The majority of polymer films are manufactured
by film blowing (blown film extrusion). A single screw extruder is used
to melt the polymer and pump it into a tubular die, as shown
in cross-section at right. Air is blown into the center of the
extruded tube and causes it to expand in the radial direction.
Extension of the melt in both the radial and down-stream direction
stops at the freeze line (frost line) due to crystallization of the melt.
The nip rolls collect the film, as well as sealing the top of the bubble
to maintain the air pressure inside. This process is used extensively
A good textbook treatment of
film blowing can be found in:
J.F. Agassant, P. Avenas, J.Ph. Sergent, P.J. Carreau,
Polymer Processing Principles and Modeling,
Hanser, New York (1986). ISBN 0-19-520864-1,
Suppliers of Film Blowing Lines
Wayne Machine and Die Company