INSULATION MATERIAL: These electrical wire connectors are made of plastic shell. The nylon shaped shell has thick wall, high hardness and is not easy to aging.  STRONG SPRING DESIGN: These Wire Caps are made of metal spring, which is inserted to ensure a stable connection to each application. Preventing the connection from falling off due to instability. Strong wiring, good conductivity. COMPACT AND PORTABLE: The electrical wire nuts are a round base with model and specification on the top of the wire connector, compact and portable. The packaging is made of transparent plastic moulded partition storage box to keep different wire connectors    separate. There are 5 kinds of nuts with different colors. Special size corresponds to special colors for easy identification and selection. Clear, easy to store and use.