SOLVENT REPLENISHMENT USING DENSITY SENSOR FOR FLEXOGRAPHIC PRINTER
A flexographic printing system uses a flexographic printing plate to produce printed patterns on a substrate. An ink recirculation system is used to reduce variability in system performance resulting from ink viscosity changes. A recirculation pump moves ink through an ink recirculation line connected to an ink reservoir. A metering pump adds a controlled flow rate of solvent from a solvent replenishment chamber into the ink recirculation line, thereby providing replenished in which is returned to the ink reservoir. A control system is used to control the flow rate of solvent provided by the metering pump responsive to an ink density measured using a density sensor.
1 . A flexographic printing system including a print module comprising:
a plate cylinder on which is mounted a flexographic printing plate having raised features defining a pattern to be printed on a substrate;
an impression cylinder that is configured to force the substrate into contact with the flexographic printing plate;
an ink reservoir containing an ink and including one or more ink recirculation ports;
an anilox roller having a patterned surface for transferring a controlled amount of the ink from the ink reservoir to the flexographic printing plate;
an ink recirculation system including:
an ink recirculation line that is connected to the one or more ink recirculation ports of the ink reservoir;
a recirculation pump for moving ink through the ink recirculation line;
a solvent replenishment chamber containing solvent;
a metering pump for pumping a controlled flow rate of solvent from the solvent replenishment chamber into the ink recirculation line; and
a mixing device for mixing the solvent and the ink thereby providing replenished ink;
a density sensor for measuring a density of the ink; and
a control system for controlling the flow rate of solvent provided by metering pump based on a measured density of the ink measured using the density sensor.
2 . The flexographic printing system of claim 1 , wherein the control system controls the flow rate of the solvent provided by the metering pump such that variations in the measured density of the ink as a function of time are reduced relative to a target density.
3 . The flexographic printing system of claim 1 , wherein the density sensor is positioned in the ink recirculation system downstream of the location where the solvent is pumped into to the ink recirculation line.
4 . The flexographic printing system of claim 1 , wherein the density sensor is positioned in the ink recirculation system downstream of the mixing device.
5 . The flexographic printing system of claim 1 , wherein the density sensor is positioned in the ink recirculation line.
6 . The flexographic printing system of claim 1 , wherein the ink recirculation system further includes a bypass line in parallel with the ink recirculation line, and wherein the density sensor is positioned in the bypass line.
7 . The flexographic printing system of claim 1 , further including a density sampling system that is independent of the ink recirculation system, and wherein the density sensor is positioned in the density sampling system.
8 . The flexographic printing system of claim 7 , wherein the density sampling system includes:
an ink line that draws ink from the ink reservoir and returns the ink to the ink reservoir; and
a ink line pump for moving the ink through the ink line;
wherein the density sensor is positioned in the ink line.
9 . The flexographic printing system of claim 8 , wherein the ink line pump is controllable to be deactivated at times when density measurements are not being made using the density sensor.
10 . The flexographic printing system of claim 1 , further including a temperature sensor for measuring a temperature of the ink, and wherein the measured density is adjusted responsive to a measured temperature.
11 . The flexographic printing system of claim 1 , wherein the mixing device includes a static inline mixer.
12 . The flexographic printing system of claim 1 , wherein the mixing device includes a dynamic mixing device.
13 . The flexographic printing system of claim 1 , further including a dynamic ink reservoir mixing device disposed within the ink reservoir.
14 . The flexographic printing system of claim 1 , wherein the recirculation pump is a peristaltic pump.
15 . The flexographic printing system of claim 1 , wherein the print module is a first print module of a plurality of print modules, and wherein at least a portion of the ink recirculation system is shared among the plurality of print modules.
16 . The flexographic printing system of claim 1 , wherein the print module is a first print module of a plurality of print modules, and wherein the flow rate of solvent is independently controlled for at least two of the print modules.
17 . The flexographic printing system of claim 1 , wherein one of the ink recirculation ports is disposed proximate to a center of the width of the ink reservoir.
18 . The flexographic printing system of claim 1 , further including:
an ink recovery tank; and
an ink recovery valve positioned downstream of the recirculation pump;
wherein when the ink recovery valve is in a first position the ink is directed through the distribution tube into the ink reservoir, and when the ink recovery valve is in a second position the ink is diverted into the ink recovery tank.
19 . The flexographic printing system of claim 1 , wherein the ink reservoir is an ink pan.
20 . The flexographic printing system of claim 1 , wherein the ink reservoir is an ink reservoir chamber.
21 . The flexographic printing system of claim 1 , wherein the ink recirculation system further includes a distribution tube for supplying the replenished ink to the ink reservoir, wherein the distribution tube includes a plurality of supply ports for supplying the replenished ink to the ink reservoir at a plurality of spaced apart locations across a width of the ink reservoir.
22 . The flexographic printing system of claim 1 , wherein a viscosity of the ink is between 10 centipoises and 20,000 centipoises.
23 . An article including a substrate with a printed pattern that has been printed by the flexographic printing system of claim 1 .
24 . The article of claim 23 , wherein the article is a touch screen display, and wherein the pattern formed on the substrate includes a set of conductive lines.