Chemistry Reference
In-Depth Information
which the bubble is stretched in the longitudinal (machine) direction
is defi ned by the take-up ratio (TUR), i.e. the ratio of the velocity of
the take-up roller (
V
f
) to the velocity of the extrudate at the exit of the
die (
V
0
). The blow -up ratio (BUR) is the ratio of the fi nal bubble diam-
eter to the die diameter, and ranges from 1.5 to 4. This transverse
expansion of the bubble is primarily controlled by the infl ation air pres-
sure or the volume of the bubble. The quench air around the bubble
cools the molten bubble and locks further expansion of the bubble after
a certain distance above the die, which is defi ned as the frost-line height
(FLH).
The dynamics of fi lm formation are more complex than those for
fi ber spinning because fi lm blowing involves biaxial stretching of the
fi lm. Bubble kinematics and temperature are often measured for cor-
relating them to mechanical properties of the fi nal fi lm. The velocity
and bubble diameter profi les are typically obtained using the video
tracer and image analysis techniques [58], and are used to solve the
momentum equations to obtain strain rates in the bubble. The stress
experienced by the bubble during fi lm blowing is proportional to the
strain rate.
Figure 3.2 presents the variation of strain rates with changes in BUR
from 0.4 to 2.0, while the TUR is kept constant at 3.8. The strain rates
along the TD (e
2
) progressively switch from negative to positive values
(biaxial distribution), while the strain rates in the machine direction
(e
1
) do not show signifi cant variation. As compared with a small BUR
4
BUR = 2.0, e
1
BUR = 2.0, e
2
BUR = 1.5, e
1
BUR = 1.5, e
2
BUR = 0.4, e
1
BUR = 0.4, e
2
3
2
1
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
-1
-2
Axial distance (m)
Figure 3.2
Machine direction (e
1
) and transverse direction strain rates (e
2
) in the
linear low-density polyethylene (LLDPE) bubble during blown fi lm extrusion for blow-
up ratios (BURs) of 2.0, 1.5, and 0.4, at a constant take-up ratio (TUR) of 3.8 (curves
represent trends, not model predictions).
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