Modular stretch film system

image With eleven possible stretching configurations and extremely flexible adjustability of individual machine components, the newly-developed high-performance MDO 600 S stretch film line from Dr. Collin GmbH (Ebersberg, Germany) is suitable not only for the production of film and sheet in a wide variety of thicknesses ranging between 200 and 3500 microns, but also for various grades of standard polyolefin material and high-temperature thermoplastics.

With an output of up to 300 kg / h, the MDO 600 S line is well-suited for development and pilot work, and can serve as a small production line for niche applications. The demand for monoaxially oriented film has risen sharply, especially recently, as this film has special properties that can be achieved specifically by stretching. Thus, the spectrum of applications for this new line is as diverse as the range of two- to nine-layer products being manufactured: Monoaxially oriented films are often needed as separator films for batteries and capacitors. This is an example of advantageous technology that can be achieved by stretching, in this case, achieving a defined porosity for a specific application.

Due to their good mechanical properties in conjunction with high transparency, monoaxially oriented films are now used primarily for optical applications. Monoaxially oriented films are usually used for TV screens and monitors, because the films must also meet ever-increasing demand for larger screen dimensions. They are expected to have excellent transparency at a minimum total thickness and superb mechanical properties to prevent, for example, screen surface "flutter".

image Special films for consumer electronics are a specialty of one of Dr. Collin’s long-standing customers, who has been operating the first MDO 600 S as a three-layer system for almost a year to their complete satisfaction. The Korean firm Kolon Industries Inc., Gumi, one of the world’s largest producers of optical films, uses the Collin equipment not only for material development, but also for optimization of its production machinery settings.

Clean room operation possible 

Cleanroom conditions possible
The new facility consists of a cast film extrusion line and an online orientation unit, both of which are adjustable to the specific material- and film characteristics. As is often the case when manufacturing optical film, a cleanroom-like production environment is needed; Dr. Collin has designed the cast film components so that a wall can separate everything from the material metering area past the extruder and up to the die from the rest of the equipment. This ensures that no unavoidable impurities in the extrusion area end up in the film area.

A dryer and a pneumatic loader ensure continuous feeding of material into the extruder. Depending on customer requirements, two to nine extrusion units with an output up to 300 kg/h can be used. These can be single- or twin-screw extruders, if direct processing of raw materials is desired. A pre-filter for coarse impurities is connected to the extruder outlet; a fine filter with a mesh size of 20 µm is attached to the outlet of the melt pump. The molten material then passes into the slot die via the adapter and the feedblock, which is designed as a multi-purpose tool in case significantly different viscosities must be processed.

Flexibility begins in the casting unit,…

The angle at which the 600-mm-thick molten film from the slot die strikes the casting roll is adjustable over a range of 45°. In addition, the distance between the machine and the 800-mm-diameter casting roll can be varied according to film thickness. As the line is designed for production of both thin film measuring 200 µm and for sheet with a thickness of up to 3,500 µm, the machine manufacturer has provided two possible roll guides in the casting unit. An air shower that can be employed as necessary at the casting roll ensures cooling of both sides of the film. It makes certain that even thicker film and sheet cool down quickly, which is very important for the mechanical and optical properties of the final product.

... continues in the stretching unit …

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For maximum flexibility in the stretching unit, its modular design provides a total of four independent mechanical and electrical modules – two preheating modules and two stretching modules. Thus, not only are the possible settings highly variable, but transport and installation are very easy as well. All preheating module rolls are cantilevered and individually driven. While the four rolls in the first preheating module are equipped with a water temperature control system for up to 150°C, the four rolls of the second preheating module have an oil temperature control system for temperatures up to 230°C. Dr. Collin has also provided two possibilities for product guides in the stretching unit – one for thin film and one for thick film and sheet. Another feature of the preheating module is the interchangeable roll jackets: depending on product requirements, the film manufacturer can attach textured, coated or non-stick roll surfaces with an easy switching tool.

A core element of the stretching unit is the third module, in which all the rolls are also oil-temperature-controlled. Two vertically mounted variable rolls provide the setting of all eleven different stretching orientations. For example, short stretching is recommended for PE and PP film, while PET is better-suited for long stretching gaps. Depending on the product, it may be recommended to reheat both sides of the film during stretching. Dr. Collin also considered these possibilities and in additional stretching modules has installed optional infrared heaters. The fourth module of the stretching unit provides film setting and cooling, and is equipped again with water-cooled rolls.
After the stretching unit, the film passes via the take-off (with either voltage or speed control) into the film accumulator, and is then wound on a dual-position winder. In the take-off unit, the edges are also trimmed and wound up.

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Different ways of guiding the films through the stretching machine allow for various stretching gaps: For PE and PP films a shorter stretching gap is recommended (here between rolls 9 and 10 or 10 and 11), while for PET film a longer stretching gap is suggested. Optional insertable IR heaters (1, 3, 4, 7, 8) provide additional film temperature control.

 

… and ends with the control unit

An important element of the new MDO 600 S monoaxial stretching units is their intelligent controls, which also reflect this modular concept. Two 19-inch touchscreens show the current guiding of the film as well as all necessary parameters of every module, such as roll temperatures, elongation rates, and stretching forces. Trend charts, error messages, analyses, and composition management are, of course, part of the control unit’s functional range.
The system operator enjoys a high degree of flexibility and security through use of additional individual operating units at the four stretching unit modules, which are joined to the main unit. Being very close to the activity is very helpful, especially when establishing the correct parameters while testing a new gap configuration or using a new material mixture. In this way, the machine operator can step in quickly and optimize the process.

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