Plexi ground glass

[sanking]

I want to make a ground glass from clear 1/8" thick plain acrylic plastic. Could someone give me foolproof directions for making a good ground glass with this material?

Sandy

[vinny]

Sandy, I have a plexi gg for my Wehman but it's not ground. It's an adhesive sheet as far as I can tell, some sort of window frosting or something.

[Sevo]

From many years of prop and diffuser making: If it is a small bit, proceed as you would for glass, but in running water - acrylic already softens at temperatures easily reached when sanding it, so it needs plenty of cooling. That also means you'll need waterproof sandpaper and cannot use loose grit. For bigger areas than your darkroom sink can handle, buy it ready blasted - it is unlikely that you'll get a similarly even result in ULF size with makeshift tools.

[Gem Singer]

Sandy,

It cannot be called a "ground glass". It can be called "frosted plexiglas" (insert smiley face here. I tried, but could not do it).

Look for someone who has the proper equipment and knows how to lightly sand blast plastic.

[J.B. Harlin]

Anyone done any research into rigid (polycarbonate or acrylic) rear projection screen materials for LF ground glass replacement? I poked around several years back and everything I found that might be suitable was prohibitively expensive for an experiment.

[J.B. Harlin]

Sandy. . . I experimented extensively with making acrylic GG several years ago. Back at the same time I was trying to find something off the shelf, possibly rear projection materials, with no luck. I made a couple of 4x10, one 8x10, and one 11x14. I used 3/16" for the larger since the thinner would not stay flat. I used a soda blaster to make mine. Never did get really even frosting, but they work so well I am still using them over two years later. My wife still has the 4x10 in one of her cameras. I did mess up several before I achieve acceptable results, but it is doable.

[Nathan Potter]

Sandy, I can't give you a foolproof way of frosting acrylic but I have used normal grit as in the case of glass for obtaining a decent grind. Acrylic is so much softer than glass that it is quite tricky to get a decent depth to the surface damage required. As Sevo mentioned heat needs to be minimized by using copious amounts of water while grinding, and going slowly.

I'm going by memory but I think I used a coarser abrasive than the usual 600 grit (probably 320) in order to get a deeper and more uniform surface damage. I used a thicker piece of glass as the lapping tool (an old optical flat) with much lighter pressure than when doing glass. The plastic scratches painfully easily which will leave unsightly lines across the finished plate so particulate contamination of the grit has to be watched carefully. I used distilled water (deionized actually and filtered) with a touch of wetting agent (Kodak Photoflo) to obtain more uniform wetting of the acrylic. I bet I spent the better part of an hour or more doing an 8X10 format screen.

I have a hunch that there may be different hardness acrylics and I may have used one that had above average Knoop hardness so that would work more satisfactorily. You will need to have some scrap to experiment on. My application was an imaging screen attached to a microscope for observing a laser scanned image of functioning semiconductor devices.

BTW sand or bead blasting is a common method but often difficult to get a uniform grind.

Nate Potter, Austin TX.

[davehyams]

I have a friend who just uses a random orbital with a 600 grit pad and light pressure. I have one on my 8X10 and its pretty even. worth a try if you have a hand sander.

[evan clarke]

I'd get some Lexan, a good sanding block and wet sand it with fine (800grit), try it and go progressively coarser if 800 is too fine. Go to an autobody paint supply store, they'll have anything you need..EC

[Tim k]

On my 11x14 I bought some pre frosted plexiglass it was pretty bad. But after taking some regular glass gritt to it, I'm pretty impressed.