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Film Stretching Lines

The biaxial stretching process on appropriate equipment has become established for the production of high-quality flexible plastic films. In a longitudinal and (usually) subsequent transverse stretching process, the plastic molecules are aligned in such a way that their morphology changes significantly. The change in molecular structure gives films a wide range of beneficial properties while reducing their thickness. Compared to other packaging materials, stretched films are lightweight, can be manufactured in an energy-efficient manner, and thus have the lowest carbon footprint during production. 

Plastic films can be stretched monoaxially, sequentially biaxially, or simultaneously biaxially. In sequential lines, an extruded “pre-film” is first oriented lengthwise, using a roller system. This lengthwise-stretched film is then oriented crosswise in an oven-like device. This is by far the most commonly used stretching method. With simultaneous technology, the film is stretched in both directions at the same time. This special process is used, for example, for ultra-thin films with thicknesses of a few micrometers. 

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Extruder
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Extruder
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Extruder
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Casting
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Casting
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MDO
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MDO
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MDO
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TDO
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TDO
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TDO
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TDO
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Pull Roll
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Winder
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Winder

The Principle of the
Sequential Stretching Process

  1. 01

    Extruder

    The extrusion system is responsible for melting and homogenizing raw material and additives in order to provide a constant and continuous melt flow. Furthermore, it is important to ensure a stable melt temperature and pressure. Depending on the application, single screw, twin screw or cascade extruders are used for the film production.

  2. 02

    Casting

    On the casting unit, the melt is cooled immediately after exiting the extrusion die via a water-cooled roll and – depending on the polymer - an additional water bath, creating a pre-film, known as cast film. Cooling both sides of the film as quickly and evenly as possible is essential for ensuring consistent film quality.

  3. 03

    MD

    The longitudinal stretching machine, or MDO, stretches the cast film lengthwise in the machine direction by a factor of up to 1:9.  
    This is achieved by increasing the roller speed in the production direction. All rollers are driven directly by gearless electric motors. (This part of the line is not required in the simultaneous stretching process.)

  4. 04

    TD

    To stretch the film transversely in a stretching oven (TDO), it is fixed at the edges with gripping devices called clips. These clips are connected by a chain and run on both sides on guide rails that diverge in the direction of the machine, resulting in the film being stretched widthwise at a stretch ratio of up to 1:10.

  5. 05

    Pull Roll

    After the TDO, the film leaves the clip system. The film is thicker at the edges and has marks caused by the clips, so the edges are cut off in the pull roll stand and returned to the process. In this part of the line, the surface is also treated to ensure optimal further processing (printing, metallization, lamination).

  6. 06

    Winder

    A winder with automatic feeding and cutting device winds the end film into large rolls. An automatic winding system ensures uninterrupted production.

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