Abstract:
A device for making a profiled insulation having an extrusion die with an extrusion tip and a polymer chamber surrounding the extrusion tip. The polymer chamber has at least one air chamber therein. The air chamber is held in place and coupled to the outside of the extrusion die by a vertical fin extending outwards from the extrusion tip. When molten polymer flows through the polymer chamber around the air chamber, an opening is introduced into the polymer such that the profiled insulation is formed as the polymer exits the extrusion die, having a longitudinal cavity therein corresponding to the location of the opening formed by the at least one air chamber.

Description:
FIELD OF THE INVENTION 
     The present invention relates to insulation in cables such as LAN (Local Area Network) cables. More particularly the present invention relates to profiled insulation in cables, having a reduced effective dielectric. 
     BACKGROUND OF THE INVENTION 
     In the field of cables, such as LAN cables, certain common insulators are used for forming the twisted pair insulation as well as the outer jacket. Common polymers used include FEP (Fluorinated Ethylene Propylene) and PE (Polyethylene). Although these insulations provide good flame resistant properties needed to meet fire safety standards, such as UL riser and UL plenum ratings, they have relatively high dielectric constants, tending to cause insertion loss in the signals propagated along the cables. 
     One approach in the prior art for reducing the dielectric constant of an insulator is to introduce air or gas into the polymer insulation during the extrusion process in order to foam the insulation. Typically, chemical or physical foaming of the insulation (dielectric) is used to provide material reduction and an improvement to the transmission properties for data communication cables. However, there are several limitations with the foaming process. 
     Physical foaming of the dielectric typically includes injecting an inert gas such as nitrogen or carbon dioxide into a molten polymer under heat and pressure while inside an extruder. The gases are injected in the extruder in a low pressure area of the screw and absorbed by the molten polymer. The gas passes through the extruder, while dissolved in the molten polymer, until the polymer exits the extruder. Once the captivated gas inside the polymer is exposed to atmospheric pressures, it combines at a nucleation point and forms bubbles within the insulation. This process requires additional equipment such as a gas pressurization unit to inject the gas at a critical velocity into the polymer and complex screw designs such as multi-stage screws and an extrusion barrel with gas injection ports. 
     Chemical foaming is also used to create bubbles within the dielectric without the need for additional equipment. However, chemical foaming is not used as frequently as physical foaming because this method also has negative drawbacks inherent in the process. Chemical foaming is done by mixing a number of additives, at a given ratio, with the main polymer. Typically, a “nucleating agent” such as Boron Nitride is added to the main polymer to provide the point at which gas bubbles are formed and grow. The nucleating agent is distributed into the polymer with or without the use of mixing elements that are located on the extrusion screw. Increasing the amount of sites available within the polymer allows for more locations for bubbles to start. Additionally, another chemical is blended into the polymer to generate the gas. These additives, known as a “blowing agents” are mixed with the nucleating agent at the same time. The blowing agent may have a melting point much lower that the main polymer, so that once the material reaches a given temperature it degrades and produces a gas (vapor) within the melt. The vapor from the degraded material forms a bubble at the closest nucleation site. Chemical foam and gas injection extrusion lines are difficult to control and run slowly with low yields. 
     Another approach to reducing the dielectric constant in a conductor is to simply create cavities in the insulation surrounding the conductors. However, prior art attempts in this area are unsatisfactory, particularly with respect to insulation for each individual conductor in a twisted pair. For example, U.S. Pat. No. 5,922,155 shows an insulation provided for coaxial cables. Here, the insulator is extruded resulting in a wheel shaped insulator surrounding the central conductor of the coaxial cable. However, such a technique is not equally applicable to placing an insulator in an individual conductor from a twisted pair which is significantly smaller in diameter. Further disadvantages of the &#39;155 patent methodology include the fact that the extrusion die used is a complicated multi-component die, requiring significant upkeep. Also, there is no ability to adjust the pressure within the cavities during extrusion without shutdown and re-tooling. 
     Thus, the prior art does not exhibit any means for both reducing the dielectric constant of the insulation, such as insulation on the individual copper conductors of a twisted pair communication cable, without the costly addition of materials need to foam the insulation. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention looks to overcome the drawbacks associated with the prior art by providing a profiled insulation for twisted pair conductors and associated jackets and method for making the same. 
     To this end, the present invention is directed to device for producing profiled insulation. A device is provided for making a profiled insulation having an extrusion die. The extrusion die has an extrusion tip and a polymer chamber surrounding the extrusion tip. The polymer chamber has at least one air chamber therein. The air chamber is held in place and coupled to the outside of the extrusion die by a vertical fin extending outwards from the extrusion tip. 
     When molten polymer flows through the polymer chamber around said air chamber, an opening is introduced into the polymer such that the profiled insulation is formed as the polymer exits the extrusion die, having a longitudinal cavity therein corresponding to the location of the opening formed by the at least one air chamber. 
     Furthermore, the present invention is directed to a method for producing a profiled insulation. A method for manufacturing a profiled insulation includes providing a molten polymer formed into an insulation in a polymer chamber of an extrusion die. The extrusion die has an extrusion tip. The polymer flows around one or more air chambers in the polymer chamber and forms a profiled insulation having longitudinal cavities that correspond to the location of the air chambers. 
     It is another object of the present invention to provide a profiled insulation having a central opening and an outer circumference having a thickness, with at least one longitudinal cavity therein, the longitudinal cavity is substantially between 0.0025″ and 0.0004.″ 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with features, objects, and advantages thereof may best be understood by reference to the following detailed description when read with the accompanying drawings: 
         FIG. 1A  through  FIG. 1F  illustrates profiled insulators in accordance with one embodiment of the present invention; 
         FIG. 2  illustrates an individual twisted pair conductor having the profiled insulation from  FIG. 1B , in accordance with one embodiment of the present invention; 
         FIG. 3  illustrates a cable with a number of twisted pairs having the profiled insulation from  FIGS. 1A through 1C  in accordance with one embodiment of the present invention; 
         FIG. 4  illustrates a cable with a number of twisted pairs having the profiled insulation from  FIGS. 1D through 1F  in accordance with one embodiment of the present invention; 
         FIG. 5  illustrates a cable having a profiled jacket with a number of twisted pairs having the profiled insulation from  FIGS. 1A through 1C  in accordance with one embodiment of the present invention; 
         FIG. 6  illustrates an extrusion die for producing profiled cable jacket and profiled insulation for twisted pairs, in accordance with one embodiment of the present invention; 
         FIG. 7  illustrates a fin for supporting an air chamber within the extrusion die of  FIG. 6 , in accordance with one embodiment of the present invention; 
         FIG. 8  is a flow chart for producing the profiled insulation from  FIGS. 1A through 1F , in accordance with one embodiment of the present invention; and 
         FIG. 9  illustrates a profiled insulator having collapsed cavities, in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment of the present invention, as illustrated in  FIGS. 1A through 1F , a profiled insulation  10  is provided. Profiled insulation generally refers to an insulator, typically for use on a conductor from a twisted pair. Unlike common solid (usually cylindrical) polymer insulation, profiled insulation  10  of the present invention has additional physical characteristics regarding its shape as discussed below. 
     Profiled insulation  10  is preferably constructed from a thermoplastic polymer insulation (dielectric), such as FEP (Fluorinated Ethylene-Propylene), however, any suitable polymer may be used according to any one of the desired insulation capabilities, fire resistance properties, mechanical strengths or the desired production rates of profiled insulation  10 . 
     Each profiled insulation  10  is provided with one or more cavities  12  that extend along the longitudinal axis of insulation  10 . Cavities  12  are disposed within the insulation itself and may have a circular cross-section, such as illustrated in  FIGS. 1A through 1C , a trapezoidal cross-section,  FIGS. 1D through 1F  or possibly an elliptical shape for additional structural strength (not pictured). 
     In one embodiment of the present invention, profiled insulation  10  is used as a coating for wires in twisted pairs. As illustrated in  FIG. 2 , a twisted pair  14  is preferably constructed of a pair of copper conductors/wires  16  twisted around one another at some regular interval. Each of the two copper wires  16  are enclosed within profiled insulation  10 . It is understood that twisted pairs  14  may be constructed of any suitable metal used for twisted pairs, however copper is used to describe wire  16  for exemplary purposes. Typically one or more twisted pairs  14  are used to form a communication cable as discussed in more detail below with respect to  FIGS. 3 through 5 . 
     As noted previously, one drawback to polymer insulations on the conductors of twisted pairs is that solid FEP has a high dielectric constant, causing disruption in any signals that travel along wires/conductors  16 . The present invention of profiled insulation  10  reduces the amount of FEP or other polymer used to insulate wires  16  of twisted pair  14 , thus reducing the effective dielectric constant relative to solid polymer insulation. Furthermore, cavities  12  reduce the amount of FEP or other polymer used in forming profiled insulation  10 , reducing the weight of profiled insulation  10  as well as the amount of polymer needed to form it relative to solid polymer insulation. 
     Thus, in one embodiment of the present invention, profiled insulation  10 , as illustrated in  FIGS. 1A through 1F , have a reduced dielectric constant relative to a solid polymer insulation of the same material. For example, a twisted pair  14  having its copper wires  16  coated with solid FEP insulation has a dielectric constant of substantially 2.095, whereas the dielectric constant of profiled insulation  10  made with FEP is substantially 1.964, as a calculated result of a reduction of substantially 15.95% in total FEP (based on 6 circular cavities  12  as shown in  FIG. 1A ). A tested version of the product has shown up to 27.70% reduction of FEP. 
     In another embodiment of the present invention, the dielectric constant of profiled insulation  10  may be further adjusted by increasing or decreasing the number of cavities  12  as shown in variations  FIGS. 1B-1C . For example, additional trials have shown a reduction to a dielectric constant of substantially 1.881 and a calculated reduction of substantially 26.61% in total FEP (based on 10 circular cavities  12  as shown in  FIG. 1B ) with the tested version showing up to a 30.87% reduction. A reduction to a dielectric constant of substantially 1.747 and a calculated reduction of substantially 41.74% in total FEP is found based on 17 circular cavities  12  as shown in  FIG. 1C . 
     Such arrangements may be useful for providing a reduced dielectric when variable physical strength requirements (mechanical strength) for profiled insulation  10  are allowed. Such arrangements may be useful when a very large reduction in dielectric constant is desired, but a physically strong insulation  10  is not essential, or vise versa. It is understood that the number of cavities  12  per profiled insulation  10  may be adjusted to any feasible number based on the diameter profiled insulation  10  meet the desired dielectric and weight specifications. 
     In another embodiment of the present invention, the shape of cavities  12  may be trapezoidal as shown in  FIGS. 1D and 1F . For example, a reduction to a dielectric constant of substantially 1.425 and a calculated reduction of substantially 68.78% in total FEP was achieved using 6 trapazoidal cavities  12  as shown in  FIG. 1D , a reduction to a dielectric constant of substantially 1.501 and a calculated reduction of substantially 63.35% in total FEP was achieved using 10 trapazoidal cavities  12  as shown in  FIG. 1E  and a reduction to a dielectric constant of substantially 1.572 and a calculated reduction of substantially 57.65% in total FEP was achieved using 17 trapazoidal cavities  12  as shown in  FIG. 1F . The following table 1 shows the calculated and some tested results for the products shown in  FIGS. 1A-1F . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Standard Solid 
                 Gatling Die 
                   
                 Reduction in 
                   
               
               
                   
                 Insulation 
                 Process 
                 Foam Process 
                 FEP 
                   
               
             
          
           
               
                   
                   
                 Hole 
                 Number of 
                   
                 Number of 
                   
                 Foam 
                   
                 Using Gatling 
                   
                   
               
               
                   
                 Product 
                 Type 
                 Holes 
                 Diele 
                 Holes 
                 Diele 
                 % 
                 Diele 
                 Process 
                   
                 Act. 
               
               
                   
                   
               
             
          
           
               
                 Cas 
                 LAN 
                 Circ 
                 0 
                 2.095 
                 6 
                 1.964 
                 10 
                 1.964 
                 15.95% 
                 est 
                 27.70% 
               
               
                 1 
                 1000 
               
               
                 Cas 
                 LAN 
                 Circ 
                 0 
                 2.095 
                 10 
                 1.881 
                 16.5 
                 1.881 
                 26.61% 
                 est 
                 30.87% 
               
               
                 2 
                 1000 
               
               
                 Cas 
                 LAN 
                 Circ 
                 0 
                 2.095 
                 17 
                 1.747 
                 27.25 
                 1.747 
                 41.74% 
                 est 
               
               
                 3 
                 1000 
               
               
                 Cas 
                 LAN 
                 Trap 
                 0 
                 2.095 
                 6 
                 1.425 
                 55.1 
                 1.425 
                 68.78% 
                 est 
               
               
                 4 
                 1000 
               
               
                 Cas 
                 LAN 
                 Trap 
                 0 
                 2.095 
                 12 
                 1.501 
                 48.2 
                 1.501 
                 63.35% 
                 est 
               
               
                 5 
                 1000 
               
               
                 Cas 
                 LAN 
                 Trap 
                 0 
                 2.095 
                 17 
                 1.572 
                 42 
                 1.572 
                 57.65% 
                 est 
               
               
                 6 
                 1000 
               
               
                   
               
             
          
         
       
     
     Such an arrangement thus results in both the reduction of the effective dielectric of insulation  10  relative to a solid FEP insulation while simultaneously significantly reducing the amount of FEP needed to produce insulation  10 . Furthermore, this process is able to achieve dielectric constants comparable to foamed FEP without the need for resorting to complicated chemical or mechanical foaming processes. For example, the profiled insulation  10  of the present invention as illustrated in  FIG. 1A  has a comparable dielectric constant to 10% foamed FEP. Furthermore, profiled insulation  10  as illustrated in  FIG. 1B  has a comparable dielectric constant to 16.5% foamed FEP, profiled insulation  10  as illustrated in  FIG. 1C  has a comparable dielectric constant to 27.25% foamed FEP, profiled insulation  10  as illustrated in  FIG. 1D  has a comparable dielectric constant to 55.1% foamed FEP, profiled insulation  10  as illustrated in  FIG. 1E  has a comparable dielectric constant to 48.2% foamed FEP, and profiled insulation  10  as illustrated in  FIG. 1F  has a comparable dielectric constant to 42% foamed FEP. 
     Thus, according to the arrangement outlined above, a profiled insulation  10  is provided for use in insulating a conductor as small as a single copper conductor from a twisted pair. It is understood that many such variations to the number and shape of cavities  12  in profiled insulation  10  may be used based on the desired weight and desired dielectric constant. 
     In another embodiment of the present invention as illustrated in  FIGS. 3 and 4 , a typical cable  20  is shown having four twisted pairs  14  within an outer jacket  22 . Each twisted pair  14 , similar to the one shown in  FIG. 2 , is comprised of a pair of wires  16  surrounded by a profiled insulation  10 , such as the profiled insulation  10  from  FIG. 1A . A cross filler element  24  is disposed within a cable jacket  22  configured to separate twisted pairs  14  from one another to reduce internal cross-talk within cable  20 .  FIG. 4  illustrates a similar cable  20 , having a jacket  22 , a cross filler  24  and four twisted pairs  14 . In  FIG. 4 , twisted pairs  14  are formed from wires  16  surrounded by a profiled insulation  10 , such as the trapezoidal profiled insulation  10  from  FIG. 1E . 
     In another embodiment of the present invention as illustrated in  FIG. 5 , a cable  30  is shown, similar to cable  20  shown in  FIGS. 3 and 4 . Cable  30  has four twisted pairs  14  within an outer jacket  32 . Each twisted pair  14 , similar to the one shown in  FIG. 2 , is comprised of a pair of wires  16  surrounded by a profiled insulation  10 , such as the profiled insulation  10  from  FIG. 1B . A cross filler element (not shown) may be disposed within a cable jacket  32  configured to separate twisted pairs  14  from one another to reduce internal cross-talk with cable  30 . 
     In this arrangement, outer jacket  32  is formed as a profiled jacket having a series of longitudinal cavities  33  running along the long axis of the jacket. This configuration of longitudinal cavities not only reduces the dielectric constant of the outer jacket, but also reduces the final weight of cable  30 , lowering manufacturing costs and improving its electrical characteristics. 
     As discussed in more detail below the process for making jacket  32  having longitudinal cavities  33  is similar to that used to produce profiled insulation  10 . 
     In one embodiment of the present invention, as illustrated in  FIG. 6 , an extrusion die  50  is provided. Extrusion die  50  is preferably constructed of a hardened metal such as nickel alloys sold under the trade names Inconel™ or Hastelloy™, either hardened or not hardened, however any suitable metal may be used. Extrusion die  50  is preferably made using a brass wire cutting technique such as brass wire erosion as well as with spark erosion however any similar effective manufacturing technique may be used. 
     Extrusion die  50  maintains an extrusion tip  52  through which a hollow cavity  53  runs there through. Hollow cavity  53  allows the substrate or item to be covered by the extruded insulation to pass through extrusion die  50 . For the purposes of illustration, extrusion die  50  and the process for applying profiled insulation  10  is discussed in conjunction with wires  16  for forming twisted pairs  14  having profiled insulation  10 . However, it is understood that a similar device and process are equally applicable for making jacket  32  with cavities  33 . 
     Extrusion die  50  further maintains a polymer chamber  54  for guiding the molten polymer into position around wires  16  as they pass through the end of hollow cavity  53  of extrusion tip  52 . As noted above, the polymer used is typically FEP, however any similar desired polymer may be passed through polymer chamber  54 . 
     As shown in  FIG. 6 , within polymer chamber  54 , a number of air chambers  56  are shown spaced evenly around hollow cavity  53 . Air chambers  56  are generally hollow tube shaped projections that are suspended within polymer chamber  54  that correspond to the formed cavities  12  in profiled insulation  10  as explained in more detail below. Air chambers  56  preferably extend from the open end of extrusion die  10  back within polymer chamber  54  of extrusion die  50  for approximately ½″ inch, but this may be extended or shortened as necessary to from cavities  12 . Furthermore, air chambers  56  are preferably 0.035″ in diameter for producing cavities  12  in profiled insulation  10  of a diameter between 0.003″ and 0.004″ as shown in  FIGS. 1A-1C . Variations in the size (diameter) of cavities  12  produced by air chambers  56 , may also be controlled dynamically based on air flow though chamber  56  as discussed below. 
     Air chambers  56  may be formed having a circular cross-section or a trapezoidal configuration resulting in cavities  12  in profiled insulation  10  as shown in  FIGS. 1A-1F . Other shapes for air chambers  56  may be used to create alternatively shaped cavities  12 . The shape of air chambers  56  generally corresponds ultimately to the shape of cavities  12  in profiled insulation  10 . 
     As illustrated in  FIG. 7 , attached to the rear end of each air chamber  56  is a vertical fin  58  that extends radially inward towards the center of extrusion die  50  via air vents  57 . Preferably, the diameter of fin  58  is 0.030″ inches, although it is not limited in this respect. Other diameters may be used based on the desired rate of air flow from the outside of extrusion die  50  into air chambers  56 . The arrangement of the present invention with thinly designed fins  58  and air chambers  56  within polymer chamber  54 , allow the polymer to flow around better, resulting in a better distribution of the polymer in the resulting profiled insulation  10 . Here the shape of fins  58  and air chambers  56  are such that flow and volume to air entering cavities  12  during extrusion can be carefully controlled through air vents  57 . 
     For example, air vent  57  allows air from the outside of extrusion die  50  to enter into air chamber  56  via fins  58 , allowing air to enter into polymer chamber  54  so as to maintain the stability of cavities  12  formed into profiled insulation  10 . This configuration allows for ambient air pressure to be placed within the airspace of cavities  12  during the extrusion process discussed below. 
     In an alternative arrangement, the outlet of air vents  57  may be further coupled to a pressurizing device  59  for introducing a positive or negative air pressure into air chambers  56 . Positive air pressure may be used to further support the structure of cavities  12  during extrusion. Alternatively, negative air pressure may be used to collapse cavities  12  formed by air chambers  56  in order to form a ridged profiled insulation  10  as discussed in more detail below. 
     Using the basic elements identified above for extrusion die  50 , the following outlines the process for producing profiled insulation  10  according to the present invention. 
     In one embodiment of the present invention as illustrated in  FIG. 8 , in a first step  100 , a user first obtains a substrate on which to apply profiled insulation  10 . For exemplary purposes, the substrate onto which profiled insulation  10  is placed is copper wires  16  as shown in twisted pair  14  in  FIG. 2  and as discussed in detail above. A similar process may be used to form profiled jacket  32  from  FIG. 5 , where the substrate would be all of the internal components of cable  30 . 
     Once the substrate, wire  16 , is selected, it is fed through hollow cavity  53  of extrusion tip  52  and pulled out of the front opening of extrusion die  50  at step  102 . Next, at step  104 , the heated molten polymer, such as FEP is introduced into polymer chamber  54  of extrusion die  50 . 
     At step  106 , as the polymer proceeds to the front of extrusion die  50  and exits out of the front end, the polymer moves around air chambers  56  (as well as vertical fins  58 ) causing a corresponding number of cavities  12  to form in the polymer  12 . 
     As an optional step  108 , air pressure is introduced or removed by air-pressure device  59  via vertical fins  58  and vents  57 , further increasing or decreasing air pressure within cavities  12 . Alternatively, vertical fins  58  simply allow ambient air around extrusion die  50  via vents  57  to enter air chambers  50  and consequently enter cavities  12  in the polymer. When pressure is introduced via air pressure device  59 , preferably either Air, Nitrogen or Helium are used, however, any useful and non-reactive gas may be used as desired. 
     In one embodiment of the present invention, air pressure device  59  in use, for example in creating cavities  12  in insulation  10  of  FIG. 1B , a pressure of 2 psi. with the volume of Nitrogen at 728 cc/min produces 10 holes at 0.003″ diameter each, at a tooling draw down ratio of 127:1 and with a calculated effective dielectric of 1.930. Changing the volume of Nitrogen to 612 cc/min at a similar pressure of 2 psi produces holes in the insulation at a 0.0025″, with a tooling ratio of 127:1, yielding an effective dielectric of 1.978. 
     This step  108  provides a distinct advantage over the prior art. Here, by introducing variable air pressure into cavities  12  via vents  57 , fins  58  and air chambers  56 , the air pressure can be dynamically changed during the extrusion process thereby varying the effective dielectric constant of profiled insulation  10 . This dynamic changing of air pressure by pressure device  59  during an extrusion eliminates the costly shut down and re-tooling of an extrusion apparatus, allowing the dielectric constant of the resultant profiled insulation to be adjusted/corrected on the fly. 
     At step  110 , both wires  16  (substrate) as well as the polymer exit the front of extrusion die  50 . It is noted here that tools of extrusion die  50  are larger than the finished profiled insulation  10  obtained by the process. The ratio of the size of the extrusion die openings to the size of the final profiled insulation product  10  is known as the draw down ratio. This size differential allows the molten polymer to “draw down” onto wires  16  at a distance away from the front exit of extrusion die  50 . Preferably the drawn down ration DDR is 120 but may vary between 50 to 200. The DDR is a ratio of the cross-sectional area of the insulation compared to the cross-sectional area of the polymer as it exits the tooling. 
     This draw down process is done to conserve the integrity of cavities  12  which would not be possible in a pressure extrusion environment. Draw down of the polymer helps with achieving smaller holes in the insulation, because the die tubes can be made to a larger outside diameter than the insulation holes have to be. Assuming the gas pressure introduced at step  108  is positive or if ambient air is allowed to flow into air chamber  56  via vertical fins  58 , the resulting product of this process is a wire  16  having a profiled insulation thereon such as that shown in  FIGS. 1A-1F . Two such wires  16  may be formed into a desired twisted pair  14  as shown in  FIG. 2  and four such twisted pairs  14  may be formed into cable  20  or  30  as shown in  FIGS. 3-5 . 
     It is noted above that negative air pressure such as the choking of ambient air flow can create a vacuum as the polymer is pulled over the tube, by choking the ambient air flow off. This results in a ridged profiled insulator  10  such as that illustrated in  FIG. 9  Such a ridged version of profiled insulation  10  will not exhibit cavities  12 , but instead will maintain a series of peaks  13  and troughs  15  that still results in a lessened amount of polymer used for the insulation, thus having the same reduced dielectric coefficient as well as reduced weight. However the alternating peaks  13  and troughs  15  provide the ridged profiled insulation  10  with a different mechanical strength profile that may be better suited for some purposes. 
     While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.