Cast Film

Cast processes are usually used to produce uniaxially oriented film, oriented in the machine direction. Physical properties of the film depend on the degree of orientation, and a film is produced with different surface properties on each side. Oriented cast polypropylene film is clear and glossy, with high tensile strength. It is about three times stiffer and stronger than low-density polyethylene film. Cast film provides good moisture barrier properties and scuff resistance at low cost. Low-temperature brittleness is a problem with homopolymer polypropylene film; this can be overcome by the use of a copolymer resin.

A water bath is sometimes used instead of a chill or casting roll; the water bath process quenches the melt on both sides at the same time, producing a film with the same surface properties on each side. The machine direction orientation in the water bath process is somewhat different than that obtained using the casting roll, and the very rapid quenching lowers the crystallinity, producing a tougher film.

Tear initiation, by impact, puncture, or ripping, is difficult in oriented polypropylene (OPP) films; once initiated, however, the resistance to tear propagation is low. Tear strength depends on grade and process conditions and on whether the tear propagates in the machine or transverse direction. A tear strip is usually incorporated in OPP film packs to facilitate opening.

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