'Double-bubble' process makes thinner, stronger films.

A new film extrusion technology under development at Windmoeller & Hoelscher Corp., Lincoln, R.I., promises to give processors the ability to generate lamination-quality films at lower cost. Called Tandex, the new process combines conventional blown-film extrusion with a second machine that orients the film in the machine direction, transverse direction, or both.

Unlike other so-called "double-bubble" systems, Tandex is not aimed at OPP film, but at products such as PE heavy-duty sacks that could be downgauged at higher stiffness. Higher-end packaging structures are another target. According to Klaus-Peter Voss, W&H's manager of R&D, coextruded film processed on a Tandex line could conceivably replace costlier laminations, thereby opening up new markets for film processors.

W&H has run a variety of materials on an R&D system, including virtually all polyolefins (even metallocene-catalyzed types), barrier resins, and polystyrene. The company is currently building a 63-in.-wide prototype line in Lengerich, Germany.

A retrofittable system

Here's how it works: The collapsed tube is drawn in a water bath that preheats the film (see illustration). From there, the material enters a holding unit, where air reinflates the tube. The film then passes a series of infrared heaters, which warm it up to its stretching temperature. At this point, the bubble is oriented in the transverse direction. An oscillating haul-off then pulls the tube out of the bath. A speed differential between the holding unit and the haul-off serves to orient the film longitudinally. The film is then thermally annealed and wound.

Film made this way offers processors downgauging and "drastically" improved tensile properties, says Voss. In coex structures, film stretching is also said to improve the bond strength between layers, which in some cases might permit a multilayer structure to replace a more expensive laminate.

The system is flexible enough to allow processors to tweak stretch ratios in order to optimize monoaxial or biaxial orientation or a balance of the two. Temperature of the infrared heaters can be adjusted to provide thickness control.

Output capacity of Tandex depends on the die size. For example, a processor wanting to orient in both directions would typically have to downsize the die.

Tandex can be retrofitted to virtually any existing blown-film system to run in-line or it can be used separately for off-line production. W&H has yet to determine pricing.

ANOTHER DOUBLE-BUBBLE PROCESS IS ON THE WAY

Macro Engineering & Technology, Mississauga, Ont., also has in the works a new double-bubble biax process. The key to this system is a new die design that permits use of temperature-sensitive PVdC as a barrier layer. Applications will include high-end barrier packaging for food, meat, and cheese.