A method of making single-sided, anti-slip film layers, each having a rough, slip-resistant, open-celled outer surface on one film side thereof, said method comprising the following steps:
- using a film blowing machine to vertically coextrude a multilayer film from a circular die to form a tube having first and second sides, wherein the tube comprises at least three coextruded layers of film including an inner layer, an outer layer, and a middle layer, said outer layer comprising a plurality of gas bubbles resulting from addition of a blowing agent to its film making material provided to said circular die, and said blowing agent causing said gas bubbles to form in said outer layer as said outer layer is coextruded from said circular die;
- drawing the resulting coextruded multilayer film tube vertically and upwardly away from the circular die at a predetermined speed;
- at a predetermined distance from the circular die, collapsing the coextruded multilayer film tube during said drawing by passing the film tube between at least two rollers positioned in close proximity to one another so that a first roller of said at least two rollers contacts said first side of said coextruded multilayer film tube and so that a second roller of said at least two rollers contacts said second side of said coextruded multilayer film tube, wherein first and second sides of the inner layer of said film tube abut one another without fusing or sticking together as a result of said collapsing, wherein surfaces of said rollers contacting said first and second sides of said film are parallel to one another, wherein said first roller is a metal or rubber nip roller, and wherein said second roller is a rubber roller, said collapsed, coextruded multilayer film having first and second edges;
- passing the collapsed, coextruded, multilayer film through a pair of heated in-line rollers, wherein at least one roller of said pair of heated in-line rollers is heated to a temperature above the glass transition temperature and below the softening point of the inner layer of said film so as not to cause fusing of said abutting first and second inner layer sides, said pair of heated in-line rollers rotating at approximately the same speed as said predetermined speed at which said coextruded multilayer film tube is drawn from said circular die;
- subsequent to passing the collapsed, coextruded, multilayer film through said pair of heated in-line rollers, passing the collapsed, coextruded, multilayer film through a pair of cooled in-line rollers rotating faster than said pair of heated in-line rollers to thereby cause the collapsed, coextruded, multilayer film to stretch in at least one direction, wherein at least one roller of said pair of cooled in-line rollers is operated at a temperature cooler than the softening point of the inner layer of said film, wherein said plurality of gas bubbles are collapsed by said stretching and cooling, and wherein the resulting, cooled multilayer film is free of stresses and of variation in thickness;
- and
- cutting the cooled multilayer film proximate said first and second edges to form two generally identical, single-sided film layers each with an inner surface and a non-slip outer surface.