Check valve system

A method of forming a check valve system, the method comprising:

• substantially simultaneously 3D printing a housing having a passageway therethrough, the passageway including a first portion and a second portion, the first portion having a diameter that is less than a diameter of the second portion so as to form a stepped portion, a blocking element disposed within the second portion of the passageway, the blocking element having a generally circular cross section and having a domed shape with an arcuate surface and a flat surface opposite the arcuate surface, and a spring disposed within the second portion of the passageway and configured to maintain the blocking element in contact with the stepped portion when under compression, the spring being formed under zero stress, so as to be initially compressing when a mixer is at least partially disposed in the second portion of the passageway, and compressed when the mixer is coupled with the check valve system, and the blocking element being formed such that when the check valve system is in a closed state the blocking element is partially disposed in the first portion, and
• forming retention clips on an internal surface of the passageway during the 3D printing.