WO1995002953A1

WO1995002953A1 - Small diameter emi gasket with conductively wrapped tube

Info

Publication number: WO1995002953A1
Application number: PCT/US1994/006976
Authority: WO
Inventors: Daniel T. Courtney; Jariwala C. Yashvant
Original assignee: Schlegel Corporation
Priority date: 1993-07-13
Filing date: 1994-06-21
Publication date: 1995-01-26
Also published as: AU7114094A

Abstract

An elongated seal (10) for blocking electromagnetic interference (EMI), and for environmental sealing against dust, air, and moisture, is arranged for production in very small diameters. The seal (10) is intended to bridge environmentally and electrically across a gap between conductive bodies (90), conforming to irregularities. The seal (10) has a compressibly resilient core (20), that can be hollow (30) and made of thermosetting or thermoplastic elastomer such as thermoplastic olefin polymer. The core (20) is formed and wrapped with a flexible, electrically conductive sheath (40), wound helically on the core in wraps of one or more yarns (42), or as an elongated strip of fabric (44). The yarns or fabric can be metalized synthetic fibers. An outer coating improves abrasion and corrosion resistance (80). Sucessive helical windings can overlap, abut (50), or define gaps. The sheath can be bonded adhesively to the core or wrapped on the core before the core has fully cured. The seal is particularly apt for very small diameters, for example for sealing in handheld electronic devices or the like.

Description

SMALL DIAMETER EMI GASKET WITH CONDUCTIVELY WRAPPED TUBE

Background of the Invention

1. Field of the Invention

The invention relates to the field of gaskets and seals for blocking passage of environmental effects and electromagnetic interference (EMI) through sealed gaps in conductive structures. In particular, the invention concerns an EMI blocking seal of indefinite length, formed by helically wrapping a conductive web of thread, yarn or fabric around a resilient core. The core can be a hollow tube of thermoplastic or thermosetting elastomer and the sheath is preferably formed by a closely wrapped twisted yarn of metallized nylon. This structure provides an effective EMI seal of a very small diameter.

2. Prior Art

EMI gaskets, seals and the like are used to bridge electrically across gaps between conductive structures in electrical and electronic equipment. The conductive structures can be fixed together by fasteners through their abutting surfaces at the gap, such as with internal shielding panels that block propagation of high frequency components between one portion of an electronic apparatus and another, or can be movable, such as the outer panels of a conductive enclosure or housing. Often it is appropriate to provide an EMI gasket between abutting portions of the housing of an electronic device, which portions are snapped together or attached using screws.

For attenuating electromagnetic fields, the seal includes an elongated resilient seal body that has conductive material extending laterally across the seal body, as needed to electrically couple between the conductive structures defining the gap. The conductive material is sometimes provided by a seal body material having a relatively low electrical resistance, and sometimes by providing a conductive coating or sheath on a non-conductive body made of rubber or a compressible polymer. Depending on the frequency of the field to be attenuated, the conductive material can have openings that render the conductive material discontinuous along the length of the seal. However, the dimensions of any openings in the conductive material of a seal intended to attenuate relatively higher frequency EMI must be relatively smaller than openings in a seal to attenuate lower frequencies.

The performance of the seal in blocking EMI is a function of its conductivity and the extent to which the conductive material fills the gap between the conductive panels, at least down to the opening size needed for the frequencies to be blocked. The seal is resiliently compressible to accommodate variations in the dimensions of the gap between the conductive structures, for example due to surface irregularities in the conductive panels, variations in the parallelism of the panel edges, etc. The seal is dimensioned such that the panels at least slightly compress the seal at the widest part of the gap, thereby achieving electrical contact and making the seal electrically conductive across the gap all along the longitudinal extension of the gap.

A seal that is compressed between panels will assume a compressed shape that remains over time, an aspect of resilient materials known as shape memory or reverse load loss. It is desirable that a seal have minimal shape memory, and return to nearly its original shape even after being compressed and relaxed a number of times. If the seal remains more and more compressed over time, the seal may become smaller than the gap or may fail to bear its conductive material against the sealed panels with sufficient force to maintain good electrical contact.

In addition to its electrical aspects, the seal advantageously makes a nearly airtight or dust-tight bridge across the gap. Where a seal is to be used between conductive panels defining an outer housing, or perhaps defining part of an air flow channel for cooling, the seal should seal environmentally against the panels to thereby block passage of moisture or dust.

A seal against moisture may be particularly important in connection with pocket size electronic devices, which may be inadvertently dropped into standing water. Miniature electronic devices such as portable radios, tape or disk players, personal organizers, palm-top computers, cellular and portable telephones and the like fall into this category. Such devices also often have high frequency oscillators for crystal clocks or phase locking tuners, that are appropriately shielded against emissions entering or escaping from their enclosures. In these very small devices, EMI and environmental sealing can present substantial problems because there is very little space for the seal and a small diameter, precisely dimensioned seal is advantageous.

US Patents 4,857,668 - Buonanno and 5,045,635 - Kaplo et al disclose EMI blocking seals that include a compressible core and a sheathing that is provided with a conductive material for achieving electromagnetic and environmental sealing. According to certain of the embodiments in these patents, a foamed polymer core of polyurethane or the like is formed in a thin sheath of fabric such as ripstop nylon that has been metallized and/or covered with a coating having suspended conductive particles.

The uncured core material can be applied to the sheath as a liquid, with a blowing agent, curing agent and any necessary thixotropic agents. The liquid polymer is placed on an elongated strip of sheath material extending longitudinally along a travelling channel mold that defines the cross section of the finished seal. The strip is slightly wider than the circumference of the finished seal and is wrapped over after application of the liquid polymer and curing agent, to form a longitudinal seam along the elongated seal. The seal is heat treated in the travelling channel mold. As the foam core expands against the fabric strip and cures, the surface of the foam contacts and binds to the fabric, resulting in a good physical attachment of the core and the sheath. The sheath, which is conductive, extends fully around the core. When the seal is engaged between conductive panels defining a gap, two thicknesses of conductive fabric traverse the gap (i.e., on the opposite lateral sides of the seal). The seal is relatively flexible, has a low shape memory, particularly when not severely compressed repeatedly, and is effective against passage of moisture and dust. However, it is difficult to make in small diameters due to the precision needed in positioning, filling and wrapping the sheath on the foam core.

Other examples of EMI seals having a resilient core and a conductive sheath are disclosed, for example, in U.S. Patents 4,720,606; 5,028,739; 3,555,168; 4,652,695; 4,098,633; and 4,857,668. The core material may be a polyurethane foam, and the sheath may be a woven or knitted fabric or a nonwoven sheet comprising conductive filaments or a conductive coating. The seals are dimensioned and made of component materials determined by the design parameters of the particular application. It is possible to achieve a range of compression and recovery characteristics by varying the seal material, the density of the foam core and the like. However, problems with complex arrangements of the conductive and resilient elements of a seal, and good control of the resilience and dimensions of the seal, are such that very small seals are difficult to make and use effectively.

It would be advantageous to provide an elongated seal that can be made dependably and inexpensively in small diameters, characterized by high EMI and environmental sealing performance, and good compression characteristics. The seal of the present invention is intended to meet the need for a small diameter conductive seal using a resilient core that preferably comprises a core of extruded thermosetting or thermoplastic elastomer. The core can be solid, hollow or foamed. The core can be homogeneous or not, for example with an axially inner core of a different durometer than an annular outer core.

Preferably the core is formed as a hollow polymer tube. A conductive outer layer or sheath is wound helically on the core. The core provides the shape memory and resilience characteristics needed to conform to irregularities. The conductive outer layer can be a sheath of metalized polymer strands, yarn or fabric, providing a flexible EMI resistant barrier using a minimum of conductive materials. The sheath can be wound on a preformed core to which adhesive has been applied, or can be wrapped on the core tube upon emergence of the tube from an extruder, while tacky and perhaps expanding, whereby the sheath is firmly affixed to the core. This arrangement effectively and inexpensively produces very small diameter EMI- environmental seals, of 0.5 to 3.5mm outside diameter, preferably 1 to 3mm outside diameter, that are particularly appropriate to handle sealing problems of small electronic devices.

Summary of the Invention

It is an object of the invention to provide an improved seal for blocking electromagnetic interference, the seal having a flexible elastomeric core, wrapped helically with a continuous length of conductive material forming a conductive sheath.

It is another object of the invention to provide a seal for blocking electromagnetic interference having a very small diameter suitable for use in microelectronic applications, particularly with high frequency components, such as cellular phones, notebook computers, phase locking tuners and the like.

It is another object of the invention to provide a seal with the ability to block EMI and environmental influences effectively over a long useful life, despite deep compression or repeated instances of compression and relaxation.

It is yet another object of this invention to provide such a seal with a durably attached core and sheath, and at minimum expense.

These and other objects of the invention are accomplished by a seal for blocking propagation of electromagnetic energy, comprising an elastomeric core, which may be a hollow tube, the core being wrapped helically with a continuous length of conductive material, thereby forming a conductive sheath. The tube can be formed, for example, of a thermoplastic or thermosetting elastomer such as a urethane polymer or a similar elastomer, and can be solid and homogeneous, foamed (i.e. with hollow cells), and/or provided with distinct inner and outer portions having different durometers, densities or other characteristics due to differences in their component compounds or processing.

The core and the conductive wrapping may be formed sequentially or co-extruded. Preferably, the core is preformed and fully cured, then coated with adhesive and wrapped with the conductive sheathing thread, yarn or fabric as the core is fed linearly through a sheathing apparatus that passes the sheathing in the form of a continuous thread, yarn or web circularly around the advancing core. Alternatively, a freshly extruded core can be conductively wrapped prior to complete curing of the surface of the polymer core, forming a durable bond, including by intrusion of the core material into interstices between fibers of the sheath.

The conductive wrapping can be placed by helically wrapping the tube with a thin strip of conductive fabric such as metallized nylon (e.g., 1/16 inch wide) as the core is extruded. The conductive fabric web can be wrapped with each successive wrap abutting the next, thereby forming a continuous sheath. Alternatively successive wraps of the conductive fabric can be lapped. For applications where the EMI frequencies are relatively low, the successive wraps can be spaced slightly along the length of the seal, defining a helical gap. Leaving a helical gap reduces the amount of conductive fabric required per unit length of tube. Gapping or simply abutting the helical wraps allows the seal to bend easily. Where the successive wraps abut, mounting the seal around a tight bend may result in a gap between the wraps on the outer radius of the bend, however, the helical wrap provides some protection and support, as well as conductivity.

According to one embodiment, the conductive wrapping can be formed by helically wrapping the tube with conductive strands of thread or yarn, e.g. of a metalized polymer. The conductive thread can also be successively wrapped in an overlapping or spaced apart fashion, using one or more strands, and can be bonded by adhesive to the core or applied to the core before the surface is fully cured to bond the core and the sheath. Preferably, the sheath is formed by helical wrapping of a twisted yarn comprising a plurality of metalized synthetic polymer fibers, e.g., of nylon.

As a further step the sheath can be coated with an electrically conductive layer comprising a suspension of conductive particles such as metal, carbon or the like. Such a coating also makes the seal, and in particular the sheath, resistant to damage due to corrosion, abrasion and general wear and tear. A thin non-conductive layer is also possible to improve corrosion and wear resistance. However, a nonconductive coating must be sufficiently thin that electron channelling can occur at least discontinuously at points spaced along the seal, for electrically coupling the seal to the conductive panels between which the seal resides.

When the seal is mounted between conductive surfaces, the resilience of the elastomer core forces the sheath into continuous contact with the conductive surfaces, blocking passage of electromagnetic interference, either into or out of a protected area. A hollow tube core is readily made sufficiently resilient, even in a very small diameter, e.g., down to 0.5mm or less, to conform to sealed surfaces with good compression and recovery characteristics. The wrapped conductive sheath or yarn tends to stiffen the core, including around sharp corners or bends where the seal may be installed. Thus the seal achieves good compression characteristics as well as good performance in sealing against passage of EMI and environmental influences. Brief Description of the Drawings

The drawings illustrate certain exemplary embodiments of the invention as presently preferred. It should be understood that the invention is not limited to the precise arrangements and instrumentalities shown and discussed as examples, and is capable of variation in accordance with the appended claims. In the drawings,

FIGURE 1 is a cross sectional view through a seal according to a first embodiment of the invention.

FIGURE 2 is side view of the seal with a conductive wrapping having successively abutting windings thereby defining a continuous sheath.

FIGURE 3 is side view of the seal with a conductive wrapping having windings spaced apart thereby defining gaps in the conductive sheath.

FIGURE 4 is a cross section through the seal as sandwiched between two sealed bodies.

FIGURE 5 is a cross section through the seal as compressed between two sealed bodies.

FIGURE 6 is a schematic illustration of a method for making the seal of the invention using conductive fabric.

FIGURE 7 is a schematic illustration of a method for making the seal of the invention using conductive yarn.

FIGURE 8 is a cross section through an alternative embodiment, characterized by a solid homogeneous core.

FIGURE 9 is a cross section through a further alternative embodiment, characterized by a core having distinct inner and outer portions.

Detailed Description of the Preferred Embodiments

A seal 10 for blocking propagation of electromagnetic energy as shown in FIGURE 1 has an elastomeric core 20. The core can be formed of a thermoplastic or thermosetting elastomer. The thermoplastic polymer can comprise, for example, one or more of various forms of polyolefins, polyethene, poly vinyl chloride, polyurethane, acrylic, polyimide, polypropene and/or polystyrene polymers and the like. Appropriate thermosetting polymers can comprise, for example, one or more of various forms of phenolic, polyester, urea, epoxide and/or melamine polymers and the like. Thermoplastic olefin polymers, which are characterized by somewhat stiffer compression characteristics than some other polymers, are advantageous in some applications, or a similar thermoplastic or thermosetting elastomer can be used. Thermosetting polyurethane is another advantageous core material. According to a preferred embodiment of the invention, the core is formed of Santoprene^® polymer, available from the Monsanto Company, St. Louis, MO.

The core can be solid and homogeneous or discontinuous, e.g. , foamed or provided with distinct inner and outer layers. However, it is advantageous in very small diameter seals to form the core as a tube, i.e. , with an associated central hole 30 extending along its length. A conductive sheath 40 is applied to the exterior surface, and is formed of metalized strands, or incorporates metal fibers in sufficient density to render the sheath conductive over a path between points on the exterior surfaces of the seal. The core material can also be conductive, for example incorporating particles of metal or conductive carbon or the like.

For a softer seal, the core material can be formed using a blowing agent, such that the core expands and cures into a celled foam upon exiting the extruder. The choice of the particular polymer is made in view of the specific application for the seal, including such aspects as the temperatures expected, whether the seal will be repeatedly compressed and relaxed or substantially left compressed, etc. To accommodate variations in applications, it may be appropriate to choose a softer or harder material, or to make the material solid and homogeneous, foamed, formed of distinct inner and outer core sections, etc. In connection with very small seals as useful for small electronic devices and the like, the Santoprene polymer provides by good compression characteristics, including resilience and resistance to reverse load loss. Where the core is a tube, one way to control the compression properties of the tube 20 for the particular intended application is to vary the relative size of the hole 30 compared to the outside diameter of the tube. Making the hole 30 relatively smaller compared to the outside diameter of the tube 20, i.e., thickening the walls of the tube, provides a seal capable of withstanding larger compression loads without collapsing. Conversely, with a relatively larger hole 30 compared to the outside diameter of the tube 20 (i.e., thinner walls), the tube is capable of withstanding smaller compression loads and is more flexible. Varying the composition of the core material, varying the density of foam cells (if the material is foamed rather than solid), varying the durometer of the material, and varying the structure of the core (e.g., providing a more or less resilient inner core and outer annulus), thus provide control of the compression and reverse load loss characteristics.

The core 20 is supported in part by the sheathing, which is wrapped helically. The seal material residing adjacent and under the conductive sheath 40 provides compliant support, allowing the conductive sheath to conform closely to irregularities of the surfaces of conductive bodies to be sealed, such as the frame and door of a cabinet, abutting portions of a conductive housing, between shielding structures and a ground plane, or at a gap defined between other such structures. The core urges the sheath material outwardly into contact with the conductive bodies to be sealed.

The invention is particularly applicable to seals of relatively small outside diameter, formed to a circular or annular cross section in order to facilitate wrapping. The structure as shown is preferably applied to seals of indefinite length having an outside diameter of about 0.5 to 3.5mm, and preferably 1 to 3mm outside diameter.

The sheath 40 can be formed using electrically conductive EMI fabric as shown in FIGURE 6, or by using electrically conductive strands or yarns as shown in FIGURE 7. The EMI fabric 44 according to FIGURE 6 is preferably a continuous strip or web approximately 1.5mm wide, for example of metalized ripstop nylon, as applied to a seal of about 3mm diameter. The fabric strip is wound securely around the outside surface of the core 20 in a helical wrap. The core can be pre-formed and coated with an adhesive for retaining the fabric on the core. Alternatively, the strip can be wound promptly after extrusion of the core, e.g., before the surface has fully cured and remains tacky, in which case the sheath can be bonded directly to the core by the intrusion of core material into interstices of the fabric. The EMI fabric can be wound with the successive helical wraps in an overlapping, abutting, or spaced apart relation. Preferably, the fabric strip is wound with a slight overlap, e.g., with an overlap of about 0.5mm as compared to a strip width of about 1.5mm for a seal of 0.5 to 3.5mm outside diameter. FIGURE 2 shows a sheath wound such that successive windings 50 substantially abut each other without overlap, thereby defining a continuous sheath.

The fabric can be nonwoven, woven or knitted, from metalized fiber or with metalization applied to the fabric after forming. As shown in FIGURE 7, the sheath 40 alternatively can be formed by helically wrapping metalized thread or yarn 42 about the outside surface of the core 20. The sheath can be formed from one or more strands of silverized synthetic fiber, wrapped around and adhered to the core, e.g., a tube. An adhesive or direct bonding between the core and the thread or yam ensures a durable attachment, adding to the strength of the seal.

According to a preferred embodiment, a 250 denier silverized nylon 6/6 yam is employed (the denier including the weight of the silver). The yam is twisted, for example at 2.5 turns per inch, in the "Z" or right hand direction. The yam is wrapped on the elastomeric core, also in the "Z" or right hand direction. It is also possible to use yam that is twisted in the "S" or left hand direction, and/or to wrap the yam in either the Z or S direction. The yam is wrapped on the core from approximately 40 to 100 turns per inch, with 50 turns per inch being preferred. The yam can be single wrapped or multiply wrapped (e.g., double). Single wrapping is preferred.

The yam can be wound in an overlapping, abutting, or spaced apart fashion. The yam can be wound on the core as a belt or web of a number of parallel strands or yarns, which reduces the orbital velocity needed for a helical wrapping arrangement that can keep up with the linear rate of extrusion or the feed rate of a pre-extruded core.

A certain amount of tension is generally applied to the yam while wrapping. The tensioned yam or strip compresses the core and preloads the core against compression. To a certain extent, the resilient tension between the core and the wrapping provides a reinforcement that stiffens the seal and increases compression resistance. Additionally, the wrapping supports the seal against the tendency of the core to fold at a sharp comer along its length. The embodiment using helical wraps of metalized yam is particularly effective in applications requiring bends, since any gapping between conductive strands is distributed around the bend instead of becoming localized at the edges of adjacent wraps. Thus any gaps in the conductive barrier are narrow and the seal remains effective for sealing against EMI at relatively high frequencies.

Whether the sheath is formed of a fabric or of separate yarns, a conductive outer coating can be applied to improve the environmental sealing aspects and durability of the seal. The sheath can be coated with an electrically conductive protective coating 80, thereby encapsulating the seal 10 and improving corrosion and abrasion resistance. The coating can be a suspension of conductive particles in a polymer, such as disclosed in US Patent 5,045,635 - Kaplo et al, over a foamed urethane core material and fabric or yam composition similar to that of US Patent 4,857,668 - Buonanno, these patents being hereby incorporated. The use of the coating makes the seal, and in particular the sheath, resistant to damage due to corrosion, abrasion and general wear and tear due to contact with the sealed surfaces or the like, while retaining the conductive aspects needed for EMI shielding. This can be an important aspect, particularly because galvanic action in the presence of moisture otherwise may corrode and roughen the respective surfaces, accelerating degradation of the seal for EMI or environmental purposes.

When the seal 20 is mounted between conductive surfaces 90, as shown in FIGURES 4-5, the resilience of the elastomeric core 20 forces the sheath 40 into continuous contact with the conductive surfaces 90, blocking passage of electromagnetic interference, either into or out of a protected area.

The seal of the invention can be produced using a variety of extrusion and molding methods. The core or tube can be extruded to form a cured structural unit which is later wrapped with a conductive sheath, and adhered to the cured unit using adhesive. The tube also can be wrapped in connection with the extrusion process, when the core is only partly cured and remains tacky, or adhesive can be used when the core is tacky as well, for a further bond. An environmental coating likewise can be applied at the same time or after the seal has been wrapped. Another possible method utilizes a continuous forming technology wherein the tube 20 and the electrically conductive sheathing material 40 are fed continuously to an injection point where the sheath 40 is wrapped around the tube 20.

The process is shown schematically in FIGURES 6 and 7. The tube 20 is formed by extruding a mixture of a liquid polymer and curing agent, preferably using a thermoplastic elastomer such as an olefin polymer, or a thermosetting polymer such as polyurethane, and preferably in the shape of a tube. A blowing agent can be added to the polymer to produce a foamed core, and additional components such as thixotropic agents can be added as well, as known in connection with plastic extrusions.

FIGURES 8 and 9 show alternative embodiments for the core portion of the seal. In FIGURE 8, the seal core is solid and homogeneous. FIGURE 9 is a cross section through an alternative embodiment wherein the core has distinct inner and outer portions. The inner and outer portions can be respectively formed of different materials, such as with a stiff er olefin polymer on the outside and a softer foamed urethane center. Variations in the compressibility also can be accomplished by confining the area of foaming in a core of a single material, for example injecting the blowing agent preferentially into the center or into the edges, or at different concentrations in these areas, so as to form a seal of the desired compressibility contour. For example, the seal can be relatively more easily compressed until the compression reaches a proportion at which the softer material is substantially compressed, whereupon further compression is resisted by a harder material.

The invention having been disclosed in connection with certain preferred embodiments, additional variations will become apparent to persons skilled in the art. Whereas the invention is intended to encompass not only the particular embodiments, reference should be made to the appended claims rather than the foregoing examples, in order to assess the scope of the invention in which exclusive rights are claimed.

Claims

I claim:

1. A seal for blocking propagation of electromagnetic energy comprising: a compressibly resilient elastomeric core; and a flexible, electrically conductive sheath formed from helically wound wraps surrounding the elastomeric core.

2. The seal according to claim 1, wherein the elastomeric core comprises foamed thermoplastic olefin.

3. The seal according to claim 2, wherein the elastomeric core is formed as a hollow tube.

4. The seal according to claim 3, wherein the conductive sheath comprises conductive yam helically wound about the tube such that successive windings substantially abut one another, thereby defining a substantially continuous sheath.

5. The seal according to claim 1, wherein the conductive sheath comprises conductive yam helically wound about the elastomeric core such that successive windings substantially abut one another, thereby defining a continuous sheath.

6. The seal according to claim 1, wherein the conductive sheath comprises conductive yam helically wound about the elastomeric core such that successive windings are spaced apart thereby defining gaps in the sheath.

7. The seal according to claim 1, wherein the conductive sheath comprises conductive fabric helically wound about the elastomeric core such that successive windings substantially abut one another thereby defining a substantially continuous sheath.

8. The seal according to claim 1, wherein the conductive sheath comprises conductive fabric helically wound about the elastomeric core such that successive windings are spaced apart thereby defining gaps in the sheath.

9. The seal according to claim 1, wherein the conductive sheath comprises one of a conductive yam and a conductive fabric helically wound in at least one wrap about the elastomeric core such that successive windings at least partly overlap.

10. The seal according to claim 1, wherein the seal has an outside diameter of 0.5 to 3.5mm.

11. The seal according to claim 10, wherein the seal has an outside diameter of about 1 to 3mm.

12. The seal according to claim 1, further comprising an environmentally protective coating applied over the sheath.

13. An electromagnetic sealing arrangement comprising: a pair of conductive bodies defining a gap; a seal for blocking passage of electromagnetic interference through the gap, the seal having a compressibly resilient elastomeric core, and a flexible, electrically conductive sheath formed from helically wound wraps surrounding the elastomeric core.

14. The sealing arrangement according to claim 13, wherein the elastomeric core comprises a hollow tube of thermoplastic olefin polymer.

15. The seal according to claim 13, wherein the elastomeric core comprises a foamed thermosetting polymer.

16. The seal according to claim 13, wherein the conductive sheath comprises at least one of a conductive yam and a conductive fabric strip, helically wound about the elastomeric core such that successive windings substantially abut one another thereby defining a continuous sheath.

17. The seal according to claim 13, wherein the conductive sheath comprises at least one of a conductive yarn and a conductive fabric strip, helically wound about the elastomeric core such that successive windings are spaced apart, thereby defining gaps in the sheath.

18. The seal according to claim 13, wherein the conductive sheath comprises at least one of a conductive yam and a conductive fabric strip, helically wound about the elastomeric core such that successive windings at least partly overlap, thereby defining a continuous sheath.

19. A method of making an electromagnetic seal, comprising the steps of: forming an elastomeric core; helically wrapping the elastomeric core with an electrically conductive material thereby defining a sheath.

20. The method according to claim 19, further comprising forming the elastomeric core in an extrusion process, and bonding the sheath to the core by at least one of an adhesive and a direct bonding between the core and the sheath by intrusion of material of the core into interstices of the sheath.

21. The method according to claim 19, further comprising wrapping the electrically conductive material such that successive windings substantially abut one another, thereby defining a continuous sheath.

22. The method according to claim 19, further comprising wrapping the electrically conductive material such that successive windings at least partly overlap.

23. The method according to claim 19, further comprising wrapping the electrically conductive sheathing material such that successive windings are spaced apart thereby defining gaps in the sheath.

24. The method according to claim 19, wherein the core is substantially fully cured prior to said wrapping, and further comprising applying an adhesive to the core before said wrapping.

25. The method according to claim 19, further comprising applying an environmentally protective coating over the sheath.

26. The method according to claim 19, comprising extruding the core and causing the core to cure at least partly after emergence from an extruder, and wherein the sheath is wrapped helically on the core under tension prior to completion of curing of the core, whereby the core and the sheath are bonded by intrusion of material of the core into interstices of the sheath.