Abstract:
An improved gasket shielding for electronic equipment is provided in the form of an integral two-layer component, namely a support layer and a cover layer, the support being strong and having excellent physical properties, and the cover layer being metal filled and having excellent electrical energy attenuation properties.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the shielding of electronic equipment and, more particularly, to gaskets for cabinet enclosures, which cabinets are adapted to receive electronic equipment, the gaskets serving to shield the electronic equipment from damage due to exterior electromagnetic radiation. 
     BACKGROUND OF THE INVENTION 
     Producers of commercial electronic equipment, not to mention defense applications, are being forced to improve the performance of shielding devices to counteract electromagnetic interference complicated by the ever higher frequencies of energy generation in and beyond the megahertz range. Openings in cabinet enclosures for access doors, hinges, etc. must be so protected by the use of suitable shielding. 
     This has been an ongoing problem and various solutions have been proposed, these prior solutions however being unsatisfactory from one or more points of view. Thus, the prior devices have been insufficient from point of view of at least one or more of the following requirements: 
     1. Attenuation of internal and external interference using gasketing with resistive values within the range of 0.1 to 0.001 ohm-cm and selectable attenuation from 30 to 90 dB. 
     2. Low compression set of the gasket so long term mechanical sealing is maintained. 
     3. Superior physical properties of the gasket to prevent failure due to tearing forces. 
     4. Resistance to reduced electromagnetic shielding due to oxidation. 
     5. Ease of cleanliness maintenance. 
     6. Freedom from free metal chips as occurs with the initial fitting and subsequent deterioration of metallic meshes and metal assemblies. 
     7. A neat appearance of the shield. 
     8. Ease of installation and replacement. 
     9. Low cost relative to alternate designs. 
     There are presently available four different types of cabinet shielding means which suffer one or more defects. These are as follows: 
     
         ______________________________________ Product              Principal Inadequacy______________________________________1.    Carbon Filled Elastomers                    Shielding ability falls                    off in the higher                    frequency range2.    Metal Filled Elastomers                    Poor physical properties                    (compression set, tear,                    etc.)3.    Metal Mesh with    Hard to cut cleanly for Elastomeric Core   corners; chips of free                    metal4.    Metallic Seals     Difficult to keep clean, (Spring-like fingers)                    oxidation______________________________________ 
    
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to overcome deficiencies in the prior art such as indicated above. 
     It is another object to provide an improved gasket shielding material for electronic equipment. 
     It is yet another object of the present invention to provide dual elastomer gasket shielding for counteracting electromagnetic interference. 
     These and other objects are achieved according to the present invention by the provision of an elastomeric gasket formed in two layers, preferably by dual extrusion, in the form of a support layer having high strength, and a relatively thin coating layer having excellent ability to counteract or attenuate electromagnetic radiation. 
     The above and other objects and the nature and advantages of the present invention will be more apparent from the following detailed description of certain specimens embodiments thereof, taken in conjunction with the drawing, wherein: 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     FIG. 1 is a perspective view of an EMI gasket configuration to which the subject of the present invention may be applied; 
     FIGS. 2a-2f are end views of other possible configurations, FIG. 2d being somewhat similar to the embodiment of FIG. 1; and 
     FIG. 3 is an enlarged partial cross-sectional view of the gasket of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1 shows an EMI gasket 10 of typical configuration, but in accordance with the present invention, having a metal clip part 12 and a rubber gasket part 14 adhered together. FIGS. 2a-2f show various other typical forms which may be used. 
     In accordance with the present invention, the gasket part 14 is formed of a non-metallic, yet conductive, elastomeric inner portion 16 to which is permanently bonded, preferably by co-extrusion within the die, and outer sheathing or covering 18 of metal-filled elastomer. From the embodiment of FIGS. 1, 3, 2a, 2b, and 2d-2f, it will be understood that such a gasket may be co-extruded and bonded during extrusion onto metallic and conductive non-metallic shapes for convenient assembly to cabinets. From FIG. 2c, it will be understood that the gasket material can be provided in other shapes which can be subsequently attached to a suitable substrate. In general, the extrusions are made in continuous length, shaped to any desired uniform cross-section. 
     The product can be formed onto a metallic carrier, for example a flat metal strip (see FIGS. 2a, 2e and 2b) or a roll-formed shape (FIGS. 2d and 2f) by passing the metal substrate through the co-extrusion tooling in a continuous process. The inner portion 16 is chemically locked to the metallic layer, thus ensuring transmission of electrical energy un-impeded between the co-elastomer and the metallic carrier. 
     The inner layer 16 is constructed of a strong elastomeric material which has good physical properties, including high tear resistance, as well as the ability to recover shape after deformation, i.e. good elasticity. This material is desirably carbon filled rubber. 
     The metal filled exterior covering 18 provides a higher degree of attenuation of electrical energy than presently available non-metal filled elastomeric shielding. On the other hand, such metal filled elastomer used alone as a shielding material suffers from poor physical properties, including low tearing resistance, and its recovery of shape after deformation is poor. The metal filler does not reinforce the elastomer and acts as a diluent. Furthermore, as the cost of the metal, usually silver, is relatively high, it is desirable to minimize the quantity of the material used. 
     By chemically bonding a metal-filled elastomer 18 to a carbon-filled elastomer substrate 16, the amount of expensive metal filler used is minimized and the tear resistance and compression recovery of the gasket 14 substantially improved. In addition, the shielding effectiveness is substantially improved over the use of carbon-filled rubber alone. The product is easily cleaned to maintain shielding effectiveness and presents a neat appearance. 
     When the two elastomers 16 and 18 are simultaneously bonded under the pressure of co-extrusion, the unity of the resultant gasket is assured and electrical and physical results are optimum. 
     It will be understood that both layers 16 and 18 are conductive, except that the outer layer 18 is considerably more conductive while the inner layer 16 is less conductive but much stronger and with better physical properties. They are bonded by co-extrusion in a molecular sharing way, and thus provide a superior electrical shield over either one used alone. 
     The inner layer 16 is substantially thicker than the outer coating, preferably on the order of 5-20 times as thick when such layer 16 is hollow as preferred, preferably at a thickness ratio of 10:1, the outer coating 18 desirably being 2-8 mils thick and the inner support layer 16 being 5-160 mils thick, preferably 20-80 mils thick. 
     The inner support layer may be formed of a variety of elastomers, but preferably it comprises an elastomeric silicone polymer containing fumed silica and carbon black as reinforcers, plus a suitable cross-linking agent. Alternatively, the inner layer 16 may be composed of a non-conductive silicone elastomer, such as one comprising silicone gum, fumed silica and a cross-linking agent. The covering layer 18 is suitably formed of an elastomeric silicone polymer filled with glass or copper beads coated with silver. 
     The following Example is offered illustratively. 
     EXAMPLE 
     Suitable materials from which to form the inner and outer cores are set forth below in Table 1, and the properties of the resultant gasket according to the present invention are set forth in Table 2 below. 
     
                       TABLE 1______________________________________       INNER CORE   OUTER CORE______________________________________Silicone Polymer         100 parts by weight                       100 parts by weightFumed Synthetic         30 parts by weight                       0SilicaCarbon Black, 20 parts by weight                       0Ketjen E300Silver Coated Copper         0             70 parts by weightDiCumyl Peroxide         2 parts by weight                       2 parts by weightCurative, 90% ActiveColorant                    SelectableMetal Bonding Agent         as applicable 0______________________________________ 
    
     
                       TABLE 2______________________________________Typical Combined Properties of Silicone &amp; Carbon WithSilver-Silicone Co-Extrusion Cover______________________________________Volume Resistivity,    0.02 ohm-cmASTM D-91Shielding Effectiveness,                  85 MinimumdB at 10 GHz Plane WaveSpecific Gravity, combined                  1.25Tensile Strength, psi  700Elongation, %          165Tear Strength, lb/inch 67Durometer Hardness, Shore A                  60Compression Set        24%Method B-22 Hrs. @ 212° F.______________________________________ 
    
     The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.