GB1599237A - Flow distribution valve - Google Patents

Description

(54) FLOW DISTRIBUTION VALVE (71) We, MOBIL OIL CORPORATION, a corporation organized under the laws of the State of New York, United States of America, of 150 East 42nd Street, New York, New York 10017, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a valve system for the control of the flow of molten polymer to extrusion dies in an apparatus for the extrusion of thermoplastic material.

It is common practice in the thermoplastic extrusion art to supply molten polymer from a single extruder to two or more extrusion outlet orifices or dies. Such an arrangement, in which a manifold is used to feed two extrusion dies, is illustrated in Figure 1 of the accompanying drawings.

The film being taken off of each of these respective dies is fed to a separate production line made up of the appropriate sequence of mechanical forming operations. It is important that the film in each such line be the same in terms of average thickness as the film in each of the other lines being supplied by the single common extruder, and this is normally adjusted by varying the speed of the nip rollers (see 15 of Figure 1), thereby controlling the rate at which the still semimolten polymer immediately adjacent to the extrusion die orifice is drawn away from the die.

In the event that the flow from the two dies is unequal (for example, because of small mechanical differences between the dies), then the rates at which the two films are drawn off by their respective nip rollers are also unequal and consequently the entire series of line components downstream from each pair of nip rollers are running at different rates. On many such dual production lines, the overall speed of the operation (i.e. the total rate of film production from the single common extruder feeding both lines) is limited by one of these downstream components. In such a case, the speed of the overall operation is restricted by the faster of the two streams (since its limiting component will be functioningatmaximumcapacity), so that the slower stream runs at less than its maximum speed and the rate of extrusion cannot be increased to compensate. It is desirable to equalize the relative speeds of the two lines, enabling the overall rate of extrusion to be increased to maximize the rate of production on both lines, and such an equalization is made possible by utiliing a flow control valve to restrict the polymer flow to the faster side of the manifold.

A typical flow control valve that has commonly been used in the prior art is shown in Figure 3 of the accompanying drawings. Molten thermoplastic material 203is caused to flow through gap 201, where it is subjected to a pressure drop. As the valve is adjusted by moving restrictor 202, gap 201 is changed and the resulting pressure drop is changed accordingly. If there are two such valves in parallel, fed from a common manifold source and discharging into identical downstream pressures, the flow rate through each of the two valves will be proportional to approximately the fifth power of the size of the gap. Since this gap is usually a small fraction of an inch, it will be seen that a very small change in the size of this gap will result in an undesirably large change in flow rate. For example, if the gap is 0-1 inch, and it is changed by a very small amount such as 0.005 inches, there will be a resulting change in flow through the gap of approximately 28 percent, provided that the supply and discharge pressure remain unchanged. Any precise flow rate control with such valves is, at best, extremely difficult. It may be seen that prior art valving means such as that described above, when used in a pair (one for each die) on a common manifold, cannot readily be used for the purpose of accurately balancing the flow since they are not sufficiently precise to permit accurate adjustment of flow distribution within the manifold. Also, since there are two separate valves to contend- with and either one is capable of fully restricting the flow of polymer to its adjacent die, numerous adjustments over a period of time may result in the inadvertent and undesirable total restriction of one side of the flow chan new in the manifold, necessitating the laborious task of readjusting and rebalancing the entire system in order to getthatsideflowingproperly once again.

The present invention is directed to a manifold valve system which balances the flow of molten thermoplastic resin (e.g.

polyethylene) between two or more dies which are being supplied with molten resin by the same extruder. The valves are located in a flow channel within the common manifold which feeds the molten polymer to the multiple dies, and the flow control is accomplished by means of adjusting the relative back pressure exerted on the polymer within that portion of the flow channel which precedes each extrusion die orifice.

According to the present invention, therefore, there is provided apparatus for controlling the flow of molten polymer from one or more feed sources to two or more extrusion die orifices, which comprises a manifold comprising an inlet port, a flow channel within the manifold communicating with the inlet port, and two or more extrusion die orifices mounted on the manifold in communication with the flow channel, and restriction means in the flow channel adjacent to and preceding each of the extrusion die orifices, each of the restriction means comprising an elongated rod of reduced diameter relative to the diameter of the flow channel in which it is positioned, which rod creates an elongated annular restriction through which the molten polymer must flow in order to pass from the inlet port to the extrusion die orifice prior to being expressed from the orifice, at least one of the rods being longitudinally displaceable in the channel to create an annular restriction of variable length and constant cross-section which serves to control the flow of molten polymer to the die orifices.

In one embodiment a first rod is inserted into the manifold between a first die and the extruder and is held in fixed position to create a first annular space of fixed length. A second rod, which is inserted into the manifold between a second die and the extruder is adjustable to provide a second annular space of varying length. Longitudinal displacement of this second rod will cause a corresponding change in the length of the annular space created by such rod, thereby causing a change in the back-pressure created by the molten polymer flowing through this second annular space relative to that created by the molten polymer flowing through the first annular space of fixed length. By increasing the length of the second annular space (and hence the relative back-pressure created thereby), the operator can decrease the amount of molten polymer flowing to the second die relative to the first die. Similarly, by decreasing the length of the second annular space the operator can decrease the relative back-pressure caused thereby and effectively increase the amount of polymer flowing to the second die.

In another embodiment both the first rod and the second rod are adjustable to provide annular spaces of varying length. In such an embodiment as this the potential sensitivity of the valving system is increased and the operator can more finely "tune" the relative extrusion rates of the dies to achieve an even higher degree of matching of the speeds of the related downstream production lines.

Other alternative embodiments include manifolds having mounted thereon more than two extrusion dies and systems in which the valve members (i.e. the "rods") are mounted vertically in the flow channel. In embodiments wherein the valve members are vertically mounted, they are positioned in the portions of the flow channel which branch off of the central channel prior to each extrusion die and the tapered end portion of each rod is directed downstream (as opposed to being directed into the flowing stream as described above) to minimize any turbulence or sudden reduction in shear rate which could cause the formation of stagnant areas in the flowing polymer stream.

One advantage to the system of the present invention is its sensitivity. The flow to the die changes much less for a given change in valve position in the present design as compared to the design of Figure 3 for two reasons: first, the length of the annular space is larger than its width, so a given number of inches of movement of the valve constitutes a smaller percentage change in the size of the restriction; and second, for a given pressure drop across the restriction the flow is proportional to the cube of the length, whereas in the valve of Figure 3 it was proportional to the fifth power of the gap.

Another advantage of the system of the present invention lies in the fact that the flow of the molten polymer cannot be inadvertently stopped through a series of valve adjustments.

In order that the invention may be more fully understood, preferred embodiments thereof and prior art arrangements will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of two conventional tubular film extrusion dies mounted on a manifold which supplies them with molten polymer from a single common source, Figure 2 is a schematic illustration of a prior art technique whereby two downstream hues are fed film from a single relatively large tubular extrudate, Figure 3 is a fragmentary view, in cross section, of a typical flow control valve that is commonly used in the prior art, Figure 4 is a schematic illustration, in cross section, of an embodiment of the flow distribution valve system of the present invention mounted within the manifold flow channel, and Figure 5 is a fragmentary cross section of a manifold showing the valve member mounted vertically in the flow channel.

Understanding of the distribution valve system of the present invention can best be facilitated by reference to Figure 4 of the drawings, which is a cross-sectional representation of a manifold feed system employing an embodiment of this invention which might typically be mounted on a conventional extrusion apparatus for the extrusion of molten polymer. The manifold 21 is adapted to accept a single stream 20 of molten polymer from the extrusion apparatus (not shown) and to divide it into two substantially equal streams which are fed to paired extrusion dies 26 and 30.

Mounted in the opposing end portions of central flow channel 23 of manifold 21 are valve members 24 and 28, which are positioned between the substantially centrally located manifold inlet port 22 and the extrusion dies 26 and 30, respectively. Valve members 24 and 28 are elongated rods of reduced diameter relative to the diameter of the flow channel in which they are positioned, each having one end portion which is directed toward the manifold inlet port 22 and which is tapered to present a streamline contour to the incoming molten polymer stream 20 so as to minimize the resistance of the end of the rod to the movement of the molten fluid and to create a minimum of turbulence within the manifold.

Each of the rods create an annular restriction through which the molten polymer must pass before being extruded through the respective extrusion dies. It is by means of manipulation of the length of one or both of these annular restrictions that the valve operates.

In the embodiment depicted in Figure 4, valve member 24 is held in fixed position in one end portion of manifold 21 and immediately preceding extrusion die 26, such that it creates an annular restriction 25 of predetermined fixed length and cross section. Valve member 28 is positioned in the opposing end portion of the manifold 21, immediately preceding extrusion die 30. It is held in fixed position relative to the walls of flow channel 23 but is capable of longitudinal movement therein, such that an annular restriction 29 is created, restriction 29 being of substantially the same cross-sectional dimension as restriction 25 but of variable length.

Valve member 28 extends through the wall of the manifold and is held in place and aligned by means of sleeve 40 and mounting bracket 41, the bracket being attached to the manifold by means of tension screws 42.

Sleeve 40 may be designed such that it can be rotated 1800 to completely close off side channel 23A from the main flow channel 23, as shown by the broken line representation, so that one production line may be stopped if required without completely stopping the extruder. The end portion of rod 28 which is outside the manifold is threaded and the threads on the rod are engaged by the threads in a nut 43 which is restrained from longitudinal movement by bracket 41. A keyway 44 in the threaded section of the rod, engaged by a key 45, prevents the rotation of the rod when the nut is rotated. By turning nut 43, either by hand or by an attached motor drive (not shown), rod 28 is made to move either in or out of the flow channel 23, thereby controllably varying the length of annular restriction 29. This, in turn, has the effect of controllably varying the back pressure exerted on molten polymer stream 20 as it flows toward extrusion die 30. Alternately, valve member 28 be actuated by means of a doubleacting hydraulic cylinder in place of the threaded nut mechanism.

Molten polymer stream 20 is supplied to manifold 21 by the extrusion apparatus (not shown) and enters inlet port 22 under pressure. Inlet port 22 directs the molten polymer to control flow channel 23 where flow diverter 24 divides the entering stream, directing a portion of it to each of the opposite ends of flow channel 23. One portion of the molten polymer stream is directed toward valve member 24, passes through the annular restriction 25, and then through side channel 23B to extrusion die 26 which is mounted at the end of manifold 21. The molten thermoplastic resin is subsequently expressed in the form of tubular film 27 through the annular extrusion orifice (not shown) of conventional die 26.

The remaining portion of the molten polymer stream is directed toward valve member 28 and passes through the annular restriction 29, then through side channel 23A to extrusion die 30 mounted at the other end of manifold 21. This portion of the molten resin stream 20 is subsequently expressed through the annular extrusion orifice (not shown) of conventional extrusion die 30 in the form of tubular film 31.

The pressure drop of the molten polymer within the manifold flow channel 23 will be dependent upon the size and length of the annulus through which it travels before being expressed into the extrusion dies 26 and 30.

Annular restriction 25, being of fixed length and cross section, will bring about a fixed pressure drop for a given molten polymer stream having a given temperature and pressure. Since annular restriction 29 is of variable length it follows that the pressure drop across that restriction is likewise variable, the pressure drop increasing for increasing length of annular restriction through which the molten polymer must flow. As the pressure drop across restriction 29 is incresed (by increasing the length of the annular space) the rate of flow of the molten polymer through that restriction is reduced relative to the rate of flow through restriction 25, and similarly as the pressure drop is decreased (by decreasing the length of annular space 29) the relative rate of flow of the molten polymer through that restriction is increased. By appropriate manipulation of valve member 28, and hence the length of annular restriction 29, it is possible to "tune" the system with a high degree of sensitivity and to adjust the rate of extrusion through extrusion die 30 to match the rate of extrusion through extrusion die 26. This "tuning" or balancing of the extrusion rates through the respective dies enables one to draw off both tubular films at the same rate while simultaneously matching their respective thicknesses by manipulation of the nip roller speeds. This in turn enables the manufacturer to realize an increased production rate from a multiple-die line in that he can now run both streams at the speed limit imposed by the limitations of their respective downstream equipment.

Such a system has the advantage of precision of adjustment. If there are two valves as described, in parallel and fed from a common manifold source and discharging into identical downstream pressures, the flow rate through the two valves is proportional to approximately the third power of the length of the annulus. When the initial length of the variable annular space is in the order of several inches, it is possible to make a substantial change in that length to achieve a fine adjustment in flow rate. For example, if the initial length of the annulus is 6 inches, and it is changed by a substantial amount, such as 0-194 inches, there will be a resulting change in the rate of flow through that space of approximately 10 percent, provided the supply and discharge pressures remain unchanged.

This permits a much more precise adjustment than was possible with the variable-gap prior art valve (see Figure 3) in which a movement of 0 005 inch produced a 28 percent change in flow rate.

The above-described embodiment has a further advantage in that there is a single control which is used by the operator to redistribute the flow. No decision is required on his part as to which knob to turn. Moving the adjustable valve member to the right decreases the average thickness of the product on the left and increases thickness on the right, and conversely.

Another embodiment of the present invention involves a manifold as described above, but utilizing two adjustable valve members instead of one adjustable and one fixed member. Such an embodiment would have the advantage of even greater sensitivity of control, as well as the ability to operate over a wider range of pressure drop variation.

Still another embodiment of the valve system of the present invention is illustrated in Figure 5, wherein the valve mechanism is the same in all respects as that shown in Figure 4 except that instead of rod 128 and associated annular restriction 129 being positioned horizontally in the main flow channel 123, it is positioned in side channel 123A and the tapered end portion of the rod is directed toward extrusion die 130.

In any of the aforementioned embodiments, the longitudinal displacement of the adjustable valve member may be initiated and controlled in the conventional manner (i.e. by an operator who is monitoring the average film thickness somewhere down stream), or by means of automatic film thickness detection device which is adapted to control the movement of the valve 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 for molten polymers, such as the extrusion of flat films, filaments, solid tubes, and foamed plastic sheets and tubes. Similarly, references in the specification and drawings to manifolds having a single feed source and two extrusion dies mounted thereon are intended to be merely illustrative of the concept disclosed herein, it being readily apparent to those skilled in the art that a single manifold can be designed to handle more than two extrusion dies or more than one feed source.

WHAT WE CLAIM IS: 1. Apparatus for controlling the flow of molten polymer from one or more feed sources to two or more extrusion die orifices, which comprises a manifold comprising an inlet port, a flow channel within the manifold communicating with the inlet port, and two or more extrusion die orifices mounted on the manifold in communication with the flow channel, and restriction means in the flow channel adjacent to and preceding each of the extrusion die orifices, each of the restriction means comprising an elongated rod of reduced diameter relative to the diameter of the flow channel in which it is positioned, which rod creates an elongated annular restriction through which the molten polymer must flow in order to pass from the inlet port to the extrusion die orifice prior to being expressed from the orifice, at least one of the rods being longitudinally displaceable in the channel to create an annular restriction of variable length and constant cross-section which serves to control the flow of molten polymer to the die orifices.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. annular restriction through which the molten polymer must flow. As the pressure drop across restriction 29 is incresed (by increasing the length of the annular space) the rate of flow of the molten polymer through that restriction is reduced relative to the rate of flow through restriction 25, and similarly as the pressure drop is decreased (by decreasing the length of annular space 29) the relative rate of flow of the molten polymer through that restriction is increased. By appropriate manipulation of valve member 28, and hence the length of annular restriction 29, it is possible to "tune" the system with a high degree of sensitivity and to adjust the rate of extrusion through extrusion die 30 to match the rate of extrusion through extrusion die 26. This "tuning" or balancing of the extrusion rates through the respective dies enables one to draw off both tubular films at the same rate while simultaneously matching their respective thicknesses by manipulation of the nip roller speeds. This in turn enables the manufacturer to realize an increased production rate from a multiple-die line in that he can now run both streams at the speed limit imposed by the limitations of their respective downstream equipment. Such a system has the advantage of precision of adjustment. If there are two valves as described, in parallel and fed from a common manifold source and discharging into identical downstream pressures, the flow rate through the two valves is proportional to approximately the third power of the length of the annulus. When the initial length of the variable annular space is in the order of several inches, it is possible to make a substantial change in that length to achieve a fine adjustment in flow rate. For example, if the initial length of the annulus is 6 inches, and it is changed by a substantial amount, such as 0-194 inches, there will be a resulting change in the rate of flow through that space of approximately 10 percent, provided the supply and discharge pressures remain unchanged. This permits a much more precise adjustment than was possible with the variable-gap prior art valve (see Figure 3) in which a movement of 0 005 inch produced a 28 percent change in flow rate. The above-described embodiment has a further advantage in that there is a single control which is used by the operator to redistribute the flow. No decision is required on his part as to which knob to turn. Moving the adjustable valve member to the right decreases the average thickness of the product on the left and increases thickness on the right, and conversely. Another embodiment of the present invention involves a manifold as described above, but utilizing two adjustable valve members instead of one adjustable and one fixed member. Such an embodiment would have the advantage of even greater sensitivity of control, as well as the ability to operate over a wider range of pressure drop variation. Still another embodiment of the valve system of the present invention is illustrated in Figure 5, wherein the valve mechanism is the same in all respects as that shown in Figure 4 except that instead of rod 128 and associated annular restriction 129 being positioned horizontally in the main flow channel 123, it is positioned in side channel 123A and the tapered end portion of the rod is directed toward extrusion die 130. In any of the aforementioned embodiments, the longitudinal displacement of the adjustable valve member may be initiated and controlled in the conventional manner (i.e. by an operator who is monitoring the average film thickness somewhere down stream), or by means of automatic film thickness detection device which is adapted to control the movement of the valve 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 for molten polymers, such as the extrusion of flat films, filaments, solid tubes, and foamed plastic sheets and tubes. Similarly, references in the specification and drawings to manifolds having a single feed source and two extrusion dies mounted thereon are intended to be merely illustrative of the concept disclosed herein, it being readily apparent to those skilled in the art that a single manifold can be designed to handle more than two extrusion dies or more than one feed source. WHAT WE CLAIM IS:

1. Apparatus for controlling the flow of molten polymer from one or more feed sources to two or more extrusion die orifices, which comprises a manifold comprising an inlet port, a flow channel within the manifold communicating with the inlet port, and two or more extrusion die orifices mounted on the manifold in communication with the flow channel, and restriction means in the flow channel adjacent to and preceding each of the extrusion die orifices, each of the restriction means comprising an elongated rod of reduced diameter relative to the diameter of the flow channel in which it is positioned, which rod creates an elongated annular restriction through which the molten polymer must flow in order to pass from the inlet port to the extrusion die orifice prior to being expressed from the orifice, at least one of the rods being longitudinally displaceable in the channel to create an annular restriction of variable length and constant cross-section which serves to control the flow of molten polymer to the die orifices.

2. Apparatus according to claim 1,

which comprises two extrusion die orifices mounted on the manifold in communication with the flow channel, and two of the rods of reduced diameter in the flow channel, one adjacent to and preceding each of the extrusion die orifices, one of the rods being held in immovably fixed position in the channel to create an annular restriction of fixed length, and the second of the rods being longitudinally displaceable in the channel to create said annular restriction of variable length.

3. Apparatus according to claim 1 which comprises two extrusion die orifices mounted on the manifold in communication with the flow channel, and two of the rods of reduced diameter in the flow channel, one adjacent to and preceding each of the extrusion die orifices, both of the rods being longitudinally displaceable in the flow channel such that each creates one said annular restriction of variable length.

4. Apparatus according to any of claims 1 to 3, in which each of the rods of reduced diameter is tapered at one end portion and the tapered end portion is directed toward the inlet port and into the flowing stream of molten polymer.

5. Apparatus according to any of claims 1 to 3, in which each of the rods of reduced diameter is tapered at one end portion and the tapered end portion is directed downstream, toward the adjacent extrusion die orifice.

6. Apparatus for controlling the flow of molten polymer from one or more feed sources to two or more extrusion die orifices, substantially as herein described with reference to Figure 4 or 5 of the accompanying draw ings.