Patent Application
20250162231
Microducts are small ducts or conduits used to install fiber optic cables. They typically range in size from about 3 mm to about 16 mm and are encased as bundles within a larger duct, sheath, or oversheath. Microducts are thus small-diameter, flexible, or semi-flexible ducts designed to provide a protective, clean, continuous, and low-friction path for optical cables. The low-friction nature of microducts is required because fiber optic cable itself has a relatively low pulling tension limit. As a general proposition, microducts should be compatible with existing construction designs and building configurations for both riser-rated and plenum-rated applications, including cable blowing apparatus. Microducts should also allow fiber optic cable to be successfully deployed through pull lines or strings using less than about 50 ft-lbs of force and via cable-blowing techniques at deployment speeds of roughly 100 to 200 feet per minute.
Microducts were developed as a solution to house fiber optic “microcables” that are much smaller in diameter that traditional fiber optic cable, but still carry significant signal capacity. For example, a traditional 144-fiber loose-tube cable is roughly 15 to 16 mm in diameter. In contrast, a comparable 144-fiber microcable is only about 8 mm in diameter and about one-third the weight of the conventional cable. The smaller size and weight allow for larger fiber counts. Thus, a host of microducts housed within one outer sheath provides multiple ducts in one structure. This allows for far easier installation of networks that include space for future expansion.
Microducts are not without their unique issues, however. One recurring issue, which arises during the manufacturing of the bundled microducts within the outer sheath, is the unintentional and unwanted welding of the sheath to the ducts inside. The outer sheath is supposed to protect the microducts bundled inside the sheath, while still allowing an access aperture to be cut into the sheath (for example, to install a connector). But when the inner surface of the sheath is accidentally welded to the outer surface of one or more microducts, it become difficult or impossible to access one or more specific microducts within the larger bundle of microducts inside the sheath.
Conventionally, during the sheathing of the microducts, the heated oversheath material is cooled by water in a series of cooling tanks. Upon experimentation, the present inventors have found that the cause of the unwanted welding is from shrinkage of the sheath polymer material due to unbalanced cooling of the inside surface versus outside surface of the sheathing material. This results in the sheath having a frozen (or suitably stiff) outer surface, but a still-molten (or too soft) inner surface. The molten/soft plastic of the inner surface of the sheath has nowhere to travel but toward the center of the bundle as the sheath cools and shrinks from its outside surface inward. The still-molten inner surface of the sheath then oozes between the microducts and solidifies, The result is unwanted (and permanent) welding of the sheath to the microducts.
It has been discovered that the onset of welding can be visually detected by ridges which form on the inner surface of the sheath. Additionally, the present inventors have determined that the higher the temperature difference between the bulk sheath material and the cooling process water, the more severe the sheath will shrink, resulting in welding.
There are several options to lower the temperature difference between the sheathing material and the cooling water, but the obvious options cause reduced production rates or other quality issues. So, for example, one could simply increase the temperature of the cooling water. But doing so unacceptably slows the production rate because it takes longer for the extruded sheath material to cool and harden around the encased microducts.
Disclosed herein are apparatus and methods for making a product comprising a plurality of microducts bundled with a sheath. The apparatus and methods are designed to inhibit thermal welding of sheath and ducts while simultaneously allowing for high production speed. The apparatus comprises: (a) a die for extruding a molten thermoplastic material (e.g., polyethylene) to form the sheath; (b) an air gap between the die and a cooling tank; and (c) a cooling tank optionally divided into two or more sections. When the cooling tank is divided, a first section of the cooling tank contains only air at a temperature lower than the temperature of the air gap, and the remaining section(s) of the cooling tank contain a cooling liquid (preferably water) and pre-selected temperatures.
Compared to conventional practices that minimize or eliminate any air gap and fill the entire cooling tank with water to cool the sheath, the extended air exposure of the present disclosure (including the air gap and at least the first section of the cooling tank) allows the sheath to be air cooled for an extended time period. Because air is a poorer conductor compared to water, the extended air exposure reduces the cooling rate of the sheath. The reduced cooling rate results in a slower shrinkage rate of the sheath. This provides additional time for the inner surface of the sheath to harden before the shrinkage of the sheath brings its inner surface into contact with the outer surface of the encased microducts. In this fashion, the inner surface of the sheath is sufficiently hard to inhibit and/or to prevent entirely any welding of the sheath to the microducts. The process thus lowers the sheath temperature before it enters the cooling tank. This yields a smaller temperature difference (ΔT) between the sheath and the cooling medium in the cooling tank. In turn, the decreased ΔT yields less shrinkage of the sheath and less welding.
To provide sufficient time for air cooling, the air gap and the first section of the cooling tank are preferably dimensioned and configured to have a total length of at least about 0.5 meters. In one specific version of the apparatus, the air gap and the first section of the cooling tank have a total length of between about 0.5 meter and about 2 meters. Longer and short air gaps, however, are explicitly within the scope of the method. The preferred air gap distance is as long as possible without incurring sag in the sheath material. Thus, determining the ideal air gap for any given situation is empirical based on the material from which the sheath is made, the temperature at which it exits the die, the thickness and diameter of the sheath, etc. The cooling tank may comprise an adjustable divider for adjusting the length of the various sections of the tank. With the extended period of air cooling, the sheath can be extruded at an increased rate, while still inhibiting welding between the sheath and the microducts. Thus, the apparatus and method disclosed herein can attain a production speed at least 10%, or at least 25%, or at least 50% higher than conventional manufacturing (wherein there is no air gap and the cooling tank has only one section filled with cooling liquid).
In one exemplary version, the apparatus is used to make a product comprising at least seven microducts, wherein the air gap has a length of about 20 cm (˜8 inches), and the first section of the cooling tank has a length of about 60 cm (˜24 inches). Using this apparatus, the production speed was increased by about 70% compared to the conventional practice, without any welding in the resulting product.
In another exemplary version, the apparatus is used to make a product comprising at least four microducts, wherein the air gap has a length of about 28 cm (˜11 inches) and the first section of the cooling tank has a length of about 2.7 meters (˜9 feet). Using this apparatus, the production speed was increased by about 60% compared to the conventional practice, without any welding in the resulting product. In this example, the microduct comprised four (4) 1.25 inch (31.8 mm) conduits.
The objects and advantages of the disclosure will appear more fully from the following detailed description of the preferred embodiment of the disclosure made in conjunction with the accompanying drawings.
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.
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.
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.
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).
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.
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.
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.
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.
As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise.
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.
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.
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.
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.
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).
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
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 (
Effects of cooling temperature on welding were tested using ambient air cooling (about 72° F./22° C.), 58° F. H2O, and 150° F. H2O 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 (
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.).
Effects of cooling time on sticking were tested for ambient air cooling alone, 10 seconds ambient air followed by 58° F. H2O, and 10 seconds 58° F. H2O 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 (
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
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 die—when 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.
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.
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
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;
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
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 (
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
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
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
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.
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.
Based on the above analysis, an improved apparatus is shown in
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
Results of the tests are summarized in
Under the condition of empty/fast/long, the test showed minor improvement (
Under the condition of extra empty/fast/long, sheath welding was greatly improved (
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
The production trial demonstrated the mechanism of sheath welding (
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.
