Patent Publication Number: US-8968974-B2

Title: Techniques for coating print media

Description:
BACKGROUND 
     Print quality, ink adhesion, rub resistance, or durability are factors that designers and users of printers consider. Such advantages are, generally, in particular desired by commercial printing customers. One manner to improve image durability is to provide a coating on a print medium. The coating may be provided over the printed image printed, which is referred to as an overcoat. Alternatively, the coating may be provided onto the surface of the print medium, which is referred to as an undercoat. An image may be then subsequently printed on the coated print medium. 
     An undercoat may be applied to enhance fixation (e.g., bonding and/or hardening) of a colorant to be subsequently applied on the print medium. If the colorant includes an ink, fixation may be desired to address coalescence, bleed, feathering, or similar effects characterized by ink or pigment migration across a printed surface. An overcoat may be applied as a protection to improve durability of the printed image. 
     Some methods for applying coatings on a substrate include roll coating, spray coating, manual application, or treatment ejection, for example, through a jetting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present disclosure may be well understood, various examples will now be described with reference to the following drawings. 
         FIG. 1  shows a block diagram of a standalone coater according to examples. 
         FIG. 2  is a flow diagram illustrating examples of operation of the standalone coater of  FIG. 1 . 
         FIG. 3  shows a block diagram of a standalone coater for providing addressable coatings according to examples 
         FIG. 4  shows a flow diagram illustrating examples of operation of the standalone coater of  FIG. 9 . 
         FIG. 5  shows a block diagram of a printing system including a standalone coating unit according to examples herein. 
         FIG. 6  shows a flow diagram illustrating examples of operation of the printing system of  FIG. 5 . 
         FIG. 7  shows a block diagram of a standalone coater according to examples. 
         FIG. 8  shows a block diagram of a standalone coater according to examples. 
         FIG. 9  is a partial cut-away view of a binary coating developer of a coater according to examples. 
         FIG. 10  shows a partial block diagram of a standalone coater according to examples. 
         FIG. 11  shows a flow diagram illustrating examples of operation of the standalone coaters of  FIGS. 7 and 8 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the examples disclosed herein. However, it will be understood that the examples may be practiced without these details. While a limited number of examples have been disclosed, it should be understood that there are numerous modifications and variations therefrom. Like numerals are used for like and corresponding parts of the various figures. While a limited number of examples are illustrated, it will be understood that there are numerous modifications and variations therefrom. 
     As set forth above, coating of print media, either as an undercoat or as an overcoat, is desired for a variety of print applications in order to improve print quality, ink adhesion, rub resistance, or durability of a printed image. In examples herein, a coating is provided by a standalone coater using a liquid electrophoretic coating composition. Thereby, it is facilitated a convenient manner of providing a coating on print media. 
     A liquid electrophoretic coating composition as used herein refers to a composition including a carrier liquid (e.g., a hydrocarbon oil) and an electrophoretic element comprised of a coating component, i.e., one or more coating components. In the following, a liquid electrophoretic coating composition is referred to as electro-coating. An electro-coating is to improve print quality, ink adhesion, rub resistance, or durability of a printed image pattern (i.e., a pattern for reproducing an image printed using a composition comprised of one or more colorants) on the print medium. More specifically, an electro-coating is characterized by being selected for providing an overcoat or an undercoat of a printed image that improves at least one characteristic of the printed image on the substrate. 
     Use of electro-coatings as described herein also facilitates providing coatings in an addressable manner so that portions of a print medium may be selectively coated. Coating units for providing addressable coatings are described below with respect to  FIG. 5 . However, an electro-coating as described herein may also be provided uniformly on a print medium. 
     A standalone coater as used herein is intended to encompass coaters in which a printing engine is configured to provide only a coating and not a printed image. This is in contrast to other systems in which a printing engine is used to provide colorant (e.g., ink) and coating on a substrate. For example, such other systems may be provided to supply ink and coating to the same chargeable surface of a photo imaging assembly (examples of photo imaging assemblies are described below). Thereby, standalone coaters as described herein may be built with a relatively simple configuration as compared to systems in which the printing engine is also to print an image pattern using one or more colorants. Thereby, cost per page of coated substrate might be kept relatively low while being able to provide a relatively good quality of a manufactured coating. 
     Some examples of standalone coaters are illustrated with respect to  FIG. 1 .  FIG. 1  is a block diagram of a standalone coater  100  including a coating supply assembly  102 , a photo imaging assembly  106  configured to coat a print medium  108 . Coating supply assembly  102  is to supply an electro-coating  104  to photo imaging assembly  106 . Photo imaging assembly  106  is to receive electro-coating  104  from coating supply assembly  102 . More specifically, during operation of coater  100 , electro-coating  104  is received on a chargeable surface (not shown in  FIG. 1 ; see, e.g., chargeable surface  702   a  in  FIG. 7 ) of photo imaging assembly  102  so as to form a coating layer  110  thereon. It will be understood that coater  100  may be comprised of further components for performing more specific functions. Some examples of further components of a coater are illustrated below. 
     Coating layer  110  is formed on the chargeable surface using electrostatic forces. In order to generate the electrostatic forces, generally, a voltage difference is imposed between the chargeable surface and an electro-coating applying element (e.g., a developer roller or an electrophoretic electrode). The coating layer may be formed according to some selected pattern. In such examples, the chargeable surface is charged in an addressable manner, as illustrated with respect to  FIG. 3 . In other examples, the coating layer is formed uniformly. In such examples, the chargeable surface may be charged uniformly. For example, a charging roller may be used to generate such uniform charging. 
     A coating supply assembly as used herein refers to an assembly suitable to supply electro-coating  104  to photo imaging assembly  106  so that coating layer  110  can be formed on a chargeable surface of photo imaging assembly  106  via electrostatic forces acting on the electro-coating. A coating supply assembly may be constituted by a binary coating developing unit configured to supply electro-coating through a developer roller, as illustrated with respect to  FIG. 9 . Alternatively, a coating supply assembly may be constituted by an electrophoretic electrode, as illustrated with respect to  FIG. 10 . Other types of coating supply assemblies are foreseen, for example, a spraying assembly that supplies electro-coating by spraying it onto the chargeable surface of the photo imaging assembly. 
     A photo imaging assembly as used herein refers to an assembly suitable to receive on a chargeable surface thereon a layer of electro-coating. In some examples, a photo imaging assembly includes a drum with a chargeable surface (see the examples below with respect to  FIGS. 7 and 8 ). In other examples, a photo imaging assembly includes an endless belt with a chargeable surface to receive the electro-coating. The endless belt is supported by a plurality of cylinders. 
     The chargeable surface may be a surface of a dielectric layer on the drum. Such a dielectric layer may, for example, have a dielectric thickness between 1 and 10 μm or, more specifically, a dielectric thickness between 2 and 7 μm. Such a dielectric layer may have, for example, a dielectric constant between 3 and 10. Such a dielectric layer may, for example, be constituted by a glass coating (e.g., a Heraeus SD2000 coating) on a layer of anodized aluminum. Such a dielectric layer may be implemented in the photo imaging assembly as a hard surface coating, which constitutes a durable, non-consumable component of the coater. Such a dielectric layer might be charged, for example, with a corona unit, or an ion gun (such components are further illustrated below). 
     In other examples, the chargeable surface may be a surface of a photoconductor film attached to the surface of a supporting element of the photo imaging assembly (e.g., a supporting drum or an endless belt, as described above). Generally, the photoconductor film is configured as a replaceable element of the photo imaging assembly. The photoconductor film may be charged by a light beam as further described below. 
     There are a variety of manners of transferring coating layer  110  to the print medium. In some examples, illustrated below with respect to  FIG. 7  coaters are configured to directly transfer coating layer  104  from the photo imaging assembly to the print medium. In other examples, illustrated below with respect to  FIG. 8 , coaters are configured to transfer coating layer  104  from the photo imaging assembly to the print medium through an intermediate transfer member. 
       FIG. 2  is a flow diagram  200  illustrating an example of operation of standalone coater  100  for manufacturing a coated print medium. At block  202 , coating supply assembly  102  supplies electro-coating  104  to photo imaging assembly  106 . At block  204 , coating layer  104  is formed on the chargeable surface of photo imaging assembly photo imaging assembly  106 . At block  206 , coating layer  110  is transferred to print medium  108 . As set forth above, coating layer  110  may be transferred directly or indirectly to print medium  108 . 
     In some examples herein, standalone coaters are configured to provide a digital coating layer, i.e., a coating that can be selectively applied to specific areas of a print medium. Such examples are illustrated with respect to  FIGS. 3 and 4 .  FIG. 3  shows a block diagram of a standalone coater  300  for providing addressable coatings  302 . Coater  300  includes a coating assembly  102  and a photo imaging assembly  106  analogous to those described above with respect to  FIG. 1 . In addition thereto, coater  300  further includes an addressable charger  202 . Addressable charger  202  is to selectively charge areas on a chargeable surface of photo imaging assembly  106 . Thereby, a coating layer can be formed on the photo imaging assembly according to a selected pattern. 
     Charger  202  may be operatively coupled with a raster image processor (not shown) to receive a raster pattern corresponding to a desired pattern of the coating on the print medium. Charger  202  may be configured to translate the received raster pattern on a latent image  204  on the chargeable surface of photo imaging assembly  106 . Latent image  204  is comprised of the selective charges imposed by charger  202 . 
     There are a variety of options for implementing addressable chargers. Generally, application of digital coatings does not require a resolution as high as the resolution to be provided for printing an image. For example, in some applications, addressability of the coating layer is to prevent coating of some specific areas which are relatively big as compared to the resolution limit of a printer for printing image patterns in commercial printing. For example, in an application, substrate edges are to remain uncoated. In other example applications, areas to receive adhesive are to remain uncoated (this can be in particular be the case for packaging applications). 
     This means that a coater as described herein may be configured relatively simply as compared to printing engines for printing image patterns. For example, such low resolution constraints facilitates a simplifying of the raster image processor for indicating areas of printing media to be coated as well as configuration of the addressable charger. In some examples, the standalone coater is configured to provide a resolution of at least 2 mm or, more specifically, 1 mm or, even more specifically, 0.5 mm. For example, a resolution of 1 mm might be sufficient for a digital coating, which corresponds to 25 dpi (dots per inch). This is to be compared to a resolution of about 800 dpi provided by some digital presses (e.g., an Indigo Digital Printing Press). This means an approximately 1000× simplification in the provision of a digital coating in comparison to the provision of a printed image pattern. 
     In some examples, the addressable charger is comprised of an addressable ion head. An addressable ion head includes one or more charge generating matrix sets that emits charges. The emitted charges are to be received by the chargeable surface of the photo imaging assembly so that a latent image can be formed thereon. A more detailed description of an addressable ion head that may be used to implement charger  202  may be found in U.S. Pat. No. 6,081,286, which is incorporated herein by reference in its entirety (to the extent in which this document is not inconsistent with the present disclosure) and in particular those parts thereof describing apparatuses for generating electrostatic charge images on a receptor surface. Ion guns facilitate a high addressability. 
     In other examples, the addressable charger is comprised of a corona unit with an addressable grid. Examples of such corona units that may be used to implement charger  202  may be found in U.S. Pat. No. 4,918,002, which is incorporated herein by reference in its entirety (to the extent in which this document is not inconsistent with the present disclosure) and in particular those parts thereof describing addressable charging. Ion guns and corona units are particularly suited for addressably charging photo imaging assemblies with a dielectric chargeable surface. Addressable corona units facilitate addressably charging that results in a relatively simple device which can be conveniently manufactured and operated. Further, an addressable corona might deliver a sufficiently high resolution. 
     In examples in which the chargeable surface corresponds to the surface of a photoconductor film, an addressable charger may be constituted of a scanning light imager configured to direct light upon the photoconductor. For example, such a scanning light imager may include a scanning laser which is moveable on the photoconductor film as it translates beneath the imager. Those portions of the photoconductor film which are impinged by the light discharge the background electrostatic charge to form a latent image upon its surface. The portions of the photoconductor that are not impinged by the laser maintain their respective background electrostatic charge. 
     It will be understood that addressable chargers are not limited to the above example and they encompass any charger device suitable to selectively charge areas on a chargeable surface of the photo imaging assembly. 
       FIG. 4  is a flow diagram  400  illustrating an example of operation of standalone coater  300  for manufacturing a coated print medium. At block  402 , addressable charger  202  selectively charges a chargeable surface of photo imaging assembly  106 . Latent image  204  is formed on the chargeable surface. At block  404 , coating supply assembly  102  deposits electro-coating  104  on the charged surface of photo imaging assembly  106  so as to form a coating layer thereon. Block  404  may be implemented analogously as block  204  described above with respect to  FIG. 2 . At block  404 , the pattern of the coating layer is determined by the selectively charging at block  402 . At block  406 , a patterned coating layer is transferred from the charged drum to print medium  108 . Block  406  may be implemented analogously as block  206  described above with respect to  FIG. 2 . 
     A standalone coater according as described herein may be a system dedicated to coating independent from a print system. More specifically, a standalone coater may be constituted as a system that is not suitable to print image patterns, but just for providing coatings as described herein. Image patterns might be (previously or subsequently) printed by an image print system physically separated from the standalone coater. Systems dedicated to coating may be of particular advantage for providing a dedicated coating system that manufacture coated substrates at a relatively low cost with a relatively high quality. In particular, as evidenced by examples herein, electro-coatings may be provided with a simplified printing engine. Further, a dedicated coater allows flexibility in the manufacturing of coated substrates. 
     In alternative examples, the coater is a standalone unit in an image printing system. In other words, a standalone coater may be integrated as a coater inline unit in an image printing system. An example of a printing system including a standalone coating unit is illustrated below with respect to  FIG. 5 . Integrating a coating unit in an image printing system as described herein facilitates increasing versatility of the printing system for providing a coating with a relatively high quality and a relatively simple, standalone, printing engine for providing the coating. 
       FIG. 5  is a block diagram of an image printing system  500  including a standalone coating unit  502  according to examples herein and a printing assembly  504 . Image printing system  500  is to provide a print medium  108  with a coating layer  510  and an image pattern  512 . 
     Coating unit  502  includes a photo imaging assembly  106  and an addressable charger  202 , which might be constituted analogously as the photo imaging assemblies and addressable chargers described above with respect to  FIGS. 1 and 3 . More specifically, photo imaging assembly  106  is to receive a liquid electrophoretic coating composition on a chargeable surface; addressable charger  202  is to selectively charge areas on the chargeable surface in a manner such that a layer of electro-coating can be formed on the photo imaging assembly according to a selected pattern. Image printing system  500  further includes a coating transfer area  506  in which the patterned coating layer  510 ′ is to be transferred over print medium  108 . 
     Printing assembly  504  is to selectively deposit colorant  514  on the print medium so as to form image pattern  512  on print medium  108 . More specifically, image printing system  500  further includes a colorant transfer area  516  in which ink is deposited on print medium  108  so as to form image pattern  512  thereon. Since coating unit  502  and printing assembly are implemented as standalone units in image printing system  500 , coating transfer area  506  and colorant transfer area  516  are spaced from each other. 
     According to examples, print medium  108  is translated between printing assembly  504  and coating unit  502  for sequentially providing coating layer  510  and image pattern  512 . Depending on the direction of translation, coating unit  502  is to provide an overcoat or an undercoat on print medium  508 . In the illustrated example, print medium  106  is translated from printing assembly  504  to coating unit  502 , i.e. along direction  518 . Thereby, coating layer  510  is provided as an overcoat on image pattern  512 . Analogously, print medium  108  might be translated from coating unit  502  to printing assembly  104 , so that a coating layer can be provided as an undercoat of the printing pattern. 
     Printing assembly  504  may be constituted by any suitable print engine suitable to form image pattern  512 . In some examples, printing assembly is to selectively deposit colorant by liquid electrophoretic (LEP) printing. LEP printing refers to printing using the principles of digital offset color technology. More specifically, printing assembly  504  may include a liquid electrophoretic (LEP) printing engine  520 . LEP printing engine  520  may be to create a printed image from digital data by forming an inked image on a photo imaging assembly using a LEP ink, transferring the inked image to a blanket element, and transferring the inked image from the blanket element to a substrate held by an impression element. Indigo Digital Printing Presses are examples of LEP printers. Combining a stand-alone coating unit with a LEP printer as stand-alone units facilitate a simple integration of coating an LEP printing in a single unit. Further, no modification in the LEP printing engine of the LEP printer is required. 
     According to some examples, printing assembly  504  is to selectively deposit ink by inkjet printing. For example, printing assembly  504  may include an inkjet printhead  522 . There are a variety of manners of configuring inkjet printhead  522  for inkjet printing an image. In some examples, inkjet printhead  522  is mounted on a movable carriage. During printing, the carriage traverses over the print medium for printing a portion of the image equivalent to a printhead swath. 
     In other examples, inkjet printhead  522  includes a full-width inkjet nozzle array. For example, the full-width inkjet nozzle array may constitute a printhead that spans the whole portion of the substrate to be printed. During printing, nozzles in a page wide array printhead are selectively fired to reproduce the image on the substrate. Such full-width inkjet presses are particularly convenient for industrial printing since they may achieve a higher productivity than scanning printheads. HP inkjet web presses are examples of such full-width inkjet presses. Coaters described herein might be particularly convenient for such industrial presses, since print quality, ink adhesion, rub resistance, or durability are factors that are particularly desirable in industrial printing. Further, combining a stand-alone coating unit with a full-width inkjet press as stand-alone units facilitate a simple integration of coating an industrial printing in a single unit, since a coater as described herein might be provided in a relatively simple configuration. 
     It will be understood that there are a plurality of manners of configuring printing assembly  504  and is not limited to the above examples. For example, printing assembly  504  may be a xerography apparatus for using a dry toner composition. Further, the constitution of colorant  514  generally depends on the used print engines. As used herein, a colorant refers to a composition suitable for reproducing an image when applied on a substrate. Examples of such a colorant are inks or dry toner. An ink, as used herein, refers to a liquid or paste that contains pigments or dyes and is usable to reproduce an image on a substrate via printing. Toner, as used herein, refers to a powder usable to reproduce an image on a substrate via xerography. 
       FIG. 6  is a flow diagram  600  illustrating examples of operation of image printing system  500  shown in  FIG. 5 . At block  402 , coating layer  510  is formed on print medium. Coating layer  512  is formed from electro-coating. Block  402  may be implemented using any of the flow diagrams illustrated above with respect to  FIGS. 2 and 4 , wherein the coater is integrated in a printed system. At block  404 , ink  514  is deposited on print medium  106  so as to form image pattern  512  on print medium  106 . As set forth above, ink  514  may be deposited using a variety of print methods such as, but not limited to, inkjet printing, or electrographic printing. Depending on the order of execution of blocks  402  and block  404 , coating layer  510  is an overcoat or an undercoat: if block  402  is executed firstly, coating layer  510  is an undercoat disposed between print medium  106  and image pattern  512 ; in contrast thereto, if block  404  is executed firstly, coating layer  510  is an overcoat disposed onto image pattern  512 . 
     In the following more specific examples of implementation of coaters  100 ,  300 ,  502  are illustrated with respect to  FIGS. 5 to 11 . It will be understood that the following examples, although depicted independently from a printing system, could also be integrated in an image printing system as illustrated above. 
     According to some examples herein, a coater is configured to directly transfer the coating layer from the photo imaging assembly to the print medium. Such examples are illustrated with respect to  FIG. 7 , which shows a block diagram of a standalone coater  700 . Coater  700  is illustrated as including a photo imaging assembly  106  comprised of an imaging drum  702 , a charging unit  704 , a coating supply assembly  102 , a heating unit  706 , a cleaning station  708 , and a charge erase station  710 . 
     Imaging drum  702  is comprised of a rotatable drum, which is configured to rotate in direction  712  so that the different processes described below for forming a coating layer  110  can be carried out on its surface  702   a  in the appropriate sequence. Surface  702   a  is a chargeable surface, which might be implemented using, for example, a dielectric material or a photoconductor, as set forth above with respect to  FIG. 1 . 
     Charging unit  704  may be implemented analogously as described above with respect to addressable charger  202 . In other examples, charging unit  704  is not addressable. For example, it might be constituted of a charging roller to generate a uniform charging on surface  702   a . A coater including a not addressable unit  704  is generally desirable for applications that merely require a uniform coating of a substrate. 
     Heating unit  706  is to promote transferring of a coating layer  110  formed onto surface  702  of imaging drum  702  to print medium  108 . For example, heating unit  706  is to heat photo imaging assembly  106 , or more specifically surface  702   a  of imaging drum  702 , so that the temperature difference between the photo imaging assembly and the print medium promotes transfer of the coating layer  110  on print medium  108 . Further, heating electro-coating may also induce that the coating layer becomes tacky, which might further promote transferring. Heating unit  706  may be to heat coating layer  110  while being carried by imaging drum  702 . For example, heating unit  706  may be to heat surface  702   a  of imaging drum  702  to approximately 100° C. to i) dry/evaporate at least a portion of the carrier (e.g., an isoparaffin oil) and ii) to cause coating carrying particles of the electro-coating to melt and blend into a smooth liquid plastic before reaching a further transfer area  506 , in which surface  702   a  of imaging drum  702  contacts print medium  108 . When heated coated layer  110  on imaging drum  702  contacts the cooler surface of print medium  108 , the electro-coating solidifies, adheres, and transfers to print medium  108 . In some examples, a heating unit may be combined with a condenser to receive a condensate carrier liquid from the electro-coating. 
     Heating unit  706  is an example of heating unit that might be implemented in heaters described herein to promote transfer of the coating layer to the print medium. There are different alternative locations for heating the electro-coating. In the example of  FIG. 7 , heating unit  706  heats the electro-coating on imaging drum  702 . In the example of  FIG. 8  (illustrated below), heating unit  804  heats electro-coating on transfer drum  806 . Moreover, a heating unit might be implemented as an external heater (see examples of  FIGS. 7 and 8 ). In other examples, heating units might be implemented as an internal heater. Internal and external heaters may also be combined in the same system. 
     The illustrated heating unit  706  is shown to be disposed externally to chargeable surface  702   a  of imaging drum  702 . Such external heating units may include further components for implementing external heating such as, but not limited to, radiation heating systems (e.g., an IR heater), air knifes, as well as air suction for collecting evaporated components of the electro-coating. In alternative examples, heating unit  706  is comprised of a heating system embedded into imaging drum  702  and configured to heat its surface  702 . Such an embedded heating unit may be comprised of an IR heater or of an array of electrical heating elements. Such an embedded heating unit may be combined with an external airflow generation system and condenser for collecting evaporated components of the electro-coating. 
     Cleaning station  708  is to clean a portion of the photo imaging assembly in an area downstream from the area in which the coating layer is transferred from the photo imaging assembly. More specifically, cleaning station  708  is to remove residual electro-coating from surface  702   a  of imaging drum  702 . Cleaning station  708  is disposed proximate to surface  702   a  and between transfer area  506  and charge erase station  710 . Cleaning station  708  is disposed downstream of charging unit  704  and coating supply assembly  102  to facilitate formation of a new coating layer on imaging drum  702 . Cleaning stations may be comprised of a combination of a blade and a wet sponge (both elements not shown) in order to collect and accumulate residual electro-coating on surface  702   a.    
     Charge eraser  710  is to remove residual charge from surface  702   a . In the illustrated example charge eraser  710  is disposed along chargeable surface  702   a  of imaging drum  702 . Further, in the example, charge eraser  710  is disposed proximate to surface  702   a  and between cleaning station  708  and charging unit  704 . Charge eraser  710  is disposed downstream of charging unit  704  to facilitate appropriate charging of imaging drum  702  so as to promote formation of coating layer  106  on its surface  702   a . In examples in which chargeable surface  702   a  forms part of a dielectric layer, charge eraser  710  may be comprised of an AC corona or an AC Charge Roller unit. In examples in which surface  702   a  forms part of a photoconductor layer, charge eraser  710  may be implemented by a light-emitting diode (LED) erase lamp. 
     According to some examples herein, a coater is configured to indirectly transfer the coating layer from the photo imaging assembly to the print medium. For example, the coating layer can be transferred to the print medium via an intermediate transfer member. The intermediate transfer member effects an offset printing of the coating layer on the print medium. Such examples are illustrated with respect to  FIG. 8 . An intermediate transfer member may be particularly advantageous for coating rough print media (e.g., print media with roughness above 0.5 μm or, more specifically, above 1 μm). An intermediate print member facilitates resilient contact with the print medium surface so that the transfer surface can better conform to the print medium surface. 
       FIG. 8  shows a block diagram of a standalone coater  800  according to examples. Coater  800  is illustrated as including a photo imaging assembly  106  comprised of an imaging drum  702 , a charging unit  704 , a coating supply assembly  102 , a cleaning station  708 , and a charge erase station  710 , which are constituted similarly as set forth above with respect to  FIG. 7 . In addition thereto, coater  800  includes i) an intermediate transfer member  802 , comprised of an intermediate drum  806 , and ii) a heater unit  804  disposed to heat intermediate transfer drum  802 . 
     Intermediate transfer member  802  is to transfer coating layer  110  from photo imaging assembly  702  to print medium  108 . Thereby, an intermediate coating layer  110   i  is formed on drum  806  in its section comprised between an intermediate transfer area  808  (in which electro-coating is transferred between imaging drum  702  and intermediate drum  806 ) and a transfer area  506  (in which electro-coating is transferred between intermediate drum  806  and print medium  108  so as to form coating layer  110 ′ thereon). Intermediate drum  806  is rotatable. In the illustrated example, during operation of coater  800 , intermediate drum  806  rotates along direction  814  (opposite to the direction of rotation of imaging drum  702 ) so that electro-coating can be conveyed from imaging drum  702  towards print medium  108 . 
     Intermediate transfer member  806  includes an exterior transfer surface  812 , which may be resiliently compressible and/or may be electrostatically chargeable. A transfer surface  812 , which is resiliently compressible facilitates that surface  812  conforms and/or adapts to irregularities on print medium  108 . Additionally, because surface  812  is configured to be electrostatically charged, surface  812  may be charged to a voltage to facilitate the transfer of electro-coating from the surface  702  of imaging drum  702  to the transfer surface  812 . In some examples, transfer surface  812  has a compressibility that reduces the likelihood of damage caused by permanent deformation of surface  812 . 
     Intermediate coating layer  110   i  may be dried and heated by heating unit  804  before transfer to substrate  108 . Further, heating unit  804  may include a hot air knife to direct hot air on the surface of transfer drum  806 . Alternatively, or in addition to heating unit  804 , transfer drum  806  may include an internal heating unit (not shown) which heats the surface  812  for promoting transfer of intermediate coating layer  110   i . Such an internal heating unit may heat surface  812  to a temperature between 70 and 150 degrees. 
     In the illustrated example of  FIG. 8 , intermediate transfer member  802  includes a drum  816  and an external blanket  818 . Drum  816  in this example is a cylinder that supports blanket  818 . The cylinder may be constructed using material(s) having a relatively low thermal conductivity and/or heat resistance. Blanket  818  in the illustrated example wraps about drum  816  and includes surface  812 . Blanket  818  may be constructed using a resiliently compressible layer and an electrically conductive layer, which facilitates transfer surface  812  to conform on the print medium surface and to be electrostatically charged. In some alternative examples, intermediate transfer member  802  includes an endless belt supported by a plurality of cylinders, including a transfer cylinder, in contact and/or in close proximity to chargeable surface  702  and the print medium  108  (or the area configured to support the print medium such as an impression cylinder). 
     Heating unit  804  is to heat intermediate transfer member  802 . Heating unit  804  is configured in a manner that the temperature difference between the intermediate transfer member and the print medium promotes transfer of the coating layer on the print medium. Heating unit  804  may be configured analogously as heating unit  706 , with the exception that it is configured to operate on intermediate transfer member  802 . 
     In some examples, as illustrated in  FIG. 8 , coaters as described herein further includes an impression member  820  to support print medium  108  against the element of the coater from which coating  110  is being transferred. In the example of  FIG. 8 , impression member  820  is comprised of an impression cylinder  822 . The example impression cylinder  822  of  FIG. 8  is a cylinder located adjacent to intermediate transfer member  802  so as to form a nip  824  between intermediate transfer member  802  and cylinder  822 . During operation of coater  800 , print medium  108  is fed between intermediate transfer member  802  and impression cylinder  822 . Intermediate coating layer  110   i  is transferred from intermediate transfer member  802  to print medium  814  at nip  824 . Although impression member  820  is illustrated as a cylinder, it may alternatively be implemented using an endless belt and/or a stationary surface against which intermediate transfer member  802  moves. An analogous impression member may be implemented in the example of  FIG. 7  to held print medium  108  against imaging drum  702 . 
     In some examples herein, a stand-alone coater further includes a fuser device to heat the coating layer on the print medium.  FIG. 8  shows, as an example, fuser  826  comprised of two rollers  826   a ,  826   b , through which print medium  108 , after receiving coating layer  110 ′ can be fed. At least one of rollers  826   a ,  826   b  might include a heating element in order to heat coating layer  110 ′ on print medium  108 . Thereby, it is facilitated to improve adhesion of coating layer  110 ′. 
     It will be understood that the configuration of elements in standalone coater  800  is merely illustrative. For example, the whole processing illustrated with respect to  FIG. 7  and  FIG. 8  may be rotated counter-clockwise 90 to 270 degree in order to simplify coating supply assembly  102 . 
     In the examples above, reference is made to a coating supply assembly. As set forth above, there is a variety of manners of constituting a coating supply assembly. According to some examples, illustrated with respect to  FIG. 9 , a coating supply assembly includes a developer roller. The developer roller is to receive a layer of electro-coating to be supplied to the photo imaging assembly. The electro-coating is supplied from the developer roller by the electrostatic forces due to a voltage difference between the developer roller and the chargeable surface in the photo imaging assembly. 
     Such a developer roller may be implemented as a binary coating developer unit as specifically illustrated with respect to  FIG. 9 . A binary coating developer unit may be implemented analogously to a binary ink developer unit as described in U.S. Pat. Nos. 5,596,996 or 5,610,694, which are incorporated herein by reference in its entirety (to the extent in which these documents are not inconsistent with the present disclosure) and in particular those parts thereof describing units for developing a liquid composition comprised of chargeable particles. 
       FIG. 9  is a partial cut-away view of an example of a binary coating developer (BCD)  900  of a coater based on electro-coatings as illustrated above with respect to  FIGS. 1 to 8 . BCD  900  is associated to imaging drum  702  of photo imaging assembly  106 , which may be constituted as described above with respect to  FIG. 7  or  8 . A tank  926  is connected to BCD  900 . Electro-coating  902  in tank  926  may be transported to BCD  900  as described in greater detail below. 
     BCD  900  may further include a reservoir  904  that stores electro-coating  909 . Electro-coating  909  may be pumped to reservoir  904  from tank  226 . A channel  906  extending from reservoir  904  enables electro-coating  909  to flow to a developer roller  908 . Electro-coating from developer roller  908  transfers to imaging drum  702  by way of electrostatic forces. 
     More specifically, developer roller  908  includes a main electrode  910  associated therewith that serve to electrically charge electro-coating  909 . Main electrode  902  is sometimes referred as the first electrode. A development voltage is applied between main electrode  902  and developer roller  908  to generate an electric current to charge electro-coating  909 . Thereby, in response to the electric current, a layer of electro-coating can be formed onto developer roller  908 . This electro-coating layer is, generally, a function of the development voltage applied by main electrode  910  relative to developer roller  908 . The electro-coating layer is to be transferred onto imaging drum  702  at a transfer region  924  according to an electrostatic latent image formed on imaging drum  702  as illustrated above with respect to  FIG. 2 . 
     Developer roller  908  rotates in a direction  912  as viewed from  FIG. 9 . As described in greater detail below, the rotation of developer roller  908  and the electric field applied between developer roller  908  and main electrode  910  enable electro-coating  909  charged by main electrode  910  to be applied to developer roller  908 . In addition, the rotation enables electro-coating to be removed from developer roller  912  and applied to imaging drum  702  as described in greater detail below. An additional transfer member (not shown) may also be provided between developer roller  912  and chargeable surface  702   a  of imaging drum  702 . 
     BCD  900  further includes a squeegee electrode  914 . Squeegee electrode  914  is configured as a roller that, in operation, rotates in a direction  916  as viewed from  FIG. 9 . Direction  916  is opposite direction  912  (rotation direction of developer roller  908 ). A voltage may be applied between squeegee electrode  914  and developer roller  908 . For example, squeegee electrode  914  may be electrically connected to a voltage supply (e.g., voltage supply  106 , depicted in  FIG. 1 ). The rotation of squeegee electrode  914  and the voltage applied to the squeegee electrode  914  facilitates further charging electro-coating on a section of developer roller  908  passing beneath squeegee electrode  914 . 
     In some examples, BCD  900  may further include a cleaner roller  920  adjacent to developer roller  908  at a region downstream (relative to the rotation direction  912  of developer roller  908 ) of an electro-coating transfer region  924 . Cleaner roller  920  is to clean any excess electro-coating remaining on a section developer roller  908  after transferring electro-coating from that section onto imaging drum  702 . Cleaner roller  920  may collaborate with further rollers  922  for conveying excess electro-coating back to reservoir  904 . Thereby, excess electro-coating may be re-utilized for forming further electro-coating development layers onto development roller  908 . 
     It will be understood that coating supply assemblies are not limited to systems based on developer rollers. For example, but not limited to, coating supply assemblies may be comprised of an electrophoretic electrode as illustrated with respect to  FIG. 10 .  FIG. 10  shows a partial block diagram of a standalone coater  1000  according to examples. Coater  1000  includes an electrophoretic electrode  1002 . Electrophoretic electrode  1002  is shown as a fixed electrode disposed in the proximity of imaging drum  702 . Electrode  1002  is shaped to partially conform surface  702   a  of drum  702 . Electrode  1002  includes a channel  1004  through which electro-coating  1006  can be fed into the space formed between electrode  1002  and surface  702   a . Electro-coating  1006  may be fed from a catch tray  1008 . 
     A voltage difference between electrode  1002  and surface  702  generates electrostatic forces that promote flowing of electro-coating towards surface  702 . Further, charges on surface  702  promotes that electro-coating adheres thereto so as to form a coating layer thereon. If charges on surface  702  have been created in an addressable manner, the coating layer is formed according to a selected pattern. 
     During operation, as imaging drum  702  rotates along direction  712 , electro-coating is distributed across surface  702   a . Residual electro-coating  1008  may flow back from electrode  1002  into catch tray  1008 . A squeegee unit  1010  may be disposed upstream from electrode  1002  to compact the coating layer formed on imaging drum  702  and/or remove any excess electro-coating  1012 . Excess electro-coating  1012  may flow back from squeegee unit  1010  into catch tray  1008 . Squeegee unit  1010  may be comprised, for example, of a squeegee roller configured to rotate against surface  702   a  of imaging drum  702 . 
       FIG. 11  is a flow diagram  1100  illustrating an example of operation of any of standalone coaters  700  or  1100  for manufacturing coated print medium  108  illustrated above with respect to  FIGS. 7 and 8 . At block  1102 , charging unit  704  charges imaging drum  702  and, more specifically, its chargeable surface  702 . At block  1104 , coating supply assembly  102  supplies electro-coating on a charged portion of imaging drum and, more specifically, at a charged portion of chargeable surface  702   a.    
     At block  1106 , heating unit  706  heats imaging drum  702 . Imaging drum  702  is heat so that the temperature difference between imaging drum  702  (and more particularly, its surface  702   a ) and print medium  108  promotes transfer of coating layer  110  from surface  702   a  to print medium  108 . Thereby, a coating layer  110 ′ is formed on the surface of print medium  108  facing imaging drum  702 . 
     At block  1108 , coating layer  110  is transferred at transfer area  506  to print medium  108  so as to form coating layer  110 ′. The manner in which coating layer  110  is transferred at transfer area  506  depends on whether coating layer  110  is transferred directly or indirectly to print medium  108 . For example, looking at  FIG. 8 , it can be understood that block  1108  may be implemented by a direct transfer from surface  702   a  of imaging drum  702  to print medium  108 . This approach facilitates a simpler implementation of the coater, which might be particularly appropriate for substrates with relatively low roughness. Looking at  FIG. 9 , it can be understood that block  1108  may be implemented by an indirect transfer from surface  702   a  of imaging drum  702  to print medium  108  via intermediate transfer member  802 . This approach is a more complex implementation of the coater, however it facilitates that the coating layer is appropriately transferred on the surface of print medium  108 . This might be particularly appropriate for substrates with relatively high roughness. 
     It will be understood that the composition of coating layer  110  on surface  702  and coating layer  110 ′ disposed on print medium  108 . For example, coating layer  110  may still include carrier components that are evaporated at the heating stage. Those components are, at least partially, not transferred to print medium  108  as part of coating layer  110 ′. 
     Coated print medium  108  may be fed into transfer area  506  with a blank surface or with an image pattern already printed thereon. In the former examples, coating layer  110  is an undercoat for subsequent image printing thereon. In the latter examples, coating layer  110  is an overcoat to cover the image pattern (see, e.g., the illustrated example in  FIG. 5 ). 
     Referring back to  FIG. 11 , at block  1110 , cleaning station  7011  cleans a portion of imaging drum  702  from which coating layer  110  has already been transferred to print medium  108 . Further, cleaning is performed before that drum portion is re-charged or re-supplied with electro-coating. At block  1110 , charge erase station  710  erase charges on a portion of imaging  702  from which coating layer  110  has already been transferred to print medium  108 . As in the illustrated example of  FIG. 11 , it might be advantageous to erase charges on a drum portion on which cleaning station  708  has already removed residual electro-coating. 
     Subsequently to a process as exemplified by flow diagram  1100 , coated print medium  108  may be further processed. For example, in case coating layer is an undercoat, an image pattern may be printed thereon either using a print assembly integrated in the same apparatus as coater  700  or in a printing system at a different location. In other examples, coated print medium  108  may be stored or shipped as a manufactured product so that a third-party may perform printing thereon. Alternatively, coated print medium  108 , might be a finished product. 
     In the following, some specific details of electro-coatings are set forth. An electro-coating is comprised of a carrier liquid (e.g., a hydrocarbon oil) and an electrophoretic element comprised of a coating component. An electro-coating may also include a surfactant/charge director to facilitate charging of the electrophoretic element. Depending on the specific application of the coating, the composition may include additional components. An electro-coating may be based on the same functional principles as a liquid electrophoretic (LEP) ink (e.g., Electro Ink, available from Hewlett-Packard). However, an electro-coating is characterized by being selected for providing an overcoat or an undercoat of a printed image that improves at least one characteristic of the printed image on the substrate (e.g., print quality, ink adhesion, rub resistance, or durability of the printed image pattern on the print medium). Further, some specific components used with LEP inks may be absent from an electro-coating (e.g., heavy oils). An electro-coating may be characterized by the absence of colorant. In some examples, an electro-coating results in a transparent coating layer. In other examples, an electro-coating results in a non-transparent coating layer. 
     According to some examples, an electro-coating comprises a carrier comprised of a hydrocarbon oil. In some more specific examples, a carrier is comprised of an alkane. Oil carriers suitable for the embodiments disclosed herein include aliphatic hydrocarbon oils. Specific examples include ISOPAR oils G through L (Exxon Mobil Corp., Fairfax Va.) as aliphatic hydrocarbon oils. Other aliphatic oils may also be suitable, such as odorless mineral spirits, or any nonconductive isoparaffin oil. In contrast to other coatings, (e.g., UV coating, which is toxic and produces bad odors) a hydrocarbon based electro-coating is not toxic both for odor and the image itself. Moreover, a hydrocarbon based electro-coating facilitates an energy-efficient coating process. 
     An LEP element of an electro-coating is characterized by being chargeable and movable within the carrier liquid such that an electro-coating can originate a coating layer using electrostatic forces as described above. Suitable LEP elements may be comprised of a polymer blend. An electro-coating may be comprised of more than one polymer blend. The polymer blend is chosen to provide a coating on the print medium after a coating process as described above is performed. Further components for interacting with ink on the print medium might be embedded into the LEP element. For example, a treatment composition for improving ink adhesion on the substrate might be included on the LEP element. An electro-coating may be constituted such that it is electrically compressible by the developing unit (e.g., a BID as illustrated with respect to  FIG. 9  or a electrophoretic electrode as illustrated with respect to  FIG. 10 ) to increase to, at least, 30% the concentration of LEP elements in the electro-coating. 
     An electro-coating may also include a charge director that imparts a charging of the LEP elements. It will be understood that there are a variety of charge directors that can be used with electro-coatings. Further, it will be understood that the charge director may impart a negative or positive charge to the LEP elements depending upon the specific configuration of the coater and, more specifically, depending upon how the chargeable surface in the photo imaging assembly is being charged. An electro-coating may also include one or more surfactant components, which might be used to improve the separation of LEP particles in the electro-coating. It will be understood that there are a variety of surfactant components that can be used with electro-coatings. Examples of polymer blends, charge directors, and surfactant components are described in, for example, U.S. Pat. No. 7,078,141, which is incorporated herein by reference in its entirety (to the extent in which this document is not inconsistent with the present disclosure) and in particular those parts thereof describing polymer blends, surfactants and charge directors. 
     The techniques for coating a print medium illustrated herein facilitate providing a coating with a relatively high quality. Further, the spatial location of the coating may be made addressable using a stand-alone coater, which can be built relatively simple as compared to a printing apparatus for printing image patterns on the print medium. For example, a stand-alone coating as described herein does not require a complex data streaming as necessary for printing an image. Further, as compared with other liquid electrophoretic engines, a stand-along coater might be built with a single developer unit. 
     Coating techniques described herein facilitate a fast and energy efficient drying of the coating. For example, drying of a 0.2 m long print medium may consume approximately 3 kW per 12 inch format width. As a comparison, aqueous coating may require a massive drying unit to dry several microns of water; for example, to achieve 1 μm of solid approximate a 10 μm coating at 10% may be needed; aqueous coating drying of a 0.2 m long print medium may consume approximately 25 kW per 12 inch format width. Moreover, water based coatings may introduce integration challenges when used in-line with a printing assembly due to insufficient drying. 
     Techniques using electro-coatings as described above also facilitate formation of a relatively thin layer. For example, a coating layer based on electro-coating may have a thickness of less than 2 μm or, more specifically, less than 1.5 μm such as approximately 1 μm. Further, since fusing is not necessarily required for transferring the coating layer and print media may be modestly warmed during coating as described herein, print media that may be damaged by a relatively high temperature can also be coated. An example of such print media are vinyl print media or low temperature acrylic media. 
     As set forth above, some examples of coating techniques disclosed herein are aimed to addressable coatings, i.e., techniques to provide a coating layer at selected portion of a print media. An addressable coating might also be referred to as digital coating. Digital coating has a wide range of applications In contrast to other digital coating methods (e.g., based on inkjet), coating techniques herein, through the use of electro-coatings, facilitate a wider range of coatings. For example, metallic coatings might be difficult to formulate for inkjet printing. 
     In an example application, a transparent print medium is to be coated. The transparent print medium might be characterized by relatively low ink absorption so that an undercoat for promoting ink adhesion to the print medium is desirable. Further, it might be desired to leave some parts of the print medium transparent and other opaque. Using techniques described herein, the transparent print medium can be selectively coated using an electro-coating that results in an opaque undercoat. Subsequently, an image pattern might be printed on the undercoated areas of the print medium. Other example application where addressability might be particularly convenient is in the coating of certain adhesive print media, where it is desired to leave some medium areas uncoated for adhesive. 
     In the foregoing description, numerous details are set forth to provide an understanding of the examples disclosed herein. However, it will be understood that the examples may be practiced without these details. While a limited number of examples have been disclosed, numerous modifications and variations therefrom are contemplated. It is intended that the appended claims cover such modifications and variations. Further, flow charts herein illustrate specific block orders; however, it will be understood that the order of execution may differ from that which is depicted. For example, the order of execution of two or more blocks may be scrambled relative to the order shown. Also, two or more blocks shown in succession may be executed concurrently or with partial concurrence. Further, claims reciting “a” or “an” with respect to a particular element contemplate incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Further, the terms “include” and “comprise” are used as open-ended transitions.