Patent Application
20160059540
This invention pertains to the field of flexographic printing, and more particularly to a method of setting an ink transfer pressure between cylinders by optical measurement of printed features.
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, printing sleeves, or printing cylinders, and are provided with raised relief images onto which ink is applied for application to a printable substrate. While the raised relief images are inked, the recessed relief “floor” should remain free of ink.
Although flexographic printing has conventionally been used in the past for printing of images, more recent uses of flexographic printing have included functional printing of devices, such as touch screen sensor films, antennas, and other devices to be used in electronics or other industries. Such devices typically include electrically conductive patterns.
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 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.
WO 2013/063188 by Petcavich et al. 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 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. Printing such narrow lines stretches the limits of flexographic printing technology. It has been found to be difficult to achieve a desired tolerance of plus or minus one micron in line width tolerance.
Line width of printed features can be affected by the ink transfer pressure for providing ink to the flexographic printing plate and also by the ink transfer pressure for printing the ink from the flexographic printing plate onto the substrate. In conventional flexographic printing applications where the tolerance on printed feature size is substantially looser than for touch screen sensor films, the operator of the flexographic printing system can simply inspect the printed image and adjust the ink transfer pressures as needed. However, for printed features such as the grid lines of a touch sensor film, there are two problems with this approach. First, a visual inspection is not sufficiently sensitive to achieve line width tolerances of plus or minus one micron. Second, by their very nature, the grid lines of a touch sensor film are intended to be difficult to see.
What is needed is a method for setting the ink transfer pressure in a flexographic printing system such that very narrow lines, which are difficult to see, can be printed with tight tolerance on line width.
The present invention represents a method of controlling an ink transfer pressure between cylinders in a flexographic printing system, the method comprising:
providing a flexographic printing plate on a printing cylinder, the flexographic printing plate including:
an image region including a plurality of raised printing elements arranged to print an image pattern having printed image features corresponding to the raised printing elements, wherein the printed image features have a smallest lateral dimension that is less than 25 microns; and
one or more pressure characterization regions outside the image region, each pressure characterization region including a plurality of raised printing elements arranged to print a pressure characterization pattern having printed characterization features corresponding to the raised printing elements;
transferring ink from an anilox cylinder to the flexographic printing plate on the printing cylinder, wherein the anilox cylinder and the printing cylinder contact each other with a first transfer pressure;
advancing a recording medium through a nip between the printing cylinder and an impression cylinder such that ink is transferred from the flexographic printing plate to the recording medium to print the image pattern and the pressure characterization patterns, wherein the printing cylinder and the impression cylinder contact each other with a second transfer pressure;
measuring an optical property of at least one printed pressure characterization pattern; and
adjusting one or both of the first and second transfer pressures responsive to the measured optical property of the at least one printed pressure characterization pattern.
This invention has the advantage that the consistency of the image characterization of the printed image patterns will be improved.
It has the additional advantage that the pressure characterization patterns and the image patterns can be designed to have similar image characteristics so that they will respond in a similar way to transfer pressure variations.
It has the further advantage that optical measurements of the pressure characterization patterns can provide a higher signal-to-noise than similar optical measurements made on the image patterns.
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. 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, 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 provide a method for setting ink transfer pressure between cylinders in a flexographic printing system, particularly for printing functional devices incorporated into touch screens. 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 is conventionally used for printing of images and it is contemplated that the pressure setting method described herein can also be advantageous for such printing applications.
The flexographic printing system 100 includes two print modules 120 and 140 that are configured to print on the first side 151 of substrate 150, as well as two print modules 110 and 130 that are configured to print on the second side 152 of substrate 150. The web of substrate 150 travels overall in roll-to-roll direction 105 (left to right in the example of
Each of the print modules 110, 120, 130, 140 includes some similar components including a respective printing cylinder 111, 121, 131, 141, on which is mounted a respective flexographic printing plate 112, 122, 132, 142, respectively. Each flexographic printing plate 112, 122, 132, 142 has raised features 113 defining an image pattern to be printed on the substrate 150. Each print module 110, 120, 130, 140 also includes a respective impression cylinder 114, 124, 134, 144 that is configured to force a side of the substrate 150 into contact with the corresponding flexographic printing plate 112, 122, 132, 142 at a nip formed between an impression cylinder and the corresponding printing cylinder. The impression cylinders 124 and 144 of print modules 120 and 140 (for printing on first side 151 of substrate 150) rotate counter-clockwise in the view shown in
Each print module 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. As is well known in the printing industry, an anilox roller (sometimes referred to as an anilox cylinder herein) is a hard cylinder, usually constructed of a steel or aluminum core, having an outer surface containing millions of very fine dimples, known as cells. How the ink is controllably transferred and distributed onto the anilox roller is described below. In some embodiments, some or all of the print modules 110, 120, 130, 140 also include respective UV curing stations 116, 126, 136, 146 for curing the printed ink on substrate 150. Also shown is controller 103 that can be used in some embodiments for controlling operation of various aspects of the flexographic printing process.
U.S. Pat. No. 7,487,724 to Evans et al. discloses inking systems for an anilox roller in a flexographic printing apparatus.
The anilox roller 18 has its circumferential surface engraved with a multitude of recessed cells, which may be of various geometric configurations, adapted collectively to retain a quantity of printing liquid in a continuous film-like form over the circumferential surface of the anilox roller 18 for metered transfer of the liquid to the image surface on the printing plate 16 of the printing cylinder 14.
The flexographic printing apparatuses of
In contrast, the flexographic printing apparatus shown in
Ink pan 160 includes a front wall 162 located nearer to impression cylinder 174, a rear wall 163 located opposite front wall 162 and further away from impression cylinder 174, and a floor 164 extending between the front wall 162 and the rear wall 163. The ink pan 160 also includes two side walls (not shown in
Fountain roller 161 is partially immersed in an ink 165 contained in ink pan 160. Within the context of the present invention, the ink 165 can be any type of marking material, visible or invisible, to be deposited by the flexographic printing system 100 (
A lip 167 extends from rear wall 163. When an upward force F is applied to lip 167 as in
As described with reference to
In order to remove excess amounts of ink 165 from the patterned surface of anilox roller 175 a doctor blade 180, which is mounted to the frame (not shown) of the printing system, contacts anilox roller 175 at contact point 182. Contact point 182 is downstream of contact point 181 and is upstream of contact point 183. For the configuration shown in
After printing of ink on the substrate, it is cured using UV curing station 176. In some embodiments, an imaging system 177 can be used to measure an optical property of at least a portion of the pattern printed on the substrate as discussed in further detail below. Also shown is controller 103 that can be used to control adjustments of ink transfer pressure according to the measured optical property in some embodiments.
Embodiments of the invention include measuring an optical property of at least one printed pattern that is outside the primary image region, and adjusting at least one ink transfer pressure between cylinders in the flexographic printing system. Controlling the ink transfer pressure is necessary to avoid performance degradations that can occur if the pressure is too low or too high. In particular, if a first transfer pressure P1 between the anilox roller 175 and the printing cylinder 171 on which the flexographic printing plate 172 is mounted is too great, ink can be transferred not only to the tops of the raised features 173 but also partially to the walls of the raised features 173 so that more ink is transferred to the flexographic printing plate 172 than is intended. This can result in broadening of the features in the printed pattern on substrate 150 to an extent that the pattern can tend to fill in, as well as increased variation in line width. Similarly if the second transfer pressure P1 between the anilox roller 175 and the printing cylinder 171 is too low, too little ink will be transferred to the flexographic printing plate 172, which can result in voids in the printed pattern on substrate 150. Likewise, if a second transfer pressure P2 between the printing cylinder 171 and the impression cylinder 174 at the nip through which the substrate 150 is advanced is too great the line width of the printed features on substrate 150 will be too large. Similarly if the second transfer pressure P2 between the printing cylinder 171 and the impression cylinder 174 is too small, the line width of the printed features on substrate 150 will be too small. Thus if the integrated optical density of the printed patterns is measured, the optical density of the printed pattern will be higher than a target value if the second transfer pressure P2 is too great, and the optical density will be lower than the target value if the second transfer pressure P2 is too small.
Herein, when it is said that the anilox roller 175 and the printing cylinder 171 contact each other with a first contact pressure P1, it is understood that the anilox roller 175 and the printing cylinder 171 indirectly contact each other through the flexographic printing plate 172. Similarly, when it is said that the printing cylinder 171 and the impression cylinder 174 contact each other with a second contact pressure P2, it is understood that the printing cylinder 171 and the impression cylinder 174 indirectly contact each other through the flexographic printing plate 172 and the substrate 150.
The anilox cylinder pressure adjustment 191 and impression cylinder pressure adjustment 193 can use various adjustment mechanisms for adjusting their respective pressures. Generally the adjustment mechanisms enable adjusting the magnitude of a force imposed on the axle of the respective cylinder (i.e., the anilox roller 175 or the impression cylinder 174) to push it toward the printing cylinder 171. In some embodiments, the pressure adjustments are made using a screw mechanism.
The flexographic printing plate 200 includes a first edge 208, which will be near first end 228 of printing cylinder 171, and a second edge 209 which will be near second end 229 of printing cylinder 171 when flexographic printing plate 200 is mounted on printing cylinder 171.
The flexographic printing plate 200 includes an image region 201 having a plurality of raised printing elements 202, 203 for printing an image pattern 231 on a surface of substrate 150. The image region 201 in the illustrated example includes an array of horizontal raised printing elements 202 and an array of vertical raised printing elements 203 for printing a grid pattern on the surface of the substrate 150.
In other embodiments, the horizontal raised printing elements 202 and the vertical raised printing elements 203 are provided on two separate printing plates, such as the flexographic printing plates 112 and 132 shown in
The flexographic printing plate 200 also includes two pressure characterization regions 204a, 204b outside the image region 201. Each pressure characterization region 204a, 204b includes a respective plurality of raised printing elements 205a, 205b arranged to print respective pressure characterization patterns 235a, 235b on substrate 150 (
With continued reference to
The use of pressure characterization patterns 235a, 235b has been found to be particularly advantageous when the raised printing elements 202, 203 have a smallest lateral dimension (e.g., line widths W1 and W2, respectively) that is less than 25 microns, and even more so when the smallest lateral dimension is less than 10 microns. This is because it is difficult to determine precisely on a sparse array of narrow features how much the ink transfer pressure needs to be increased or decreased in order to provide the desired feature width.
In a preferred embodiment, the raised printing elements 205a, 205b in the pressure characterization regions 204a, 204b, that are used for printing the pressure characterization patterns 235a, 235b have a smallest lateral dimension which is substantially equal to the smallest lateral dimension (i.e., W1 and W2) of the raised printing elements 202, 203 that are used for printing the image pattern 231. “Substantially equal” in this context means within ±20% or ±1 micron, whichever is larger. However, the characteristic spacings (unlabelled) between raised printing elements 205a, 205b in the pressure characterization regions 204a, 204b are typically significantly smaller than characteristic spacings (i.e., line spacings S1 and S2) between the raised printing elements 202, 203 that are used for printing the image pattern 231. For example, the line spacings S1 and S2 between the raised printing elements in the plurality of raised printing elements 202, 203 can be about 200 microns to about 500 microns and the characteristic spacing between raised printing elements 205a, 205b can be less than 100 microns. The line widths W1 and W2 can be about 5 microns. As a result of the higher density, measurements of optical properties of printed pressure characterization patterns 235a, 235b can be performed with a better signal-to-noise ratio than similar optical properties of printed image pattern 231.
In the exemplary embodiment illustrated in
A transfer ink from anilox roller to printing plate step 260 is used to transfer ink to the flexographic printing plate 200. The ink is transferred at a nip formed between the anilox roller 175 (
A transfer ink from printing plate to substrate step 270 is used to transfer ink from the flexographic printing plate 200 to the substrate 150 (
A measure optical property step 280 is used to measure an optical property 285 of at least one of the printed pressure characterization pattern(s) 235. The measure optical property step 280 can use a variety of types of optical measurement to measure a variety of different types of optical properties 285 in various embodiments of the invention. Within the context of the present invention, an “optical property” is one that can be measured with an optical device (e.g., a densitometer or a digital camera).
In an exemplary embodiment, the optical property 285 is an integrated optical density. In this case, an optical densitometer can be used to measure an integrated optical reflection density or an integrated optical transmission density. Alternately, the optical densitometer can be used to measure an integrated optical reflectance or an integrated optical transmittance. Note that there is a simple mathematical relationship between reflection density (DR) and reflectance (R) given by DR=−log(R), so these quantities can be viewed as representations of the same quantity. Likewise, there is a simple mathematical relationship between transmission density (DT) and transmittance (T) given by DT=−log(T). Optical densitometers are well known in the art, and generally include a light source which illuminates the substrate 150 with a uniform region of light and measures the light that is either reflected from or transmitted through the substrate to determine the corresponding optical density value.
In some embodiments, the integrated optical density properties (e.g., the integrated optical reflection density, the integrated optical transmission density, the integrated optical reflectance or the integrated optical transmittance) can be measured within a single field-of-view within the pressure characterization pattern(s) 235. In other embodiments, a plurality of integrated optical density measurements can be made using different fields-of-view within the pressure characterization pattern(s) 235. The resulting optical density values can then be averaged to reduce measurement noise and the resulting average optical density can be used as the optical property 285.
In other embodiments, rather than measuring an integrated optical density, the local optical density properties (e.g., the local optical reflection density, the local optical transmission density, the local optical reflectance or the local optical transmittance) of the printed characterization features 234a, 234b (
In other embodiments, the measured optical property 285 can be a geometric characteristic of the printed pressure characterization pattern(s) 235 that is determined by analyzing a digital image of the pressure characterization pattern(s) 235 captured using an appropriate image capture device such as a digital camera. For example, the geometric characteristic can be a lateral dimension of the printed characterization features 234a, 234b (
In a preferred embodiment, the measure optical property step 280 is performed using an optical measurement device which is integrated into the flexographic printing system (
In an alternate embodiment, the measure optical property step 280 can be performed offline by removing a piece of substrate 250 from the web and bringing it to a separate optical measurement device (not shown). An operator can then perform a manual measurement to determine the optical property 285.
An adjust transfer pressure(s) step 290 is used to adjust one or both of the first transfer pressure 265 and the second transfer pressure 275 responsive to the measured optical property 285 determined for at least one of the pressure characterization pattern(s) 235. Any appropriate process control process known in the art can be used to adjust the transfer pressures. In an exemplary embodiment, the measured optical property is compared to a predefined target optical property 295 and the difference can be used to determine a transfer pressure adjustment. The transfer pressure adjustments can be made using, for example, the anilox cylinder pressure adjustments 191, 192 or the impression cylinder pressure adjustments 193, 194 described above with reference to
For cases where pressure characterization patterns 235a, 235b (
In some embodiments, the transfer pressure adjustments can be performed automatically using automated adjustment mechanisms (e.g., motors or hydraulic systems) that are controlled by the controller 103. In other embodiments, the transfer pressure adjustments can be performed manually by an operator. In some embodiments, adjust transfer pressure(s) step 290 can include an automatic analysis step which compares the measured optical property 285 to the target optical property 295 and determines an appropriate transfer pressure adjustment. A user interface can then be used to communicate the recommended transfer pressure adjustment to the operator, who can then manually adjust the transfer pressure controls to apply the transfer pressure adjustment.
In some embodiments, a plurality of different optical properties 285 can be determined and used in the process of adjusting the transfer pressures. For example, both the line width and the local optical density of the printed characterization features 234a, 234b (
Alternatively in some embodiments conductive pattern 350 can be printed using one or more print modules configured like print modules 110 and 130, and conductive pattern 360 can be printed using one or more print modules configured like print modules 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.
