This DIY PTFE hotend tube will be most of all useful for RigidBot / RigidBotBig extruders, but also will be useful for J-head hotend, Delta Rostock extruder and may be some others but this all I know.
The idea is to use 2 PTFE tubes one inside another, first is ID 2mm and OD 4mm, second is ID 4mm and OD 6mm.
Tubes can be purchased on ebay for 5$ per meter which is waaaaaay cheaper then 4$ per 20mm tube, and then you can make as many as you need.
How to do:
Insert smaller tube into larger, align ends of tubes. measure 16 mm for RigidBot hotend and 20mm for J-Head, cut with exacto knife! Just make sure you cut is straight and even, both tubes ( inner and outer is same width. ) You can even sand both ends on 220 grit sand paper just to smooth it if necessary.
I don’t have long term test at the moment I just replaced this in my hot end, first impression is pretty good. Will do some complex prints and will post results.
Also currently I print exclusively with PLA, so max temperature is 200C. but still original PTFE tube was shrinking even at this relatively low temperature, so I pretty optimistic with this design.
Edit ( Jan 27th, 2015 ):
After running this tube design for almost 2 month, I can say this is successful design and worth doing, especially if your Teflon tube is busted. Also PTFE shrinking is much less with composite tube.