Causes of edge beads in cast films

Cast polymer films are extruded through a uniform thin slit onto a quench roll. Between the die and the quench roll, thick edges called edge beads form, which have to be trimmed from the film, and are often scrapped. The relation between edge beads and neck‐in is shown. Three causes of edge beads are discussed: surface tension, die swell, and an edge stress effect. Surface tension and die swell effects can be important in low viscosity and elastic materials respectively. The predominant cause of edge beads is an edge stress effect which occurs because the film is stretched between the die and the roll. The edge elongates in uniaxial stress while the center material elongates in plane strain.