Sunday, January 4, 2009

Hot Stuff! First Extrusion with New Extruder Design.

It's very late, but I have to put *something* up, because I just extruded my first plastic!

Not only that, I did so using a new design for my extruder's "hot zone." The design uses an Aluminum heater barrel, with inside-out nozzle inserts (made of brass.) The melted-plastic zone is under an inch long. The stem is stainless steel, with a reduced cross section just above the (threaded) junction of Al/Stainless, to confine the heat to the hot zone.

Above: Exploded view of the key parts of my extruder's "hot zone." The melt zone is deliberately kept short, and the heat is confined by the stainless steel stem, and its reduced cross section, just upstream of where it mates with the Aluminum barrel. The coarse threads on the barrel are to accept the nichrome wire.

Above: Here is the hot-zone assembly, after adding the nichrome and coating it with my DIY ceramic glue. It's not as clean looking as I'd like, but it seems OK mechanically and electrically. Aesthetics will have to wait for awhile....

But here's the real news (drum roll please.....):

Above: First extrusion with my new extruder design.
I fed the ABS welding rod by hand, and controlled the temperature (open loop) by varying the PWM duty cycle, while monitoring the thermistor resistance with a DVM (to the left of the extruder.) The temperature is approx. 200 deg. C. Note that the barrel is not yet insulated, except by the thin coating of ceramic glue.

Above: With some ceramic-fiber blanket to insulate the heater barrel, I can get to higher temperatures, and extrude HDPE. Based on the thermistor's resistance (visible via the DMM), the temperature here was ~245 deg. C. The PWM was set to 150; so the duty cycle was 150/255 or ~59%

Above: A (very, perhaps too) candid photo of my "highly organized" electronics workbench in my basement. I could clean it up, but I'd rather work on my repstrap!
The alert viewer may discern the beginnings of Cerberus' horizontal mechanism (left-hand side of the photo), based on a compound (X, Y) table, fitted with two stepper motors. More about that later.

That's all for now. I may add more photos and details later.
Next step should be closed-loop temperature control, based on the thermistor reading.


  1. Hi Larry,

    Nice simple design. How hot is the ss tube above the thermal break?

    The reason I ask is I made a similar design but without the narrowing and it did not work well. When the filament is stationary for some time it softened and jammed. Whereas I can see the narrowing would help I would be suprised if it cures the problem. Maybe a heatsink just above the break would though.

  2. Nop,

    I'll have to measure that temp., but the very top remained safe to touch, even while extruding at ~245 deg. C. (I know, because my hand slipped on the HDPE, while feeding it.) I had planned to put a heatsink on the stainless steel "stem" (just above the reduced dia. section), but my upper connection to the nichrome proved too big/bulky to permit that. (Should have made a CAD assy., but at present, I'm faster cutting metal than making CAD files.)

  3. Larry,

    Congratulations! That looks really good. I'm considering a similar design. Although, are you sure that you want to use stainless steel for the nichrome wire section? I would have thought you might want to use aluminum due to its greater thermal conductivity. (about 4-8 times better)
    Keith Ostrom

  4. Keith,

    Thanks for your kind words.
    The section wound with nichrome is aluminum, precisely for the higher conductivity. The stainless steel is the narrower, threaded piece that threads into the Al. The SS "stem" has a reduced cross section to act as a heat "resistor." FYI, more photos of the pieces are in my public picassa albums:

    -- Larry

    -- Larry

  5. Larry,

    I'm Sorry, my mistake. I guess I was up too late mis-reading stuff... I also didn't realize the stem threaded into the Al (heater) section. Your idea for the reduced diameter (surface area) for the heat "resistor" is really good! BTW, Have you thought of using Kapton tape to insulate the heater? I've had good results so far, even with uninsulated nichrome wire and about 15+ hours of operation.

    Keith Ostrom

  6. Larry, you might be interested a blog I just posted. I put together a thermal model that shows why your extruder works well. Anyway, happy trails.


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