Film Manufacturing Process
Shrink Film Manufacturing

Extrusion

An extruder is constructed of a hollow cylindrical barrel which is fitted with heaters around the exterior. A screw is fitted into this barrel which is driven by a motor.

The polymer resin is loaded into the rear of the extruder and conveyed through the barrel by the screw turning motion. As it travels through the heated barrel the resin is compacted and melted. The screw then pushes the melted resin through a circular die.

Coextrusion is accomplished by using multiple extruders that feed their melt through a feed block or multicavity die. The die is designed so that the melt from each of the extruders are layered in the structure.

Orientation

The orientation of the molecules in a film plays an important role in its performance in both mechanical and optical characteristics. The orientation of a film or tape can be made in either the longitudinal direction or transverse direction in a simultaneous or two phase operation. The bubble process which Cryovac uses is a method in which a tubular film or tape is heated after extrusion to the orientation temperature and stretched by inflation with air between upper and lower sets of pinch rollers operating at different speeds. The speed ratio of the pinch rollers controls the orientation in the longitudinal direction, while the orientation in the transverse direction depends on the ratio between the bubble diameter and the pre-inflation tape diameter.

Inflating the tube and stretching it equally in the longitudinal and transverse direction results in biaxial orientation. Biaxial orientation improves the tensile strength, ball burst impact strength, flexibility, and optics.

Preferentially (unequally) oriented film is usually produced by winding at a faster rate than the rate of extrusion. This results in a film which is stretched more in the longitudinal direction then the transverse direction.

Converting

After orienting, the film is converted into finished goods by one of the following methods:

• Singlewound Film : After collapsing the bubble, the film is edge ripped and wound into two large Singlewound rolls called mill logs. These mill logs are then slit to the finished film widths. Occasionally, mill logs may contain taped splices due to an interruption in the manufacturing process. This is why customers may experience film splices in rolls.
• Mechanically Centerfolded Film : Singlewound film is folded into centerfold film using specially constructed frames over which the film is rewound.
• Natural Centerfolded Film : Natural Centerfolded film is wound in-line, directly from extrusion.

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