Background

[0001] Inkjet printers are, in general terms, controllable fluid ejection devices that propel droplets of ink from a nozzle to form an image on a substrate wherein such propelling can be achieved by different technologies. In all of such technologies, the primary cause of inkjet printing problems is ink drying on the nozzles, causing the pigments and dyes of the ink to dry out and form a solid block of hardened mass mat may clog the ink passageways. ftrfof Dgftfripfion offfifr Qm\m*

[0002] Examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0003] Figure 1 shows a perspective view of an example of a printhead and a cleaning mechanism.

[0004] Figure 2 is a perspective view of an example of a cleaner.

[0005] Figure 3 shows a detailed view of an example cleaner acting on a printhead.

[0006] Figure 4A is a schematic view of a set of nozzles and an example of a cleaning mechanism in a first position.

[0007] Figure 4B is a schematic view of the set of nozzles and the cleaning mechanism of figure 4A with the cleaning mechanism in a second position.

[0008] Figure 5 is an example of control signal send to the nozzles.

[0009] Figure 6 is a flow diagram of a printhead cleaning method according to an example.

Detailed Description

[0010] Common fluid ejection technologies are divided into two main categories: fluid ejection through thermal drop generation or piezoelectric drop generation. Piezoelectric drop generation is performed by using a piezo actuator for modifying the pressure of a chamber thereby controlling the expelling of ink through an opening on such chamber and thermal drop generation is accomplished by using a heater to increase the temperature of ink within a chamber thereby generating a bubble that, in turn, in- creases the internal pressure of a chamber and ejects drops.

[0011] In thermal drop generation is inherently harder to control the pressure inside the chamber whereas on piezoelectric drop generation the actuator can be used to modify the internal pressure more accurately and, furthermore, the pressure can be set to specific values throughout the printing process. Nonetheless piezoelectric drop generation is much more expensive and technically complex than thermal drop generation. A cleaning method that may be used for, at feast, both piezoelectric and thermal drop generation printing systems would be advantageous.

[0012] As mentioned above, hereby it is disclosed a cleaning mechanism allows to prevent dogging of the nozzles within a printhead. The cleaning mechanism allows to clean printheads irrespective of whether the drop generation is performed thermally or by piezoelectric means and, also, a method is disclosed wherein it is prevented that air may flow through the nozzle into the printhead and may help prevent the generation of air bubbles in the nozzle.

[0013] Figure 1 shows an example of a printhead carriage 1 that comprises a set of receptacles to receive printheads 10, 11 , 12. Each of such printheads 10, 11 , 12 comprises an inner container with printing fluid and a set of nozzles to controijably expel such printing fluid through the nozzle in an expelling direction as to generate an image on a substrate located in such expelling direction.

[0014] As mentioned above, the expelling of the printing fluid through the nozzles is performed by pressurizing a firing chamber within the printhead upstream the nozzle, such firing chamber comprising a determined amount of printing fluid. This pressurize- tion causes the ejection of droplets of the printing fluid thought the nozzle and may be performed, e.g., by using a piezo actuator or a heater. In the case of the piezo actuator, the pressure of the firing chamber can be controlled to several values since the vibration of such piezo actuator may be controlled by a control signal issued by a controller. On the other hand, in the case of heaters, the printing system is either pressurized (with the heater activating generating bubbles on the printing fluid) or non-pressurized {with tiie heater being deactivated) with no intermediate pressurizatton.

[0015] To dean the nozzles of the printheads 10, 11, 12 it is envisaged using a cleaning mechanism 2 comprising a wiper 21 attached to a wiper carriage 20 being such wiper carriage 20 movable along the printhead carriage 1. In particular, the wiper carriage 20 is to move below 8» printhead carriage while contacting the nozzles as to provide a rubbing effect. The movement of the cleaning system is, in an example, performed in a cleaning direction Di.

[0016] Figure 2 shows an example of cleaning mechanism 2 according to an example. The cleaning mechanism 2 of figure 2 comprises a casing 20, a wiper container 21 , a wiper 22 at least one loomed wheel 200.

[0017] The wiper 22 may be comprise an absorbent such as, e.g., a cloth or foam to withdraw printing fluid residues that may located on the nozzle and/or its surroundings. Further, the cleaning mechanism 2 may comprise or be connected to a motor for moving the cleaning mechanism 2 at least linearly along the printhead carriage 1. Such motor may, for example, be connected to a belt that engages the toothed wheels 200 as to move the cleaning mechanism. Furthermore, the cleaning mechanism may comprise or be connected to at least one position sensor such as, e.g., a laser distance sensor, an optical encoder and/or a mechanical encoder. In an example, the cleaning mechanism comprises a laser distance sensor and is connected additionally to a mechanical encoder attached to the belt thereby achieving an improved accuracy.

[0018] Furthermore, the cleaning mechanism 2 may comprise an elastic member 23 located below at least a section of the wiper 22. The effect achieved by such elastic member 23 is to perform a rubbing force in a direction with a component in a direction perpendicular to the wiper, i.e., in the direction towards the nozzles and/or with a component a direction opposed to the firing direction of the nozzles. Examples of this elastic elements may be foam and/or rubber. In an example, the wiper 22 and the elastic member 23 may be a single element. [0019} To perform the cleaning, the cleaning mechanism 2 may comprise consumables that may be replaced during the lifetime of the cleaning system 2. For example, the wiper 22 may be a cloth that may comprise a storage 21 for clean cloth and a storage for used cloth 220.

[0020] Referring now to figure 3 a perspective view is shown of the cleaning mechanism 2 as it moves along the printhead carriage 1 carrying at least one printhead 10. Also, it is shown a detailed of a longitudinal section showing the interaction between the cleaning mechanism 2 and a set of nozzles 100.

[0021] In figure 3 it can be seen that the cleaning system is to move in a cleaning direction Di. In an example, the cleaning direction Di is a direction along a linear trajectory following a bidirectional movement longitudinally along the prtnthead 1 and, in a further example, the wiper 22 has a width enough to cover transversaiiy at least the width of the printhead comprising nozzles. In this manner, a single pass of the cleaning system in the cleaning direction Di may clean all the nozzles 100 within the printhead 1.

[0022] in an example, the cleaning system comprises a wiper 22 and at least a portion of such wiper 22 is located over an elastic member 23. This configuration helps to perform a force with a component in a direction perpendicular to the cleaning direction Di as to exert a force towards the nozzle 100 providing a rubbing action between the wiper 22 and the nozzle 100.

[0023] Figures 4A and 4B is a schematic longitudinal section illustrating a cleaning action performed by an example of cleaning mechanism 2. In a first cleaning position, as shown in figure 4A, the cleaning mechanism 2 is moved by a motor 200 and instructed by the controller 30 by means of a movement signal 201 to be positioned at determined a distance for a reference point A controller 30 determines that the actual position Xc of the cleaning mechanism 2 corresponds to the position of a first nozzle 100, e.g., by receiving a positioning signal 240 from an encoder 24. Once it has been determined that the actual position Xc corresponds to a first nozzle 100, the controller 30 issues a control signal 3 towards a first actuator 101 that increases the pressure of a first firing chamber 102 thereby expelling printing fluid located within the chamber through the first nozzle 100. In an example, the controller 30 only issues the firing signal to the first actuator 101 so that a second actuator 101' and a third actuator 101 "are not energized and printing fluid is not expelled to their respective second nozzle 100' and third nozzle 100". f 0024] The expelling action while the cleaning mechanism is located in tie expelling direction of the nozzle accomplishes two main objectives. First, it removes any blockage that may be located within the nozzle preventing a proper expelling of fluid and, second, it prevents air bubbles to be generated within the nozzle given that there is a fluid pressure to the outside of the nozzle 100.

[00251 Referring now to figure 4B, the cleaning mechanism 2 has moved along a cleaning direction Di from below the first nozzle to a position wherein the wiper 22 is located in the expelling direction of the second nozzle 100'. In this actual position Xc the controller determines which nozzles have a expelling direction towards the actual position Xc of the cleaning mechanism 2. in the example of figure 48, only the second nozzle 100' has a expelling direction towards the wiper 22 and, therefore, the controller issues a control signal 3 so that the second actuator 101 ^* pressurizes the second firing chamber 102' thereby expelling printing fluid through the second nozzle 100' while maintaining the actuators that correspond to the nozzles for which the expelling directions are not towards the actual position Xc of tile wiper 22 inactivated, i.e., maintain its firing chambers at a pressure wherein there is no expelling of fluid through the nozzle.

[0026] in the particular example of figure 4B the first actuator 101 and a third actuator 101 " have nozzles which expelling directions are not towards the wiper 22 of trie cleaning system 2. Therefore, the first actuator 101 and the third actuator 101" are inactive. In the case of piezo electric actuators, the actuators do not vibrate or, at least, do not vibrate with a frequency and/or amplitude enough to cause fluid to be expelled through the nozzle and, in the case of thermal actuators, the nozzles are not heated, or, at least, not heated at a temperature enough to cause bubbles within the firing chamber. On the other hand, the second actuator 101 ^* is activated thereby expelling fluid through the second nozzle 100'.

[0027] As for the encoder 24, tine cleaning mechanism 2 may comprise an internal encoder or It may be external to the cleaning mechanism 2, e.g., an encoder attached to a belt that is to move the cleaning mechanism 2. In alternative embodiments an optical system may be used to determine the position of the cleaning system or even several positioning mechanisms may be used in order to improve the accuracy of the positioning signal 240.

[0028] in the example of figure 4A and 4B it can be seen mat a synchronization to activate the actuators associated to nozzles which expelling direction correspond to the position of the cleaning mechanism is advantageous for the health of the nozzles. Also, maintaining inactive the actuators tor which the cleaning mechanism 2 (or the wiper 22) is not on its expelling direction helps optimize the use of ink and maintain clean other parts of the printing system.

[0029] Figure 5 shows an example of control signal 3 issued by the controller 30 to activate the actuators. The control signal is preferably a low frequency signal. In the example of figure 5, the frequency of the signal is 1 kHz, however, it is envisaged the use of any frequency in a range between 500 Hz and 1500 Hz.

[0030] The control signal 3 of figure 5 is an example of trigger pulse to be used with a thermal actuator for a predetermined time or may be configured to be maintained until the wiper 22 is not in the expelling direction of the nozzle.

[0031] Figure 6 shows a flow diagram further explaining the cleaning process according to an example.

[0032] In the example of figure 6 the controller 30 is to receive a signal from a carriage encoder 62 or any positioning device associated to the cleaning carriage and, also, to receive the positions wherein nozzles are located. The nozzle positions 64 may be, e.g., obtained from a database. [0033] The controller 30 is to control the movement of the carriage 60 in a cleaning direction, i.e., longitudinally along the printhead carriage 1. By means of tile carriage encoder 62, the controller determines the carriage position Xc 61, then, the controller 30 runs a comparing action 63 as to determine if the actual position Xc of the carriage corresponds to a expelling direction of at least a nozzle. If there is at least a nozzle mat fulfils such condition, the controller sends a control signal as to pressurize the nozzles 65 that comply the condition. Even though, in the example of figures 4A and 4B only one nozzle was activated at a time, in the example of figure 6 it can be seen that several nozzles may fulfil the condition of having its expelling direction towards the wiper 22, i.e., having the wiper below them or in contact with them.

[0034] Finally, the controller 30 checks if the cleaning carriage is at its final position 66. If it is, then the cleaning is finished and, if it is not the movement of the carriage is continued.

[0035} In essence, it is disclosed a method for cleaning a printhead comprising:

• moving a wiper along a plurality of nozzles of the printhead; and

• pressurizing the nozzles within the printhead, the pressurization of each nozzle causing ink to be expelled in an expelling direction;

wherein a controller is to detect the position of the wiper and to selectively pressurize at least some of the nozzles for which the position of the wiper corresponds to its expelling direction.

[0036] In an example, the wiper is to move along the plurality of nozzles exerting a rubbing action on the plurality of nozzles. This is performed by having, e.g., an elastic element below or forming part of the wiper.

[0037] In a further example, the wiper comprises an ink absorbing element, e.g., a cloth or a foam,

[0038] The pressurization of the nozzles may be performed, in the case of a thermal drop generation by activating a heater in the vicinity of the nozzle or associated to such nozzle. [0039] Furthermore, the controller may be to detect the position of the wiper by receiving a position signal from an encoder, an optical sensor, a laser or any other type of positioning mechanism associated to the cleaning carriage.

[0040] As for the expelling of printing fluid, the pressurization of the nozzles may be done at a frequency in the range of 500 Hz to 1500 Hz, tor example at 1 kHz.

[0041] Also, it is disclosed a printing system comprising:

• a printhead receptacle adapted to receive a printhead, wherein the print- head contains ink comprises a plurality of nozzles as to expel ink in an expelling direction;

• a carriage adapted to receive a wiper; and

• an encoder to determine the position of the carriage

wherein the printing system comprises a controller to move the carriage along the plurality of nozzles and to issue a control signal as to selectively expel ink through a nozzle when the wiper is located in its expelling direction.

[0042] In an example, the wiper comprises an elastic member as to perform a rubbing action on the nozzles.

[0043] Furthermore, the system may comprise an encoder or an optical detector to determine the position of the carriage.

[0044] In a further example, the printhead comprises heaters on the plurality of nozzles and the control signal to selectively expel ink through a nozzle is an electric signal that activates such heaters.

[0045] Furthermore, the carriage may comprise an optical drop detector.