Why exactly is Kreene used in making photopolymer plates? I assume it’s non porous enough to create the same vacuum as glass so is it simply that the matte or fog of the plastic is needed to bend the light thus building the shoulders? I’m thinking it’s similar to what strippers used to do to build spreads in films. If so why not vacuum the film and photopolymer to glass and use a fog sheet over the glass? will this not create enough of a shoulder? Granted we were only looking for a couple of mills of spread in film, but the width was controlled with time and on the rarity a layer of mylar between the film and contact for something really wide.
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Glass frames will work, but the kreene closely follows the contour of the edge of the plate material and hold tight at every point against the negative. If dealing with a large glass in a vacuum frame, the thickness of the plate material amy cause some slight bending of the glass which will reduce the contact pressure in the middle of the form. It doesn’t seem possible, but it does happen, and you will get undercut images in the center of your work. You can counter that by placing spacers the same thickness as you plate/negative sandwich around the edges of the plate you are burning, which will level the glass out.
which style of frame would do that? The one I have has a bladder under the glass. It fills with air while the air between it and the glass is vacuumed out. The bladder is soft and contours to anything between it and the glass, usually just a plate board though.
Is it the nature of the material that when it’s exposed it creates the shoulders or do you still need something to help bend the light to create them?
My understanding is that a translucent medium rather than a transparent medium is necessary for the rays to penetrate through the negative and into the photopolymer plate correctly. Apparently glass refracts the light in a manner that is at cross purposes to the process. I don’t know the exact technical explanation or rationale but since this is basic industrial practice I figure folks who actually do know the engineering basis of it all made the appropriate decisions in regard to Kreene some many many years ago.
I don’t know of any of the common photopolymer platemaking machines that do not come equipped with a Kreene sheet (though the digitally rendered photopolymer plates that have been developed in recent years require neither film nor vacuum drawdown sheet).
so then the question is the kreene just to bend the rays of light in order to develop shoulders? could I get the same result under glass using the “fog” sheets we use to make chokes and spreads with?
I’m not in a position where I can remove the glass from my frame is why I ask.
I can’t discuss the use of diffused glass/plastic when speaking of artificial UV sources, but I have found that lightly frosted glass does indeed improve plates exposed to sunlight. I’ve switched over to “non-glare” glass used for picture framing for all of my exposures, and noticed more consistency from plate to plate.
Matte krene may well help diffuse light, but most exposure units using krene have a close bank of lamps so there is already a diffuse lightsource. Most glass vacuum frames are used with a more distant single lamp, so there is less diffusion, only that inherant in the photopolymer emulsion itself; light does spread as it passes into the emulsion. However, the cheapest photopolymer exposure units are glass-and-foam pressure “suitcases” with a bank of tubes, so it is not unheard of to use glass and photopolymer commercially, and indeed it is standard with liquid material.
The matte of the krene may also aid drawdown. Drawdown in a vacuum can be problematic if one does not use either matte film or matte plate material. Slick film and slick plate can stick together and air bubbles can be trapped between film and plates causing localized spread. That may be the cause of jhenry’s plate problems, rather than any bending of the glass (the rubber blanket and the plate are far more flexible than the glass after all). The boards suggested will relieve pressure at plate edge so air can be sucked out rather than trapped. If the vac frame doesn’t have a bleed valve, it is a tough to get proper contact. The appearance of Newton’s rings between glass and film are an important indicator of proper contact, and also show presence of dust, hair, etc.
so it sounds like I should have no trouble with my UV plate exposure vacuum frame to make ploymer plates. Just a matter of getting some material and testing some exposures then…