Method and equipment for the dual orientation of multiple-layer thermoplastic materials

Abstract

This invention describes a process for orienting and drawing multiple-layer thermoplastic materials, in particular of a tubular type, wherein a) the tubular exiting from the heating tunnel is brought up to the orienting temperature of a first material having the highest orienting temperature, while the material of the remaining layers is still in a plastic state; b) said first material is subjected to a first partial drawing process, while the remaining layers plastically deform to follow its expansion; c) the temperature of the tubular is dropped to the orienting temperature of a second layer; d) this second layer is drawn, while at the same time continuing to draw the previous layer; e) the phases c) and d) are repeated for the subsequent layers, until the maximum allowable degree of drawing is achieved for each layer.

1 . A process for orienting and drawing multiple-layer thermoplastic materials, in particular of a tubular type, characterized in that it provides for the following phases: 
 a) the tubular exiting from the preheating tunnel is brought to the orienting temperature of a first material having the highest orienting temperature, while the material of the remaining layers is still in a plastic state;    b) a first partial drawing of said first layer is performed, while the remaining layers are plastically deforming to follow its expansion;    c) the temperature of the tubular is dropped down to the orienting temperature of a second layer;    d) the drawing of this second layer is performed, while continuing to draw the previous layer;    e) the phases c) and d) are repeated for the subsequent phases, until the maximum allowable degree of drawing is achieved for each layer.    
     
     
         2 . A method according to  claim 1 , in which during the partial drawing phase of said first material a drawing value is reached which equals the difference between the maximum obtainable drawing value of said first material and the maximum drawing value of the following material.  
     
     
         3 . A method according to the foregoing claims, characterized in that the orienting of said film occurs by using the “bubble” system.  
     
     
         4 . A plant for orienting and drawing some multiple-layer thermoplastic materials of a type comprising: 
 an annular die for extruding a multiple-layer tubular, equipped with the relative cooling system;    devices capable of effecting the longitudinal drawing of said tubular;    devices capable of blowing air into said tubular to effect its transversal drawing,    characterized in that it provides for: 
 devices capable of creating a temperature gradient on said tubular in the drawing area, so as to bring said tubular from a first temperature corresponding to an orienting temperature of the layer constituted by the material having the highest orienting temperature;  
 devices for regulating the amount of drawing in a first stretch of said drawing area, so as to partially draw said first layer, while the other layers are plastically deforming;  
 devices for subsequently bringing said tubular to a temperature corresponding to the orienting temperature of a second layer having an orienting temperature lower to that of the former;  
 devices for performing the drawing of the second layer, while simultaneously continuing the drawing of the first layer,  
 devices capable of repeating the phases c) and d) for the subsequent layers, until all other layers are drawn to reach the maximum allowable drawing ratio for each of them.