Page 284 - Plastics Engineering

Processing of Plastics 267

Initially the bubble consists of molten plastic but a jet of air around the outside
of the tube promotes cooling and at a certain distance from the die exit, a freeze
line can be identified. Eventually the cooled film passes through collapsing
guides and nip rolls before being taken off to storage drums or, for example,
gussetted and cut to length for plastic bags. Most commercial systems are
provided with twin storage facilities so that a full drum may be removed without
stopping the process.
The major advantage of film blowing is the ease with which biaxial orien-
tation can be introduced into the film. The pressure of the air in the bubble
determines the blow-up and this controls the circumferential orientation. In
addition, axial orientation may be introduced by increasing the nip roll speed
relative to the linear velocity of the bubble. This is referred to as drawdown.
It is possible to make a simple estimate of the orientation in blown film
by considering only the effects due to the inflation of the bubble. Since the
volume flow rate is the same for the plastic in the die and in the bubble, then
for unit time
TtDdhdLd = TtDbhbLb
where D, h and L refer to diameter, thickness and length respectively and the
subscript ‘d’ is for the die and ‘b’ is for the bubble.
So the orientation in the machine direction, OMD, is given by

Ddhd
Lb
hd
OMD = - = - -
-
-
Ld hbDb hbBR
where BR = blow-up ratio (Db/Dd)
Also the orientation in the transverse direction, OTD, is given by
Therefore the ratio of the orientations may be expressed as

(4.16)

Example 4.3 A plastic shrink wrapping with a thickness of 0.05 mm is to
be produced using an annular die with a die gap of 0.8 mm. Assuming that
the inflation of the bubble dominates the orientation in the film, determine the
blow-up ratio required to give uniform biaxial orientation.

Solution Since OMD = OTD
-
then the blow-up ratio, BR = /%
hb

Common blow-up ratios are in the range 1.5 to 4.5.