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9486Re: [PPLetterpress] Re: platemaking issues

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  • typetom@aol.com
    Mar 12, 2008
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      In a message dated 3/11/2008 4:36:05 P.M. Mountain Standard Time,
      Megalonyx@... writes:

      Tom, are you talking about a commercially made vacuum frame, a
      pressure frame, or something homemade?
      I don't see how glass would bend in a NuArc vacuum frame. There is
      a flexible rubber blanket beneath the glass, and as the air is sucked
      out, the blanket is pulled up to the glass. The glass does not distort
      in my experience. Proper use of relief valve will leave no air between
      glass, neg, and plate, and full vacuum is indicated by appearance of
      Newton's Rings.
      I could understand this problem if you were talking about a
      pressure frame like those Amergraph units using foam rubber, or
      something not using the normal vacuum blanket.
      But if air bubbles do form in a vacuum frame, it may be from not
      using either matte-emulsion film or a matte plate, or failing to
      back-trim plates to remove burrs and swollen edges.
      --Eric Holub, SF



      Hi Eric,
      I am using an old NuArc vacuum frame (with a homemade bank of UV bulbs in a
      box placed on top of the glass), not sure of the model or any possible NuArc
      variants of design. The problems I had were not due to air bubbles or burrs.
      The plates often had an irregular small area that printed more boldly than the
      rest of the plate -- the end of a long line, one edge of a text block. No
      apparent rhyme or reason, no regular pattern as to where the bolder edge would
      appear. The irregular differences in the plate were hard to see when looking
      at the surface of the plate, but it showed on printing. At first I thought it
      was a problem with my rollers or my trucks, perhaps causing irregular inking
      more heavily down the sides of the type. New rollers, metal trucks, very
      careful adjustments, no difference.

      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 material
      within the frame completely solved the problem. The mat board gives support for
      the glass across the size of the frame. I can see no other explanation. While
      the rubber blanket beneath the glass is pulled up by the vacuum, the glass
      apparently distorts slightly around the edges of the plate material if it is
      not supported. Differences in the positions of plate materials in the frame
      may account for the irregular location of the problem on different plates.
      Maybe my rubber blanket is old enough to be too stiff to shape to the contours of
      the plate?

      (I'm not clear what you mean by: "Proper use of relief valve will leave no
      air between glass, neg, and plate, and full vacuum is indicated by appearance
      of Newton's Rings." I'm not aware of a "relief valve" in my system. I do
      think I know what you mean by Newton's Rings... I suspect there is some
      difference in use of a vacuum pump at Denver's elevation and thin air, but it seems to
      maintain sufficient full contact between the negative and the plate material
      except for the problem described above.)

      Meanwhile, thanks for your on-going excellent observations on all aspects of
      photopolymer plates and printing. Best wishes,
      Tom

      Tom Parson
      Now It's Up To You Publications
      157 S. Logan, Denver CO 80209
      (303) 777-8951 home
      (720) 480-5358 cell phone
      http://members.aol.com/typetom



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