Andy Pye discovers how solid silicone rubber flat sheet EMI gaskets embedded with Monel or aluminium metal wires can achieve RFI/EMI/EMP and environmental sealing in a single gasket.
Excellent RFI/EMI/EMP shielding performance is achieved, as the material has a wire density of up to 140 wires/cm2 in solid silicone, providing an environmental and RFI/ EMI seal when clamped between two metallic surfaces.
These wires are crimped to aid compression and are chemically bonded to ensure their retention. They will also penetrate most light oxide layers, ensuring a low contact resistance path when suitably compressed.
For applications with severe joint unevenness, low closure forces and where greater compressibility is required, Kemtron has developed new grades 470/480 of material to match the performance of grades based on orientated wire in silicone sponge but with improved environmental sealing qualities. This is achieved by using a very soft solid silicone with a reduced wire count of 100 wires/cm2, the same as silicone sponge grades.
The advantages of these materials over silicone sponge are that it is possible to manufacture sheet widths of 225mm with a minimum thickness of 0.8mm. Material consistency is a great advantage over silicone sponge, as there is no uneven cell structure to consider – this can affect moisture ingress and closure force.
Kemtron offers a wide variety of options and thicknesses, as the material is cut in-house from large blocks, up to 225mm wide, using a high-speed slicer: die- cut gaskets; large fabricated gaskets; virtually any flat shape; sheet material; strip material in continuous lengths; and with self-adhesive backing to allow for easy assembly. The material can be fitted with compression limit stops or collars and small gaskets can be punched in one operation, keeping production costs to a minimum.
Larger gaskets can be produced cost effectively and without the constraint of sheet size limitation from strips of material fabricated into the required finished shape, thus avoiding waste material from the centre of the gasket.
* It is important that this material is not over-compressed. If the design of the equipment does not allow for any mechanical method of preventing over-compression, the gasket should be fitted with built-in compression limiters, either metal stops fitted to the gasket, or metal collars fitted around each fixing hole.
* There is no need for a conductive connection where strips are joined as the wires forming the EMC contact run vertically though the material; a waterproof seal is achieved by vulcanising the join with silicone.
* The material is not suitable for frequent opening or sliding applications.
* Recommended compression: Solid – 15% to 20%, Sponge – 15% to 25%.
* Fluorosilicone: self-adhesive backing is not recommended for use with this type of elastomer.
* Minimum material width should not be less than 2mm or at least the material thickness in any part of the gasket. If this cannot be achieved around fixing holes consider using a slot. Particular attention is required if specifying compression collars in holes.