Loading ...
Sorry, an error occurred while loading the content.

9472Re: [PPLetterpress] Re: platemaking issues

Expand Messages
  • Peter Fraterdeus
    Mar 7, 2008
    • 0 Attachment
      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

      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.
    • Show all 22 messages in this topic