Biaxial stretching of flat films

During the stretching of tubular films, as discussed in the previous section, longitudinal and transverse stretching are usually carried out simultaneously. This is also possible in flat-film stretching by means of a tenter frame. 

The heating of the film must be very accurately controlled in order to 

The difficulty with these tenter frames is that mechanically they are very complicated and other systems have been proposed in patent literature, for instance the manufacture of films with beaded edges. The beads slide in diverging slots or are gripped by rubber strips longitudinal orientation is realised by a difference in speed between feeding and take-off rolls. 

If the film is stretched too much in the first step, it may be too splitty for the second stretching process. This is especially a drawback for materials with stretch by necking-in since the first stretching cannot be reduced at will in order to prevent unstretched regions. In order to reduce the splitting tendency, the draw-down of the melt and the first stretching step are carried out in narrow gaps so that some orientation in the transverse direction is obtained. 

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