Casting of polymeric film

Claim 1

A process for quenching a molten polymeric film, which comprises forming a molten polymeric film and quenching said film on a cooled casting surface which is electrically earthed, the molten film being passed across an electrode located in spaced relation from the film surface and in the proximity of the region of first contact of the molten film and the casting surface, applying an alternating voltage to the electrode, said voltage being an asymmetrical wave form, having positive and negative portions with the positive peak having a greater magnitude than the negative peak, and exceeding 4.0 kilovolts peak to peak, the peak to peak voltage being at least equal to the A.C. corona discharge inception voltage and having a frequency less than that at which positive and negative electrostatic charges alternately emitted by the electrode are unable to cross the space between the electrode and the molten film before the polarity of the voltage on the electrode changes, so as to produce an rms pinning current to the electrode, including leakage current, in the range 0.2 to 20 milliamperes whereby the electrostatic charges emitted by the electrode are deposited by the applied electric field upon the molten film to cause it to adhere to the casting surface.