Patent Application
20110128338
Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Docket No. 95526) filed ______ entitled “MEDIA TRANSPORT SYSTEM FOR NON-CONTACT PRINTING”, by Muir et al.
The present invention generally relates to printing apparatus for web media and more particularly relates to a printing apparatus having an arrangement of components that do not require precision alignment for feeding a continuous web of media from a supply through one or more printing sections and to a take-up section.
Continuous web printing allows economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of paper or other substrate material is fed past one or more printing subsystems that form images by applying one or more colorants onto the substrate surface. In a conventional web-fed rotary press, for example, a web substrate is fed through one or more impression cylinders that perform contact printing, transferring ink from an imaging roller onto the web in a continuous manner.
Proper registration of the substrate to the printing device is of considerable importance in print reproduction, particularly where multiple colors are used in four-color printing and similar applications. Conventional web transport systems in today's commercial offset printers address the problem of web registration with high-precision alignment of machine elements. Typical of conventional web handling subsystems are heavy frame structures, precision-designed components, and complex and costly alignment procedures for precisely adjusting substrate transport between components and subsystems.
The problem of maintaining precise and repeatable web registration and transport becomes even more acute with the development of high-resolution non-contact printing, such as high-volume inkjet printing. With this type of printing system, finely controlled dots of ink are rapidly and accurately propelled from the printhead onto the surface of the moving media, with the web substrate often coursing past the printhead at speeds measured in hundreds of feet per minute. No impression roller is used; synchronization and timing are employed to determine the sequencing of colorant application to the moving media. With dot resolution of 600 dots-per-inch (DPI) and better, a high degree of registration accuracy is needed. During printing, variable amounts of ink may be applied to different portions of the rapidly moving web, with drying mechanisms typically employed after each printhead or bank of printheads. Variability in ink or other liquid amounts and types and in drying time can cause substrate stiffness and tension characteristics to vary dynamically over a range for different types of substrate, contributing to the overall complexity of the substrate handling and registration challenge.
One approach to the registration problem is to provide a print module that forces the web media along a tightly controlled print path. This is the approach that is exemplified in U.S. Patent Application No. 2009/0122126 entitled “Web Flow Path” by Ray et al. In such a system, there are multiple drive rollers that fix and constrain the web media position as it moves past one or more ink application printheads.
Problems with such a conventional approach include significant cost in design, assembly, and adjustment and alignment of web handling components along the media path. While such a conventional approach may allow some degree of modularity, it would be difficult and costly to expand or modify a system with this type of design. Each “module” for such a system would itself be a complete printing apparatus, or would require a complete, self-contained subassembly for paper transport, making it costly to modify or extend a printing operation, such as to add one or more additional colors or processing steps, for example.
Various approaches to web tracking are suitable for various printing technologies. For example, active alignment steering, as taught for an electrographic reproduction web (often referred to as a belt on which images are transported) in commonly assigned U.S. Pat. No. 4,572,417 entitled “Web Tracking Apparatus” to Joseph et al. would require multiple steering stations for continuous web printing, with accompanying synchronization control. It would be difficult and costly to employ such a solution with a print medium whose stiffness and tension vary during printing, as described above. Other solutions for web (or belt as referred to above) steering are similarly intended for endless webs in electrophotographic equipment but are not readily adaptable for use with paper media. Steering using a surface-contacting roller, useful for low-speed photographic printers and taught in commonly assigned U.S. Pat. No. 4,795,070 entitled “Web Tracking Apparatus” to Blanding et al. would be inappropriate for a surface that is variably wetted with ink and would also tend to introduce non-uniform tension in the cross-track direction. Other solutions taught for photographic media, such as those disclosed in commonly assigned U.S. Pat. No. 4,901,903 entitled “Web Guiding Apparatus” to Blanding are well suited to photographic media moving at slow to moderate speeds but are inappropriate for systems that need to accommodate a wide range of medias, each with different characteristics, and transport each media type at speeds of hundreds of feet per minute.
In order for high-speed non-contact printers to compete against earlier types of devices in the commercial printing market, the high cost of the web transport should be reduced. As such, there is a need for an adaptable non-contact printing system that can be fabricated and configured without the cost of significant down-time, complex adjustment, and constraint on web media materials and types.
It is an object of the present invention to advance the art of web media handling in an imaging system. With this object in mind, the present invention applies kinematic design principles to transport a continuous web of media through a non-contact printing system.
According to one feature of the present invention, a digital printing system for printing on a continuous web of print media includes a first module and a second module that guide a continuous web of print media under tension through the printing system. At least one of the first module and the second module include a digital printhead for placing marks on the print media as it travels through the module. The first module includes a first support structure. A first mechanism is affixed to the first support structure and includes structure that positions the print media in a cross track direction. A second mechanism is affixed to the first support structure and includes structure that sets a tension of the print media. The second module includes a second support structure and is positioned downstream from the first module. A mechanism that kinematically connects the continuous web of print media traveling through the first module to the continuous web of print media traveling through the second module is affixed to the support structure of the at least one of the first module and the second module. A third mechanism includes structure that sets an angular trajectory of the print media and is affixed to the support structure of the at least one of the first module and the second module.
According to another feature of the present invention, a method of printing on a continuous web of print media includes guiding a continuous web of print media under tension through a printing system using a first module and a second module positioned downstream relative to the first module by: positioning the print media in a cross track direction using a first mechanism located in the first module; setting a tension of the print media using a second mechanism located in the first module; kinematically connecting the continuous web of print media traveling through the first module to the continuous web of print media traveling through the second module using a mechanism located in at least one of the first module and the second module; setting an angular trajectory of the print media using a third mechanism located in at least one of the first module and the second module; and selectively placing marks on the print media as it travels through the printing system using a digital printhead located in at least one of the first module and the second module.
One advantage of the present invention is that it allows the web media transport components to self-align to the continuously moving web in order to maintain registration of the printing media. Another advantage of the present invention is that it allows non-contact printing or, more generally, the application of fluid onto the media surface at high speeds, without applying an over-constraining force or pressure that might inadvertently damage the media, cause image misregistration, or otherwise inhibit proper drying or curing of applied inks and other fluids.
The invention and its objects and advantages will become more apparent in the detailed description of the example embodiments presented below. The invention is defined by the claims.
In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
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 method and apparatus of the present invention provide a modular approach to the design of a digital printing system, utilizing features and principles of exact constraint for transporting continuously moving web print media past one or more digital printheads, such as inkjet printheads. The apparatus and method of the present invention are particularly well suited for printing apparatus that provide non-contact application of ink or other colorant onto a continuously moving medium. The printhead of the present invention selectively moistens at least some portion of the media as it courses through the printing system, but without the need to make contact with the print media.
In the context of the present disclosure, the term “continuous web of print media” relates to a print media that is in the form of a continuous strip of media as it passes through the printing system from an entrance to an exit thereof. The continuous web of print media itself serves as the receiving print medium to which one or more printing ink or inks or other coating liquids are applied in non-contact fashion. This is distinguished from various types of “continuous webs” or “belts” that are actually transport system components rather than receiving print media and that are typically used to transport a cut sheet medium in an electrophotographic or other printing system. The terms “upstream” and “downstream” are terms of art referring to relative positions along the transport path of a moving web; points on the web move from upstream to downstream.
Kinematic web handling is provided not only within each module of the system of the present invention, but also at the interconnections between modules, as the continuously moving web medium passes from one module to another. Unlike a number of conventional continuous web imaging systems, the apparatus of the present invention does not require a slack loop between modules, but typically uses a slack loop only for media that has been just removed from the supply roll at the input end. Removing the need for a slack loop between modules or within a module allows addition of a module at any position along the continuously moving web, taking advantage of the self-positioning and self-correcting design of media path components.
The apparatus and methods of the present invention adapt a number of exact constraint principles to the problem of web handling. As part of this adaptation, the inventors have identified ways to allow the moving web to maintain proper cross-track registration in a “passive” manner, with a measure of self-correction for web alignment. Steering of the web is avoided unless absolutely necessary; instead, the web's lateral and angular positions in the plane of transport are exactly constrained. Moreover, other web support devices used in transporting the web, other than non-rotating surfaces or those devices purposefully used to exactly constrain the web, are allowed to self-align with the web. The digital printing system according to this invention includes one or more modules that guide the web of print media as it passes at least one non-contact digital printhead. The digital printing system can also include components for drying or curing of the printing fluid on the media; for inspection of the media, for example, to monitor and control print quality; and various other functions. The digital printing system receives the print media from a media source, and after acting on the print media conveys it to a media receiving unit. The print media is maintained under tension as it passes through the digital printing system, but it is not under tension as it is received from the media source.
Referring to the schematic side view of
Downstream from first module 20 along the path of the continuous web media, second module 40 also has a support structure, similar to the support structure for first module 20. Affixed to the support structure of either or both the first or second module 20 or 40 is a kinematic connection mechanism that maintains the kinematic dynamics of the continuous web of print media in traveling from the first module 20 into the second module 40. Also affixed to the support structure of either the first or second module 20 or 40 are one or more angular constraint structures 26 for setting an angular trajectory of the web media.
Still referring to
Within the printing apparatus of the present invention, the web is guided along its transport path through a number of rollers and curved surfaces. For each web span, both lateral constraint 64 and angular constraint 66 are necessary. However, adding an additional mechanism to achieve lateral or angular constraint can easily cause an over-constraint condition. Thus, for each web span that follows an initial lateral constraint along the web path, the constraint method employed by the inventors attempts to use, as its lateral “constraint”, the given cross track position of the web as it is received from the preceding web span.
Over each web span, then, an angular constraint is provided by a roller mechanism, as described in more detail subsequently. Not every roller along the web path applies angular constraint; in many cases it is advantageous to provide a castered roller or a stationary curved surface that is arranged to provide zero constraint.
Following principles such as these, the inventors have found that an arrangement of mechanisms can be provided to yield the stable constraint arrangement described with respect to
The schematic side view diagram of
Table 1 that follows identifies the lettered components used for web media transport and shown in
The first angular constraint is provided by in-feed drive roller B. This is a fixed roller that cooperates with a drive roller in the turnover section and with an out-feed drive roller N in second module 40 in order to move the web through the printing system with suitable tension in the movement direction (x-direction). The tension provided by the preceding S-wrap serves to hold the paper against the in-feed drive roll so that a nip roller is not required at the drive roller. Angular constraints at subsequent locations downstream along the web are often provided by rollers that are gimbaled so as not to impose an angular constraint on the next downstream web span.
The web plane diagram of
Table 2 that follows identifies the lettered components used for an alternative embodiment of the web media transport shown in
In this embodiment, an angular constraining fixed roller has been located at G, immediately after the print zone containing the printhead 16 and dryer 34, rather than in location F immediately preceding the printhead as in the first embodiment. To eliminate an over constraint condition in the span from roller F to G, fixed roller F of the previous configuration has been replaced with a gimbaled roller. In a similar manner the angular constraining fixed roller has been moved from location L to location M. This places the angular constraint on the print media in the print zone immediately after printhead 16. To eliminate an overconstraint condition in this configuration between the fixed roller M and the fixed drive roller O, a zero constraint castered and gimbaled roller N has been placed between those two fixed rollers.
In either the first or the second embodiment, the angular orientation of the print media in the print zone containing one or more printheads and possibly one or more dryers is controlled by a roller placed immediately before or immediately after the print zone. This is critical for ensuring registration of the print from multiple printheads. It is also critical that the web not be overconstrained in the print zone. This has been done by placing a constraint relieving roller at the opposite end of the print zone in each case; a castered roller following the print zone in the first embodiment and a gimbaled roller preceding the print zone in the second embodiment. As a result of the transit time of the print drops from the jetting module to the print media, variations in spacing of the printhead to the print media from one side of the printhead to the other, it is desirable to orient the printheads parallel to the print media. To maintain the uniformity of this spacing between the printhead and the print media, preferably the constraint relieving roller placed at one end of the print zone is not free to pivot in a manner that will alter the printhead to print media spacing. Therefore the gimbaled roller preceding the print zone in the second embodiment should not have a caster pivot as well. Similarly, the castered roller following the print zone in the first embodiment should preferably not include a gimbal pivot. The use of nonrotating supports under the media in the print zone as shown in
The top view of
The system of the present invention is adaptable for a printing system of variable size and allows straightforward reconfiguration of a system without requiring precise adjustment and alignment of rollers and related hardware when modules are combined. The use of exact constraint mechanisms means that rollers can be mounted within the equipment frame or structure using a reasonable amount of care in mechanical placement and seating within the frame, but without the need to individually align and adjust each roller along the path, as would be necessary when using conventional paper guidance mechanisms. That is, roller alignment with respect to either the media path or another roller located upstream or downstream is not necessary.
A digital printing system 50 shown schematically in
Load cells are provided in order to sense web tension at one or more points in the system. In the embodiments of
The configurations of
Annotation in
Each module in this sequence provides a support structure and an input and an output interface for kinematic connection with upstream or downstream modules. With the exception of the first module in sequence, which provides the edge guide at A, each module utilizes one edge of the incoming web media as its “given” lateral constraint. The module then provides the needed angular constraint for the incoming media in order to provide the needed exact constraint or kinematic connection of the web media transport. It can be seen from this example that a number of modules can be linked together using the apparatus and methods of the present invention. For example, an additional module could alternately be added between any other of these modules in order to provide a useful function for the printing process.
Using the apparatus and methods of the present invention, module function can be adapted to the configuration of the complete printing system. In many cases, rollers and components can be interchangeable, including rollers at the interface between modules, moved from one module to another as best suits the printer configuration. Frames and other support structures for the different modules can use a standard design and dimensions or can be designed differently according to the contemplated application. This also helps to simplify upgrade situations.
The perspective view of
There are a number of ways to track web position in order to locate and position inkjet dots or other marking that is made on the media. A variety of encoding and sensing devices could be used for this purpose along with the necessary timing and synchronization logic, provided by control logic processor 90 or by some other dedicated internal or external processor or computer workstation. Such encoders or sensing devices are typically placed just upstream of the print zone containing the one or more printheads, and are preferably placed on a fixed roller so as to avoid interfering with self aligning characteristic of castered or gimbaled rollers.
In order to provide a digital printing system for non-contact printing onto a continuous web of print media at high transport speeds, the apparatus and method of the present invention apply a number of exact constraint principles to the problem of web handling, including the following:
An active steering mechanism could be used within a web span, such as where the web span length of an overhang exceeds its width, so that the web no longer has sufficient mechanical stiffness for exact constraint techniques. This can happen, for example, where there is considerable overhang along the web span, that is, length of the web extending beyond the angular constraint for the span. This is the case for modules 72 and 78 in the embodiment described with respect to
Kinematic connection between modules 20 and 40 follows the same basic principles that are used for exact constraint within each web span. That is, cross-track or edge alignment is taken from the preceding module. Any attempt to re-register the media edge as it enters the next module would cause an overconstraint condition. Rather than attempting to steer the continuously moving media through a rigid and potentially over-constrained transport system, the media transport components of the present invention self-align to the media, thereby allowing good registration at high transport speeds and reducing the likelihood of damage to the media or misregistration of applied ink or other colorant to the media.
Where multiple modules are used, as was described with reference to the embodiment shown in
It can be seen that the method of the present invention can be applied for handling continuous web media transport within and between one, two, three, or more modules applying exact constraint techniques. This flexibility allows a web transport arrangement that provides good registration and repeatable performance at high speeds commensurate with the requirements of high-speed color inkjet printing. As has been shown, multiple modules can be integrated to form a printing system, without the requirement for painstaking alignment of rollers or other media handling components at the interface between two modules.
It has been found that web transports systems as described above maintain effective control of the print media in the context of a digital print system where the selected portions of the print media are moistened in the printing process. This is true even when the print media is prone to expanding in length and width and to becoming less stiff when it is moistened, such as for cellulose based print media moistened by a water based ink. This enables the individual color planes of a multi-colored document to be printed with good registration to each other.
The digital printing systems having one or more printheads that selectively moisten at least a portion of the print media as described above include a media transport system that serves as a support structure to guide the continuous web of print media. The support structure includes an edge guide or other mechanism that positions the print media in the cross track direction. This first mechanism is located upstream of the printheads of the digital printing system. The print media is pulled through the digital printing system by a driven roller that is located downstream of the printheads. The systems also include a mechanism located upstream of printheads of the printing system for establishing and setting the tension of the print media. Typically it is also located downstream of the first mechanism used for positioning the print media in the cross track direction. The transport system also includes a third mechanism to set an angular trajectory of the print media. This can be a fixed roller (for example, a non-pivoting roller) or a second edge guide. The printing system also includes a roller affixed to the support structure, the roller being configured to align to the print media being guided through the printing system without necessarily being aligned to another roller located upstream or downstream relative to the roller. The castered, gimbaled or castered and gimbaled rollers serve in this manner.
As noted earlier, slack loops are not required between or within modules. Slack loops can be appropriate where the continuous web is initially fed from a supply roll or as it is re-wound onto a take-up roll, as was described with reference to the printing apparatus of
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
