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Polyethylene films are
made by 2 processes; Blown and Cast.
Globally, the Blown process is more widely used, with the exception
of North America, where Cast film lines are preferred due to higher
production outputs. PE film business is increasingly becoming fiercely
competitive, where merely additional benefit of 3-5% material savings
makes the difference to profitability.
To reduce costs, faster packaging lines are increasingly employed,
demanding very tight thickness controls. Additionally, those film
processors who also convert the films into bags would make a savings
on material due to better thickness controls. Only those film converters
who sell films on weight basis may not bother too much about tighter
thickness controls.
Inherently, the type of PE material influences the thickness
controls :
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LDPE, due to broader molecular weight distribution and the presence
of long chain branches could provide the best control on thickness
variation ( between 4-6%). |
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LLDPE (C4) typically gives 8-10% thickness variation. |
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The higher alpha olefin LLDPE would provide thickness variation
between 6-8% on account of longer side chains compared to C4LLDPE. |
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Among all the Polyethylenes, HDPE and HMHDPE are the hardest to
kness controls, with variations in the range
of 15-20%. |
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The basic thickness of blown film also influences the thickness
variation. Thicker films are always easier to control compared to
thinner films. |
To achieve better thickness controls, significant developments have
taken place over the last two decades. European machinery suppliers
like Windmoeller & Hoelscher, (W&H) have pioneered the thickness
control system emanating from segmented dies with individual control
of heating in each segment. Control of heating is by cartidge heaters
to obtain better thickness controls. In fact W&H's patents on
segmented dies expired in 2000. The segmented die system is now
offered by Alpine Brampton, Kiefel, Kuhne, Maachi, Reifenhauser
and several others. The segmented die system is more effective for
LDPE & LLDPE films rather than highstalk HDPE film produced
with very high Blow Up Ratio (BUR)
While W&H pioneered gauge control concept with segmented dies,
Reifenhauser and Kuhne introduced segmented air ring system, particularly
for HDPE. Infact, the cost of segemented air rings is lower compared
to segmented die system. They can be easily retrofitted to the exisiting
equipments that do not have an oscillating die.
Both of these newer thickness control devices improves the base
tolerance of thickness variation by atleast 50%, compared with the
blowing with normal system including the IBC (Internal Bubble Cooler)
or oscillating die, or rotating haul off etc.
No wonder that most of the well reputed blown film equipment suppliers
more so of the European ones, now offer both the types of thickness
control systems. Infact these improved film equipments are being
used by the leading PE film suppliers to remain afloat in the current
difficult times. These guage control systems have enabled the film
processors to manufacture the PE films with the confidence level
of 95.46% (2 Sigma). Infact 70% of the blown film equipment manufacturers
assure supplying the equipment with 2 Sigma (Standard Deviation
Range). Some even try to achive 99.73% ( 3 Sigma ) accuracy in the
thickness although it is extrmely difficult to achieve it.
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