When executing photo-based printmaking processes it is helpful to have previous examples and testing completed prior to look at and make exposure time determinations from. This is especially true with letterpress photopolymer plates. With these plates, the image determines the exposer time. Where half-toned photographic image might only take a minute or so, a graphic image requires a longer exposer time. For this reason, I created an exposer calculator that you can find in the letterpress shops. I will outline here the way that was created, the process I went through, and outcomes that I observed.
In screenprinting exposer calculators are used often in the early stages of setting up a shop, or to test new emulsions. Anthem Screen Printing has a nice exposer calculator that can be downloaded for free and outlines the steps that are required to take to determine an accurate exposer time. I used this exposer calculator as a point of departure for the exposer calculator that I made. Where in screenprinting you are looking to determine one single exposer time, in polymer plates with hand washing at ASU it was important to understand what would happen with a variety of image structures at different times. For this reason, I included…
After making a right-reading negative film on an Epson 3880 inkjet printer on Pictorico film I was ready to expose my plate. I set up the exposer unit as normal by removing the protective cover from the photopolymer plate, placing the film on the plate emulsion to emulsion, and using the plastic covering to create a vacuum over the sandwiched plate/film combination. I then used a piece of chipboard to cover all but one of the rows of images. I set the timer for 20 seconds and exposed the plate. Once the timer went off I moved the chipboard down a row leaving the first and second row open and exposed for another 20 seconds. Added together the first row had now been exposed for 40 seconds and the second row for 20 seconds. I kept repeating this processes until all 12 rows had been exposed in varying increments of 20 seconds. The range of exposer went goes from 20 seconds to 4 minutes. I then washed out the plate using a brush, warm water, and a sticky plate like normal. Then post-exposed the plate for 12 minutes to harden the plate for printing.
After printing, I was able to make a few observation. A good exposer for a photographic half-toned image sits around 1:00-1:20. A good exposer for line work sits at 2:40. A graphic image is pretty good after 1:20. Lines that have a stroke size smaller than .5pt are too delicate to hold up to washing. Font size smaller than 8 point has a really hard time holding up to washing, though you might be able to do 6pt if you are very delicate.
I hope this helps many of you moving forward with calculation your photopolymer plate times.