ANDRITZ Biax two-step sequential film stretching
Well-proven biaxially oriented film stretching technology for precision and high productivity
Sequential stretching is the most common method of producing biaxially oriented films. With the exception of highly specialized products, sequential stretching is used for more than 95% of all biaxially oriented films. ANDRITZ Biax supplies large lines (capacity of more than 40,000 tons/year) for the production of commodity films made from PP and PET.
Machine Direction Orienter (MDO)
The cast film formed by the extrusion and casting unit is stretched in machine direction inside the MDO.
The MDO consists of three groups of rolls: preheating, stretching, and annealing. The configuration of the MDO (number of rolls, diameters, surfaces, and heating media) and the stretching method are determined separately for each film material and application.
In the preheating zone, the film is heated up to stretching temperature. The heat must be precisely controlled so that the film surface does not stick or slip on the rolls. ANDRITZ Biax supplies the Caloduc system for direct-heated rolls (infrared heaters and a heat transfer liquid), which provides efficient and uniform heat transfer. The Caloduc system ensures temperature accuracy of +/-0.5° C.
Since every roll is heated individually, the film is heated gradually and gently. Each roll is driven by an individual motor so that film tension can be controlled carefully to avoid any slipping, sagging, or scratching of the film’s surface. The roll surfaces are either chrome-plated or coated with special non-stick materials.
The film is stretched in machine direction by consecutive stretching rolls driven with an increasing speed profile. BOPP is usually stretched in a four-roll configuration with two stretching gaps, while BOPET film is stretched with either one or two stretching gaps. Infrared heaters are used when stretching BOPET to maintain ideal stretching conditions. Nip rolls are arranged in the optimum position to assist stretching of the film.
The stretched film passes through heated annealing rolls to stabilize the orientation of the molecules. Depending on the type of polymer, heating is achieved either by heat transfer oil, water, or electricity. A combination of heating media can be used for preheating, stretching, and annealing. For specific film types requiring a very clean environment (e.g. capacitor films), the ANDRITZ Biax Caloduc system can be utilized for all MDO rolls.
Transverse Direction Orienter (TDO)
In the second stretching sequence, the film is stretched in transverse direction in the TDO. The TDO is divided into preheating, stretching, annealing, and cooling zones. Each zone can have a number of sections ranging from 1.5 to 4 m in length. The arrangement and number of zones is tailored to the film type, polymers used, and throughput of the line.
At the inlet, the film is engaged by stretching clips riding on the chain track system. ANDRITZ Biax offers two types of chain track systems: sliding clips PP or PS, and a 9-roller bearing clip for PET. Special products, like capacitor films, require a clean environment and roller bearing clips are used.
A set of two clips is mounted on a common clip body with a common jaw pin to ensure a perfect contact line across the width of the clip. The bottom part of the jaw is designed with a specially calculated profile to accommodate the film thickness. The clip design encloses the chain link and protects the film from contamination. The bearings of the roller bearing clip are located inside the clip body along with the chain to compensate for the forces resulting from film stretching and chain tension. This increases the life of the bearings dramatically, ensuring system reliability.
In addition, a lubrication system for the sliding and roller bearing chain track systems reduces oil consumption while maintaining maximum service life and durability.
The monorail is the core of the tenter frame, as it guides the chain and the clips smoothly and without interruptions. At each pivot point, the monorail can be adjusted by +/-20°. This unique capability of the ANDRITZ Biax technology makes it the most flexible system in the market. It allows a wide range of width adjustments to optimize the stretching profile. The monorail frame and the oven frame are separated from each other to avoid any thermal influences.
The clips are mounted on a special chain with 60 mm pitch and a breaking strength of 150 KN. The chain system is driven by large diameter outlet sprockets. Additionally, the sliding chain track system is equipped with driven inlet sprockets to reduce forces considerably. Clips are closed by non-contact magnets and opened by a wheel mounted on the outlet sprocket.
The ANDRITZ Biax TDO oven ensures efficient and uniform heating of the film. Equal distribution of air at a consistent temperature over the entire film width is essential to ensure uniform film thickness. Air distribution plenums distribute the air equally to individual nozzle boxes (seven per plenum) located above and below the film. Each heating zone is equipped with independent air circulation fans, in a staggered arrangement along both sides of the oven. Air curtains separate the air flow between each TDO zone and avoid leakage at the inlet and outlet of each zone.
The oven design minimizes heat losses to ensure low specific energy consumption and compensate for thermal expansion. An optional heat recovery system to return exhaust heat to the TDO airing system provides considerable energy savings. The preferred fuel for oven heating is electricity when cleanliness is critical. However, the oven can also be heated by means of gas or oil.
In the pull roll stand, the stretched film exiting the TDO is cooled, measured, trimmed, and treated. The film is then wound into mill rolls at the winder.