This happened to me a few days ago.
Fortunately, the actual part underneath all that mess was just fine.
I'm not sure what the material was. It was a former piece of form-tooling that I'd saved when the mold was replaced. It was some sort of plastic, that I know. Any ideas how I should adjust the feeds and speeds to get cleaner chips?
Lower the speed, use an endmill with fewer flutes, take deeper cuts with less overlap.
Just assuming the stuff is somehow melting and fusing together...