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181 PLASTICS IN FOOD PACKAGING For increased strength and improved barrier properties, film can be stretched to realign, or orient, the molecules in both the machine direction (MD), and across the web in the transverse (TD) or cross direction. In the Stenter-orienting process, transverse stretching of the cast flat sheet is carried out using clips which grip and pull the film edges, so that the width increases. Stretching in the MD can be achieved with several sets of nip rolls running at faster speeds. With the blown, or tubular, film process, orienting is achieved by increasing the pressure inside the tube to create a tube with a much larger diameter (Fig. 7.4). Film stretched in one direction only is described as being mono-oriented. When a film is stretched in both the directions, it is said to be biaxially orientated. Packing the molecules closer together improves the gas and water vapour barrier properties. Orientation of the molecules increases the mechanical strength of the film. Cast films and sheets which are not oriented are used in a range of thicknesses and can be thermoformed by heat and either pressure or vacuum to make the bottom webs of pouches and for single portion pots, tubs, trays or blister packs. Cast films are also used in flexible packaging because they are considered to be tough; if one tries to tear them, they will stretch and absorb the energy, even though the ultimate tensile strength may be lower than that with an oriented equivalent. Annealing zone Extrusion die ter ten gs ratin le e Acc s clip Cooling roll Extruder Casting rolls Acc eler atin g st ente r cli ps Heated oven Wind-up Figure 7.4 TD orientation by Stenter and MD orientation by acceleration in machine direction (courtesy of The Institute of Packaging).