PTFE
PTFE, also known under the brand name Teflon, has the lowest coefficient of friction of any known solid material and melts between 260°C and 327°C. It's fairly hard and fairly stiff too, so it's ideal for RepRap parts including things low friction bushings to main assembly parts.
Thermoplastics often swell just before they melt, so PTFE inserts are used in many extruder hot ends so that the swelled plastic slides on this insert, instead of wedging itself in, preventing extrusion.
PTFE is used in lubricants, and PTFE grease highly recommended by many in the community as being low friction and inert.
PTFE is also sold in very thin tape, known as plumbers tape or thread-sealing tape.
Availability
PTFE rods suitable for RepRap extruders can be purchased from:
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