In order to finally get my Mendel operational, I reactivated the insulatorless extruder I built last December. Back then, I didn’t get it working. The ABS got stuck shortly after starting an extrusion.
For the Mendel, I tried it again (with PLA this time). I cleaned the whole thing, rewound the nichrome wire and attached the extruder to a special variant of the Printruder II:
This Printruder II has a special mounting hole to directly screw the insulator-less extruder to the body with a single captive M8 nut.
Although the whole setup looked very nice and I even was able to extrude PLA at 185°C by pushing the filament manually, it got stuck again as soon as I tried to extrude more than 5cm with the motor.
I really don’t know why the extruder doesn’t work, but I guess the problem is the long brass barrel. Probably, the plastic melts way too early in the barrel and forms some kind of plug.
It seems I have to count the insulator-less extruder as a fail after all.
Speaking of fail: I finally recieved two new stepper motors: SY32STH47-1683B from Zapp Automation. This motor is recommended on the RepRap site for use in the Mendel Extruder 2.0. However, after connecting it to a standard MakerBot extruder controller and trying to drive it with the MakerBot firmware (recompiled for driving a stepper extruder motor), it turned out that even when just driving the bare stepper motor (no gears, no filament, no nothing), both A3949 motor driver chips on the extruder board heat up to 70-100°C in just a few seconds and start to smell funny. The stepper motor turns as expected during this time, so I’m quite sure the 4 wires are connected in the correct order.
I switched of the whole thing quickly, so I’m not sure if 100°C is the top temperature or if the chips would just burn out (I really don’t want to know…).
Do I something wrong? When reading the Mendel documentation on the RepRap page, it looks like there is no additional electronics needed. Just hooking up the 4 stepper wires to the 1A/1B/2A/2B connectors on the extruder board and go.
When asking for help on the RepRap forum, nophead answered that I’d need to limit the current by using a smaller PWM value than 255. I tried that already, but the chips still get very (!) hot and if using a lower PWM value, the stepper motor looses a lot of its torque, of course.
Does anyone successfully use a stepper motor in an extruder with MakerBot firmware? I’d really appreciate any help on this.
But back to today’s main feature :)
When reading about a “Concept for Extruder” in the Makerbot mailing list some days ago, I really liked the first illustration, Brent Crosby (“baxsie”) attached to his post:
This sketch shows a heater section where a PTFE tubing goes all the way down from the extruder body to shortly before the hot zone in the extruder tip. This should reduce friction in the extruder significantly. And since it seems, that too much friction killed my insulator-less extruder design, I decided to give this design a try.
Please also have a look at the “Concept for Extruder” mail thread in the MakerBot mailing list. Brent documents there the build process of a slightly different extruder design (derived from the above, but using a rather large melting chamber).
Here’s a drawing of what I try to build:
Although I used the general idea of Brent’s design, I changed it in order to get an extruder I could easily use with a standard MakerBot extruder body (and of course with a Printruder II).
To avoid leaking plastic, I designed the PTFE tubing to be threaded at the end where it goes into the brass hot part. This also makes the whole thing somehow more rigid.
The brass nozzle
Here’s the nozzle blank, before drilling the stepped bore:
The completed nozzle. Although out of focus, you can see the M6 threads inside the nozzle:
Another shot of the finished nozzle, this time in focus (kind of):
The PTFE tubing
I presume, that the PTFE tubing in Brent’s original design is meant to be a piece of simple PTFE tube. But since this part is slightly more complex in my design (and I don’t have any PTFE tubes lying around), I turned this from a piece of 15mm PTFE rod.
The outher PTFE shell
This part was rather easy to build. It’s simply a piece of 15mm PTFE rod with a 9mm bore in it:
Once the three parts are manufactured, the assembly of the extruder is straight forward:
1. Screw the PTFE tubing into the end of the brass nozzle
2. Press the above part into the outer PTFE shell
3. Insert the holder
I build the holder (my version of the MakerBot Retainer Washer) from a 2mm thick piece of aluminum bar. The part has a centered 6mm hole for the nozzle and two 3mm holes for the M3 bolts (holding the whole thing on the extruder body). Using the aluminum bar instead of a large washer also allows you to use such an extruder nozzle in a standard Mendel carriage.
Finally, I wound the nichrome wire onto the brass nozzle:
And here’s the completed extruder nozzle, after attaching a thermistor and some insulated wires to the nichrome:
So far so good. I hadn’t a chance to test the new nozzle, yet.
I need to print another Printruder II first, since I don’t want to risk dissasembling my current (and only only working) extruder to test the new nozzle. I hope, I find the time to test it tomorrow.
I’ll let you know, as usual…