[sledzeppelin]

I'm thinking of building a metal halide exposure unit and considering my options for the top. I've seen a lot of DIY vacuum versions, but I was wondering why that's more popular than having a solid lid and a piece of foam inside the screen frame that's taller than the frame so that the clamped down lid would compress it, and force the mesh against the film and glass. It seems like it would be much simpler to build, cheaper, and potentially give better results. I'd think stiff foam would give more pressure than a vacuum, but maybe I'm wrong.

Your thoughts?

 

[PhilR]

It's not the amount of pressure that matters, but the consistency. Plus, foam puts a lot of pressure on the glass. Now it takes a lot to bust tempered glass, but it will probably pick up scratches over time and with repeated heat and pressure it only takes a scratch to make tempered glass go pop.

Anyways, with vacuum you can use multiple screen sizes and even multiple screens without needing an assortment of foam blocks.

 

[tpitman]

The vacuum draws the emulsion-coated mesh against the film and the glass with much more compression than some foam and weight can possibly do, yielding much better contact of the film to the emulsion.
A vacuum blanket is one of the best things you can do to improve stencil image detail.

 

[sben763]

Although I agree with vacuum being best with proper compression there is no quality difference in the final product. I used just weight for 3 years, compression for 2 years and now have a vacuum lid. It convenient and without putting direct force on the glass you apply a tremendous amount of force due to gravity. Vacuum is better if you have a warped screen and will make a positive contact where as compression can loose contact in the high areas.

 

[sledzeppelin]

Thanks everyone. Vacuum it is then. I guess I'm underestimating how strong a pressure the vacuum method can exert.

 

[DNeeld]

It has nothing to do with pressure. It has to do with how intimate the contact between the emulsion and the film positive is. Technically, if you can achieve 100% contact between the film and the emulsion, you don't even need a cover. However, without advanced equipment, this isn't a reality.

The vacuum or pressure doesn't have to be strong. It simply has to bring the film into complete contact with the emulsion. Vacuum does accomplish this better than the compression method, simply because any air between the film and emulsion is vacated.