I’m looking for an art board that would hold 30+lbs of brass for a project. Once the project is complete I was going to get it framed. So I would need to find something that would be easy to frame too. It’s project for my husband that my kids and I are doing for fathers day. Thanks 
Thick foam core is very strong.
You need to support it on all edges, or it will bend over time, though.
“Art board” usually refers to card boards and foam boards, but for 30 lbs of weight I’d just use plywood. You can paint it or stain it, drill it, glue it.
Wood working stores like Rockler will carry a good variety of quality plywood, but if you’ll be painting it so the grain doesn’t show, then Home Depot will sell you what you need. Depending on the size and design of the piece I’d use 5/8 or 3/4".
The title… Well, I’m glad it’s a typo. 
Thread title edited to fix typo (art broad to art board).
Use simple shallow shadow box.
1/8 inch fiber-reinforced plastic can be had at Lowe’s. I bought a sheet and have used segments for everything from a commercial building sign backerboard (had the sign printed on adhesive PVC) to smaller signs and so forth. They have a cheap one that has one pebbled side and one smooth side.
Then say it three times fast.
If plywood wood be too thick/heavy, there’s also masonite (hardboard). Your local hardware store will have big sheets for ~$20 that they will cut to size for you. Then you just rough it up with sandpaper and prime and paint however suits you. I don’t know how big your project is, but I would guess it would be stable holding 30 lbs of metal as long as the dimensions are under 24". Anything bigger, go with plywood.
I would not use foam core for this.
ETA: Whatcha making?
I agree that plywood or masonite would work and it’s easy. If you must, find someplace that can sell you 1/2 inch Gatorboard cut to size. It looks like foamcore, but is much stronger and more durable. Not the easiest to work with, either.
Thank you everyone!! U guys really helped a lot. I think I’m going to check out plywood. The project size is 26" x 35.7". I was just worried that plywood would bow overtime.