How to cut open cell foam?

My local gym was replacing their padding and I picked some up for my garage wall. The dimensions of the foam are 4’x6’x18”. I don’t need an 18 inch thick pad to land on and I’m trying to cut it into two 9 inch thick pads but it’s proving quite difficult. Any suggestions on how to cut through such a huge piece of foam?

An exacto knife doesn’t do a great job, and right now I’m trying a bread knife while wedging pieces of wood to try to keep the two sides of the foam apart. 

Hot nichrome wire and mind the smoke. 

Nice! How would you recommend heating up 6 feet of nichrome wire?

We used an electric carving knife that worked good. Not sure it wouldn't be hard to do what you are trying to do though. 

You can easily make your own.

See: https://www.instructables.com/5-DIY-Hot-Wire-Styrofoam-Cutter/

Or any number of YouTube videos

Aaron Liebling wrote:

You can easily make your own.

See: https://www.instructables.com/5-DIY-Hot-Wire-Styrofoam-Cutter/

Or any number of YouTube videos

That was the exact one my engineering buddies fabbed up. I’ve used it and it works great!

Red lightsaber.

Hot wire method is best, but if you don't want to/can't make one, this can be done with a regular blade. The foam dulls edges quickly, so use a box cutter or something with a replaceable blade and change it often. For a piece that size I would expect to go through at least a dozen blades. Take your time to keep it even

Strength through patience

Try a cheap bread knife, the sawing action worked great for me.

I've had pretty good luck with a 18" crosscut or pull saw

A very sharp long chef's knife works, especially if you have someone else pulling the 2 sides apart as you go.  The tension allows the knife to make fast progress slicing.
If you don't pull like this, any cutter will put the foam in compression, which makes for very slow cutting.  Resharpen as needed.

The biggest problem is the knife will wander, making one side thicker than the other if you are not careful.  So keep checking the thickness of each side during the process.

I used to work a place that made custom foam mattresses, and did custom foam cushions for boats, RVs, etc.  We used an electric carving knife, like the one you use to carve a turkey.