1. The basics of the calendering process

Process description

The calendering process is based on a high-temperature flow of viscoelastic thermoplastic material between at least two driven rolls. In fact, experience has shown that only four-roll calendering enables film or sheet quality to be properly controlled. In a four-roll calender (figure 1 ), the plastic melt coming from the gelling mixer passes from one roll to the next through three increasingly narrow gaps, resulting in reflux before each gap, at the same time as an expansion of the sheet. These refluxes form what are known as beads. Only the first bead, called the feed bead, is limited in width by material guides. Beads 2 and 3 have a free width and a cylindrical shape, except at the ends where the shape becomes ovalized. The...