This can eliminate the majority of EMI emissions. However circuit design and layout for EMC may not be sufficient to attenuate emissions to the required levels and shielding of the enclosure may be necessary.
This is where the enclosure design engineer is required to design the enclosure with sufficient thought to using EMI screening gaskets at joint interfaces. Although his brief may be to make the enclosure as small as possible, if this is not taken into consideration early on then this can result in an enclosure with no landed area on which to place a EMI screening gasket. This would require very expensive redesign or the use of more complex and expensive gasket materials and configurations.
Enclosures can be made of any material provided it is electrically conductive and will form a Faraday cage, metallic enclosures have this characteristic, however all plastic enclosures need to be made conductive by applying a conductive coating to the inner surface. This can be done using conductive paints, electroplating or vacuum metallization.