What's the difference between a vacuum lid and a compression lid?Updated 2 years ago

The purpose of the vacuum is to create the best, tightest positive contact between the light source, glass, film, and emulsion. Having tight, strong contact to the point where the black ink in the film is almost embedded into the emulsion means that light cannot wrap around the film positive very well. The end result — a straight, clean burn with sharp edges. Vacuum units provide the best detail resolution. 

Compression exposure units require a bit more work. To replicate the downforce of a vacuum unit, you'll need to put weight on top of the lid or foam to put compression onto the film, screen, emulsion, etc. in order to create the tightest contact possible. Compression exposure units will not achieve the same level of positive contact as vacuum units, but they still do a good job. Edges may be a little rough from time to time, but the stencil will still rinse out well and produce an acceptable print. For best results, printers should use emulsions with diazo. The diazo is a resolution and detail enhancer, it helps sharpen edges. Diazo solves all exposure problems issue.

To learn more about the differences between the two, read this blog.