Blown film extrusion: In this process the extruded material follows through a tubular dies, which is likes a combination an offset die and a pipe die in that the material turns as it flows into the die, in that the material turns as it flows into the die, thus providing access to the back of the die and all oaring the extrudate to travel upward. Melt emerging from extruder is inflated by air pressure (3 to 4 kgs/cm2). Bubble is properly stabilized and cooled, Wound on the winder, In Blown Film Extrusion a tube of plastic material is extruded out of the die, while hot it is blown into a bubble, and then cooled. The bubble is inflated by air pressure contained in between the die and the seating provided by the nip rolls. The bubble is flattened by a pair of collapsible frames before it passed through the nip rollers. The film blowing operation can be accomplished theoretically in any of the following conditions: =>Horizontal, Vertically Upward, and Vertically Downward Choice of any one or the other of the three methods is dictated by the plastic material and process limitations. The horizontal direction is very rarely used. The vertical upward blowing is preferred, e.g. for PE and PVC. The vertical downward blowing is used for the manufacture of high clarity PP film. This process requires water quenching of the bubble for fast cooling which is rendered convenient by this position.
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The Process
1. Plastic melt is extruded through an angular slit die placed vertically to form a thin walled tube.
2. Air is introduced via a hole in the centre of the die to blow up the tube like a balloon. A high-speed air ring blows onto the hot film to cool it.
3. 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.
4. 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).
5. 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.
6. The expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter.
7. 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.
8. 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 including PP, PA, EVOH.
Applications:
1. 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.
Industry packaging (e.g. shrink film, stretch film, bag film or container liners),
2. 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)
3. 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),
4. Films for the packaging of medical products,
5. Agricultural film (e.g. greenhouse film, crop forcing film, silage film, silage stretch film).
Advantages:
1. Low initial Cost,
2. Adjustable die opening,
3. Will handle low flow materials.
4. Produce tubing (both flat and gusseted) in a single operation.
5. 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.
6. Eliminate end effects such as edge bead trim and non uniform temperature that can result from flat die film extrusion.
7. capability of biaxial orientation (allowing uniformity of mechanical properties)
8. Very high productivity.
9. Permits the combination of a number of different materials and properties.
Disadvantages: Mandrel deflects with extrusion rate, necessitating die adjustment, Die opening changes with pressure, Non-uniform melt flow, Cannot be rotated, One weld line in film.
Blown film: Tougher than Cast Film, More Stiffer, Cheaper, A High output Tubular film requires high tower & Bigger building to accommodate, Easily changeable film width by changing air pressure, Less gloss & clarity.
Blow film dies:
Side feed die:
Advantages:
1. Low initial Cost
2. Adjustable die opening
3. Will handle low flow materials
Disadvantages:
1. Mandrel deflects with extrusion rate, necessitating die adjustment
2. Die opening changes with pressure
3. Non-uniform melt flow
4. Cannot be rotated
5. One weld line in film.
Centre feed die:
Advantages:
1. Positive die opening
2. Can be rotated
3. Will handle low flow resins
Disadvantages:
1. High initial cost
2. Very hard to clean
3. Two or more weld lines in film
Spiral flow die:
Advantages:
1. No weld line in film
2. Positive die opening
3. Easy to clean
4. Can be rotated
5. Improved Film Optics
Disadvantages:
1. High head pressure
2. Will not handle low flow resins without modification.
Blown film extrusion:
Upward blown film - LD, HD, PVC, Nylon etc.
Downward blown film - PP (Mainly to get clarity)
Extrusion: In principle, the plastic raw material is plasticated by means of a screw plastication unit and the molten material is continuously pumped out through a standard orifice (die) in order to take the shape and then the shape is set by cooling/sizing system.
Example: Film, Pipe, Tube, Profile, Monofilament, Box Strapping etc.
Batch – Type:
1. Ram Extruders
2. Reciprocating screw extruders
Continuous –Type:
1. Screw less Extruders
2. Disk Extruders
3. Drum Extruders
4. Other Extruders
5. Screw Extruders
6. Single-Screw Extruders (SSE)
7. Twin-Screw Extruders (TSE)
8. Multi-Screw Extruders
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