Film Extrusion Process

Film extrusion is the best way to manufacture any kind of bag, such as bread bags, grocery bags, or any one of thousands of different shapes and sizes. Polyethylene’s (HDPE, LDPE and LLDPE) are the most common resins in use.

Methods

Two basic methods are used for making polyolefin film: cast film extrusion and blown film extrusion. In both methods, the resin is first melted by subjecting it to heat and pressure inside the barrel of an extruder and finally forcing the melt through a narrow slit in a die (also referred to as the die gap). The slit may be either a straight line or ring-shaped. The resulting thin film has either the form of a sheet (cast film) or a tube, also called a “bubble” (blown film). As the film comes out of the die, it is cooled and then rolled up on a core.

The extruder consists of a resin feeding hopper, a heated barrel, a constant rotating screw, a screen changer, a die adapter and a base. Polyolefin resins are dropped into the extruder feed throat through a round or square funnel, called the hopper. An automatic loader on top of the hopper periodically feeds resin into it.

A motor-driven screw rotates within the hardened line of the barrel. As the screw rotates, the screw flights force the resin in the screw channel forward. As the screw channels become shallower, the resin is heated, melted, thoroughly mixed and compressed. Good mixing of the melted resin is essential for obtaining high clarity film with no defects or blemishes.

An adapter guides the resin melt from the barrel to the die as thoroughly and consistently as possible. The film extrusion die is attached to the adapter. A good die designs ensures smooth and complete melt flow, thus preventing resin degradation from overheating. The die forces the melt into a form approaching its final shape. Also it maintains the melt at a constant temperature and meters the melt at a constant pressure and rate to the die land for uniform film gauge, with allowance for gauge reduction.

Air is introduced through a hole in the centre of the die to blow up the tube like a balloon. Mounted on top of the die, a high-speed air ring blows onto the hot film to cool it. The tube of film then continues upwards, continually cooling, until it passes through nip rolls where the tube is flattened to create what is known as a ' lay-flat' tube of film. This lay-flat or collapsed tube is then taken back down the extrusion ' tower' via more rollers. On higher output lines, the air inside the bubble is also exchanged. This is known as IBS (Internal Bubble Cooling).  
 
The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage.

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film which is drawn down along the extrusion direction only.