Blown Film Extrusion
Blown film extrusion is the most common process used to make plastic films for the packaging industry. This process relies on extruding a tube of molten plastic and continuously inflating it to form a thin tubular film.
The Extrusion Process
The plastic melt is extruded through a die, usually vertically, to form a thin-walled tube. Air is introduced through a hole in the middle of the head to blow up the tube like a balloon. To cool the film, an air ring installed above the die strikes the hot film. The tube is then moved upward, cooled by air, until it passes through nip rolls where it is flattened into a "flat" film tube. At higher-throughput exit lines, the air inside the bubble is also exchanged — a technique known as IBC (Internal Bubble Cooling). The flat film is either kept as-is or cut along its edges to produce two flat sheets wound onto spools.
Technical Parameters
The expansion coefficient between the die and the blown film tube is typically 1.5 to 4× the die diameter. Thickness variance is controlled in both radial and longitudinal directions by adjusting air volume inside the bubble and exit velocity — giving blown films a superior balance of properties vs. traditional cast film. Ampac Packaging operates both 3-layer and 5-layer blown film lines.