I use an old (yard sale...2 or 3 bucks, probably an Ebay 1960's collectible now) electric carving knife. The kind that has a little motor/handle with two interlocked blades about 12" long that oscillate in opposite directions. Cuts all kinds of foam and other stuff very cleanly with little loss and no mess.

Scrollsaw (for inside stuff like lettering) or 14" bandsaw (as long as the piece isn't too huge) with fine blade.
A good quality jigsaw with variablt speed like a Bosch with a fine blade works very well. The really cheap ones vibrate too much.
I use all three of these tools.

A big scrollsaw (I have a Delta) with a deep throat would be my first choice but I use my bandsaw for long sweeping cuts, it's fast. It will also cut foam up to a foot thick (not that I have tried that yet) which is cool.

If you are going to buy a jigsaw, one that allows the blade to turn free from the saw would allow very fine and tight curves. I wish I had that type.

Hi Tango
You can't cut very tight curves with that knife though.:)
I do lettering (fonts) in 1/4 thick styrofoam as small as 1 inch high with my scrollsaw.
I just did a set of numbers 3 inches high in 1/2 styrofoam.
I'm planning to cast the numbers and use them for house addresses on faux stones.

Rick, I use a long razor blade that's designed for polymer clay. Billie

I was doing a search on other hobbiest that cut styrofoam. One hobby that uses it alot and are very picky and need to have percision are the RC model builders. Here is what they do:
"A thin wire of any kind will work, as long as you can run enough voltage through the wire to melt or actually to cut the foam. Something like a gutiar string...one of them that has the smaller wire wrapped around the center cord."
I thought this was interesting. I know about RC model guys. They are very creative. My dad is one and still building large scale at the age of 89.

Jo

Yes, the knife is only good for simple things like the form that Rickharmer was asking about. Making a "hotwire cutter" is a piece of cake and will cut detail as small as any foam can hold. Had one several years ago. Just 2 galvanized pipe sections & an elbow attached to a 4'x4' piece of ply with a threaded baseplate. Powered by an old electric train transformer that heated a very taught guitar (Top E) string (attach one of the low-voltage leads to the top and one to the bottom and make sure the cutting wire is electrically insulated at each end). The transformer lets you dial in the current as needed and you can cut 2 or 3 foot thick blocks with ease. Worked like a champ.

Blue polystyrene comes in sheets from 1/2 inch thick to 2 1/2 inches thick.
It doesn't come in blocks.
For CARVING blocks a hand held hot wire is great. But for cutting flat sheets it's not. The deeper the throat the more difficult it is to handle. You won't get a smooth cut
I had stationary bench top model used for lettering in sheet stock for several years. Never used it much but it was handy.

If Rick is gluing these sheets into blocks I recommend a good respirator if he's planning to use a hotwire.
some of the glues are extremely toxic.

I ALWAYS use a respirator when I play in the mud.I doubt that I'll be playing with a heater,though.Me and my jigsaw,or an old electric carving knife.Won't be doing fine detail.Planning on building septic tank covers(to go over the ubiquitous green cover)that look something like a low rock.Here on the coast,septic is a way of life.And need!
Cheers from here!

Rick,
I do not understand why you want/need clean cuts. Are you wanting to release the styro mold, or are you using it as a support to build your cover, then removing it. Either one you likely would not need clean cuts.
Maybe I really am just a little tired and just don't get your plan,......a little more explaination.
dena

Rick even cutting the blue stuff with a fine jigsaw is pretty nasty. It floats in the air and sticks to you and everything else. Much worse than cryogenic perlite. It even sticks to vertical surfaces. I guess it's static electricity.
I use a razor utility knife with a long blade as well to avoid the sticky dust but they don't last long. Styrofoam takes the edge right off after just a few cuts.
I have a huge pile of styrofoam molds now and I hate cutting the stuff.
But it makes great molds and templates for concrete.

After writing this the hot wire with it's toxic fumes doesn't sound all that bad.:)

I use a long, thin serrated knife. My best one has beveled serrated edges on both sides, & is about a foot long with about a half-inch-wide blade. I think it cost me $6 new.

Sue

I also use dollar store serrated knives or the infamous Ginso (sp) knife that I got from the fair over 20 years ago, you know it cuts a tin can and a tomato and blue foam.
Dena

Would a wood burning tool work? Once you get it cut or burnt a bit, it should break clean?
bonnie

Rickharmer, here is your next new toy for cutting your styrofoam. I think it will cut tufa too!

Here is a link that might be useful: ultimate styrofoam cutter