Blown film extrusion

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Blown film extrusion is the process by which most commodity and specialized plastic films are made for the packaging industry. The film blowing process basically consists of a extruding a tube of molten thermoplastic and continuously inflating it to several times initial diameter, to form a thin tubular product that can be used directly, or slit to form a flat film.

Contents 1 The Process 2 materials 3 advantages 4 applications

The Process

Blown Film Extrusion (Film Blowing)Plastic melt is extruded through an annular slit die, usually vertically, to form a thin walled tube. Air is introduced via a hole in the center 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 bubble to stabilize the bubble and cool it. This allows manufacturers to run at higher rates without loosing bubble stability. On higher output lines, the air inside the bubble is also exchanged. This is known as IBC (Internal Bubble Cooling). The tube of film then continues upwards, and is preflattened using a collapsing frame (also known as v-boards or A-frame) until it passes through nip rolls. The flattened bubble is now known as a lay flat tube. This lay-flat or collapsed tube is then taken back down the extrusion ' tower' via idler rollers.

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.

materials

Polyethylenes (HDPE, LDPE and LLDPE) are the most common resins in use, but a wide variety of other materials can be used as blends with these resins or as single layers in a multi-layer film structure. these include pp, pa, evoh. In some cases, these materials do not gel together, so a multi-layer film would delaminate. To overcome this, small layers of special adhesive resins are used in between. These are known as “tie layers”.

advantages

• Produce tubing (both flat and gussetted) in a single operation
• Regulation of film width and thickness by control of the volume of air in the bubble, the output of the extruder and the speed of the haul-off
• Eliminate end effects such as edge bead trim and non-uniform temperature that can result from flat die film extrusion
• capability of biaxial orientation (allowing uniformity of mechanical properties)
• Very high productivity
• Permits the combination of a number of different materials and properties

applications

Blown film can be used either in tube form (e.g. for plastic bags and sacks) or the tube can be slit to form a sheet. Typical applications include Industry packaging (e.g. shrink film, stretch film, bag film or container liners), Consumer packaging (e.g. packaging film for frozen products, shrink film for transport packaging, food wrap film, packaging bags, or form, fill and seal packaging film), Laminating film (e.g. laminating of aluminium or paper used for packaging for example milk or coffee), Barrier film (e.g. film made of raw materials such as polyamides and EVOH acting as an aroma or oxygen barrier used for packaging food, e. g. cold meats and cheese), films for the packaging of medical products, Agricultural film (e.g. greenhouse film, crop forcing film, silage film, silage stretch film).