Summary

UPDATE: I posted the STEP version of this object for easy remixing / modification.

UPDATE: I've recently updated my E3D v5 extruder and fan duct (http://www.thingiverse.com/thing:533029) and significantly improved the fit of the hotend into the extruder. I translated these improvements to my E3D v6 extruder so inserting the hotend into the extruder won't require as much force and will hold the hotend tighter due to better fit. This update has improved my print qualities over the previous extruder version, because the hotend nozzle doesn't have as much play. I've also renamed the part with the 2.0 mm ID PTFE upper opening to accurately reflect the inner diameter of the upper inlet.
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Here is my extruder for version 6 of the E3D hotend for the MakerGear M2 printer. To design this I used my reference model for the E3D v6 (http://www.thingiverse.com/thing:433258). While this design is intended for use with my E3D v6 hotend duct (http://www.thingiverse.com/thing:494430), here is a plain version of it for use with other cooling ducts.
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I was inspired by both rsilvers’ M2 extruder design (http://www.thingiverse.com/thing:267137) and cmenard’s adaptation for the E3D v6 (http://www.thingiverse.com/thing:349859).
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I’ve made two versions of the extruder, one to fit 2.0 mm I.D. PTFE (at the top) and one to fit 3.0 mm I.D. PTFE. The reason for the later is because I prefer 3.0 mm I.D. PTFE as it allows room for filament with kinks in it. I’ve uploaded a version of each with and without supports / pads. Most of the supports and pads peel away easily or can be crushed and plucked off with a pair of needle nose pliers. There should be no need for extensive cleanup. I suggest that you pass a 2.0 mm diameter circular file down the length of the filament shaft of the extruder. Sometimes there are minor blockages that can lead to filament jams.
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I brought the heat sink higher up into the body and this makes for a tighter junction between the hot end and the extruder as the extruder is holding onto more of the hotend. There is very little if any wobble in the hotend, which I’ve found can develop with time as the extruder experiences the stresses of printing. The only wobble in the system now is in the tightness of the rail carriage that the extruder motor and extruder ride on. The tolerances are pretty tight but the E3D heatsink should snap into place pretty firmly and might be a challenge to take back out. It will require a fair degree of force to get into the extruder and I recommend the pressing this in with the rubberized handle of a hand tool or small block of wood.
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Please comment if you have any design suggestions or requested fixes.