Cutting Plastic Sheet

A professional grade paper cutter - like those used for cropping photographs. There are several types, from the chopping-lever type to the roller-knife type I have.
We have a heavy-duty, lever action, geared blade one at work that can cut through a on inch stack of paper that would be the ultimate. It has a clamp which fixes the stock in place during cutting. A good feature to look for.

I would recommend the rolling knife. I have a lever action and for heavy cardboard or thinner plastic, the piece being cut has a tendency to move toward the knife as the knife blade comes down along it. I think there would be less tendency to do this with the other type, especially if you could adjust the pressure on the cutting edge and maybe make a could of passes.
-- John The history of things that didn't happen has never been written. . - - - Henry Kissinger

I score a line with X-acto or utility knife and break along the edge. I then sand to make good glue joint on thicker stock. I have cut even 1/8 thick stock this way, but it works better with 1/16 or narrower. If I am cutting out a small piece from a large stock sheet, I use a bandsaw with a fine tooth blade. Yeah, there is some melted crud that builds up along the edge, but it can be easily sanded or scraped off.
I love the bandsaw. For years I used a jig saw for model cutting, but hardly use it now that I got the used bandsaw. I like a bandsaw so well I intend to buy a new, better one soon. I use a lot of wood in scratchbuilding and modifying kits, so a power saw of some sort is almost an essential.

I generally use the score-and-break method myself. For thicker stock, I'll make several scoring passes to make a deep groove, then put some waterproof ink in the groove and wipe away the excess. When I break it and start filing & sanding the edge smooth, the ink provides a reference point so I don't go too far.
For power saw work, you need a speed control or a slow-moving blade. My variable speed scroll saw can cut through 1/8" styrene scrap without any signs of melting. Variable-speed rotary tools (Dremel et al) can cut styrene without melting it if you set the speed low enough; just remember that the larger the bit's diameter, the lower you have to set the speed to avoid melting.
I've been thinking of bulding a mini table saw, using my Dremel with a circular saw blade and flex-shaft attachment. It ought to be good for both plastic and small-scale woodworking projects.

I use the score method too with a scribe and metal straight edge but after scoring I trap the plastic between 2 pieces of wood, usually 2x4's, press tightly and snap along the scribe mark. If for any reason it decides to wander away form the score, the wood keeps it on track. Never had one go anyplace else using 2 pieces of wood. Grandpa John

For large pieces I find that a good straight edge and a single edged razor blade using the score & snap method work best. For small pieces something like The Chopper with heavy duty single edge razors works up to 0.030" thick with no beveling (note, it takes a little practice).

Agreed - this is why looking for one with a built-in clamping device for holding the stock in place is a must if you go for the lever type.
I'd REALLY like to get hold of one of the heavy-duty guillotine ones like we have at work...but I'll bet it's expensive. It's simply the best I've seen. Not only does it clamp the stock, but the blade comes straight down, and moves latterally as it does. Produces VERY precise, even cuts through the thickest of stock.

Yes - I do the same, but use a steel rule or a set of flat jawed pliers (for small breaks). You could also build a tool of sorts (similar to a bending brake) from plexi.
Hmmnnnn....

The Chopper rocks!

I have something like a Chopper but I forget the name...Miter Cut or some such. Actually works a little better that The Chopper. Those 0.009" thick razor blades are a must on either.

You didn't say how thick your stock is. I'm working up a project and plan on using a friend's table saw. He suggested a laminate blade for cutting thicker sheet. hth
The Keeper (of too much crap!)

I'm with Don. I've used a Sears 10" bandsaw for 20 years. Its speed is slow enough that it doesn't melt plastics, & I use it for sheet styrene & up to to 1/4" Lexan & Plexiglass, as well as wood & soft metals.
D> I score a line with X-acto or utility knife and break along the edge.