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Bi-axial orientation is a method of stretching cast film or sheet in two directions or axis (bi-axial) to create a film or sheet of the desired thickness in a width that is greater than that of the original cast film or sheet.
The process starts the same as with cast film or sheet, from this point on the term "web" will be used to refer to the sheet or film. Resin is melted in the extruder and forced through the slit in the die onto the cast roll. Depending on the polymer used, after the cast roll the web may travel through a series of "chill rolls" which cool the plastic even more.
The web then travels into a machine called the Machine Direction Orienter (MDO). The MDO usually consists of two sets of heated rolls. The first set usually run around the same speed as the cast roll or chill rolls. The later set, called the stretch, run faster than the first causing the web to be pulled and stretched in the direction of travel. The stretch rolls usually run 1.5x-2x faster than the first set of rolls, depending on the polymer used and the desired amount of stretching.
Leaving the MDO the web then enters the Trans Direction Orienter or TDO, also known as the tenter. The TDO is basically a large oven with openings at each end. At the in-feed the web is gripped on both sides by a chain of clips that then transport the web into the oven. Around one-third of the way in the clips start to move outwards, stretching the web across its width to a little larger than the desired finished width.
Upon leaving the TDO the web then travels through a set of two or more knives to trim the rough edges, and in some cases split the web in two, before being finally wound onto the final roll.