The present invention relates to a fluid transfer device (1) comprising an outer casing (6), a proximal end (8), a piercing member (3), a pierceable seal (4) arranged against said piercing member (3) and a distal end (D) with a number of locking members (5). The device (1) further comprises a spring member (11) made of an elastic material extending from the proximal end (8) longitudinally through the distal end (D) in a manner surrounding the piercing member (3) in fluid tight fashion and forming an inner through channel (23) therein. Said spring member (11) comprises an upper portion (12), a sealing portion (13), and a number of locking members (14) in close proximity to the said distal end (D), that are integrally molded as a one piece unit.
