Experimental investigation and modelling of film
Jean-François Agassant, Yves DEMAY, Pierre LAFLEUR, Johann LAFFARGUE
Ecole des Mines de Paris - CEMEF
France

Keywords: blown film, modeling, instability

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Most polymer films are produced by film blowing. With the development of new polyethylene resins (LLDPE, metallocene) marked processing instabilities are encountered even at low rates (low draw ratio and low blow up ratio); Both axisymmetric and helical instabilities have been precisely investigated using and original optical system developped at Ecole Polytechnique de Montreal. It allows to define precisely and without ambiguity the processing conditions for the development of each instability. This results in constructing real bubble stability maps.
A new film blowing model is proposed. All the equations are written in the laboratory frame (instead of using a frame linked to the membrane geometry as in the models developped up to now) and then developped as a function of an adimensional parameter. Mass balance, force balance and temperature balance equations are then deduced at successive orders of this (small) dimensional parameter. In the stationnary case one obtain the classical film blowing equations early developped by Pearson and Petrie and others.
But it is then possible to develop linear stability analysis, first in the axisymmetric case and then in the non axisymmetric case. This was difficult in a frame linked to the membrane geometry because, when the process is unstable, both the frame and the different parameters defined in this frame were simultaneously varying. When the eigenvalue with the greatest real part becomes positive, the process is unstable and the period of the instability is linked to the imaginary part of the eigenvalue.
The results of the model are in qualitative agreement with the experimental results