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13310Re: [PPLetterpress] Photopolymer with 'moving' parts

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  • Gerald Lange
    Dec 3, 2012
      Lola

      Your exposure time seems about right to me. But you really do need to
      test the exposure time with a 21 steps grey scale. With a hardness
      rating of 67 you should get a reading of 14-15 as solid—that means the
      14-15 boxes are not washed out or corroded (13 or below should be washed
      out). Exposure times can vary dependent upon the condition of your
      bulbs/ballasts. Fine lines or dots might need a slightly longer exposure
      time.

      Basically, you are not giving enough time to build a decent structural
      relief if your exposure/washout are out of sync and imaging is drifting.
      You do not have to back-exposure this plate, the manufacturer has
      already done that.

      You washout can vary quite a bit dependent upon bath temp, machine,
      brushes, etc. Mainly you want a clean floor with no undercutting of the
      surface imaging.

      Post exposure should be the same or longer than the main exposure. I
      usually do about 1.5x.

      It is difficult to "read" the validity of a plate immediately after
      washout because clinging moisture can cause optical distortion. Your
      drying temp seems high. Specs sheet recommends 60-70 C. I'm not sure if
      that is the problem but lower the temp and see if that helps.

      Gerald
      http://BielerPress.blogspot.com



      On 12/3/12 1:58 PM, tinglado_letterpress wrote:
      > Dear list members,
      >
      > I am using KF95 PP, processing the plate as follows: exposure: 200 s, wash out: 240 s / 21 ºC, dry out: 10 min at 80 ºC and post exposure 300 s.
      >
      > The main problem is that some letters (body size 6 pt) are pushed out of its possition in the PP. Please, check this picture to see what I mean (http://flic.kr/p/dyigw8). I checked it after wash out and they seem to be at the correct position. Do you think that it could be something related to the dry out process? Do you think that a bit of back exposure could fix the exposed photopolymer to the plastic base?
      >
      > The problem only happens with the '1' and the 'l' characters.
      >
      > I am in trouble, because it happens every time I try to process this work.
      >
      > Please, help!!!! :-)
      >
      > Thank you in advance and regards,
      >
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