Method and apparatus for in-situ digital image forming fountain solution thickness measurement
A coefficient of friction (COF) sensor on a carrier roll surface wetted with fountain solution transferred from an imaging member measures COF of the wetted carrier roll surface in real-time, even between or during printing operations. The transferred fountain solution may be concentrated and/or chilled to solidify before the measurement. The measured COF is used in a feedback loop to actively control the fountain solution layer thickness by adjusting the volumetric feed rate of fountain solution added onto the imaging member surface during an imaging or other printing operation to reach a desired uniform thickness for the printing. This fountain solution monitoring system may be fully automated.
1. A method of controlling fountain solution thickness on an imaging member surface of a rotating imaging member in an image forming device, comprising:
(a) transferring fountain solution from a fountain solution layer dispensed at a fountain solution dispense rate onto the rotating imaging member onto a carrier roll having a surface with a coefficient of friction that varies as a function of fountain solution layer thickness to allow the transferred fountain solution to wet the carrier roll surface and alter the coefficient of friction to a wetted coefficient of friction;
(b) measuring the wetted coefficient of friction of the wetted carrier roll surface as a measured coefficient of friction;
(c) modifying the fountain solution dispense rate to a modified fountain solution dispense rate based on the measured coefficient of friction; and
(d) applying a subsequent fountain solution layer at the modified fountain solution dispense rate onto the imaging member surface for an image forming operation thereon.
2. The method of claim 1 , further comprising, before Step (a), dispensing the fountain solution from a fountain solution applicator at the fountain solution dispense rate onto the rotating imaging member to form the fountain solution layer.
3. The method of claim 1 , further comprising, before Step (a) positioning the carrier roll adjacent to and spatially separate from the imaging member.
4. The method of claim 3 , further comprising, after step (b), withdrawing the carrier roll away from the imaging member and removing the transferred fountain solution from the carrier roll.
5. The method of claim 1 , further comprising, in Step (a), transferring the fountain solution as fountain solution vapor from the fountain solution layer dispensed onto the rotating imaging member.
6. The method of claim 5 , further comprising, between Step (a) and Step (b), solidifying the transferred fountain solution on the carrier roll, and after Step (b), vaporizing the solidified fountain solution.
7. The method of claim 1 , further comprising, after Step (b), comparing the measured coefficient of friction of the wetted carrier roll surface to a target coefficient of friction, the target coefficient of friction corresponding to a predetermined fountain solution layer thickness on the rotating imaging member, and Step (c) includes modifying the fountain solution dispense rate based on the comparison.
8. The method of claim 1 , further comprising, after Step (b), estimating a thickness of the fountain solution layer dispensed onto the imaging member based on the measured coefficient of friction of the wetted carrier roll surface, and Step (c) includes modifying the fountain solution dispense rate based on the estimated thickness of the fountain solution layer.
9. The method of claim 8 , further comprising comparing the estimated thickness of the fountain solution layer dispensed onto the imaging member with a target thickness, and Step (c) further includes modifying the fountain solution dispense rate based on the comparison between the estimated thickness of the fountain solution layer and the target thickness.
10. The method of claim 1 , further comprising before Step (b) moving a coefficient of friction sensor into contact with the carrier roll, during Step (b) measuring the wetted coefficient of friction of the wetted carrier roll surface with the coefficient of friction sensor, and after Step (b) moving the coefficient of friction sensor away from contact with the carrier roll.
11. The method of claim 1 , further comprising, during Step (a), rotating the carrier roll at a rotational speed slower than a rotational speed of the rotating imaging member, with the transferred fountain solution forming a layer on the carrier roll thicker than the dispensed fountain solution layer.
12. The method of claim 1 , further comprising, after Step (a), solidifying the transferred fountain solution on the carrier roll.
13. A method of controlling fountain solution thickness on an imaging member surface of a rotating imaging member in an image forming device, the image forming device printing a current image onto a print substrate, the printing including applying a fountain solution layer at a fountain solution dispense rate onto the imaging member surface, vaporizing in an image wise fashion a portion of the fountain solution layer on the imaging member surface to form a latent image, applying ink onto the latent image, and transferring the applied ink to the print substrate, the method of controlling fountain solution thickness comprising:
(a) transferring fountain solution from the fountain solution layer dispensed at the fountain solution dispense rate onto the rotating imaging member onto a carrier roll having a surface with a coefficient of friction that varies as a function of fountain solution layer thickness to allow the transferred fountain solution to wet the carrier roll surface and alter the coefficient of friction to a wetted coefficient of friction;
(b) measuring the wetted coefficient of friction of the wetted carrier roll surface as a measured coefficient of friction; and
(c) modifying the fountain solution dispense rate to a modified fountain solution dispense rate based on the measured coefficient of friction for a subsequent printing of a subsequent image by the image forming device using the modified fountain solution dispense rate.
14. An image forming device controlling fountain solution thickness on an imaging member surface of a rotating imaging member, comprising:
a carrier roll having a surface with a coefficient of friction that varies as a function of fountain solution layer thickness;
a heat source configured to evaporate fountain solution from a fountain solution layer dispensed onto the rotating imaging member at a dispense rate for transfer onto the carrier roll, the transferred fountain solution wetting the carrier roll surface via condensation thereon and altering the coefficient of friction to a wetted coefficient of friction;
a sensor including a sensor head adjacent the carrier roller and in contact with the transferred fountain solution, the sensor configured to measure the wetted coefficient of friction; and
a controller in communication with the sensor and a fountain solution applicator to modify the dispense rate based on the measured wetted coefficient of friction for a subsequent dispense of a subsequent fountain solution layer at the modified fountain solution dispense rate onto the imaging member surface for an image forming operation thereon.
15. The image forming device of claim 14 , wherein the carrier roll surface is engineered to have a root-mean-squared (RMS) surface roughness greater than 0.2 μm.
16. The image forming device of claim 15 , wherein the carrier roll surface includes a plurality of engineered surface sections, wherein one of the plurality of engineered surface sections is engineered to have a root-mean-squared (RMS) surface roughness different than another one of the plurality of engineered surface sections.
17. The image forming device of claim 14 , further comprising the subsequent fountain solution layer, wherein the carrier roll surface is engineered to have a root-mean-squared (RMS) surface roughness greater than a thickness of the subsequent fountain solution layer.
18. The image forming device of claim 14 , wherein the heat source includes an LED bar.
19. The image forming device of claim 14 , wherein the carrier roll surface is chilled to a temperature below a freezing temperature of the fountain solution to solidify the transferred fountain solution.
20. The image forming device of claim 14 , the fountain solution applicator configured to dispense the fountain solution layer onto the rotating imaging member at the dispense rate and to dispense the subsequent fountain solution layer at the modified fountain solution dispense rate onto the rotating imaging member for the image forming operation thereon.