A blown film line is quite different from a flat film line. In a blown film line a tubular film is extruded vertically upwards as shown here. The tube expands due to internal air pressure. The ratio of the bubble diameter and the die diameter is called the blow-up ratio. Typical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0:1 to 2.5:1. When the bubble has cooled sufficiently, the bubble is flattened in a collapsing frame and pulled through a set of nip rolls at the top of the collapsing frame. From there, the layflat is guided over several idler rollers to the winder where the film is rolled up over a core. Some lines perforate and heat seal the layflat prior to wind-up.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes, the plastics is extruded downwards to produce films with special properties.
EXTRUSION COMPOUNDING LINES
Compounding lines come in many shapes and sizes. Compounding can be done on single screw extruders, twin screw extruders, reciprocating single screw compounders, batch internal mixers, and continuous internal mixers. The configuration of the line will be determined by the ingredients that have to be combined in the compounding extruder. The downstream equipment typically consists of a pelletising system. Some pelletisers cut extruded strands that are cooled in a water bath; these are called strand pelletisers and form cylindrical pellets. Dicers cut an extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletisers cut the material right at the die exit; these are called die face pelletisers. The pellet shape from a die face pelletiser is often more spherical than cylindrical.
Different pelletisers:
• Strand pelletisers
• Dicers
• Water ring pelletisers
• Under water pelletisers
Compounding extruders can also be combined with direct forming systems downstream. In some lines, a gear pump is placed at the discharge end of the extruder to generate the diehead pressure and to control the throughput. An example of a combination compounding/sheet extrusion line is shown here.
The plastics is introduced to the first feed port of the compounding extruder, the filler is introduced to the second feed port, and the volatiles and air entrapment are removed from the vent port. A gear pump is placed between the compounding extruder and the sheet die. The sheet is fed to a roll stack, from there it is handled as in a normal sheet line as discussed earlier.
PROFILE EXTRUSION LINES
Many extrusion lines are used for the production of profiles. Profile lines also come in many shapes and forms. A typical extrusion line consists of an extruder, a calibrating unit, a cooling unit, a measurement device, a haul-off, and a coiler, cutter, or saw.
On some profile lines, a film or foil is laminated to the extruded profile. The number of profiles that are extruded is enormous; some examples of extruded profiles are shown here.
