Blown film extrusion

Claim 1

In a method of making polyolefin film comprising the steps of extruding molten polyolefin material through an annular die to form a tubular film, withdrawing said tubular film from said die in a machine direction, and expanding said tubular film in a direction transverse to said machine direction by maintaining a positive gas pressure therein, said tubular film defining first and second regions generally coaxial with said die, said first region being adjacent said die and extending therefrom in said machine direction and having a maximum diameter substantially equal to the diameter of said die, said second region extending in said machine direction from said first region at a point of intial expansion of said film and having a diameter substantially greater than those of said die and said first region with a frost line located downstream in said machine direction from said point of initial expansion, the improvement wherein said polyolefin material is selected from homopolymers of ethylene made under high pressure conditions, copolymers of ethylene and one or more members of the group consisting of vinyl acetate and butyl acrylate made under high pressure conditions, and high density copolymers of ethylene and 1-olefins having three to eight carbon atoms made under low pressure conditions, said polyolefin material having an elongational viscosity of at least about 350,000 poise at 190° C., and which is substantially independent of applied stress at the temperature of extrusion and wherein a first air ring coaxial with said die and said tubular film is positioned about said tubular film adjacent said die to provide air flow in a direction parallel to said machine direction to stabilize said film without allowing substantial expansion thereof, and a second air ring is positioned about said tubular film at said point of initial to provide suction force in said transverse direction to assist in said expansion and cooling and maintain the stability of said film.