You can score and snap plexi using a scoring blade meant for formica. You can find these in the formica section of your local Lowes or HomeDepot.
You need to make several passes, and I like doing so on both sides if it is very thick. The thin stuff does not require it. It is easy to align as you can see the score on through the plexi.
Do not use Styrene. It scratches really easy and shatters like glass.
Remove the protective film where you are going to score.
The scoring blade is useful for formica too. I have this one:
http://www.amazon.com/Hyde-Tools-45730-P...B000C027ZEMake sure your clamp your straight edge or you will make several score lines and you only want one. Or a rubber backed straight edge.
I think this does a better job than a table saw.
I use plexi for childrens' rooms, for areas that might see abuse (like a den where kids might be unsupervised(, and when I have to ship.
Glass for everything else. Once you get used to cutting glass it is the easiest thing in the world. Use one stead (not overly hard) pass with water or oil. With the score facing up, place a dowel directly under the score and press down on both sides. Make sure your score goes all the way to the end of the sheet. You don't really need the dowel but it is easier for first timers.
The ability to cut glass will come in handy over time. So it is a worthwhile skill to acquire. Glass is cheap, does not scratch easily and does not attract dust like plexi does.
But at all costs avoid the styrene glazing sheets. They really suck.
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