1. Polymer extrusion as seen from the flat-film process

1.1 Process description

Flat film extrusion involves extruding a polymer melt through a flat die, stretching it in air over a short distance and cooling it rapidly on a thermostatically-controlled roller. The resulting film is then stabilized in contact with successive rollers and wound up, after first trimming the edges. Depending on film thickness and application, it's important to distinguish between thin films (10 to 50 µm), thick films (100 to 300 µm) and sheets for thermoforming (200 to 2,500 µm). The first two categories of film are actually obtained by stretching the material in the molten state, as described above. Sheets for thermoforming are obtained by a process akin to sheet calendering. In the latter case, the dimensions are achieved by the calender gap, and stretching...