Method for printing a container, and device for printing a container

Abstract

A method for printing a container (130), such as a bottle, with a printing image (150) using one inkjet print head per ink (111-114), the method comprising determining an ink quantity to be applied to the container (130) in a manner which is dependent on the printing image (150) to be applied for each ink, and applying the ink quantity to be applied by each inkjet print head (111-114), wherein at least one of the inkjet print heads applies the ink quantity to be applied to the container (130) in two printing operations, wherein an ink part quantity is applied to the container (130) in each printing operation in a manner which is dependent on the ink quantity to be applied, wherein the ink part quantity in each printing operation corresponds to up to the maximum ink quantity which can be ejected by the inkjet print head.

Description

[0001] Method for printing a container and device for printing a container

[0002] The present invention relates to a method for printing a container, such as a bottle, with a print image using one inkjet print head per color according to claim 1 and to a device for printing containers, such as bottles, with a print image, wherein the device comprises one inkjet print head for dispensing a color and a control unit for controlling the dispensing of color by the respective inkjet print heads, according to claim 11.

[0003] State of the art

[0004] Devices for printing containers, such as bottles, are generally known from the prior art. Inkjet print heads can also be used for this purpose, which distribute ink droplets of the appropriate size, depending on the amount of ink to be applied, with pixel precision across the surface of the container. These inkjet print heads comprise a multitude of separately controllable and actuated nozzles, each of which can eject a single ink droplet. This enables pixel-precise application of printing ink.

[0005] Inkjet print heads apply the required amount of ink by accumulating smaller ink droplets into a larger ink droplet, for example, by actuating the respective print nozzle or a piezoelectric element associated with it. For this purpose, a control signal can be transmitted to the piezoelectric element, causing it to move mechanically, causing ink to exit the nozzle. Depending on the control signal, both large and small ink droplets can be generated.

[0006] In principle, this allows for variable adjustment of the amount of paint applied and thus the amount of paint produced on the container.

[0007] In multi-color printing, different inkjet print heads are usually used, with each inkjet print head printing exactly one color. For example, one inkjet print head can print white or black (hereinafter also referred to as colors), while other print heads can print magenta or yellow. There are also inkjet print heads that can print multiple colors, for example two or four. Since several small ink droplets are combined to create one ink droplet, the production of a specific color is only possible in a range from 0 (no ink applied) to the maximum amount of ink that can be printed by the print head (i.e. the amount of ink that can be printed by each nozzle). The ink droplet size cannot be adjusted continuously; instead, the smaller droplets that make up the large droplet can only be produced in a certain discrete volume.

[0008] This limits the color space achievable with inkjet print heads (i.e., the gradation from 100% to 0, i.e., no applied color), which allows only limited flexibility when creating printed images, especially their color gradation, on containers. At the same time, on transparent substrates (e.g., PET containers), as well as on colored or dyed substrates or colored backgrounds, the achievable opacity or color intensity of the printed colors may be too low for part of this color space, which can adversely affect the quality and recognizability of the applied printed images.

[0009] Task

[0010] Based on the known state of the art, the technical problem to be solved is to provide a method for printing on a container that is as flexible as possible while simultaneously ensuring sufficient quality of the applied print images. The problem may also include improving the opacity of the applied print images and / or expanding the color space available for applying print images.

[0011] Solution

[0012] This object is achieved by the method for printing a container, such as a bottle, with a printed image according to claim 1 and the device for printing containers, such as bottles, with a printed image according to claim 11. Advantageous developments of the invention are covered in the subclaims.

[0013] The method according to the invention for printing a container, such as a bottle, with a print image using one inkjet print head per color comprises determining an amount of ink to be applied to the container depending on the print image to be applied for each color and applying the amount of ink to be applied by each inkjet print head, wherein at least one of the inkjet print heads applies the amount of ink to be applied to the container in two printing processes, wherein in each printing process a partial amount of ink is applied to the container depending on the amount of ink to be applied, wherein the partial amount of ink in each printing process corresponds to the maximum amount of ink that can be applied by the inkjet print head.

[0014] In the following, an inkjet print head is understood to mean, on the one hand, inkjet print heads already known from the prior art. These have a number of print nozzles that can be individually actuated to eject ink droplets of different sizes. The inkjet print head is typically assigned a single ink supply, via which all nozzles of this inkjet print head are supplied with ink of exactly one color.

[0015] In addition, the term "inkjet print head" should also be understood to refer to a configuration in which a print head comprises a number of inkjet print nozzles as a structural unit, but wherein each group of inkjet print nozzles is assigned an ink supply, so that the print head comprises a first group of inkjet print nozzles that receive printing ink of a first color from a first ink supply, and a second group of inkjet print nozzles that receive printing ink of a possibly different second color from a second ink supply. In the following, the respective groups of inkjet print nozzles are also referred to as "inkjet print heads."

[0016] In the following, the amount of ink to be applied or the partial amount of ink to be applied always refers to the amount of ink to be applied by a respective nozzle of an inkjet print head to create a respective color pixel of the (optionally multi-colored) print image. Since this can vary from pixel to pixel and thus also from nozzle to nozzle of the inkjet print head, the total amount of ink to be applied does not have to correspond to an integer multiple of the amount of ink applied by a specific nozzle of the inkjet print head. For the sake of simplicity, however, the following refers to the amount of ink or partial amount of ink to be applied by the inkjet print head in each printing process, although the amount of ink or partial amount of ink to be applied by a respective nozzle of the inkjet print head is also meant.

[0017] The partial ink quantity in each print process corresponds to the maximum amount of ink that can be applied by the inkjet print head (or a respective nozzle). This means that in the two consecutive print processes, the maximum amount of ink that can be applied can be output by each nozzle. The maximum amount of ink that can be applied can be determined either by structural constraints of the print nozzle or print head or by programming (e.g., depending on a control signal).

[0018] With the method according to the invention, it is now possible to expand the achievable color space and thus apply a wider variety of printed images, or to improve their quality in terms of contrast, etc. At the same time, the maximum amount of ink that can be applied, which corresponds to twice the maximum amount of ink that the respective inkjet print head can apply in a single printing process, can improve the opacity of the applied printed images, which is advantageous when printing on transparent substrates as well as colored or dyed substrates.

[0019] In one embodiment, it is provided that at least two inkjet print heads eject an ink drop from a print nozzle in a printing process and the size of the ink drop is determined depending on the partial ink quantity to be applied, wherein the size of the ink drop can be adjusted in discrete partial steps from 0 up to a maximum ink drop size depending on the partial ink quantity to be applied.

[0020] With this embodiment, the advantage according to the invention can also be realized for different colors.

[0021] Furthermore, the method may comprise adjusting the amount of ink emitted by the at least two inkjet print heads.

[0022] This adjustment can be performed in a single step, especially before the actual printing process, and only once. Different inkjet printheads, especially inkjet printheads of different models or different manufacturers, do not necessarily produce the same maximum ink quantity, and may vary in terms of the droplet sizes they can produce and also in the intermediate sizes that can be achieved. This deviation, which could otherwise lead to reduced print quality, can be compensated for by, for example, linear color scaling or linear adjustment, and possibly a color profile adjusted based on the applied ink droplet quantities.

[0023] In particular, it can be provided that the at least two inkjet print heads can eject different maximum ink droplet sizes, and the adjustment is carried out based on the maximum ink droplet sizes. This embodiment allows for the leveling of ink droplets of different sizes that can be ejected by different inkjet print heads.

[0024] It can further be provided that the adjustment includes changing a control signal (also called a waveform) for at least one inkjet print head. Control signals are typically used here to control piezo elements assigned to individual print nozzles, which in turn directly influences the ejection of ink droplets. By specifically changing these control signals, the size of the ink droplets can be adjusted, for example.

[0025] In one embodiment, the determining comprises deriving color values ​​from the print image to be applied, wherein the color values ​​correspond from 0 to a maximum color value corresponding to the maximum amount of ink that can be applied by the inkjet print head, and the determining further comprises doubling the derived color values, and based thereon the amount of ink to be applied is determined.

[0026] While a print image to be applied can itself specify a specific color value, which would also require a specific ink droplet size, the amount of ink to be applied can also be increased by additionally doubling the desired color value (through control technology). Since a total of twice the amount of ink can be applied in the two consecutive printing processes, the relative color intensity of the resulting print image can be kept constant, while simultaneously improving the opacity of the applied print image.

[0027] Furthermore, it can be provided that the determination comprises deriving color values ​​from the print image to be applied, wherein the color values ​​correspond from 0 to twice a maximum color value corresponding to the maximum amount of ink that can be applied by the inkjet print head, and based on this, the amount of ink to be applied is determined.

[0028] With this embodiment, the color space is enlarged because the color space is not defined as the maximum amount of ink that can be applied by the inkjet print head, but rather as at least twice the maximum amount. This method can also be advantageously continued by selecting values ​​between 0 and 3 times, 4 times, 5 times, or generally n times the maximum amount of ink as color values, and determining the amount of ink to be applied based on these values. In particular, determining the amount of ink to be applied can further comprise determining the partial amount of ink to be applied depending on the amount of ink to be applied.

[0029] This means that the amount of ink to be applied can be determined individually for each printing process.

[0030] In one embodiment, the amount of ink to be applied corresponds to at least twice the minimum amount of ink that can be applied by the inkjet print head, and the partial amount of ink for each printing process corresponds to at least the minimum amount of ink that can be applied by the inkjet print head. Alternatively, it can be provided that the amount of ink to be applied corresponds to at least twice the minimum amount of ink that can be applied by the inkjet print head, and the partial amount of ink for a printing process is selected to be as large as possible depending on the amount of ink to be applied and the maximum amount of ink that can be applied by the inkjet print head. If the amount of ink to be applied corresponds to more than the maximum amount of ink that can be applied by the inkjet print head, the difference between the amount of ink to be applied and the maximum amount of ink that can be applied is defined as the partial amount of ink in the second printing process.

[0031] The first variant achieves a preferably even application of the entire amount of ink. For example, if the amount of ink to be applied is six times the minimum amount of ink that can be applied by the inkjet print head, three times the minimum amount of ink that can be applied by the inkjet print head can be applied as ink droplets in each printing process, resulting in two layers of equal thickness. Undesirable color reflections due to ink layers of different thicknesses can thus be avoided. The second embodiment ensures that the majority of the amount of ink to be applied is applied in one of the printing processes. Effects that have a particularly detrimental effect on small applied ink droplets (e.g. wind) can thus be reduced.

[0032] In one embodiment, the method further comprises curing the applied partial amount of ink using a curing device after each printing operation. Curing does not have to occur after each printing operation. Alternatively, curing can also be provided only after a second printing operation or, if provided, a third printing operation. Providing curing after each or even only after every second printing operation can be based on the ink used and in particular its properties, such as viscosity, and can also be provided depending on the substrate, the printing speed, or similar.

[0033] By curing after each printing process, the printing ink already applied is prevented from smearing, splashing, running or mixing with other colors by the subsequent printing process or the printing ink applied during this process, so that the quality of the overall printed image is improved.

[0034] The device according to the invention for printing containers, such as bottles, with a multi-colored print image comprises an inkjet print head for applying a color and a control unit for controlling the application of ink by the respective inkjet print heads, wherein the device is designed to carry out a method for printing a container with a multi-colored print image, the method comprising determining an amount of ink to be applied to the container depending on the print image to be applied for each color and applying the amount of ink to be applied by each inkjet print head, wherein at least one of the inkjet print heads applies the amount of ink to be applied to the container in two printing processes, wherein in each printing process a partial amount of ink is applied to the container depending on the amount of ink to be applied,where the partial ink quantity in each printing process corresponds to the maximum ink quantity that can be output by the inkjet print head.

[0035] This device allows for more flexible and better quality printing.

[0036] In particular, the device can comprise a curing device arranged such that a partial amount of ink applied can be cured after each printing process. By providing a corresponding curing device, splashing of printing ink applied during a first printing process can be avoided.

[0037] Short description of the characters

[0038] Fig. 1 shows a device for printing a container according to an embodiment

[0039] Fig. 2 shows a flow diagram of a method for printing a container according to one embodiment. Figs. 3a and 3b show further embodiments for determining the amount of ink to be applied for each color.

[0040] Fig. 4 shows a flow chart of a process for adjusting the ink quantities of different print heads

[0041] Detailed description

[0042] Fig. 1 shows a device 100 for applying a multi-color printed image to a container 130, such as a bottle or a can. In the embodiment shown here, the device comprises a turntable 131 on which a container can be positioned and rotated about the rotation axis R. The rotation axis R can, in particular, be parallel to the longitudinal axis of the container from the bottom to the mouth of the container 130 or coincide with this longitudinal axis. Other embodiments of the positioning and movement of the container are also conceivable, so the invention is not limited in this regard.

[0043] While methods and devices for printing three-dimensional objects, in particular (beverage) bottles, tubes, drinking glasses, or cans, are described below, the described embodiments are not limited to printing on curved surfaces of containers. In particular, flat and curved surfaces can be printed using the described methods and devices. Furthermore, substrates other than containers, such as paper or film, can also be printed.

[0044] Furthermore, the device 100 comprises at least one or more inkjet print heads 111-114 (hereinafter also referred to as print heads), each having a plurality of print nozzles 115 that can be controlled separately and eject ink droplets of variable size. Each of the inkjet print heads can be provided to eject precisely one color of the multi-color print image. For example, the inkjet print head 111 can be designed to eject black, the inkjet print head 112 to eject cyan, the inkjet print head 113 to eject magenta, and the inkjet print head 114 to eject yellow. For this purpose, ink reservoirs can be assigned to the inkjet print heads in a known manner, since the ink can be supplied to the inkjet print heads.

[0045] While exactly four print heads 111-114 are shown here, more or fewer print heads can be provided. Thus, depending on the number of colors to be applied or the area to be printed on the container, more or fewer print heads can be provided. Instead of being arranged one after the other in the direction of rotation of the container, as shown here, the print heads can also be arranged vertically offset from one another and / or on different levels. In particular, a first number of print heads can be arranged on a vertical level and a second number of print heads can be arranged on a second level, which can apply different or even the same colors.

[0046] Optionally, the device 100 may also comprise at least one curing device 120, which can cause the printing inks to dry, for example, by applying UV radiation or infrared radiation or heat.

[0047] The device 100 further comprises a control unit 140, which is connected via suitable data connections 170 at least to the inkjet print heads 111-114 and optionally to the curing device 120 and optionally to the turntable 131 in order to transmit control signals to these respective devices for control purposes.

[0048] The control signals for the inkjet print heads 111-114 can, in particular, be control signals that can selectively control piezo elements or other actuating elements assigned to each print nozzle 115. Thus, in conventional inkjet print heads, each print nozzle is assigned a piezo element that is actuated according to a control signal (e.g., a voltage curve) and can mechanically and / or acoustically cause the ejection of one or more small ink droplets, which together form an ink droplet corresponding to the amount of ink ejected by this print nozzle. The inkjet print heads can typically eject ink quantities (per print nozzle) starting at a value of 0 (i.e., no ink ejected by a print nozzle) up to a maximum ink quantity corresponding to the maximum droplet size that can be ejected by a print nozzle.Between these extreme values, the resulting ink droplet size ejected by a print nozzle can usually be adjusted in steps. For example, the piezo element can be actuated by suitable control signals so that it can eject seven intermediate steps and thus seven different droplet sizes. However, print heads are also conceivable that do not implement any gradation, i.e., can eject either no color (value 0) or the maximum droplet size (in this case, the only droplet size). The invention is not limited with regard to the number of intermediate steps.

[0049] The control unit 140 can comprise a data storage device (not shown here), in which, for example, the multi-color print image to be applied to the container can be stored in the form of a file. The control unit can, in particular, be configured to derive color values ​​from the multi-color print image or file 150 in order to determine the ink quantities to be dispensed by the individual inkjet print heads, and in particular their print nozzles 115, based on these color values.

[0050] Based on the amount of ink to be dispensed or applied to the container (each per print nozzle), the control unit can then send corresponding control signals to the respective inkjet print heads, which then cause the amount of ink to be applied to the containers to be dispensed through the respective print nozzles. The invention provides that each of the inkjet print heads is configured to dispense the maximum amount of ink that can be dispensed by the inkjet print head (i.e., each print nozzle of the print head), even if the container moves past the respective print head several times.

[0051] According to the invention, at least one of the inkjet print heads performs two printing operations on the container to apply a portion of the multi-color print image, in particular the ink to be applied to the container by this inkjet print head. According to the invention, this inkjet print head can thus apply ink to a specific point on the container that corresponds to up to twice the maximum amount of ink that can be applied by the inkjet print head per printing operation.

[0052] For this purpose, the container can, for example, be rotated twice completely around the rotation axis R on the turntable 131, so that the same surface area of ​​the container is moved past the print head twice and can be printed on. It can also be provided that the curing device 120 causes drying or generally curing of the printing ink (for example, but not necessarily with UV light, infrared light, or visible light) on the container 130, so that the printing ink applied in the subsequent printing process does not cause smearing, splashing, running, mixing, or merging of the printing ink already applied in the previous printing process. While only one curing device is shown here, a curing device can also be arranged between each two adjacent print heads.

[0053] Fig. 2 shows a method for controlling the inkjet print heads for dispensing quantities of ink through the individual print nozzles based on a print image to be applied to the container. The method comprises, in a first step 201, obtaining a print image. This obtaining can be effected, for example, by storing the print image 150 in the control unit 140 or a memory associated with it. Based on the obtained print image, the amount of ink for each color to be applied is then determined in step 202. As already mentioned, this amount of ink refers to the amount of ink to be applied by a respective print nozzle at a specific point on the container and can be different for each print nozzle as well as for specific points on the container for one and the same color. The determination of the amount of ink to be applied is usually based on the color information of the print image.The information encoded in the image, such as the pixel values ​​regarding brightness and color for individual pixels, is evaluated, and the amount of ink to be applied is determined based on this. A pixel of a certain color with, for example, a maximum color value also corresponds to a maximum amount of ink that can be applied to that pixel; lower color values ​​correspond to smaller amounts of ink.

[0054] Using the information thus obtained, each inkjet print head and each print nozzle of the respective inkjet print head are then controlled 203 so that the required amount of ink is applied to the correct position on the container. This can typically occur while the container is rotated on the turntable 131 and can be configured as a continuous process. Alternatively, the container can be fixed in a specific position, and the respective print heads can apply the ink quantities intended for this position to the appropriate locations on the container.

[0055] This procedure applies generally. However, if one of the inkjet print heads performs two prints on the container (for example, by rotating the container so that the same surface passes the print head twice), partial ink quantities are determined for each of the prints according to this procedure, as further described with reference to Fig. 3.

[0056] A more specific process compared to Fig. 2 is described in the flow diagram of Fig. 3a.

[0057] According to this method, in step 301, the color values ​​are first derived from the print image to be applied. However, instead of using these directly to determine the amount of ink to be applied, these color values ​​are first doubled (step 302). Alternatively, the original color values ​​(e.g., Sbit) can be used together with a doubled number of ink drops. From these doubled color values ​​or the original color values ​​together with the doubled number of droplets, the ink quantities to be applied are then determined in step 303 for each color and, accordingly, each print head or the individual print nozzles.

[0058] Since the invention provides that at least one of the print heads can perform two consecutive printing operations on a container with up to the maximum amount of ink, this configuration with doubled color values ​​can increase the opacity of the respective color on the container without, however, changing the relative strength of the colors to each other. The embodiment of Fig. 3a can in principle be combined with all other embodiments.

[0059] It should also be noted that instead of doubling the color values, it is also conceivable to triple or quadruple the color values, or generally multiply the color values ​​by n. The same applies to doubling the number of ink droplets. To achieve these color values ​​according to the required ink quantity, the container must be printed by the print head with three, four, or n print cycles. This can be done using the same methods as described for doubling the color values ​​and requires an additional print cycle each time, which can be performed with up to the maximum ink quantity.

[0060] Fig. 3b shows a further flow diagram of a method for determining the ink quantities to be applied in the individual printing processes. The ink quantities to be applied in individual printing processes are also referred to as partial ink quantities. As already described, digital print heads can usually eject ink droplets with an ink droplet volume or ink droplet size that can vary in discrete sub-steps, starting with a minimum ink quantity or ink droplet size up to a maximum ink quantity or ink droplet size of the respective print nozzle. For simplified explanation, it is assumed that these discrete sub-steps orPartial ink quantities differ by a constant minimum ink quantity, so that a partial ink quantity that can be dispensed by a print nozzle in one printing process corresponds to any integer multiple of the minimum ink quantity and is less than or at most equal to the maximum ink quantity for one print droplet.

[0061] For real print heads, this assumption generally does not hold, so the discrete, differently sized achievable ink droplets do not exhibit the same size differences. However, this can be compensated for by adjusting the ink quantities, as described with reference to Fig. 4. For a simplified explanation, however, we assume the idealized notion that the difference between different sized ink droplets is an integer multiple of the minimum ink quantity.

[0062] The method of Fig. 3b begins with step 311, in which the total amount of ink to be applied for each color is determined. This amount can, for example, be less than the maximum amount of ink that can be dispensed by a print nozzle in one printing process, but can also be greater, approximately up to twice the maximum amount of ink that can be dispensed.

[0063] Subsequently, in step 312, the partial ink quantities to be applied in the individual printing processes for the corresponding color are determined.

[0064] Several variants are conceivable here. In a first case 313, the amount of ink to be applied is n times the minimum amount of ink (but in these embodiments, it is less than twice the maximum amount of ink that can be applied by the print head).

[0065] If the ink quantity is an even multiple n=2m of the minimum ink quantity (case 331), the partial ink quantity for each print run can be set to the value m 332. This means that the partial ink quantity corresponds to m times the minimum ink quantity. This is less than or at most equal to the maximum ink quantity that can be applied in a single print run. In this case, the same-sized ink drops are applied by the respective print nozzle in each print run.

[0066] In another case 333 the color set is an odd multiple of the minimal color set n, n=2m+1.

[0067] In this case, the partial ink quantity to be applied in one of the printing processes is set to the value m and the ink quantity to be applied in the other printing process is set to the value m+1, so that in total 2m+1=n as the ink quantity (measured in minimum ink quantities) has been applied to the container after completion of the two printing processes (step 334).

[0068] This design ensures that ink droplets of approximately the same size or approximately the same size are ejected from the print nozzle during each printing process. This results in ink droplets of approximately the same size being produced in each printing process, thus avoiding or reducing potentially adverse effects on print quality, such as insufficient spreading of the ink on the substrate. Particularly in embodiments in which the container is continuously rotated, this can prevent unintentional blurring of a specific pixel of the printed image, even if two printing processes are necessary to produce the required amount of ink.

[0069] In an alternative embodiment, the determined total amount of ink to be applied to a pixel is again an n-fold multiple of the minimum amount of ink 314.

[0070] In this case, however, the partial ink quantities ki and k2 for the respective printing processes are not determined in such a way that the amount of ink applied by the print nozzle of the print head in each printing process is the same or as equal as possible, but the partial quantities are determined in such a way that in one of the printing processes, as much or as much as possible of the total amount of ink to be applied is applied.

[0071] Therefore, in this embodiment, the partial ink quantity ki is set to the value n (step 315) if the total ink quantity to be applied is less than or equal to the maximum ink quantity that can be applied by the print head in one printing operation. Otherwise, if the ink quantity to be applied is greater than the maximum ink quantity that can be applied in one printing operation, the partial ink quantity ki is set to the maximum ink quantity.

[0072] The partial color set k2 is accordingly set to the value 0 (step 315) if the partial color set ki already represents the total color set n (first case in the discussion of ki). Otherwise, the partial color set k2 is set as the difference between the color set n to be applied minus the partial color set ki.

[0073] The indices 1 and 2 do not indicate a chronological order of the respective printing processes, so that the partial ink quantity ki can be applied either in the first printing process or in the second printing process. The partial ink quantity k2 is accordingly applied either in the second or the first printing process. This embodiment ensures that the majority of the ink quantity to be applied (except in the case where the ink quantity to be applied corresponds to twice the maximum ink quantity) is applied in one of the printing processes. This ensures that the largest possible ink droplet is generated in this printing process and applied to the surface of the container, which reduces negative influences on the printed image, such as those caused by airflow.The smaller partial ink quantity k2 may then be subject to such effects, but due to its comparatively small contribution to the total amount of ink to be applied, it leads to less impairment of the result.

[0074] As already mentioned above, the variation in droplet size of a print head, but also of different print heads, can deviate from the case discussed with reference to Fig. 3b, so that the difference in the size of two different ink droplets does not correspond to an integer multiple of the minimum ink droplet size. For example, while in the ideal case the print nozzle of an inkjet print head can eject ink droplets with a size of 0 (no ink droplet) to 35 pl, and do so with discrete sub-steps of 5 pl, in reality a deviation from this exactly linear case can occur without compensation of the control signals. For example, the ink droplets can have a size of 6 pl, 13 pl, 16 pl, 21 pl, 23 pl, 31 pl, and 37 pl.

[0075] Corresponding deviations from the ideal linear distribution of ink droplet sizes can also occur with other print heads, ultimately resulting in ink droplets of different sizes being ejected despite essentially the same amount of ink being applied, or in ink droplets of different sizes of one color not differing by the desired amount of ink. This leads to undesirable color deviations, particularly in designs where two (or even more) prints are performed to achieve the highest possible color saturation or the largest possible color space, since the resulting differences can increase with the number of prints or are not large enough.

[0076] Fig. 4 shows a method by which these deviations can be corrected either as part of the method of one of the previous embodiments or separately, for example when commissioning the device according to Fig. 1.

[0077] First, in a first step 401, the maximum ink droplet size of a print head and / or each print head is determined. Subsequently, the (maximum) ink quantities that can be dispensed by the inkjet print heads are adjusted 402.

[0078] This can be accomplished in different ways. For example, if it is determined during the determination of the maximum ink droplet sizes that these differ for different printheads, the maximum ink droplet size can be adjusted in step 403. For example, if it is determined that a first printhead ejects ink droplets with a maximum size of 30 pl, while a second printhead ejects ink droplets with a size of 32 pl, the maximum ink quantity of the printhead capable of ejecting 32 pl ink droplets can be reduced, or the ink droplet size of the printhead capable of ejecting 30 pl ink droplets can be increased.This can be achieved by changing the control signals supplied to the inkjet print heads so that, for example, the piezo elements are operated differently, for example by having different activation times in order to influence the ejection of ink drops from the individual print nozzles.

[0079] It can also happen that different print heads provide a different number of sub-steps or different discrete ink droplet sizes. For example, two print heads that can produce the same maximum ink droplet sizes can differ in terms of the intermediate sizes of the ink droplets. For example, a first print head may allow five intermediate sizes, while a second print head may allow up to seven. When determining the color values, however, this means that the second print head can display color differences in more detail that the first print head cannot, so that the first print head assigns the same ink droplet size to a larger range of pixel color values ​​in a digital print image, while the other print head can use different ink droplet sizes and therefore different amounts of ink.

[0080] This can have a detrimental effect on the quality of the generated print images and can be achieved by disabling sub-steps 404 of the print head, which can eject a larger number of ink droplets of different sizes. For example, only five different ink droplet sizes can be realized instead of seven by appropriately adjusting the control signals.

[0081] If, instead or additionally, the sizes of the ink droplets differ in the individual sub-steps, these can also be adjusted by changing the control signals. This applies not only to different print heads, but also to a single print head. For example, if the factory setting of a single print head means that the ink droplet size changes do not occur in integer multiples of 5 pl (see the example above), changing the control signals can compensate for or at least minimize these differences.

[0082] Such linearization can also be performed between different print heads, so that the ink droplets from the different print heads can be dispensed in as uniform a sub-size as possible, each of which is identical in size for the print heads. Using the adjusted ink quantities, the control signals for the individual print heads can then be adjusted in step 406.

[0083] With control signals adapted in this way, an amount of ink to be applied can then be determined, for example based on the print images obtained in step 201 (see Figure 2), and ultimately the inkjet print heads can be controlled.

Claims

Claims: A method for printing a container, such as a bottle, with a printed image using one inkjet printhead per color, the method comprising determining an amount of ink to be applied to the container depending on the printed image to be applied for each color and applying the amount of ink to be applied by each inkjet printhead, wherein at least one of the inkjet printheads applies the amount of ink to be applied to the container in two printing operations, wherein in each printing operation a partial amount of ink depending on the amount of ink to be applied to the container is applied, wherein the partial amount of ink in each printing operation corresponds to the maximum amount of ink that can be applied by the inkjet printhead.A method according to claim 1, wherein at least two inkjet printheads dispense an ink droplet from a print nozzle in a printing process, and the size of the ink droplet is determined depending on the partial amount of ink to be applied, wherein the size of the ink droplet is adjustable in discrete increments from 0 up to a maximum ink droplet size depending on the partial amount of ink to be applied. A method according to claim 1 or 2, wherein the method further comprises equalizing the amount of ink dispensed by the at least two inkjet printheads. A method according to claim 3, wherein the at least two inkjet printheads can dispense different maximum ink droplet sizes, and the equalization is based on the maximum ink droplet sizes. A method according to claim 3 or 4, wherein the equalization comprises modifying a control signal for at least one inkjet printhead.A method according to any one of claims 1 to 5, wherein the determination comprises deriving color values ​​from the printed image to be applied, wherein the color values ​​range from 0 to a maximum color value corresponding to the maximum amount of ink that can be applied by the inkjet printhead, and the determination further comprises doubling the derived color values, and the amount of ink to be applied is determined based thereon. A method according to any one of claims 1 to 5, wherein the determination comprises deriving color values ​​from the printed image to be applied, wherein the color values ​​range from 0 to a maximum color value. twice the maximum color value corresponding to the maximum amount of ink that can be applied by the inkjet printhead, and the amount of ink to be applied is determined based on this. A method according to claim 6 or 7, wherein determining the amount of ink to be applied further comprises determining the partial amount of ink to be applied depending on the total amount of ink to be applied.A method according to any one of claims 1 to 8, wherein the amount of ink to be applied is at least twice the value of the minimum amount of ink that can be applied by the inkjet printhead, and the partial amount of ink for each printing operation is at least equal to the minimum amount of ink that can be applied by the inkjet printhead; or wherein the amount of ink to be applied is at least twice the value of the minimum amount of ink that can be applied by the inkjet printhead, and the partial amount of ink for a printing operation is selected to be as large as possible depending on the amount of ink to be applied and the maximum amount of ink that can be applied by the inkjet printhead, wherein, if the amount of ink to be applied is greater than the maximum amount of ink that can be applied by the inkjet printhead, the difference between the amount of ink to be applied and the maximum amount of ink that can be applied is defined as the partial amount of ink in the second printing operation.Method according to any one of claims 1 to 9, wherein the method further comprises curing the applied partial amount of ink by means of a curing device after each printing process.Device for printing containers, such as bottles, with a printed image, the device comprising an inkjet printhead for applying one color and a control unit for controlling the application of color by the respective inkjet printheads, the device being configured to carry out a method for printing a container with a multi-colored printed image, the method comprising determining an amount of ink to be applied to the container depending on the printed image to be applied for each color and applying the amount of ink to be applied by each inkjet printhead, wherein at least one of the inkjet printheads applies the amount of ink to be applied to the container in two printing operations, wherein in each printing operation a partial amount of ink depending on the amount of ink to be applied to the container is applied, wherein the partial amount of ink in each printing operation corresponds to the maximum amount of ink that can be applied by the inkjet printhead. Device according to claim 11, wherein the device comprises a curing device arranged such that a portion of the applied ink can be cured after each printing process.

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