Wednesday, December 24, 2008

 

Extruder Madness!

Building on Nophead's previous post on the slow thermal gradiants within the heater barrels and the havoc this causes, I decided to try an experiment to see if there was another way to rid ourselves of the PTFE thermal break since I didn't currently have one anyway. To this end I simply removed it (it being already gone) and did not replace it with anything. I attached the heater barrel to the PTFE clamp--sans PTFE--like so:





























This setup is made with a 2" flat washer sandwiched between two 3/8-16 square bolts on the heater barrel. In the flat washer are two M3 holes which will be used to hold the heater a specified distance away from the extruder body. Basically, the air has become our thermal break thus changing the slow thermal gradient to a very fast thermal gradient.















I then threaded the ABS filament down through the pump body and into the heater barrel assembly. I was sorta doubtful about this setup, it seemed too easy, and I was right.

Upon initially heating up the barrel, the filament had a tendency to pull to the side once the plastic got warm enough to deform if there was any lateral forces cause by misalignment or by a curvature in the filament itself. This isn't a show stopper, however, because a guide can be constructed to hang down from the extruder to align the filament a few millimeters form the heater opening.

Once the heater got up to temp--which took longer and required more power due to the huge increase in surface area presented by the nuts and washer--I turned on the extruder and this is what happened:





























Here it is pulled out of the heater assembly:




























It seems the opening on the heater barrel is too big for the filament and when you press it into the heater it mushrooms up around it. Now, it did extrude a little but I did not want to unreasonably foul my heater so I stopped it. This expulsion of material from the top of the heater isn't good. A positive side effect of removing the thermal break is that the pressure needed to push the filament into the heater barrel is trivial. A hard look would push it through.

I'm wondering if this effect can be mitigated by a tighter tolerance on the heater barrels bore? What tolerance, with respect to the filament, might this require?

I stopped my experimenting here as I had other stuff to do but I thought I would post so others can be thinking about the problem too. Let me know what you think.

Demented

Labels: , , , ,


Comments:
I am beginning to think that the heater barrel should be as short as possible. I think most of the resistance to extruding is caused by the partially molten plastic.

Perhaps just a nozzle and with a heater around it. Then a short section of PTFE or PEEK and then an aluminium heatsink. Stainless steel bolts through the lot to hold it together. I will try that over Christmas.
 
The stainless steel is definitely necessary. I just used normal bolts with no ill side effects but they got very hot at the extruder body end...Was not a good idea.

I would really like to eliminate the PTFE or PEEK altogether as it is not a common item.

Looking forward to seeing your progress.

Demented
 
You may be interested in this post on my blog here.
It seams we had some similar ideas at the same time.
 
This comment has been removed by the author.
 
f you can eliminate PTFE that'd be great. Otherwise you could try reducing the cost a bunch by using teflon tubing. Teflon tubing of 7 AWG gauge, an inside diameter of 3.75 mm, can be found for a dollar a foot at:
http://www.westfloridacomponents.com/
mm5/merchant.mvc?Screen=PROD&Store_Code=
wfc&Product_Code=TF025

On the plus side, one foot of tubing would be enough for a few extruders. On the downside, since it is just a tube you'd have to somehow put the teflon tube fully supported inside something else and mate it carefully to the heater barrel.
 
That's a better idea, Enrique! It would also require less tooling to do. A simple tube inside a tube could work out well. I'll have to look into that.

@geo01005

I'll take a look at your post.

Demented
 
FYI, commercial machines avoid the exotic insulator by using cooling air around the place the filament enters.

the idea is to have a very short temperature transition zone-- on commercial machines, on the order of 1/2". this keeps friction to a minimum while keeping the incoming filament stiff.

I have seen several people use heaters to keep the build platform temp elevated to minimize shrinkage. Perhaps a stream of air over the extruder inlet, then directed at the build area, would accomplish both purposes?
 
That's an interesting idea. Shouldn't be too hard to try that out either. I've seen a bit of speculation about how commercial printers do this but no one has really been sure. Interesting.

Demented
 
Post a Comment

Links to this post:

Create a Link



<< Home

This page is powered by Blogger. Isn't yours?

Subscribe to
Posts [Atom]