US 7,467,587 B2
Method for accurate exposure of small dots on a heat-sensitive positive-working lithographic printing plate material
Joan Vermeersch, Deinze (Belgium); Wim Sap, Bruges (Belgium); and Marc Van Damme, Bonheiden (Belgium)
Assigned to Agfa Graphics, N.V., Mortsel (Belgium)
Filed on Apr. 21, 2005, as Appl. No. 11/114,210.
Claims priority of provisional application 60/567691, filed on May 03, 2004.
Claims priority of application No. 04101647 (EP), filed on Apr. 21, 2004.
Prior Publication US 2005/0235854 A1, Oct. 27, 2005
Int. Cl. B41C 1/10 (2006.01); B41M 1/06 (2006.01); G03F 7/00 (2006.01)
U.S. Cl. 101—467  [101/463.1; 101/453; 101/451; 430/270.1] 19 Claims
OG exemplary drawing
 
1. A method for making a lithographic printing plate comprising the steps of
(a) providing a heat-sensitive positive-working lithographic printing plate precursor which comprises a support and a coating thereon, wherein the coating comprises a novolac, a resole, or a polyvinylphenol;
(b) exposing a halftone image comprising microdots having a size of 25 μm or less on the plate precursor by means of infrared light; and
(c) processing the plate precursor in a developer, thereby removing non-image areas of the coating from the support;
wherein the infrared light has an energy density in the range from CP to 1.5*CP, wherein CP is the clearing point which is defined as the minimum energy density that is required to obtain, after the processing step, an optical density of the coating at fully exposed areas equal to 0.05*Du, and wherein Du is the optical density of the coating in the unexposed state.