I've been meaning to mention what I was told many years ago about using rigid
plastics such as acrylics or polycarbonates for vacuum chamber parts. My then
boss, who has a PhD in materials told me sternly that they may work for a
while, but will degrade on the vacuum side and eventually fail, since this will
make a slight veining of that surface. At some point the plastic will start
cracking and implode.
Of course the bigger the span the plastic covers the sooner it will happen.
Not absolutely sure how true it is, but that's what he told me.
--- Mike Brose <puppetsnprops@...
> Tom and Rod....., thanks! My guess is, that from an engineering
> standpoint, a 16"-18" diameter is just too big a span for 1/2" acrylic or
> lexan (stresses the material beyond it's limits), whether is has holes
> drilled in it or not. I have a 10" chamber (my first chamber I built)
> that has a 3/4" acrylic lid which has never failed in 10-12 years of use.
> The lid really does not flex (it does have holes in it however, as I
> didn't even think about that back then). My other chamber is 12" diameter
> with a 1/2" lexan lid. It flexes some, but not very much. I suspect
> there's probably a rule of thumb or formula (an engineer could probably
> tell us what it would be) for how thick a (non metal) lid needs to be for
> a given diameter, to be within a safe range.
> I do however understand how Rod feels about it after 3 lids imploding.
> And that last one. Yikes!
> thanks again for the feedback guys.
> Mike Brose
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