The air knife is used to ensure intimate contact between the molten polymer web and the casting roll. As the polymer exits the die and contacts thee roll, an air stream from the air knife strikes the polymer. An air blower supplies air. The air velocity across the face of the knife should be equal. A good air knife must not only have a good distribution at low pressure, but at a variety of different pressures.
Uneven air velocity leads to uneven web pinning and gauge variation or gauge bands. Uneven web pinning often causes defects in films such as chevrons fisheyes and blotches. The air is typically at room temperature although it can be heated or cooled depending upon the application. The room temperature or cooled air provides additional cooling to the film and hence creates a more amorphous film leading to higher film clarity.
The air knife body is constructed from cold drawn seamless steel tubing. The lips should be highly machined and constructed of a corrosion resistant material such as stainless steel and be removable as well as adjustable. The gap is typically set in 0.5~2.0 mm range. Air should enter at both ends of the air knife in order to minimize the pressure drop across the length of the knife. Air can exit either perpendicular or tangential to the roll.
The tangential manner is generally preferred as it allows the air knife to be tucked in closer to the die/cat roll geometry to shorten the melt curtain distance. This design also allows the air to exit along the cast roll thereby limiting the air disturbances to the fragile melt curtain.
Uneven air velocity leads to uneven web pinning and gauge variation or gauge bands. Uneven web pinning often causes defects in films such as chevrons fisheyes and blotches. The air is typically at room temperature although it can be heated or cooled depending upon the application. The room temperature or cooled air provides additional cooling to the film and hence creates a more amorphous film leading to higher film clarity.
The air knife body is constructed from cold drawn seamless steel tubing. The lips should be highly machined and constructed of a corrosion resistant material such as stainless steel and be removable as well as adjustable. The gap is typically set in 0.5~2.0 mm range. Air should enter at both ends of the air knife in order to minimize the pressure drop across the length of the knife. Air can exit either perpendicular or tangential to the roll.
The tangential manner is generally preferred as it allows the air knife to be tucked in closer to the die/cat roll geometry to shorten the melt curtain distance. This design also allows the air to exit along the cast roll thereby limiting the air disturbances to the fragile melt curtain.
