Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FLOW CONTROL VALVE
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This invention relates to a flow control valve
for an extrusion system for thermoplastic resins ~uch as
polyethylene.
In our eo-pending Canadian Patent Application
No. 291,899 (corresponding to U.S. Application Serial
No. 753,747, Mobil Case F-9299, we have described a flow
control valve for a twin outlet extrusion system. The
8y8tem comprises a flow channel with a longitudinally
(i.e.axially) movable flow restrlctor. The flow channel
has a central inlet and outlets at oppo~ite ends; the
flow restrictor i~ a rod which has at lea t one portion
of increased diameter along its length. In one
embodiment, the rod has a central portion with the
increa3ed diameter; in another embodiment, the two end
portions of the rod are larger. Axial movement o~ the
rod varie~ the pressure drop between the inlet and the
outlets to the extruders in such a manner that as tbe
pre~sure drop between the inlet and one outlet increases
there is a corresponding decrease in the pressure ~rop
to the other outlet. Because the flow rate is dependant
on the pre~sure drop, the flow rate to the outlets will
be ~aried acoording to the po~ition of the rod.
However, becau~e an ~ncrease in the pressure drop
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between the inlet and one outlet is balanced by a
countervailing decrease in the pressure drop to the
other outlet, the total pressure drop remains constant.
The extruder therefore operates against a constant
pressure and accordingly maintains a constant total flow
rate.
We have now devised a flow control system for
more than two extrusion dies. According to the present
invention, the distribution valve comprises a flow
channel with at least one inlet, and outlet at each end
of the flow channel and on or more intermediate outlets
disposed between the end outlets. A flow restrictor of
the type described in Canadian Application 291,899 (Mobil
Case F-9299) is disposed in the flow channel for
longitudinal movement within the channel. This
restrictor comprises a rod of smaller cross-section than
that of the flow channel but with at least one portion
of enlarged cross-section to control the flow to the end
outlets. A flow restrictor is also disposed in the flow
channels which are connected to the intermediate
outlets. Again, this flow restrictor is smaller than the
channel it is in. Movement of this restrictor creates a
restriction of variable axial length in the flow channel
so as to control the pressure drop and hence, the mass
flow rate, to the associated extrusion die.
The present invention, then, provides an
apparatus for controlling the flow of molten polymer
from one or more feed sources to three or more extrusion
die orifices, said apparatus comprising:
~a) a manifold comprising one or more inlet openings;
a primary flow channel within said manifold in communication
with said inlet openings; secondary end-branching flow channels
branching off from each of the opposing end portions of said
primary flow channel; one or more intermediate secondary flow
channels in communication with said primary flow channel and
disposed in between said end-branching secondary flow channels;
and an individual extrusion die orifice for each of said
secondary flow channels and in communication therewith;
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(b) flow restriction means in said primary flow
channel, said means comprising a rod of re~uced cross section,
relative to the cross section of said channel, said rod being
axially displaceable in said primary flow cl~annel and further
characteri~.ed by having at least one portion of its cross-
sectional area enlarged whereby axial displacement of said
rod controls the flow of said molten polymer to said end-
branchi.ng secondary flow channels; and
(c) restriction means, disposed in each of said one
or more intermediate secondary flow channels, comprising an
axially displaceable elongated rod of reduced cross section,
relative to the cross section of its respective secondary flow
channel, whereby said rod creates a restriction of variable
length in said secondary flow channel through which restriction
said molten polymer flows prior to being expressed from the die
orifice in communication with said channel.
Further f~atures o~ tlle invention are described
below with reference to the accompanying drawings in which:
Figure 1 is a sectional elevation of a
flow control system for three
extrusion dies,
Figure 2 is a section along line 2-2'
of Figure 1,
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Figure 3 is a section along line 3-3'
of Figure 1, and
Figure 4 is a section along line 4-4'
of Figure l.
The flow control system shown in the
accompanying drawings employs a single inlet feeding to
three extrusion dies. However, more than one inlet may
be provided and more than three outlets leadin~ to their
associated extrusion dies.
; The flow control system comprise a manifold M
for supplying molten thermoplastic polymer (e.g., a
polyolefin such as polyethylene) to three extrusion dies
(A, B, C) from a common feed source (not shown) for the
extrusion of tubular fiIms F-1, F-2 and F-3. Dies A and
C are substantially identical and are mounted at
opposite ends of manifold M. Die B, which is mounted on
manifold M intermediate dies A and C, is substantially
the same as dies A and C except that it accommodates
valve member 60, as will be described below.
FIGURE 2 shows that the molten polymer P
enters manifold M through entry port 40 and flows into
primary flow channel 41. Branching off from primary
~ flow channel 41 are three secondary flow channels 42, 43
and 44 which serve as conduits for the polymer P to dies
A,B and C, respectively. The molten polymer i9
distributed through the associated outlets in the
primary flow channel to secondary channels 42, 43 and
44, flows upwardly through dies A, B and C and is
extruded from the die orifices in the form of tubular
~ilms F-1, F-2 and F-3, respectively (FIGURE 1)-. The
films are thereafter drawn away from the dies in the
conventional manner and fed to the appropriate
downstream processing sequences. Valve members 50 and
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60 redistribute the flow of the molten polymer P through
manifold ~ to each of the dies A, B and C in a
controllable manner.
Valve member 50 comprises an elongated rod
mounted in primary flow channel 41 and extending through
the opposing end walls of manifold M. ~od 50 is
centrally positioned along the longitudinal axis of
primary flow channel 41 so that channel 41 has an
annular configuration along its entire length. ~od 50
is substantially symmetrical in configuration, having a
relatively small diameter at its central portion 51
which, at points spaced longitudinally from the
center, gradually tapers to increased diameter sections
52 and 53. Sections 52 and 53 thereafter remain
constant in cross-sectional area as they pass through
the end walls of manifold M and extend beyond the
manifold. Bracket 54 is attached to one end of manifold
M and holds moveable nut 55 so that it is free to rotate
but is restrained from significant lateral movement.
Nut 55 in turn engages the threaded end portion 56 of
rod 50. By rotating nut 55, rod 50 is displaced
axially, thereby repositioning the relatively narrow
section 51 within flow channel 41 in relation to dies A
and C.
The pressure drop of the molten polymer within
primary flow channel 41 will be dependent upon the
cross-sectional area and length of the restriction
through which the polymer travels before entering
extrusion dies A and C on opposite sides of valve rod
50. The pressure drop per unit length of molten polymer
travel is greater in the region surrounding the larger
diameter portion of rod 50. Accordingly, it will be
seen that if rod 50 is shifted to the left, i.e, in the
direction of extrusion die A, the length of the larger
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diameter portion to which the flowing thermoplastic
material is exposed is reduced on the left end, and
simultaneously increased on the right end this causes
a reduction in the pressure drop on the left end and in
a simultaneous increase in the pressure drop on the
right end. The flow rate through the left end then
increases until the pressure drop is the same as it was
before rod 50 was moved, and likewise, the flow rate
through the right end decreases until the original
pressure drop is attained. After that has happened, the
pressure in primary flow channel 41 is substantially the
same as it had been previously and the increase in flow
rate at the left end has been equalled by the
countervailing decrease in flow rate at the right end.
Valve member 60, which may be viewed to
advantage in FIGURE 4, comprises an elongated rod 61
inserted through the wall of intermediate extrusion die
B and/or manifold M and into the secondary flow channel
43. Rod 61 may be inserted either into the upwardly
rising section 43b of the secondary flow channel (as
shown in FIGURE 4) or, alternatively, in its horizontal
section 43a. If it is in the horizontal section, the
end portion of the rod will be directed into the flowing
molten polymer P entering flow channel 43. The end
portion 62 of rod 61 is preferably tapered to minimize
turbulence in the stream of molten polymer and, in the
case of th~e alternative embodiment in which the rod is
directd into the flowing polymer, so that it will
present a streamlined contour to the incoming polymer
stream so as to minimize the resistance of the end of
the rod to the movement of the molten fluid.
Rod 61 creates an elongated restriction in
secondary flow channel 43 through which the molten
polymer must pass before reaching the die orifice of
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extrusion die B. The rod 61 is held in fixed position
relative to the walls of flow channel 43 but is capable
of longitudinal movement in the channel so that the
elongated restriction created by the rod is
substantially constant and uniform cross section but of
variable length. Valve member 60 is adjustably attached
to the body of the unit by means of a bracket and
rotating nut arrangement (see 64 and 65) which engages
threaded end portion 63 of rod 61 in the manner
previously described with respect to bracket 54 and
rotating nut 55.
A portion of the molten polymer stream P,
after entering primary flow channel 41 of manifold ~q,
flows into secondary flow channel 43 and is subsequently
extruded from the orifice of extrusion die B as tubular
film F-2. Prior to reaching the die orifice, the molten
polymer stream passes through the elongated restriction
caused by valve member 60, thereby increasing the
pressure drop on the stream as it passes through channel
43 and reducing the flow rate. By varying the length of
the restriction (and thereby simultaneously changing the
volume of the flow channel) it is possible to regulate ~-
the pressure drop, and hence the flow rate, with a high
degree of controllability. It is therefore possible to
adjust the extrusion rate of the polymer, so as to
control the thickness and extrusion rate of film F-2,
this enables the operator to match the characteristics
of film F-2 to the simultaneously extruded films F-1 and
F-3-
The manner of balancing the extrusion rates
and thicknesses of the three films is as follows. The
extruder is turned on and allowed to equilibrate in the
usual manner. After extension of the films has begun,
the extruder and downstream nip roller speeds are
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adjusted to give the desired average total product
weight for the three webs F-1, F-2 and F-3. When this
is satisfactory, rod 50 is adjusted to achieve
substantially identical product weight for the two outer
dies A and C. The intermediate valve member 60 is then
adjusted to make the product weight from intermediate
die 3 the same as that from the two outer dies A and C.
While it is conceivable that the total extruder output
may change by a small amount when the flow rate to the
intermediate die B is adjusted, any resulting change in
back pressure on the extruder, and thus in extruder
output, will be very small. This is because any
reduction in flow through the centr die which results
from an adjustment of its valve memter will generally be
redistributed between the two outer dies, so that there
will be an increase in output through each of the outer
dies that is approximately equal to half of the
reduction in flow through the center die. The back
pressure through one of the outer dies is approximately
proportional to the cube root of the flow, so the
resulting change in back pressure on the extruder is
very small. That is, the increase in back pressure on
the extruder is about proportional to the cube root of
the increase in flow through each of the outer dies,
which in turn is equal to only half of the decrease in
~low through the center die. As an approximate example,
a 10% reduction in flow through the center die results
in a 5% increase in flow through each outer die, which
then results in about 1.6% (i.e. the cube root of 5%)
increase in back pressure on the extruder.
Consequently, readjustment of the extruder or nip speeds
is, in usual practice, generally unnecessary
` The system described above has the advantage
of precision of adjustment. Although the system has
been described with reference to a manif'old having three
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dies mounted thereon, it will be readily apparent that
such a system may be easily expanded by providing a
multiplicity of intermediate dies patterned after the
aforedescribed intermediate die B.
In any of the aforementioned embodiments, the
axial displacement of the adjustable valve members may
be initiated and controlled in the conventional manner
(i.e. by a human operator who is monitoring the average
film thickness somewpere downstream), or by means of
automatic film thickness detection devices which are
adapted to control the movement of the valves to
compensate for any differential in the gauge of the
various extruded films.
Although the present invention has been
described with reference to the extrusion of tubular
films of thermoplastic material, it has applicability to
other extrusion techniques, such as the extrusion of
flat films, filaments, splid tubes, foamed plastic
sheets and tubes and also to extrusion coating.
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