Elastomeric stator with modified fiber orientation

Abstract

A stator for use in a positive displacement motor or a progressing cavity pump. The stator comprises an elastomer mix preferably including rubber and a fiber reinforcement. The fiber reinforcement includes a plurality of fibers. The elastomer mix is formed into a stator via an injection molding process. The injection molding process includes a shear flow step in which shear flow is induced in the elastomer mix while the elastomer mix is in an uncured state. The shear flow modifies the orientation of the fibers into an advantageous modified fiber orientation. Shear flow is induced preferably via differential rotation of injection mold assembly elements during the injection molding process. Methods of manufacturing the stator are also disclosed.

Claims

1. A method for manufacturing a stator for use in a positive displacement motor or a progressing cavity pump, the method comprising the steps of: (a) assembling a mold assembly, wherein the mold assembly is generally cylindrical in shape with a longitudinal cylindrical axis, the mold assembly including a cylindrical outer tube element and an inner core element, the inner core element disposed within the outer tube element so as to provide a space between the inner core element and the outer tube element; (b) injecting an uncured elastomer mix into the space, the elastomer mix including rubber and fibers; (c) following step (b), but while the elastomer mix is still in an uncured state, inducing a shear flow in the elastomer mix by rotating the outer tube element about the cylindrical axis while holding the inner core element substantially stationary, wherein the shear flow has a shear flow direction generally transverse to the cylindrical axis, such that the shear flow modifies an orientation of at least some of the fibers in the elastomer mix to at least partially realign with the shear flow direction; and (d) following step (c), curing the elastomer mix.

2. The method of claim 1, in which the inner core element has helical pathways formed thereon.

3. The method of claim 1, in which the outer tube element is rotated at a speed in a range between about 0.1 RPM and about 5 RPM.

4. The method of claim 1, in which, during step (c), the elastomer mix is maintained in the space at a temperature in a range between about 150° F and about 260° F.

5. The method of claim 1, in which the elastomer mix includes greater than about 1.0 phr of fibers.

6. The method of claim 1, in which the elastomer mix includes a blend of unchopped elongate fibers and chopped fibers.

7. The method of claim 1, in which the elastomer mix further includes trimellitate acting as a plasticizer.

8. The method of claim 1, in which, prior to step (b), a bonding agent is applied to an interior surface of the outer tube element.

9. The method of claim 1, in which step (c) further includes an amount of the rotation being in a range of about 0.5 revolutions to about 50 revolutions.

10. The method of claim 1, in which the outer tube element is rotated for a time period in a range between about 10 seconds and about 30 minutes.

11. A method for manufacturing a stator for use in a positive displacement motor or a progressing cavity pump, the method comprising the steps of: (a) assembling a mold assembly, the mold assembly including an outer tube element and an inner core element, the inner core element disposed within the outer tube element so as to provide a space between the inner core element and the outer tube element; (b) injecting an uncured elastomer mix into the space, the elastomer mix including rubber and fibers; (c) following step (b), but while the elastomer mix is still in an uncured state, inducing a shear flow in the elastomer mix by rotating the outer tube element while holding the inner core element substantially stationary, wherein the shear flow has a shear flow direction, such that the shear flow modifies an orientation of at least some of the fibers in the elastomer mix to at least partially realign with the shear flow direction; and (d) following step (c), curing the elastomer mix.

12. The method of claim 11, in which, during step (c), the elastomer mix is maintained in the space at a temperature in a range between about 150° F and about 260° F.

13. The method of claim 11, in which the elastomer mix includes greater than about 1.0 phr of fibers.

14. The method of claim 11, in which the elastomer mix includes a blend of unchopped elongate fibers and chopped fibers.

15. The method of claim 11, in which the outer tube element is rotated at a speed in a range between about 0.1 RPM and about 5 RPM.

16. The method of claim 11, in which step (c) further includes an amount of the rotation being in a range of about 0.5 revolutions to about 50 revolutions.

17. A method for manufacturing a stator for use in a positive displacement motor or a progressing cavity pump, the method comprising the steps of: (a) assembling a mold assembly, the mold assembly including an outer tube element and an inner core element, the inner core element disposed within the outer tube element so as to provide a space between the inner core element and the outer tube element; (b) injecting an uncured elastomer mix into the space, the elastomer mix including rubber and fibers such that the elastomer mix includes greater than about 1.0 phr of fibers; (c) following step (b), but while the elastomer mix is still in an uncured state, inducing a shear flow in the elastomer mix by rotating the outer tube element while holding the inner core element substantially stationary, wherein the shear flow has a shear flow direction, such that the shear flow modifies an orientation of at least some of the fibers in the elastomer mix to at least partially realign with the shear flow direction; and (d) following step (c), curing the elastomer mix.

18. The method of claim 17, in which the elastomer mix includes a blend of unchopped elongate fibers and chopped fibers.

19. The method of claim 17, in which the outer tube element is rotated at a speed in a range between about 0.1 RPM and about 5 RPM.

20. The method of claim 17, in which step (c) further includes an amount of the rotation being in a range of about 0.5 revolutions to about 50 revolutions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a more complete understanding of embodiments described in detail below, and the advantages thereof, reference is now made to the following drawings, in which:

(2) FIG. 1 is a flow chart depicting an exemplary embodiment of a conventional PDM injection molding process for an elastomer and fiber mix;

(3) FIG. 2 is a flow chart depicting an exemplary embodiment of a PDM injection molding process, including rotation, for an elastomer and fiber mix in accordance with this disclosure;

(4) FIG. 3 is a schematic diagram depicting expected fiber orientation after conventional injection molding consistent with FIG. 1;

(5) FIG. 4 is a schematic diagram depicting expected modified fiber orientation after injection molding consistent with FIG. 2;

(6) FIG. 5A is a section through an exemplary PDM stator mold assembly 500 depicting expected fiber orientation after conventional injection molding consistent with FIG. 1;

(7) FIG. 5B is a section through an exemplary PDM stator mold assembly 510 depicting expected generally radial fiber orientation after magnetic force treatment consistent with the disclosure of U.S. Pat. No. 8,944,789 to Butuc et al. (“Butuc”);

(8) FIG. 5C is a section through an exemplary PDM stator mold assembly 520 depicting expected modified fiber orientation after injection molding consistent with FIG. 2; and

(9) FIGS. 6, 7 and 8 are schematic diagrams illustrating alternative exemplary injection molding protocols consistent with FIG. 2, in which:

(10) FIG. 6 illustrates single end injection;

(11) FIG. 7 illustrates dual end injection; and

(12) FIG. 8 illustrates multi-point injection.

DETAILED DESCRIPTION

(13) The following description of embodiments provides non-limiting representative examples using Figures, diagrams, schematics, flow charts, etc. with part numbers and other notation to describe features and teachings of different aspects of the disclosed technology in more detail. The embodiments described should be recognized as capable of implementation separately, or in combination, with other embodiments from the description of the embodiments. A person of ordinary skill in the art reviewing the description of embodiments will be capable of learning and understanding the different described aspects of the technology. The description of embodiments should facilitate understanding of the technology to such an extent that other implementations, not specifically covered but within the knowledge of a person of skill in the art having read the description of embodiments, would be understood to be consistent with an application of the disclosed technology.

(14) Reference is now made to FIGS. 1 through 8 in describing the currently preferred embodiments of the disclosed transmission. For the purposes of the following disclosure, FIGS. 1 through 8 should be viewed together or in comparative context. Any part, item, or feature that is identified by part number on one of FIGS. 1 through 8 will have the same part number when illustrated on another of FIGS. 1 through 8. It will be understood that the embodiments as illustrated and described with respect to FIGS. 1 through 8 are exemplary, and the scope of the inventive material set forth in this disclosure is not limited to such illustrated and described embodiments.

(15) As disclosed herein, elastomer (or rubber) composites including elastomer and fiber are injection molded into a variety of different form factors and components. FIG. 1 is a flow chart depicting an exemplary embodiment of a conventional PDM injection molding process 100 for an elastomer and fiber mix. When a stator is injection molded, a mold assembly is assembled (block 111 in mold assembly phase 110). Stator mold assemblies preferably include at least an inner core element and an outer tube element with a space (or void) between for receiving the injected rubber mix. These elements (or components) are assembled together around a common central (cylindrical) axis. As illustrated on FIG. 1, the inner core and outer tube may be locked together (block 112). In Borne embodiments, the ends of the mold assembly may be capped, sealed, or otherwise closed to contain and/or control the flow rubber once it is injected.

(16) The injection phase may begin once the mold has been assembled (see injection phase 120 on FIG. 1). The elastomer is pre-heated to a temperature which causes the rubber to become a flowable liquid (block 121). Reinforcing fibers and other additives are typically added at this time to form the rubber mix to be injected. Once the rubber mix has reached the desired viscosity, it is injected into the mold assembly (block 122). The mold assembly is then removed, as a unit, from the injection machine (block 123). The removed mold assembly unit includes outer tube and inner core, with injected mix deployed therein.

(17) As noted above, when rubber is injected into the mold, the rubber tends to develop a grain in the direction of the flow path. Fiber reinforced rubbers establish a grain direction in substantially the same manner and any reinforcing fibers become oriented in generally the same direction as the rubber grain created by the injection flow path.

(18) With further reference to FIG. 1, the mold assembly (including injected rubber mix) now undergoes curing phase 140. The mold assembly including injected rubber mix is heated. to a vulcanizing and/or curing temperature (block 141). This initiates a chemical reaction, thereby causing the rubber cure and solidify with the reinforcing fibers in the orientation established by the injection flow.

(19) As noted above, the technology described in this disclosure relates to the modification of the orientation of reinforcing fibers after rubber has been injected into a mold and before the rubber has been cured. FIG. 2 is a flow chart depicting an exemplary embodiment of a PDM injection molding process 200, including rotation phase 230, for an elastomer and fiber mix in accordance with this disclosure. Injection molding process 200 on FIG. 2 is substantially the same as injection molding process 100 described above with reference to FIG. 1, except with addition of rotation phase 230 on process 200 after injection phase 220 and before curing phase 240. A mold is assembled (block 211 in mold assembly phase 210). The inner core and outer tube are locked together (block 212). The elastomer is pre-heated to a temperature which causes the rubber to become a towable liquid (block 221 within injection phase 220). Reinforcing fibers and other additives are typically added at this time to form the rubber mix to be injected. Once the rubber mix has reached the desired viscosity, it is injected into the mold assembly (block 222). The mold assembly is then removed, as a unit, from the injection machine (block 223).

(20) As shown in FIG. 2, rotation phase 230 now follows injection phase 220. Once the mold assembly is removed from the injection machine, the mold assembly may be unlocked in some embodiments (block 231) to permit rotation of a mold assembly element (block 232). A mold assembly element as described on FIG. 2 is nominally either the mold outer tube or the mold inner core. In block 232 on FIG. 2, the outer tube and/or inner core may be rotated relative to each other, such rotation about the mold assembly's cylindrical axis, thereby creating a shear flow (and preferably, a generally transverse shear flow) within the injected but uncured rubber mix deployed between outer tube and inner core. In some embodiments, the inner core may be kept substantially stationary while the outer tube is rotated. In other embodiments, the outer tube may be kept substantially stationary while the inner core is rotated. Once the desired amount of rotation has been reached, the mold assembly may be re-locked (block 233).

(21) The curing phase 240 on injection molding process 200 on FIG. 2 is substantially similar to curing phase 140 as described above with reference to FIG. 1. The mold assembly including injected rubber mix is heated to a vulcanizing and/or curing temperature (block 241). This initiates a chemical reaction, thereby causing the rubber to cure and solidify with the reinforcing fibers in the modified orientation established first by the injection flow in injection phase 220, and then realigned in rotation phase 230.

(22) As noted, rotating the inner core and/or outer tube of the mold assembly on FIG. 2 while the rubber is in an uncured “green' state creates a shear flow (and preferably, a generally transverse shear flow) within the rubber in the mold. The reinforcing fibers are impacted by the shear flow and will change orientation to at least partially realign with the shear flow direction. It will be appreciated that the degree to which the fiber orientation is modified may be impacted by the degree of shear flow created. The degree of fiber reorientation may also be impacted by the nature of the reinforcing fibers including, but not limited to length, diameter, aspect ratio, and/or fibrillation. In some embodiments, a blend of shorter “chopped” fibers may be used in addition to longer elongate unchopped fibers. The degree of reorientation may also be impacted by the nature of the fiber components in a blend including the proportion of the various fiber components.

(23) FIG. 3 is a schematic diagram depicting expected fiber orientation after conventional injection molding consistent with injection molding process 100 on FIG. 1. FIG. 3 illustrates the orientation of reinforcing fibers on conventional stator 300 after a rubber and fiber mix 301 is injected into a stator mold in a conventional manner (such injection represented by arrow 303). The helical nature of the inner core causes the rubber to flow in a generally helical path, denoted on FIG. 3 by helix angle 306 (as opposed to being oriented parallel to the longitudinal axis of the stator). This injected rubber flow path causes the reinforcing fibers 304 to be oriented at a bias, typically at a fiber orientation angle 305 somewhat less than helix angle 306.

(24) FIG. 4 is a schematic diagram depicting expected modified fiber orientation after injection molding consistent with injection molding process 200 on FIG. 2. FIG. 4 illustrates the modified orientation of reinforcing fibers in an exemplary embodiment of the disclosed technology. Similar to FIG. 3, FIG. 4 illustrates the orientation of reinforcing fibers 304 on stator 400 after a rubber and fiber mix 301 is injected into a stator mold (such injection represented by arrow 303). As on FIG. 3, the helical nature of the inner core causes the rubber to flow on FIG. 4 in a generally helical path, denoted on FIG. 4 by helix angle 306.

(25) However, stator 400 on FIG. 4 has undergone fiber orientation modification consistent with rotation phase 230 on FIG. 2. Rotation of the mold assembly (as described above with reference to FIG. 2) realigns reinforcing fibers 304 responsive to a shear flow (and preferably, a generally transverse shear flow) induced into the uncured rubber mix during rotation phase 230 on FIG. 2. As seem on FIG. 4, reinforcing fibers 304 now have a fiber orientation angle 405 whose fibers are realigned in a more transverse direction to the stator's longitudinal axis as compared to fiber orientation angle 305 on the conventionally injection-molded stator on FIG. 3.

(26) It will he appreciated that, consistent with FIGS. 2 and 4 as described above, reinforcing fibers may be oriented and/or reoriented in substantially three dimensions or any combination of these three dimensions. Further, modified fiber orientations consistent with FIGS. 2 and 4 described above are still generally circumferentially aligned.

(27) FIGS. 5A, 5B and 5C are cross sections of stator mold assemblies 500, 510 and 520 respectively. FIGS. 5A and 5C are cross sections of stator mold assemblies 500 and 520 more generally described above with reference to FIGS. 1 and 2 respectively. FIG. 5A illustrates stator mold assembly 500 immediately prior to curing phase 140 on FIG. 1, depicting expected fiber orientation after conventional injection molding consistent with FIG. 1. FIG. 5C illustrates stator mold assembly 520 immediately prior to curing phase 240 on FIG. 2, depicting expected modified fiber orientation after injection molding consistent with FIG. 2.

(28) FIG. 5B is a section through an exemplary PDM stator 510 depicting expected generally radial fiber orientation after magnetic force treatment consistent with the disclosure of U.S. Pat. No. 8,944,789 to Butuc et al. (“Butuc”);

(29) In more detail, FIG. 5A illustrates a cross section of stator mold assembly 500, in which injected elastomer and fiber mix 501 separates outer tube 502 from inner core 506. The orientation of fibers 504 shown in FIG. 5A will be understood to be generally as shown on FIG. 3, with a bias generally consistent with a fiber orientation angle expected to be somewhat less than the helix angle.

(30) FIG. 5C illustrates a cross section of stator mold assembly 520, in which injected elastomer and fiber mix 521 separates outer tube 522 from inner core 526. In contrast to FIG. 5A, the orientation of fibers 524 on FIG. 5C will be understood to be generally as shown on FIG. 4, with a bias generally consistent with a modified fiber orientation angle, modified into a more longitudinally transverse direction as compared to the orientation angle of fibers 504 on FIG. 5A. Note that the orientation of fibers 524 on FIG. 5C is still generally circumferentially aligned. The degree to which fibers 524 on FIG. 5C are circumferentially aligned is at least partially related to the degree of shear flow induced in the uncured rubber mix during a rotation phase 230 as described above with reference to FIG. 2.

(31) FIG. 5B illustrates a cross section of a stator mold 510, in which injected elastomer and fiber mix 511 separates outer tube 512 from inner core 516. Fibers 514 on FIG. 5B are oriented as expected after magnetic force treatment consistent with the prior art disclosure of Butuc reference (described above in the “Background” section). Butuc discloses use of magnetically-responsive particles to be included with the reinforcing agents and dispersion substances infused into the elastomer and fiber mix. In FIG. 4C and associated disclosure, Butuc activates a magnetic source through the longitudinal center of the stator during curing of the rubber composite, with the goal of causing the magnetically-responsive particles to align the reinforcement fibers towards the source. As a result, the fiber orientation in such stators is as shown on FIG. 5B, with fibers 514 oriented substantially uniformly in the radial transverse direction, orthogonal to the stator's cylindrical axis. As noted above, a drawback to this magnetic alignment process in Butuc is that the process yields radially aligned transversely-oriented fibers, as opposed to circumferentially aligned fibers with at least a partial orientation in a cylindrical longitudinal direction as described with reference to FIGS. 2 and 4 in this disclosure. Fiber orientation according to the Butuc disclosure thus significantly compromises reinforced rubber properties in both the circumferential and cylindrical longitudinal directions.

(32) FIGS. 6, 7 and 8 are schematic diagrams illustrating alternative exemplary injection molding protocols consistent with FIG. 2, in which FIG. 6 illustrates single end injection, FIG. 7 illustrates dual end injection, and FIG. 8 illustrates multi-point injection. FIGS. 6, 7 and 8 illustrate sections through mold assemblies 600, 700, and 800 respectively. Each mold assembly 600, 700, 800 includes: outer tube element 610, 710 and 810 respectively; inner core element 615, 715, 815 respectively; and space 620, 720 and 820 respectively (each space 620, 720, 820 to receive injected rubber mix). Each of FIGS. 6, 7 and 8 depict rubber mix entry with black arrows 625, 725 and 825, and rubber mix exit with white arrows 630, 730 and 830 respectively.

(33) Stators are frequently manufactured using a single end injection molding process as shown on FIG. 6. In single end injection, liquid rubber mix is injected into one end of the mold assembly and flows through the mold assembly. Some rubber mix may be allowed to exit (bleed out) of the opposite end of the mold assembly in order to ensure the mold assembly is entirely filled. Consistent with FIGS. 1 and 3 as described above, single injection molding creates a generally consistent grain direction and fiber orientation throughout the injected rubber mix.

(34) In some embodiments, other manufacturing considerations may make it optimal to inject liquid or flowable rubber mix from two opposing ends of a mold assembly as shown on FIG. 7. In embodiments such as illustrated on FIG. 7, the rubber mix exits (bleeds out) from one or more points in the middle of the mold assembly.

(35) In other embodiments, yet further manufacturing considerations may make it optimal to inject liquid or towable rubber mix from one or more points in the middle of the mold assembly as shown on FIG. 8. In embodiments such as illustrated on FIG. 8, the rubber mix exits (bleeds out) from one or both ends of the mold assembly.

(36) When multi-point injection points are used such as in injection protocol embodiments illustrated on FIGS. 7 and 8, injection flow of the rubber mix may be turbulent within the mold assembly, thereby causing regions of reinforcing fibers to be oriented in near random directions. Such random fiber orientations, even when limited to regions thereof, may not be optimal to predict rubber performance in an operational stator. In some embodiments, use of the mold assembly rotation technique described in this disclosure may be advantageous to remediate such regions of random fiber orientation. Shear flow (and preferably, generally transverse shear flow) created by mold assembly rotation, consistent with this disclosure, may realign fibers as desired (including regions of randomly-oriented fibers) into a more uniform, circumferentially-aligned fiber orientation angle.

(37) The following disclosure describes exemplary embodiments of injection molding consistent with FIG. 2 as described above. It will be appreciated that such embodiments are exemplary only, and that the scope of this disclosure is not limited to such exemplary embodiments.

(38) When rubber material is initially heated in order to be injected into a mold, the rubber is typically heated to a temperature of between about 100° F. to about 330° F., and preferably to a temperature between about 150° F. to about 260° F. This temperature range allows the rubber mix to reach a desirable viscosity so that the rubber becomes flowable. This viscosity allows the rubber mix to be injected into a mold assembly. This viscosity allows shear flow (and preferably a transverse shear flow), consistent with rotation phase 230 on FIG. 2, to be developed within the rubber mix by rotating one element of the mold assembly relative to another. If the rubber temperature is raised further, the curing processes may be initiated, thereby prematurely curing and/or vulcanizing the rubber mix and preventing the rubber mix from achieving the desired degree of shear flow.

(39) It will be appreciated that different elastomer and/or rubber compositions achieve desirable viscosities at different temperatures. It will also be appreciated that different cure packages and/or curing agents may initiate the curing process at higher or lower temperatures. In some embodiments, a sulfur cure package may be used. In some embodiments, a peroxide cure package may be used. It will be appreciated that existing cure packages, with varying activation temperatures may be utilized.

(40) The shear flow generated by rotation of the mold assembly while the rubber mix is in an uncured state may generate heat. In some embodiments, the rubber mix may be heated to a minimum flowable temperature in order to account for heat produced during rotation of the mold assembly and to reduce the likelihood that heat generated by a shear flow initiates the curing process. In some embodiments, a cure package may be selected and/or modified to avoid prematurely initiating the curing process if a higher degree of shear flow is desired. In some embodiments, the speed, acceleration, and/or duration of the disclosed rotation may be selected in order to reduce the rate of heat generation and/or amount of heat generated within the rubber mix.

(41) As discussed, the degree to which the fiber orientation is modified is related to the amount of shear flow created within the mold assembly. The amount of shear flow created may depend at least in part on the speed, duration, acceleration and/or amount of relative rotation between the outer tube and the inner core.

(42) The degree of shear flow is at least partially dependent on the number of relative rotations between the outer tube and the inner core. In some embodiments, the shear flow is generated by an amount of rotation of at least about 0.5 of a revolution of the outer tube relative to the inner core, or at least about 1 revolution, or at least about 3 revolutions, or at least about 5 revolutions, or at least about 10 revolutions or at least about 20 revolutions, or at least about 50 revolutions. In some embodiments, the shear flow is generated by at most about 0.5 of a revolution of the outer tube relative to the inner core, or at most about 1 revolution, or at most about 3 revolutions, or at most about 5 revolutions, or at most about 10 revolutions or at most about 20 revolutions, or at most about 50 revolutions.

(43) In some embodiments, the outer tube may be rotated relative to the inner core for at least about 10 seconds, or at least about 30 seconds, or at least about 1 minute, or at least about 3 minutes, or at least about 5 minutes, or at least about 10 minutes, or at least about 20 minutes, or at least about 30 minutes. In some embodiments, the outer tube may be rotated relative to the inner core for at most about 10 seconds, or at most about 30 seconds, or at most about 1 minute, or at most about 3 minutes, or at most about 5 minutes, or at most about 10 minutes, or at most about 20 minutes, or at most about 30 minutes.

(44) In some embodiments, the outer tube and/or inner core of the mold assembly are rotated using a hydraulic chain vice. in some embodiments, the outer tube may be rotated indirectly by rotating an injection head and/or other component which is rotationally fixed to the outer tube. It will be appreciated that either mold assembly component (or mold assembly element) may be kept substantially stationary and/or rotated relative to the other. In some embodiments, particularly those in which a greater speed and/or number of rotations are implemented, the ends and/or other openings in the mold assembly may be sealed in order to avoid flowing the rubber mix material out of the ends of the mold assembly. In some embodiments, the rubber mix within the mold assembly may be cured with the ends uncapped.

(45) In some embodiments, a bonding agent may be applied to the inside the outer tube element of the mold assembly. The bonding agent is applied in an uncured “green” state and facilitates bonding between the injected rubber mix material and the metal outer tube. Currently preferred embodiments of the disclosed techniques generate a shear flow within the rubber material sufficient to modify the orientation of reinforcing fibers, but not sufficient to have a negative impact on the bonding agent. If a strong shear flow is generated, the bonding agent may be swept away from the interface between the injected rubber mix material and the inner surface of the metal outer tube. If the bonding agent is swept away from the rubber/tube interface, the beneficial effects of bonding the rubber mix to the metal outer tube will be reduced and/or eliminated. Additionally, in some embodiments, the bonding agent may dilute or contaminate the injected rubber mix material.

(46) In some embodiments, rotating the inner core element of the mold assembly while keeping the outer tube substantially stationary may reduce the amount and/or proportion of shear flow generated at the outer regions of the injected rubber mix and thereby reducing impact of rotation on the bonding agent layer.

(47) In some embodiments, rotating the inner core element of the mold assembly while keeping the outer tube substantially stationary may result in a greater shear flow being generated relative to the inner core. If the acceleration of the inner core relative to the uncured rubber mix is too high, the liquid rubber may cavitate in any reduced pressure areas created by the rotating lobes of the inner core. Cavitation may occur has the liquid rubber mix flows and may, under excessive rotation speeds and/or accelerations, may cause the rubber mix to detach from the profile of the inner core. Additionally, in some rubber composites, any low viscosity liquid additives may outgas under reduced pressure, thereby creating a gas bubble and/or blister defect in the cured rubber mix. Optimal speed, acceleration, and amount of rotation utilized may vary depending on the formulation of the rubber composite, nature of the reinforcing fibers, dimensions of the injection mold assembly, and/or shape of the injection mold assembly. In some embodiments, a reduced rotational RPM may be utilized in order to achieve the desired reorientation of reinforcing fibers while avoiding potential defects caused by cavitation and/or disruption of an outer bonding agent.

(48) In some embodiments, to achieve the desired material properties, the outer tube element of the mold assembly may be rotated relative to the inner core at a speed of at least about 0.1 revolution per minute (RPM, or at least about 0.25 RPM, or at least about 0.5 RPM, or at least about 1 RPM, or at least about 2 RPM or at least about 5 RPM. In some embodiments, the outer tube element of the mold assembly may be rotated relative to the inner core at a speed of at most about 0.1 RPM, or at most about 0.25 RPM, or at most about 0.5 RPM, or at most about 1 RPM, or at most about 2 RPM, or at most about 5 RPM.

(49) In some embodiments, optical microscope views may be analyzed to show particle and grain boundaries. In some embodiments, anisotropy may be analyzed using ASTM tensile coupons from a cross section of a stator. In some embodiments, the disclosed rotation techniques may be utilized to impact the grain and/or fiber orientation as well as the associated anisotropy within a stator or other rubber composite.

(50) In some embodiments, although not specifically illustrated herein, rotation phase 230 on FIG. 2 may be executed during injection phase 220, rather than after injection phase 220. It will be understood that in such embodiments, shear flow (and preferably, generally transverse shear flow) caused by rotation may assist with injection of the rubber and fiber mix directly into a modified fiber orientation (modified as compared with the fiber orientation expected in a corresponding injection protocol without rotation).

(51) Embodiments have been described in this disclosure with reference to exemplary deployments in stators for use in positive displacement motors (PDMs). It will be appreciated, however, that exemplary deployments may also include stators for use in progressing cavity pumps (PCPs).

(52) Embodiments as described in this disclosure may provide a method of modifying fiber orientation in uncured rubber, the method comprising the steps of: (a) injecting a material comprising rubber into a mold assembly wherein the mold assembly comprises an inner core and an outer tube; (b) rotating the inner core relative to the outer tube, thereby inducing a shear flow in the material; and (c) curing the material. in some preferred embodiments, the material also comprises a plurality of reinforcing fibers. In some embodiments, the inner core may be rotated at between about 0.1 and about 1 RPM relative to the outer tube. In some embodiments, the material may be maintained at a temperature between about 150° F. and about 260° F. while the inner core is rotating relative to the outer tube. In some embodiments, the step of curing the material may comprise heating the material to a vulcanizing temperature.

(53) Embodiments as described in this disclosure may also provide a stator for use in a positive displacement motor, the stator comprising: a rubber compound formed into a stator tube; the rubber compound including a plurality of fibers, the plurality of fibers oriented in a grain direction, wherein at least a portion of the fibers are oriented circumferentially around a central longitudinal axis of the stator. In some embodiments, the rubber compound may comprise a fiber loading of greater than 1.0 phr of fibers. In some embodiments, the rubber compound is formed into a stator tube by (1) injecting a flowable rubber into a mold assembly including an inner core and an outer tube disposed about a common longitudinal axis; (2) rotating the outer tube relative to the inner core about the common longitudinal axis; and (3) curing the rubber compound in the mold. In some embodiments, the rubber compound may be maintained at a temperature between about 150° F. and about 260° F. while the outer tube is rotating relative to the inner core. In some embodiments, the rubber compound may further include a trimellitate plasticizer. In some embodiments, the plurality of fibers includes a blend of elongate unchopped fibers and chopped fibers.

(54) Although the inventive material in this disclosure has been described in detail along with some of its technical advantages, it will be understood that various changes, substitutions and alternations may be made to the detailed embodiments without departing from the broader spirit and scope of such inventive material, some embodiments of which are recited in the appended. claims.