Platemaker Exposure Issues
I work in a shop where we primarily print from photopolymer plates. We seem to be having problems with our PA 350 N2 (its the first one here on the Jet site: http://www.jetusa.com/letterpress_equipment.html )
The job I am working on is business cards running 4 up and a single business card, same design, different contact, 1 up. The type is black and the logo is a blind deboss.
The type is set in a very fine serif. The plate for the 1up plate exposed and washed out properly, and then printed fine. However on the 4up plate the type does not expose to the same level of detail as the first one. The type is loosing its delicate strokes and is becoming bulky. We have remade the plate several times, each time making sure the kreen is applied tight and all the air bubbles pushed out. The plate made once correctly, but due to my error the film was backwards. We have tried remaking it several times, cutting the film down to two and then one, to be the same as the first single plate that worked fine.
We have decided it is likely something wrong with the exposure in the platemaker because the blind plate for the 4up was remade due to the fact that the impression was not as crisp as on the first single plate. Seeing this we cut out one of the sections from the 4 up plate and tried to print it alone to see if it had anything to do being run as a multiple. On the same stock, two different plates, same image, we are getting two very different results.
What can be going wrong in the exposure that is causing one plate to have a hard crisp edge and another to have a soft rounded edge? Has anyone ever had this happen and what did you do to resolve it?
If I were you I’d check the amount of vacuum you’re getting with an actual gauge (being that those don’t have built in gagues). You can disconnect the vac. hose underneath the exposure drawer when its pulled out (should be a just a simple press barbed fitting).
My guess is that perhaps you have weak vacuum & while the machine might create enough for a small plate, maybe its not sufficient to draw your film tight for a large plate.
-Jason
Light bulbs - they need to be replaced once in a while, I run a 24 x 30 unit with 18 bulbs and every 1000 hours they need to be replaced
When you said the plate made once correctly but the film was backwards, do you mean the image quality was OK but the plate came out right reading instead of wrong reading?
If that is the case then the film is probably wrong. The film must be wrong reading with the emulsion down when you expose the plate. Your film is probably right reading with the emulsion down, and that is why it came out OK when it was backwards……by coincidence the emulsion was actually on the correct side to make a good plate.
If you expose a plate with the polyester film base down against the plate instead of the emulsion down, the light will go sideways through the polyester somewhat, and expose a larger image size than what is on the film. That sounds like what is happening in your case.
To check the film to determine which is the emulsion side, take a sharp point like an X-Acto knfe and scratch the film on an edge, away from the image area. The emulsion side will be the side where you can easily scratch away the black (emulsion) layer.
Geoffrey’s right:
That’s most likely the problem. I went back & re-read your post, there was part I don’t think I followed the first time I read through.
Vacuum is important & I’ve had to rebuild the pump in this exact same model. I wasn’t real impressed with the stock plumbing/pump design, which leave it very susceptible to dust/debris (that foul the check valve in the pump & create weak vacuum) - dissemble the pump, clean it & install a filter in the suction line if this ever becomes a problem (rebuild kits are available if the seals have become brittle).
However, to avoid confusing the issue - follow Geoffrey’s advice first (and yes, make sure you have good bulbs - but they shouldn’t have changed drastically from one exposure to the next & the change should be in the direction of plates becoming under-exposed, leading to details/features missing after wash-out).
to 10workingfingers et al:
This is a bit out of my territory, but I like to extrapolate from other areas of skill.
I taught myself to do most simple jobs of electric welding, and also using a sewing machine, by starting out on the simplest tasks I could devise and observing if the results were within the range of acceptable. None of my welds broke.
I suggest the same applies to photopolymer exposure times. Usually (in any system) there are tests possible to check if any variables need adjusting. At one place I worked, we had a problem with the adjustment of the intensity of the light used (the time was not adjustable), and we found a simple test which gave a [go/no-go] result. This was not photopolymer.
At present I am trying to read a paperback book printed by a very large company; my particular copy is several editions on, and also the colophon shows 15 reprints of the edition. I am ashamed to record that my copy is printed in Australia, by a large printing company, and it looks as if they photocopied from pages already printed; most of the letters are blurry, letters like e and o and d have almost no white space inside them; the width of the strokes making up the characters appears to have suffered a good deal of “gain”.
One of the first lessons I was taught by an old-time printer was the story of how a typeface for a newspaper was chosen, by trying all that could be easily found, and then reading them by the light the customers (subscribers to the newspaper) would be using; the typeface was chosen because it seemed “to jump up off the paper”. It was Linotype Ionic No. 5 (from memory).
Persevere.
Alan.
10 workingfingers
Reading your post as carefully as I can I assume the film for your 4-up is bad. As far as I can tell you remade your plates from this same film, correct? Have the film regenerated and try again?
If your exposure problems end up being the culprit, check not only your lamps but also your ballasts. Many lamp related problems can be tracked to faulty wiring or an errant ballast. If your lamps are darkened on one end far more than on the other, you’ve got a wiring/ballast problem.
See:
http://bielerpressxi.blogspot.com/2006/03/on-scales-and-lamps.html
Gerald
http://BielerPress.blogspot.com