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Posted by Bruce Stewart 
November 12, 2008 05:19PM
Where can i buy a one inch plastic augur feed screw?
Re: Augers
November 13, 2008 03:19AM
I'm experimenting with some augur drills i got from a cheap tool shop. i'll post more details when my hands are better (rsi)
Re: Augers
November 13, 2008 03:17PM
I've had a bit of a play with using standard wood cutting auger bits. I've not posted about it before as it was all a little haphazard and then got put on a back burner when the BfB kit arrived.

In case it's of any use here's what I tried:
I used 15mm bits inside 15mm (1/2 inch) copper tube, the same stuff used for cold water plumbing. I only tried PLA, as it saved having to deal with a heater. The last inch or two were submerged in near boiling water. I used a variable speed hand drill to turn the auger (backwards compared to drilling). The torque on the drill didn't seem like much but I don't have any way to quantify it

The initial tests were done with an 8mm orifice. I filled the thread of the auger by hand with granules and then heated the end in hot water before starting the drill. After an initial pause it would extruded 10 to 20mm before either running out of granules or the water going cold.

Problems with this setup were:
The auger would rise within the barrel.
The pressure at the tip was high enough that the first orifice, which was just hammered into the tube, was pushed out.
Only enough granules for about 10mm could be fitted in the auger.

It tended to 'run out' of granules not in terms of there being no granules left in the barrel, just not enough in there to produce the pressure needed to extrude.

The second had the following 'improvements':
A skate bearing was soldered into a 22mm 15mm converter which in turn was soldered onto the top of the barrel. This stopped the auger rising.
Near the top of the barrel large holes were drilled through the tube to allow more granules to be added despite the now blocked top.
A 3mm orifice was soldered into the end of the barrel.
As both ends were blocked by soldered components I added a straight 15mm compression fitting to the barrel.

The good news is that it produced filament, but only about 10mm before the compression joint failed.

The kit from BfB arrived shortly after this. Once I've got a working RepStrap/RepRap I intend to have another go. I would probably use a top and bottom plate linked by three pieces of threaded rod with the barrel in the middle. That would allow the top of the barrel to be open making getting granules in simpler and also removing the weak point of the compression fitting.

Good luck in your endeavors and don't under estimate the pressures, which seems to have been my recurring mistake ;-}>
Re: Augers
November 13, 2008 06:52PM
Sorry .. just an idea:

Why turning the auger backwards?
It has a specific shape to transport material much better in the other direction (at least the ones I've seen have)

And because the filament can leave the tube sideways, there is no need to "mount" the drill on the input side, is there?

Re: Augers
November 14, 2008 09:06PM
That might work, although you would need to seal off the bearing/shaft from the screw part and of cause blunted the edges so it doesn't cut into the tube.

Filling the thread with metal epoxy or similar for the last inch would probably be enough to stop leakage, assuming the clearance between the drill and the tube was small enough.

Filling the thread reminded me of an idea. The thread could be partially filled to produce an effect similar to the different thread pitches and depths used in professional single screw extruders. Up to six stages are used, although most machines only seem to use three.

Below are some brief notes on the zones. This link is about twin screw extruders but nearly all of it applies to single screw extruders as well.

The 'feed zone' moves the granules from the hopper to the heated 'transition zone' and or the 'pump zone'. Interestingly this is normally cooled to avoid the problems Adrian Bowyer had with granules melting in the hopper and blocking the entrance to the screw. [dev.www.reprap.org]
The screw has a constant thread width and depth.

The 'melting' or 'transition zone' takes in granules and pumps out molten plastic. It can do this via a combination of effects. The most obvious is heating the barrel, but heat from friction and the work done pressurizing the plastic also add the the heating. The hard part is getting a length that totally melts the granules.
This section is heated. The thread of the screw either gets shallower or the pitch rises during this section. The initial thread pitch and depth match the feed zone and the final pitch and thread match the metering stage.

The 'metering' or 'pump' zone moves the molten plastic, increasing it's pressure to push it out of the end. The molten plastic also gets mixed together in the process.
This section can be either heated or cooled depending on how much heat the pressure and friction produce. The thread is normally constant in width and depth, matching the end of the melting zone.

The other zones can probably be ignored for now, but for completeness:

A 'depressurizing' or 'venting' zone is used with some materials to remove trapped gasses. I think this sits between the melting and metering zones, but some machines have more than one venting zone so I'm not sure what happens then, possibly an extra metering stage is added between the vents.

A 'mixing' zone seems to refer to a section where granules, of possibly immiscible materials are either broken into smaller chunks or mixed together.
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