13313Re: [PPLetterpress] Re: Photopolymer with 'moving' parts
- Dec 3, 2012Rick
My understanding is that Boxcar recommends back-exposure only for the
KF152 plate (the so called deep relief plate). Not the same animal as
the KF95 or the steel-backs KM95/KM152, which all have harder ratings.
Specs for letterpress plates generally seem to indicate hardness ratings
around 67. This, combined with its thickness, seems to plague the KF152.
There have been a number of reported problems relating to loss of detail
and drift and the solution proffered has been back exposure. Though it
too is back-exposed by the manufacturer. Most sheet photopolymer is
back-exposed prior to backing.
My A&V rep told me the KF152 plate was made for molding purposes and
recommended I not buy it for letterpress. He never steered me wrong so I
follow his advice. I had a client who actually needed a thick molding
plate, a polyester-backed plate with a thickness of .100" (the KF152 is
.60"). It is softer as well and, except for its insane cost and poor
shelf life, it is perfect for metal clay work and similar.
On 12/3/12 3:05 PM, letterpress_man wrote:
> I use the same plates as you are using, I buy them from Boxcar Press and they recomend 25 sec back exposure. They sell and process probibly more plates than anyone one here so there advice should hold some weight. I also agree with the other comments Gerald made.
> --- In PPLetterpress@yahoogroups.com, Mª Dolores Jiménez Mondragón<doloresj3@...> wrote:
>> Thank you Gerald for you advices. I will try to improve those parts of the
>> process and I will be back with the results.
>> 2012/12/3 Gerald Lange<Bieler@...>
>>> 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
>>> 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.
>>> 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 ÂºC, dry out: 10 min at 80 ÂºC 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
>>>> Please, help!!!! :-)
>>>> Thank you in advance and regards,
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