Thursday, January 08, 2009


A design for sealing the leaks

Hello everyone.

I had my first real printing experience with my McWire based RepStrap yesterday. I am printing ABS with the BitsFromBytes Lasercut Extruder. The first layer worked out pretty well, I think:However I was forced to stop printing at this point. The extruder drived stalled due to the enormous amount of leaked ABS:

I am sorry, I had to use the flash on my camera. You can see the huge mushroom of ABS leaking out of the space between the PTFE insulator and the halfnut on the big washer. This is not the first time this happens, and I have to disassemble the entire thing, remove the pressure bearing on the filament to be able to pull the hole mushroom-insulator-and-heater-barrel thing. Which gets annoying after doing it five times. As you might also see, I tried sealing the leak using PTFE tape and a hose clamp, which did not work.

Since I spend some of my christmas holiday hunting down replacement parts for the heater section and I have someone who can machine parts for me on a lathe and a milling machine, I thought up this part, which might be able to seal the leak properly:
Now unfortunately the lathing-and-milling-person has called in sick today, so I cannot try it out this week.

What do you think about the design? I hope the drawing is not too bad. The dotted lines should be threads.
Could it seal the leak? Should I be using aluminum and not stainless steel? I only have a very small piece of brass thread protruding out of the big washer (the black line on bottom of the picture), so I would not like to thread the PTFE directly into it. I think it would not hold and would get very hot.

Anyways: I have a first real printing result! This is so great :) Thanks everyone who helped me so far :)

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Cool results. Honestly, any result is good even if it means a little more work before it all pays off.

Your idea looks sound in principle to me. I have just got a mini lathe and am currently experimenting with alternative heater barrels. My thoughts had been to use a stainless steel upper section too.

I am currently looking for a supplier of sensibly priced stainless steel studding connectors (Long Nut's) to try some of this with as I don't think my mini lathe is up to too much work on stainless. (It is very hard and my lathe tools are cheap)

This may be a useful route for you too to get the stainless steel bit but without needing a machinist and machine tools.


Le Kirby
Daniel, Andy et al,

@Andy: your "studding connectors" are more commonly called "coupling nuts" at least in the US. Here is the URL of one such that matches the thread (3/8-16) used for the acorn-nut nozzles sold by the store:

If the link gets mangled go to and search for partNumber 64827371 YFI, these are USD 3.12 each.

To minimize heat xfer, you could turn the outside down, either over the entire length, or just a groove. The former is more work, but better for confining heat. Worth trying without machining first, esp. if you don't have access to a lathe.



What purpose does the "big washer (the black line on bottom of the picture)" serve? It seems to me that it'll mainly take heat away from your brass, and leak it into the air. Consider omitting it, and letting the SS outer piece be the sole connection between your PTFE and brass.

my $02,

@Andy: Coupling nuts might be an idea. I'll see if I can find ones with two different thread diameters in my hardware store...

@Larry: Thanks for the "translation" . The big washer is stainless steel and is screwed to the upper half of the extruder. Its purpose is to provide a little bit of pressure on the PTFE. Oh, and it is cemented to the rest of the heater section. Maybe you get an idea on the picture.

I cannot omit it, as the PTFE is not screwed into the top of the extruder design. The PTFE and the heater section cannot be connected without it. It would just fall off.
Trapping the PTFE inside something, rather than something inside the PTFE, definitely works, I tried it here.

The PTFE swells as it expands and closes the seal.
that was my inspiration :)
PTFE is definitely something you don't want to depend on mechanically at elevated temperatures.
True that, Forest! Stuff is garbage for any sort of mechanical dependability.

I tap the brass heater barrel into the PTFE rod by about 6mm, and also wrap it with PFTE tape before assembly. Did you do either of these?

Vik :v)
@Vik: I have only about 3 mm of brass that the PTFE could be threaded into. That might be a bit to little. And I would have to find another way to feed the filament into the top of the PTFE. This is because of the restrictions of the bfb extruder design.
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