The material used to encase sausage filling is called sausage casing, sausage skin, or just casing.
Natural Casings had traditionally been manufactured from either animal intestines or skin, but artificial casings made from collagen and cellulose were introduced in the early 20th century. The material is moulded via a continuous extrusion process, yielding a single sausage casing of infinite length, which is then sliced into appropriate lengths, typically while the extrusion process is still ongoing.
Collagen casings are mainly produced from the collagen in beef or pig hides, and the bones and tendons. It can also be derived from poultry and fish. They have been made for more than 50 years and their share of the market has been increasing. Usually the cost to produce sausages in collagen is significantly lower than making sausages in gut because of higher production speeds and lower labor requirements.
Cellulose casings Clear, durable casings for wieners and franks are extruded from viscose, a kind of cellulose often derived from cotton linters or wood pulp. They are shirred for convenience and can be coloured to create “red hots.” Skinless franks are made by removing the casing after cooking.