PETG printing issue

Hello,
Have a PETG printing issue that refuses to go away. Smaller items print fine, and on larger items the first layer is near perfect, but additional layers exhibit a grainy, incomplete coverage. As a result the part looks terrible and is fragile. I've attached a picture with the problem illustrated.

How it started:
Initially large parts printed fine. Then the problem started seemingly randomly, with the first layer still coming out fine or almost fine, but subsequent layers exhibiting the grainy coverage that seems to get worse throughout the print. I thought it was due to a faulty thermocouple, but after changing multiple of those and the hotend itself the problem persists.

What I've tried:
Changing thermocouple and hotend
Changing speeds (slowing down considerably, even though it printed fairly fast before just fine).
Changing temps (tried about the whole range for PETG, no improvement).
Changing nozzle height for first layer (in case it was clogging due to being too close, no improvement)
Changing filament (tried a different manufacturer in fact, right now I have eSun, before had MatterHackers brand)
Simply extruding with the extrude command right after stopping the print when noticing the grainy pattern (seems to extrude fine, so apparently not clogged)

What I haven't tried:
Changing plastics (I have a new nozzle on the way and will test it with ABS to see if problem persists, don't want to cross-contaminate this one)
Yet another hotend, which I will try with the PETG if it arrives and works fine with ABS

Any thoughts?

Are you cooling the part with a fan? PETG likes to be cooled quite a lot.
I had similar bad results with to high speeds and with an extruder that did not work continously.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

I'm only cooling the nozzle with a fan, I have no fan for the part though I could point the nozzle fan down at it and try. Does seem like an extrusion issue so not sure if part cooling is consistent with that.

I'm actually thinking of turning the fan off altogether and trying to print like that since it's the first layer that prints perfectly (no fan), will report back on how that goes tomorrow along with pointing the fan at the part.

If you don't mind me asking, why was your extruder not working consistently? Stepper motor issue, slipping, poor heating, something else? Sounds like that could conceivably be a problem for me.

At that time it was to little idler pressure if i remember correctly.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

As far as I have seen, PETG likes to go slow. The slower you print, the stronger the parts. I've had to slow all my infill down to 30mm/s to match the perimeters, otherwise it looks very spotty and fragile. I've seen this with eSun PETG as well as Hatchbox PETG. I've also had to increase the number of perimeters for certain parts. Smaller parts print fine, but larger parts with a lot of infill seem to turn out fragile and slowing the speed minimized this affect on my parts.

Right, idler tension doesn't sound awfully likely to me, I've adjusted that enough to think so anyway. Thanks though.

It's not the fan. I notice the second layer starts ok (a couple of good solid lines) then goes to thin, drippy lines that don't completely fill the model, even with fan off.

I'm trying out the speed suggestion right now: slowed it down to 20mm/s. Would be a shame if that's it since it printed fine before at 60mm/s for infill (same settings, same filament), and all prints would now take 3x the time.

Edited 1 time(s). Last edit at 12/28/2016 08:34PM by anvoice.

Here's the latest report:
20mm/s worked better: more of the surface was evenly covered. Attached is an example of the good region. Unfortunately there is also a poorly covered region (attached also) on the same print, again with the thin, drippy lines. The bad news is that I've run into the lower limit of how slow I'm willing to print this.

It's fairly clear it's an extrusion issue, but not sure what's causing it. I'll try the new hotend once it arrives (couple of weeks at worst), but for now I'm out of ideas. Still gladly taking suggestions.

Maybe a better heater element would help. Standard is 12V/25W but at the same size you can find 12V/40W elements. You'd have to do autoPID tuning with the new element.
I believe the slow print speed is just a workaround to fully melt the energy hungry PETG.

It's tough to judge extrusion on clear filament - I'd recommend getting some solid colour while you're testing. The big question is what happens to the filament/extruder when the under-extrusion sets in. When you know what's happening you'll be closer to figuring out the solution. We can assume that the slicer commands the same amount of extrusion throughout the print, so there are only a certain number of things that can happen to cause less filament to end up on the print.

a) The motor stalls - depending on type you'll see the gear jump backwards and hear the clicking noise (what sort of extruder are you using?)
b) The teeth slip on the filament, typically grinding some of the filament away into shavings that build up between the teeth and around the extruder housing.
c) Some part of the extruder drive chain slips on its axle.

edit: I just thought of a d) - filament buckles between drive gear and throat and makes an unholy mess inside the extruder. That one's real obvious too

a) is normally obvious, so I'm guessing you would have noticed that. b) usually goes straight to no extrusion at all, with a semi-circle chewed out of the filament, so doesn't seem likely with the symptoms you describe. On the other hand c) fits the bill pretty well, a gear slipping on the drive shaft will typically catch for a while on each rotation, and often seems to get worse as the print progresses (not sure why - heat?). You could disassemble the extruder and look for witness marks from a slipping set screw, or you could add alignment marks across the ends of the axles onto the gear so you can see if it slips.

Edited 1 time(s). Last edit at 12/30/2016 10:18AM by JamesK.

I think it's becoming a bit clearer now. Heat creep was a likely issue: contrary to what I thought, the fan was not working on subsequent layers after the first, and perhaps the nozzle was getting partially jammed by expanding filament.

Extruder is unlikely as an issue, I did look for lost steps, etc. (simple Wade extruder). Also I agree that filament grinding usually leads to complete jams.

Currently testing on higher speeds to see if the issue goes away completely, but it's looking positive right now. I'll report back the results once I'm done with the latest print. For now, thanks to everyone for the thoughtful input.

Edit: prints fine at slow speed, but higher speed results in bad patches as in the original pictures. Considering I can at least get a decent print out now, I think I can call this more or less solved. Solution was to use low speed and turn the nozzle fan on.

Perhaps this filament just doesn't print at 60mm/s.

Edited 1 time(s). Last edit at 12/30/2016 01:01AM by anvoice.

Heat creep or other forms of nozzle clogging increase the force required to extrude but don't tell you anything about how the system failed to deliver sufficient filament. Until you figure out which mechanism limits your extrusion force you will always be dancing around the edges of your problem instead of getting to the root of it.

Edit: missed your edit Yeah, I can't print petg at 60 either, 30 is about my limit with typical nozzle sizes and layer heights.

Edited 1 time(s). Last edit at 12/30/2016 10:16AM by JamesK.