Home > MacroLetter > 7.1 - Autumn 2001 > Collapsing of Barrier Films

Collapsing of Barrier Films

Extrusion today has gained a complexity previously unrealized for blown film. Much advancement has taken place in resins and additives leading to more sophisticated multilayer barrier films.  Both have led to creative solutions involving die and screw design.  It would be remiss, however, if things were simply left at that.  The coextrusion process, needs to address this complexity within all facets of machine design including the downstream equipment and the materials of manufacture.  These considerations must be assessed with a keen awareness of market trends towards increased line speeds and downgauging of finished products. 

Different resins, in either monolayer or coextruded structures, offer unique collapsing problems.  Various alternatives for blown film collapsing exist: full width rollers, segmented rollers and air board collapsers.  Each collapsing alternative needs to be considered in terms of the materials, the structure and the application.  It is important to understand that there is no one correct solution or universally acceptable approach to bubble collapsing.

First, however, lets review the basics of blown film collapsing.  There is an inherent incompatibility in collapsing a tube of plastic material from a tubular (circular) cross-section to a flattened form.  The objective of bubble collapsing is to produce a film with as few scratches and wrinkles as possible.  This is attempted by maintaining bubble roundness and stability prior to collapsing, as well as by providing a uniform heat history through the use of side stabilizers during collapsing.  The end result should be a wrinkle free roll of film at windup with more uniform web tension and improved optical properties.

The collapsing surface of choice is largely a function of the materials being run and line speed.  Segmented rollers are the collapsing surface chosen by many producers of extensible films produced with LDPE-LLDPE blends and metallocenes.  They work extremely well with rigid barrier films since they provide the best film flatness and prevent wrinkle formation.  Macro has introduced a patented arched segmented Teflon roller collapsing surface with positive results for a plethora of different structures and materials, including coextruded structures with Nylon, EVOH, PVDC, COC, tie resins and all types of polyethylene.

Full width aluminum or composite rollers provide the best results for high take-off speeds (above 400 FPM or 120 m/min), films with high additive loads or very tack films.

Air board collapsers provide another alternative allowing the processor to run higher line speeds as well as particularly tacky, cling films (PVC or EVA) and stretch films with PIB, which builds up on other collapsing surfaces.  Turbulence and unequal air flow causing bubble instability and non-uniform cooling has proven to be their limitation.

In addition to the collapsing surface, processing parameters play an important role in collapsing a bubble successfully.  Stiffer materials need to be collapsed while they are still warm and flexible, such as Nylon, EVOH and HDPE.  Polystyrene (PS) is no exception to this rule with collapsing occurring almost immediately after the bubble is formed. 

In the end, the final choice will be a function of products to be run and price. Processors must decide between the trade-offs inherent with any collapsing surface alternative and the final product quality they wish to achieve.  Whatever tension issues, center sag or scratches are imparted to the film at the collapsing stage will have to be accounted for at the winder and beyond.