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12871Re: Laser Engraved Photopolymer Plates

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  • Eric
    Oct 23, 2011
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      --- In PPLetterpress@yahoogroups.com, "Gerald Lange" <Bieler@...> wrote:
      > A major concern would be maintaining relative reverse relief depth, which is fairly uniform when exposing photopolymer plates specified for letterpress applications, no matter what the thickness of plate. For instance, the relief depth of the counter of a small point size lowercase e or o is not going to be open to the depth of the floor of the plate. It is halted with maximum exposure at a specific range, about .30 mm; relief depth is not uniform [which is actually the best technical argument refuting the practice of extreme impression]. I don't see how this could be controlled with a laser cutter.

      Gerald, this common structure to thinner and thicker plates would be due to equal diffraction of light as it penetrates the photopolymer layer?
      A diagram in an old GAM article about the earliest photopolymer plates suggests there was some reflection from the steel back which helped to form the base, along with diffraction. However the plates I use have a yellow layer above the back that should absorb some of the UV light, just as yellow masking sheets do (even if red mask is preferred). I've seen others with a greenish layer.
      --Eric Holub, SF
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