Plastic casings are extruded like most other plastic products. They also can be flat or shirred. Generally, smoke and water do not pass through the casing, so plastic is used for non_smoked products where high yields are expected. The inner surface can be laminated or co_extruded with a polymer with an affinity for meat protein causing the meat to stick to the film, resulting in some loss when the casing is peeled, but higher overall yield due to better moisture control.