Depending on your set up, several tests be done:
If you are using an integrator instead of a timer, place the probe in the center
of the frame and time the exposure. Move the probe to the edge and compare the
time to the first exposure.
If you are using a timer, then expose a 21 step scale in the center of the frame
and then move the tissue to the edge and and make a second exposure next to the
first ( with the respective halves protected from light). Transfer the tissue to
copper, develop and evaluate the two scales. Any exposure variation should be
visible in the resist without having to etch and sacrifice any copper.
The exposure tests are assuming you are using continuous tone positives. If you
are using digital halftone positives then a suspect will be the contact pressure
in your frame. Poor contact due to a vacuum seal at the edge of a large sheet
or short draw down times will cause under cutting of the halftone structure
especially in the highlights and produce overexposure in the center and thus a
slower etch.
If it is an etching problem, then a longer pre-soak in 45 baume ferric chloride
should even out the wetting of the resist. If there is residual moisture in the
resist, this will accelerate etching in those moist areas but would probably be
blotchy and not repeatable. This is unlikely, as the pattern you describe is a
classic light fall off symptom, especially if what you see is repeating.
Other clues can be based on the type of UV light source (fluorescent or halide),
light source distance to the frame, amount of vacuum (inches of mercury) in the
contact frame, and how long is the draw down before exposure.
Good Luck, (I may soon query you with my own frustrations.)
Russ Dodd
> Recently I have been making some large (16 x 20 and bigger) photogravure
> plates and have encountered a vexing problem which never seemed to happen when
> I was working smaller: When etching, the center area of the plate always seems
> to be a bit behind the rest. This results in what looks like an oval shaped
> hot spot, usually most visible in underetched highlights. These hot spots are
> not in my positive. I have etched small test plates using the same positive
> and have not encountered this problem, but when I go for the full Monty the
> problem reappears. My guess is that it has to do with the rate of penetration
> of the ferric chloride into the resist and perhaps uneven moisture in the
> resist (due to it's large size?) leading to uneven penetration. This is a
> particularly frustrating problem (and an expensive one- been going through
> alot of copper). Has anyone else out there had a similar problem and/or
> suggest a remedy?
>
> Thanks,
> Jonathan Higgins