Cast Film Lines for Barrier Films
- Published: November 01, 2003, By Gerhard Winkler, Windmöller & Hölscher
PEER-REVIEWED TECHNICAL PAPER: Following is an expanded summary of a complete paper available on the TAPPI web site at www.TAPPI.org. On the page, click “the PLACE” in the section designated “Journals.”
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Application: Besides the blown film process, cast film technology is the most significant production method for manufacture of barrier films ranging from 20 to 300 microns. The number of layers, the selection of polymers, the layer configuration, and the thickness of the individual layers depend on the intended film use.
Barrier films are multi-layer film structures produced in huge quantities with the cast film or blown film process. Barrier films provide a high degree of protection against gases and maintain good flavor and aroma. Polyamide and ethylene vinyl alcohol polymers have almost exclusive use for these applications. Combining them with polyethylene and polypropylene in layer configurations gives excellent barrier properties against water, steam, and moisture.
Applications focus on packaging of food, pharmaceutical products, and chemicals with the goal of extensing shelf life and improving the hygienic conditions for handling the packaged products.
Production of barrier films typically uses five- and seven-layer extrusion lines to make either symmetrical or asymmetrical compositions. The main difference between these films is that asymmetrical structures have an outer polyamide layer. Cast film lines produce various barrier films meeting specific requirements including laminating film, bag film, tray film for thermoforming applications, and lid film. Making barrier films uses processing lines up to 3000 mm wide with preferred film width of 1800-2500 mm.
Cast Film Line
Modern cast film lines comprise the following specific components:
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Gravimetric dosing equipment
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Extruder
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Feedblock
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Sheet die
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Chill roll unit
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Thickness gauge
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Winder.
Gravimetric dosing is an indispensable component of any state-of-the-art cast film line. Depending on the function of the individual extruders within the line, one to five additional dosing units for the secondary components allow for sophisticated blends. The gravimetric metering principle ensures dosing of each polymer component to an accuracy of ± 0.5%.
The number of extruders used and their respective sizes depend on the structure of the composite being produced and the required line output. A special demand for barrier film extruders is the universal use of the line for a broad spectrum of applications. For example, the use of only one feed screw capable of processing polyethylene, polypropylene, and polyamide is an absolute requirement for economically efficient and highly flexible production.
The core of an extrusion line is the die assembly consisting of a co-extrusion adapter — the “feedblock — and a sheet die such as a mono-layer die. The feedblock determines the number of layers and their configuration. Product change-over uses special distributor systems such as selector plugs to ensure minimum downtime and rapid start-up of the line after product changes. State-of-the-art feedblocks are necessary to meet specific demands such as flexibility, universal use, etc.
With the newer feedblock technology, multi-channel dies are now less important. In the cast film sector, coat-hanger dies or so-called T-dies find use today. Automatic dies designed to control film gauge tolerances have become a standard feature in cast film production. The principle is that individual expansion bolts spaced approximately 25 mm across the width of the die lip are actuated by a thickness gauge and influenced thermally. The resulting linear expansion of the bolts triggers a specific gap adjustment.
Short die cleaning time is always important. Special provisions incorporated in the line allow die cleaning without removal and complete disassembly. This eliminates damage to delicate die parts. An operator simply flips the die open for effortless cleaning.
The chill roll unit normally has two high-capacity chill rolls with different diameters. The melt tail impingement angle on the primary chill roll is either 90° — vertical position of the die — or 45°. The primary chill roll of 600-800 mm has a chromed, slightly matte finished surface. Adjacent is the secondary chill roll. An essential requirement is constant temperature across the entire face width of the rolls within tolerances of ± 1°K. Barrier films require a higher temperature for cooling to adjust the properties of the product to its specific application.
Radiometric thickness gauges have primary use for recording the total film thickness, recording and controlling gauge tolerances, or both. Barrier films require that the individual barrier layers are not subject to any major variations. An infrared thickness gauge allows selective measurement to display and monitor polyolefins, polyamide, or other polymer.
Barrier films require highly sophisticated winding systems that provide sensitive web tension control and give a choice of different wind modes. The industry predominantly uses center winding. Winding and web tension determine hardness of the finished film reel and therefore its subsequent conversion. Crystalline resins such as polyamide or polypropylene can re-crystallize and shrink resulting in the film becoming more rigid during storage. Incorrect settings of the winder or inappropriate winder specification may lead to excessive scrap rates. Modern winding systems feature a final cooling section, multiple-reel slitting capability, and automatic loading and unloading systems for the reel shafts and finished reels.
Barrier Film Quality
Barrier films must meet high quality demands because of their intended use in the food and pharmaceutical sector where stringent requirements are necessary. State-of-the-art cast film systems therefore feature a high degree of automation and production monitoring capability to visualize and record all essential functions of the line. The operator can always maintain constant production conditions and fix problems immediately. Linking the production line through serial interfaces with higher level operating systems allows storage of machine and production data.
Layer thickness tolerances are major challenges to a machine builder and a film manufacturer. Any film can only fulfil its intended function when not only the total film thickness but also the individual layers are within tight tolerances. This is an essential requirement impacting on barrier properties, thermoforming or deep-draw capability, and economically efficient operation of the extrusion line. Unnecessarily high use of valuable and expensive raw materials such as polyamide or other resins in the multi-layer structure as a “safety allowance” can increase the share of material costs in a product. Laboratory evaluations using measurement of microtome cuts with a microscope are necessary to control consistency of individual layers within multilayer film structures.
Conclusion
The superiority of the cast film process in the manufacture of barrier films considers the high outputs and outstanding film transparency that are possible and the ability to use moderately priced standard polymers. Mature technology and a profound knowledge of the polymers used, the process, and operational related matters are an indispensable prerequisite for a film converter to manufacture barrier films meeting the highest quality standards. These include an even layer thickness distribution, superb film surface properties, and excellent roll quality.
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