Method and apparatus for exposure of flexographic printing plates using light emitting diode (LED) radiation sources

A method for preparing a plurality of printing plates and optimizing the user-definable time delay, the method comprising:

• (a) providing an apparatus for preparing a printing plate comprising a photosensitive polymer activated by exposure to ultraviolet (UV) radiation, the printing plate having opposite sides, including a non-printing back side and a printing front side, the front side having a mask for defining an image to be printed, the printing plate having first and second lateral dimensions, the apparatus comprising:
• at least two sources of UV radiation, including a front source of radiation comprising an array of light emitting diodes (LEDs) positioned to emit a radiation field incident on the front side of the plate and a back source of UV radiation comprising an array of LEDs positioned to emit a radiation field incident on the back side of the plate;
• a substrate, at least partially transparent to the radiation, positioned to receive the printing plate in a fixed configuration between the front source and the back source;
• means for causing movement of at least the front radiation field relative to the substrate in a travel direction along the second lateral dimension of the plate fixed on the substrate;
• and
• a controller connected to the front source, the back source, and the means for causing the movement;
• (b) creating a number of printing plate samples, each sample created using a different user-defined time delay during an exposure step for the sample;
• (c) printing a print using each of the number of printing plate samples;
• (d) identifying an optimum user-defined time delay (t) corresponding to the print printed in step (c) which (i) holds the smallest printing dots or (ii) has a minimum value for a smallest printed minimum dot diameter coinciding with a maximum value for dot ground diameter for a range of time delay values;
• (e) utilizing the optimum user-defined time delay (t) during at least one exposure step for the plurality of printing plates, wherein the apparatus is configured to sequentially commence a first exposure step of a first side of the plate with one of the at least two sources and then commence a second exposure step of a second side of the plate opposite the first side with the other of the at least two sources with the optimum user-definable time delay (t) between commencement of the first exposure step and commencement of the second exposure step, and wherein the first exposure step comprises an exposure of the back side of the plate with the back source and the second exposure step comprises an exposure of the front side of the plate with the front source.