Plastic flatbed knitting machines are mostly mid-gauge or bulky but there are some standard gauge machines as well. They were manufactured by Japanese and European companies (Singer, Studio, Silver Reed, Brother, Phildar, Passap, etc.) Some are still being manufactured and are very popular among machine knitters.
If you are new to machine knitting on flatbed machines, and you just got a machine, consider the beginner-friendly projects I recommended to all my students and customers.
Brother KM100, basic/manual, mid-gauge
Brother KH341, standard, manual/basic, foldable
Brother KX350, mid-gauge, basic
Brother KX355, 7 mm, basic/manual, plastic bed
Brother KH360, 4.5/9 mm, basic/manual
Brother KH370, mid-gauge and/or bulky, basic/manual
Brother KH380, mid-gauge and/or bulky, basic/manual
Brother KH390, 4.5/9 mm, basic/manual
Brother KH395, 4.5/9 mm, basic/manual
Brother KX395, 4.5/9 mm, basic/manual
Brother KH400, 4.5/9 mm, basic/manual
Empisal HK100 (Hobby 100), 8 mm, basic/manual
KnitKing Simple Seven, mid-gauge, basic
Silver Reed LK110, mid-gauge, manual/basic
Silver Reed LK150, mid-gauge, manual/basic
Silver Reed Amimumemo La-La LK200, mid-gauge, manual/basic
Silver Reed LK310, mid-gauge, manual/basic
Silver Reed LK330, mid-gauge, manual/basic
Silver Reed KB360, mid-gauge, manual, basic
Silver Reed LK360, mid-gauge, plastic bed
Silver Reed SK2200, mid-gauge, manual/basic
Silver Reed LK100, 9 mm, manual/basic
Silver Reed LK100N, 9 mm, manual/basic, foldable
Silver Reed LK100T, 9 mm, manual/basic, foldable
Singer MK70, punchcards, foldable
Singer HK100, bulky, basic
Singer LK100, bulky, basic