Today, I cut the foam cores with box cutter and the edges are all
rough, not smooth.
I am thinking about getting a HOT KNIFE.
Would anyone recommend what brand or name of hot knife do I need to buy?
THNAKS.

Hmmm..Maybe Michael's craftstore has some kind of hot knife.When we were kids the shop teacher would make a cutter using wire and batteries..The cutter would melt through styrofoam to cut it.Neat idea!

Michaels should have something called a styrofoam cutter. not bad, but definitely delicate. I broke mine the second time I used it!

Thank you!
I will check at Michaels.

I bought a hot knife at Michael's - it's NOT the styrofoam cutter. This has blades like an exacto knife and you attach the blade to the handle and plug it in.
It works GREAT on foamboard....
I found it at Michael's by the woodworking section. I think it was $16.99
Don't forget your coupon!!

Oops! Should have put more info. The knife is called:

Walnut Hollow Designed for Her Tools Creative Hot Knife

You're welcome...happy to help! Don't forget - it was by the woodworking tools....

Yes! That's what I used it on....you have to go slow, but it cuts through easily without sawing and making the edges ragged.