7499Re: [PPLetterpress] Re: Plastic Plates Curling...
- Feb 7, 2007Tom
I think this is fairly accurate. Don't know about exactly about the
drying time thing or the humidity factor, nor have ever experienced the
curling factor. But I love the word "impressionistic" in this regard
(applies to machine processing as well as hand processing, by the way).
PP isn't voodoo, it's just a technical process. Thus, I'm confused about
the thinking on drying in the thread. Drying just removes the moisture
left over from washout and has nothing to do with the innards of the
matrix. Water doesn't actually get into the polymer. And while moisture
must be maintained at a certain rate to prolong the longevity of plates
I suspect far too much is being made of this.
Your description of the photopolymerization process is as on the mark as
it gets except that during extended exposure the relief grows upward
(shallower) as the molecular structure continues to grow and interlock
(because of the extended UV exposure).
I'm also thinking there is confusion (in the thread) over the term
hardness as opposed to eventual loss of tack and resilience, resulting
in brittleness. Post-exposure simply ensures complete
photopolymerization of the subsurface relief (the surface is already
stabilized); it is suggested that it can prolong deterioration but
common practice would indicate not to reuse plates (as per your
practice)—it has not seemed a beneficial practice to me as well. I
suspect if one waits a month or so to print from plates, well, one has
missed the window of optimum opportunity.
Fresh, seems to be a fairly politically correct term these days. "Fresh
plates are good for your printing"?
> In a message dated 2/6/2007, harold@... writes:
> In thinking this morning about John's suggestion of drying the plate
> longer, I realized this is a good suggestion during the Iowa summer.
> The humid weather then requires a longer (or hotter) drying to expel
> the right amount of moisture. I wonder, John, if you find yourself
> using a longer dry time more frequently in winter or in summer?
> I agree with John about drying time affecting curl. Here in Denver the air
> is normally so dry a plate seems to need almost no drying time. I discovered
> this is deceptive -- if I don't dry long enough, there is still moisture in
> the plate which eventually dries, which seems to cause the plate to curl since
> the surface shrinks more (or faster) than the sub-surface of the polymer.
> Additional drying time (which I do with a hand-held hair drier) seems to solve
> most of my curling problems.
> The exact timing needed will vary according to humidity -- my methods are
> somewhat impressionistic (or should I say learned by craft and experience),
> usually blow-drying the plate until it is almost too hot to hold, and then a
> little longer.... I also added a little additional post-exposure time, for what
> it's worth, which seems to me to dry the plate further and maybe fix it
> better in its original flat condition throughout the full depth of the polymer.
> Larger surfaces still seem more prone to curl, and long storage still
> results in a more brittle and often more curled plate. (I don't know if the brittle
> quality is a result of an increased "hardness" of the polymer as it is
> exposed to further UV, or dry air, or some other kind of deterioration in the
> polymer such as Gerald suggests happens because of Ozone). But the result is that
> I don't often reuse old plates, but save the negative and remake the plate.
> Seems to me these variations may be better recognized when making plates by
> hand, rather than by setting timers on a machine, but maybe that's my own
> preference for hand-work showing. I did have to learn that some of the timing
> measurements have to be mechanically and very precisely followed, especially
> exposure times for differing kinds of images, and washout time limits.
> (Regarding the extra exposure needed to preserve very fine lines and dots, I
> would say that the polymer is not getting "harder" but may be hardening
> further all the way to the base, which also allows sub-surface material to harden
> more widely than the surface image -- which will provide additional support
> and protection for the fine surface lines or dots. An opposite process is
> involved in determining exposure time needed for printing a reverse line in a
> solid surface -- too much exposure time will allow the sub-surface to harden
> and widen to over-fill the detail of the reverse, thus leaving it without
> enough relief for printing -- so this kind of image needs to be under-exposed to
> protect the fine details. These processes are a result of the fact that the UV
> light does not just enter the negative/plate in a directly vertical
> direction, but angles through the image in the negative and thus exposes a wider area
> below the surface, and thus can harden an expanding sub-surface area of
> polymer the longer it has the chance.)
> Best wishes, Tom
> Tom Parson
> Now It's Up To You Publications
> 157 S. Logan, Denver CO 80209
> (303) 777-8951 home
> (720) 480-5358 cell phone
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