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12639Re: [PPLetterpress] Re: Problem with PP crop marks

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  • Gerald Lange
    Jun 15, 2011

      Yes, for the most part this is correct re: exposure and temperature.

      There is a certain latitude available in regard to exposure. Increase it
      and you will likely have better results. Especially since there is a
      given wane to bulbs over time. Increasing exposure should probably be
      your first attempt at correction...

      As you point out, sheet photopolymer plates are already back-exposed. It
      will likely not help to add to this though, if you even can, re your
      term "safely" (since it has already been done at the manufacturer),
      though I have read it proffered for "deep relief" polyester-backed
      plates as a fix (but only from one source). Not sure how valid this is,
      these plates are not formulated for letterpress in the first place. Hard
      to make a silk purse out of a pig's ear.


      On 6/15/11 3:07 AM, KalleP wrote:
      > Hi,
      >> During the drying process, our crop marks are curling upwards. It is as though they no longer want to be part of the bottom of the polymer.
      > Not having done any plates personally I still have thoughts on your problem.
      > Assuming most of the other variables are similar this looks like the polymer does not have a chance to expose all the way to the base to 'attach' the printing face to the plate. The thin crop mark has to expose all the way down through the narrow slit in the negative without help from adjacent text to raise the floor.
      > Two fixes suggest themselves, as others have mentioned increase the exposure time, and secondly to add a bit of back exposure across the whole plate or even only around the crop masks with a cardboard or paper mask. Photopolymer has a factory exposure from the back to bond it to the film and this can be safely added-to at the expense of the plate relief depth. Metal backed plates will not work with this work around.
      > The thought was that your light output is less than usual and bulb age and temperature are suspected. I have read in many scanner manuals that they take 3 minutes to stabilise in light intensity before they reach the specified even level if they use fluorecent illumination. I also know that old tubes may fail to light up in very cold weather so temperature is certainly implicated here if you have had a cold spell. To fix it, warm up your exposure unit with a heater or run a 5 or 10 minute dummy exposure cycle just before you do the plate to bring the tubes up to temperature.
      > It may be an indication of ageing tubes but increasing the exposure is a safe fix usually if the level of illumination is still even across all the tubes.
      > Regards
      > Kalle
      > --
      > Idyllic Press - Johannesburg, South Africa
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