US3710440A - Manufacture of coaxial cable - Google Patents

Abstract

A metal tape is advanced through a zone where it is formed into a tube surrounding the cable core passing through this zone, the core comprising the inner conductor and longitudinally spaced turns of solid insulation for spacing this conductor from the tube constituting the outer conductor. A foam-forming composition is coated on at least one of said core and tape in advance of the tube-forming zone and is expanded in the formed tube to provide an insulating foam filling the spaces between the turns of the solid insulation.

Description

United States Patent 1 Nevin et al.

[ 51 Jan. 16, 1973 [54] MANUFACTURE OF COAXIAL CABLE [75] Inventors: John J. Nevin, Orange, Conn.; Leo

G. Dumire, Stony Point, N.Y.

[73] Assignee: Phelps Dodge Copper Products Corporation, New York, N.Y.

[22] Filed: Jan. 16, 1970 [21] Appl. No.: 3,286

Related U.S. Application Data [60] Division of Ser. No. 801,568, Feb. 24, 1969, abandoned, which is a continuation-in-part of Ser. No. 660,791, Aug. 15, 1967, abandoned.

[52] U.S. Cl. ..29/624, 29/203 C, 124/36, 124/107, 124/109, 156/47, 156/48, 264/47,

[51] Int. Cl. ..H0lb 13/00, H05k 3/00 [58] Field of Search ..29/624, 203 C; 174/36, 107, 174/109; 156/47, 79, 48; 18/13 RR, 13 H; 264/47; 425/1 19, 190

[5 6] References Cited UNITED STATES PATENTS l/1964 Snelling ..264/47 X 3,309,458 3/1967 Yoshimura et a1. ..264/47 3,344,228 9/1967 Woodland et a1. ..156/79 X 3,411,182 11/1968 Nevin ..18/13 RR 3,452,434 7/1969 Wagele ..29/624 FOREIGN PATENTS OR APPLICATIONS 149,184 10/1950 Great Britain ..156/48 Primary ExaminerCharles W. Lanham Assistant ExaminerRobert W. Church. Attorney-Davis, I-loxie, Faithfull & l-lapgood [5 7] ABSTRACT A metal tape is advanced through a zone where it is formed into a tube surrounding the cable core passing through this zone, the core comprising the inner conductor and longitudinally spaced turns of solid insulation for spacing this conductor from the tube constituting the outer conductor. A foam-forming composition is coated on at least one of said core and tape in advance of the tube-forming zone and is expanded in the formed tube to provide an insulating foam filling the spaces between the turns of the solid insulatron.

6 Claims, 4 Drawing Figures E is-iaii imme mwi smsomarqvx PATENTEDJAN 16 1915 3.710.440

sum 2 or 2 v va INVENTORS JOhI'L, J.=Nev in Leo G. DUITHYE y 09M; 7%, F41 WW I I ATTOR EYS IFI 6E6 luli 4% MOE 856m mp3 MANUFACTURE OF COAXIAL CABLE The present application is a division of our application Ser. No. 801,568 filed Feb. 24, 1969, now abandoned which is a continuation-in-part of our application Ser. No. 660,791, filed Aug. 15, 1967 (now abandoned).

This invention relates to the manufacture of coaxial cables and more particularly to a method of making a coaxial cable characterized by a relatively low effective dielectric constant and high physical stability.

An ideal coaxial cable would employ an inner conductor coaxially supported within an outer conductor only by air. The dielectric constant for such a construction would be 1.0, and a cable of such configuration would exhibit little loss and could be made of any desired characteristic impedance. However, except for short lengths, coaxial cables cannot be fabricated in such a manner.

Accordingly, prior art low loss cables have been of two basic types. In one construction, longitudinally spaced turns of a substantially solid dielectric plastic material have been interposed for mechanical separation purposes between the two cable conductors. Such cables thus employ relatively little' of such material between the conductors and exhibit low effective dielectric constants which approach the ideal case. However, pressurization is required to provide long term electrical stability for coaxial cables of this type.

The other basic form of prior construction employs a continuous insulating foam to maintain the spaced concentric relationship between the cable conductors. However, foams with relatively high effective dielectric constants are required to provide a physically stable cable. The thin foam densities having low dielectric constants associated therewith do not possess the mechanical strength required to keep the center conductor uniformly supported in a coaxial relationship within the outer conductor.

By retaining the longitudinally spaced turns of solid dielectric material between the two cable conductors, but filling the spaces between the turns with an insulating foam of low density, the cable is provided with a relatively low effective dielectric constant and yet has the mechanical strength required for high physical stability. However, a problem is presented in the manufacture of this improved cable and particularly in filling the foam into the spaces between the turns of solid dielectric material.

An object of the present invention, therefore, is to provide an efficient method of manufacturing the improved cable.

According to the invention, the cable core comprising the inner conductor with the longitudinally spaced turns of substantially solid insulation is passed through a zone where a metal tape, also passing through this zone, is formed into a tube surrounding the core to provide the tubular outer conductor. A foam-forming composition is coated on either the core or the tape (or both) in advance of the tube-forming zone, and this composition is subsequently expanded in the formed tube to provide an insulating foam filling the spaces between the turns of the solid insulation.

For a better understanding of the invention, reference may be had to the following detailed description in conjunction with the accompanying drawings, in which FIGS. 1 and 2 are longitudinal sectional views of two different forms of the improved coaxial cable, and

FIGS. 3 and 4 are schematic views of two different forms of apparatus for making such cable by the new method.

Referring now to FIG. 1, there is illustrated a coaxial cable 10 employing concentric tubular center and outer conducting elements 11 and 13. A helical web 15 of any suitable solid plastic insulating material spirals about the inner conducting element 11 and defines the spaced relationship between the conductors 11 and 13. The helical web 15 extends radially outward from and is integral with a thin layer 16 of the solid insulating material which forms a continuous dielectric barrier around the inner conductor, for a purpose to be described presently. As noted above, the partially airspaced construction of the elements 11l513 has a low effective dielectric constant associated therewith, typically less than 1.25. The helical web 15 is preferably extruded around the inner conductor 11 simultaneously with the continuous dielectric barrier 16, and for this purpose an apparatus may be used as disclosed in US. Pat. No. 3,411,182 (John J. Nevin) granted Nov. 19, 1968.

To obviate any requirement for pressurizing cables which employ only a helical spacer, the air space in the cable 10 between the turns of the helix 15 spacing the conductors 11 and 13 is filled with a foam material 17, for example, foam polyethylene or styrene. Since the helical web 15 provides a portion of the requisite mechanical stability for the cable 10, the foam 17 may advantageously be made relatively thin (low density) so that it is also characterized by a relatively low effective dielectric constant, for example 1.25 or less. The composite non-pressurized cable 10 in FIG. 1 is physically stable because of the supporting action of both the foam 17 and the plastic helix l5, and is also characterized by the desired low dielectric constant. For example, the plastic helix 15, the thin barrier 16 and the low-density foam 17 may provide the composite cable with an effective dielectric constant of less than 1.3. Moreover, it is observed that the composite cable 10 is relatively impervious to external vibrations. The foam material 17 will' not migrate when vibrated, hence avoiding any discontinuities in the cable.

FIG. 2 depicts a second embodiment 20 of the im-' proved coaxial cable which employs concentric conductors 11 and 13 having a foam l7 therebetween. Also disposed between the conductors 11 and 13 are a plurality of regularly spaced insulating plastic discs or beads 25 which closely surround the continuous dielectric barrier 16. The properties and advantages obtaining with the cable 20 are similar to those discussed above regarding the cable 10, with the air spacing discs 25 simply replacing the similarly functioning helix 15.

In both FIG. 1 and FIG. 2, the thin dielectric barrier 16 provides a continuous covering over the inner conductor 11 so that even if cracks should develop in the mechanically unstable foam material 17, there can be no direct passage from the inner conductor 11 to the outer conductor 13. Thus, the barrier 16 performs the important function of preventing voltage break-down in the event of such cracks, and yet it is sufficiently thin that it does not appreciably reduce the volume available for the foam filling material 17.

The insulating material forming the helical spacer 15 and the underlying continuous barrier 16 may be any of the polyolefins, whether cross-linked or not, such as polyethylene, polypropylene, polytetrafluorethylene (Teflon) and polysulfone.

Referring now to FIG. 3, the reference numeral 30 designates a tube-forming device for continuously forming a metal tape 13a into a tube 13 closely surrounding a core, with lapping edges of the tape adhered to each other. The core, consisting of the inner conductor 11 with the dielectric barrier 16 and the helical spacer l extruded thereon, as previously described, is fed through a mist chamber 31 containing a plurality of nozzles 32 which are supplied by respective pipes 33 with components for producing a plastic foam. The streams of these components are blended together as they are sprayed from the nozzles 32, and the combined components deposit as a coating on the exposed surfaces of the core parts -16 passing through the mist chamber 31. This foam-forming coating 170 has a composition such that the foam formation occurs after a time interval dependent upon the temperature to which the coating is subjected, such compositions being well known in the art.

The core 11, 15-16 thus coated and the metal tape 13a pass continuously to the tube-forming zone 30, where the tape is folded width-wise around the core to form the concentric outer conductor 13. The tape 130 has an adhesive bonded to at least one surface so that the lapping edges of the tape are firmly adhered to each other when pressed together in the tube former 30, thereby providing the tube 13 with a longitudinal seam.

When the tubular outer conductor 13 has been formed in the former 30, the coating 17a on the core parts 15-16 is expanded into the foam 17 by passing the cable assembly through a heater 34. The expanded foam completely fills the space between the turns of the helical web 15, the foam being confined by the outer conductor 13.

The tube-forming device 30 and the mechanism for passing the core and the metal tape therethrough may be of conventional construction, as disclosed, for example, in U. S. Pats. Nos. 3,332,138 and 3,379,821, dated July 25, 1967, and Apr. 23, 1968, respectively. Also, the adhesive for binding the lapping edges of the tape together may be as disclosed in said patents.

The plastic foam 17 may be produced from any of a variety of thermosetting or thermoplastic resins, such as the urethanes, phenolics, urea formaldehydes, vinyls, polystyrenes and polyolefins. The components to be blended in the mist chamber 31, including the additives for timing the foam-forming action under temperature control, may be selected from formulations known in the art. In the case of flexible urethane foams, for example, their preparation is based upon controlled reactions of diisocyanate with a polyol and blowing agent (usually water), and minor quantities of a catalyst, a foam stabilizer and an activator are usually added to facilitate processing. The selection of these components determines the time interval before foaming occurs at a predetermined temperature; and-it will be understood that the heating zone 34 serves to heat the coating 17a to a temperature which induces the foam formation.

Referring now to FIG. 4, the core consisting of the inner conductor 11 and the extruded helix 15 and barrier layer 16 is passed directly to the tube former 30, and the foam-forming composition is applied to the tape 13a by a dispenser 35 as the tape approaches the tube former. This composition is deposited on the upper surface of the tape, which forms the inner surface of the tubular outer conductor 13. The composition is applied by a procedure generally referred to as frothing, whereby foam in a preexpanded form is produced by mixing a volatile liquid blowing agent, usually dichlorodifluoromethane (R-12), with the other components of the foam-forming composition under pressure. As shown in FIG. 4, this mixing is effected in the dispenser 35 which is in the form of a head providing a mixing chamber adapted to be pressurized.

The components of the foam-forming composition, including the blowing agent such as R-l2, are metered and thoroughly mixed in the head 35. The discharge nozzle 35a of the mixing head is provided with a reducing valve (not shown) which restricts the discharge so as to maintain a back-pressure in the head greater than the vaporization pressure of the R-12 blowing agent. In this way, all of the components are kept in liquid form for the mixing in head 35. As the mixture discharges through nozzle 35a to atmosphere, it is partially expanded and is caused to froth by the resulting pressure reduction. More particularly, the blowing agent vaporizes in the mixture so as to cause immediate expansion and form a liquid foam or froth similar in appearance to shaving cream dispensed from an aerosol can. The amount of this initial expansion, as well as the final density of the foam, depend upon the amount of R-l2 blowing agent metered into the mixture.

The coating of froth deposited on the metal tape from dispenser 35 is shown at 17b in FIG. 4. After the tape 13a has been formed into a tube in the forming device 30, with the lapping edges of the tape bonded together, the froth is heated in the zone 34 and expands to the final density of the foam 17, thereby completely filling the spaces between the turns of helical spacer 15. The final density of the foam 17 may be from 2 to 10 pounds per cubic foot, depending upon the amount of fluorocarbons other than the R-12 blowing agent in the formulation.

In both FIG. 3 and FIG. 4, the cable assembly is shown as passing directly to heater 34 from the tubeformer where the lapping edges of the tape 130 are seamed. Of course, this longitudinal seaming of the tube 13 may be effected downstream from the tube former 30, rather than in the tube former; but in any case the heater 34 is located downstream from the tube seaming operation so that the foam is fully expanded only when the tube has been seamed.

A typical urethane foam suitable for use in the present invention is derived from a mixture having the I following composition, the parts being by weight:

Polyols I00 pans Water 2 parts Freon l l 5 parts Freon l2 3 parts Tin Catalyst 0.15 parts Amine Catalyst 2 parts Surfactant 2 parts Toluene I00 Index Diisocyanate of stoichemistry (TDI) to index The foregoing composition is better suited for the HO. 3 embodiment, where the various components are introduced into the mist chamber 31. However, by increasing the proportion of the low boiling Freon (Freon 12), the composition is better adapted for the FIG. 4 embodiment. For the latter embodiment, the proportion of low boiling Freon may be increased to as much as 15 parts.

A specific example of the foam-forming composition for introduction into mist chamber 31 (FIG. 3) is Callery Foam System 1 l0, sold by Callery Chemical Company of Callery, Pennsylvania.

A specific example of the foam-forming composition for introduction into the dispenser head 35 is Callery Foam System 310, sold by the above-noted company.

We claim:

1. ln the manufacture of a coaxial cable having a tubular outer conductor surrounding a core which'includes an inner conductor and longitudinally spaced turns of substantially solid insulation on the inner conductor for spacing said conductors from each other, the method comprising the steps of advancing said core and a metal tape through a tube-forming zone and there forming the tape into a tube surrounding the core, to provide said tubular outer conductor, said tape having an inner surface forming the inside of said tube, coating a foam-forming composition on substantially the entire area of at least one of said core and tape surface in advance of said tubeforming zone, and expanding said composition in the formed tube to provide a foam filling the spaces between said turns of'the core.

2. The method as in claim 1, in which said foamforming composition is coated on the core.

3. The method as in claim 2, in which said coating is effected by passing the core through a mist of the foamforming composition.

4. The method as in claim 1, in which said foamforming composition is coated on that surface of the tape which becomes the inner surface of said tubular outer conductor.

5. The method as in claim 4, in which said composition prior to said coating includes a volatile liquid blowing agent and is maintained under pressure sufficient to prevent vaporization of said agent, said coating being effected by releasing said pressure and depositing the composition in the form of a froth on said surface of the tape.

6. The method as in claim 1, in which said core prior to the coating step is provided with a thin layer of substantially solid insulation integral with said turns and forming a dielectric barrier completely covering the inner conductor between said turns.

Claims (6)

1. In the manufacture of a coaxial cable having a tubular outer conductor surrounding a core which includes an inner conductor and longitudinally spaced turns of substantially solid insulation on the inner conductor for spacing said conductors from each other, the method comprising the stePs of advancing said core and a metal tape through a tube-forming zone and there forming the tape into a tube surrounding the core, to provide said tubular outer conductor, said tape having an inner surface forming the inside of said tube, coating a foam-forming composition on substantially the entire area of at least one of said core and tape surface in advance of said tube-forming zone, and expanding said composition in the formed tube to provide a foam filling the spaces between said turns of the core.

2. The method as in claim 1, in which said foam-forming composition is coated on the core.

3. The method as in claim 2, in which said coating is effected by passing the core through a mist of the foam-forming composition.

4. The method as in claim 1, in which said foam-forming composition is coated on that surface of the tape which becomes the inner surface of said tubular outer conductor.

5. The method as in claim 4, in which said composition prior to said coating includes a volatile liquid blowing agent and is maintained under pressure sufficient to prevent vaporization of said agent, said coating being effected by releasing said pressure and depositing the composition in the form of a froth on said surface of the tape.

6. The method as in claim 1, in which said core prior to the coating step is provided with a thin layer of substantially solid insulation integral with said turns and forming a dielectric barrier completely covering the inner conductor between said turns.

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