Gasket junction design and selection requires an understanding of the concepts of electromagnetic interference. Gasket design fails when the designer does not treat the mating surface with the same attention as the selection of the gasket material.
A gasketing surface that is rigid, as conductive as possible, and recessed to house the gasket is ideal. If the reaction to the environment is inevitable, the reaction products should also be electrically conductive or permeable by mechanical abrasion.
A gasket is necessary when there is an imperfect mating surface, and the more imperfect the surface, the more critical the gasket. Since the perfect mating surface is expensive, designers must consider cost and performance in gasket design.
For reasons of economy, weldability, strength, and resistance to corrosion, flanges are usually made of the same material as the enclosure. If possible, the flange should also be made of the highest conductivity materials. Paint can be applied to outside surfaces, but the flange mating surfaces should not be painted.
Usually, the more highly conductive the material, the more susceptibility there is to oxidation due to the hardness of the oxides. Some oxide layers stay thin while others build up in thickness quickly. This buildup forms insulating between the gasket and flanges but can be overcome by using slow-oxidizing materials or coatings, such as nickel, tin, or zinc.
Advantages of Grooved Designs
Gaskets tend to over-compress in the areas of the bolts since flange surfaces cannot be held uniformly. Proper groove design can remedy this problem. A groove can also reduce contact resistance by providing metal-to-metal contact between flange members.
While a single groove will be sufficient for most designs, adding a second groove increases the overall performance. Adding more than two grooves, however, does not significantly increase the gasketing effectiveness.
Flange Design Considerations
Designers are often faced with space limitations, particularly with the gasket seam. The junctions are often made more compact by adding complex fasteners.
The ideal flange includes a groove for the fastener to limit the deflection of a gasket. The need for gaskets under the fasteners is eliminated by mounting the screw or bolt fasteners outboard of the gasket.
Grooves should be held to a machined tolerance of 0.002 in., while holes drilled into machined parts should be held to within a 0.005 in. tolerance. Location of punched holes should be within a 0.010 in. tolerance.
The three factors that must be considered with waveguide flanges are to ensure maximum transfer of electromagnetic energy across the flange interface, to prevent RF leakage, and to maintain pressurization of the waveguide. For both an electrical and seal function, conductive elastomeric gaskets are the choice. For flat cover flanges, a die-cut sheet gasket is best. The die-cut sheet gasket incorporates expanded metal reinforcement that provides an excellent seal to control gasket creep into the wavelength opening.
The peak power handling abilities of wavelength flanges are limited by misalignment and sharp edges of flanges and/or gaskets. Average power handling is minimized by the heating effects caused by junction resistance, or the contact resistance of the flange-gasket interface.
Gasket junction design and selection necessitates that every effort is made in understanding the concepts of electromagnetic interference. Gasket design fails when the designer does not select the most rigid material possible for the application while still considering any design constraints or specifications. The resistance between the flange and gasket should be as low as possible to guarantee the best design for each project.