Foolproof extruder?

Hi, much to my dismay I cannot get the bitsfrom bytes extruder to work well, and in trying I have damaged it. I was getting relatively good performance for a while, but then the ptfe deformed, splurging hot hdpe on to the top of the large washer. I also had problems with the hdpe filament not being gripped by the thread. I have since cut a new short length of threaded rod with a die which works brilliantly, I can barely stop the filament moving with pliers. However, I am completely stumped as to how to get a good heater working. The ptfe rod and tape I have tried starts cooking before the hdpe becomes liquid enough to extrude, and the bitsfrombytes heater barrel evidently produces too much friction for extrusion to occur. Having build a working cartesian bot, and all the electronics, I hate being stuck at this last hurdle, and would massively appreciate any help to get my reprap up and running.

I don't know if it applies to this extruder but I keep the PTFE from expanding with a pipe clip. I also seal the joint with PTFE tape.

You should be able to extrude HDPE easily at 240C, and I have had the PTFE survive 290C recently, due to an accident. PEEK seems to be a good substitute but I haven't got round to trying it yet.

I think sharpening the thread with a split die is essential, it makes a big improvement in grip. Also angling the drive screw so that it bites gradually reduces the friction a lot.

The point where the HDPE is only just molten, i,e. is very viscous, needs to occur inside something slippery. I found it works well in PTFE but does not work in stainless steel.

HDPE is the most difficult plastic to extrude. ABS is a doddle in comparison.

Edited 1 time(s). Last edit at 11/15/2008 07:43PM by nophead.

---

[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
>
> The point where the HDPE is only just molten, i,e.
> is very viscous, needs to occur inside something
> slippery. I found it works well in PTFE but does
> not work in stainless steel.
>
I go from filament to completely melted in the copper extruder barrel in Tommelise's extruder. Never had any trouble
>
> HDPE is the most difficult plastic to extrude. ABS
> is a doddle in comparison.
>
That's good to hear. All my serious experience heretofore has been with HDPE, which I found fairly easy to extrude and polypropylene, which requires a lot higher barrel pressure.

Quote

I go from filament to completely melted in the copper extruder barrel in Tommelise's extruder. Never had any trouble

Yes I never understood that. When I tried to push HDPE through a stainless steel barrel with ~260C at one end and ~40C at the other, 3.6mm bore and no nozzle on the end I could hardly do it by hand, the force required was enormous.

I think I have just realised why (it's amazing what beer can do):

Maybe because SS is such a poor conductor it could not source heat to the melt zone fast enough. You must have roughly the same thermal gradient along your copper, but a much lower thermal impedance, if such a thing exists. I.e. at the point, roughly half way down, where the temperature is at the melting point of the HDPE it will be very viscous until the temperature rises well above the melting point. To raise the temperature requires considerable heat flow because of the high specific heat capacity of plastic. SS has low conductivity so supplying the heat will take time. That means the plastic can only flow slowly, any attempt to move it quickly will cause the temperature to drop and it will freeze.

So perhaps a copper or brass barrel with a much more powerful heater and a big heatsink at the top will achieve my original aim of getting rid of the PTFE and allow temperatures way above 300C so we can do things like PET and AL.

Edited 1 time(s). Last edit at 11/15/2008 08:56PM by nophead.

---

[www.hydraraptor.blogspot.com]

Interesting. I'm having trouble with my lasercut extrder as well. I sharpened the drive bolt a bit, but not enough apparently. How long is your copper barrel Forest? Simpler would be bettter!

Nophead, et al,

Your comments about the thermal gradient and thermal impedance lead me to wonder if it's possible (and desirable) to shape the gradient to go from unmelted to freely flowing plastic in as short a space as possible. That is, confine the "hot zone" by reducing the area of metal conducting heat up (towards the PTFE) by thinning the outer diameter of the barrel down. At the pressures we're currently working at, a thin wall could stand the pressure, but conduct less heat out of the melt zone.

I hope to get started on some experiments along these lines, as soon as I receive the materials I ordered (brass, stainless and Aluminum rod stock.)

-- Larry

Wade Wrote:
-------------------------------------------------------
>
> How long is your
> copper barrel Forest?
>
The one I used the most was 3" long with 1/4" of that acting as a sleeve in the PTFE thermal break.

I made it out of 5/32 inch OD hard copper tube that you can by in the specialty metals rack at your local hardware store in 1 foot lengths. That stuff is just a touch over 3 mm ID. That gives you wall thickness of your barrel of just a bit under 0.5 mm. The barrels I make heat up and cool off VERY quickly, in a matter of a few seconds.

[3dreplicators.com]

Here you can see the whole jury-rigged extruder that I made out of the kit of plastic parts that Adrian sent me years ago.

[3dreplicators.com]

You see that only the bottom two compression springs are actually used. That was subsequently found to be a good idea. All I knew was that it worked using the bottom two and didn't when I tried to use all four.

Note that I kept the nichrome heating wire to the last inch before the extruder orifice.

[3dreplicators.com]

In practice, the brass collar rarely got hotter than 80 C. Mind, I was extruding at about 190-200C according to my IR non-contact thermometer. It was probably a bit more than that given that the field of view of the thermometer was a bit bigger than the heated portion of the barrel even with the IR thermometer only a few mm from the barrel.

Edited 2 time(s). Last edit at 11/24/2008 09:40AM by Forrest Higgs.

Critical to good filament drive is sharpening of the thread, we also now push the end of the heater barrel into the PTFE so a pipe clip can be used especially if the temperature is being pushed up high, I also preffer to use 6 ohms of nichrome as it heats up a lot quicker. We are in the process of redesigning the hot end to make assmebly easier and less chance to leak whatever the temperature or pressure i.e. a design where extra pressure creates a stronger seal.

---

Ian
[www.bitsfrombytes.com]