Stretching Device and Method for Stretching a Plastic Film in the Transport Direction Thereof

Abstract

The invention describes a stretching device for stretching a plastic film in its transport direction, having a first roller that can be driven by a first drive and can be rotated at a first rotational speed, and having a second roller that can be driven by a second drive and can be rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film. The device is characterized by means of varying the amount of air between the plastic film and flat elements of the first roller and/or the second roller on which the film rests.

1 . A stretching device for stretching a plastic film in its transport direction, having a first roller that can be driven by a first drive and can be rotated at a first rotational speed, and having a second roller that can be driven by a second drive and can be rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film, characterized by
 means for changing the amount of air between the plastic film and surface elements of the first roller and/or of the second roller on which the film rests.   
     
     
         2 . A stretching device according to  claim 1 ,
 characterized in that   the means for varying the amount of air comprise depressions in the surface of the first roller and/or the second roller, the air being receivable in the depressions.   
     
     
         3 . A stretching device according to  claim 1 ,
 characterized in that   the means for varying the amount of air comprised through openings in the surface of the first roller and/or the second roller said through openings providing a fluid-communicating connection between the outer surface and a cavity within the first roller and/or the second roller.   
     
     
         4 . A stretching device according to  claim 1 ,
 characterized in that   the means for varying the amount of air comprised through openings in the surface of the first roller and/or the second roller, wherein at least part of the through openings can be provided by a porous material, in particular by a microporous material.   
     
     
         5 . A stretching device according to  claim 1 ,
 characterized in that   the means for varying the amount of air comprise through openings in the surface of the first roller and/or the second roller, at least a part of the through openings being provided with a porous material, in particular by a microporous material, the porous material being a sintered material, in particular, a sintered metal.   
     
     
         6 . A stretching device according to  claim 1 ,
 characterized in that   the means for varying the amount of air comprised through openings in at least part of a roll shell of the first roll and/or the second roll, at least part of the through openings being formed by bores.   
     
     
         7 . A stretching device according to  claim 1 ,
 characterized in that   the first and/or the second roller comprise at least one cavity which can be subjected to an air pressure that is lower than the ambient pressure.   
     
     
         8 . A stretching device according to  claim 1 ,
 characterized in that   the first and/or the second roller comprise at least one cavity which is divided into at least two segments in the axial direction and/or in the circumferential direction by means of at least one separating element, wherein at least one segment can be subjected to an air pressure which is reduced or increased in comparison with the ambient pressure.   
     
     
         9 . A stretching device according to  claim 1 ,
 characterized in that   the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests comprises at least one suction device with which air can be sucked out of the region in which the plastic film comes into contact with the first and/or the second roller in the transport direction.   
     
     
         10 . A stretching device according to  claim 1 ,
 characterized in that   the suction device is divided into sections transversely to the transport direction of the plastic film.   
     
     
         11 . A stretching device according to  claim 1 ,
 characterized in that   the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller, on which the film rests, comprises at least one upper-pressure device, with which the surface of the plastic film facing away from the roller can be acted upon with air under upper pressure.   
     
     
         12 . A stretching device according to  claim 11 ,
 characterized in that   the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests comprise at least one electrode with which the plastic film can be electrostatically charged upstream or at the line of contact of the plastic film with the roller.   
     
     
         13 . A method for stretching a plastic film in its transport direction with a first roller, which is driven by a first drive and rotated at a first rotational speed, and with a second roller, which is driven by a second drive and rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film,
 characterized in that   the quantity of air present between the plastic film and surface elements of the first roller and/or of the second roller, on which the film rests, is changed by means of changing the quantity of air.   
     
     
         14 . A blow film installation comprising at least one extruder for producing a plastic melt, a die head for producing a film tube, a flattening device for converting the film tube into a double-layer plastic film, a driven extraction device for drawing off and further transporting the double-layer plastic film, and a winding device for winding up at least one layer of the double-layer plastic film,
 characterized by   at least one stretching device according to  claim 1 .