NuArc and Photopolymer exposure
I teach printmaking and the university just purchased a brand new NuArc unit for screens. I was told we could also expose photopolymer on this unit. I cannot seem to get the suction tight enough for these plates. I recently tried to take one of the suction tubes and run it across the back of the plate not along side of it as I had already tried, but it shows on the image, but the suction is better. Could I weight it with a copper plate and then run the tube along the back? Has anyone used these exposure units for polymer?
Bleuacier-
Someone will tell you that Kreene is the answer, and while they’re right, if you have to go with what you’ve got you will indeed need more airstrings. What is happening is that the rubber is closing around the plate and contacting together, sealing off the plate, and keeping a tight contact from ocurring-because the air is not able to escape the plate and go to the vacuum/evacuation point.
Kreene is great because you can see (and feel) when this occurs and check things before you close the drawer and expose, and if the kreen isnt’ stretched tight like a drum- rather it’s a bit floppy and the neg isn’t contacting the plate all the way- you know you need to evacuate some air still because the neg is NOT contacting.
So you can check for this with Kreene.
Not so through glass. You can visually inspect if you have a flip-up unit like I do- but if you have a closed container unit or whatever it’s not really going to be that easy to know and you’ll have to do everything you can to be SURE contact has ocurred; this is not something an air gauge will tell you, either!
What you need is not necessarily increased airflow, but rather you need to increase the amount of evacuation points around the plate for the air to escape from and keep the rubber from pinching off the draw before it can pull the air out.
So that the air can flow from more avenues and therefore empty out from between the plate and glass.
Make sense?
OK:
Why? As you may already know- the way this type of contact system works, it’s not pressure forcing the two parts together so much as it is a vacuum between them PULLING them together.
So, no- copper or weight won’t really help.
One thing that will help is to crisscross a few airstrings to different points all around the edges of the plate, making sure to pull air from all points and edges every 1/4 length of the plate. Like a grid.
If you get some netting- cheap twine or rope netting like this http://www.mandmcycles.com/default.aspx?PartNo=100010&gclid=CLGg58fPt7cC...
http://images.mandmcycles.com/CartImages/Products/BikersChoice/10/10-001...
(without the hooks of course!)- you can net the back of the plate as long as it goes over all the edges- and add the airstring (or, hopefully, airstrings- I use 2-3 in my largest exposure unit for larger format 43X54 silkscreens) behind this.
HOWEVER- If you’re really careful with the positioning of your airstring and a little bit of tape, you can seem it all the way around the edges of the plate so that the airstring reaches/stays in contact with all sides and creates an evacuation point for the air.
Either of these should help to pull air from every available avenue and create more uniform contact between the plate, glass, and neg.
If it’s still not working, you may have an under-powered vacuum or a porous blanket (is your blanket neoprene rubber, like wetsuit material? or some sort of opaque, totally non-porous rubber like the kind rubber gloves are made from?)
Best of luck.
I used a NuArc 26-1K unit for years (though not the deeper version for screen exposure) and although the light output is not optimium for the thicker emulsions of relief photopolymer, vaccum should be excellent if you understand how to use a vaccuum frame.
If you do not back-cut your plate material (that is, after making the first cut, you must take the material that was outside the cut and do another cut to finish that edge) you are working with a distorted edge that acts as a gasket against draw-down when placed against the glass of the frame. Then it is not a good idea to use negatives smaller than the plate material; again that edge will be a gasket against draw-down.
Getting the materials right, you then need to use the bleed valve so the air vaccuum is reached slowly, the material approaching the glass before final vacuum is achieved. If the valve is closed when the pump is activated, the material will just slap against the glass without complete draw-down.
Finally you need to examine the sandwich of materials after the vaccuum gauge shows correct pressure. When the air is fully drawn out, you will see Newton’s Rings (an interference pattern similar to gas on water) between glass and top of negative. Where the pattern is broken you will also see indications of any dust, hair or other foreign matter that will interfere with contact or exposure.
A clean room, yellow safelights, lots of glass-cleaner and film-cleaner and lintless wipe, these are all investements that will reduce the need to remake bad plates. And bad plates are expensive far beyond their raw material cost.