[MakerBot] Concept for Extruder

[Brent Crosby]

After many idle moments thinking of what an extruder needs, here is my latest concept.

http://img534.yfrog.com/img534/2860/g98.png

I have a big old chunk of Torlon (12"x3/4"), so Torlon plays heavily in this design. It may be that PEEK would work.

The main ideas are:

1) minimize the heated volume (from nophead's no excuses)

2) maximize the heat transfer from the nichrome to the ABS

3) minimize the heat transfer to anything except the ABS

4) reduce complexity

5) have the brass (lower thermal coefficient of expansion) seal around the Torlon, so the hotter this thing gets, the tighter the press-fit seal is.

6) make it fit in the MakerBot (hence the long main barrel and long screws)

My main doubts:

A) The transition zone will be in the Torlon, I am not sure if it will be slippery enough. Perhaps I can find a really small reamer and make the last bit of hole in the main barrel flared slightly like nophead's stainless barrels. I may be able to polish the inside of the bore a bit (rolled sand paper, then compound on a fat string?)

B) The melt area/volume is small, I am not sure that the filament will have time to melt before its tip touches and clogs the extrusion hole. The plan is to have it exit off-center, I do not know if this will be enough. What it needs is for the filament to enter from the side, not the top, tangential to the inner diameter of the heater section. Then the filament would have awesome contact with the hottest part of the extruder.

C) My ability to run my lathe may be below the ability needed to create these parts.

Please give me your thoughts. If you can think of a "stopper" reason it will not work, I may bag building a prototype.

Thanks in advance for any insight you can offer.

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[baxsie]

I finally got some lathe time today. I have machined out the Torlon
cap and retaining washer. Images here:

http://yfrog.com/5dtluj

Comments welcomed.

[baxsie]

Huh, yfrog's "next" does not seem to be working like I expected. Here
is the full set of images:

http://yfrog.com/5dtluj
http://yfrog.com/jqm66j
http://yfrog.com/9glgj
http://yfrog.com/j5ia5j
http://yfrog.com/2r7efj
http://yfrog.com/jy5zkj
http://yfrog.com/jus6gj
http://yfrog.com/0g3ipj

[Brent Crosby]

I got a bit of time to work on the prototype for this extruder. Lots
of photos here:

http://yfrog.com/0cv2kj
http://yfrog.com/e8dq7j
http://yfrog.com/5m4taj
http://yfrog.com/0vpzyj
http://yfrog.com/5naw4j
http://yfrog.com/ei8fij

Comments / observations / encouragement / mocking welcomed.

[Brent Crosby]

Photo collage of MakerBot Concept Extruder parts.

[Roger Walsberg]

Interesting! The idea of the rod being introduces into the warmer zone makes sense, but the extruded filament is now being produced from the "colder" part of the heating chamber(the center). Would it make more sense to make the heat chamber smaller? You also have an undercut between the wire and the heat chamber which would be difficult to clean out if it comes down to it. I like the idea of making it shorter than the stock one. Run it and let us know how it works.

--
Thanks. Roger

[Brent Crosby]

We are finally getting close to trying this thing out. Need to go to the hardware store to get some longer bolts.

We did a compressed air check of the interference fit. No bubbles in the soap :)

Lots of other pics here:

http://yfrog.com/6257pj

http://yfrog.com/0yl5uj

http://yfrog.com/09yowj

http://yfrog.com/6t7ykj

http://yfrog.com/0rnz4j

http://yfrog.com/5mu8cj

http://yfrog.com/20r3qj

http://yfrog.com/0gpuj

http://yfrog.com/0anruj

http://yfrog.com/2mh65j

Comments welcomed.

[Joel Chia]

Looking pretty sweet.

-Joel

On Fri, May 14, 2010 at 2:43 PM, Brent Crosby <br...@crystalfontz.com> wrote:

We are finally getting close to trying this thing out. Need to go to the hardware store to get some longer bolts.

We did a compressed air check of the interference fit. No bubbles in the soap :)

Lots of other pics here:

http://yfrog.com/6257pj

http://yfrog.com/6257pjhttp://yfrog.com/0yl5uj

http://yfrog.com/09yowj

http://yfrog.com/6t7ykj

http://yfrog.com/0rnz4j

http://yfrog.com/5mu8cj

http://yfrog.com/20r3qj

http://yfrog.com/0gpuj

http://yfrog.com/0anruj

http://yfrog.com/2mh65j

Comments welcomed.

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[Brent Crosby]

We got this extruder finished and it is printing better than anything
we had tried with the stock extruder.

Here are pics:

http://yfrog.com/06n58j
http://yfrog.com/6w8b2j
http://yfrog.com/5sa0nj

What have I learned from this? Here is the summary (keep in mind this
was all done with ABS, natural and black, and it is still early):

1) The melt zone can be very short ( http://yfrog.com/20r3qj ). The
heated section in this design is really small. The entire heated brass
element is only 15mm or 0.6 inches long, and of that only 10mm or 0.4
inches is in contact with the ABS. There is NO REASON to have a long
brass tube heated and clogging.

2) Having the heated zone very small gives the default heating drive
circuit and nichrome element plenty of heating authority. The light
blinks happily as it is printing, where in the original design the
heater seemed to be full on all the time. It comes to temperature
quickly and controls nicely. There is NO REASON to have a huge metal
washer thermally coupled to the heated element.

3) Pressing the low COE brass OVER the higher COE plastic (Torlon
tested, I think there would be similar success with PTFE and PEEK)
seals and does not leak over temperature. Hotter = tighter if the low
COE material is on the outside. The original design puts the PTFE with
its high COE on the outside, and the hotter it gets the looser it gets
(and not in a good way like on spring break), plus the PTFE gets soft
at high temps.

4) To lower the active thermal mass, I used the Torlon "cap"
http://yfrog.com/0anruj to cover the heated brass element, and then
also used it to thermally insulate the steel washer from the brass. I
expected the steel washer to be fairly cool, but it was not nearly as
cool as I would have liked. This means that Torlon is more thermally
conductive (poorer insulator) than I had expected. I will need to re-
think insulating of the heated element.

5) I need to get a photo, but it appears that the melt zone is very
short. Certainly within 1/2 inch of where the brass is pressed onto
the Torlon. This means that the whole assembly should be able to be
reduced in size greatly. This prototype was made to fit in a stock
MakerBot, so the top Torlon is long. I think a much shorter length can
work, and it would reduce the "compression spring" bit between the
plug and the extruder pinch wheel. Shortening the length should make
the responsiveness between the motor commands and actual extrusion
more coupled.

6) I did not make a dedicated place for the thermistor. In the next
version I will extend the extruder so there is a hole to put the
thermistor into.

7) Not shown on the drawings, but I made several counter-bores on the
back side of the nozzle to keep the 0.45mm bore as short as possible.
The idea is to reduce the friction of pushing the plastic through a
long skinny hole. Trying to keep that "to the 4th" term small. (ref:
http://hydraraptor.blogspot.com/2009/03/rheology.html )

8) I think the small flat area around the extrusion hole is too small.
It looks like skeinforge uses this area to smooth the top layers and
mine is so small it leaves grooves. (one of Nophead's designs
http://hydraraptor.blogspot.com/2009/11/no-compromise-extruder.html
has a very large flat area)


Coincidently, Richard is working on a very promising design:
http://www.makergear.com/blogs/frontpage/1636862-peek-ptfe-hybrid-insulator-v2
which looks very like it might work well.

Intentionally, Eberhard is working on the "Complextruder":
http://pleasantsoftware.com/developer/3d/2010/05/15/complextruder/
Which also looks very promising.

My plans for the next revision:

1) Reduce complexity
2) Try out lower-cost materials PEEK or maybe even PTFE rather than
the super expensive Torlon
3) Reduce the size (this will require new or modified "dinos")
4) Accommodate the thermistor (which will likely increase the size,
meh)
5) Re-think the insulation around the heated element

The end goal is to have a design that is simple to manufacture and
assemble, has very few components and those are as low-cost as
possible, works every time, and never, ever sticks.

Now that I can actually create objects in MakerBot #1024 (aka
MiseryBot), it may be a while till I get back to trying to improve on
this design.

As usual, comments are welcome.