ANDRITZ Biax technologies for production of biaxially oriented film
Innovative, biaxially oriented sequential or simultaneous stretching processes
Polymers like polypropylene (PP), polyester (PET), polyamide (PA), polystyrene (PS), and others are extruded, cast, and mechanically stretched under the influence of heat. The film is then measured, treated, and wound onto jumbo rolls. Films can be produced with one layer or multiple functional layers. Stretching modifies the molecular structure and thickness of the film, giving the biaxially oriented films (BOPP, BOPET, etc.) specific properties.
Biaxially oriented plastic film is stretched in two directions: machine direction (lengthwise) and transverse direction (across the film sheet). Stretching can be performed in two steps (sequential) or in one single step (simultaneous, as ANDRITZ Biax’s MESIM technology).
Cast film production
Dosing, conveying, and feeding
Polymer resins stored in silos are conveyed pneumatically to high-precision gravimetric dosing and mixing units, where the resins are metered to the extruder while being mixed with additive master batches and edge trim recycle.
Main and co-extrusion
Twin- and single-screw extruders melt the materials. The melt is metered with gear pumps before being conveyed via heated melt pipes to the die system. Vapor heat transfer technology ensures a uniform temperature along the melt pipe. An automatic extrusion die with fast changeover feed block or multi-layer design for different layer configurations forms the melt into a cast film.
The melt is cast onto the polished chill roll. Air knife or advanced electrostatic systems pin the film on the chill roll. Depending on the polymer, the hot film is either cooled by the chill roll alone or in combination with a water bath enveloping the lower perimeter of the chill roll. The pinning and cooling techniques determine the efficiency of the casting process.
Biaxially oriented stretching
The cast film is stretched/oriented by one of the following two processes:
Biaxially oriented sequential stretching
MDO – Machine Direction Orienter
Film is oriented first of all in machine direction. Direct-driven rolls heat the cast film to optimum stretching temperatures. Depending on the material and film type, a single- or multi-gap configuration is used to orient the film in the machine direction. Consecutive annealing rolls stabilize the orientation.
TDO – Transverse Direction Orienter
The MD-oriented film enters the TDO and is carried by the clips of the chain track system, which performs the transverse (cross-machine) stretching step. Hot air is applied through nozzle boxes above and below the film. The TDO is separated into preheating, stretching, and annealing zones arranged specifically to match the requirements of the material or type of film. The TDO is heated either by electricity, gas, or oil.
Biaxially oriented simultaneous stretching(MESIM - MEchanical SIMultaneous stretching)
The cast film enters the MESIM oven (similar to a TDO oven) and is carried by the clips of the pentagraph system. The pentagraph system controls the clip movement in machine direction, while the adjustable diverging tracks determine the orientation in transverse direction. This enables simultaneous stretching in both machine direction (MD) and transverse direction (TD). The unique mechanical system also allows controlled shrink ratios in machine direction.
The simultaneous system may be used for specialty films (e.g. films requiring 100% isotropic properties), for films where roll contact during stretching is not desirable, or films made from materials that cannot be stretched otherwise.
Subsequent production steps
Pull roll stand
After stretching, the film is fed into the pull roll stand. Individually driven rolls, edge cutters, high precision X-ray systems, and corona and/or flame treatment systems are used to cool, measure, trim, and treat the surface of the film.
Winding of the finished film is the final process step. A constant winding angle and precise control of all winding parameters ensure wrinkle-free jumbo rolls up to 10 m width.
The entire stretching line is equipped with an advanced process control system providing comprehensive and user-friendly adjustment of all process parameters.
ANDRITZ Biax offers state-of-the-art equipment and complete lines for the production of biaxially oriented films, including the unique MESIM technology.