Patent ID: 8252218

Claim:
A porous film manufacturing method comprising: thermally melting polyethylene powder by using oil or liquid paraffin as a solvent; performing phase separation to form a raw film; and thereafter forming micropores by successive biaxial stretching, in which the raw film is stretched longitudinally by a longitudinal stretching machine comprising a plurality of rolls such that the raw film contacts each of the plurality of rolls, thereafter the raw film is transferred from the longitudinal stretching machine to a transverse stretching machine, and then the raw film is stretched transversely by the transverse stretching machine, wherein a tensile force applied to the raw film positioned between an outlet section of the longitudinal stretching machine and an inlet section of the transverse stretching machine as the raw film is transferred from the longitudinal stretching machine to the transverse stretching machine is set not less than a stretching force necessary for the longitudinal stretching by the longitudinal stretching machine, so that the raw film is prevented from slipping on a contact surface between the raw film and each of the plurality of rolls of the longitudinal stretching machine; the plurality of rolls of the longitudinal stretching machine comprise a heating roll arranged in an inlet side of the longitudinal stretching machine, a cooling roll arranged in an outlet side of the longitudinal stretching machine, and a stretching roll arranged between the heating roll and the cooling roll; the circumferential speed of the stretching roll is set to a speed that is 4 to 6 times as fast as the circumferential speed of the heating roll, and the circumferential speed of the cooling roll is set to a speed that is 4 to 6 times as fast as the circumferential speed of the stretching roll, so that the raw film is prevented from slipping on the contact surface between the raw film and each of the heating roll, the stretching roll, and the cooling roll of the longitudinal stretching machine; the longitudinal stretching machine comprises a plurality of nip rolls, each nip roll being mounted in a swing arm, and each nip roll including a soft rubber on its roll surface, the soft rubber having a rubber hardness of not more than JIS-A60 and a thickness of not less than 3mm; and the raw film is nipped by each ni roll with a linear pressure of not more than 2 Kg/cm in a manner such that each nip roll mounted in a swing arm is pressed against the stretching roll when the swing arm is swung by a cylinder device.