Patent Application
20080083562
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
Turning now to the drawings in greater detail, it will be seen that in
In sharp contrast to the gasket 12 affixed to a major surface defining the panel 10 of
More specifically, EMC gasket 112 includes a silicone foam core 130. An entire outer surface defining the silicone foam core 130 is covered with an electrically conductive fabric 132. In exemplary embodiments, the conductive fabric is composed of a polyester, polyamide or other suitable thermoplastic resin or cloth overplated with either nickel or silver
The panel 100 is an electrical housing substrate molded of a thermoplastic having an electrically conductive network of fibers above a percolation limit of the fibers. The thermoplastic is a base resin including the electrically conductive network of fibers. In exemplary embodiments, the conductive fibers include stainless steel fibers, for example, but is not limited thereto. The thermoplastic includes Faradex®. Faradex® is a ready-to-mold thermoplastic commercially available from LNP (a GE Plastics Company) that consists of a base resin plus highly conductive stainless steel fibers. The fibers form an electrically conductive network above the percolation limit. Coupled with their high aspect ratio, Faradex fibers provide adequate EMI shielding at very low filler levels (on the order of 0.7-1.4 vol. %).
Faradex® products are available in a blend of polycarbonate and acrylonitrile butadiene styrene (PC/ABS) yielding high strength and high flow for thinwall molding PC/ABS blends, the workhorse resin for business equipment and electronic housings. Covers or enclosures molded from Faradex® PC/ABS blends provide an electrically conductive substrate to which a traditional EMC gasket can be mated. By insert molding a silicone core, fabric-over-foam gasket into electrical housings molded from Faradex®, traditional PSA backed EMC gaskets can be eliminated. An integrated gasket provides greater surface contact with the enclosure (e.g., along the entire bottom surface 120 of the gasket 112, as well as partially up the sidewalls 122) and virtually eliminates the potential for shear failure of the gasket. With a traditional PSA-backed gasket, the shear force only needs to be greater than the PSA bond strength (typically less than 1 lb/in width). In order to shear an exemplary integrated gasket 112 from the enclosure 100, the tear strength of the fabric 132 and foam core 130 must be exceeded. The tear strength of the gasket 112 is well over an order of magnitude greater than PSA bond strength of the conventional gasket 12. However, if additional retention strength is required, the profile of both the gasket and the molded cavity can be modified in order to meet design specifications.
Faradex® compounds provide electromagnetic and radio frequency interference (EMI/RFI) attenuation in applications from electronics to material handling. Conductive fibers form the conductive network required for EMI/RFI shielding (e.g., EMI/RFI shielding capabilities between 40-60 dB and higher from 30-1000 MHz). Faradex® compounds can also be used in applications where electrostatic discharge (ESD) is required. Faradex® compounds provide mechanical properties, part weight and a design freedom similar to standard unfilled base resins. They avoid costly secondary steps, offering total system cost reduction.
Accomplished with electrically conductive stainless steel fibers at modest loading levels, the panels 100 formed of Faradex® compounds can also be used in applications where electrostatic discharge (ESD) is required. The mechanical properties of Faradex® compounds are similar to standard unfilled base resins. If needed, additional glass or carbon fiber reinforcement is available to enhance strength and stiffness or control mold shrinkage. In addition, Faradex® compounds can be compounded for flame retardancy (FR), including non-halogenated FR compounds.
Use of Faradex® compounds helps control costs in that EMI/RFI attenuation occurs throughout the part, thereby eliminating the need for secondary conductive coatings (e.g., a conductive paint or vacuum deposited metallization), or attachment of conductive fabrics or sheet metal. Design freedom of injection molding offers advantages for intricate part design, where metallic sprays or conductive netting are far less effective. Because the conductivity permeates the entire part, it cannot be disrupted by surface scratches or nicks.
In order to minimize a resin-rich surface and ensure that the surface resistance values of Faradex® are low enough to provide adequate grounding of the in-mold gasket, the Faradex® can be molded on the cool side of the melt specification and the mold temperature can be decreased in order to freeze out the resin prior to packing it on the surface. Such trivial process modifications can decrease the surface resistivity two orders of magnitude. In this case, the surface of the part provides adequate grounding in the as-molded state (i.e., no post-molding, secondary operation is required to expose the stainless steel fibers to ensure grounding). Further, if it is deemed necessary, features can be incorporated into the mold to disrupt the melt front to create a fiber-rich surface. Alternatively, a snap-off runner or zipper can be molded into the gasket channel which, upon removal, would expose the fibers and guarantee intimate metal-to-metal contact (e.g., between gasket 112 and panel 100). Should this be required, the gasket must be inserted following the molding process.
It will be recognized that in alternative exemplary embodiments that the foam core material may include silicone, polyether urethane, polyester urethane, ethylene propylene diene monomer (EPDM), thermoplastic elastomers, or a combination including at least one of the foregoing materials. Further the plastic used for the housing substrate may include polycarbonate, acrylonitrile butadiene styrene (ABS), polyvinyl chloride (PVC), Noryl (polyphenylene oxide), polyamides or combinations of at least one of the foregoing materials. Lastly, the conductive network of fibers may include stainless steel, nickel powder/flake, carbon fiber, carbon nanotubes, silver (Ag) powder/flake or combinations of at least one of the foregoing materials.
Utilizing the above approach to form an exemplary integrated EMC gasket with an electrical enclosure results in a multitude of benefits. First, elimination of the PSA from traditional EMC gaskets provides a part cost savings, as well as reliability enhancement by eliminating potential for corrosion of metal-coated fabric in high temperature and humidity environments. Secondly, the exemplary integrated EMC gasket eliminates a secondary operation of application of the gasket to the substrate via insert molding. Thirdly, the exemplary integrated EMC gasket eliminates expensive plastic metallization processes (e.g., either vacuum metallization, plating, or conductive paint). Fourthly, the exemplary integrated EMC gasket provides enhanced reliability as the insert molded gasket has a tear strength much greater than the bond strength of a conventional PSA-backed gasket. Fifthly, the exemplary integrated EMC gasket provides increased EMI shielding due to greater surface contact with the electrical enclosure having a recess to receive the integrated EMC gasket therewith. By integrating a gasket into the enclosure, surface contact with the enclosure is increased (thereby increasing gasket effectiveness) and the tendency to shear the gasket from the enclosure is effectively eliminated or at least substantially reduced. Lastly, the exemplary integrated EMC gasket enhances design flexibility, as the insert molded gasket can be positioned around bosses, standoffs, retaining features, etc.
While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.
