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, 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. Plastic casings are generally made from polymers such as Polyamide, Polypropylene or Polyethylene. Polyamide (Nylon) plastic casings are the most commonly used in production of cooked sausages and hams such as luncheon meat and bologna. Polyamide casings come in two main varieties: Oriented and non_oriented. The oriented polyamide are shrinkable casings and will shrink during the cooking process thereby reducing the water loss. Non_oriented polyamide casings remain the same diameter during the cooking process and thereby allow for expansion of the meat during cooking.