Nichrome wire, also known as Nickel Chromium wire, is a type of resistive heating element used in just about any electric heating device such as an Extruder and Heated Bed. RepRap uses Nichrome 80 which is composed of 80% Nickel and 20% Chromium.
Insulated Nichrome wire is generally preferable to prevent a short circuit, but uninsulated wire should work as long as extra measures are taken to insulate the wires from each other. Kapton Tape or alternatively Teflon Tape is utilized for extra insulation.
An image of Nichrome wire and insulation in the Casainho extruder.
You can buy Nichrome wire online, and it's a good idea to use a recognized vendor to ensure you get the correct type. It's also possible to find Nichrome wire and insulation at local shops, typically the ones that sell parts for electric machines.
- Nichrome Wire on the forum