A rib forming apparatus has an elongated support table, a supply of thermoplastic sheet material over the table, a transporting means having at least two transverse carriages mounted on the table to slide along longitudinal edges thereof, a thermal means mounted on one of the carriages for locally heating the sheet material in preparation of a molding press, and at least one press module mounted on another carriage and having a reciprocating press blade positioned at one flat side of the sheet material and a couple of swiveling press blades positioned at the opposite flat side of the sheet material and movable toward the reciprocating press blade to mold the heated section of the sheet material by closing the above three press blades toward each other, whereby a series of vertical rib folds are formed transversely of the sheet material and advanced by the transport means through the closed press blades.