Sunday, November 06, 2005


New Extruder

As Vik mentioned below, I have got a new extruder design working (though he was too modest to say it is based on an original idea of his). Full documentation and downloads are available from the Extruder Version 2 section of the RepRap wiki.

Here's a picture:


A geared electric motor (A) drives a screw chamber (B) in which a threaded rod is forced against a rod of polymorph. As the thread turns, the polymorph is pushed downwards to a heated nozzle (C) and is extruded out of the bottom. The control electronics is shown at (D).

This Version 2 design has many fewer parts than Version 1, those parts are cheaper and easier to obtain, and the overall mass of Version 2 (just under 200 g) is less than half the mass of Version 1.

Here's a picture of it building a small thin rectangular block.


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Brilliant! Things have really been picking up!

Let me see if I've got this right. You're talking about the extruder being able to produce a maximum of about 2.83cm³/hr of polymer while drawing about 6 watts for the heater and .5 watts for the gearmotor?

I'm assuming that you're not extruding a bead larger than that .5 mm diameter by jacking up the extruder and slowing down the stage.
That's right. But it's pretty easy to make different nozzle diameters, and the volume rate goes as the square of that, of course (or more - flow restriction reduces as diameter increases too). Given that ease, we are thinking of having a coarse nozzle for infill and a fine one for detail. The nozzle itself sweeps the bead flat, so getting two different diameters to make a single vertical thickness but different horizontal splurge comes for free.
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Are you able to get the thread of hot polymer from the extruder to adhere to existing cool polymer or are you having to maintain the whole accreting object in a heated enclosure as I have seen some 3D prototyping machine people do?

I was working with polyethylene some years ago and found the that new polymer applied to existing surfaces that had had a chance to cool to would tend not to adhere.
The thin layer of deposited material conducts sufficient heat from the nozzle to melt the surface of the underlying layer. With Polymorph at least, there's no problem getting the stuff to stick together. It's not just squirting out a bead of plastic.

Vik :v)
Encouraging developments. I was hoping some of the complexity would drop out of the extruder.
To add to what Vik just said, the main cooling problem is the need to pause between layers to allow everything to drop in temperature to the point where it won't all turn into a big splurge, especially for small objects. Maybe we'll stick an old processor fan on the side of the extruder to cool the workpiece...
This is what I did with the EVA Meccano prototype. You need some ducting (Gaffer tape :) to keep the airflow off the heating element though, or it may have difficulty in keeping up with heat demands without vapourising the nearest PTFE.

Vik :v)
I wonder if there is some sort of insulating material that we can use that has a higher failure temperature that that PTFE tape. I'd like to be able to extrude polypropylene. As best as I can see, however, the PTFE failure temperature is within about 20 degrees of the extrusion temperature of polypropylene. I've been thinking about glass cloth or possibly baking a ceramic skin on the extruder. Anybody have any ideas?
Plaster of paris and insulating fibreglass?

Vik :v)
That's just a guess, by the way. I've been meaning to try it out but keep losing track of my sample of fibreglass insulation :)

Vik :v)
We've gotten some reasonably priced ceramic fiber solid insulations from McMaster-Carr. Excellent insulating properties, can be machined with regular tooling. They do tend to erode if maintained at red heat for many hours, though. But they should last essentially forever at extrusion temperatures.
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