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The new plastic casing product line is called Nova. It’s extruded from a variety of raw materials thoroughly combined in each layer to provide every single type of Nova casing with its unique properties. Each raw material is melted at an individual extruder to further form a multilayer structure for the required processing performance (mechanical and physical properties). Once formed, it passes through a cooling stage. The cooled casing is then bi-oriented, i.e., in both directions–transverse (TD) and longitudinal, or machine direction (MD)–to align its polymer chains for better properties. Finally, the casing is stabilized through a process called annealing to create the required shrinkage memory needed for each application.
Types and properties of Nova casing
Oriented or Non-Oriented (Straight) Casings
Casing orientation considerably enhances its mechanical and barrier properties. It becomes less stretchable but stronger and stable. For the same reasons, an oriented casing is normally thinner than a non-oriented one. Biaxial orientation builds up a shrink force in the casing, which can be released by exposing the casing to heat, e.g., during sausage processing. The shrinkage combined with a special plastic resin in the inner layer contributes to the adhesion between the meat and the casing during processing and to the elimination of wrinkle formation later on. An oriented casing normally forms straighter shoulders than a non-oriented casing, and this translates to increased yields and throughputs in sliced applications.
When manufactured in a line that includes only one extruder feeding the die, the resultant tubing consists of only one layer and forms a casing called monolayer or single-layer. It can be either non-oriented or oriented, and it can still be combined with several components to improve its properties.
When manufactured in a line that includes several extruders feeding the die, the resulting tubing consists of several layers and forms a casing called multilayer or coextruded. The main component of a multilayer casing is often polyamide which provides such properties as oxygen barrier, mechanical resistance, expandability and shrink ability. Polyolefin provides a water barrier and acts as a tie layer or an adhesive that creates the bond between the adjacent layers and keeps them together.
ViskoTeepak casings have a multilayer structure design that can be tailor-made to fit any application or production process through the combination of different materials in each layer for the required properties. A single layer of material that alone has all the required performance properties is extremely rare. Co-extrusion allows us to combine, in one casing, the good properties of various raw materials such as mechanical strength, oxygen and water-vapor barrier for increased shelf life and zero weight loss, lower water-vapor permeability for reduced weight loss during processing and storage, etc. Moreover, each casing can be adapted to suit the customer’s unique processing conditions.
Soaking to change the properties
Nova can be stuffed dry or soaked depending on the customer’s procedures. For compliance with the latest regulations on hygiene, it’s strongly recommended to exclude soaking from the process in order to avoid any contamination. However, if it’s still necessary, cold water should be used at a recommended maximum temperature of +20°C (68°F) for no longer than 20 minutes. Shorter soaking times won’t allow the casing to absorb enough water for stuffing to the maximum diameter. Longer soaking times won’t improve the performance, but they don’t negatively affect the casing, either. The casing can be stuffed to a maximum diameter of 2% larger than the recommended stuffing diameter (RSD). Overstuffing can result in casing breakage during stuffing or thermal processing. If there’s a delay between soaking and stuffing, it’s recommended to store the casing in a sealed plastic bag or in the soaking water. During the soaking phase, the casing must be completely submerged in and filled with water. Soaking isn’t recommended for Nova HM because it’s more sensitive to moisture and, if soaked or stored in a humid environment, can begin to shrink. Consequently, it will be less shrinkable during processing. The casing should be stored in a sealed plastic bag as long as possible before use.
Stuffing to perfection
Nova has a wide range of calibers and properties. Nova X and L must be stuffed to the recommended stuffing diameter (RSD). It’s easy to remember because both types are sold by their RSD in mm. For example, a Nova X 065 BRS must be stuffed to 65 mm. Stuffing too loosely can result in the formation of wrinkles on the products after thermal processing and cooling. The product might expand up to 2 mm (0.08 in.) in final diameter, but it depends on the emulsion and machinery used and should therefore be noted during testing. Nova HM is referred to and sold by its flat width in mm, because it’s normally used for products processed in a mold. The casing must usually be 5% to 10% longer than the mold. For example, if the mold has a length of 100 cm (39.37 in.), the casing length should be 105-110 cm (41.34 to 43.31 in.) measured from clip to clip. If it’s too short, the final products will have rounded ends making the first slices unusable. If it’s too long, the final product may have deep wrinkles, thus rendering the affected slices unusable.
It’s also important to use a suitable stuffing tube according to the casing specifications as well as the production process.
Water-vapor transmission rate
Water-vapor transmission rate (WVTR) is the amount of water vapor that passes through the casing at a specific temperature and relative humidity. The test conditions vary but Nova is continuously tested at 37.8°C (100°F) and RH of 100% as set forth in ASTM E-398 and/ or ASTM D-1653. In practice, this value correlates with the water (i.e., weight) loss over storage of a specific product at specific temperature and air pressure. The lower the rate, the lower the weight loss. However, the rate varies depending on the actual material thickness, temperature and pressure.
Oxygen Transmission rate (OTR)
Oxygen transmission rate (OTR) is the amount of oxy – gen gas that passes through the casing over a certain period of time at a specific temperature, relative humidity and air pressure. The test conditions vary but Nova is continuously tested at 23°C (73°F) and RH of 0% as set forth in ASTM D-3985. The lower the rate, the less oxygen passes through the material.
The shrinkage of Nova is the ability of its nylon layers to return to their initial state with a certain force. The casing shrinks longitudinally (in the machine direction (MD) along the length) and transversally (in the transverse direction (TD) across the width).
The overall thickness of Nova is composed of all of its layers. It’s the average of measurements taken at different points throughout the casing (excluding its edges).
The flat width tolerance indicates the maximum variation in the width of the casing.