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9473Re: [PPLetterpress] Re: platemaking issues

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  • Gerald Lange
    Mar 7, 2008
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      Peter

      I'm not sure about this. I don't know that coloration has an effect
      though it may very well. The BASF plates have a very dense coloration to
      them.

      The plate materials have not changed since the book was written,
      regardless of which edition you have. The only change I have seen in the
      last decade and a half or so was the recent reformulation of the the
      Toyobo Printight brand. There is a slight change in coloration (which I
      cannot detect) and they expose quicker than the older formula.

      That's about it, except for the newer filmless direct to plate sheet
      photopolymer, not much has changed.

      Gerald
      http://BielerPress.blogspot.com

      Peter Fraterdeus wrote:
      > Hi Cody, et alia
      >
      > Richard wrote:
      >
      >
      >> The characteristic
      >> slope of a well made photopolymer plate is the result of how
      >> photopolymer bends the light ray at the point it enters the
      >> photopolymer.
      >>
      >
      >
      > Photons do travel in a straight line, and thus a broad light source
      > will create a light cone, regardless of diffraction.
      > Your argument implies that the source is an infinitely distant point.
      > While the ray may bend due to the change in medium, I don't think
      > that's the reason for the slope of the shoulder.
      >
      > As far as it goes, I am finding that the plate material from Boxcar
      > produces very vertical shoulders. I think this is as much due to the
      > fact that the yellow color absorbs the UV, and therefore the photons
      > coming at an angle do not spread much beyond the point of entry. Those
      > which enter at a shallow angle are absorbed before reaching the base.
      >
      > It seems to me that the shoulder angle is related to the photo-
      > sensitivity of the polymer.
      > I've been trying to find more in-depth technical info on all this,
      > rather than apocryphal say-so, but the manufacturer's documentation
      > (what little there is) seems to have been translated from Japanese
      > into English leaving much to the imagination. Perhaps I'm looking up
      > the wrong trees. Google likewise provides little elucidation. Of
      > course there's Gerald's book, but why not reinvent the wheel every
      > generation? Also, the plate materials seem to have changed since the
      > copy I have was published.
      >
      > The problems I was having previously with loss of points and moving on
      > the base had to do with too long wash-out, which would undermine the
      > exposed points.
      >
      > I'm now using about 3.5 minute wash instead of five, which has done
      > wonders for the stability of the fine points and rules. With the 'deep-
      > relief' base it needs another minute or so to clean the base, but I'm
      > washing as short a time as possible.
      >
      > Leaders and isolated dots and periods are solid cylinders. Not much
      > shoulder, but they're hard as rocks, firmly seated on the base and
      > seem to be printing fine. In fact, the older plate material I was
      > using produced a wide shallow shoulder, the current stuff, with the
      > clear untinted plastic base produces a nearly vertical shoulder.
      >
      > YMMV, of course....
      >
      > Peter Fraterdeus
      > http://ExquisiteLetterpress.com
      > http://dubuquebookarts.com
      >
      >
      >
      >
      >
      > On 7 Mar 2008, at 10:07 AM, richard seibert wrote:
      >
      >
      >> All light, even after being diffused, travels in a straight line.
      >> When light travels out of one substance into another, its speed
      >> changes. This changes the angle at which the ray travels. Think of a
      >> pencil in a glass of water. Every substance has an "index of
      >> refraction" which determines what this angel is. The characteristic
      >> slope of a well made photopolymer plate is the result of how
      >> photopolymer bends the light ray at the point it enters the
      >> photopolymer. This is also why film needs to be emulsion down.
      >> Nothing that happens to the light in-between being emitted by the
      >> electrons in the lamp and entering the film will effect what happens
      >> at the boundary between film and polymer. (Unless the intervening
      >> substance absorbs or reflects the necessary frequencies.)
      >>
      >> Too steep an angle is the result of not enough photons (or not enough
      >> photons of the right frequency) to fully polymerize the material.
      >>
      >>
      >
      >
      >
      >
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