| 摘要 |
<p>A crystallisable polymer is used which has a distribution of molecular weights such that the ratio between its weight-average molecular weight and its number-average molecular weight is at least equal to 10. During its extrusion, the molten polymer is subjected to biaxial drawing at a substantially constant drawing rate by passing through a divergent annular die having a hyperbolic profile, the molten polymer penetrating the said die being thermally conditioned beforehand to a temperature greater than its melting temperature but less than its temperature of crystallization under the biaxial drawing induced by the said die, the melting and crystallisation temperatures being determined for the value of the pressure prevailing at the inlet of the divergent annular die, and then the biaxially oriented structure thus produced is subjected to rapid cooling in order to bring its temperature to a temperature below its crystallization point at atmospheric pressure, this cooling being carried out, in the absence of deformation, by the structure passing through a subsequent annular straight die section so as to perfect the crystallisation induced during the biaxial drawing.</p> |
