Patent Publication Number: US-11029619-B2

Title: Print sequence in an electrophotographic printer

Description:
BACKGROUND 
     Electrophotographic printing refers to a process of printing in which a printing substance (e.g., a liquid or dry electrophotographic ink or toner) can be applied onto a surface having a pattern of electrostatic charge. The printing substance conforms to the electrostatic charge to form an image in the printing substance that corresponds to the electrostatic charge pattern. An electrophotographic printer may use digitally controlled lasers to create a latent image in a charged surface of an imaging element such as a photo imaging plate (PIP). In this process, a uniform static electric charge is applied to the photo imaging plate and the lasers dissipate charge in certain areas creating the latent image in the form of an invisible electrostatic charge pattern corresponding to one “separation” of the image to be printed. An electrically charged printing substance, in the form of dry or liquid toner, is then applied and attracted to the partially-charged surface of the photo imaging plate, recreating a color separation, in the form of a layer of printing substance, of the desired image. 
     In certain electrophotographic printers, a transfer member, such as an intermediate transfer member (ITM) is used to transfer developed toner to a print medium. For example, a developed image, comprising toner aligned according to a latent image, may be transferred from a photo imaging plate to a transfer blanket of an intermediate transfer member. This transfer occurs via predominantly electrical and mechanical forces that exist between the charged toner and the intermediate transfer member which is often biased at a particular voltage level. Pure mechanical force, using zero electrical potential difference between the blanket of the intermediate transfer member and toner produces poor print quality. From the intermediate transfer member, the toner is transferred to a desired substrate, which is placed into contact with the transfer blanket. 
     At least two different methodologies may be used to print multi-color images on an electrophotographic printer. These involve the generation of multiple separations, in the form of multiple layers of a printing substance, where each separation is a single-color partial image. When these separations are superimposed, they result in the desired full color image being formed. In a first methodology, a color separation layer is generated on the photo imaging plate, transferred to the intermediate transfer member and is finally transferred to a substrate. Subsequent color separation layers are similarly formed and are successively transferred to the substrate on top of the previous layer(s). This is sometimes known as a “multi-shot” imaging sequence. In a second methodology, a “one-shot” imaging process is used. In these systems, the photo imaging plate transfers a succession of separations to the transfer blanket on the intermediate transfer member, building up each separation layer on the blanket. Once a predetermined number of separations are formed on the transfer blanket, they are all transferred to the substrate together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features of the present disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate features of the present disclosure, and wherein: 
         FIG. 1  is a schematic diagram showing a cross section of a print engine in a liquid electrophotographic printer according to an example; 
         FIG. 2  is a flow diagram showing a method of printing images in a liquid electrophotographic printer, according to an example; 
         FIGS. 3 a  and 3 b    show a one-shot print sequence, according to an example; 
         FIGS. 4 a -4 c    are tables showing example print sequences for four, three and five color separations, respectively; 
         FIGS. 5 a -5 c    are tables showing example print sequences for four, three and five color separations, respectively, in which a longer voltage rise or fall than that of  FIGS. 4 a -4 c    occurs; and 
         FIG. 6  is a non-transitory computer readable storage medium comprising a set of computer-readable instructions to be carried out by a processor, according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details of certain examples are set forth. Reference in the specification to “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least that one example, but not necessarily in other examples. 
     As described herein, an example electrophotographic printer in the form of a liquid electrophotographic (LEP) printer comprises an imaging element such as a photo imaging member, which can be referred to as a photo imaging plate (PIP). The photo imaging plate may be implemented, for example, as a drum or a belt. A charging element charges the photo imaging plate and a latent image is generated on the photo imaging plate. At least one image development unit deposits a charged layer of printing fluid onto the photo imaging plate. In one example, each image development unit deposits a different colored layer of printing fluid onto the photo imaging plate. Those skilled in the art will appreciate that some areas of the photo imaging plate will be charged, and charge in some other areas will have been dissipated by the lasers in generating the latent image. The areas where the layer of printing fluid is applied will form the inked image and the remaining areas will be background areas which do not contain printing fluid. An example printing fluid in the form of liquid toner comprises ink particles and a carrier liquid. The ink or pigment particles are charged and may be arranged upon the photo imaging plate based on a charge pattern of a latent image. The inked image comprises ink particles that are aligned according to the latent image. In an example, the ink particles may be in the order of about 1-2 microns in diameter. 
     An intermediate transfer member (ITM) receives the inked image from the photo imaging plate and transfers the inked image to a print substrate. In order to transfer the image from the photo imaging plate to the ITM, the photo imaging plate and the ITM may engage one another and move relative to one another. For example, the photo imaging plate and the ITM may rotate relative to one another. In one example, the ITM is heatable. The ITM may comprise a drum or belt wrapped with a blanket. In an example, the ITM is supplied with a high voltage, such as +500V to +600V, in order for the first electrical transfer of printing fluid from the PIP to the blanket. A second transfer, from the blanket to a print substrate, takes place as the ink comes into contact with the substrate, owing to a temperature differential between the blanket, which has been heated, and the cooler substrate; the ink solidifies, sticks to the substrate and peels off the blanket, leaving the blanket clean and ready to accept a new ink layer. However, in the case of printing to a metallized substrate, electrostatic discharge issues can occur owing to the high voltage that is applied to the ITM drum. 
     In order to allow printing on a conductive substrate, cumbersome workarounds may be employed in comparative systems to prevent the occurrence of high voltage breakdown between the biased ITM and the substrate. These voltage breakdowns are exhibited as violent sparks on the substrate, which can damage it. Comparative solutions may involve the use of insulating ITM drum bearings which are expensive. Furthermore, these bearings have a short life span meaning difficult, regular maintenance is involved. 
     In order to mitigate such discharge issues, the high voltage applied to the ITM drum can be turned off when the second transfer is taking place. However, this is not practical when a “two-page” print is being carried out by the ITM, that is, when two separate images are being developed on separate portions of the ITM. In such a situation, two portions of the ITM are in different stages of image development at a given moment, and a first image cannot be transferred to a conductive substrate simultaneously to the ITM receiving a color separation of a second image from the PIP. 
     In the present examples, a sequence of separation printing, which includes “null” separations between ink color separations, allows a first transfer to take place when there is no print substrate in contact with the ITM blanket (and conversely, the print substrate is printed to during the null separation when there is no “first transfer” taking place between the PIP and the blanket). A null separation occurs when there is no transfer of a color separation from the PIP to the ITM blanket as the PIP and ITM move, e.g. rotate, relative to one another. For example, a null separation may involve a period where there is no latent image on the PIP or no image development unit is engaged with the PIP, such that no liquid toner is applied by the image development units. This is turn leads to a period where there is no developed image (e.g. in the form of a layer of ink) to transfer from the PIP to the ITM. The null separations are inserted to eliminate the electrostatic discharge issues noted above, while ensuring an efficient print cycle in a two-page print process. Such a print sequence can also take into account the rise and fall time of the high voltage power supply provided to the ITM, e.g. may allow the voltage to be reduced or turned off for longer than the exact substrate contact time. 
       FIG. 1  is a schematic diagram showing a liquid electrophotographic (LEP) printer  100  in accordance with an example, although it should be appreciated that other examples may be printers that use a dry printing substance. Liquid electrophotography, sometimes also known as Digital Offset Color printing, is the process of printing in which printing fluid such as liquid toner is applied onto a surface having a pattern of electrostatic charge (i.e. a latent image) to form a pattern of liquid toner corresponding with the electrostatic charge pattern (i.e. an inked image). This pattern of liquid toner is then transferred to at least one intermediate surface, and then to a print medium or substrate. During the operation of a digital liquid electrophotographic system, ink images are formed on the surface of a photo imaging plate. These ink images are transferred to the blanket of an intermediate transfer member and then to a print medium. 
     According to the example of  FIG. 1 , a latent image is formed on a photo imaging member, which can be referred to as a photo imaging plate (PIP)  110  by rotating a clean, bare segment of the PIP  110  under a charging element  105 . The PIP  110  in this example is cylindrical in shape, e.g. is constructed in the form of a drum, and rotates in a direction of arrow  125 ; however, a photo imaging member or photo imaging plate may be planar or part of a belt-driven system. The charging element  105  may include a charging device, such as corona wire, a charge roller, scorotron, or any other charging device. A uniform static charge is deposited on the PIP  110  by the charging element  105 . In one example, a voltage of between −900V and −1100V is applied to the charging element  105  to enable charging. As the PIP  110  continues to rotate, it passes an imaging unit  115  where one or more laser beams dissipate localized charge in selected portions of the PIP  110  to leave an invisible electrostatic charge pattern that corresponds to the image to be printed, i.e. a latent image. In some implementations, the charging element  105  applies a negative charge to the surface of the PIP  110 . In other implementations, the charge is a positive charge. The imaging unit  115  then locally discharges portions of the PIP  110 , resulting in local neutralized regions on the PIP  110 . 
     In the described example, printing fluid such as ink is transferred onto the PIP  110  by at least one image development unit  120 . An image development unit may also be referred to as a Binary Ink Developer (BID) unit. There may be one image development unit  120  for each ink color. During printing, the appropriate image development unit  120  is engaged with the PIP  110 . The engaged image development unit  120  presents a uniform film of ink to the PIP  110 . The ink contains electrically-charged pigment particles which are attracted to the opposing charges on the image areas of the PIP  110 . The PIP  110  now has a single color ink image on its surface, i.e. an inked image or separation. In other implementations, such as those for black and white (monochromatic) printing, one or more ink developer units may alternatively be provided. 
     The ink may be a liquid toner, comprising ink particles and a carrier liquid. The carrier liquid may be an imaging oil. An example liquid toner ink is HP ElectroInk™. In this case, pigment particles are incorporated into a resin that is suspended in a carrier liquid, such as Isopar™. The ink particles may be electrically charged such that they move when subjected to an electric field. Typically, the ink particles are negatively charged and are therefore repelled from the negatively charged portions of PIP  110 , and are attracted to the discharged portions of the PIP  110 . The pigment is incorporated into the resin and the compounded particles are suspended in the carrier liquid. The dimensions of the pigment particles are such that the printed image does not mask the underlying texture of the print substrate, so that the finish of the print is consistent with the finish of the print substrate, rather than masking the print substrate. This enables liquid electrophotographic printing to produce finishes closer in appearance to offset lithography, in which ink is absorbed into the print substrate. 
     The ink is transferred from the PIP  110  to the ITM  130 . The ITM  130  may also be known as a blanket cylinder or a transfer element and may take the form of a rotatable drum, belt or other transfer system. In the example of  FIG. 1 , the ITM  130  rotates in the direction of arrow  135 . The transfer of an inked image from the PIP  110  to the ITM  130  may be known as the “first transfer”, which takes place at a point of engagement T 1  between the PIP  110  and the ITM  130 . The first transfer of the layer of liquid toner is affected by the potential difference that exists between the liquid toner and the ITM  130 . In an example, the voltage applied to the ITM  130  is between +500V and +600V. 
     Once the layer of liquid toner has been transferred to the ITM  130 , it is transferred to a print substrate  145 . This transfer from the ITM  130  to the print substrate may be deemed the “second transfer”, which takes place at a point of engage T 2  between the ITM  130  and the substrate  145 . The impression cylinder  140  can both mechanically compress the substrate  145  in to contact with the ITM  130  and also help feed the substrate  145 . In one example, the impression cylinder  140  is grounded. The present electrophotographic printer is capable of printing on either conductive or non-conductive substrates. Non-conductive substrates may include: sheets of metal; metal-coated paper or cardboard; or substrates with metal areas or parts. 
     In an example, the ITM  130  is used as a “two-sided” or “two-page” intermediate transfer drum to develop two images on different portions of the ITM  130  at a time. Image development units  120  deposit respective first and second sequences of color separations onto the PIP  110 . The ITM  130  has a first portion (an example of which is shown as portion A in  FIG. 1 ) to receive the first sequence of color separations from the PIP  110  and a second portion (an example of which is shown as portion B in  FIG. 1 ) to receive the second sequence of color separations from the PIP  110 . The PIP  110  and ITM  130  can be rotatable drums that rotate relative to one another, such that the color separations are transferred during the relative rotation. 
     The print method may be a “one-shot” imaging process as described previously. The sequences are controlled so that, during the second transfer of the first developed image to a conductive substrate  145 , there is no first transfer of a color separation of the second image from the PIP  110  to the ITM  130 , and conversely, no image is printed to the conductive substrate when a first transfer of a color separation between the PIP  110  and the ITM  130  is taking place. 
     Controller  150 , discussed in more detail below, controls part, or all, of the print process. A memory  160  may comprise a set of computer-readable instructions stored thereon to perform functions such as controlling a voltage  170 , inserting a null separation  172 , reducing a voltage  174  and transferring an image  176 , as explained further below. Alternatively, these functions may be implemented in dedicated circuitry. For example, the controller  150  can control the voltage level applied by a voltage source  155 , for example a power supply, to the ITM  130  in accordance with the rotation of the ITM  130 . The ITM  130  voltage is selectively applied such that the ITM  130  receives each color separation from the PIP  110 . The controller  150  inserts at least one null separation into the second sequence of color separations during the development of the second image. During a period for the null separation, the controller  150  controls the voltage source  155  to reduce the voltage applied to the ITM  130 , and to transfer the first image to the conductive substrate  145 . The voltage source  155  is reduced to a low enough voltage in order that electrostatic charging/discharging issues are not introduced when printing to the conductive substrate  145 . The voltage source  155  may be reduced to approximately 0V, for example by turning off an associated power supply. 
     It will be appreciated that the controller  150  can also control any other, or all of the components of the printer  100 , however connections between those elements and the controller are not shown in  FIG. 1  for clarity. Furthermore, controller  150  may also be embodied in one or more separate controllers. The controller  150  may comprise a microprocessor and a memory. The LEP printer  100  comprises electronic circuitry to receive a control signal from the microprocessor and, in response, to cause the voltage source  155  to reduce the voltage applied to the ITM  130 . 
       FIG. 2  shows an example method of printing images in an LEP printer  100 . At block  202 , a voltage is applied to the ITM  130  during receipt (at block  204 ) of each color separation from the PIP  110 . As described previously, the first sequence of color separations is received from the PIP  110  to develop the first image on a first portion of the ITM  130 , while the second sequence of color separations is received from the PIP  110  to develop a second image on a second portion of the ITM  130 . At block  206 , during the developing of the second image, at least one null separation is inserted by the controller  150  into the second sequence of color separations. This insertion may include generating control data that includes the null separation, e.g. as compared to control data that does not include the null separation. At block  208 , during a period for the null separation, a voltage applied to the ITM by the voltage source  155  is reduced by the controller  150 , and the first image is transferred (block  210 ) a conductive substrate. 
       FIGS. 3 a  and 3 b    show a more detailed example method of printing images in an LEP printer  100 .  FIG. 3 b    is a continuation of  FIG. 3 a    over predetermined and equal time periods t 0  to t 26 . Each time period corresponds to a half a rotation of the ITM  130 , that is, an 180° rotation of the cylindrical drum shown in  FIG. 1 . In this example, each image may take up approximately 150° of the perimeter of the ITM  130  blanket. A voltage level that is supplied to the ITM  130  using voltage source  155  is shown to be HIGH/ON or LOW/OFF in accordance with times t 0 -t 26  shown on the horizontal axis. The vertical axes of  FIGS. 3 a  and 3 b    indicate: a first transfer (at the point of engagement, T 1 , between the PIP  110  and the ITM  130 ) to a first portion of the ITM  130  (blanket A); a first transfer (at point T 1 ) to a second portion of the ITM  130  (blanket B); a second transfer (at the point of engagement, T 2 , between the ITM  130  and the conductive substrate  145 ) to the first portion of the ITM  130  (blanket A); a second transfer (at T 2 ) to a second portion of the ITM  130  (blanket B). Each transfer is represented by a block indicating an action at a particular time, where P 1  is a first image to be printed, P 2  is a second image to be printed, and S 1 -S 4  represent the individual color separations that are transferred for each respective image, as explained further below. In this example, there are four color separations, but images comprising fewer or more color separations can also be printed using the printing method of  FIG. 2 . 
     Referring to  FIG. 3 a   , at time t 0 , the voltage is applied to the ITM (for example, by turning a power supply attached to the ITM  130  up or on) as the development of images onto the ITM  130  begins. The PIP  110  and ITM  130  rotate at constant process velocities relative to one another, and at time t 1  block P 1 S 1  indicates that a first color separation of a first image is transferred from the PIP  110  to a first portion, blanket A, of the ITM  130 . At time t 2 , the high voltage level is maintained but there is no transfer of a color separation to the ITM  130 . This can be referred to as a “dummy” phase and ensures that in subsequent color separation transfers, separations of the same color are not transferred to portions A and B of the ITM  130  at adjacent times t x , t x+1 . For example, if separation S 1  is magenta and separation S 2  is cyan, it can be seen from  FIG. 3 a    that by inserting the dummy phase at time t 2 , blocks P 1 S 1  and P 2 S 1  are spaced from one another, and blocks P 1 S 2  and P 2 S 2  are correspondingly spaced, which eases pressure on the system and allows the appropriate image development unit  120  to prepare for the next color separation transfer. 
     At time t 7 , block P 1 S 4  indicates that the fourth separation of the first image is transferred onto the first portion of the ITM  130 . As each image in this example has four color separations, the transfer of the first image onto the ITM  130  blanket is now complete, and the first image is ready to be transferred to the conductive substrate  145 . As can be seen from  FIG. 3 a   , the transfer of the first image to the conductive substrate  145  occurs when a subset of the second sequence of color separations have been received on the second portion of the ITM  130 . In this example, the first and second color separations (S 1 , S 2 ) of image P 2  have been transferred to blanket B. 
     At time t 8 , the controller  150  inserts a null separation into the second sequence of color separations, so that no color separation transfer occurs between the PIP  110  and the ITM  130 . During the null separation, the controller  150  also reduces the voltage applied by the voltage supply  155  to the ITM  130  to the LOW/OFF level. The second transfer of the first image (T-P 1 ) from the ITM  130  to the conductive substrate (in this example, substrate A) can then take place during the null separation. A second null cycle can be introduced at time t 9 , because in the example of  FIG. 1 , the location T 2  at which the ITM  130  meets the substrate  145  is not directly opposite the location T 1  of the first transfer between the PIP  110  and the ITM  130 . 
     As shown in  FIG. 3 b   , second transfers of a second image (T-P 2 ), a third image (T-P 3 ) and a fourth image (T-P 4 ) can also take place during subsequent null separations that are inserted into the print cycle at appropriate times by the controller  150 . These times may be the optimum times at which to transfer the respective images, based on the final separation for the respective images being received on the ITM  130  blanket and the position of each portion of the ITM  130  drum. 
       FIGS. 3 a  and 3 b    also show that there may be a time period during which the voltage decreases and increases once the controller has instructed the voltage source to reduce or increase, respectively, the voltage applied to the ITM  130 . This rise and fall time of a high voltage power supply means that the power supply may be enabled to lower or turn off the applied voltage for longer than the exact ITM-substrate contact time during the second transfer. The insertion of appropriate null separations by the controller  150  ensures that the second transfer takes place when the voltage is at a suitably low level, and that no transfers take place during the voltage rise and fall periods. 
     As shown by blocks P 3 S 1 -P 3 S 4 , a third image P 3  can be developed on the first portion (blanket A) of the ITM  130  by receiving a third sequence of color separations from the PIP  110  after the first image P 1  has been transferred to a conductive substrate. In this example, the term “substrate A” is used to show that the third image is developed from blanket A, that is, the first portion of the ITM  130 ; however, it should be appreciated that the third image may, in practice, be printed onto a different physical substrate to the substrate to which the first image P 1  has been printed. During the development of the third image, at least one null separation is inserted by the controller into the third sequence of color separations. During a period of time for the null separation, the ITM  130  voltage is reduced and the second image P 2  is transferred at block T-P 2  to a second conductive substrate. The second conductive substrate may be separate to, or part of, the first conductive substrate. For example, the first and second substrates may be first and second portions, respectively, of a continuous web substrate. As shown in  FIG. 3 b   , similar print cycles may be repeated for subsequent images, with up to two images being developed on the ITM  130  at any given time. 
       FIG. 4 a    is a table illustrating the example sequence of  FIG. 3 ; the numbers indicate a color separation number that is received at each of blankets A and B, running in time order from the top to the bottom of the table. The term “n” indicates that a null separation is inserted into the print cycle, while “dummy” indicates the insertion of a dummy phase.  FIGS. 4 b  and 4 c    illustrate similar tables in the case of an image having three color separations and five color separations, respectively. 
       FIGS. 4 a -4 c    provide example print cycles in which the voltage rise and fall is relatively fast. By contrast,  FIGS. 5 a , 5 b  and 5 c    show examples of print cycles having 4, 3 and 5 color separations, respectively, which may be employed in the case of a longer duration of voltage rise or fall. 
     Referring to  FIG. 6 , an example of a non-transitory computer readable storage medium  605  may comprise a set of computer-readable instructions  600  stored thereon. The instructions are executed by a processor  610  which may form part of the controller  150  of the example LEP printer of  FIG. 1 . The instructions are executed by the processor  610  and cause it to carry out the illustrated tasks. At block  620 , the processor  110  receives print data for at least a first image and a second image to be printed to the conductive substrate  145 . At block  630 , the processor  610  instructs development of first and second images by depositing color separations of printing fluid from at least one image development unit  120  onto a PIP  110  of the LEP. The processor  610  then instructs, at block  640 , transfer of the color separations from the PIP  110  to the ITM  130  in accordance with the respective first and second separation development sequences. The first and second separation development sequences comprise one or more null separations to delay development of the second image. During the one or more null separations, the processor  610  (i) instructs (at block  650 ) a reduction in the voltage applied by the voltage source  155  to the ITM  130  and (ii) instructs transfer (at block  660 ) of the first image from the ITM  130  to the conductive substrate  145 . 
     While certain examples have been described above in relation to liquid electrophotographic printing, other examples can be applied to dry electrophotographic printing. 
     The preceding description has been presented to illustrate and describe examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any other of the examples.