What does BF stand for?

Blown Film

Extrusion of a continuous thin walled tube of plastic through a round die and inflating it to form a bubble. The bubble is then collapsed flat between rollers and slit into layflat film which is wound onto rolls. OR Process involves extruding a continuous thin-walled tube of plastic and inflating it immediately after it leaves the die. The pressure is such that the tube stretches, increasing its diameter and reducing its wall thickness to desired gauge.