Patent Application
20110129275
Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Docket No. 95529) filed ______ entitled “MODULAR MEDIA TRANSPORT SYSTEM”, by DeCook et al.; to commonly-assigned copending U.S. patent application Ser. No. ______ (Docket No. 95526) filed ______ entitled “MEDIA TRANSPORT SYSTEM FOR NON-CONTACTING PRINTING”, by Muir et al.
The present invention generally relates to printing apparatus for continuous web media and more particularly relates to a transport system that has reduced sensitivity to irregularity in edge characteristics of a continuous web of media.
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 and servo controlled web steering mechanisms. 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.
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 be difficult and costly to employ such a solution with a print medium whose stiffness and tension vary during printing, and whose edge irregularities are of a much greater magnitude than that found on web belts, as described in the reference cited 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 greatly 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.
Additionally, in order to provide registration accuracy for a continuous web media transport system, proper edge guidance is important. An edge guide establishes the lateral position of the web media for such a system. The combination of edge guidance with suitable angular constraint for the moving web allows kinematic principles, or “exact constraint” to be applied to the transport problem, helping to minimize the need for complex web handling apparatus and minimize the likelihood of inducing undesirable stress or constraint in the web handling mechanics.
One problem with continuous kinematic web media handling, however, relates to edge defects. The edge of the web that requires guidance may have various types of defects due to mishandling, slitting operation variability during manufacture and consequent edge weave, splicing, and other causes. These edge defects, inherent to large rolls of printing paper, for example, make it difficult to rely on a single edge guide for maintaining proper registration, particularly needed when printing multiple color separations. Mis-registration in the printing area is likely, since the lateral motion of the web media is dependent on the quality and straightness of the media edge.
One way of addressing this problem has been the use of web steering techniques, using a servo motor with a steering roller and employing a i“deadband” that filters out some high-frequency web edge effects. This is the approach used, for example, for an electrophotographic belt transport in commonly assigned U.S. Pat. No. 4,572,417 entitled “Web Tracking Apparatus” to Joseph et al. While this type of active control approach using a servo can be appropriate for a closed-loop transport belt, however, the problems presented by a continuous web of print media moving at high speeds are more acute, particularly since the edge of the web itself is used to maintain register accurately.
It is an object of the present invention to advance the art of continuous web media handling for printing and other applications. With this object in mind, the present invention provides a digital printing system for printing on a continuous web of print media. The media transport system of the printing system includes a support structure that guides a continuous web of print media under tension through the printing system. The support structure includes a first mechanism, a roller, and a section in which cross track motion of the continuous web of print media is not desired. The first mechanism is affixed to the support structure and includes structure that positions the print media in a cross track direction. The roller is affixed to the support structure and sets an angular trajectory of the print media. The roller is located downstream relative to the first mechanism. The section in which cross track motion of the continuous web of print media is not desired is located downstream relative to the roller.
According to another feature of the present invention, a digital printing system for printing on a continuous web of print media includes a support structure that guides a continuous web of print media under tension through the printing system. The continuous web of print media includes an edge. The support structure includes a first mechanism, a filter mechanism, and a section in which cross track motion of the continuous web of print media is not desired. The first mechanism is affixed to the support structure and includes structure that positions the print media in a cross track direction. The filter mechanism sets an angular trajectory of the print media, is located downstream relative to the first mechanism, and passively filters fluctuations in position of the edge of the continuous web of print media. The section in which cross track motion of the continuous web of print media is not desired is located downstream relative to the filter mechanism.
According to another feature of the present invention, a method of transporting a continuous web of print media through a digital printing system includes guiding a continuous web of print media under tension along a media path of the printing system, the continuous web of print media including an edge; positioning the print media in a cross track direction at a first location along the media path; establishing an angular trajectory of the print media at a second location of the media path after positioning the print media in the cross track direction to passively filter fluctuations in position of the edge of the print media and to define the cross track position of the print media at the second location of the media path, the second location being at a distance downstream from the first location; and causing the print media to travel through a section of the media path in which cross track motion of the print media is not desired after passively filtering fluctuations in position of the edge of the print media at the second location of the media path.
One advantage of the present invention is that it reduces or eliminates the negative impact of web edge defects on registration in a web transport apparatus. The present invention is compatible with kinematic design principles, supporting a web transport system in which components self-align to the continuously moving web in order to maintain excellent web tracking performance without precise alignment of the web transport components and in order to maintain accurate 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 transport of the media web.
The apparatus and methods of the present invention improve overall print quality by allowing more consistent color to color registration on print media and more consistent inspection capability for verification and color control throughout a print job.
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 example 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 utilize 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. Where used relative to the web path, the term “after” means occurring downstream or at a position later in the paper path. When used in this context, the term “after” does not necessarily imply that the cross track positioning has ended, been stopped, or temporarily discontinued.
A fixed roller is non-pivotable and may or may not be a drive roller. The fixed roller may spin on its axis, but is not movable in other directions.
Kinematic web handling is provided throughout the web transport apparatus of the printing system of the present invention as the continuously moving web medium passes from one portion of the system to another. In doing this, 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
These include the following:
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
It is important to maintain the center-line of the moving web at the same relative position along each roller, regardless of edge defects in order to maintain accurate registration, for example, color registration during printing. Even with edge guide alignment, center-line registration should be maintained. Subsequent figures and description show how center-line registration applies along the path of the web media.
As noted earlier in the background section, the task of providing a lateral constraint along the edge of the moving web is complicated by irregularities in the web edge, either inherent to the slitting and manufacturing process for the roll of media or due to stacking or handling problems. The plan views of
For web transport systems that incorporate and edge guide to set the lateral position of the web, the crosstrack motion of the web's centerline will typically follow the irregularities of the web's edge. It can be readily appreciated that some type of compensation is needed in order to limit the effects of each of the types of edge irregularities shown in
Referring to the schematic diagram of
Shown on the right side of
The graph of
Span length relates to the distance between the angular constraint and the location where the upstream lateral position of that span is determined. In terms of spatial frequency, where the span length L equals or exceeds 16% (½π) of the period λ of the defect, attenuation caused by the web span is significant. For example, approximately 16 dB attenuation is provided when the span length L is about the same as the characteristic length λ of the defect. An attenuation of 16 dB results in reducing the centerline motion due to web edge irregularity by approximately 85%. As a practical minimum for providing the passive filtering of the present invention, the span length should be at least greater than about 0.13 times the characteristic length, and preferably greater than the characteristic length for effective filtering of a periodic defect in the print media, for example, an edge weave 110. A span length of at least 0.13 times the characteristic length reduces the centerline motion due to web edge irregularity by approximately 20%. However, span lengths that are less than 0.13 times the characteristic length still filter periodic and quasi-period defects, for example, indentation defects 114, or non-periodic defects such as step defects 112. At the other extreme, practical considerations such as maintaining cross-track stiffness of the web in the span may place some limits on the maximum length of a web span.
It should be noted that a span can be folded over a surface, for example; span length refers to the actual distance traveled by the media and does not necessarily correspond to the linear distance between the angular constraint roller and the location where the upstream lateral position of that span is determined. Even though it is generally preferable to have a span length that is greater than the characteristic length of the periodic irregularity, a span length that is less than the characteristic length will filter the irregularity, although the amount of filtering will be less. As such, when the span length is less than the characteristic length of the irregularity, it can be particularly useful and desirable to provide more filter mechanisms.
The passive filtering effect of the web span is further improved by cascading. Referring to the schematic diagram of
The graph of
Embodiments of the present invention apply the principles described with reference 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 in the embodiment of
The web plane diagram of
Roller C, a castered and gimbaled roller in the embodiment of
The web plane diagram of
Segment L2, extending between the angular constraint of roller B and roller C, is an overhang section that contributes an amplification factor to the response of web transport filter 1. As a general observation, an overhang portion that extends beyond the angular constraint in the downstream direction detracts from the filtering function of its web span.
The graph of
The graph of
The graph of
The following notes apply for embodiments of the present invention:
Advantageously, a passive filter is formed when the web span has the arrangement described, with the roller that sets angular trajectory downstream from the structure that sets cross track position. No additional motors or other actuators are needed to provide the filtering effect.
The digital printing system 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. However, using a second edge guide will eliminate the filtering effect of that span. 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.
