Kemtron manufacture a range of conductive Elastomers as gaskets and gasket strip for EMI shielding of electronic devices
The base Elastomer is silicone for normal environments and Fluorosilicone for fuel and oil resistance, both offer a wide temperature range of -40°C to +160°C and up to +200°C for some grades. The conductive fillers are available in silver-plated aluminium, silver-plated copper, pure nickel and nickel-coated graphite.
The manufacture of a conductive Elastomer requires a balance of conductive particle loading and distribution throughout the silicone base. The distribution must be sufficient to ensure that the particles are in contact with each other allowing a good conductive path through the Elastomer, but the loading must not be so great to cause the silicone to lose its elastomeric properties. In general, the conductive Elastomer will have a shore A hardness of between 60 and 75, depending on grade, and still maintain good tensile strength. This ensures the gasket will deflect sufficiently when under compression, giving a good EMI and environmental seal. The electrical properties of the conductive Elastomer are measured in volume resistivity – ohms-cm and shielding effectiveness stated in db. However, the two do not directly correlate with each other. The volume resistivity of silver-plated aluminium in silicone will be max 0.008Ωcm, giving a shielding effectiveness at 2Ghz of 102db, whereas nickel-coated graphite will have a volume resistivity more than 10 times greater, but exhibits a similar shielding effectiveness.
Nickel-coated graphite in silicone is the most cost effective conductive Elastomer, giving excellent shielding characteristics even though the material is much more resistive than precious metal-plated particle Elastomers. This excellent performance can be attributed to the fact that the nickel graphite particles are very irregular in shape and have sharp edges. When the gasket is put under compression between two surfaces, the particles dig into the surface giving very low contact resistance. Graphite is also a good microwave absorber thereby enhancing the shielding performance. Nickel graphite in silicone costs approximately 60% less than silver-plated aluminium in silicone and 70% less than silver-plated copper in silicone. Kemtron produce a flame-retardant version of nickel graphite in silicone (tested and approved to the international standard UL94V-0 by Underwriters Laboratories for flame retardancy, file number E344902).
Kemtron’s process capabilities include extrusion, moulding and conductive vulcanised jointing. Many extrusion profiles are available including cords, tubes, hollow D, solid D, etc. Custom profiles are available. Extrusions are supplied in continuous lengths, cut to length or made into large or small diameter O rings which are produced by joining extruded profiles with the same conductive compound as the base material, the join is heat vulcanised ensuring a strong conductive joint. Mouldings include sheet stock, O rings, custom shapes and gaskets. Sheets can be die cut to the customer’s design, and a conductive PSA can be applied to aid assembly.
Kemtron also deposit liquid-conductive silicones directly to customers hardware as a form in place gasket. This process can deposit very small (0.4 high × 0.5 wide) gaskets on complex paths with narrow land.
Kemtron are based in Braintree, UK. In addition to conductive Elastomers, they manufacture a full range of shielding gaskets and components providing a comprehensive EMI shielding service to the electronics and enclosure industry.