CA1093934A

CA1093934A - Flow distribution valve system for control of extrusion from multiple die orifices

Info

Publication number: CA1093934A
Application number: CA315,314A
Authority: CA
Inventors: Fox J. Herrington
Original assignee: Mobil Oil Corp
Current assignee: Pactiv LLC
Priority date: 1977-11-07
Filing date: 1978-10-31
Publication date: 1981-01-20
Also published as: DE2847470A1; IT1160300B; FR2407813A2; IT7829473A0; FR2407813B2; AU519111B2; NL7810687A; GB2008278B; JPS611286B2; ES474827A2; DE2847470C2; CH633218A5; GB2008278A; JPS5477666A; AU4135678A

Abstract

- 1 -FLOW CONTROL VALVE ABSTRACT A valve for regulating the flow of molten polymer to a number of extrusion dies connected to a common manifold. The manifold has a primary flow channel which has outlets at each end to secondary flow channels which lead to the extrusion dies. At least one intermediate secondary channel is also disposed between the ends of the manifold, leading to another extrusion die or dies. A flow restrictor is disposed in each of these secondary extrusion channels to control the flow of polymer. F-9592

Description

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FLOW CONTROL VALVE
.

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 . ' ,'' '.;''' .',`'` ' ', ,~' ' ~, 3~:134 " ~ ~
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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~ ' '~ ' ., - 2a - ~0~3~34 (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, ,,.~ ., ~3934 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 .

~1 - ~ , ' , ~ , , , -~ ~0~3~34 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 ~.

, .

~0'~3~334 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 .

~t~ 3~

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 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 ~3934 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 ~05~3934 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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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;
(b) flow restriction means in said primary flow channel, said means comprising a rod of reduced cross section, relative to the cross section of said channel, said rod being axially displaceable in said primary flow channel and further characterized 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-branching 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.

2. Apparatus according to claim 1 in which the flow restrictor rod in the primary flow channel has a central portion of reduced cross-sectional area.