a double mold assembly including an upper mold half and a lower mold half conjointly defining a partition plane whereat said mold halves can be separated from each other;

said upper mold half and said lower mold half defining a common longitudinal axis and said mold halves having corresponding inner walls defining corresponding approximately bottle-shaped hollow spaces which are mirror images of each other so that said hollow spaces conjointly form a common hollow space having a bellied expanse for receiving the air spring flexible member blank therein;

a pressure bellows unit including a pressure bellows extending axially in said common hollow space within said air spring flexible member blank along a length corresponding approximately to the length of said blank and said pressure bellows enclosing a pressure space;

said pressure bellows unit including first and second end pieces joined to said pressure bellows at corresponding longitudinal ends thereof;

means for supplying pressurized air or gas to said pressure space of said pressure bellows to cause said pressure bellows to expand laterally to press said blank against the composite inner wall defining said common hollow space whereby said pressure bellows shortens in length; and,

said end pieces being movably mounted in said double mold assembly so as to permit said end pieces to follow said pressure bellows as said pressure bellows shortens with the supply of said pressurized air or gas.