly, such that the maintenance unit is not interfering with normal printing activities.

16. The inkjet system according to claim 15, wherein a printing direction is defined which corresponds to a direction in which substrates pass the print head assembly for printing purposes, and wherein the maintenance unit is moveable in a horizontal direction perpendicular to the printing direction.

17. A method to perform maintenance on a print head of a print head assembly, said print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the print head towards a substrate, said method comprising the following steps: providing a wiper which is moveable along the surface of the print head to remove ink from said surface; moving the wiper along the surface of the print head while pressing the wiper against the surface of the print head with a force actuator; determining a wiping force with which the wiper is pressed against the surface of the print head by the force actuator; and driving the force actuator based on the determined wiping force in order to press the wiper with a predetermined wiping force against the surface of the print head.

18. The method according to claim 17, further comprising the following steps: urging the wiper to an unreachable position inside the print head with a force actuator while moving the wiper along the surface of the print head; and while urging the wiper to said position, keeping the maximum by the force actuator applied force below a predetermined value.

19. The method according to claim 17, wherein ink fluid is purged out of the nozzles prior to moving the wiper along the surface of the print head.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0111] The invention will be explained in more detail with reference to the appended drawings. The drawings show a practical embodiment according to the invention, which may not be interpreted as limiting the scope of the invention. Specific features may also be considered apart from the shown embodiment and may be taken into account in a broader context as a delimiting feature, not only for the shown embodiment but as a common feature for all embodiments of falling within the scope of the appended claims, in which:

[0112] FIG. 1 depicts an inkjet system according to an embodiment of the invention;

[0113] FIG. 2 depicts a part of the inkjet system of FIG. 1 and shows schematically a maintenance unit according to an embodiment of the invention;

[0114] FIG. 3A depicts in more detail a part of a maintenance unit according to an embodiment of the first subaspect of the fourth aspect of the invention, which is suitable to be used in the inkjet system of FIG. 1;

[0115] FIG. 3B depicts in more detail a part of a maintenance unit according to an embodiment of the second subaspect of the fourth aspect of the invention, which is suitable to be used in the inkjet system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0116] FIG. 1 depicts an inkjet system IS according to an embodiment of the invention for depositing ink fluid in a desired pattern on a substrate S by jetting liquid droplets DR of the ink fluid in a jetting direction JD towards the substrate S. The inkjet system is preferably a drop-on-demand inkjet system in which a droplet is only jetted when required. This is in contrast to continuous inkjet systems in which droplets are continuously jetted at a predetermined frequency and wherein droplets required to form the pattern are directed towards the substrate and the remaining droplets are captured and thus prevented from reaching the substrate.

[0117] The inkjet system of FIG. 1 is an industrial inkjet system, for instance an inkjet system used to deposit resist material as a mask layer on a printed circuit board (PCB) as an alternative to the more traditional process of providing a mask layer using lithography. Because the mask layer can be deposited directly by the inkjet system, the amount of process steps can be reduced dramatically and thus the time for PCB manufacturing. However, such an application requires a high droplet placement accuracy and a high reliability (substantially every droplet counts).

[0118] To provide a high accuracy inkjet system, the inkjet system IS comprises a force frame FF which supports a metrology frame MF from the ground GR. Between the force frame FF and the metrology frame MF a vibration isolation system VIS is provided to support the metrology frame MF from the force frame FF while isolating the metrology frame MF from vibrations in the force frame FF. As a result, a relatively steady and quiet printing environment can be created on the metrology frame MF which is advantageous for accuracy.

[0119] The inkjet system further comprises a print head assembly with one or more print heads PH which are held by a print head holder H, and a substrate holder SH to hold the substrate S. The print heads PH each comprise one or more, typically dozens of, nozzles from which droplets DR can be ejected towards the substrate S. The nozzles are preferably arranged in an array, i.e. in one or more rows. The print heads together define a printing plane perpendicular to the jetting direction JD, said printing plane indicating where the substrate has to be positioned in order to receive jetted droplets from the print heads.

[0120] The substrate holder SH is moveable relative to the print heads PH in a printing direction PD parallel to the Y-direction and thus parallel to the printing plane in order to let a substrate S pass below the print head assembly. In this application a distinction is made between passing the print head assembly while moving from left to right in FIG. 3, i.e. moving the substrate holder in the positive Y-direction, and passing the print head assembly while moving from right to left, i.e. moving the substrate holder in the negative Y-direction. The right to left movement will be referred to as a forward swath and the left to right movement will be referred to as a backward swath.

[0121] In order to be able to cover the entire top surface TS of the substrate S, many configurations are possible. In a first configuration, the printing plane in the X-direction is at least as large as the largest possible dimension in X-direction of a substrate S that can be held by the substrate holder SH. In that case, a single swath of the substrate holder SH may suffice to cover the entire top surface with droplets. In a second configuration, the printing plane in X-direction is smaller than the largest possible dimension in X-direction of a substrate S that can be held by the substrate holder SH. In that case, multiple parallel swaths are necessary to cover the entire top surface TS of the substrate S. To allow multiple parallel swaths, the print head assembly and/or the substrate holder SH is moveable in the X-direction perpendicular to the printing direction PD.

[0122] In case of the printing plane in the X-direction being at least as large as the largest possible dimension in X-direction of a substrate S that can be held by the substrate holder SH, multiple swaths may still be necessary in order to obtain the required printing resolution, because the nozzles in the print heads PH may be arranged at a larger distance than the corresponding pitch from each other, e.g. to prevent or reduce cross talk between adjacent nozzles. The substrate is then passing the print head assembly multiple times, wherein each time the substrate has been moved in X-direction corresponding to the resolution in order to print the entire pattern.

[0123] In this embodiment, the print head assembly has a printing plane with a dimension in X-direction at least as large as the largest possible dimension in X-direction of a substrate the substrate holder SH can handle. As a result, the print head assembly can be mounted stationary with respect to the metrology frame MF.

[0124] In the embodiment of FIG. 1, the substrate holder SH is supported by a substrate positioning stage PS, which substrate positioning stage PS in turn is supported by the metrology frame MF. The substrate positioning stage PS is supported by the metrology frame such that it is moveable in the printing direction PD, thereby allowing to position the substrate holder SH and thus the substrate S in the Y-direction. Positioning of the substrate positioning stage PS is done using a stage positioning device SD, which is able to apply forces F between the substrate positioning stage PS and the force frame FF. As a result, the forces F do not introduce disturbances to the metrology frame MF, but are transmitted to the ground via the force frame FF, which results in a higher obtainable accuracy of the inkjet system.

[0125] Between the substrate positioning stage PS and the substrate holder SH, a holder positioning device HD is provided in order to position the substrate holder SH in one or more degrees of freedom, preferably at least in the printing direction PD, relative to the substrate positioning stage PS. Using this configuration, the stage positioning device SD can be used for coarse positioning the substrate holder SH in the printing direction while the holder positioning device HD can be used for fine positioning of the substrate holder in the printing direction relative to the print head assembly. If required, the holder positioning device HD may also be used for fine positioning of the substrate holder in other directions as well, e.g. the X-direction and/or the Z-direction, and may even fine position the substrate holder in rotational directions such as Rx, Ry and Rz as well. Preferably, the holder positioning device HD is able to position the substrate holder relative to the substrate positioning stage in six degrees of freedom.

[0126] Position information about the substrate holder SH relative to the metrology frame MF is measured by a measurement system MS. The measurement system is at least configured to measure a position quantity, i.e. actual position, velocity or acceleration, of the substrate holder in the printing direction PD. In an embodiment, the measurement system measures position information about the substrate holder in six degrees of freedom, depending on the level of control that is applied/required.

[0127] The output of the measurement system MS is provided to control electronics CE. The control electronics are here depicted as a black box that controls all processes in the inkjet system IS. As an example, the output of the measurement system MS can be used by the control electronics to drive the stage positioning device SD and the holder positioning device HD (as shown in dashed lines) in order to position the substrate holder accurately relative to the print head assembly. The control electronics may further send driving signals to the print heads PH (see dashed lines) in order to print a desired pattern on the substrate while the substrate S passes the print heads PH.

[0128] The inkjet system IS further comprises a droplet detection device DD which measures the position of placed droplets on the substrate, e.g. by emitting light towards the substrate and detecting the reflected light. The obtained information is also send to the control electronics, which may comprise a calibration unit in order to adjust the position of the print heads relative to each other based on the droplet position information obtained by the droplet detection device. The droplet detection device DD may further be used to calibrate the timing for firing the nozzles.

[0129] The FIGS. 1, 2, 3A and 3B relate in particular to the invention.

[0130] The inkjet system IS as shown in FIG. 1 further comprises a maintenance unit MU (see FIG. 2) configured to remove ink fluid from the surfaces SU of the print heads PH in which the nozzles are arranged, because ink fluid may accumulate on said surface during printing which reduces the obtainable accuracy and reliability.

[0131] The surfaces SU of the print heads PH are shown with reference to FIG. 2 in which the print head assembly is shown from below. The printing direction PD is also indicated by the respective arrow to indicate the transport direction of the substrates for printing. Only a few print heads and a few surfaces SU are indicated by the respective reference numerals PH and SU for clarity reasons.

[0132] Also schematically shown in FIG. 2 are a wiper support frame WSF of the maintenance unit MU that can be moved between a non-operational position NOP as shown in FIG. 2 in which no maintenance can be performed on the print heads and a maintenance position MP (see dashed box) below the print heads in which the maintenance unit is able to perform maintenance actions on the print heads. For this purpose guides G1, G2 are provided along which the wiper support frame is able to move between the non-operational position and the maintenance position. Movement of the wiper support frame may be caused by a respective actuation system provided between the wiper support frame and the guides G1, G2.

[0133] The non-operational position of the wiper support frame is in this case adjacent the transport area of the substrates, i.e. the movability of the wiper support frame is in a direction D1 perpendicular to the printing direction PD, which has the advantage that the maintenance unit can be moved to a position in which the maintenance unit does not interfere with the printing activities, i.e. does not collide with passing substrates or substrate holders.

[0134] The maintenance unit MU further comprises multiple wipers with respective wiper moving devices to move the wipers in a direction D2 relative to the wiper support frame WSF. Direction D2 is in this embodiment parallel to the longitudinal direction of the surfaces SU of the print heads PH. The wipers and wiper moving devices are schematically indicates by dashed boxes W as they are operational at the other side of the wiper support frame, i.e. the side of the wiper support frame facing towards the surfaces of the print heads when being in the maintenance position MP.

[0135] This configuration allows the wiper support frame to be positioned in the direction D1 such that the wipers are aligned with the surfaces SU of a first column of print heads after which the wipers are subsequently moved by the wiper positioning devices along the surfaces of the print heads. After performing the wiping action, the wipers can subsequently be positioned properly with respect to a second column of print heads for a next wiping action, and so forth until all print heads of the print head assembly are wiped clean. In such a case, the wiper support frame is moved stepwise and the wiping action is performed by the wiper moving devices while the wiper support frame is kept stationary relative to the print head assembly. It will be apparent to the person skilled in the art of maintenance units for inkjet systems that other configurations for moving the wiper are also envisaged.

[0136] So far, the maintenance unit can be according to the first or second subaspect of fourth aspect of the invention. An example of a maintenance unit according to the first subaspect of the fourth aspect of the invention will be given with reference to FIG. 3A, and an example of a maintenance unit according to the second subaspect of the invention will be given with reference to FIG. 3B.

[0137] FIG. 3A schematically depicts a part of a maintenance unit MU according to an embodiment of the first subaspect of the invention, which maintenance unit can be used in the inkjet system of FIGS. 1 and 2. Shown are a wiper support frame WSF which moveably supports a frame FR. In between the frame FR and the wiper support frame WSF, a wiper moving device WMD is operable to generate a force F1 to position the frame FR relative to the wiper support frame WSF.

[0138] Arranged on the frame FR is a wiper W1 to be moved along the surfaces of the print heads. Movement of the wiper W1 is guided by a guide with two parallel leaf springs LF which together form a linear guide allowing the wiper only to move up and down. Connected to the wiper W1 is a permanent magnet PM as part of a force actuator. The permanent magnet is arranged inside a coil CO being another part of the force actuator, so that supplying a current I to the coil by an appropriate energy source, e.g. a current source, will generate a force on the permanent magnet due to the interaction between the respective magnetic fields of the magnet and coil. This force can be used to position the wiper in a direction perpendicular to a surface SU of a print head PH with respect to the surface SU of the print head PH, which print head is shown in dashed lines.

[0139] The position of the wiper W1 relative to the surface SU is indirectly measured using a position sensor PS based on the assumption that the distance between frame FR and surface SU is substantially the same each time. The output of the position sensor is fed to a controller CON which based on the output of the position sensor provides driving signals to a current source CS to apply a current I to the force actuator, and to the wiper moving device WMD. In order to provide a predetermined wiping force to the surface SU, the maintenance unit comprises a set point generator SG which provides a set point corresponding to a location of the wiper W1 inside the print head PH as shown by wiper W1. However, the wiper W1 is not able to reach that location, so that the controller will continuously urge the wiper W1 to the position W1 using the force actuator. The controller comprises a limiter LI which keeps the maximum applicable force applied by the force actuator within a predetermine value, in this embodiment by limiting the maximum current that can be generated by the current source. As a result, substantially the same wiping force is applied to the wiper independent of occurring variations in properties of the wiper.

[0140] FIG. 3B schematically depicts a part of a maintenance unit MU according to an embodiment of the second subaspect of the fourth aspect of the invention, which maintenance unit can be used in the inkjet system of FIGS. 1 and 2. Shown are a wiper support frame WSF which moveably supports a frame FR. In between the frame FR and the wiper support frame WSF, a wiper moving device WMD is operable to generate a force F1 to position the frame FR relative to the wiper support frame WSF.

[0141] Arranged on the frame FR is a wiper W1 to be moved along the surfaces of the print heads. Movement of the wiper W1 is guided by a guide with two parallel leaf springs LF which together form a linear guide allowing the wiper only to move up and down. Connected to the wiper W1 is a permanent magnet PM as part of a force actuator. The permanent magnet is arranged inside a coil CO being another part of the force actuator, so that supplying a current I to the coil by an appropriate energy source, e.g. a current source, will generate a force on the permanent magnet due to the interaction between magnet and coil. This force can be used to press the wiper against the surface of the print heads during a wiping action.

[0142] Preferably, the force actuator is configured such that a substantially constant current-force relationship is obtained in the working range of the wiper. This allows an open loop kind of control, wherein controlling the current through the coil properly controls the force applied to the wiper by the force actuator. The current can be measured using a measurement resistance R1 and measuring the voltage V1 over the resistance R1. The measured current can be supplied to a controller CON, which is able to control the current source CS based on said measured current.

[0143] In case the leaf springs of the guide do not apply a significant force to the wiper while guiding the wiper in the working range of the wiper, the force applied by the force actuator corresponds to the wiper force with which the wiper will be pressed against the surface of the print heads independent of the stiffness of the wiper, the actual position of the wiper etc. In some embodiment, it may be necessary to overcome a known or determinable constant force, e.g. gravity, but this constant force can easily be compensated for.

[0144] In case the forces applied by the guide to the wiper are significant and non-constant or when the current-force relationship is not constant, the mentioned open-loop control may not suffice. Usually, the current-force relationship is dependent on the position of the permanent magnet inside the coil, so that adding a position sensor PS for determining the position of the magnet can be beneficial for accurately determining the force applied to the wiper by the force actuator.

[0145] The position sensor PS can alternatively or additionally be used to determine the position of the guide. In case the stiffness of the leaf springs in vertical direction is too high, the disturbance force applied by the guide to the wiper is also dependent on the position of the wiper relative to the guide. Hence, measuring the position allows to determine the disturbance force of the guide which when fed to the controller can be compensated for.