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In a cast film extrusion process, a thin film is extruded through a slit
onto a chilled, highly polished turning roll, where it is quenched from one
side. The speed of the roller controls the draw ratio and final film
thickness. The film is then sent to a second roller for cooling on the other
side. Finally it passes through a system of rollers and is wound onto a
roll. Thicker polymer sheets can be manufactured similarly. A sheet is
distinguished from a film by its thickness; by definition a sheet has a
thickness exceeding 250 mm. Otherwise, it is called a film. The cast film
process is used for very tight tolerances of thin film, or for lowviscosity
resins. Most flat dies are of T-slot or coat hanger designs, which contain a
manifold to spread the flowing polymer across the width of the die, followed
downstream by alternating narrow and open slits to create the desired flow
distribution and pressure drop. Most cast film lines manufactured today are
coextrusion lines, combining layers from as many as 7 extruders into the
product through multimanifold dies, or else single manifold dies with the
aid of feedblocks. |
Molding
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