CONTROLLING COOLING RATE TO AVOID THERMAL WELDING OF SHEATH AND DUCT AT HIGHER SPEED

Abstract

An apparatus for making a product comprising a plurality of microducts bundled within a protective outer sheath, wherein the apparatus functions to inhibit thermal welding of the sheath and the microducts, and a corresponding method of making the bundled microducts.

Claims

1. An apparatus for making a product comprising a plurality of microducts bundled within a sheath and inhibiting thermal welding of the sheath and the microducts, the apparatus comprising: a die dimensioned and configured to extrude a molten material to form the sheath; and an air gap defined between the die and a cooling tank; wherein the cooling tank divided into a plurality of sections wherein a first section of the cooling tank contains air at ambient temperature, and any remaining section of the cooling tank contains water at a temperature lower than a temperature of the molten material.

2. The apparatus of claim 1, wherein the air gap and the first section of the cooling tank have a total combined length of at least about 2 feet (about 0.61 meters).

3. The apparatus of claim 1, wherein the air gap and the first section of the cooling tank have a total combined length of at least about 5 feet (about 1.52 meters).

4. The apparatus of claim 1, wherein the cooling tank comprises an adjustable divider for adjusting the length of the first section.

5. The apparatus of claim 1, wherein the air gap has a length of about 8 inches (about 20 cm), and the first section of the cooling tank has a length of about 2 feet (about 0.61 meters).

6. The apparatus of claim 1, wherein the air gap has a length of about 11 inches (about 28 cm), and the first section of the cooling tank has a length of about 9 feet (about 2.74 meters).

7. The apparatus of claim 1, wherein the water has a temperature of about 58 F.

8. The apparatus of claim 1, wherein the molten material comprises polyethylene.

9. A method of making a product comprising a plurality of microducts bundled within a sheath, the method comprising: (a) extruding a roughly cylindrical sheath material having an inner surface around a plurality of microducts each having an outer surface in such a fashion that the inner surface of the sheath material does not contact outer surface of the microducts, wherein the sheath material is extruded at a temperature T.sub.1; (b) passing the sheath material and microducts through an air gap for a distance, a time, and at an air temperature T.sub.2 which is lower than T.sub.1; and then (c) passing the sheath material and microducts through a cooling medium, for a distance, and time, and at a cooling medium temperature T.sub.3 that is lower than T.sub.2; wherein the temperatures T.sub.2 and T.sub.3 are pre-selected to inhibit welding of the inner surface of the sheath material to the outer surfaces of the microducts.

10. The method of claim 9, wherein in step (b) the air gap has a length of at least about 2 feet (about 0.61 meters).

11. The method of claim 9, wherein in step (b) the air gap has a length of at least about 5 feet (about 1.52 meters).

12. The method of claim 9, wherein in step (b) the air gap has a length of at least about 9.8 feet (about 3 meters).

13. The method of claim 9, wherein in step (c) the cooling medium comprises water.

14. The method of claim 9, wherein T.sub.1 is no greater than about 450 F. (about 232 C.) and T.sub.2 is no greater than about 100 F. (about 38 C.).

15. The method of claim 14, wherein in step (c) the cooling medium comprises water and T.sub.3 is no greater than about 70 F. (21 C.).

16. The method of claim 14, wherein in step (c) the cooling medium comprises water and T.sub.3 is no greater than about 60 F. (16 C.).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0014] FIG. 1 compares conditions of the sheath material (orange) at 350 F., 400 F., and 450 F., at a pressure of firm handpress and with air cooling. The sheath at 350 F. (left) was not sticky; the sheath at 400 F. (middle) was sticky; and the sheath at 450 F. (right) was welded to the microduct (blue).

[0015] FIG. 2 compares conditions of the sheath (orange) at pressures of firm handpress (left) and bodyweight press (right). The sheath is at a temperature of 400 F. and with air cooling. The sheath was sticky under both conditions.

[0016] FIG. 3 compares conditions of the sheath (orange) under various cooling temperatures. The sheath cooled with air (left) was sticky. The sheaths cooled with 58 F. H.sub.2O (middle) or 150 F. H.sub.2O (right) were welded to the microducts (blue).

[0017] FIG. 4 compares conditions of the sheath (orange) under various cooling times. The sheaths cooled by 10 seconds of air alone (left) or 10 seconds of air followed by 58 F. H.sub.2O (middle) were sticky, while the sheath cooled for 10 seconds with 58 F. H.sub.2O followed by air (right) was welded with the microduct (blue).

[0018] FIG. 5 shows shrinkage of sheath cooled by 15 seconds in air (top), cooled by 15 seconds in air followed by water (second from top), and cooled by immediately after heating (third from top). The bottom panel is a schematic diagram showing that uneven cooling causes shrinkage toward hotter surface.

[0019] FIG. 6 is a top plan photograph of an exemplary version of the apparatus disclosed and claimed herein. The apparatus is shown bundling seven (7) microducts (i.e., legs) inside an orange, extruded sheath. Machine direction is from left to right.

[0020] FIG. 7 is a table showing test results of the process for making the 7-way microduct bundle using the apparatus depicted in FIG. 6 under various water temperature/speed/air gap conditions. Sheath thickness above each of the outer six ducts was measured and is shown in the table. The lower left panel shows the color coding of the six outer microducts (i.e., legs; those microducts in direct contact with the sheath). Three conditions were recorded: Green=slide out, i.e., the microducts could be slid out (axially) from the sheath, with ease. Orange=sticky, i.e., the microducts could be slid out (axially) from the sheath, but only with considerable force. Red=welded, i.e., the microducts were welded to the sheath and could not be slid out. The lower right panel shows duct numbering and color coding for the six microducts in direct contact with the sheath.

[0021] FIG. 8 shows the condition of the seven (7) microducts bundled with a sheath produced under the water temperature/speed/air gap condition of chilled/fast/short. The top panel shows all seven microducts immediately after cutting into the sheath. The bottom panel shows the three (3) microducts that were welded in place and could not be removed from the sheath.

[0022] FIG. 9 shows the condition of the seven (7) microducts bundled with a sheath produced under the water temperature/speed/air gap condition of chilled/fast/long. The top panel shows all seven (7) microducts immediately after cutting into the sheath. The bottom panel shows the one (1) microduct that was welded in place and could not be removed from the sheath.

[0023] FIG. 10 shows the condition of the seven (7) microducts bundled with a sheath produced under the water temperature/speed/air gap condition of hot/slow/short. The top-left panel shows all seven (7) microducts immediately after cutting into the sheath. The top-right panel shows the sheath after the microducts were removed. The bottom panel shows microbubbles that formed on the sheath surface under these conditions.

[0024] FIG. 11 shows the condition of the seven (7) microducts bundled with a sheath produced under the water temperature/speed/air gap condition of hot/fast/short. Again, the top-left panel shows all seven (7) microducts immediately after cutting into the sheath. The top-right panel shows the sheath after the microducts were removed. The bottom panel shows microbubbles that formed on the sheath surface under these conditions.

[0025] FIG. 12 shows the conditions of the seven (7) microducts bundled with a sheath produced under the water temperature/speed/air gap condition of empty+air/fast/short. The top panel shows all seven microducts immediately after cutting into the sheath. The bottom panel shows one of the microducts welded to the sheath.

[0026] FIG. 13 shows the condition of the seven (7) microducts bundled with a sheath produced under the water temperature/speed/air gap condition of empty/fast/short. The top panel shows all seven microducts immediately after cutting into the sheath. The bottom panel shows the sheath alone after removal of the microducts.

[0027] FIG. 14 shows conditions of the seven (7) microducts bundled with a thinner sheath than the sheath shown in FIGS. 8-13. This bundle was produced under the water temperature/speed/air gap condition of chilled/slow/short. The top panel shows all seven microducts immediately after cutting into the sheath. The bottom panel shows the sheath alone after removal of the microducts.

[0028] FIG. 15A shows a sheath having shrunk when its outside surface was stiff/frozen, but its inside surface was still molten. As shown in the figure, this caused portions of the molten inside surface to ooze in between the microducts. FIG. 15B shows a duct having deformed during the cooling process, but without shrinkage. The top-left panel shows how the sheath deformed about the outer surface of the microducts. The top-right panel shows how the upper surface of the sheath has sagged due to gravity. The bottom panel shows that while the sheath has sagged due to gravity, the thickness of the sheath remained consistent.

[0029] FIG. 16 shows machine direction thickness (MDT), transverse direction thickness (TDT), and sticky factor (SF) of sheath produced under various water temperature/speed/air gap conditions.

[0030] FIG. 17 is a top-plan photograph of an improved apparatus design comprising an adjustable divide for optimized air gap.

[0031] FIG. 18 is a top plan photograph of another version of the apparatus dimensioned and configured for manufacturing a four-microduct bundle.

[0032] FIG. 19 is a table showing test results of the process for making 4-way microducts bundled with a sheath under various water temperature/speed/air gap conditions. Sheath thickness above each of the four ducts were measured and are shown in the table. The lower left panel shows color code of the leg conditions in terms of slide out, sticky, and welded. (See description for FIG. 7.) The lower right panel shows the duct numbering.

[0033] FIG. 20 shows conditions of the 4-way microducts bundled with a sheath produced under the water temperature/speed/air gap conditions of chilled/fast/long (upper left; two microducts welded to sheath), empty/fast/long (upper right; one microduct welded to sheath), and extra empty/fast/long (lower left; no welding).

[0034] FIG. 21 shows one version of the system (upper panel) in which a spray hose is used to cool the outer surface of the sheath. The lower panel shows the final condition of a 4-way microduct bundle produced using this apparatus under the water temperature/speed/air gap condition of spray/fast/long.

[0035] FIG. 22 is a schematic of a conventional sheath welding mechanism. 1: The speed of the molten sheath plastic exiting the die increases when it meets the ducts. The difference in speed between the sheath material and the encased microducts causes the sheath to stretch; the elongational stress causes polymer molecules in the molten sheath to align in the machine direction. Because HDPE (conventionally used as a sheath material) is a semicrystalline polymer, it shrinks more in the transverse direction (anisotropic), becoming more susceptible to bind to the microducts and cause sticking. 2: A wet sponge freezes/solidifies the outer layer of the sheath to avoid tearing. 3. While the outer layer of the sheath is solidified from the sponge, plastic on the inner surface of the sheath is still molten and shrinks inward as it cools, thereby strongly gripping the ducts and causing welding.

[0036] FIG. 23 shows surface area contact between sheath and ducts for the 4-way microduct bundles (upper) and the 7-way microduct bundles (lower). Each microconduit is 18 mm in diameter.

[0037] FIG. 24 is a fishbone diagram showing factors that may contribute to sheath welding.

[0038] FIG. 25 is a schematic diagram illustrating how to improve the process to prevent welding.

DETAILED DESCRIPTION

[0039] Provided herein is a method of preventing thermal welding of sheath and ducts in the making microduct bundles by carefully controlling the cooling rate of the sheath as it is disposed about the microducts. A key and unintuitive technique is to delay the cooling process by having the extruded sheath air cooled for an extended time period. Air being a generally poor cooling medium, the heat from the sheath surface is shed more slowly and steadily and shrinkage of the sheath then occurs more evenly. Once the temperature of the outer surface of the sheath falls below about 390 F. (about 199 C.), cooling the sheath with chilled water does not cause welding, regardless of throughput, sheath thickness, outer diameter, or machine type. For reference, the surface temperature of the sheath material (typically HDPE) was about 450 F. (232 C.) immediately upon exiting the extrusion die. (This exemplary temperature, however, is machine-specific. For example, extruding the same material in a larger extruder will lower the melt temperature due to the lower RPM of the screw and corresponding decreased shear rate. Conventionally, the sheath was provided to 6 inches (15.2 cm) of air exposure. In one version of the process, the method sues about a 900% increase of air exposure to lower the sheath surface temperature to the desired target. Contrary to conventional practice and thinking, increasing air cooling to 5 feet (1.75 meters) yields a dramatic increase in production rate without causing welding.

[0040] Conventional processes of bundling microducts with an oversheath comprise extruding molten material (e.g., polyethylene) from a die to shape the material into a roughly cylindrical form that covers the microducts. Conventionally, the sheath extrudate exiting the die is exposed to air (i.e., a short air gap) before entering a series of cooling tanks to be cooled with water to reduce the temperature of the product. The temperature difference between the sheath and water results in unbalanced cooling of the outside and inside surfaces of the sheath, such that the outer surface of sheath is stiffened, but the inner surface of the sheath remains molten. The molten plastic has nowhere to travel but toward the center, flowing between the ducts as it cools and hardens. When the inner surface of the sheath and the outer surface of the microducts come into thermal equilibrium, the sheath becomes permanently welded to the microducts. This is a highly unfavorable and unwanted outcome.

[0041] Thus, disclosed herein is a method to prevent thermal welding of sheath and duct by controlling the cooling rate of the sheath material. This is achieved by extending the length of the air gap, and/or by emptying a first section of a cooling tank for extended air exposure. The system may comprise an adjustable divider in the cooling tank to optimize the length of air exposure. To provide sufficient time for air cooling, the total length of the air gap and the first section of the cooling tank is at least about 2 feet, and in some embodiments, at least about 3, 4, 5, 6, 7, 8, 9, or 10 feet (i.e., about 0.6 meters to about 3.05 meters).

[0042] With the extended length of air exposure, the system allows for higher production speed without welding compared to the conventional practices. In some embodiments, the production speed is increased by at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100% compared to the conventional practices.

[0043] The forgoing detailed description of the disclosure is to aid those skilled in the art in practicing the present method. Those of ordinary skill in the art may make modifications and variations in the embodiments described herein without departing from the spirit or scope of the present disclosure.

[0044] Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described. While the disclosure will be described in conjunction with the exemplary versions, it will be understood that it is not intended to limit the disclosure to those versions. To the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the disclosure as defined by the appended claims.

[0045] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the disclosure pertain. Many methods and materials similar, modified, or equivalent to those described herein can be used in the practice of the embodiments of the present disclosure without undue experimentation.

[0046] As used herein, the singular forms a, an, and the include plural referents unless the content clearly dictates otherwise.

[0047] Numerical ranges as used herein are intended to include every number and subset of numbers contained within that range, whether specifically disclosed or not. Further, these numerical ranges should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 2 to 8, from 3 to 7, from 5 to 6, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and so forth.

[0048] As used herein, the term about refers to 10% of the variable referenced. As used herein, the term or is an inclusive or operator and is equivalent to the term and/or unless the context clearly dictates otherwise. Unless otherwise clear from context, all numerical values provided herein can be modified by the term about. The elements and method steps described herein can be used in any combination whether explicitly described or not, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.

[0049] All combinations of method steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.

[0050] The system disclosed herein may comprise, consist of, or consist essentially of the various steps and elements disclosed herein. The disclosure provided herein may be practiced in the absence of any element or step which is not specifically disclosed herein.

EXAMPLES

Example 1: Identify Contributors to Welding

[0051] The sheath of microduct bundles is prone to permanently adhering to (i.e., welding to) to the microducts (also called legs) in the following situations: (1) relatively thick sheath; (2) high production speed; and (3) high temperature of sheath up exit from extruder. Lab-scale mock tests were performed for root cause analysis (RCA) via replicating the problem. The goal was to identify the biggest contributor(s) to welding/sticking by varying the process conditions and improving the process conditions based on the findings. Test specimens were convention: sheath thicknesses of 0.07 (1.78 mm) and 0.2 (5.1 mm) (nominal) and 12/7 microduct (i.e., 12 mm O.D./7 mm I.D. nominal thickness).

[0052] Effects of sheath temperature on sticking were tested at 350 F. (177 C.), 400 F. (204 C.), and 450 F. (232 C.) with pressure of a firm handpress and air cooling. Higher temperatures cause thermal welding. See FIG. 1. The 0.2 sheath is sticky at typical melt temperatures out of the die (400 F.) without cooling.

[0053] Effects of pressure on sticking were tested at pressures of a firm handpress and bodyweight press at 400 F. and with air cooling. The sheath is sticky under both conditions (FIG. 2), indicating that pressure does not have a significant effect on welding.

[0054] Effects of cooling temperature on welding were tested using ambient air cooling (about 72 F./22 C.), 58 F. H.sub.2O, and 150 F. H.sub.2O with the initial sheath temperature of 400 F. and pressure of a firm handpress. The sheath was welded at a typical chilled-water temperature of 58 F./14.4 C. with 0.2 sheath thickness (FIG. 3). In contrast, 0.07 sheath material does not cause welding under these same conditions. This result demonstrates that the thickness of the sheath material is a causative factor for welding.

[0055] Sudden, large temperature differences between the molten sheath material and the cooling medium also cause rapid shrinkage, causing sheath to grip the ducts as it cools, again leading to welding. Microbubbles were formed in the sheath material when the cooling water was set at 150 F. (65.5 C.).

[0056] Effects of cooling time on sticking were tested for ambient air cooling alone, 10 seconds ambient air followed by 58 F. H.sub.2O, and 10 seconds 58 F. H.sub.2O followed by ambient air with the initial sheath temperature of 400 F. and pressure of a firm handpress. The longer the air cooling was, the less sheath became sticky and the less it welded to the microducts (FIG. 4). Thus, air cooling ties into temperature differences and shrinkage rates as causative factors in welding.

[0057] Sheath shrinkage was also compared between ambient air cooling alone, water cooling after 15 seconds in air, and water cooling immediately after the sheath exited the extruder. Samples were heated to 400 F., and then placed in/on the 58 F. water (free-floating) until cool to the touch. See FIG. 5. As shown in FIG. 5, uneven cooling causes shrinkage toward the hotter (inner) surface of the sheath. Air cooling had the least amount of shrinkage (top panel of FIG. 5; 47 of shrinkage on the outer surface versus 62 on the inner surface), compared to water cooling immediately after heating (third panel from top in FIG. 5; 114 shrinkage on the outer surface versus 164 shrinkage on the inner surface.). Because air cooling is much slower than water cooling, the shrinkage amount can be decreased by decreasing the rate of cooling. At the same time, the air cooling decreases the T between the sheath and the cooling medium, which likewise decreases shrinkage of the sheath (which decreases the incidence of welding).

[0058] The above tests show that sheath temperature and shrinkage are the biggest contributors to welding. A thinner sheath experiences a faster rate of cooling via natural convection in air. When the line speed is increased, the sheath has less time in air. This explains, in part, the welding issue. Because air is poor heat conductor compared to water, the sheath warps more in cooling water even when the water is at the same temperature as the air and the exposure times are the same. Welding thus occurs in the first moments after the sheath material exits the diewhen its inner surface is still molten, but cooling and shrinking far more than the outer, already-stiffened surface. Thus, to minimize warping of the inner surface of the sheath (and thus to minimize welding), the cooling rate must be controlled and exposure to air needs to be extended (via a longer air gap or slower machine speed). The goal is to achieve more even cooling of the inner and outer surfaces of the sheath.

[0059] Based on the initial tests, production trials were conducted to confirm the findings and look for additional process improvements that decrease welding while maintaining (or increasing) production speed.

Example 2: Production Trial of 7-Way Microduct Bundles

[0060] The goal of the trial was to increase the production speed without causing duct welding by controlling the shrinkage rate through varying water temperature, air gap length, and cooling method. See Table 1 below for specific conditions. The extruding equipment was set up with a pin of 3.03 OD, bushing of 3.25 ID, and 0.194 lb/ft of extruded sheath. Test specimens were 7-way 18/14 with 0.07 sheath. A photograph of the apparatus used is shown in FIG. 6.

TABLE-US-00001 TABLE 1 Specific conditions of water temperature, speed, and air gap for production trial of 7-way microduct bundles. Category Specific Condition Water Temperature Chilled 58 F. Hot 125 F. (2)/100 F. (4)/80 F. (6) Empty 2 section of first tank drained Empty + Air Compressed air in 2 section Speed Slow 35 ft/min Fast 60 ft/min (Slow + 70%) Air gap Short 8 Long 18

[0061] FIG. 7 shows a summary of the results. Under the water temperature/speed/air gap condition of chilled/fast/short, the ducts get sticky because the speed (60 ft/min) is higher than the standard speed (35 ft/min). See FIG. 8. When the air gap was increased from 8 (20.3 cm) to 18 (45.7 cm) (the chilled/fast/long condition) helps to relieve welding. See FIGS. 7 and 9. However, when the air gap is long, there is a higher risk of the sheath tearing at the air gap due to increased elongational stress.

[0062] Under the condition of hot/slow/short, no visible difference was shown with sticking for the outer six ducts. The results showed the same performance as chilled water with the same air gap (the chilled/slow/short condition; FIG. 7). However, higher water temperature causes unwanted microbubbles on the sheath surface (FIG. 10). Similar results were obtained for the condition of hot/fast/short. Higher water temperature helps with sticking. Only one duct became sticky compared to three sticky and three welded with chilled water under the same speed and air gap (FIG. 7). But higher water temperature still causes unwanted microbubbles on the sheath surface. See FIG. 11.

[0063] Under the condition of empty+air/fast/short, although air has slower rate of cooling than water, forced air increased the cooling rate enough to start the sheath to shrink and cause welding. See FIGS. 7 and 12.

[0064] The condition of empty/fast/short addressed the risk of sheath tearing by placing a wet sponge to the sheath after it exits the extruder and then immediately exposing sheath to the air for gradual cooling. This condition performed the best among the tested conditions with fast speed (FIGS. 7 and 13).

[0065] The condition of chilled/slow/short condition was also tested with a thin sheath. Lb/ft was decreased from 0.194 to 0.123 to test minimum sheath thickness needed to avoid tearing the extruded sheath. With a thinner sheath there is less plastic to cool, which allows more even cooling, and thus less likelihood of welding. See FIG. 14.

[0066] Another test was conducted to run at a speed of 5 ft/min and then air cool for about 30 seconds. But when the sheath is left in the molten stage for too long, it starts to sag onto the ducts, yielding a petal-like cross-sectional shape to the sheath (aka Daisy Duct). However, because the rate of cooling is gradual, there is no shrinkage of the sheath, making the thickness constant throughout. See FIGS. 15A-15B.

[0067] Sheath samples from each trial were cut in the same direction and reheated at 200 C. for 2 minutes. Sheaths produced under various water temperature/speed/air gap conditions were evaluated for machine direction thickness (MDT), transverse direction thickness (TDT), and sticky factor (SF). The results are shown in FIG. 16. Shrinkage is much greater length-wise (machine direction) than height-wise. This is because molecules are oriented in machine direction due to the extrusion pressure and thus harder to shrink. There were no detectable correlations of thickness, width, height, ratio, and machine/transverse direction shrinkage between the different processes.

[0068] In sum, the trials showed that a key factor in inhibiting welding is gradual cooling, rather than maximized cooling/agitation. Fast cooling increases the rate of shrinkage, and thus tightly welds the sheath to the ducts. By gradual cooling, production speed was successfully increased by 70% without sticking. This is highly counter-intuitive because fast cooling would seem to be conducive to faster production speeds. Using hotter cooling water somewhat helps to inhibit welding, but also induces the formation of microbubbles on the sheath surface. Forced air cooling exacerbates the welding problem as compared to un-aided cooling in ambient air. A balance must be struck, however, to avoid deformation or tearing of the extruded sheath material. Longer exposure times in ambient air inhibits welding. But excessive exposure times in air causes the sheath to deform or tear. Thus, optimization is required.

[0069] Tooling size optimization tests were conducted to minimize elongational stress. This allows the sheath to be stable in a longer air gap while still moving at a faster production speed. The use of weight of sheath per linear feet of sheath (wt/ft) to dial in sheath thickness is recommended over extruder screw speed (RPM) because it proved harder to correlate machine speed with sheath thickness.

[0070] Based on the above analysis, an improved apparatus is shown in FIG. 17. The improved apparatus comprises an adjustable divider for optimizing the length of air exposure. The improved system was run with bushing size increased from 3.25 to 3.35, which resulted in larger die gap, increased draw ratio balance, increased area draw ratio, and midway speed ratio. See Table 3 below for conditions tested for the updated apparatus.

TABLE-US-00002 TABLE 2 Conditions tested for the updated system. Water Speed Air Gap Pin Bushing Empty 35 ft/min 8 3.03 3.35 Empty 35 ft/min 18 3.03 3.35 Empty 60 ft/min 8 3.03 3.35 Empty 60 ft/min 18 3.03 3.35 Chilled 35 ft/min 18 3.03 3.35 Chilled 60 ft/min 18 3.03 3.35

Example 3: Production Trial of 4-Way Microduct Bundles

[0071] Tests of water temperature/speed/air gap conditions were also applied to produce 4-way microducts bundled within a sheath. See Table 3 below for specific conditions. The goal of this testing was to show that the production speed could be increased by increasing the air gap and also to determine if the sheath functions in the same way when subjected to the longer air gap. The equipment setup included a pin of 4.625 OD, bushing of 5.00 ID, and 0.349 lb/ft. Test specimens are 4-way 1.25 SDR 11 with 0.07 sheath (ADDD 1619). A photograph of the apparatus is shown in FIG. 18.

TABLE-US-00003 TABLE 3 Specific conditions of temperature, speed, and air gap for production trial of 4-way microduct bundles. Category Specific Condition Water Temperature Chilled 58 F. Empty Tank drained per length shown in FIG. 19 Spray 3 section drained and replaced with spray hose Speed Slow 10 ft/min (PRP standard) Fast 16 ft/min (Slow + 60%) Air gap Short 5 Long 11

[0072] Results of the tests are summarized in FIG. 19. Under the water temperature/speed/air gap condition of chilled/fast/short, the ducts get sticky because the speed (16 ft/min) is higher than the standard speed (10 ft/min) (FIG. 19 and FIG. 20, upper left). These findings are consistent with the well-known concept that the higher the speed, the sticker the ducts get.

[0073] Under the condition of empty/fast/long, the test showed minor improvement (FIG. 19 and FIG. 20, upper right), showing that using a longer air gap is beneficial.

[0074] Under the condition of extra empty/fast/long, sheath welding was greatly improved (FIG. 19 and FIG. 20, lower). The result was consistent with the trial of the 7-way 18/14. A longer gap may be more beneficial under these conditions (as compared to the 7-way 18/14 bundle) due to heavier wt/ft of the sheath (i.e., the product has a larger bundle diameter and thus a larger wt/ft of sheath needed to wrap the bundle) and the increased area of the sheath in direct contact with encased conduits.

[0075] Under the spray/fast/long condition, a spray hose was placed above and below the midway of the sheath in the emptied 3 section of the tank. See FIG. 21, upper. Spray cooling caused the sheath to cool too fast, causing severe welding on all ducts (FIG. 19 and FIG. 21, lower).

[0076] The production trial demonstrated the mechanism of sheath welding (FIG. 22). Together with the production trial of the 7-way 18/14, conditions that cause increased welding include: [0077] (1) thinker sheath (0.1>0.07>0.05); [0078] (2) higher weight per foot of the sheath (0.5 lb/ft>0.3 lb/ft>0.25 lb/ft); and [0079] (3) greater surface area contact between sheath and microducts (between the same wt/ft; 4-way 18/14=44% contact; 7-way 18/14=34% contact; see FIG. 23).

[0080] FIG. 24 shows a fishbone diagram of factors that may contribute to stick of ducts, including factors of machines, methods, materials, manpower, measurements and ambient environment. Based on the analysis, an improvement of the process is shown in FIG. 25. The top half of the figure are photographs of an exemplary device used to practice the method. The lower panel of FIG. 25 is a schematic of the process. Both upper and lower panels of FIG. 25 have corresponding reference numerals identifying key aspects of the method. At the far left of both upper and lower panels is the extruder. The air gap is shown at 1. As noted in the top panel, it is preferred (but not required) that the air gap 1 be kept as long as possible without incurring sag in the sheath. Reference numeral 2 the inlet to the first section of the cooling tank, which contains air at ambient temperature. Reference numeral 3 shows the first section of the cooling tank. Reference numeral 4 shows the sheath-wrapped microduct after passing through the air gap and nearing the end of the first section of the cooling tank. At this point, at reference numeral 5, the microduct enters a second section of the cooling tank (and optionally additional sections of the cooling tank, designated 6, 7, and 8). A take-up reel 9 spools the finished product onto cores.

[0081] In sum, the trials confirmed that the key factor in inhibiting welding is gradual cooling in air achieved using a longer air gap. A short period of cooling in water or some other cooling medium is still required to impart the desired outer surface finish to the sheath. By using longer air gap, the production speed of the 4-way 18/14 bundle was successfully increased by 60% without any welding of the sheath to the microducts. The length of air gap needed to reduce welding depends on (a) the sheath temperature as it exits the die; (b) the wall thickness of the conduit (most easily controlled by controlling lb/ft); (c) the extrusion rate (ft/min); and (d) the area of the inner surface of the sheath in direct contact with the outer surfaces of the encased microducts. The improved process disclosed herein is robust enough to leave the annealing gap the same for all products.