I've taken some shots to show how the extruder will (hopefully) work. The hot parts will be milled from brass and insulated with a nylon barrel as normal. Lego won't take the heat. The extruder insulator will be bolted into a lego assembly, and the feed rod will be drawn in using lego gears, motor, and a pair of pulley wheels that pinch the rod. The lego motor also needs a reduction gearbox (easy to make with lego). In these images, the extruder is shown partly dismantled because I'm rebuilding the Z axis and the way the extruder is mounted.
The extruder is basically a sandwich of about 7 layers. From the motor, we reduce the rotation speed then use gears to drive 2 pulley wheels in opposite directions with a small gap between them. The gap is a great fit for my plastic feed rod (a short piece is shown, I have a reel) but I had planned to put rubber bands on the pulley wheels if it was not a tight fit.
In this video you can see that the mechanism pulls the rod through the extruder and is quite stiff (I can lift the extruder via its grip on the rod). With luck, this will be stiff enough to extrude. If I need to change the gearing, this is easy in Lego. The Reprap electronics allow speed control, and according to this page my Technic motor will not draw too much current at 9v in the worst case (a stall) (however, I might be running it at 12v, but the rod will slip before the motor will stall? I hope.)
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