WO2018192658A1

WO2018192658A1 - Printed cleaner sheets

Info

Publication number: WO2018192658A1
Application number: PCT/EP2017/059375
Authority: WO
Inventors: Ido FINKELMAN; Nir Mosenson
Original assignee: Hp Indigo B.V.
Priority date: 2017-04-20
Filing date: 2017-04-20
Publication date: 2018-10-25

Abstract

In an example, at least one production sheet is printed onto a first substrate by transferring an ink image onto an intermediate transfer member and then applying the ink image from the intermediate transfer member onto the first substrate. A cleaner sheet is printed, at predetermined intervals, onto a second substrate by transferring a patch of ink onto the intermediate transfer member and then applying the ink patch from the intermediate transfer member onto the second substrate. The applied ink patch in the cleaner sheet has a thickness lower than that of the applied ink image on the production sheet.

Description

PRINTED CLEANER SHEETS

BACKGROUND

[0001] There are many types of printing devices in common use and these devices implement various printing technologies. One class of printing device prints with a liquid ink. An ink image is formed on a plate or plate cylinder. The ink is then transferred from the plate or plate cylinder to an intermediate transfer member or "blanket" (which may be made of a flexible material such a rubber). Finally the ink image is transferred from the blanket onto a print medium (such as paper). This printing technique is termed offset printing due to the presence of an intermediate transfer member.

[0002] In certain printing devices, the liquid ink used contains electrically charged, pigmented (i.e. ink) particles dispersed in a carrier liquid. Digital presses, such as the HP[RTM] lndigo[RTM] range of offset printing devices, make use of such inks - including inks sold as Electrolnk [RTM]. By controlling the application and location of electrical fields to a plate cylinder, any desired ink image can be formed on the plate cylinder.

[0003] The quality of prints tends deteriorate as ink residue and/or particulate contaminants accumulate within the printer. To maintain high quality prints, many printing presses include a cleaner function, in which a cleaner (e.g. a wiper) wipes out dirt and ink residues from the blanket, or intermediate transfer member, at a preset or predetermined frequency.

[0004] Modern printing machines, for example, HP Indigo Digital Press, implement an auto-cleaner function in which ink is transferred as a single solid-color patch covering all or most of the print area of a sheet (or portion) of the print medium. Such color patches are used to ensure that ink residues are removed from the blanket onto the print medium.

BRIEF INTRODUCTION OF THE DRAWINGS

[0005] Illustrative examples are further described hereinafter with reference to the accompanying drawings, in which:

[0006] Figure 1 shows a schematic diagram of a printing system according to some examples;

[0007] Figures 2A and 2B illustrate elements of a printing apparatus suitable for implementing auto-cleaner operation in accordance with an example; [0008] Figure 3 is a cross-sectional diagram of a binary image development unit according to one example of the liquid electrophotographic ink developer unit described in this disclosure;

[0009] Figures 4A and 4B show flow charts illustrating methods for cleaning a printer according to respective examples; and

[0010] Figure 5 illustrates a machine readable medium according to some examples.

DETAILED DESCRIPTION

[0011] Electro-photography (EP) printing devices form images for transfer to a print medium by placing a uniform electrostatic charge on a photoreceptor and then selectively discharging the photoreceptor in correspondence with the images. In certain cases, the selective discharge is performed by exposure of the photoreceptor to the output beam(s) from a scanned array of laser diodes. These lasers may be controlled by a raster image processor which converts instructions from a digital file into "power" instructions for the lasers.

[0012] In dry EP (DEP) printing devices, toner is used as the colorant. Toner particles are electrically charged and thus more strongly attracted to regions of the photoreceptor where electrostatic charge persists. The colorant toner is transferred to the print medium as the print medium passes below the photoreceptor. The toner is then fixed in place as it passes through heated pressure rollers.

[0013] In liquid EP (LEP) printing devices, ink is used as the colorant instead of toner: the ink comprises electrically charged ink particles (with diameters typically in the order of micrometers) dispersed in a carrier liquid. In printing presses, such as the HP Indigo range from Hewlett-Packard Company, LEP is used in an offset arrangement.

[0014] Here, as in other offset techniques, an ink image is formed on a plate cylinder, transferred to a blanket (i.e. an intermediate transfer element) and then transferred a second time to a substrate. Throughout the present disclosure, the term "substrate" refers to print media of various physical and chemical compositions (e.g. paper, plastic or fabric). The substrate may be formed as individual sheets (typically of a standardized dimension) or as a continuous web of print medium (often drawn from a roller). Ink images may be applied to one side of the planar substrate (i.e. simplex) or they may be applied to each side of the planar substrate (i.e. duplex). [0015] In LEP, the ink image is formed on the photoreceptor (for example, an electrophotographic Photo Imaging Plate, PIP, mounted on an imaging cylinder) through a combination of electrostatic charge and selective exposure to light. The surface of the photoreceptor is subjected to a uniform electrostatic charge. The output from a multi-beam laser scanning unit is then selectively applied to the surface, locally discharging the static charge on the photoreceptor. The selective discharging forms a latent image on the photoreceptor: the latent image being an invisible electrostatic charge pattern conforming to the image to be printed.

[0016] The liquid ink, comprising electrically charged particles in a liquid carrier, is then developed onto the photoreceptor (upon which the latent image as been formed). Image development (i.e. the temporary fixing of ink to the photoreceptor) is performed by Binary Ink Developer (BID) units. In certain cases, a respective BID unit is provided for each ink used. Typically four different inks are used to achieve the gamut of color; there is however no upper limitation on the number of different inks. The BID units include at least one BID roller and are arranged to prepare a thin film of highly electrically charged liquid ink on the surface of the BID roller. During printing, the appropriate BID roller engages with the photoreceptor (i.e. the PIP cylinder). The electrical fields between the PIP and the BID result in ink becoming attracted from the BID roller to the image area and repelled from the non-image areas, shearing the ink film accurately and instantaneously. Ink (i.e. colorant) is thus developed onto the latent image of the photoreceptor.

[0017] Inks of different colors may be developed, offset and then deposited on the substrate in successive layers, also referred to as "color separations". Each color separation may be semi-transparent. When separations are overlaid different hues can be produced. Depending upon the desired outcome, one or more color separation (of the same or different color of ink) may be applied to the same print area of the substrate to generate a final printed image.

[0018] In certain examples of LEP devices using an offset arrangement, an ink image developed on the photoreceptor is offset (i.e. transferred) to an intermediate transfer element, (i.e. the blanket), where it is heated and solidified. This image layer is then transferred to the surface of the substrate in the form of an image or text, for example. The voltage applied to the BID roller is known to correlate monotonically with the optical density, OD, of the resulting ink film transferred to the surface of the substrate, i.e. the OD varies according to a monotonically increasing (typically linear or near-linear) function of the voltage. [0019] The transfer of the developed ink image from the photoreceptor to the blanket (i.e. intermediate transfer element) is driven by a "nip" contact (between the blanket and an PIP cylinder) and an electric field created by a bias voltage applied to the blanket.

[0020] In some cases, the blanket is a heated rubbery layer covering an intermediate roller. Further heating may be supplied from a source external to the intermediate roller. In each case, however, the ink becomes solid (or nearly 100% solid) upon contact with the substrate (which is at a substantially lower temperature) and transfers from the blanket to form a solidified ink image (i.e. a color separation) adhering to the surface of the substrate.

[0021] Ink splitting, where ink residue accumulates on the blanket, is well known in offset printing. While the second transfer (i.e. from the blanket to the substrate) is very nearly 100% in LEP offset printing, it is still possible for ink residue and other particulate contaminants to accumulate on the blanket. When this happens, the quality of printed output tends to deteriorate as dirt (e.g. dust) or ink residue accumulate within the printer. In certain cases, the accumulation of ink residue and/or foreign particles (i.e. particulate contaminants such as dust, dirt, etc.) may even be addressed by replacing the blanket entirely: during replacement, the printing press cannot run production jobs.

[0022] To maintain high quality prints while maintaining production, many printing presses deploy a cleaner function, in which a cleaner (e.g. a wiper) wipes out dirt and ink residues from the blanket, or intermediate transfer member, at a preset or predetermined frequency.

[0023] Some printing presses also implement an auto-cleaner function in which an auto- cleaner print is printed as a single solid-color patch covering all or most of the print area of a sheet (or portion) of the substrate (referred to hereafter as a "cleaner sheet" for brevity). Such color patches may be used to ensure that ink residues are removed from the blanket onto the substrate, as the residues preferentially adhere to the ink patch. The term auto- cleaning refers to the deliberate printing of a cleaner sheet using a solid-color block or patch of ink to "clean off" any accumulated residue. Cleaner sheets allow easy spotting of any unwanted contaminants (either automatically using post-print imaging or by visual checking by the operator). The use of solid-color patches covering all or most of the print area of a sheet does however consume a significant quantity of ink when compared to production printing, with ink coverage of 200% (in duplex printing) compared to the 15% coverage typical for a production print.

[0024] To reduce ink consumption by cleaner sheets, it would be possible to reduce the number of cleaner sheets printed and/or to reduce the frequency at which cleaner sheets are inserted (during print runs). However, as parameters such as cleaner sheet number and insertion frequency are set to promote good blanket cleaning, such reductions lead to reduction in cleaning and may lead to long-term damage to blanket functionality and deterioration in print quality.

[0025] Auto-cleaning is typically performed at a predetermined frequency under the control of a print controller, but may also be performed upon instruction or at a predetermined phase of a production job (e.g. at the beginning or end of a production run, or before/after application of a cleaner function). Cleaner sheets may, for example, be printed: after a pre-set number of (production) separations (this may happen more than once for longer print jobs); at the end of a print job; or when switching between jobs. In one example, two auto-cleaner sheets are output every 1000 separations. The operator of the printing press may configure the application of auto-cleaning manually, typically through a graphical user interface with the print controller. Typically, auto-cleaner frequency for such presses is predetermined with respect to the type of substrate which is currently used for printing. If this frequency is not optimal for a given substrate, the operator may encounter print quality issues, that may consequently lead the operator to replace blankets at a high rate to address the quality issue.

[0026] The development of ink (i.e. from developer units, BIDs, to photoreceptor) is currently controlled in order to satisfy color calibration conditions for production printing. Color calibration may be configured with default configuration values for developer voltage values to be applied in a BID unit to achieve a visually acceptable optical density (i.e. an OD that delivers high print quality and/or good color accuracy) in the finished printed article. Using post-print optical analysis, print controllers may perform a color calibration procedure in which an empirical relationship between measured optical density of sample production separations with the developer voltage applied in the BID unit is established (i.e. plotted and then recorded, in a mapping table, for example). Satisfying such color calibration conditions is associated with the consumption of a corresponding volume of ink (per unit area) that is observed to be significantly greater than a volume of ink (per unit area) found to be adequate for cleaning accumulated residue in auto-cleaning. Once the relationship between developer voltage and OD is known or established, the print controller may determine a BID developer voltage that results in any arbitrary target OD by consulting the recorded mapping table. By reducing the volume of ink developed for transfer to the blanket to a volume sufficient to clean the blanket, the volume of ink transferred during auto-cleaning may be reduced. [0027] The property of the ink in the ink patch of a cleaner sheet that makes it sufficient to clean the blanket is its thickness perpendicular to the plane of the substrate. For an acceptable level of adhesion to blanket residue, the thickness exceeds a threshold thickness. The colorant layer thickness correlates to the optical density of that layer. For production sheets it is desirable that the optical density of that layer be calibrated to achieve a desired color output; by contrast desired color output is not a consideration in cleaner sheet printing.

[0028] In certain cases, the volume of ink transferred during auto-cleaning may be reduced by applying a weighting factor to the developer voltage (i.e. the voltage applied to the developer roller in each BID unit) that would be used when meeting color calibration conditions (thereby reducing thickness of the or each color separation, or optical density which varies with thickness, by a related factor).

[0029] In certain cases, the appropriate developer voltage (and thus by inference the volume of ink developed for transfer to the blanket) for auto-cleaning may be calculated from a table provided (i.e. plotted and recorded) specifically for auto-cleaning. The ink volume table may, in this case, include data obtained empirically so that volume of ink indicated is sufficient (i.e. thick enough) to achieve a desired blanket cleaning effect without reference to the color calibration conditions. The degree of blanket cleaning may be measured as a function of the number of foreign particles over a threshold diameter counted within a sample area, for example.

[0030] The examples below describe aspects of apparatuses and methods to reduce ink consumption when printing cleaner sheets.

[0031] It will be appreciated that a print job may include one or more printed instances of a printed article (itself comprising one or more color separations). A print job may also be referred to as a print run. The printed instances are the actually printed out examples of the print job/run. The printed instances may comprise (initial) test examples, and (subsequent) final versions of the print job. Test examples may be substantially identical in form and content to a final version, the difference being merely they are labelled test versions, usually since they are an early sample for quality and reproduction accuracy assessment prior to carrying out the bulk of the print job run. A print job may include or be referred to as one or more print runs. Equally, a print run may include one or more print jobs. Likewise, the printing of one or more printed instances for delivery as a product, rather than for testing, calibration or cleaning purposes, may be referred to as a production job/run - an instance of a printed article from a production job may be referred to as a "production sheet". Throughout the present disclosure, the term "sheet" is to be interpreted broadly to include a sheet of substrate delivered to the printing device in individual sheets (such as paper from a stack of cut paper sheets) but also a portion of a substrate fed into the printing device as a continuous web.

[0032] Figure 1 shows a schematic diagram of a Digital Press type of printing system 100 according to an example. The printing system 100 comprises a print controller/digital front end (DFE) 101 that controls printing apparatus 120 arranged to print on a substrate 140. While the substrate 140 is shown as a continuous web, other examples include substrate delivered as a series of individual sheets.

[0033] The print system 100 may further include a "down-stream" imaging apparatus 170 arranged to image (e.g. scan) the print job instances as they come out from the print apparatus 120. The illustrated imaging apparatus 170 includes an in-line scanner 150, which, for example optically scans the outputted print instances using camera 155.

[0034] The substrate 140 may be conveyed along by rollers 180 (e.g. the substrate 140 may be drawn through the printing system 100 by rollers 180, past both the printing apparatus 120 and the imaging apparatus 170).

[0035] The print controller 101 may comprise a processor 1 10, data storage 1 15 for storing printing programs and the data used during printing, such as but not limited to the printing parameters of the printing apparatus 120.

[0036] The print controller 101 shown in Figure 1 also includes a combined display and input device 130, for example in the form of a touchscreen display presenting graphical user interface. This touchscreen is used by an operator to control, configure and/or oversee the printing process(es) of the printing system 100.

[0037] Printing apparatus 120 operates to print defined print jobs 20 onto a substrate 140 or portions thereof, to form one or more particular instances 10 of the or each print job 20.

[0038] In some examples, in line camera 155 may image the printed substrate 140 and provide the captured image data to the in-line scanner (I LS) 150 portion of scanning system 170, and the print controller 101 . In some examples, the imaging apparatus 170 may analyze the image data captured by the inline scanner 150 and provide printing parameters or other data to the print controller 101. In some other examples, the imaging apparatus 170 may provide raw image data to be analyzed by the print controller 101 and/or the operator of the print system. The imaging apparatus may further include an optical sensor such as a spectrophotometer and/or optical densitometer 160, which receives imaging output from a further in line camera 165.

[0039] Print controller 101 may cause the printing apparatus 120 to perform an auto- cleaner operation periodically by printing at least one cleaner sheet 15. The printed cleaner sheet 15 may be scanned by ILS 150 and the scanned image data from ILS 150 may be forwarded to print controller 101 , where it may be analyzed to determine existence of artifacts indicative of dirt and/or residue ink. For example, when analyzing the scanned image data, it may be determined whether the artifacts discovered are above or below a predetermined threshold. This may be accomplished, for example, by measuring an optical characteristic associated with the overall appearance of the cleaner sheet, across the entire print or a portion thereof, using, for example, optical sensor 160. The measured optical characteristic may be, for example, the optical density, OD, the color or the glossiness of the cleaner sheet 15 or a portion thereof. Hereinafter the measuring of OD is described, but it is understood that other optical characteristics may apply and may, therefore, be measured.

[0040] The print controller 101 may modify the frequency of the auto-cleaner operation, depending on the analyzed scanned image data of the auto-cleaner print (i.e. the cleaner sheet 15).

[0041] For example, if the OD of the discovered artifacts is above a predetermined threshold, the frequency of the auto-cleaner operation may be increased, whereas if the OD of the discovered artifacts is below the predetermined threshold, the frequency of the auto- cleaner operation may be decreased.

[0042] Typically a cleaner sheet may comprise a simple image. For example, a cleaner sheet may comprise one or more single-color patch covering the entire or most of the print area. Typically a yellow patch may be used (in which case the OD of the artifacts could be measured in the blue channel to increase sensitivity). In some examples, ink of a particular color, such as yellow or magenta ink, may be used because that ink is known to adhere more strongly to ink residue than other inks. Other colors may be used too. In some examples, ease of spotting of any unwanted contaminants through post-print image analysis or visual checking by operators may additionally or alternatively determine which color(s) of ink be used in printing cleaner sheets.

[0043] Ink is transferred as solid-color patches - rather than as dots - because patches spread the adhesive ink more evenly, increasing the surface area to which ink residue may adhere. Where the cleaner sheet comprises more than one single-color patch, each patch may be of the same or different color (although a single-color patch is considered adequate for many auto-cleaner purposes). "Color" in the context of the present specification refers to a single shade of any of the color components of the color space or a combination thereof. In some examples, more than one shade of color may be used; in other examples a single- color shade consistent with the stock color of the ink is used. The thickness of the production sheets 10 and the cleaner sheets 15

[0044] Figures 2A and 2B illustrate a printing apparatus elements of a printing apparatus (i.e. printing "device" or "press") suitable for implementing auto-cleaner operation in accordance with an example.

[0045] Printing apparatus 120 may be operated to print on substrate sheets 140a, 140b, 140c which consecutively progress through a printing element assembly 202, which may include, for example, a developer roller 212, a photoreceptor 214, a blanket 216 and an impression cylinder 218 (the arrows 204 shown in these figures indicate the direction of motion of the substrate sheets). Feeder 210 may be provided, to feed the substrate sheets into the printing apparatus 120.

[0046] Shown in Figure 2A are sheet 140a which still lies within feeder 210 and awaits its turn to be fed into the printing apparatus 120, sheet 140b (a cleaner sheet) which is currently being printed upon by the printing element assembly 202, and sheet 140c (a production sheet) which has emerged from the printing apparatus 120 after it had been printed upon. An output tray or hopper may be provided to collect printed matter such as sheet 140c. The thickness of the ink image in the illustrated production sheet 140c is seen to be thicker than the patch of ink in the cleaner sheet 140b.

[0047] Alternatively the press may be fed a continuous web of substrate.

[0048] In Figure 2B, certain elements of an offset printing apparatus are illustrated. Ink developed on a developer roller 212 is applied to the photoreceptor 214, upon which a latent image has been formed. An ink image generated on the photoreceptor 214 by deposition of developed ink from the developer roller 212 is offset (i.e. transferred) to a blanket 216, where it is heated and substantially solidified. This offset image layer is then transferred to the surface of the substrate 140, the substrate being fed through a nip contact formed between the blanket 216 and the impression cylinder 218.

[0049] As noted previously, the voltage 250 applied to the developer roller 212 is known to correlate monotonically with the optical density, OD, of the resulting ink film transferred to the surface of the substrate 140. The thickness of the ink image in the illustrated sheet is seen to have a thickness that differs according to the applied voltage 250 (Vi, V2 etc.).

[0050] Figure 3 is a cross-sectional diagram of an example of an ink development unit. In this example, the ink development unit is a binary image development (BID) unit 305. The binary image development unit 305 comprises a developer roller 320 arranged to rotate about an axis fixed relative to a BID unit housing 380. The binary image development unit 305 may also comprise a number of other static parts and rollers which cooperate with the developer roller 320 to transport an amount of ink from the binary image development unit 305 to a photoreceptor 315 (i.e. a photo imaging plate, PIP) on a photo imaging drum 310 (i.e. a PIP cylinder). A BID unit 305 as shown in Figure 3 may be included within a liquid electrophotographic (LEP) printer device 120 (such as the printing apparatus of Figures 1 and/or 2). The LEP printer device 120 may include any number of BID units 305 as needed, each BID unit 305 containing a different color or type of ink with which to apply to the photoreceptor 315. While not illustrated, transfer members may take physical shapes other than cylindrical drums or rollers, the terms "drum" and "roller" may be understood to include alternative shapes of transfer member such as, for example, transfer belts and plates (curved or planar) etc.

[0051] In addition to the developer roller 320, the binary image development unit 305 may include a back electrode 350, a main electrode 345, a squeegee roller 325, a cleaner roller 330, a wiper blade 335, a sponge roller 340, an ink chamber 355, an ink reservoir 360, an ink inlet 370, and an ink outlet 385. The LEP printer device 120 therefore may include the BID unit 305 mentioned above as well as a photoreceptor 315 coupled to a photo imaging drum 310. Each of these will now be discussed in more detail.

[0052] The BID unit 305 selectively coats the photoreceptor 315 with an amount of ink. To accomplish this, separate ink tanks may be used to hold and control the desired properties of the ink such as the ink's density and conductivity. One ink tank may be used for each color. In an idle stage, for example, before printing starts, the BID unit 305 may be empty (i.e. devoid of ink). To start developing ink, the BID unit 305 may be provided with a flow of ink pumped from ink tanks (not shown) through the ink inlet 370 that allows a continuous supply of ink in the development area or zone (i.e. the respective gaps 373, 375 between developer roller 320 and electrodes 350, 345). The electrically charged ink particles may be positively or negatively charged. For purposes of simplicity in illustration, the ink within the BID unit 305 in Figure 3 is described as if it is negatively charged. Still further the ink may contain varying amounts of solids within the ink solution. In one example, the ink may be comprised of 2-3% solids. [0053] As the ink is pumped into the ink chamber 355 via the ink inlet 370, two electrodes, the main electrode 345 and the back electrode 350, apply an electric field across respective gaps 373, 375. A first gap 373 is located between the main electrode 345 and the developer roller 320, and a second gap 375 is located between the back electrode 350 and the developer roller 320. The potential difference in electric charge across these gaps 373, 375 (i.e. the "developer voltage" 250) causes the ink particles to be attracted to the more positively charged developer roller 320. The applied voltage 250 may be varied to increase or decrease the volume of ink drawn across the gaps onto the developer roll 320.

[0054] The developer roller 320 may be made of a polyurethane material with an amount of conductive filler, for example, carbon black mixed into the material. The choice of material may give the developer roller 320 the ability to hold a specific charge having a higher or lower negative charge compared to the other rollers 325, 310, 330 with which the developer roller 320 directly interacts.

[0055] In one example, the electrical bias between the electrodes 345, 350 and the developer roller 320 produces an electric field between the electrodes 345, 350 and the developer roller 320 that is of the order of 800-1000 volts. With a gap 373, 375 of about 400-500 μηη, the electric field becomes relatively high and the negatively charged ink particles are attracted to the developer roller 320. This creates a layer of ink over the developer roller 320.

[0056] As the ink particles are built up on the developer roller 320, the squeegee roller 325 may be used to squeeze the top layer of oil away from the ink. The squeegee roller 325 may also develop some of the ink onto the developer roller 320. In order to accomplish these two objectives, the squeegee roller 325 may be both more negatively charged relative to the developer roller 320 and may abut the developer roller 320 creating a nip. As the squeegee roller 325 comes in contact with the developer roller 320, the ink layer on the developer roller 320 may become more concentrated. In one example, the squeegee roller 325 may develop the ink layer and remove enough oil or organic solvent from the ink such that the concentration of ink particles in the (oil-based) carrier is increased. In one example, the resulting ink concentration may be around 20% to 25% colorant concentration.

[0057] After the ink on the developer roller 320 has been further developed and concentrated by the squeegee roller 325, the ink may be transferred to the photoreceptor 315. In one example, the photoreceptor 315 may be coupled to a photo imaging drum 310. In another example, the photo imaging drum 310 may incorporate the photo imaging plate 315 such that the photo imaging drum 310 and photoreceptor 315 are a single piece of photoconductive material. However, for the purposes of simplicity in illustration, the photoreceptor 315 and photo imaging drum 310 are separate pieces thereby allowing the photoreceptor 315 to be selectively removed from the photo imaging drum 310 for replacement if needed.

[0058] In one example, prior to transfer of ink from the developer roller 320 to the photoreceptor 315, the photoreceptor 315 or, alternatively, the photo imaging drum 310 and photoreceptor 315, may be negatively charged with a charge roller. A latent image may, therefore, be developed on the photoreceptor 315 by selectively discharging selected portions of the photoreceptor 315 with, for example, a laser (not shown). The discharged area on the photoreceptor 315 may now be more positive as compared with developer roller 320, while the charged area of photoreceptor 315 may still relatively be more negative as compared with developer roller 320. When the developer roller 320 comes in contact with the photoreceptor 315, the negatively charged ink particles may be attracted to the discharged areas on the photoreceptor 315 while being repelled from the still negatively charged portions thereon. This can create an image on the photoreceptor 315 which may then be transferred to another intermediate transfer element (i.e. a blanket) or directly to a sheet or web of print media such as a piece of paper.

[0059] Because a portion of the ink is transferred from the developer roller 320 to the photoreceptor 315, the excess ink may be removed from the developer roller 320 using a cleaner roller 330. The cleaner roller 330 may have a more positive bias compared to the developer roller 330. As such, the negatively charged ink particles may be attracted to the cleaner roller 330 and thereby removed from the developer roller 320. The wiper blade 335 and sponge roller 340 may subsequently remove the ink from the cleaner roller 330.

[0060] Figures 4A and 4B show a respective flow chart illustrating a method for cleaning a printer according to examples of the disclosure.

[0061] As part of a typical print run, the method 400 begins with a printer device (such as the printing apparatus 120 of Figures 1 and/or 2) receiving instructions from a digital front end (DFE) (such as the print controller 101 of Figures 1 and/or 2) to initialize a print job (operation 410).

[0062] In the example illustrated in Figure 4A, the print job comprises a production job in which one or more production sheets are printed (operation 420), followed by an auto- cleaning function in which one or more cleaner sheets are printed (operation 440). Thereafter, the process flow may be discontinued (operation 480). [0063] Although not illustrated, these jobs may be performed in another temporal order: for example auto-cleaning may be performed before the onset of any production job. Furthermore, additional jobs such as the generation of test sheets, distinct from production and auto-cleaning, may be performed before, interspersed between, and after the production job and auto-cleaning function.

[0064] In performing the production job (operation 420), at least one production sheet is printed onto a first substrate (e.g. a substrate sheet of a portion of a web of substrate) by transferring an ink image onto an intermediate transfer member (for example, a blanket 216 in the printing element assembly 202 of Figure 2B) and then applying the ink image from the intermediate transfer member onto the first substrate, the applied ink image having a first predetermined thickness, θι.

[0065] Implementing the auto-cleaning function (operation 440) may include, after a predetermined time (or, in an alternative example, after a predetermined number of production sheets have been printed), the printer device printing at least one cleaner sheet onto a second substrate different from the substrate upon which the at least one production sheet is printed (e.g. a further substrate sheet or a further portion of a web of substrate). Printing the cleaner sheet is achieved by transferring a patch of ink onto the intermediate transfer member and then applying the ink patch from the intermediate transfer member onto the second substrate. However, the ink patch applied to the second substrate has a second predetermined thickness, Θ2, lower than the first predetermined thickness, Θ1.

[0066] The reduction in thickness may be achieved by controlling an auto-cleaning operation developer voltage (at which ink is developed in preparing the patch of ink) to take a value lower than the production operation developer voltage at which ink is developed in preparing the ink image for the production job.

[0067] In certain cases, as illustrated by the example method 400' in Figure 4B, the print job again comprises a production job in which one or more production sheets are printed (operation 420), followed by an auto-cleaning function in which one or more cleaner sheets are printed (operation 440) as they were in the example in Figure 4A. Thereafter, however, at least one sample sheet from the production sheets and/or the cleaner sheets may be subjected to post-print analysis (operation 450). In this analysis, an image of the sample sheet is scanned and values for at least one auto-cleaning metric are measured in the scanned data. The measured values for the at least one auto-cleaning metric are compared to corresponding reference values (operation 460). The auto-cleaning metric may include a metric of an optical characteristic such as a count of the number of artifacts in the scanned image data, the measured optical characteristic may be, for example, the optical density, OD, the color or the glossiness of a sample area of the sample sheet.

[0068] If the measured values for the auto-cleaning metric in the scanned image fulfil a criterion (for example the optical density of the sample image exceeds (or drops below) a predetermined threshold), the printer controller 101 may modify the configuration of the printer device for future print jobs, for example by altering the predetermined frequency of the auto-cleaner operation (operation 465).

[0069] Next it may be determined (operation 470) whether to continue the print run (for example, to continue printing more instances of a previously printed production sheet (at operation 420) or to print one or more instances of at least one further production sheet (e.g. the next page in a sequence of pages in a magazine or book). If the print run is to continue, the flow returns to operation 420 (with the ink image for the production sheet(s) being the same or different from that in the first iteration). Otherwise the process flow may be discontinued (operation 480).

[0070] In certain cases, the patch of ink is a single ink separation.

[0071] In certain cases, the patch of ink comprises an ink having adhesive characteristics. Ink residue present on the intermediate transfer member may therefore be lifted off the intermediate transfer member due to said adhesive characteristics.

[0072] In certain cases, the first predetermined thickness is determined by a color calibration technique. This color calibration technique is provided to ensure high color fidelity in production sheet printing. In certain cases, the second predetermined thickness (i.e. the thickness of the cleaner sheet ink separation(s)) is determined by applying a weighting factor to the first predetermined thickness, the weighting factor being less than 1.

[0073] In a further example, the transfer of ink onto at least one substrate in a printer having a transfer member, may comprise transferring at least one ink separation at a first optical density, OD, from the transfer member onto a first substrate; and at predetermined intervals, printing a cleaner sheet by transferring a patch of ink from the transfer member onto a second substrate, the cleaner sheet being printed at an optical density lower than the first OD.

[0074] It is noted that values of the developer voltage (i.e. the voltage applied to develop ink onto the developer roller for application to the photoreceptor) may be mapped to the resulting thickness of the ink layer in a given ink separation (or equivalently to the optical density of that ink layer. [0075] In certain cases, the volume of ink transferred during auto-cleaning may be reduced by applying a weighting factor, ω, to the developer voltage (i.e. the voltage applied to applied to the developer roller in each BID unit) that would be used when meeting color calibration conditions (thereby reducing thickness of the or each color separation, or optical density which varies with thickness, by a related factor).

[0076] In order to print one or more ink separation at a first optical density, OD and the patch of ink at a second optical density, that is lower than the first OD, the print controller 101 may control the developer voltage to take a corresponding lower value during cleaner sheet printing than that taken during production sheet printing.

[0077] In accordance with a further example, a printer may be cleaned by printing at least one production sheet onto a first substrate by transferring an ink image from a print plate onto an intermediate transfer member and then applying the ink image from the intermediate transfer member onto the first substrate, the applied ink image being at a first optical density, OD, and, at predetermined intervals, printing a cleaner sheet onto a second substrate by transferring a patch of ink from the print plate onto the intermediate transfer member and then applying the ink patch from the intermediate transfer member onto the second substrate, the applied ink patch having an optical density lower than the first OD.

[0078] In accordance with yet another example, a printing apparatus for transferring ink onto at least one substrate, may be provided in which the apparatus comprises: a processor; an ink developing unit; and an intermediate transfer member; wherein the processor controls the ink developing unit to develop ink for at least one production sheet as an ink image for transfer onto the intermediate transfer member and then controls the intermediate transfer member to apply the developed ink image to a first substrate, the applied ink image having a first predetermined thickness, and wherein, at predetermined intervals, the processor controls the printing apparatus to print a cleaner sheet onto a second substrate by controlling the ink developing unit to develop a patch of ink for transfer onto the intermediate transfer member and then applying the developed ink patch from the intermediate transfer member onto the second substrate, the applied ink patch having a second predetermined thickness lower than the first predetermined thickness.

[0079] Methods described herein may be implemented using one or more processors. Instructions for causing the one or more processors to carry out the methods may be stored on computer readable medium (such as memory, optical storage medium, RAM, ROM, ASIC, FLASH memory, etc.) The medium may be non-transitory or transitory. [0080] Figure 5 illustrates a machine-readable medium (such as the computer readable storage medium 1 15 in Figure 1 ) according to some examples. The computer readable storage medium 1 15 stores at least one block of machine readable code, with each block including instructions that, when executed, cause a processor 1 10 or other processing device to perform particular operations. The computer readable storage medium 1 15 includes a developer unit control block 510 including instructions that, when executed, cause a processing device 1 10 to control the operation of at least one developer unit in accordance with auto-cleaner configuration data. The computer readable storage medium 1 15 also includes a calculation block 520 including instructions that, when executed, cause the processing device 1 10 to calculate a developer voltage based on a first target OD for production sheets and an auto-cleaning function developer voltage based on a second target OD for cleaner sheets. The blocks of the computer readable storage medium 1 15 may cause a processing device 1 10 to operate in accordance with any of the examples described herein.

[0081] In one example, the target OD for cleaner sheets is changed by multiplying the target OD for production sheets by a weighting factor, ω. An example of the weighting factor may be ω = 0.8 - representing a 20% reduction in the target OD used for auto-cleaning operation.

[0082] In another example, the target OD for cleaner sheets is changed by establishing an auto-cleaning relationship between developer voltage to be applied by a given BID unit and an optical density of a cleaner sheet ink patch sufficiently thick to adhere to ink residue, and then looking up the target developer voltage.

[0083] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or operations.

[0084] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, arrangement or example of the present disclosure are to be understood to be applicable to any other aspect, arrangement or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the operational integers of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or integers are mutually exclusive. The present disclosure is not restricted to the details of any foregoing arrangements. The present disclosure extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the operational integers of any method or process so disclosed.

Claims

1 . A printing apparatus for transferring ink onto at least one substrate, the apparatus comprising:

a processor;

an ink developing unit; and

an intermediate transfer member;

wherein the processor controls the ink developing unit to develop ink for at least one production sheet as an ink image for transfer onto the intermediate transfer member and then controls the intermediate transfer member to apply the ink image to a first substrate, the applied ink image having a first predetermined thickness, and

wherein, at predetermined intervals, the processor controls the printing apparatus to print a cleaner sheet onto a second substrate by controlling the ink developing unit to develop a patch of ink for transfer onto the intermediate transfer member and then applying the developed ink patch from the intermediate transfer member onto the second substrate, the applied ink patch having a second predetermined thickness lower than the first predetermined thickness.

2. The printing apparatus of claim 1 , wherein the patch of ink comprises a single ink separation.

3. The printing apparatus of claim 1 , wherein the patch of ink comprises an ink having adhesive characteristics.

4. The printing apparatus of claim 1 , wherein the ink includes electrically charged ink particles.

5. The printing apparatus of claim 1 , wherein the first predetermined thickness is determined by a color calibration technique.

6. The printing apparatus of claim 5, wherein the second predetermined thickness is determined by applying a weighting factor to the first predetermined thickness, the weighting factor being less than 1.

7. The printing apparatus of claim 1 , wherein the ink developing unit includes a developer roller and at least one electrode.

8. The printing apparatus of claim 7, wherein the processor controls the ink developing unit to develop ink at the first predetermined thickness by controlling a developer voltage applied between the at least one electrode and the developer roller to take a first developer voltage value, the first developer voltage value corresponding to the first predetermined thickness.

9. The printing apparatus of claim 8, wherein the processor controls the ink developing unit to develop ink at the second predetermined thickness by controlling the developer voltage take a second developer voltage value, the second developer voltage value corresponding to the second predetermined thickness, and

wherein the developer voltage is controlled to take the second developer voltage value by applying a weighting factor to the first developer voltage value, the weighting factor being less than 1 .

10. The printing apparatus of claim 7, wherein the processor controls the ink developing unit to develop ink at the first predetermined thickness by controlling a developer voltage applied between the at least one electrode and the developer roller to take a first developer voltage value, the first developer voltage value corresponding to a first optical density, OD, and

wherein the thickness of the applied ink image varies according to the OD of the applied ink image.

1 1 . The printing apparatus of claim 10, wherein the processor controls the ink developing unit to develop ink at the second predetermined thickness by controlling a developer voltage applied between the at least one electrode and the developer roller to take a second developer voltage value, the second developer voltage value

corresponding to a second optical density, OD,

wherein the thickness of the applied ink patch varies according to the OD of the applied ink patch, and

wherein the developer voltage is controlled to take the second developer voltage value by applying a weighting factor to the first OD, the weighting factor being less than 1 , thereby printing the cleaner sheet at an optical density lower than the first OD.

12. The printing apparatus of claim 1 , wherein the intermediate transfer member is a heated blanket.

13. The printing apparatus of claim 1 , wherein the printer is a digital offset print press.

14. A method for cleaning a printer, the method comprising:

printing at least one production sheet onto a first substrate by transferring an ink image onto an intermediate transfer member and then applying the ink image from the intermediate transfer member onto the first substrate, the applied ink image having a first predetermined thickness, and

at predetermined intervals, printing a cleaner sheet onto a second substrate by transferring a patch of ink onto the intermediate transfer member and then applying the ink patch from the intermediate transfer member onto the second substrate, the applied ink patch having a second predetermined thickness lower than the first predetermined thickness.

15. Machine-readable instructions provided on at least one machine-readable medium, the machine-readable instructions, when executed, to cause processing hardware to control a printing apparatus:

to print at least one production sheet onto a first substrate by transferring an ink image onto an intermediate transfer member and then applying the ink image from the intermediate transfer member onto the first substrate, the applied ink image having a first predetermined thickness, and

at predetermined intervals, to print a cleaner sheet onto a second substrate by transferring a patch of ink onto the intermediate transfer member and then applying the ink patch from the intermediate transfer member onto the second substrate, the applied ink patch having a second predetermined thickness lower than the first predetermined thickness.