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Extruder thermal standoff/barrel....

Posted by mars bonfire 
Extruder thermal standoff/barrel....
May 03, 2011 10:50AM
Only a week or so into planning my 3D printer build....if this is old news, hit the delete and please let me know if this post is not appropriate for this forum.

Anyhow, I have been researching the various extruder designs and specifically, the material used for the thermal "standoff" i.e. the barrel section between the feed pusher and the hot tip. The material of choice seems to be PTFE (teflon, trademark). There is some discussion that it can distort...this is not surprising since it looks like ABS requires a tip temp of maybe up to 240 to 250 degrees C. That is getting real close to the max working temp of PTFE. It looks like you can buy some more temperature margin using Rulon LR rod stock. It is typically used for plain bearing (bushing) stock (low friction) and has a working temp maybe another 20 degrees C or so over pure PTFE. Rulon is some sort of alloy or composite that contains some kine of fluorocarbon plastic/PTFE. It is not cheap but neither is PTFE. You can get 6" inch rod lengths from Small Parts Inc. (no affiliation) or 12" lengths from a variety of other web online vendors...something like $30 for 1/2" diameter, 6"...would make two extruders. I have machined it and it cuts like butter.
Re: Extruder thermal standoff/barrel....
May 03, 2011 05:12PM
That is pricy; I picked up 6 feet of PTFE for roughly that much. How does the thermal conductivity compare to PTFE?

Re: Extruder thermal standoff/barrel....
May 03, 2011 09:37PM
I've been using PEEK lined with PTFE to reduce friction (Adrian's first PEEK design) and it's been working great.
Re: Extruder thermal standoff/barrel....
May 04, 2011 03:05PM

I think you got a good deal on the PTFE and I got a not so good price on the Rulon LR. Actually, I just ordered a 12" long rod from Small Parts (3/4" diameter) for $26...it almost looks like a misprint...smaller rods are more expensive!. I was thinking about PEEK but what tilted me toward the Rulon LR was that (in addition to it being inherently low friction), it has a working temperature typically listed at 500 to 550F, whereas both PTFE and PEEK seem to be listed at 400F. Seems hard to believe the numbers...www.mcmaster.com lists them...hopefully I got the numbers right! Seems that the addition of whatever to the the PTFE to be Rulon (a "composite") ups the working temp significantly. To your question on thermal conductivity...I didn't worry about it but I just looked it up:

Rulon LR - 2.3 BTU/HR/Ft-sq/in
Unfilled PTFE - 1.7 BTU/HR/Ft-sq/in
Unfilled PEEK - 1.75 BTU/HR/Ft-sq/in

My guess is that PTFE has a more abrupt glass transition temperature or whatever it is called i.e. it softens over a wider range of temp near and into its max working temp than PEEK which stays stiffer making it better for structural components.

So, PTFE is the best insulator, but I would think not significantly for this application. Also, any of the "filled" versions of PTFE and PEEK have higher conductivities (no surprise, that is the Rulon LR story).

So, lots of ways to cook the cat...just thought I would share some info that may be of interest.


P.S. I take it you are the Wade of extruder fame...I am in the process of rolling a variation on your design...stand on the shoulders of those who come before sort of thing...
Re: Extruder thermal standoff/barrel....
May 04, 2011 03:11PM
I use only PTFE held onto the extruder body using four set screws.


It works well on my machines because I don't put too much current through the extruder stepper driver. This allows the filament to spring back the gearing when the extrude rate exceeds the nozzle output.

I've printed six (6) machines with this design on a single extruder, with no problems.

I did have an issue when I bought some stronger steppers, and tried to print ABS on a Wade's extruder body that was made out of PLA, but it was the extruder boddy that warped, not the PTFE tube.
Re: Extruder thermal standoff/barrel....
May 04, 2011 04:26PM
Sorry...made a mistake. Looks like PEEK works to 480F, not 400F as I stated. McMaster lists Rulon LR to 500F and others list Rulon LR to 550F...bottom line, PTFE and PEEK work, supported by specification and actual use, Rulon LR by spec and maybe, hopefully, by actual use. I will check in once I get something working...
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