Blown film extrusion's future balloonsBlown film extrusion's future balloons

The big driver in producing film by the blown film process is evolving technology fueled by market demands for increased outputs, and extended flexibility of applications.

Add to this reduced film gauge tolerances, and the ongoing trend toward more layers, and the picture clearly points to one direction. The continuously growing competitive pressure facing film producers and the engineering industry is bound to speed up developments. The following review highlights the current trends.

Throughput/output

Apart from an extruder''s capacity, output achievable on blown film lines is dependent on bubble stability, which can be improved by fast, efficient melt cooling as it exits the die gap. The arrival of new technologies in this area over the last 30 years has led to remarkable increases in output.

This trend will continue with new developments like extruders with lower melt temperatures and improved melt cooling systems like the dual-deck air ring.

Flexibility

In addition to output capacity, flexibility with product specifications has become increasingly important. The ongoing trend toward reduced order volumes and specialty products calls for equipment designed for high output, while at the same time providing short startup and changeover times and minimum labor requirements.

Modern blown film die heads, especially for barrier films, therefore need a compact design with short flow paths, less resin in the die, and high flow rates to permit faster product changeovers.

Automatic gauge profile control

While automatic gauge profile control has become a standard feature, there are still blown film lines in operation that are operated without them.

Frequently, when talking about the usefulness of gauge profile controllers, some important advantages of the system are still ignored or neglected. Obviously, the ability to automatically reduce gauge tolerances by 50% or more is a significant result and worth considering. Equally important, however, is the fact that these tight tolerances are evenly distributed across the film width. In particular, when it comes to slitting at the winder for multiple roll production, consistent straight running of the individual webs is essential and is, in fact, a requirement for trouble-free processing on the downstream converting equipment.

Film thickness is measured by capacitance thickness gauges, today a standard feature of blown film lines, despite the fact that they contact the film bubble while measuring the thickness. Current R&D efforts are working to develop a noncontact capacitive system; this would offer obvious advantages when processing tubular film with tacky outer surfaces.

Apart from these systems, noncontact radiometric thickness gauges used in cast film production are increasingly being used on blown film lines, downstream from the bubble-collapsing station. They provide definite advantages for barrier films. Special control algorithms are also available to ensure that optimum gauge tolerances are achieved quickly.

Coextrusion

The multilayer coextrusion market in Europe is growing rapidly. A decade ago, about 20% to 25% of all blown film lines installed were for coextrusion; this figure has since doubled, and there is every indication that it will continue at a faster rate. As a result, coextrusion today accounts for as much as 80-90% of all new extrusion systems installed.

The demand for more layers also shows no sign of letting up. Monolayer products are disappearing. The recent trend is to replace three-layer products with five layers. A five-layer structure allows the properties of polyolefin combinations to be improved in a specifically targeted manner, while at the same time reducing costs.

Winder

When it comes to winding film, surface and turret winders have been the primary choices. The quality of the film roll is undoubtedly the most important criterion; it is not so much influenced by the winding principle, but rather by the specific features of the winder. Today''s winders are still either surface or turret types, but can be specifically designed and equipped to meet the applications they are intended for.

Regional differences that influence winder type can be expected to vanish gradually. While the turret winder is predominantly used in North America, Europe and Asia seem to prefer the surface winder and would select the turret design only for specific or more sophisticated applications. Consequently, turret winders account for about 50% in the cast film sector, but for only about 20% in blown film extrusion.

Another major step toward improved winding and roll quality, especially when using surface winders, is the ability to eliminate fold-back of the web tail when starting a new roll. The newly developed vacuum contact cutter drum is currently regarded as the only available system to offer this feature. The web tail is held on the contact drum until the moment it is applied to the core and is therefore prevented from folding back. The integrated knife of the vacuum cutter drum cuts the film web while lying on the drum surface, ensuring a clean right-angled cut across the entire web width, eliminating fringy edges and incomplete cuts.

Robert Wirtz, general manager extrusion equipment, Windmöller & Hölscher, [email protected]

Contact information

Windmöller & Hölscher

Lengerich, Germany

+49-5481-14-0

www.wuh-lengerich.de