The title of this article is meant to address a common mistake platemakers make when performing both a main exposure and back exposure step test. We see this happen more frequently on main exposure tests because the lamp cycle is much longer than a back exposure step test. With conventional bank light exposure systems, we find it necessary to perform these tests monthly and when we change photopolymer batches.
Making an Optimized Repeatable Product
We have all heard that we should not build or stack times when performing a main or back exposure step test. Each step should be performed with one interrupted exposure to UV-A. Building or stacking partial exposure lengths to obtain one complete face exposure introduces a new factor to the process and would either have to be compensated for or eliminated. Not only do we strive to make a plate that has been optimized for performance and longevity, but we are also looking to make a consistent or repeatable product. This reasoning supports our decision to eliminate the factor in question versus trying to measure and compensate for the lost energy caused by each interruption.
Testing this theory is simple. First, expose a piece of polymer one complete uninterrupted exposure cycle. Expose a second piece of material interrupting the cycle by turning the lamps on/off multiple times until the predetermined exposure time has been reached. Then process, dry, post expose, and detack the plate. If doing a back exposure, measure each plate sample for floor thickness. If comparing differences in main exposure, measure differences in minimum dot held or differences in plate shoulder width—which affects the support of the image area and will affect dot gain.
- Note: The testing we performed yielded a difference to floor thickness of 7% and differences to shoulder width of 14%.
Polymer and Pizza
A colleague once described to me the reasoning behind this best practice. He said it is like putting a pizza in an oven to bake for 25 minutes, but pulling it out every 5 minutes to check the pizza. The pizza, or plate, losses momentum and will require additional time to be processed. Interrupting an exposure cycle starts a chemical chain reaction that will then require additional time to build that energy back up.
With this in mind, we must now look towards some of our older exposure units that are equipped with analog timers that may max out at only 15 minutes. Many of today’s high-performance polymers that are used for high line counts require main exposure times in excess of 15 minutes. With the limitations of some analog timers, this requires the platemaker to build up the main exposure by being forced to interrupt the cycle. Some polymer manufactures suggest increasing the typical exposure time an additional 10%. In order to eliminate this issue, the analog timers can be replaced with modern digital timers.
If you want to learn more about this subject, please feel free to reach out to us by calling 1-800-445-4017, or by e-mail at [email protected].
All Printing Resources has formed our Technical Solutions Group (TSG) to encompass our full range of expertise in al critical areas of the flexo process. This team is made up of industry professionals dedicated to being up-to-date on new technologies along with best practices. They are armed with the latest in diagnostic tools and are experienced in problem-solving that can achieve sustainable results. The TSG has walked in your shoes, and have felt your pain.
We offer a variety of equipment services, including annual and semi-annual preventative maintenance programs for digital and conventional photopolymer platemaking systems. We also offer services to replace and install digital timers, replacing your old analog timers on your exposure, post-exposure and detack units. To obtain information on these programs and services please contact TSG Sr. Equipment Service Technician, C.J. Marriner at [email protected] or call (630) 715‐6338