Blown Film's Manufacture
One of the most common methods of film manufacture is Blown Film
(also referred to as the Tubular Film) Extrusion. The process involves extrusion of a plastic through a circular die, followed by "bubble-like" expansion. The principal advantages of manufacturing film by this process include the ability to:
· Produce tubing (both flat and gussetted) in a single operation
· Regulation of film width and thichness by control of the volume
of air in the bubble, the output of the extruder and the speed of
· 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)
Blown Film Extrusion can be used for the manufacture of co-extruded, multi-layer films for high barrier applications such as food packaging.
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 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.
3 layer die head with air ring and internal bubble cooling
Film bubble going into a collapsing frame
Control panel graphic from an automated blown film line
Taking an edge trim from film web
The layflat tube is separated into 2 single sheets
Blown Film Line In Action
Polyethylenes (HDPE, LDPE and LLDPE) and PVC 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”.
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, packagingbags, 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).
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