Traditionally, shielding materials are created by molding engineering thermoplastics that are either painted with conductive films or filled with conductive and metallic additives to provide electro-static dissipation (ESD) and electro-magnetic interference shielding (EMI), notes Raj Mathur, the compounder's business development manager. . Carbon fiber is naturally conductive and thus a popular additive. To increase shielding efficiency, carbon fibers are often coated with metals such as nickel. However, these materials only achieve EMI shielding efficiencies in the 30 to 40 dB range, according to PlastiComp. —[email protected]
Coating the fibers with nickel or other metals usually leads to a loss of mechanical properties and shielding efficiency for the carbon fibers. In addition, adding nickel and secondary operations involving conductive paint add cost to the process.
For a recent project PlastiComp was tasked to supply compounds able for unidentified EMI applications. These parts had to possess a stiffness of 35 GPa or higher, provide 50 dB or better EMI shielding and possess a surface resistivity of 5 ohms/square, or lower. There were restraints on unit costs, as well. Thicknesses varied in part sections and were 1.5 mm or less. PlastiComp went through structural and mold flow analyses before it selected a long carbon fiber-filled nylon that was tested to 50-70 dB shielding effectiveness. The compounding was such that long fiber lengths could be retained; this, along with the use of some proprietary additives, were principally responsible in creating a conductive network of the reinforcing fibers, leading to the high shielding effectiveness as well as the high stiffness.