Method and apparatus for forming double-layer hollow film

An apparatus for forming a flexible double-layer hollow film, said apparatus comprising:

• an extruding die having an outlet including a pair of inner and outer annular slits arranged concentrically closely adjacent each other and a plurality of annularly spaced leg slits extending between said pair of annular slits, said die being arranged to extrude downwardly through said outlet a molten thermoplastic film including inner and outer concentric annular film layers connected by a plurality of annularly spaced legs with a plurality of annularly spaced chambers being defined between said annular film layers and said legs;
• said extruding die including means for feeding pressurized air downwardly into said chambers;
• an inner cooling vessel positioned beneath said extruding die;
• an outer cooling vessel, separate from said inner cooling vessel, positioned beneath said extruding die and surrounding said inner cooling vessel;
• said inner and outer cooling vessels respectively having outer and inner annular side walls defining therebetween an annular gap through which said molten thermoplastic film passes with said inner and outer annular film layers respectively contacting said outer and inner annular side walls;
• means for supplying separate flows of cooling liquid into said inner and outer cooling vessels and for causing said cooling liquid to fill said cooling vessels and to overflow said outer and inner annular side walls thereof and to directly contact said inner and outer annular film layers, respectively;
• and
• means for adjusting the levels of said separate flows of cooling liquid overflowing above said outer and inner annular side walls of said inner and outer cooling vessels such that said levels are equal to each and such that the liquid pressure exerted by said cooling liquid on said inner and outer film layers is balanced with the air pressure of said pressurized air within said chambers.