Blow Up Ratio (BUR)
The blow film process is a method to convert a plastic polymer into a plastic bag by an extrusion process, where the extrudate is formed through a circular or annular die. The extrudate formed by the die is immediately blown to form an inflated film while giving a transversal orientation to the plastic. The inflated film is cooled by air through the air ring to become a vertical blown tube. For certain types of plastic polymers the cooling is done by water. At the opposite end of the die, a guide roll clamps and pulls the bubble to form a flattened tubular film, simultaneously giving the machine direction/longitudinal orientation to the plastic. The tension that is formed during the blown tube making vanishes as soon as the sheet is rolled by the wind-up roll.
The blown tube, generally, has a bigger diameter than that of the die. The ratio of the tube to the die diameter is called Blow Up Ratio (BUR). The width of the tubular sheet after the guide roll is around 1.57 times larger than the tube diameter. This relation can be used to estimate how much BUR is needed to geta certain sheet width.
Generally the BUR for a PE film is around 2 - 4, depending on the expected width of the sheet.
Various types of polymers are used to make plastic bags, like HDPE, LLDPE, and mixtures of LDPE and LLDPE. To process PP flims a significantly higher cooling is needed than for PE, due to its slower crystalization rate. Even the die position is turned vertically downwards to enable water cooling.
Hereunder are the heating conditions used generally in the barrel for the 2 PE polymers, often used in the production of bags:
| Zona 1 ( C ) | Zona 2 | Zona 3 | Zona 4 | Adapter | Die | |
|---|---|---|---|---|---|---|
| HDPE | 190 | 195 | 200 | 200 | 215 | 220 |
| LLDPE | 195 | 195 | 195 | 195 | 215 | 215 |
An HDPE film process has a "high stalk" tube shape, which is shaped like a wine glass.
The height of the blown tube rod is around 7 - 9 times the die diameter. Because of this height, the frost line position is high enough. The frost line is the boundary where the melted material starts to solidify, which can be observed by visual change from the film transparency to a more opaque condition. With a higher frost line, a good equilibrium of mechanical properties, both the transversal and longitudinal orientation can be achieved.
If the frost line position is too low, higher energy is required to orientate the film. However if the frost line position is too high, in case of too high die temperature and/or screw rpm, the blown tube becomes unstable.
HDPE has a linear structure and is thus easier to orientate in the machine direction, compared to LLDPE and LDPE which have branch structures.The height of the blown tubewith HDPE helps to provide sufficient cooling time so that it is cool enough for having the required air pressure to achieve the maximum blown tube diameter. Transversal orientation begins when the bubble starts to inflate.
Cooler bubble will experience greater transversal orientation. This condition also reduces the relaxation effect. Therefore transversal orientation does not reduce due to the relaxation effect of hotter material. After passing the frost line the film experiences longitudinal orientation because of the pull of the nip roll. However transversal orientation is well maintained because the temperature of the film is cool enough. Imagine that the blown tube rod does not exist. If the transversal orientation occurs when the temperature is too high, the molecule will relax and the transversal orientation will decrease. Thus if HDPE film is processed without a blown tube rod, the film will have poor transversal mechanical properties.
For LLDPE film making, the BUR is around 2 - 3.5, depending on the required tubular sheet width. Too high a BUR will cause the bubble to become unstable, and if the BUR is too low, the ratio of longitudinal/transversal mechanical properties become too big, resulting in unreliable film strength.