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Re: PlexiGlass (sic)
> Date: Tue, 15 May 2001 17:15:45 EDT
> From: Dennis8425 at aol_com
> Subject: Re: PlexiGlass
> The thing is there are multiple products sold under the name
> of plexiglass. Some will warp to extremes others like Lexan have
> a minimimum warpage problem and still others in the Acrylic family
> have no warpage near water. The big secret is knowing which type
> to use for what application.
"Plexiglas" (like "Acrylite," "Lucite," and, I think maybe "Perspex" in the
UK) is a trademark for acrylic plastic (polymethylmethacrylate). "Lexan" is
a trademark for polycarbonate plastic, a totally different (and more
expensive) class of clear material.
Acrylic makes great aquariums. I think it has been well covered in the Krib.
It makes quite poor covers, for the warpage is not controlled by the shape
as in a welded tank. Weld some vertical ribs on it, and it suddenly becomes
a well-behaved cover. Likewise, all big acrylic tanks have an inner rim
around the top to prevent the sidewalls and ends from bowing and warping.
Acrylic can be brittle and can craze or crack following cutting, if
stressed. Good annealing at near the boiling point of water (190F+) is
advisable. Polycarbonate seems tougher and does not need that step.
Every kind of acrylic I have ever used can show the moisture induced
warpage, and I think the way it is formed or formulated has very little to
do with it, unless the shape resists it. It is such a wonderful, cheap
material that it is worth understanding how it behaves and using it rather
than banishing it from wet places.
Thin sheets warp, if wetter on one side.
Thick sheets warp less in the same circumstances.
Thin or thick sheets with ribs can be made that do not warp, noticeably.
If possible, anneal by heating to near 200F and very slowly cooling. It will
retard potential future crazing or cracking.
Don't scratch it. It can really crack if under internal stress and not
Put the final product between crossed sheet polarizers to easily see the
rainbows of internal stresses. Anneal to reduce or remove them.
Wright Huntley, Fremont CA, USA, 510 494-8679 huntley1 at home_com
"The whole aim of practical politics is to keep the populace alarmed -- and
thus clamorous to be led to safety -- by menacing it with an endless series
of hobglobins, all of them imaginary." -- H.L. Mencken