Andy:
The school where I teach needed to replace an aging exposure unit last June - we could no longer get switches for the older halogen unit. We bought a 4 UV/2 floresent unit (Vastex E2227) it was good because of the drying cabinet that was available. We set the machine up and started doing test exposures.
We're using Ulano QTX emulsion in a light-safe environment and we've tried 1/1, 2/1 and 1/0 coatings allowing drying between coats. Nothing we do seems to allow us to get the easy washouts we had with our older unit. Even with low exposure times around 30 seconds, clearing detail is nearly impossible - the unexposed emulsion seems not to liquify as it's washed out - it washes out more as solid particles.
Our last two options seem to be increasing the wattage of the UV bulbs or changing the type of emulsion we're using. Any recommendations or advice would be greatly appreciated.
Thanks
Posted by Al Tucker on November 19, 2007
Hi Al;
I don't know the specific model, I do know Vastex make decent machines and I would assume the unit does the work it is supposed to.
I would start at the beginning.
Positives are solid black? Test with some rubylith or proper imagesetter film that is completley black and dense if you are using laser or inkjet posi's - if the emulsion is solid in the exposed (clear areas) maybe the light is so strong it is buring through weak positives?
2. use an exposure calc, and settle on a coating pattern. what is typical coat 1/1? Start with that and test. If your stencil is hard after washout, you can keep cutting your time back. I am not familiar with the emulsion, but if it is highspeed and the lamps are good you may be overexposing, even at 30 seconds. Me, I use dualcure - not as fast, and a wide exposure latitude combined with water resistance, solvent resistance, and good detail.
3. How old is the emulsion? (is this a onepot high speed, no diazo added?) maybe you need some new. 'Hard to washout' is a sign of old/bad emulsion. Ask your supplier or ask Vastex what they recommend. shouldn't be too hard to get and try some new emulsion.
4. Vacuum is working fine? sometimes if the posi is not tight, you can get light bounce starts to expose the emulsion behind the black on the film.
try these things. Changing one part of your process sometimes means changing others. Exposure should be a slamdunk and work every time if you have your proceedures worked out.
Posted by andy MacDougall on November 20, 2007
