Patent ID: 7833468
Filing Date: 2010-11-16
Classification: B29C,B29L

Abstract:
1. In a process of forming a tubular oriented film of at least one thermoplastic polymer film material having a crystallisation range by extruding a flow of said at least one molten thermoplastic polymer material from a circular extrusion die in the form a continuous hollow tube, in which process said flow after leaving a circular exit orifice in the die is cooled and is oriented at least in the longitudinal direction while still being at least partly molten, said longitudinal orientation taking place by a pulling force applied to the tubular flow by moving draw-down means, and in which process the flow while still at least partly molten on its travel between the exit orifice and the draw-down means passes and is in frictional contact with a surface of an annular friction device, which can be arranged inside or outside said tubular flow, the improvement wherein said longitudinal orientation is enhanced and determined by controlling the temperature of said surface of said frictional device by means of a fluid heat-exchange medium to impart to the flow a temperature within or slightly above the crystallization range of said thermoplastic material, and controlling the frictional resistance resulting from said frictional contact by creating through holes or micropores in said surface of said frictional device which is in contact with said tubular flow a selected gaseous pressure differential acting on at least portions of a surface of said flow contacting the surface of the frictional device and/or the opposite surface of said flow, the degree of frictional resistance imposed by said frictional device surface being adapted to produce between the frictional device and said draw-down device and while said flow is at the said temperature, a controlled contribution to the longitudinal orientation of said flow.