Re: platemaking issues
- --- In PPLetterpress@yahoogroups.com, typetom@... wrote:
>Tom, here's another way to look at the problem of photopolymer plates
> . . .
> The answer is that the glass was bending in my vacuum frame, allowing
> some slight distortion of contact with the negative and thus an
> irregular exposure and thus swelling in the plate. Mat board placed
> around the plate materialwithin the frame completely solved the
> problem. The mat board gives support forthe glass across the size
> of the frame.
in a glass-faced vacuum frame. Many photopolymer plates are smooth and
tacky, and stick easily to smooth-surfaced films. If the drawdown in
the frame is fast, a bond can form at one or more edges of the plates
before the air is drawn out of the interior of the plate, causing
out-of-contact bubbles. The rubber blanket pulling the plate up might
also apply more pressure at the edges. Using the bleed valve allows
for a soft contact, removing more air before hard contact is achieved.
Your cardboard is definitely relieving the pressure at the edge, and
would also be allowing more air to escape. But I think the forces at
work are moving upward to the glass, not downward to the plate.
The problem of plate tackiness is addressed in many manufacturer's
instructions, specifying matte-emulsion film, or detacking powder
(silicon spray is another possibility). I'm now using the Rigilon HX
which itself has a matte-emulsion for better drawdown to any kind of
film. This isn't an issue with a crene-covered frame, since you can
brayer or burnish out any air bubbles, but with glass it is definitely
a problem to overcome, with various possible solutions.
--Eric Holub, SF
- Something new (something old & new) every day. Thanks, Eric.
My nuArc vacuum frame has a name plate but no model number, no bleed valve,
but a pressure gauge on the line. It has a glass top hinged within a metal
frame, with a vacuum area about 21.5 x 25.5. Not clear whether it ever had a
light attached -- it appears to have been part of a larger apparatus. The
vacuum space below the glass is sufficient to handle plate material, more than a
just contact prints.
Meanwhile, I do dust the plate material with powder to prevent it from
grabbing the negative. I don't think air bubbles are the cause of the exposure
irregularities I experienced. Uneven pressure on the plate and negative at the
edges of the plate (which I have assumed involves some slight bending of the
glass) seems the likely explanation. Mat board around the plate seems to solve
the problem completely. Whatever works!
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