Hi richardwen, They are both adjectives - folding chair or foldable chair (also ‘folded’ chair) - and I would say they are interchangeable in this function. 'Folding', however, is also the present participle verb form - I am folding this foldable chair and when I’m finished it will be folded.

Folding is the verb form. Foldable is the adjective form. He is folding the laundry. That dress is not foldable. Hope this helps. ~Samantha :)