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9485Re: platemaking issues

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  • parallel_imp
    Mar 11, 2008
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      --- In PPLetterpress@yahoogroups.com, typetom@... wrote:
      >

      > If you use a glass top vacuum frame, you need to add matboard around
      > the plate material within the frame, to support the glass across the
      > whole surface. Otherwise, glass will bend under the vacuum, and
      > allow some light to bleed irregularly around edges of the negative
      > (which is not held in tight contact due to the bend in the glass).
      > Without the matboard support, the plate will have swollen lines that
      > print irregularly bold.

      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
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