Ground Foam

Er, I wouldn't bother. Very messy business. Clogs up the food processor something dreadful, too. It's not ordinary plastic foam, BTW, but rubber foam such as is used in mattresses and seat cushions. Woodland Scenics, Scenic Express, and several other manufacturers make ground foam in many different colours. Check Walthers catalog or website under Scenery.
HTH

I would only add that the shaker bottles Woodland Scenics are a bit overpriced. I use used and empty plastic spice jars (eg the big ones that come loaded with powered garlic or powered onion, etc.). You buy the foam in large plastic bags and fill up the jars with pure or mixtures as needed.
Basically any used, empty, dry, plastic container with a removable lid that has holes in it that some sort of powder came in will work.

Wolf spake thus:
Then don't use a food processor! I use an ordinary hand-cranked meat grinder which works great.
Yes, it's ordinary foam rubber; what did you think it was?

Ed Walsh spake thus:
Use a hand-cranked meat grinder. You know--the old-fashioned type, cast metal, usually comes with several grinding wheels. Just stuff chunks of foam (rubber) in the hopper and crank away.
I've also had considerable success dyeing foam, both before and after grinding. (Gives different color saturations.) Use ordinary Rit dye. You can produce a great range of colors for very realistic landscaping.

[...] I know what it is, but OP's original question was ambiguous -- "ground foam" could be anything, and "foam" these days usually means plastic foam as used in insulation board. I was attempting to prevent any possible misconception on OP's part.

My experience is a little different than most others' who have posted here, but I did my own in the blender and it turned out pretty well. I bought one of those "egg carton" matress pads from a big box store, then cut it up into pieces about an inch square. I "loosely' filled an old blender with them then put in PLENTY of water. Put on the lid & let 'er rip. I ended up with a very slushy mixture that I poured through a cloth to remove most of the water.
After I had ground up a sufficient amount, I mixed some green latex paint in water and then stirred in the foam. This part is kind of messy, but I managed to wring the excess water out of it, then let it dry. I did this in the summer so the garage was warm enough for it to dry out in about a week. From time to time, I'd stir it up to expose all of it to the drier air. When it was completely dry, I let it dry some more then stored it in zip lock bags.
I've also used a similar process on sawdust, but screened it first and used dye instead of the latex paint. Now I have quite a collection of "stuff" for texturing & scenicing my railroad.
dlm

[...]
Thanks for this info, Dan.

I had given up the idea of making some until I saw the post about using an old hand crank meat grinder. Turns out I have just such a grinder somewhere up in the wasteland we call our attic. I'm going to dig it out and give it a try.
Thanks for the info. Ed