WO1994014309A9

WO1994014309A9 - Emi-shielding gasket including electrically conductive, resilient foam profile

Info

Publication number: WO1994014309A9
Application number: PCT/US1993/011554
Authority: WO
Filing date: 1993-11-30
Publication date: 1994-08-04

Abstract

A gasket for shielding electromagnetic interference passing through a seam between two electrically conductive bodies includes an electrically conductive, extruded foam profile which is elongated along a longitudinal axis and which is suitable for being secured to one of the electrically conductive bodies. The gasket further includes an electromagnetic shield embedded in the foam profile for shielding electromagnetic interference passing through the seam. The gasket enables a wide variety of materials and configurations to be used for the electromagnetic interference shield because the electrically conductive foam profile in which the shield is embedded protects the shield from contact with the bodies defining the seam but still allows energy absorbed by the seam to be dissipated to the bodies and, thereby, to ground.

Description

EMI-SHIELDING GASKET INCLUDING ELECTRICALLY CONDUCTIVE, RESILIENT FOAM PROFILE

Background of the Invention The invention relates generally to the field of gaskets for shielding electromagnetic interference (EMI). In particular, the invention concerns EMI shielding gaskets which include resilient foam profiles which are electrically conductive. Most electronic equipment generates unwanted electromagnetic energy during operation which, if not properly shielded, can interfere with other electronic equipment by way of radiation and/or conduction. This can include electromagnetic energy of wavelengths along various points of the spectrum such as radio frequency interference. As used herein, therefore, the term electromagnetic interference pertains to interfering electromagnetic energy of any wavelength. To avoid the problems associated with electromagnetic interference, all sources of electromagnetic energy should be properly shielded and grounded. Shielding should be designed to prevent both incoming and outgoing interference. In the case of equipment permanently contained in a housing, effective shielding can be accomplished through proper construction of the housing. It is difficult to shield effectively, however, equipment having access panels or doors. This is because the seams between the access panels or doors and the housing body provide natural escape and entry routes for electromagnetic interference.

In the case of metal housings, seams can also inhibit the beneficial Faraday Cage effect by presenting conductivity discontinuities in the electrically conductive surfaces of the housings. Moreover, these discontinuities can cause the seams to act as slot antennae and result in the housing becoming a secondary source of EMI radiation. The seams also reduce the efficiency of the ground conduction path. In the case of electrical apparatus housings which include seams, therefore, an effective shielding mechanism should be configured both to provide sufficient EMI shielding characteristics and to allow proper closure of the door or access panel. Various configurations of gaskets have been developed for serving these purposes which are intended to provide maximum EMI shielding, while minimizing the force required to close the door or access panel.

To present an effective shield to electromagnetic interference, a gasket should be capable of absorbing and/or reflecting electromagnetic interference and of establishing as continuous an electrically conductive path as possible across the seam in which the gasket is positioned. Typically, metallic structures are used for EMI shielding because of their high degree of electrical conductivity. Because that conductivity is not infinite, however, part of the electromagnetic field being shielded is transmitted across the shield and supports a current in the shield. Accordingly, an effective EMI shielding gasket should provide an electrically conductive path for dissipating this current to ground, otherwise the current could serve to radiate an electromagnetic field on the other side of the shield. Known EMI shielding gaskets have been unable to combine effectively these qualities with the other requirements of suitable gaskets, such as resisting compression set while being soft and yielding so as to present a consistent, minimal closure resistance. Compression set refers to a gasket's ability to resume its initial shape after having been subjected to a compressive load. Another important characteristic of EMI shielding gaskets is that they not break down due to galvanic corrosion, such as can occur when dissimilar metals are in contact with one another.

Accordingly, it is an object of the invention to provide an EMI shielding gasket which affords improved shielding of electromagnetic interference passing through seams in the housings of electronic apparatus. Another object of the invention is to provide such a gasket which presents an acceptable level of resistance to the closing of doors and access panels in those housings. Yet another object of the invention is to provide an EMI shielding gasket which enables the use of a wide variety of shielding materials.

Summary of the Invention

These and other objects are achieved by the present invention which features a gasket for shielding electromagnetic interference passing through a seam between two electrically conductive bodies. The gasket includes an electrically conductive, resilient foam profile which is typically formed of an extruded thermoplastic elastomeric material and an embedded EMI shield for reflecting and/or absorbing electromagnetic interference.

An electrically conductive material, such as carbon particles, is blended with the thermoplastic elastomer in a proportion sufficient so that the foam profile is electrically conductive enough to be at least electrostatic dissipative. In most embodiments, however, the profile is not electrically conductive enough to act independently as an effective electromagnetic interference shield. This is because increased electrical conductivity of the profile comes to a degree, at the expense of increasing the profile's stiffness. Also, a high degree of conductivity is not necessary for dissipating absorbed energy. Accordingly, electrically conductive material is blended with the profile material to an extent sufficient so that energy absorbed by the shield is dissipated without unnecessarily increasing the profile's stiffness so as to unacceptably increase door or access panel closure force requirements. A variety of materials and configurations are suitable for use as the electromagnetic interference shield, such as copper metallized nonwoven fabrics, metal foils, strands of various types of metal, and metallized yarns. Combinations of these materials and configurations may be utilized as well.

Where the EMI shield is formed of metal strands or metallized yarns, the strands or yarns can be variously arranged in many ways within the foam profile for optimizing EMI shielding. For example, while in some embodiments it may be desirable to arrange EMI shielding strands along a straight line, in other applications the strands may follow a jagged or curved path. Specific arrangements are selected based upon considerations such as the degree of EMI shielding needed and the expense of utilizing a given shielding material and configuration. Clearly, while more shielding will be provided by increasing the amount of shielding material, this will typically also increase the expense of manufacturing the gasket. Advantageously, therefore, the invention teaches that effective EMI shielding can be achieved with a minimal amount of properly configured shielding material.

In some embodiments, the inventive EMI shielding gasket includes a protective skin which surrounds all or part of the foam profile. The skin is abrasion resistant and, therefore, protects the foam profile against mechanical wear. This is useful in applications in which the gasket is subjected to friction caused by contact with moving parts, such as where the gasket interacts with sliding doors or access panels. Additionally, the skin protects the foam profile from corrosive environmental elements which may be present such as moisture.

In some embodiments of the invention the abrasion resistant skin is electrically conductive. For example, in situations in which the the skin is interposed between the electrically conductive foam profile and the electrically conductive body of the equipment with which the gasket is being used, it is usually desirable that at least the interposing portion of the skin is electrically conductive. This enables energy absorbed by the EMI shield to be dissipated to the grounded electrically conductive body

In applications where the EMI shield reflects most of the passing electromagnetic interference, as opposed to absorbing it, however, the ability of the shield to dissipate absorbed energy to ground is less critical. Accordingly, in such applications it is acceptable to interpose a non-electrically conductive, abrasion resistant skin between the electrically conductive foam profile and the seam-defining electrically conductive body.

In some embodiments of the invention the gasket includes a relatively rigid carrier coupled to the foam profile for securing the gasket to one of the electrically conductive bodies. The carrier is rigid in relation to the foam profile and may include different types of mechanisms such as arms and/or barbs for enabling attachment of the gasket to a flange or channel of the electrically conductive body. If necessary for energy dissipation or the preservation of electrical conductivity continuity, the carrier may be formed of an electrically conductive compound. These and other features and objects of the invention will be more fully appreciated by reference to the following detailed description which is to be read in conjunction with the attached drawing.

Brief Description of the Drawings FIGURE 1 is a perspective view of an inventive EMI shielding gasket including an EMI shield embedded in an electrically conductive foam profile,

FIGURE 2 is a perspective view of an inventive EMI shielding gasket including another type of EMI shield embedded in an electrically conductive foam profile,

FIGURES 3A through 3C are cross-section views taken along line 3-3 of FIGURE 2 depicting several embodiments of the invention shown in FIGURE 2,

FIGURE 4 is a cross-section view of an inventive EMI shielding gasket arranged between two electrically conductive bodies and including an abrasion resistant skin surrounding an electrically conductive foam profile,

FIGURE 5 is a cross-section view of an inventive EMI shielding gasket including an electrically conductive foam profile attached to a relatively rigid carrier, and

FIGURES 6A through 6E are cross-section views depicting inventive EMI shielding gaskets having a variety of foam profile configurations.

Detailed Description

As stated, the invention features a gasket for shielding electromagnetic interference passing through a seam between two electrically conductive bodies. The gasket includes an EMI shield embedded in an electrically conductive foam profile, which construction provides many advantages, several of which are discussed below.

One embodiment of the invention is shown in FIGURE 1 which depicts in perspective an EMI shielding gasket 10 including a resilient foam profile 12. The foam profile 12, and thereby the gasket 10, has a circular cross-section and is elongated along an axis L. The gasket 10 further includes an EMI shield 14 embedded in the foam profile 12. While typical gaskets include foam profiles ranging in size from approximately 0.1 inches to 0.5 inches in diameter, it will be clear to those ordinarily skilled that gaskets encompassing the features of the invention can be formed in a variety of sizes and profile configurations. Indeed, gaskets have been constructed for some applications having foam profiles as small as 0.05 inches in diameter and as large as one inch in diameter. ,

An advantage of the extruded foam profile of the invention is that it can be manufactured with great versatility. Known EMI shielding gaskets which are continuously molded such as polyurethane type gaskets, and the gaskets described in United States patents nos. 5,045,535 and 4,857,668 can not be manufactured with as much versatility because with molded gaskets the entire molding process must be changed to produce profiles of different shapes. Manufacture of extruded profiles, on the other hand, is very flexible because profile changes are made simply by changing the extrusion die. In most applications, two components are desired for proper EMI shielding, a shielding mechanism for reflecting and/or absorbing electromagnetic interference, and a conductive path to ground for dissipating energy absorbed by the shield. While grounding of the gasket enhances its shielding capacity, it may not be absolutely essential in applications involving the shielding of a high impedance wave (high electric field) with a low impedance shield (metal) because in such cases most electromagnetic energy is reflected rather than absorbed. Conversely, for shielding of low impedance waves (high magnetic fields) low impedance shields such as metal absorb a lot of energy, which energy should be dissipated to ground. Otherwise, the energy absorbed by the shield can become so great that it arcs in the form of electrostatic discharge from the shield to a nearby electrical conductor. If the absorbed charge arcs to a circuitry component of the electrical apparatus the gasket is supposed to be protecting from electromagnetic interference, serious damage to the apparatus can occur.

It is a significant feature of the invention, therefore, that the foam profile 12 is formed of a material which is electrically conductive. For example, an electrically conductive grade of

Santoprene™ brand resin, which is a thermoplastic elastomer resin sold by Advanced Elastomer Systems Company, can be used to form the foam profile 12. Santoprene™ is a thermoplastic rubber (elastomer) which may be foamed with a blowing agent to produce extruded profiles having a substantially closed cell foam structure. The characteristics and properties of Santoprene™ are described in United States patents nos. 4,130,535 and 4,311,628 assigned to Monsanto Company. It will be apparent, however, that other thermoplastic elastomer resins can be used as well for forming the gaskets of the invention.

Still, suitable samples embodying the invention have been made by foam extruding 199-89 grade Santoprene™ with water as a blowing agent to a density of 0.323 grams per cubic centimeter (g/cc). As indicated by its designation, this resin has a Shore A durometer hardness value of 89. Resins of various hardness values can be used for producing foam profiles having hardness values which vary analogously. Details of grading the hardness values of elastomers are described in ASTM D2240.

Resin hardness values and extruded foam density combine with profile configuration to determine a gasket's sealing qualities. As discussed above, to effectively seal a seam in a housing against environmental elements, a gasket should be soft and yielding so as to present a minimum of closure resistance to a door or access panel. The gasket must also show good compression set resistance so that it resumes its original shape after the removal of compressive forces, such as during opening and closing of a door or access panel. To achieve these objects, the foam profiles of the invention should be formed of a resin having a Shore A durometer hardness value between 30 and 90, preferably between 50 and 70 Shore A durometer hardness scale. Moreover, their extruded density should be between approximately 0.1 g/cc and approximately 0.5 g/cc, preferably between approximately 0.15 g/cc and approximately 0.25 g/cc. As noted, the starting thermoplastic elastomer resin is blended with an electrically conductive material so that the extruded foam profile 12 is itself electrically conductive. Electrically conductive carbon particles are suitable for this purpose and a blend of approximately 85% by weight resin with approximately 15% by weight carbon particles has been found to produce an acceptable compound.

The EMI shield 14 may comprise a variety of materials and configurations. For example, in one embodiment the shield 14 consists of a membrane formed of a polyester non-woven fabric which has been metallized on both sides with copper. Other commonly known materials such as metallized yarns and metal foils may be used as well. Examples of appropriate foils include aluminum, tin, silver and nickel. An advantage of the invention, such as the embodiment shown in FIGURE 1, is that the EMI shield 14 does not come into direct contact with the housing of the electronic apparatus with which the gasket is being used. As a result, there are no concerns of galvanic corrosion stemming from the intimate contact of dissimilar metals and, therefore, few restrictions with respect to the materials which can be used to form the shield 14. Accordingly, in appropriate applications materials having magnetic properties may be used. Additionally, the embedded shield 14 is insulated from environmental elements which can result in oxidation and other forms of decay. Another embodiment of the invention is shown in FIGURE 2 in which an EMI shielding gasket 20 includes an electrically conductive resilient foam profile 22 and a number of embedded strands 24. The strands 24 extend through the length of the profile 22. Copper wire approximately 0.005 inches in diameter can be used for the strands 24 and provides an inexpensive, effective shield to electromagnetic interference. Other acceptable strands include steel wire with copper and/or tin plating. FIGURES 3A through 3C depict the gasket 20 in cross-section and show some of the ways in which the strands 24 can be embedded in the foam profile 22. As illustrated, the strands 24 may be arranged to be parallel to the longitudinal axis L (FIGURE 3A) or they can be parallel to one another but follow a curved path along the gasket's length (FIGURE 3B). Additionally, the strands can be embedded in the foam profile 22 so as to intersect with one another as they follow a tortuous path along the gasket's length (FIGURE 3C). Many configurations for the strands 24 will be apparent and a given configuration can be selected based upon considerations such as the amount of EMI shielding needed and the expense of the configuration.

An embodiment of the invention is shown in FIGURE 4 which depicts an electromagnetic interference shielding gasket 40 arranged between two electrically conductive bodies Bl and B2 and including an electrically conductive foam profile 42 and an embedded EMI shield 44. The gasket 40 differs from those discussed above in that the gasket 40 further includes an abrasion resistant skin 46 which surrounds the foam profile 42. The skin 46 protects the foam profile 42 in applications where the profile 42 is arranged in contact with moving parts or where the profile 42 is exposed to potentially harmful environmental elements such as moisture.

Typically, the abrasion resistant skin 46 is electrically conductive so that any electrical current flow between the electrically conductive foam profile 42 and the housing of the electrical apparatus with which the gasket is being used is not interrupted. Additionally, if the skin is electrically conductive it serves to preserve continuity of electrical conductivity across the housing seam in which the gasket is arranged, thereby preserving the benefits of the Faraday Cage effect as discussed above. The skin 46 can be applied to the profile 42 with a standard crosshead coating die mounted on an extruder. Commonly assigned United States patent application serial no. 07/650,974 to Mertinooke et al., the teachings of which are hereby incorporated herein by reference, described a suitable method and apparatus for this purpose. Various materials such as thermoplastic elastomers and, if necessary, electrically conductive additives, can be used as the skin 46.

For some applications the skin 46 does not need to be electrically conductive such as, for example, where the skin 46 does not completely surround the foam profile 42 but, rather, only covers portions of the profile 42. As long as some point of the profile 42 is arranged for direct contact with the apparatus housing, energy absorbed by the EMI shield 42 can be dissipated through that point. Additionally, as stated above, if the gasket of the invention is used in an application where little energy is absorbed by the EMI shield 42, no.conductive path to ground is required and a dielectric skin can be used.

Still another embodiment of the invention is shown in FIGURE 5 which depicts in cross-section an EMI shielding gasket 50 including a carrier 58. The carrier 58 is generally rigid relative to the resilient foam profile 52. For example, polypropylene is often used for forming such carrier structures. The carrier 58 can be formed in a variety of profiles for anchoring the gasket 50 to appropriately analogous substrates or flanges. If it is needed to provide a conductive path to ground for the dissipation of energy absorbed by the EMI shield 54, the carrier 58 can be formed of an electrically conductive compound.

FIGURES 6A through 6E depict in cross-section a variety of gaskets 62A through 62E having variously configured profiles 62A through 62E. Metal strands 64 are embedded in each of the profiles for EMI shielding. The depicted profiles represent merely a small sample of the possible configurations which can be utilized in connection with the inventive gaskets. The profiles shown in the figures, as with any of the foam profiles discussed herein, can be formed by way of a number of known extrusion techniques. For example, the commonly assigned United States patent no. 4,898,760 to Halberstadt et al., the teachings of which are hereby incorporated herein by reference, describes a suitable process. Accordingly, the process is not described here in great detail.

Generally, however, to extrude a profile, a thermoplastic elastomer resin is pumped with a blowing agent such as water or a chlorofluorocarbon, at high temperature through a suitable extrusion die. The resin and blowing agent are thoroughly mixed together under pressure and travel through an adaptor prior to passing into the extrusion die. Upon exiting the die, the pressure of the mixture rapidly drops, thereby engendering a substantially closed cell structure having a cross-section matching that of the die, such as that shown in any of FIGURES 6A through 6E. , After extrusion, the profile is cooled using, for example, a standard conveyor system with a heat resistant belt. Typically, the conveyor belt moves at a line speed of between approximately forty and eighty feet per minute.

The metal strands 64 can be fed through the extrusion die with the resin by utilizing a standard crosshead die arrangement. Other configurations of EMI shields can be embedded in the profile this way as well. Alternatively, the profile can be extruded with a cleft into which an EMI shield is placed after extrusion. The cleft would then be sealed around the shield using a hot melt adhesive such as the Numel 5430 brand which is a polypropylene based adhesive sold by the Baychem Company of Houston, Texas.

While various embodiments of the invention have been set forth in detail, it should be understood that the above description is intended as illustrative rather than limiting and that many variations to the described embodiments will be apparent to those skilled in the art. The invention is to be described, therefore, not by the preceding description, but by the claims that follow.

What is claimed is.

Claims

1. A gasket for shielding electromagnetic interference passing through a seam between first and second electrically conductive bodies, the gasket comprising an electrically conductive, extruded foam profile elongated along a longitudinal axis and suitable for being secured to one of the electrically conductive bodies, and a shield embedded in the foam profile for shielding electromagnetic interference passing through the seam.

2. A gasket as set forth in claim 1 wherein the shield comprises a metallic strand.

3. A gasket as set forth in claim 1 wherein the shield comprises a plurality of metallic strands arranged substantially parallel to one another within the foam core.

4. A gasket as set forth in claim 1 wherein the shield comprises a plurality of metallic strands arranged to intersect with one another within the foam core.

5. A gasket as set forth in claim 1 wherein the shield comprises a metallized fabric.

6. A gasket as set forth in claim 1 further comprising attachment means coupled to the foam core for securing the gasket to the first conductive body.

7. A gasket as set forth in claim 1 further comprising an abrasion resistant skin covering at least part of the resilient foam profile.

8. A gasket as set forth in claim 7 wherein the abrasion resistant skin is electrically conductive.

9. A gasket as set forth in claim 1 wherein the resilient foam profile comprises a thermoplastic elastomeric material.

10. A gasket as set forth in claim 9 wherein the resilient foam profile is of a density between approximately 0.1 grams per cubic centimeter and approximately 0.5 grams per cubic centimeter.

11. A gasket as set forth in claim 9 wherein the resilient foam profile is formed of a resin having a hardness value less than approximately 90 on a Shore A durometer hardness scale.

12. A gasket as set forth in claim 11 wherein the resilient foam profile is formed of a resin having a hardness value between approximately 30 and approximately 90 on a Shore A durometer hardness scale.

13. A gasket as set forth in claim 1 wherein the resilient foam profile is formed of a resin having a hardness value between approximately 50 and approximately 70 on a Shore A durometer hardness scale.

14. A gasket as set forth in claim 1 wherein the foam profile has a maximum width dimension perpendicular to the longitudinal axis between approximately 0.05 of an inch and approximately one inch.

15. A gasket for shielding electromagnetic interference passing through a seam between first and second electrically conductive bodies, the gasket comprising an extruded profile of electrically conductive thermoplastic elastomeric material, the profile being being elongated along a longitudinal axis and adapted for attachment to one of the electrically conductive bodies, and an shield embedded in the profile and extending in a direction parallel to the longitudinal axis for shielding electromagnetic interference passing through the seam.

16. A gasket as set forth in claim 15 wherein the shield comprises a metallic strand.

17. A gasket as set forth in claim 15 wherein the shield comprises a plurality of metallic strands arranged substantially parallel to one another within the foam core.

18. A gasket as set forth in claim 15 wherein the shield comprises a plurality of metallic strands arranged to intersect with one another within the foam core.

19. A gasket as set forth in claim 15 wherein the shield comprises a metallized fabric.

20. A gasket as set forth in claim 15 further comprising attachment means coupled to the foam core for securing the gasket to the first conductive body.

21. A gasket as set forth in claim 15 further comprising an abrasion resistant skin covering at least part of the resilient foam profile.

22. A gasket as set forth in claim 21 wherein the abrasion resistant skin is electrically conductive.

23. A gasket as set forth in claim 15 wherein the profile is of a density between approximately 0.1 grams per cubic centimeter and approximately 0.5 grams per cubic centimeter.

24. A gasket as set forth in claim 15 wherein the profile is formed of a resin having a hardness value less than approximately 90 on a Shore A durometer hardness scale.

25. A gasket as set forth in claim 24 wherein the profile is formed of a resin having a hardness value between approximately 30 and approximately 90 on a Shore A durometer hardness scale.

26. A gasket as set forth in claim 15 wherein the resilient foam profile is formed of a resin having a hardness value between approximately 50 and approximately 70 on a Shore A durometer hardness scale.

27. A gasket as set forth in claim 15 wherein the foam profile has a maximum width dimension perpendicular to the longitudinal axis is between approximately 0.05 of an inch and approximately one inch.