Patent Application
20150328879
Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. ______ (Docket K001800), entitled “Drive gears providing improved registration in printing cylinder systems” by K. Peter et al; to commonly-assigned, co-pending U.S. patent application Ser. No. ______ (Docket K001789), entitled “Precision Registration in a Digital Printing System” by Peter et al.; and to commonly assigned, co-pending U.S. patent application Ser. No. ______ (Docket K001799), entitled “Drive gears providing improved registration in digital printing systems” by K. Peter et al, each of which is incorporated herein by reference.
This invention pertains to the field of printing cylinder systems, such as flexographic printers and offset printers, and more particularly to a web transport design for improved registration of printed patterns from different printing stations in a roll-to-roll web printing system.
Flexography is a method of printing or pattern formation that is commonly used for high-volume printing runs. It is typically employed for printing on a variety of soft or easily deformed materials including, but not limited to, paper, paperboard stock, corrugated board, polymeric films, fabrics, metal foils, glass, glass-coated materials, flexible glass materials and laminates of multiple materials. Coarse surfaces and stretchable polymeric films are also economically printed using flexography.
Flexographic printing members are sometimes known as relief printing members, relief-containing printing plates, or printing sleeves, and are provided with raised relief images onto which ink is applied for application to a printable material. The relief printing member is typically mounted on a plate cylinder. The combination of a relief printing member and a plate cylinder onto which it is mounted form a printing cylinder. While the raised relief images are inked, the recessed relief “floor” remains free of ink.
Offset printing presses also include a printing cylinder onto which a master image is directly formed. Ink rollers transfer ink to the printing cylinder. The image is then transferred to a blanket cylinder and from the blanket cylinder to a web of print media that is fed from a supply roll to a take-up roll.
Although flexographic and offset printing have conventionally been used for the printing of images, more recent uses have included functional printing of devices, such as touch screen sensor films, antennas, and other components to be used in electronics or other industries. Such devices typically include electrically conductive patterns. Whether for printing of images or for functional printing of devices, a plurality of printing stations can be included in a flexographic or offset printing system. For example, for printing a color image on a side of a web, four printing stations are typically used for printing cyan, magenta, yellow and black inks. If suitable color-to-color registration is not maintained in the printing system, print defects such as color halos at the edges of multicolor features can result. For duplex printing, another similar set of four printing stations can be used for printing on the other side of the web.
Similarly, functional printing of devices can be done in multiple successive steps using a plurality of printing stations. If suitable registration is not maintained between printing stations, the performance of the printed device can be degraded. In many cases, the required registration tolerances for functional printing can be tighter than what is required for image printing.
Touch screens are visual displays with areas that may be configured to detect both the presence and location of a touch by, for example, a finger, a hand or a stylus. Touch screens may be found in many devices including televisions, computers, computer peripherals, mobile computing devices, automobiles, appliances and game consoles, as well as in other industrial, commercial and household applications.
A capacitive touch screen includes a substantially transparent substrate which is provided with electrically conductive patterns that do not excessively impair the transparency—either because the conductors are made of a material, such as indium tin oxide, that is substantially transparent, or because the conductors are sufficiently narrow that the transparency is provided by the comparatively large open areas not containing conductors. As the human body is also an electrical conductor, touching the surface of the screen results in a distortion of the screen's electrostatic field, measurable as a change in capacitance.
Projected capacitive touch technology is a variant of capacitive touch technology. Projected capacitive touch screens are made up of a matrix of rows and columns of conductive material that form a grid. Voltage applied to this grid creates a uniform electrostatic field, which can be measured. When a conductive object, such as a finger, comes into contact, it distorts the local electrostatic field at that point. This is measurable as a change in capacitance. The capacitance can be changed and measured at every intersection point on the grid. Therefore, this system is able to accurately track touches. Projected capacitive touch screens can use either mutual capacitive sensors or self capacitive sensors. In mutual capacitive sensors, there is a capacitor at every intersection of each row and each column. A 16×14 array, for example, would have 224 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or styli can be accurately tracked at the same time.
Self-capacitance sensors can use the same x-y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, but it is unable to resolve accurately more than one finger, which results in “ghosting”, or misplaced location sensing.
International Patent Application Publication WO 2013/063188 by Petcavich et al., entitled “Method of manufacturing a capacative touch sensor circuit using a roll-to-roll process to print a conductive microscopic patterns on a flexible dielectric substrate” discloses a method of manufacturing a capacitive touch sensor using a roll-to-roll process to print a conductor pattern on a flexible transparent dielectric substrate. A first conductor pattern is printed on a first side of the dielectric substrate using a first flexographic printing plate and is then cured. A second conductor pattern is printed on a second side of the dielectric substrate using a second flexographic printing plate and is then cured. In some embodiments the ink used to print the patterns includes a catalyst that acts as a seed layer during subsequent electroless plating. The electrolessly-plated material (e.g., copper) provides the low resistivity in the narrow lines of the grid needed for excellent performance of the capacitive touch sensor. Petcavich et al. indicate that the line width of the flexographically printed material can be 1 to 50 microns.
To improve the optical quality and reliability of the touch screen, it has been found to be preferable that the width of the grid lines be approximately 2 to 10 microns, and even more preferably to be 4 to 8 microns. In addition, multiple successive printings can be done on each side of the substrate to fabricate the touch sensor. Registration of 20 microns or tighter is needed in some instances between the different portions of the device that are printed by different printing stations.
One approach is to use in-situ measurement techniques on the printed web such that the registration of layers can be monitored and controlled to be within the required tolerance. U.S. Pat. No. 4,534,288 to Brovman, entitled “Method and apparatus for registering overlapping printed images,” discloses registration control in the context of offset printing of multicolor images. Registration marks are printed on the web at the same time that each color layer of the image is printed. The registration marks are monitored by a register control system and mechanical adjustments are made to the printing process. For example, positioning of a color plane of the image along the web motion direction (the in-track direction) to register it with portions of the image previously printed with one or more other colors can be done by introducing a phase shift of the plate cylinder relative to the web.
U.S. Patent Application Publication 2009/0283002 to Schultze, entitled “Method for printing correction,” discloses registration control in the context of flexographic printing. The position of at least one registration mark is detected using at least one sensor, and evaluation is performed in the register control unit by comparing each detected position of a printing mark with a respective reference position in order to control a relative movement of the web of printing material to the printing cylinder. A relative movement of the printing cylinder to the web means that the tangential velocity of the printing cylinder differs from the linear velocity of the printing material. The tangential speed of the printing cylinder or the linear speed of the printing material can be changed in order to achieve a relative movement. Typically the adjustments are made when the web is in contact with the printing cylinder in the margins outside of the printing region of interest to prevent ink smearing within the print. In some instances small corrections can be made within the printing region without introducing an unacceptable level of smearing.
Although methods exist for registering portions of the print that are successively printed by different printing stations, what is needed for precision printing is to design the web transport for a printing cylinder system in such a way that the size of registration errors introduced in the printing system is reduced.
The present invention represents a printing system for printing on a web of media traveling along a web transport path, comprising:
a plurality of print stations located along the web transport path, each print station including a printing cylinder for printing on the web of media at a corresponding print location;
a plurality of web-transport rollers to guide the web of media along the web transport path;
wherein each of the printing cylinders has a circumference that is substantially equal to a specified printing cylinder circumference, and wherein each of the plurality of web-transport rollers has a roller circumference that is substantially equal to an integer fraction of the specified printing cylinder circumference.
This invention has the advantage that disturbances in the motion of the web of media caused by any run-out or other imperfections in the web-transport rollers are made more consistent by keeping the rollers all in phase with each other.
It has the additional advantage that registration errors between image data printed by the different print stations are reduced.
It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale.
The present description will be directed in particular to elements forming part of, or cooperating more directly with, an apparatus in accordance with the present invention. It is to be understood that elements not specifically shown, labeled, or described can take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements. It is to be understood that elements and components can be referred to in singular or plural form, as appropriate, without limiting the scope of the invention.
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. 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.
The example embodiments of the present invention are illustrated schematically and not to scale for the sake of clarity. One of ordinary skill in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention.
As described herein, the example embodiments of the present invention relate to web transport systems for use in a printing cylinder system such as a flexographic printing system or an offset printing system, for example for printing functional devices such as touch screen sensors. However, many other applications are emerging for printing of functional devices that can be incorporated into other electronic, communications, industrial, household, packaging and product identification systems (such as RFID) in addition to touch screens. Furthermore, flexographic printing and offset printing are conventionally used for printing of images and it is contemplated that the web transport systems described herein can also be advantageous for such printing applications.
The flexographic printing system 100 includes two print stations 120 and 140 that are configured to print on the first side 151 of the web of media 150, as well as two print stations 110 and 130 that are configured to print on the second side 152 of the web of media 150. The web of media 150 travels overall in roll-to-roll direction 105 (left-to-right in the example of
Each of the print stations 110, 120, 130, 140 located along the web transport path includes a set of similar components including a respective plate cylinder 111, 121, 131, 141, on which is mounted a respective flexographic printing plate 112, 122, 132, 142, respectively. Collectively, the plate cylinder 111, 121, 131, 141 and the respective flexographic printing plate 112, 122, 132, 142 can be referred to as a printing cylinder 117, 127, 137, 147. Each flexographic printing plate 112, 122, 132, 142 has raised features 113 defining an image pattern to be printed on the web of media 150. Each print station 110, 120, 130, 140 also includes a respective impression cylinder 114, 124, 134, 144 that is configured to force a side of the web of media 150 into contact with the corresponding flexographic printing plate 112, 122, 132, 142. The impression cylinders 124 and 144 of print stations 120 and 140 (for printing on the first side 151 of the web of media 150) rotate in a counter-clockwise direction in the view shown in
Each print station 110, 120, 130, 140 also includes a respective anilox roller 115, 125, 135, 145 for providing ink to the corresponding flexographic printing plate 112, 122, 132, 142. Within the context of the present invention, the term “ink” is used broadly to refer to any substance with is printed onto the web of media 150. The ink may or may not include pigments or other colorants that are visible to a human observer. As is well known in the printing industry, an anilox roller is a hard cylinder, usually constructed of a steel or aluminum core, having an outer surface containing a large number of very fine dimples, known as cells. Ink is controllably transferred and distributed onto the anilox roller by an ink pan and fountain roller (not shown) or by an ink reservoir chamber (not shown). In some embodiments, some or all of the print stations 110, 120, 130, 140 also include respective UV curing modules 116, 126, 136, 146 for curing the printed ink on web of media 150.
In a flexographic printing system or in an offset printing system, the repeat length L of printed images is substantially equal to the circumference of the printing cylinder 117, 127, 137, 147. In print station 110 of flexographic printing system 100, for example, the circumference of the printing cylinder is the outer circumference of the flexographic printing plate 112 wrapped around the plate cylinder 111. In an offset printing system (not shown) the circumference of the printing cylinder 117, 127, 137, 147 is simply the circumference of the cylinder on which the master image is formed. The length of the actual printed image is typically less than the repeat length L. There is typically a margin of unprinted substrate between two successive printed images. A complete revolution of the printing cylinder 117, 127, 137, 147 prints one repeat length L of web of media 150.
In order for the printed image 254 to have the same repeat length L as the printed image 252, the circumference of printing cylinder 220 needs to be the same as the circumference CP of printing cylinder 210. In the example of
In general, the size of the printing cylinders 210 and 220 needs to be large enough to accommodate the largest printed image length (i.e. the largest repeat distance L) of interest. Conventionally, the size of the web-transport rollers 270 is determined by the size and weight of the web-based media, as well as the intended web tension and the wrap angle of the media around the roller. If a web-transport roller 270 has too small a diameter, it will have insufficient strength to support the web of media 150 without flexing and causing conveyance non-uniformity.
Embodiments of the invention provide design criteria for printing systems having a plurality of print stations located along a web transport path, where each print station includes a printing cylinder, in order to reduce disturbances in the motion of the web of media as it is conveyed through the printing system. By reducing such disturbances there is greater reproducibility and registration precision in the composite printed patters that are formed by the plurality of print stations.
In particular it is observed that web-transport rollers along the web transport path (e.g., the web-transport rollers 106 and 107 in
With reference to
C
R
=L/N=C
P
/N (1)
where N is a positive integer. By substantially equal it is meant that the roller circumference CR of each of the web-transport rollers 270 is equal to an integer fraction of the specified printing cylinder circumference CP to within 1.0%, and more preferably to within 0.1%.
In accordance with the present invention, any non-uniformities in the motion of the web of media 150 caused by irregularities in the web-transport rollers will be consistent at each print location, thereby reducing relative registration errors between the image content printed by the different printing cylinders 210, 211 (e.g., color-to-color registration errors). Furthermore, the registration errors for the image content printed by a particular printing cylinder 210, 211 will be much more consistent and predictable from one frame to another since the rollers will all be in consistent angular positions for a given location within the frame. As a result, the registration errors can be characterized as a function of position within the image frame (for example by using a quality control sensor to sense the position of registration marks printed in the margin of the printed image), and can be compensated for by providing a correction function which specifies compensating shifts to be applied during the process of printing the image data. For example, electronic cam gearing can be used for the printing cylinders 210, 211 to make small adjustments in the tangential velocity of the printing cylinders 210, 211 within the image frames to compensate for the measured registration errors.
It is not required that the web-transport rollers 270 all have the same roller circumference as each other, only that each web-transport roller 270 has a circumference that is an integer fraction of the printing cylinder circumference CP. However, the case where all web-transport rollers 270 have the same circumference CR can be advantageous from the standpoint of commonality of parts.
Preferably the impression cylinders 260, 262 are also selected to satisfy the design criteria that their circumference CI be an integer fraction of the printing cylinder circumference CP. Typically, the impression cylinder circumference CI will be the same as the printing cylinder circumference CP (i.e., N=1), however, this is not a requirement. Similarly, in a flexographic printing system the anilox rollers 115, 125, 135, 145 (
Typically, all of the printing cylinders 210, 220 will have the same printing cylinder circumference CP, but this is not a requirement. However, if some of the printing cylinders 210, 220 have different sizes, it is preferable that all of the printing cylinders 210, 220 have circumferences that are integer fractions of the largest printing cylinder circumference CP.
Transport roller size has previously been considered in different ways for web transport in a printing system. For example, Kodak's NexPress line of color electrophotographic printers has a seamed transport web for advancing cut sheets of receiver media past a series of electrophotographic print modules. All rollers used in this assembly, including the main drive roller, tension roller, steering roller, detack roller, touch down roller, guide rollers, and paper transfer rollers are designed in a way that their circumference matches an integer fraction of the print module-to-module spacing. So, for example, the main drive roller rotates exactly 3 times while the transport web moves from one print module to the next while, the receiver media being firmly attached to the transport web. In consequence, all periodic variations due to roller run-out or unbalance that might cause an in-track timing problem stay in phase between the print modules and do not show up as a print registration problem. Line spacing might vary from the ideal 600 lines per inch, but registration is not affected because the variation occurs in the same way in all print modules. Although the motivation of improving the precision registration is similar in the present invention, the design criterion is different for web-based printing systems using printing cylinders because the fundamental distance which is used to determine the allowable roller sizes is the circumference of the printing cylinders rather than the module-to-module spacing.
Other differences in design criteria in embodiments of the invention result from a roll-to-roll printing system architecture. With reference to
In this exemplary embodiment, the flexographic printing system 100 includes a media guiding subsystem 160 downstream of supply roll 102. The media guiding subsystem 160 can move side to side and helps to guide web of media 150 to start down a desired path as it unwinds from supply roll 102, and generally includes one or more web-transport rollers 161 and other components such as edge guides and control systems.
An out of round supply roll 102 will cause disturbances in the motion of the web of media 150 at increasing frequency as the web is unwound. A front-end motion isolation mechanism, such as an S-wrap tensioning subsystem 170 is commonly provided to buffer such disturbances and allow a steady motion of the web of media 150 at controlled tension throughout the flexographic printing system 100. The S-wrap tensioning subsystem 170 generally includes two or more web-transport rollers 162 which define an S-shaped media path. In alternate embodiments, other types of motion isolation mechanism can be used such as slack loops or festoons. Additional web-transport rollers 171 are located along the web transport path between the S-wrap tensioning subsystem 170 and the print location associated with the first print station 110.
On the output side of the flexographic printing system 100, a main drive roller 180 driven by a motor 183 is generally used to pull the web of media 150 at a predetermined tension as measured with a load cell associated with a web-transport roller 175. The main drive roller 180 also serves the function of a back-end motion isolation mechanism to isolates the print stations 110, 120 from the take-up roll 104. In alternate embodiments, other types of motion isolation mechanism can be used such as slack loops or festoons. Additional web-transport rollers 181 and other components are also typically located along the web transport path between the print location of the last print station 120 and the take-up roll 104.
The design rule stated above that the circumference of each of the web-transport rollers 106 and 107 located along the web transport path between print locations associated with successive print stations 110 and 120, can also be applied to some or all of the rollers located along the web transport path between the supply roll 102 and the print location associated with a first print station 110 (e.g., web-transport rollers 161, 162, 175). Likewise, the design rule can also be applied to some or all of the rollers located between the print location associated with the last print station 120 and the take-up roll 104 (e.g., main drive roller 180 and web-transport rollers 181, 182). There is particular benefit to constraining the web-transport rollers 171 between the S-wrap tensioning subsystem 170 and the first print station, as well as the web-transport rollers 175, 162 in the S-wrap tensioning subsystem 170, to be selected according to the aforementioned design criterion. Since the S-wrap tensioning subsystem 170 serves to effectively isolate the supply roll 102 and media guiding subsystem 160 from the print stations 110, 120, the benefit of constraining any web-transport rollers 161 upstream of the S-wrap tensioning subsystem 170 to conform to the design criteria is reduced. Likewise, it is preferable that the main drive roller 180, as well as any web-transport rollers 181 between the last print module and the main drive roller 180, be constrained to satisfy the aforementioned design criterion. Since the main drive roller 180 effectively isolates the print stations 110, 120 from the take-up roll 104, the benefit of constraining the web-transport rollers 182 downstream of the main drive roller 180 to conform to the design rule is reduced.
Motors 211, 221 are used to drive the printing cylinders 117, 127. In some embodiments, the printing cylinders 117, 127 are driven by the respective motors 211, 221 using a direct servo drive. In other embodiments, a driven gear can be affixed to one end of each of the printing cylinders 210, 220 and gear trains including one or more drive gears are used to transfer torque from the motors 211, 221 to the respective printing cylinders 210, 220. For example,
For the same reasons that were discussed earlier with respect to the diameters of the web-transport rollers, it is desirable that each of the gears (e.g., driven gear 190 and drive gear 191) in these printing cylinder gear trains should rotate an integer number of times for each rotation of the printing cylinders 117, 127. This can be achieved by constraining the gear ratios of the gears (e.g., driven gear 190 and drive gear 191) in the printing cylinder gear trains such that the gears rotate an integer number of times for each rotation of the printing cylinders 117, 127. In this case, the driven gear 190 is affixed to the end of the printing cylinder 117. Consequently, the driven gear 190 will rotate 1× for every rotation of the printing cylinder 117. In accordance with the present invention, the gear ratio for the drive gear 191 is preferably constrained to satisfy the design criteria that it rotates an integer number of times for every rotation of the printing cylinder 117. For example, if the driven gear 190 has 3× the number of teeth as the drive gear 191 (i.e., a 3:1 gear ratio), the drive gear 191 will rotate 3× for every rotation of the printing cylinder 117.
In some embodiments, the impression cylinders 114, 124 (
As was discussed with respect to
In the some embodiments, such as the example shown in
D=M×L=M×C
P (2)
where M is a positive integer.
In the example shown in
Alternatively in some embodiments conductive pattern 350 can be printed using one or more print stations configured like print stations 110 and 130, and conductive pattern 360 can be printed using one or more print stations configured like print stations 120 and 140 of
With reference to
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 spirit and scope of the invention.
