I’m not the most experienced person around here, so it might be better for someone with more experience to jump in here.
However, let my try to think this through:
I see two possibilities depending on the direction of travel:
If the direction of travel is in the direction of the taper, then that would suggest the flow rate is gradually being choked off before suddenly surging back to normal. My first thought was a partial clog. I’ve never had one that I’m aware of, but a partial clog would cause filament to back up in the hot end until the pressure popped the clog out, resuming flow. Although, as I think that through (again, no direct experience), a couple of problems with that theory jump to mind: a clog would be a sudden event, as would its release; a clog would also be entirely random both in when it occurs and how long it lasts.
If the direction of travel is opposite to the taper, that would suggest a sudden loss of pressure followed by a gradual resumption. Is the extruder skipping? Try letting the filament slide through your fingers at the point where it enters the extruder. That’s going to be difficult on a direct drive. I’m wondering if you can feel changes in filament feed rate.
I suppose there’s a third possibility: that the filament has a an inconsistent diameter. I’ve heard of some filament having poor quality control with respect to diameter resulting in wildly different feed extrusion rates, but the fact that there appears to be a pattern to it suggests that isn’t the case.
I note that the problem doesn’t appear completely random. For each incidence, the distance from the point where the filament first tapers to the point where it is widest again seems to be roughly the same. The occurrences are random, but the behaviour within each occurrence seems consistent. If the clog theory were true, I would expect the behaviour during a “clog” to be variable as I would expect the clogging particle to not only appear randomly, but be of random sizes and hence produce random recoveries. It’s the consistency that has me puzzled.
Is this consistent in both the X and Y directions?
I can’t tell from the photo, but is the height consistent? I’ve been assuming the extrusion rate is being affected, but as I think about it, if the height of the bed was changing (let’s say, due to dirt on the rollers), then the extrusion rate would remain constant, but the line width would be thinner and taller vs. shorter and fatter. Still. dirt wouldn’t produce a tapered pattern and the taper wouldn’t be of such consistent lengths.
So, here’s what I think I can eliminate:
- partial clogs will appear randomly, but don’t resolve themselves so consistently
- variable filament widths will result in variable extrusion rates, but again, that taper is damn consistent in length. A filament with such a sawtooth profile you would almost be able to feel.
- dirt on a track would produce inconsistent layer thickness (and hence, width), but it would appear at the same point across all the lines and again, it doesn’t produce a consistent taper.
- dirt on a wheel might be more random in location given that the wheel may slide a bit before rotating over the dirt, but again, while it can produce variable layer thickness, it doesn’t produce the taper.
- dirt on an extruder gear will produce variable extrusion rates and might even produce a taper, depending on how the contamination is distributed among the extruder gear’s teeth, but the occurrences will have a pattern.
I suppose I should have thought of this at the start: is the direct drive new? If so, I take it you didn’t have this problem when it was a Bowden setup? Have you used this filament with the Bowden setup? If ‘yes’ to all of the above, it eliminates inconsistent filament thickness, and contamination of the wheels and track.
Has the direct drive always behaved this way? Have you inspected the direct drive gear to confirm it’s clean?
Is the direct drive’s gear tightly attached to the shaft? I’m thinking that a gear slipping on a shaft could cause random changes in hot end chamber pressure which would then fairly consistently ramp-up to normal again.
I hadn’t considered the above initially because stepper motors generally have D-shaped shafts that allow gears to be connected tightly, but I just had a look at my extruder, and the extruder gear (the part that engages the filament) actually has a round hole in it, not a D-shaped hole as I would have expected. Therefor it would be vulnerable to slippage which I would imagine would give results similar to what your photo shows.
At the moment, that’s my most plausible theory.
If you have a fine-point sharpie, put a line across the extruder motor’s shaft and continue it across the gear, then check after a while to see if they are still aligned. If you don’t have a fine-point sharpie, use an exacto knife to make a scratch across both pieces to use as an alignment indicator.