Method for cleaning the nozzle plate and the nozzles of at least one inkjet printhead with a rinsing medium in an inkjet printer

Abstract

Method for cleaning the nozzle plate and the nozzles of at least one inkjet printhead in an inkjet printer, wherein in a rinsing process the inkjet printhead (2) is rinsed with a rinsing medium, the rinsing quantity of which is determined from a predetermined rinsing quantity (mmin) dependent on the printing duration of the inkjet printer or from a rinsing quantity (m) derived from a rinsing quantity curve (k) which indicates for an ink used in printing the dependence of the rinsing quantity (m) required for rinsing the inkjet printhead (2) on the elapsed time (t), depending on which rinsing quantity has the larger value, and wherein the nozzle plate (5) of the inkjet printhead (2) is subsequently cleaned with a cleaning device (7).

Description

Inkjet printers can be used for printing in one or more colors on a substrate, such as a single sheet or a tape-like recording medium made of various materials, e.g., paper. The design of such inkjet printers is known, e.g., EP 0 788 882 B1. Inkjet printers that operate on the Drop on Demand (DoD) principle have one or more printheads with nozzles containing ink channels. Activators controlled by a printer controller stimulate ink droplets toward the substrate, directing them to create printed dots for an image. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically. In an inkjet printer, the ink's physical and chemical composition is specifically adapted to the printhead, for example, its viscosity. During low-volume printing, not all printhead nozzles are activated; many nozzles experience periods of inactivity (printing pauses), resulting in the ink not moving within the ink channel of those nozzles. Due to evaporation from the nozzle opening, the ink's viscosity can change. This can impair the ink's flow within the ink channel and cause it to leak from the nozzle. In extreme cases, the ink dries out completely within the ink channel, clogging it and rendering that nozzle unusable. The drying out of ink in the nozzles of a printhead during printing breaks is a problem that can be prevented by flushing a cleaning medium, such as ink or cleaning fluid, through all nozzles within a predefined cycle. This flushing cycle can be adjusted according to the printing workload. From US patent 6,578,945 B2, it is known how to prevent the nozzles of an inkjet printer with a print unit consisting of multiple printheads from drying out by sealing the printheads with protective caps. The ink released from the nozzles during cleaning is collected by the protective caps. To apply the protective caps to the printheads, the print unit with the printheads is moved upwards away from the substrate. The protective caps are then moved into the space between the print unit and the substrate, during which the printheads are cleaned. Spring force then moves the protective caps upwards onto the printheads, covering them. The protective cap assembly remains in this position until the print unit is ready to be used for printing again. During cleaning, the cleaning medium, e.g., ink, is forced or drawn through the nozzles and ink channels of the printhead using positive or negative pressure. This cleaning medium is then wiped off with a wiping tool (blade, squeegee), e.g., a rubber lip or several rubber lips. For this process, the printhead can be moved over the wiping tool or vice versa. Precise positioning of the printhead relative to the wiping tool is necessary to ensure consistent overlap between the wiping tool and the printhead. When using a cleaning medium to clean the nozzles of a printhead, the following problems, for example, must be considered: - The cleaning medium used for cleaning the nozzles is generally not reusable and is considered a waste. One goal is therefore to reduce the amount of cleaning medium used for cleaning as much as possible. - The necessary amount of cleaning medium depends on the condition of the ink used in the printhead during printing. If the printhead has not been used for a long time, the ink is more dried out, and a larger amount of cleaning medium is required to adequately clean the nozzles. - Furthermore, additional cleaning of the printhead nozzles is necessary after a prolonged period of printing. During printing, a small amount of ink mist (small droplets) is always produced, which settles on the nozzle plate. These droplets can cause problems.Firstly, such a droplet at a nozzle opening can deflect the ink droplets. Secondly, these droplets can dry out and enter the nozzle as disruptive particles during the next cleaning. From DE 697 07 962 T2 and DE 693 11 397 T2, a method is known according to which the consumption of rinsing ink is adjusted to the usage of the printing device prior to cleaning, e.g., during printing operation or during printing pauses. According to DE 693 11 397 T2, the duration of the printing pauses is measured, and the number of pulsed rinsing cycles, during which the rinsing ink is flushed through the printhead nozzles, is set accordingly. According to DE 697 07 962 T2, the duration of the printing pauses is also measured, and the number of pulsed rinsing cycles is determined based on the duration of the printing pauses. The number of rinsing cycles also depends on the behavior of the ink during the printing pause. US 6,042,218 A describes a method for determining the amount of rinse water used when cleaning one or more inkjet printheads, with the aim of saving rinse water. A table specifies the required rinse water volume for a printhead based on the time elapsed between the completion of a rinse cycle and the next print command. During printing, the time between a rinse cycle and the next print signal for the printhead is measured, and the required rinse water volume is then determined from the table based on this measured time. EP 1 013 435 A1 describes a method for determining the amount of rinse required per rinse cycle when rinsing an inkjet printhead in an inkjet printer. The amount of rinse depends on the type of ink used, the time the printhead is covered with a protective cap, or the printing time. If this rinse is insufficient, a cleaning process is carried out in which a cleaning agent is drawn through the nozzles. This occurs when the printing time or the time the printhead is covered with a protective cap is too long. In JP 7 047 696 A (Abstract), after a printing process in an inkjet printer is completed, the number of print pulse signals during that print job is used to determine the number of purge cycles before the next print job. The number of purge cycles can be reduced if the number of print pulse signals was high, in order to decrease the amount of purge fluid used. The problem to be solved by the invention is to provide a method for cleaning the nozzles and ink channels of a printhead in an inkjet printer, in which the amount of cleaning medium used for cleaning the printhead is determined taking into account the duration of printing pauses and other parameters of the printing operation. Such parameters include, for example, the properties of the ink used for printing, or whether the printhead is covered with a protective cap or not during a printing pause. Another parameter is the presence of ink droplets on the nozzle plate during printing, which must be removed. The aforementioned problem is solved by a method according to the features of claim 1. The amount of cleaning medium required to clean a printhead is determined as follows: - A cleaning volume curve is established for the printhead of the printing device by measurement. This curve indicates, for a given ink used in printing, the dependence of the cleaning volume required for cleaning the printhead on the elapsed time, i.e., for one cleaning cycle. - Before a new cleaning cycle is performed, the corresponding cleaning volume is determined from the cleaning volume curve based on the time elapsed since the previous cleaning cycle, and this volume is used to clean the printhead. The following terms are used in the explanation of the method according to the invention: - The required flushing quantity is the amount of flushing medium necessary and sufficient for cleaning the nozzles with ink channels. - An ink or a cleaning fluid can be used as the flushing medium. - During a flushing process, the nozzles of a printhead are flushed with the required flushing quantity and thereby cleaned. - The time interval between two successive flushing processes (the flushing cycle) can be specified by the operator of the printing device. - The method can, for example, be stored as a table or software in a printer control unit of the printing device. Further developments of the invention result from the dependent claims. The method according to the invention thus has the following advantages: - The amount of flushing medium for flushing the nozzles and ink channels of a printhead is determined based on the duration of the printing pause and the parameters listed above; this allows the amount of flushing medium used to be reduced. - A high level of reliability is achieved, since all nozzles are ready for operation due to the use of the determined necessary amount of flushing medium. - The method can be implemented without any modification of the printing device's hardware. The invention is further explained with reference to Figs. 1, 2, 3, 4, 5, 6 to 7. Figure 1 shows a schematic representation of a known print bar with five printheads; Figure 2 shows a schematic representation of a cleaning device for the nozzle area of ​​a printhead; Figure 3 shows a graphical representation of the rinsing volume for a rinsing process for a printhead when using different inks as a function of the time interval between two successive rinsing processes (rinsing volume curve); Figure 4 shows a graphical representation of the rinsing volume curve for a printhead when using one ink as a function of the time interval between two successive rinsing processes, wherein the printhead is temporarily covered with a protective cap or uncovered.Fig. 5 a graphical representation of the flushing volume curve for a printhead when using an ink as a function of the time interval between two successive flushing processes, wherein the printhead is temporarily covered with a protective cap and uncovered to illustrate the calculation of the necessary flushing volume, Fig. 6 a flowchart illustrating the determination of the necessary flushing volume for a printhead, Fig. 7 an arrangement for dosing the flushing volume per printhead. Fig. 1 schematically shows a print bar 1 of a printing unit with inkjet printheads 2, a known component of an inkjet printer. The printheads 2 have nozzles with ink channels (not shown; for brevity, the unit nozzle and ink channel will be referred to as "nozzle") which, for example, can generate ink droplets according to the DoD principle. These droplets are directed onto a substrate 3 to create a printed dot. The printheads 2 work together with a printer controller (not shown), which derives control signals for the individual nozzles of the printheads 2 from a data stream representing the print image. Control signals are derived from the data stream that activate those nozzles of the printheads 2 that are to create a printed dot on the substrate 3. Fig. 2 shows, as an example, a cleaning device for a printhead 2 arranged on a printhead carrier 4, whose nozzle surface 5 is to be cleaned. Ink droplets 6 are deposited on the nozzle surface and are to be removed. The cleaning device includes a cleaning element 7, which provides a cleaning blade, e.g., a rubber blade. By moving the cleaning blade 7 along the nozzle surface 5 in the direction of arrow PF1, the ink droplets 6 are wiped off, and the nozzle surface 5 is cleaned. The nozzles are cleaned using a rinsing medium, e.g., a rinsing ink, which is flushed through the nozzles of the printhead 2. The object of the invention is to determine the quantity "m" of rinsing medium (referred to as the rinsing quantity "m") that is necessary and sufficient to flush and clean the nozzles of the printhead 2. To minimize the amount "m" of cleaning medium required, the parameters that influence the amount "m" of cleaning medium needed to clean printhead 2 must be determined. For example, it must be taken into account that, as shown in Fig. 1, printhead A is fully utilized during printing, printhead B is partially utilized, or printhead C remains unused. Additionally, it must be considered whether the unused printhead 2 is covered by a protective cap to prevent the ink from drying out in the nozzles of printhead 2. To determine the quantity "m" of the cleaning medium for cleaning the nozzles of a printhead 2, at least the following parameters must be considered: - The idle time t of a printhead 2 of the printing device, i.e., the time a printhead 2 of the printing device is not in use when it is covered with a protective cap. This also includes the time during which the printing device is switched off. In this case, too, the printhead 2 can be protected from rapid drying out by a protective cap in a parked position. - During printing, a printhead 2 may be partially (printhead B, Fig. 1) or completely (printhead C, Fig. 1) not in use. This time t, during which the printhead 2 is not covered by a protective cap, leads to relatively rapid drying of the nozzles of the printhead 2. Consequently, the quantity of cleaning medium must be increased more rapidly depending on the time interval between successive cleaning processes.- Since the parameters mentioned above affect all nozzles of a printhead 2 equally, a partially used printhead 2 (printhead B, Fig. 1) must be treated like a completely unused printhead 2 (printhead C, Fig. 1). - Furthermore, the print time tprint of printhead 2 must be taken into account because of the undesirable settling of ink droplets on the nozzle plate. Therefore, the following parameters must be considered to determine the amount of cleaning medium "m": m = amount of cleaning medium (=cleaning quantity) required to clean the nozzles of a printhead 2; this depends on the time between two cleaning cycles. In formula (1), ink is used as the cleaning medium. This time can be selected by the printer operator. t = time starting from a cleaning cycle; k = correction factor To account for the effect of the different drying rates of the inks in a printhead 2, correction factors kencapsulated and kungencapsulated are introduced. These must be determined empirically for the printing device used and may depend on the ink used in the printing process. As an approximation, in a printing device, the dependence of the flushing volume "m" on time t, determined by measurements for two different inks, can be approximated by the flushing volume curves k1, k2 in Fig. 3. In this embodiment, the flushing volume "m" can be modeled by an exponential function (Fig. 3), which reaches different maximum values ​​mmax1, mmax2 depending on the inks under investigation. mmax is the maximum flushing volume that can be achieved with a longer time interval between flushing processes. If the time interval is chosen to be shorter, the necessary flushing volume "m" can be smaller than mmax, corresponding to the curve shape in Fig. 3. For an ink, depending on whether a printhead 2 was covered with a protective cap or not (encapsulated or unencapsulated) during the period of non-use, a flushing volume curve k3 for the flushing volume m2 = m encapsulated or k4 for the flushing volume m1 = m unencapsulated can be measured according to Fig. 4. It can be seen that for a printhead 2 that is not encapsulated during its printing pause, the maximum flushing volume mmax is reached after a shorter time t compared to a printhead 2 that is encapsulated during its printing pause. For the curves k3, k4 of Fig. 4, the following functions (2), (3) can be assumed for the quantity "m" of flushing medium in the exemplary embodiment, depending on whether the printhead 2 is covered with a protective cap or not: for the encapsulated state and for the unencapsulated state, respectively. Since a printhead 2 can be both encapsulated (e.g., in the park position) and unencapsulated (in the print position) between two rinsing cycles, both components m1 and m2 must be considered when calculating the rinsing volume "m". This case can be illustrated graphically in Fig. 5. In this example, the printhead 2 was unencapsulated for the period Δt1, i.e., from time t=0 to time t=t1. This means that the printhead 2 in the unencapsulated state dried out relatively quickly, and the required rinsing volume m2 increased rapidly (rinsing volume curve k4). Subsequently, the printhead 2 was parked for the period Δt2 and was in an encapsulated state (rinsing volume curve k3). The state of the printhead 2 at the time of parking must be considered when calculating the further rinsing volume "m2". For this purpose, an equivalent time t1* is determined, which corresponds to the state of the encapsulated printhead 2 (i.e., m2 = m1). Subsequently, "m" follows the curve k3.The time interval Δt2, during which printhead 2 was encapsulated, is then added to t1* to obtain t2. t2 thus corresponds to a time equivalent that encompasses both the unencapsulated and the encapsulated period of printhead 2. The required flushing volume can then be calculated from this using formula (2). From (3) m1 can be calculated for Fig. 5, where m1 is the flushing quantity when the printhead 2 is without a protective cap. The time equivalent t2 for an encapsulated printhead 2 can then be calculated using m1 as follows: With t2= Δt2+ t1* it follows: m2 is in the example of Fig. 5 the amount of rinse required to clean a printhead 2 that was sometimes covered by a protective cap and sometimes uncovered. This calculation can be extended analogously to multiple state changes; state changes are periods during which the printhead 2 was, for example, covered with a protective cap or uncovered. The diagram in Fig. 6 shows the calculation process for several of these state changes: The process begins, for example, with a subsequent rinsing cycle at time t=0 (step S1) and ends with a rinsing cycle (step Sx). Between these times, the printhead 2 may be temporarily covered with a protective cap or uncovered; accordingly, the rinsing volume "m" required for cleaning the printhead 2 changes, e.g.: - S2: Calculation of the rinsing volume m1 for time t1, the printhead 2 is, for example, uncovered. - S3: Calculation of the rinsing volume m2, the printhead 2 is covered for the period Δt2. - S4: Calculation of the rinsing volume m3, the printhead 2 is uncovered again for the period Δt3, etc. As described at the beginning, disruptive residues also accumulate on the nozzle plate 5 of a printhead 2 during printing, which must be regularly cleaned by a rinsing process. A minimum quantity of rinsing medium (mmin) is required for this. The number of ink droplets ejected during printing is crucial for determining when the next cleaning of the printhead 2 is necessary. Since a printhead 2 can have a large number of nozzles that may be significantly separated, counting ink droplets per nozzle would be the most accurate method of qualification. However, as such a process would be very complex, the printing time (tprint) can instead be used as an approximate parameter for the rinsing quantity (mmin). If a certain printing time (tprint) is exceeded, the printhead 2 should be cleaned before the next print job. The larger of the two rinsing quantities (mmin or mx) can be used as the rinsing quantity "m" (according to Fig.6) be used. The optimal time to rinse printhead 2 is immediately before printing begins. Therefore, when prompted to start printing, the printer should calculate the required rinsing volume "m" and then perform the rinsing process before printing starts. However, it is not absolutely necessary to perform a rinsing process before every print job. The dosage of the quantity "m" of cleaning medium for cleaning the printhead 2 can be carried out as shown in Fig. 7: A valve 9 is arranged between a reservoir 8 for a cleaning medium and the printhead 2. Pressure is exerted on the reservoir 8 by a compressed air source 10, so that the cleaning medium is supplied to the printhead 2 when the valve 9 is open. The quantity "m" of cleaning medium supplied to the printhead 2 can be adjusted by controlling the valve 9. For this purpose, the valve 9 can be controlled by the printer control system according to the method according to the invention. The method according to the invention has been described for a single printhead 2; it can readily be transferred to a plurality of printheads as shown in Fig. 1. In this case, the quantity of rinsing medium required for cleaning each printhead 2 can be determined, and then each printhead can be cleaned with the determined quantity of rinsing medium. For this to work, for example, in the case of a print bar 1 according to Fig. 1, the necessary quantity of rinsing medium must be determined for each printhead 2, and this printhead 2 can be rinsed separately from the other printheads 2 with the determined quantity of rinsing medium. Reference symbol list 1 Pressure bar 2 Printhead 3 Substrate 4 Printhead carrier 5 Nozzle plate 6 Ink droplet 7 Cleaning blade 8 Reservoir 9 Valve 10 Compressed air source m Amount of rinsing medium PF Arrow k Rinsing quantity curve t Time

Claims

Method for cleaning the nozzle plate and the nozzles of at least one inkjet printhead in an inkjet printer, wherein in a rinsing process the inkjet printhead (2) is rinsed with a rinsing medium, the rinsing quantity of which is determined from a predetermined rinsing quantity (mmin) dependent on the printing duration of the inkjet printer or from a rinsing quantity (m) derived from a rinsing quantity curve (k) which indicates for an ink used in printing the dependence of the rinsing quantity (m) required for rinsing the inkjet printhead (2) on the elapsed time (t), depending on which rinsing quantity has the larger value, and wherein the nozzle plate (5) of the inkjet printhead (2) is subsequently cleaned with a cleaning device (7). Method according to claim 1, wherein the flushing quantity curve (k) is stored as a table in a printer control of the inkjet printer. Method according to claim 1, wherein a mathematical formula approximating the flushing quantity curve (k) is stored in a printer control of the inkjet printer. Method according to one of claims 1, 2 or 3, wherein in an inkjet printer in which, during the printing pause of an inkjet printhead (2), this inkjet printhead (2) is covered with protective caps, a flushing quantity curve (k3) for the covered inkjet printhead (2) is used to determine the necessary flushing quantity. Method according to one of the preceding claims, wherein in an inkjet printer in which, during the printing pause of an inkjet printhead (2), this inkjet printhead (2) is not covered with protective caps, a flushing quantity curve (k4) for the uncovered inkjet printhead (2) is used to determine the necessary flushing quantity. Method according to claim 5, wherein, in the event that an ink printhead (2) is only temporarily covered with a protective cap between two successive rinsing processes, a portion of the rinsing quantity curve (k4) for the unencapsulated ink printhead (2) and a portion of the rinsing quantity curve (k3) for the encapsulated ink printhead (2) are used to determine the necessary rinsing quantity for the unencapsulated period of the ink printhead (2). Method according to one of the preceding claims, wherein a rinsing process is carried out at a pressure start. Method according to one of the preceding claims, wherein the printing time is determined during the printing of the inkjet printer and a predetermined flushing quantity (mmin) dependent thereon is determined. Method according to one of the preceding claims, wherein in an inkjet printer having a plurality of inkjet printheads (2), the flushing quantity is determined individually for each inkjet printhead (2) and each inkjet printhead (2) is flushed with its assigned flushing quantity.

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