BACKGROUND

[0001] A printer, for example, an inkjet-printer or a 3<limensional printer, ejects drops of a printing fluid from a print head towards a pixel on a medium. Printing a pattern may include the continuous firing of the print head of the printer, which may cause a loss of drop velocity and drop weight of a drop of the printing fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0002] FIG. 1 shows an example of different patterns to illustrate decei and the associated misalignment of the drops when being placed on a medium, wherein FIG. 1(A) shows a first pattern printed at a first firing frequency, FIG. 1{B) shows a second pattern printed at a second firing frequency, and FIG. 1(C) is an enlarged representation of the vertical straight lines of the first and second patterns.

[0003] FIG. 2(A) shows ten examples of different patterns printed by firing nozzles of an inkjet print head to eject a specific amount of ink towards a medium. [0004] FIG. 2(B) describes an example to print the patterns (1) and (2) shown in FIG 2{A) during a calibration for a diagnostic for decei.

[0005] FiG. 3 shows an example of the results of ^: a measurement of pattern (2} of FIG, 2(A) indicating the ink densities in different regions measured by a densitometer.

[0006] FfG. 4 js a schematic representation of a printer in accordance with an example.

[0007] FfG.5 shows a carriage inlqet-printer in aocord

[0008] FiG. 6 is a flow diagram of an example of a method described herein.

DETAILED DESCRIPTION

[0009] in the following description, details are set farthto Drovicfe an urKlerstand&)gof the examples described herein. However, it is to be understood that the examples may be practiced without these details. While a limited number of examples are disclosed, it is to be understood that there are numerous modifications and variations therefrom. Similar or equal elements in the figures may be indicated using the same numeral. [0010] A printer uses a printing fluid, also referred to as a printing material, such as an ink or a fusing agent. The printer may by an inkjet-printer, such as a thermal inkjet- printer or a piezoelectric inkjet-printer. Depending on the pattern to be printed, the print head may be operated continuously for some tone, i.e., drops of the printing fluid are ejected at a high firing frequency, in such printing conditions, a transitory phenomenon may occur that is referred to as deed or decei-effect. Decei may occur when repeatedly firing or activating a nozzle of a print head of an inkjet printer for long time periods. In such a situation, a thin film of ink components may form and deposit over a nozzle resistor. When the nozzle stops printing, the film disappears after some time, however, when the nozzle keeps printing for a long period of time, the film does not disappear. The film causes energy to be transmitted to the nozzle less efficiently so that the drop which is fired from the nozzle has less momentum when compared to a drop fired by a nozzle in which no such film is present The reduced momentum causes the drop weight and the drop velocity to be reduced so that the drop is decelerated, when compared to the situation in which no thin film is formed and in which the drop has a nominal weight and velocity. Drops of the printing fluid with less drop velocity than the nominal drop velocity cause the drop to have a longer flying time so that it lands on the medium with a misalignment when compared to a nominal drop. This misalignment may result in visible artefacts, for example in a printed image, or may result in the application of a fusing agent in areas of a powder layer in a 3D printing process which is not to be solidified. [0011] FIG. 1 shows an example of different patterns to illustrate decei and the associated misalignment of the drops when being placed on the medium. FIG. 1(A) shows a first pattern printed at a first firing frequency. The first pattern has a first section 10a printed continuously, followed by a second section 10b including seven pairs of parallel, vertical lines. FIG. 1(B) shows a second pattern printed at higher firing frequencies when compared to the first pattern. The second pattern includes a first section 12a and a second section 12b. The first section 12a has a first subsection 12ai and a second subsection 12t>2, in the first subsection 12ai the density of the applied ink is higher when compared to the first pattern. The second section 12b includes, in a similar way as the first pattern in FIG. 1(A), seven pairs of parallel vertical straight lines. FIG. 1(C) is an enlarged representation of the second sections 10b and 12b of the first and second patterns. Using the vertical straight lines printed for the different pattern, a misalignment due to deed may be seen. For example, in a situation in which no decel is triggered by the second pattern the vertical pairs of straight lines of the first and second patterns are aligned at position 0 in FIG. 1(C), also referred to as a reference position. However, in the example depleted in FIG. 1 the second pattern, due to its higher firing frequency of the nozzles to apply the increased amount of Ink In the first and second subsections 12βι, 12a2« causes decel, and the vertical straight lews in sections 10b, 12b are no longer aligned at position 0, rather, the vertical pair of straight lines are aligned with respect to each other at position -2 in FIG. 1(C). Thus decel when printing the second pattern leads to a misalignment of the ink drops with respect to the reference position 0. Decel may be addressed by providing correction factors when printing. The corrections may be fixed and may be applied at any time of printing so that visible misalignments due to decel are avoided.

[0012] it was found that mere patterns in which nozzles are fired continuously but which do not trigger decef. For such patterns applying a decel correction may cause visible deteriorations in the printout as the correction is applied although decel is not triggered. Such patterns may include, for example, the printing or small letters or when printing a minimum saturated area fill. For example, when applying the decel correction for printing small characters, due to the correction the dots are not placed correctly and the characters may appear blurred.

[0013] FIG. 2(A) shows ten examples of different patterns printed by firing nozzles of an inkjet print head to eject a specific amount of ink towards a medium. Pattern (1) is printed at a low firing frequency. On the right side of the pattern a vertical line is printed. The vertical line has three elements ail of which are in alignment with the reference position. The pattern (1) does not cause a misalignment i.e., decef is not triggered when printing pattern (1). The patterns (2) to (10) are printed with different firing frequencies when compared to pattern (1 ) to obtain the more saturated areas in the respective pattern. On the right side of the pattern (2) a vertical line is printed. The vertical line has two elements which are not in alignment with the reference position which is indicated by the vertical dotted line. The pattern (2) causes a misalignment, i.e., decel is triggered when printing pattern (2). Also the patterns (3) to (5) trigger decel, as may be seen from the misalignment of the vertical line elements from the reference position. The patterns (2) and (3) cause a misalignment which is smaller man the misalignment in patterns (4) and (5). The patterns (6) to (10) do not cause a misalignment as may be seen from the vertical line elements at the right end of the pattern which are in alignment with the reference position, i.e., the patterns (6) to (10) do not trigger deed. [0014] FIG.2(B) describes an example to print the patterns {1 ) and (2) shewn in FIG 2(A) during a calibration for a diagnostic for decel. FIG 2(A). the patterns (1) and (2) are shown individually. The pattern (1} does hot trigger decel, while the pattern (2) triggers decel. With each pattern the vertical line elements are printed together with the pattern. Trie three vertical line elements of the pattern (1) and the two vertical fine elements of the pattern (2) are printed. With no deed triggered by the pattern (2) the vertical line elements of both patterns match and form one straight line. When deed is triggered, a stepped vertical line is revealed because of the different drop velocity. To evaluate the pattern (2), both patterns (1 ) and (2) are printed on the medium without advancing the medium so that the patterns overlap as shown by pattern (1 )+{2) in FIG. 2(B). As may be seen from the combined pattern (1)+(2), the vertical line elements do not match but are stepped, i.e., pattern (2) provokes decel. With the determined misalignment of the vertical line elements, a correction measure to overcome the drop velocity change driven by decel may be determined. In accordance with examples, more than one vertical line may be printed for determining a misalignment of different patterns printed, for example, seven pairs of vertical lines may be printed, as is shown in FIG. (1). The pattern (1) may include less than three or more than three vertical line elements. The other patterns may include less than two or more man two vertical line elements.

[0015] The patterns described with reference to FIG. 2(A) may be analyzed with respect to the ink distribution along the pattern, and FIG. 3 shows an example of the results of a measurement of, for example, pattern (2) of FIG. 2(A) indicating the ink densities in the different regions measured by a densitometer, which may be internal to a printer. The printer is a carriage printer and the pattern (2) is printed as the print head is moved along the scan axis to apply the ink. In the areas of the pattern (2) having high densities, the print head is fired with a higher frequency than in those areas where the density is low. The scan axis may be segmented into 15 segments or portions, and each number in FIG. 3 indicates the ink density or ink level, for example in drops*100/600ths. Whether a print pattern triggers decel or not is determined using this information. During a calibration process patterns as shown in FIG. 2(A) may be printed and measured, for example by a densitometer, to determine, using the densities and ink distributions, those patterns which trigger decel and those patterns which do not trigger decel. During the calibration process, correction factors may be calculated to correct the amount of misalignment caused. [0016] in accordance with the techniques described herein, misalignments due to a decei correction, in situations when no decet is triggered, are avoided by monitoring a pattern to be printed, tor example by monitoring an amount of ink to be fired per pass to determine whether a printing follows a specific pattern in terms ink amount and ink distribution along a scan axis, to see whether the printed pattern causes or triggers decei. Dependent on whether decel is triggered or not _r a oVnar^

avoid misalignment in sensitive areas,

[0017] FIG. 4 is a schematic representation of a printer in accordance with an example. The printer 100 includes a print head 102 including a plurality of nozzles. The print head 102 may be a thermal print head in which respective resistors are provided in the nozzles to cause ejecting a drop of an ink or other printing fluid by energizing the resistor responsive to control signals provided by a controller 104 via a lino 106. The respective print head nozzles, when being energized, cause the ejection of ink drops 108 to print a specific area or pixel on a print medium 110 that is supplied to the printer 100. The print head 102 may be mounted to a carriage to be move^

medium 110, as is indicated by the arrows pointing to the left and to the right in FIG.4. in accordance with other examples, the printer 100 may be a page wide array printer in which the print head Is stationary and spans the entire width of the print medium 110. In accordance with other examples, the print head may be a piezoelectric print head in accordance with which the ink drops 108 are generated by deflecting a membrane formed in the nozzle due to the piezoelectric effect by applying an activation signal over the line 106 from the controller 104. In accordance with yet other examples, the printer 100 may be a 3-D printer. 3-dimensional printing, also referred to as additive manufacturing, rapid prototyping or solid free-form fabrication, is a technology for manufacturing a variety of objects. The printer 100 may generate objects through a selective solidification of successive layers of a build material, such as a powder material or a fiber material. A fusing agent which is applied a layer of the build material to define areas in the non- soiidifled build material layer to be solidified by applying energy to the layer.

[0018] in accordance with examples of the tecrmofegv described herein, the printer 100 comprises the print head 102 to print a pattern. Printing the pattern includes applying an amount of printing material, such as ink, to the medium. The controller 104 includes a memory to store a correction measure to modify the printing of the pattern to compensate for decel. The memory 152 may also store calibration data that may be used for analyzing print data for a patterns triggering decei. The controller includes a processor to analyze print data for a pattern that, when printed using the print head, triggers decel, control the print head to eject drops of the printing fluid in accordance with the print data, v«'hen the analysis indicates that the pattern to be printed triggers decel, apply the correction measure upon printing the pattern, and when the analysis indicates that the pattern to he printed does not trigger decel, not apply the correction measure upon printing the pattern. Misalignments of drops on the medium are avoided by applying the correction measure in situations in which decs! actually occurs, thereby providing a better image quality of the printed image in terms of grain, banding and color differences. A correction of decel is avoided when it is determined that the pattern to be printed does not cause decel so that any corrections leading to other visible effects in a printed image are avoided. For example, avoiding the decel correction when printing patterns including, e.g., small characters, the characters are printed without blur, in other words, in accordance with the techniques described herein, counterproductive correction measures are avoided, when no decel is triggered. The techniques described herein determine whether decel is present and whether a correction is to be applied or not. in case decel actually occurs, the image quality is maintained by dynamically changing the alignment position.

[0019] In accordance with further examples, also the print head usage or printed lifetime may be monitored, as decel may be more prominent with an increase in the print head age and there may be situations where the decel correction is no longer possible by modifying the correction measures. A user of the printer may be alerted accordingly to indicate that dece! may no longer be overcome and that additional actions may be considered. [0020] FiG. 5 shows a carriage inkjet-printer in accordance with another example. The printer 100 includes a carriage 112 with a print head receiving assembly 114 to receive the print head 102. The carriage 112 is to transition the print head 102 across the width of the print medium 110, also referred to as a substrate, along the print head transition directions 116, 118. For example, a drive 119 may be coupied to the carriage 112 to effect the carnage transition. The printer 100 may perform printing across the width of the medium 110 by a corresponding translation of the carriage 112. In the example of FIG. 5, the print head 102 includes a plurality of ink print head units 120 to 126. Each of the ink print head units 120 to 126 includes respective ink nozzle array arrangements 120a to 126a to eject ink drops 128. The ink print head units 120 to 126 are fiuidically connected to an ink reservoir system 132. The ink reservoir system 132 includes ink reservoirs 132a to 132d for providing ink to the respective ink print head units. In accordance with examples, the ink reservoirs 132a to 132d may store cyan Ink, magenta ink, yellow ink and black ink. Base colors may be reproduced on the medium 110 by depositing a number of ink drops of one of the inks at a specific medium location, also referred to as a pixel. Other colors may be reproduced by combining inks from different ink prirtt head units, for exampte,

dot locations in the pixel. In accordance with other examples, less ink print head units as depicted in FIG. 5 or more ink print head units and associated ink reservoirs may be provided, for example, a CcMmKY printer may include further ink reservoirs for light cyan (c) and light magenta (m). The reservoirs may be mounted on the carriage 112 in a position adjacent to the respective print heads, in accordance with other examples, the reservoirs may not be mounted on the carriage 112 and may be connected to the print head units on me carnage by means of a fluid supply. Such a system is also referred to as an off-axis system. In accordance with other examples, fluids different from inks may be provided through one of the print heads or by print head units, for example, a pre- treatment fluid, such as a fixer, and/or a post-treatment fluid, such as a coating.

{0021] The print head units may be monolithically integrated in the print head or may be modularty implemented in the print head so that each print head unit 120 to 126 may be individually replaced. The print head 102 may be a disposable print element or a fixed print element designed to last for the whole operating life ofthe printer 100.

[0022] The printer 100 includes a sensor 144 attached to the carriage 112 to move together with the carriage 112.The sensor 144 may be provided to measure a cotori metric parameter associated with an ink drop number, such as lightness, chroma values or optical density. The sensor 144 may measure values of the cotori metric parameter on medium locations or pixels across the medium 110. in accordance with examples, the sensor 144 may include a spectrophotometer, a densitometer or any other optical sensor for measuring a oolorimetric parameter. [0023] The controller 104 of the printer 100 is operatively connected to the previously described elements of the printer. The controller 104 includes a memory 152 and a processor 154 to process print jobs received from a print job source. The memory 152 may include computer readable instructions for controlling the processor 154 to execute methods as described herein. [0024] in the printers according to the of examples FIG. 4 and FIG. 5, the correction measure rnay include applying a correction factor which cause an activation signal to be shifted with respect to a reference time, the shift may be realized by changing a print r>ead encoder postfwatong a prirt

moving the print head across the medium may be decreased, or a speed of moving the medium past the print head may be decreased.

[0025] in accordance with other examples, the correction measure may be a precursor activation signai that is applied in the case of determining that decei is triggered by a specific pattern. The precursor activation signal is applied prior to the activation signai or main activation signai which causes a drop of printing fluid to be fired from the print head. Applying the precursor prior to the actual activation signai reduces or avoids decei. In accordance with the techniques described herein, precursor activation signal may be applied selectively dependent on whether a pattern to be printed is determined to trigger deed or not.

[0026] in accordance with yet other examples, decei may be addressed by providing a decei -alleviating component to the ink. For example, in the printer of FIG. 5, two of the ink reservoirs may be provided to hold ink of, for example, the same color, however, in one reservoir the ink is provided without the dece!-a!!eviating component whereas in the other reservoir the ink includes the decel-alleviating component. Dependent on whether it is determined that a pattern to be printed triggers decei or not, the respective ink including the decei-aileviating component or the ink not including this component is selected tor printing the pattern so that potential issues that may result from adding the decel- alleviating component do not occur when printing patterns in which decei does not occur or is not triggered.

[0027] FIG. 6 illustrates a method in accordance with the techniques described heron. Print data, received by a printer controller, is analyzed 300 for a pattern that, when printed, triggers deed. The analysis may be performed on the fly, i.e., upon receiving the print data and prior to printing an image. For example, the analysis may be performed for print date that is to be printed later, e.g. a number of passes after the current print pass. The print head is controlled 302 to eject drops in accordance with the received print data, it is determined 304 whether the analysis shows that the pattern triggers decei and in case that it does, when printing the pattern a correction measure is applied 306. When it is determined that the pattern does not cause deed, no correction measure 308 is applied when printing the pattern.

[0028] Although some aspects of the techniques described herein are described in the context of an apparatus, these aspects may also represent a description of the COTespondirtg method _* where a block or device corresponds to a method block or a feature of a method block. Analogously, aspects described inthe ccfttextcf a rnethod also represent a description of a corresponding block or item or feature of a corresponding apparatus.

[0029] Examples described herein may be realized in the form of hardware, machine readable instructions or a combination of hardware and machine readable instructions. Any such machine readable instructions may be stored in the form of volatile or nonvolatile storage, for example, a storage device such as a ROM, whether erasable or rewritable or not, or in the form of a volatile memory, for example, RAM, memory chips device or integrated circuits or an optically or magnetically readable medium, for example, a CD, DVD, magnetic disc or magnetic tape. The storage devices and storage media are examples of machine readable storage that is suitable for storing a program or programs that, when executed, implement examples described herein.

[0030] All of the features disclosed in this specification, including any accompanying claims, abstract and drawings, and/or all of the method or process so disclosed may be combined in any combination, except combinations where at least some of the features are mutually exclusive. Each feature disclosed in this specification, including any accompanying claims, abstract and drawings, may be replaced by features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example of a generic series of equivalent or similar features.