I'm wondering if anybody has ever encountered a contact printer that has any sort of "airbag" as part of it. I am competing for a contract in which my incumbant is integral with the contact printing of Aerial negs. I have been in a darkroom professionally for 45+ years. Never heard of such an accessory or its use. Anybody clue me in. Learn something new every day!
I believe it was designed to maintain full contact between the paper and the negative against the platen. It would provide more even pressure than conventional springs which would be important in critical applications such as aerial work and photogrammetry.
In serigraphy (screen printing), I've used UV exposure units that have a vacuum back of sorts that involves a piece of vinyl or rubberized fabric of sorts connected to a vacuum to suck it flat against the glass, keeping the sensitized screen and the stencil tightly against one another. I don't see why you couldn't use something similar (whether a vacuum sucking things flat or an airbag pushing things flat) for contact printing.
Vacuum printing frames don't exert much strain on the glass. An air bag would. The frames I once used for 32x40 film would have exerted over a ton of pressure on the glass if they used a bag instead of vacuum. Glass under constant or repeated stress can be permently distorted.
A friend is helping me to build a pneumatic contact print frame, even as I type. My design doesn't use an air bag, but a diaphragm, and more closely resembles a diaphragm pump than the bladder-type aero contact printers. My design is only good for developing out processes, as it's not a split back, but for its intended purpose it works superbly. I don't know how it would fare at 32x40, but at 14x17 it works great. We're working now to incorporate a registration system, which will be useful for multiple transfer carbon printing, and many other applications, and an integrated light source.
Years ago I had a contact printer with an air bag. It was pumped up with bicycle pump. The light source(s) were 20 argon or xenon lamps, each with its own switch so it could be used for burning and dodging to some extent. I don't remember he maker. It was a pleasure to use, but large, bulky and hot.
You need an aerial film contact printer. The air bag acts as a pressure plate to keep the negative pressed against the paper or diapositive film. Look at http://www.egoltronics.com/markv.html or perhaps look for military surplus.
Kevin
A vacuum system will work far better than any air bladder.
Atmospheric pressure is 15 lb/sq in. Even a 14x17" vacuum frame will exert a clamping force of 3,500 lb. while putting very little strain on the glass. A 32x40 vacuum frame has a total clamping force of 19,000 lb - close to 10 tons!.
Needless to say if you used an air bladder and tried to pump it up to more than 1psi you would break the glass. At 1 psi the bladder would exert 238lb of force in a 14x17" frame. Small systems might use a bladder if contact isn't critical as the mechanics are simpler (cheaper) and it can use shop air, saving the price of a pump. Another reason for using a bladder is if one is printing frames from a long roll of film - the film and paper can't 'flow through' a vacuum frame because it would break the vacuum seal.
But even 1 psi of evenly distributed clamping force is more than a spring-back frame exerts. If a 3-spring 14x17 frame has 30lb of clamping force the average pressure on the material is a puny 1/8 psi - distributed unevenly so some areas have almost no force.
If you are going to build a DIY air bladder system it might be a very good idea to use a piece of thick tempered safety glass. Tempered glass is quite strong and safety glass will crumble into pebbles rather than sharp shards when (er, hopefully 'if') it breaks.
At a former job we used a '50s? vintage contact printer made by Miller-Holzwarth for the USAF. It was a console-sized box with a lamp below and a glass top. Aerial long-roll negatives ran across this emulsion-up, and copying film or paper was placed emulsion-down on top of the neg. There was a lever-clamping setup, not unlike a dry-mount press, on top, that forced the air bag down onto the film to make proper contact- the bladder was filled with a bike pump. It was a workable artifact in the '80s, I'm sure the company is long gone now.
I just picked up a 9"x18" contact printer for aerial negs, 40's-50s era, uses an air bladder as pressure-plate.
Photos tomorrow.
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