The invention relates to an injection system (10) including a longitudinal barrel (11), a distally projecting tip (13), a collar (20) provided with an inner thread (21), said collar (20) being securely engaged around said distally projecting tip (13), said distally projecting tip (13) having a lateral surface (16) and a fluid passageway (14) extending therethrough, and a tip-cap assembly (30) comprising an elastomeric inner cap (50), and a rigid outer cap (40) that is or can be securely disposed around said elastomeric inner cap (50). The rigid outer cap (40) of the tip cap assembly comprises an outer thread (44d) intended to be screwed with the inner thread (21) of the collar (20), a frustoconical extension (44a), a radial recess (44e) between the outer thread (44d) and the frustoconical extension (44a). The frustoconical extension (44a) is intended to ensure a circumferential sealing of the distally projecting tip (13) around the lateral surface (16) in order to act as a sterility skirt when the tip-cap assembly (30) closes the passageway (14) of the projecting tip (13).
