Patent Grant
8840105
Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. 13/956,668, entitled “Charger providing non-uniform electrostatic holding force” by Priebe; and to commonly assigned, co-pending U.S. patent application Ser. No. 13/956,692, entitled “Controlling recharging to restore electrostatic holding force” by Priebe, each of which is incorporated herein by reference.
This invention pertains to the field of printing and more particularly to retaining receiver media on a transport belt using an electrostatic holding force during printing.
In a digitally controlled printing system, for example an inkjet printing system, a receiver medium is directed through a series of components. The receiver medium can be a cut sheet or a continuous web. A transport system physically moves the receiver medium through the printing system. As the receiver medium moves through an inkjet printing system, liquid, (e.g., ink), is applied to the receiver medium by one or more printheads through a process commonly referred to a jetting of the liquid. The jetting of liquid onto the receiver medium introduces significant moisture content to the receiver medium, particularly when the system is used to print multiple colors on a receiver medium. Due to its moisture content, the receiver medium can undergo various physical changes, which can include change in electrical resistivity as well as changes in physical dimension and mechanical stiffness.
Cut sheet transport systems typically employ a holding force to hold the receiver medium to a transport mechanism, such as a belt or a drum. The holding force on the receiver medium can be applied using vacuum or electrostatic force, for example. Transport systems using electrostatic force can be advantageous because of their simplicity, and are frequently used in printing systems using dry printing processes, such as electrophotography. For example, U.S. Pat. No. 5,918,875 to Masley et al., entitled “Zero clearance handle,” describes a paper feeding system in which a copy sheet is charged so that the sheet is electrostatically secured (or “tacked”) to a photoreceptor belt. The sheet is then provided with an opposite charge so that it can be removed (or “detacked”) from the transport belt.
For printing systems where a liquid ink is applied to the receiver medium, the resulting change in electrical resistivity of the receiver medium can adversely impact the reliability of using electrostatic holding for the receiver medium. Moist paper can dissipate charge relatively quickly. Moreover, connected regions of liquid permit charge to move through them, potentially redistributing charge on the receiver medium. Evaporation of components of liquid ink can also result in ionization that neutralizes some of the charge on the receiver medium. Still, some inkjet systems can be compatible with electrostatic holding of the receiver medium. U.S. Patent Application Publication No. 2011/0109037 to Kunioka entitled “Sheet feeder and image forming apparatus incorporating same,” describes an ink-ejecting printer having recording heads mounted on a carriage over a transport belt. The transport belt electrostatically holds a receiver sheet and transports it to a position facing the recording heads. In a carriage printer, the printheads are moved by the carriage to print a swath of an image and advance the print medium between swaths in order to form the image swath by swath. In this application, the transport belt is able to provide sufficient electrostatic holding force for the portion of the cut sheet receiver medium that has not yet been printed. After the entire image is printed, the sheet is transported only a short way to an output tray.
High volume cut sheet printing systems typically print one color of an entire line of the image essentially all at once, for example using a page-width printhead or other page-width printing processes in a printing station for that color. The cut sheet is advanced past the printing station as page-width lines of the same color are printed sequentially. To print all colors (typically requiring at least cyan, magenta, yellow and black), the receiver medium is moved past a sequence of printing stations, one for each color. Dryers are typically provided between printing stations for evaporating volatile components of ink from the receiver medium, thereby increasing ionization due to evaporation. This can weaken the electrostatic holding force such that holding of the receiver medium is no longer reliable. If the receiver medium is no longer tightly held to the belt, registration of the colors printed by successive printing stations can be lost, thereby degrading print quality. Loosened receiver medium can also cause paper jams.
There remains a need for an electrostatic holding system that provides a reliable electrostatic holding force for the transport of sheets of receiver media through a cut sheet printing system. In addition, what is needed is an electrostatic holding system that accommodates dimensional changes in receiver medium that has absorbed liquid during the printing process.
The present invention represents a printer comprising:
a transport belt having an electrically non-conducting surface adapted to transport a receiver medium along a transport path from an upstream position to a downstream position;
a charging subsystem configured to add charge to the transport belt or the receiver medium, thereby providing an electrostatic holding force between the transport belt and the receiver medium;
an inking subsystem positioned downstream of the charging subsystem that deposits a pattern of ink on the receiver medium;
a dryer positioned downstream of the inking subsystem adapted to dry the inked receiver medium; and
a recharging subsystem positioned downstream of the dryer configured to restore at least some charge dissipated while the receiver medium is transported between the charging subsystem and the recharging subsystem.
This invention has the advantage that the recharging subsystem restores charge dissipated during the printing process so that the receiver medium can be reliably transported along a downstream portion of a transport path.
It has the additional advantage that it enables electrostatic receiver medium holding systems to be used in inkjet printers where charge dissipates during the printing process.
The attached drawings are for purposes of illustration and are not necessarily to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.
The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense.
Ink printing processes can be embodied in single-function printers or in multi-function devices that also provide functions such as copying, scanning and facsimile transmission or reception. Ink printers operate by depositing marking material on a receiver medium (e.g., paper). Inkjet printing is a non-contact application of an ink to a receiver medium. Typically, one of two types of ink jetting mechanisms is used: drop-on-demand inkjet printing or continuous inkjet printing.
The first inkjet printing technology, drop-on-demand inkjet printing, provides ink drops that impact upon a recording surface using a pressurization actuator, for example, a thermal, piezoelectric, or electrostatic actuator. One commonly practiced drop-on-demand inkjet printing type uses thermal energy to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to form a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed thermal inkjet. A second commonly practiced drop-on-demand inkjet printing type uses piezoelectric actuators to change the volume of an ink chamber to eject an ink drop.
The second inkjet printing technology, commonly referred to as continuous inkjet printing, uses a pressurized ink source to produce a continuous stream of liquid ink by forcing ink, under pressure, through a nozzle. The stream of ink is perturbed using a drop forming mechanism such that the liquid jet breaks up into drops of ink in a predictable manner. One continuous inkjet printing type uses thermal stimulation of the liquid jet with a heater to form drops that eventually become print drops and non-print drops. Printing occurs by selectively deflecting either the print drops or the non-print drops and catching the non-print drops. Various approaches for selectively deflecting drops have been developed including electrostatic deflection, air deflection, and thermal deflection.
The invention described herein is applicable to both drop-on-demand and continuous inkjet printing technologies. As such, the term inkjet printhead, as used herein, is intended to be generic and not specific to either technology. The invention can also be applied to other types of printers such as electrophotographic printers.
Inkjet inks typically include colorants (e.g., cyan, magenta, yellow or black pigments or dyes) and a carrier fluid (e.g., water or alcohol), together with other components such as biocides, humectants and surfactants for reliable jetting and for proper interaction with the receiver medium. The carrier fluid helps to convey the colorant to the receiver medium, but once the ink drops hit the receiver medium, the carrier fluid needs to be managed through absorption into the receiver medium or evaporation from the receiver medium. If too much carrier fluid from one color of ink remains at the surface of the receiver medium when another color of ink is deposited in the same location, the wet inks can bleed into each other, thereby degrading print quality. If too much carrier fluid is absorbed into the receiver medium, the receiver medium can become limp or change shape, making it more difficult to transport through the printing system. Mid-volume to high-volume inkjet printing systems, which can print more than 100 pages per minute, typically include dryers between successive printing stations that print the different colors of ink.
A printer typically includes a digital front-end processor, a marking engine (also referred to in the art as a “print engine”) for applying marking material to the receiver medium, and optionally one or more post-printing finishing systems (e.g., a UV coating system, a glosser, or a laminator). The digital front end processor converts input electronic files into image bitmaps for the marking engine to print, and permits operator control of the output. The marking engine takes the image bitmap and renders the bitmap into a form that can control the printing process. The finishing system applies features such as protection, glossing, or binding to the prints. A transport unit moves the receiver medium through the printer.
Transport systems of the present invention include transport belts 10 (
With reference to
Returning to a discussion of
The receiver medium sheets 21a, 21b, 21c, 21d (e.g., paper) typically have high resistivity (such as around 1×1013 Ω-cm) when it is dry, so that it can hold a charge. However, its resistivity can drop by several orders of magnitude when it is moistened, which can cause charge to dissipate.
A first inking subsystem 31 is positioned downstream of charging subsystem 50 and deposits a pattern of ink on receiver medium sheet 21a as it moves past the first inking subsystem 31. If printer 100 is an inkjet printer, first inking subsystem 31 can include a drop-on-demand or continuous inkjet printhead that ejects drops of ink 32. Initially, the deposited ink provides a wet ink pattern 41 on the surface of the receiver medium sheet 21a. Subsequently, a portion of the carrier fluid in the ink will typically be absorbed into the body of the receiver medium sheet 21a or into surface coatings on the receiver medium sheet 21a.
The printer 100 optionally includes one or more dryers (i.e., first dryer 61 and second dryer 62) for at least partially drying the deposited ink on the receiver medium. Receiver medium sheet 21b represents a piece of receiver medium that has advanced along the transport path 15 to a position in proximity to a first dryer 61 that is positioned downstream of first inking subsystem 31. First dryer 61 is represented graphically in
Charge on the receiver medium sheet 21b can be dissipated as the receiver medium sheet 21b becomes moistened by the ink and as volatile components of the ink evaporate. For example, evaporated water molecules can ionize and carry off charge as they leave the receiver medium sheet 21b, being driven away by the net electric field between the charged surface of receiver medium sheet 21b and the neutral free space above it. As the first dryer 61 drives faster evaporation (e.g. by elevated temperature), the charge on the receiver medium sheet 21b can become progressively dissipated in this region, thereby weakening the electrostatic holding force.
According to embodiments of the invention a first recharging subsystem 51 is positioned downstream of the first inking subsystem 31. For embodiments including first dryer 61, the first recharging subsystem 51 is preferably positioned downstream of first dryer 61. First recharging subsystem 51, which is shown as a corona charging unit 80 in
A control system 90 is adapted to provide control over the various functions of printer 100, including controlling the first inking subsystem 31 and the second inking subsystem 33 to deposit patterns of ink on the receiver medium to form an image. Control system 90 can include a microprocessor incorporating suitable look-up tables and control software executable by control system 90. It can also include a field-programmable gate array, programmable logic device, microcontroller, or other digital control elements. It can include memory for storing control software and data.
One or more charge sensing devices 71, 72 are preferably positioned downstream of the first inking subsystem 31 and are used to sense a level of charge on the surface of receiver medium sheet 21b, or on the surface of transport belt 10. Charge sensing devices 71, 72 can be solid state electrometers or ammeters, for example, that provide a signal to control system 90. In the illustrated configuration, the first dryer 61 is positioned between first inking subsystem 31 and charge sensing devices 71, 72. Control system 90 can be adapted to control the first recharging subsystem 51 responsive to a sensed level of charge on the receiver medium sheet 21b as characterized by one or both of the charge sensing devices 71, 72. In some embodiments, the control system 90 adjusts a voltage on conductive grid 83 (
Charge sensing device 71 is positioned upstream of first recharging subsystem 51. In some embodiments, the control system 90 determines an amount of charge to be provided by first recharging subsystem 51 responsive to a difference between a level of charge sensed by charge sensing device 71 on receiver medium sheet 21b and an aim level of charge that is known to provide sufficient electrostatic holding force. The control system 90 then controls first recharging subsystem 51 to provide all or part of the difference in charge to the receiver medium sheet 21b. This approach provides a feed-forward mechanism to control the charge level provided by the first recharging subsystem 51.
Charge sensing device 72 is positioned downstream of first recharging subsystem 51. In some embodiments, the charge sensing device 72 is used in conjunction with control system 90 to determine a charge difference between the level of charge sensed by charge sensing device 72 and an aim level of charge that is known to provide sufficient electrostatic holding force for receiver medium sheet 21b. The control system 90 determines an amount of charge to be provided by first recharging subsystem 51 to a subsequent sheet of receiver media (e.g., receiver medium sheet 21a) responsive to the determined charge difference in order to provide all or part of the difference in charge to subsequent receiver medium sheet 21a. This approach provides a feedback mechanism to control the charge level provided by the first recharging subsystem 51. Various embodiments of the invention can include either or both of the charge sensing device 71 and charge sensing device 72.
Second inking subsystem 33, located downstream of the first recharging subsystem 51, is controlled by control system 90 to deposit a second pattern of a second ink on receiver medium sheet 21c as receiver medium sheet 21c moves past it. If printer 100 is an inkjet printer, second inking subsystem 33 can include a drop-on-demand inkjet printhead or a continuous inkjet printhead that ejects drops of ink 34. Initially, the deposited ink provides a wet ink pattern 44 on the surface of the receiver medium sheet 21c. For clarity, the wet ink pattern 44 is shown adjacent to an ink pattern 43 that was deposited by first inking subsystem 31. However, in practice, the wet ink pattern 44 will typically overlap the ink pattern 43. Even though the wet ink pattern 44 is touching the ink pattern 43, the wet ink pattern 44 and the ink pattern 43 do not bleed into each other because ink pattern 43 is substantially immobilized due to absorption and evaporation of portions of carrier fluid. Subsequently, a portion of the carrier fluid in the wet ink pattern 44 will typically be absorbed into the body of the receiver medium sheet 21c or into surface coatings on the receiver medium sheet 21c.
Receiver medium sheet 21d represents a sheet of receiver medium that has advanced along transport path 15 to a position close to a second dryer 62 positioned downstream of second inking subsystem 33. The second dryer 62 is represented graphically in
Charge on receiver medium sheet 21d can be dissipated as receiver medium sheet 21d becomes moistened by ink and as volatile components of ink evaporate, as described above relative to receiver medium sheet 21b. According to some embodiments of the invention a second recharging subsystem 52 is positioned downstream of second inking subsystem 33 and the second dryer 62. Second recharging subsystem 52, which is shown as a corona charging unit 80 in
One or more charge sensing devices 73, 74 can be positioned downstream of the second inking subsystem 33 and upstream or downstream, respectively, of the second recharging subsystem 52. The charge sensing devices 73, 74 operate in conjunction with control system 90 for controlling the second recharging subsystem 52 responsive to a sensed level of charge on the receiver medium sheet 21d in a similar manner to the charge sensing devices 71, 72 described above relative to the first recharging subsystem 51.
As described above control of first recharging subsystem 51 and second recharging subsystems 52 can be performed by control system 90 in response to sensed level of charge on the receiver medium. In some embodiments, other control factors can be used by control system 90 for determining how much charge should be directed by first recharging subsystem 51 and second recharging subsystems 52 toward the receiver medium sheets 21b, 21d. Examples of other control factors (in addition to charge information from charge sensing devices 71, 72, 73, 74) are shown schematically in
With regard to information on deposited ink pattern 91, in some embodiments, the control of one or both of the first recharging subsystem 51 and the second recharging subsystems 52 can be responsive to the amount of ink deposited on the receiver medium by the previous inking subsystems, or by the cumulative amount of ink that has been deposited. The more ink that has been deposited, the greater amount of charge that will be required for restoring the electrostatic holding force. In some embodiments, the control of one or both of the first recharging subsystem 51 and the second recharging subsystems 52 can be responsive to the spatial ink distribution deposited on the receiver medium. In this case, more charge can be applied to regions of the receiver medium that have been more heavily inked.
With regard to receiver characteristics 92, in some embodiments, the control of one or both of the first recharging subsystem 51 and the second recharging subsystems 52 can be responsive to characteristics of the receiver medium. Examples of receiver characteristics 92 that can influence the amount of charge that is required for restoring the electrostatic holding force include material type, material thickness, porosity, roughness, capacitance, resistivity, and dielectric strength. In some embodiments, the effect of the various receiver characteristics 92 on the amount of charge that is required can be characterized ahead of time, either in the form of a parametric model, or as a measured characteristic for a set of available receiver medium types.
With regard to ink characteristics 93, in some embodiments, the control of one or both of the first recharging subsystem 51 and the second recharging subsystems 52 can be responsive to characteristics of the ink. Examples of particular ink characteristics 93 that can influence the amount of charge that is required for restoring the electrostatic holding force include electrical conductivity, carrier fluid volatility and type, and concentration of components such as surfactants and carrier fluid. In some embodiments, the effect of the various ink characteristics 93 on the amount of charge that is required can be characterized ahead of time, either in the form of a parametric model, or as a measured characteristic for a set of available ink types.
With regard to environmental attributes 94, in some embodiments, the control of one or both of the first recharging subsystem 51 and the second recharging subsystems 52 can be responsive to attributes of the environment such as temperature and humidity, which can be sensed using environmental sensors. In some embodiments, the effect of the environmental attributes 94 on the amount of charge that is required can be characterized ahead of time, either in the form of a parametric model, or as a measured characteristic for a set of typical environmental conditions.
With regard to operating conditions 95, in some embodiments, the control of one or both of the first recharging subsystem 51 and the second recharging subsystems 52 can be responsive to operating conditions of the operation of the printer 100 (
Returning to a discussion of
In some embodiments some or all of the charging subsystem 50, the first recharging subsystem 51 and the second recharging subsystem 52 are self-limiting chargers adapted to provide an aim level of charge on the receiver medium, where the aim level of charge corresponds to a receiver surface voltage that approaches the grid voltage of the corona charging unit 80. A self-limiting charger ideally charges the receiver surface to a voltage level corresponding to 100% of the control grid voltage of the corona charging unit 80. More typically the receiver surface does not receive enough charge to reach 100% of the grid voltage. As the receiver surface receives charge, its surface voltage increases and begins to approach the grid voltage. As a result, the electric field between the receiver medium and the conductive grid 83 decreases and provides less of a driving force to move ions to the receiver medium. More ions strike the body 81 of the corona charging unit 80, so that the charging rate of the receiver medium decreases. The surface voltage of the receiver medium approaches the grid voltage and reaches a value that is determined by the charging efficiency. Some factors affecting the charging efficiency include speed of the transport belt 10 and width of the corona charging unit 80 (related to residence time of the receiver medium sheet 21a, 21b, 21c, 21d under the corona charging unit 80); design of the conductive grid 83 (including grid conductor spacing periodicity and total number of conductors in conductive grid 83); and characteristics of the receiver medium including resistivity, dielectric constant and thickness or capacitance.
In the embodiments described above, corona charging units 80 were used in the charging subsystem 50, the first recharging subsystem 51 and the second recharging subsystem 52.
In some embodiments, some or all of the charging subsystem 50, the first recharging subsystem 51 and the second recharging subsystem 52 are adapted to provide a non-uniform charge on the receiver medium (e.g., receiver medium sheet 21a).
In an inkjet printer, the carrier fluid that is absorbed in the receiver medium sheet can cause it to expand. Also, when the receiver medium sheet is exposed to the dryers 61, 62 (
The examples shown in
In other embodiments, the charge distribution may be non-uniform in both the cross-track and the in-track directions.
In order to provide non-uniform charge distributions that vary in the cross-track direction, the charging subsystem 50 (as well as the first and second recharging subsystems 51 and 52) must be controllable to provide different charge levels at different cross-track positions. In some embodiments this can be done by using a plurality of independently controllable corona charging units 80 that span different cross-track portions of the receiver medium sheet 21. The charge provided by each of the corona charging units 80 can be controlled by various means such as adjusting the charger current, or the grid potential of the conductive grid 83 (
U.S. Pat. No. 8,408,539 to Moore, entitled “Sheet transport and hold down apparatus,” discloses an inkjet printer having a transport belt that primarily provides holding force for sheets of receiver medium using vacuum. At the two opposing side edges of the sheet, an electrostatic holding force is applied to further aid in holding the sheet in a flat position. Although the printer described by Moore has a charging subsystem providing a non-uniform charge on a sheet of receiver medium, the function is not to hold receiver medium more tightly at the side edges (where the electrostatic holding force is applied) than it is in the rest of the sheet where it is held by vacuum. Rather, the intent is to tightly hold the receiver medium across the entire width, including along the edges where the vacuum system was found to not provide a tight holding force. Since the receiver medium is held tightly across the entire width, it does not provide the advantage of enabling the receiver medium to expand and contract freely to avoid wrinkling of the receiver medium.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5918875 | Masley et al. | Jul 1999 | A |
| 7651212 | Kadomatsu et al. | Jan 2010 | B2 |
| 8408539 | Moore | Apr 2013 | B2 |
| 20020092735 | Greive et al. | Jul 2002 | A1 |
| 20110109037 | Kunioka | May 2011 | A1 |
| Entry |
|---|
| Barthakur et al., “Evaporation rate enhancement of water with air ions from a corona discharge,” International Journal of Biometeorology, vol. 31, pp. 29-33 (1995). |
