The stretching of rigid PVC films

The LT films are usually stretched with the aim of improving the mechanical properties in the machine direction. This is necessary for products like sound-recording tapes and adhesive tapes. The thickness and mechanical properties must be within very narrow tolerances and, in addition, the strength in the transverse direction must be high enough to prevent splitting. This can be achieved by various methods which will now be discussed. 

If a round filament of plastic is stretched at a ratio of 1 4, the diameter decreases by a factor of 2, if minor variations in density due to, e.  

This effect can be understood by the following simplified reasoning (see Fig. 2). The film is transported by the feeding rolls at a rate ki and stretched by the drawing rolls at a rate F2 Vi. The stretching ratio, A, equals K2/K1. At the boundary of the film (e.g. the points on the curve AC) the material is only subjected to a tension in the direction of the tangent to the boundary. The stress is here monoaxial so that the material contracts freely both in the y-direction (transverse) and the z-direction (perpendicular to the film). This will result in monoaxial orientation. 

In the middle of the sheet (points on the line EF) the same situation would occur if the material between C and D would be able to contract freely along the drawing rolls as this is not the case the transverse contraction will not be complete and a stress will be present in the y-direction which reduces the film thickness and causes a transverse orientation. The way in which this stress increases as a function of y from the edge towards the middle, depends on the overall geometry if /, the free length of the film between the rolls, is comparable to or greater than b. 

It has appeared advantageous to carry out the stretching operation in a number of steps. This can be realised by arranging a large number of heated rolls closely together as indicated in Fig. 4. The transverse forces in each step are much lower than in a single-step operation so that transverse contraction is largely prevented by the friction between the film and the rolls. The result is a film of almost constant thickness. 

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