Viscoelastic effects in film casting

Film casting is an important process in the polymer industry. A thin film of thermoplastic is extruded through a slot die; the sheet is stretched and either falls on a chilled metal roll or passes into a water bath. The flow rate imposes the velocity of the melt at the exit of the slot die while the rate of rotation of the chill roll imposes the final velocity of the sheet. The thickness of the sheet is much smaller than the distance between the die and the chill roll; that distance is of the same order as the width of the sheet. Along the stretching direction, a neck-in phenomenon occurs due to the fact that contact forces vanish along the edges. One of the objectives of the process is to generate sheets of uniform thickness across their width. However, one observes the formation of thick edges called edge beads which need to be trimmed from the film. Such edge beads surround a central portion of the sheet where the thickness is essentially uniform. A description of the film casting process may be found in Pearson [1] while the causes of edge bead were discussed by Dobroth and Erwin [2].