I think that those of us who have beta tested the dual extruder system agree that it (and basically every other dual extruder system, with the exception of the new E3D "cyclops", and that has its own limitations) has a substantial drawback in being very drip-prone. Software (namely Cura) that can generate drip shields and wipe towers can help, but these additional structures use additional plastic, increase the risk of print failure, and do not work for every situation.
I have a proposal for a mounting system that I think will be drip-free, and which uses no additional motors or electronics. See the attached diagram. The key point of the system is that the extruder motors and hot ends are in a separate mount that rotates on a hinge that is a bit above the motors. The motor mount is held between this hinge and a gear rack passing under the stepper motor gears. The motor mount can swing from side to side. When it swings left, it reaches the end when the right extruder is pointed straight down. In this position, the right extruder is "active", and the left extruder is "parked". The parked extruder's nozzle rests on a wipe plate that prevents it from oozing while parked. When the motor mount swings right, it reaches the end when the left extruder is pointed straight down, and the left extruder is "active" and the right extruder is "parked".
The trick that makes the system work is that the gear rack is designed such that only the parked extruder's stepper motor engages the gears. The active extruder's stepper motor can turn freely for normal extrusion and retraction. When it is time to change extruders, the following things happen (which are coded entirely in g-code): (1) The active extruder retracts a small amount of filament (say, 2mm or so) as it would normally do. (2) The parked extruder extrudes about 10mm of filament. The act of extruding swings the whole motor mount, because the parked extruder's stepper motor is engaging the gear. This brings the extruder out of park. (3) Both extruders run at the same rate, with the (previously) parked extruder extruding an additional 10mm of filament, while the (previously) active extruder retracts an additional 10mm of filament. In the middle of this action, both extruders are briefly engaging the gear rack. At the end of it, the previously parked extruder is free of the gear, and the previously active extruder is engaging the gear. (4) The previously active extruder retracts a final 10mm of filament. This brings it into the parked position and puts the other extruder into the active position. (5) The newly active extruder extrudes 2mm (or so) as it would normally do to get started again.
The 20mm or so retraction is large but not so large as to be ridiculous; as it is, I'm regularly setting the retractions up into the 10+ mm range to keep the thing from dripping all over the place (which it does, anyway).
This system has the additional advantage of keeping the unused extruder out of the way of the plate where it won't be in the way when only one extruder is being used.
I can't think of any disadvantages that are not already present in the existing dual extruder setup (e.g, the right extruder can't move all the way to the left side of the plate). The only unknown is that I'm not sure how well the stepper motor gear can be made to engage the gear rack; I think the position of all the gear teeth should be known to within the precision necessary to keep the gears from binding.
Any comments would be appreciated.
Note that I don't actually have the technical expertise (or equipment) to build this thing. . .
- Dual extruder system
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Last edited by Tim
on Fri Jan 02, 2015 11:49 am, edited 1 time in total.