To the best of my knowledge, all foldable knitting machinces are plastic. Their huge advantage is their compactness when stored and travelled with as well as lighweightness.
Brother KH341, standard-gauge
Empisal HK160, mid/bulky gauge, punchcards
Empisal KnitMaster Hobby, mid/bulky gauge, punchcards
Riccar, LECLE, standard-gauge, punchcards
Silver Reed LK100N, bulky gauge
Silver Reed LK100T, bulky gauge
Singer MK70, standard-gauge, punchcards
Singer HK160