A two part nozzle for use in expressing thick, viscous materials therethrough comprises a nozzle base and a nozzle tip, and screw means for mounting the tip to the base. The base projects within the tip, and is proportioned to interfere with the inside wall of the tip around a narrow, continuous path, thereby providing an internal seal with the interior wall surface of a pouch to which the nozzle is coupled to prevent the back-flow of material through the screw means. Preferably, the nozzle also provides a seal on the external wall surface of the pouch, thereby reducing the possibility of material leaking both from and into the pouch. Gripping ribs are provided to facilitate the application of a manual force to couple and decouple the nozzle parts.
