i3 Pro C Dual Extrusion Problem Fix!
Here was my problem. The hotend would intermittently just stop outputting plastic. This was most obvious on the base layer. There would be areas with no plastic. I could watch the print process and see where the plastic just stopped extruding. Other areas would be very thin. Manually pushing the filament into the servo helped the problem. This is not to be confused with filament not sticking to the bed. You can see the difference if you look closely.
One way to test. Heat up the extruder. Mark your filament with a sharpie (gold or silver works well on dark filament) so you can tell if it's moving. Also mark the shaft of the feed servo, visible in the hole in the center just to the side of the fan. This way you can easily see if the servo is turning to feed filament. Heat up the extruder and raise it off the bed (Z axis) by a good amount so you can see. Use Repetier to advance 100mm of filament. Watch the mark on the servo and the mark on the filament. Both should be advancing smoothly, with a steady stream of plastic coming out of the hotend within a few seconds. The filament should be moving smoothly with no jerkiness of stops. If you hear a thumping coming from the extruder, then you probably have a clogged head. Otherwise, continue on.
The problem was that the filament would not feed continuously. It would stop during the feed. I could push it in manually and it would work better for a short time but would then stop. The servo was turning correctly, but didn't appear to be advancing the filament.
The solution is two part. Both involve increasing the pressure exerted on the filament by the feed wheels.
First, we need to increase the tension on the spring attached to the release lever. This is addressed in this thread: http://www.geeetech.com/forum/viewtopic ... 19&t=18507
If you don't like using paper like they did, there are shims available on Thingiverse. I used paper to get mine running long enough to print the shims. http://www.thingiverse.com/thing:1936289
Secondly, we need to decrease the distance between the gear and the idler wheel that actually push the filament into the hotend. You need to trim some plastic off the idler arm. On mine, I took enough off so that the idler wheel rests against the gear. It works fine
You can also replace your nozzles with 0.4 mm ones instead of the stock 0.3 mm nozzles. You get better detail with 0.3, but 0.4 are more forgiving and you will be able to print faster.
Repeat this process for the second extruder.
Now, you need to level and set the distance. Follow the instructions in the manual for leveling the first extruder. If the tip of the second extruder is lower than the first, you may need to move it up and out of the way if it is pushing on the bed and affecting the bed height. Note, when you are doing the coarse adjustment involving the Z-axis end switch, you will need to re-home the Z-axis after every adjustment to the screw.
Once you have the first extruder gapped and set and working, you can move on to the second one. The gap from the tips of the nozzles to the print bed needs to be very close on both extruders. There is a video on how to make the adjustment, but don't do it this way. Note how the nut is loosened and how the height is adjusted, however. They are using a 10 mm wrench.
Here's the trick. In the video, they turn the hotend a full turn and everything is still nice and straight. In reality, this won't happen. You will likely need to make a partial turn to get the gap just right. Home the Z-axis, then slide a business card under the first tip and then the second tip. You should have a similar amount of drag (just a little) from both tips. It's best if you only adjust the SECOND extruder and leave the first one alone. If there is more drag on 2 than on 1, you will need to loosen the nut and screw the barrel in a little more. If it's too loose, the you'll need to loosen the nut and screw the barrel out a little. Start with quarter turns. You'll likely need to rearrange the heater wires to do so. You can make the adjustments with the extruder assembly still mounted if you are patient with the wrench (it's a little tough, but it can be done). Be careful with the wires. I ended up breaking one going to the thermistor causing a MINTEMP ERROR and had to re-solder it.
Here's how mine ended up looking:
I had to adjust the first extruder a little because I couldn't quite get the second one into the right position because of the wires and the backplate interfering.
The gaps need to be very close in size. If the gap on #2 is too low, it can bump the print (you can hear this as it crosses onto the print) and cause print problems or even scrape the layers off the bed. If it's too high, your print may not adhere or may get scraped off by the first nozzle. Getting this right may take you some time, but it can be done.
I hope this helps some folks avoid the frustration I had!