Help. Extrusion limited with both hotends

My Setup:
Modified Prusa i3 with dual bowden
360W 24V power supply
1.1b RAMBo
E3D V6 hotends (Genuine)
MK8 drive gear

Lately, I've been trying to tune in my extrusion amounts since Slic3r has gotten to the point that perimeters and infill won't touch with a 15% overlap setting, as well as Cura even having some issues in that regard, though better than Slic3r, but I've run into a problem. If I remove the bowden tubing and hot end from the equation, the mk8 drive gear extrudes perfectly at 155 steps/mm. I tell it to extrude 100mm, and it does. This is repeatable and reliable. The same goes for adding the bowden tube to the equation. However, when I add the E3D V6 hotend with a 0.4mm nozzle to the equation, it will only extrude 88-90mm of filament when told to extrude 100. at first I thought, ok, maybe I just need to tighten the idler, but that either had no effect or made the problem worse when tightened enough to deform the filament so much during extrusion. I've tried this with PETG at 245C as well as ABS at 250C and get the same results. I have verified the hotend temps are accurate and have disassembled and cleaned the hotends as well as performed a couple cold nylon pulls that revealed no internal issues with the hotend, ie, no gaps or pits for filament to have problems with. I've verified the voltage at the board at 24.8V. The motors I have are rated for 56oz.-in. holding torque at full power, 1.2A, but I'm running them at .75A. Which gives me a holding torque of 35oz.-in. I have tried increasing the current to the motors to both .85A and 1A in the off chance it as getting too much resistance from the hot end, but the extrusion results remained the same. For these tests, I've tried extrusion speeds of 10mm/min, 50mm/min, 100mm/min, and 150mm/min. All giving the same extrusion amounts plus or minus 2mm at the most. Yes, you read that right mm/min. At 10mm/min, I figured there shouldn't be hardly any resistance at all from the hot end to prevent extrusion, yet it still only pushed through 90mm. This is happening on both hotends each with separate motors. I even tested the holding torque of the motor and found it consistent with the expected number based on amperage and holding torque at full amperage. I've done that test for .75A, .85A and 1A. The failure mode of the extrusion was filament stripping. regardless of idler tightness. Does anyone know what might be causing this? The bowden tubes are about 18" in length in case anyone thought that might be an issue. Any help with this would be greatly appreciated! Thanks!

Edited 1 time(s). Last edit at 04/23/2016 03:37PM by zborschuk.

It looks like you are a methodical person, which is good to find a problem origen and fix it, but some times you need to understand that you are dealing with an imperfect machine that uses shortcuts and simplifications to operate at minimum cost.

Your printer is just fine and it's only dealing with real life physic. With the resistance from the hotend the filament will snake in the bowden tube, the filament will expand inside the hotend before cold material can push it out and some other 100 different factors will work against you.......

I always calibrate my extrusion with operational conditions so when I ask it to extrude 100 mm, it will do it in real life conditions.

So if you didn't catch what I just said: adjust your steps per mm until you get a 100 mm with real life conditions. If you think the shortcomings of a bowden system are greater than its advantages, change to a direct drive system.

Good luck and enjoy 3d printing.

So with the hot ends working its pushing less filament. That's the product of the resistance and causes grinding.
Since you are printing at low speed it has to be an issue with the hot ends or from the pressure.
One thing to check is the lead in chamfers on the Bowden tube fittings. Especially if you have a metal insert near the hot end. Don't forget the filament will deflect under load so can snag on a sharp edge.
Check the Bowden does align correctly with hot end.
Other issues I have heard of include shaving of the filament that causes a build up of dust in the Bowden leading to jamming but I don't think you will have this over the small amounts you are feeding for calibration checks.
Make sure you have reasonable alignment of the Bowden to the drive gear, not a big gap or it may bend before it gets in the Bowden tube and not offset.

You say it is not extruding enough.
Is that in general or at the beginning of a print?
I am asking because the bowden system will need quite some length before you reach an even pressure.
Same goes for retractions, which need to be longer.
If you push 100mm without the hotend and have no issues to the point that the filament is stripping when you block it then there is not much left.
Watch carefully what happens when you start a print by making some markings on the filament every few mm.
This should happen:
1. Nozzle purge until the filament runs out good.
2. Print starts and might bit a tiny bit under extruded if you need a lot force in your hotend.
The filament will move evenly through the system.
3. till the end of the print there should be no issues.

If you notice the filament is not moving evenly through the bowden system while the extruder is turning you have a resistance problem.
If you see the filament stopping in the tube while the extruder still runs it means you might have a sharp edge somewhere that is snagging the marks from the extruder on the filament.
If you see the filament running at an even speed, the extruder is fine but you can hear clicking noises than there is high chance your filament is being caught on the tube connectors or an edge in the extruder.

If you can double check your nozzle is actually the size it should be.
I had a few .4mm ones ordered and received a collection instead.
Smallest is just 0.35mm, the widest 0.46mm - seems like a cheap production job.
With these tiny diameters the extrusion multiplicator becomes important and if the real nozzle diameter is smaller than what is set in the slicer it will always overpressure and have a more or less destinct gap between lines.
Looking quite similar to what you would expect when you z-axis move up too much.