Adam Kimball (akimball@finebrand.com)
Sun, 04 Apr 1999 15:12:27 -0700
Well, I thought I had finally made some real progress of the photogravure
front. I did a plate with a digital screen that I was quite happy with. It
was a very small and had a lot going on within it. However, a friend and I
have been working with the screen and have noticed real problems with its
use. First, a little background may help.
I purchased a stochastic screen from the service bureau that was handling my
digital negative output. It is a 70% tint (positive) printed at 100ppm f
memory serves me right. Emulsion down, etc. Everything looks great with
it. No uneven densities.
Anyway, what we have noticed is that during the screen exposure, we are
getting a very random and very murky spotting of the resist. These spots may
be 5mm across, usually smaller. They are *not* in focus. They affect shadow
densities the worst, today's test revealed little ability to discern them in
the highlight areas (note gravure requires a positive, so when I say
highlight I am refering to areas on the positive with little density). These
spots throw a major wrench into the process, and I can't figure out their
origin. I've tried..
1. Bleeding the vacuum pressure to 10 bars. No luck.
2. Emulsion side to the glass (per a long-ago discussion on Newton's Rings
found in the archive)
3. Mylar on top.
I have yet to try dusting the screen with talc or cornstarch, or using an
Anti-Newton Ring product. Everything is closed on Easter :(
Some gravurists I have talked to have mentioned that dust on the screen can
cause these spots. That doesn't make sense to me.. how could a piece of dust
(microns wide) cause a 5mm spot? But, lots of stuff about photogravure makes
no sense at this stage.
It is important to note that these spots DO NOT occur when I am exposing a
positive film to the tissue. I've made a number of carbon transfers with
this tissue that show NONE of these problems. It only occurs when using the
screens.
Right now I am leaning toward Newton Rings as the problem, followed by dust,
followed by suspicion of the imagesetter's film stock.
Anybody have these sorts of problems before? Any suggestions for tests?
Thanks (as always),
Adam
This archive was generated by hypermail 2.0b3 on Thu Oct 28 1999 - 21:39:30