Odd extrusion failure...

[cobnut]

I’m having an odd failure of extrusion on a print (see photos). There are two of these locating pins on the top of the model and they’ve failed because filament (PLA) stopped extruding. On inspection, the hobbed gear was choked and the filament had the usual gouge in it, but why did it happen right at that point? This was a 12 hour print, and these two pins probably only take 10 minutes of that time right at the end (they’re the highest point). This isn’t a one off either, this has happened twice now, with the failure at exactly the same point.

So the question is, why is the extruder perfectly happy for 11 hours 50 minutes, producing a perfect print, then clogging in the last 10 minutes on these pins? I was wondering if it is something to do with retraction, maybe switching between the pins (which are quite small, maybe 4mm diameter) is causing too much back and forth on the filament while extruding very little? The printer (Original Prusa i3 Mk2), quite happily prints detail far finer than this normally.

Any ideas? I’ve another print going right now, and there’s 10 hours left before we reach this point. At the moment the hobbed gear is perfectly clean.

[wirlybird]

What are the settings? Speed temps etc.? Is the spring tension set ok? I think the distance is 13mm.

[cobnut]

Hi Chris, I’m pretty sure it’s not a speed or temp issue; I’ve been using this filament supplier with the Prusa for over 6 months and the prints are all fine, normally. The spring tension is a good call though and something else I checked this morning before starting this latest print (which, by the way, is still going fine with no obvious clogging on the gear after 3 hours printing). A week or so ago I had a similar failure to extrude (I’ve just remembered), and I “fiddled” with the tension springs. This morning I had to take them out entirely to clean the gear and set them carefully to 13mm when re-assembling, so maybe the problem is already fixed…

It’s still weird though that it would only happen when printing these pins, isn’t it?

[Perry_1]

Well cobnut, you might want to cancel that print, and start over, because that little nob is going to be a glob, if you have not changed your cooling settings.

This is PROBABLY a cooling issue, plain and simple. The tiny locating pins are too small to print at the same speed and temp as the rest of the large print. So the layers are not cooling to a solid enough before the next layer gets put on it, so it starts globbing.

Increase cooling settings, or minimum layer time, or whatever. In simplify3d you have several options. (sorry, not familiar with other slicer)–>

Under the cooling tab, crank up your “adjust printing speed for layers below” to a larger number, and set the “allow speed reductions down to” to be a lot lower.
If your slicer allows it, which S3D does, you can actually create multiple print processes. Stop your normal print process when you get to that height, then use a process that is A LOT slower while you increase cooling settings for that process. Not sure how to do this in other software.

ONE OTHER OPTION may be your accelleration settings. If you can lower those, you may find that the knob prints better.
OH YEAH! The pin, in the stl, could have such high resolution, (dont know what slicer you use) that it is making the printer stutter trying to do all the resolution. Its usuallly not noticable on large cylinders, but can be noticable on small high resolution cylinders. The printer always takes a circle and turns it into line segmeents.

My money is on cooling though. Cut that model off, just use the very top part, and experiment. Stop doing a 12 hour print to test a 5 minute part of it, lol!

[Perry_1]

I know you are an experienced printer, but I am still going to put my money on cooling!

[Perry_1]

OK, maybe resolution of the pin… maybe…
The problem with cylinders unlike most other shapes, including things like letters and such is cylinders in modeling packages are often just described within the modeling package as a center and radius. So when the software gets exported in STL, it can have a LOT of straight lines making up the cylinder, causing issues with x/y motion, or trying to do so much with tiny segments that the printer stumbles…

[wirlybird]

Is the gear tight and the set screw on the flat part of the shaft? Seems to be common.

[wirlybird]

Is the filament feeding smoothly off the roll on the small part?

[Perry_1]

[jfield]

You should try just printing the top surface of this part only. With S3D, you can just drop the part down below the print surface. Maybe just print from that notched area up.

[Perry_1]

@cobnut Checking in with you to see how it is going?

[cobnut]

Hi folks, well, I think @wirlybird was on the money asking about the temperature; this print was “standard” PLA which, from my supplier on this printer, I print at 205C, but I’d accidentally used a profile for PLA+, which prints at 220C, way too hot for that filament, so I suspect there was some “heat creep” and the filament up by the hobbed gear was getting too soft. It doesn’t really explain why the rest of the print worked OK, possibly because of a higher throughput during the main body, but I watched the pins being printed last night and there is a lot of shuffling to and fro, so maybe that, combined with filament that had got soft, was the cause.

Whatever the reason, the last print came out perfectly (when set at 205C), right down to the slightly domed top of the pins and the hobbed gear remained clear.

@Perry_1 it being PLA, and a 50 micron, high detail print at that, the profile is already using 100% fan so if it had been cooling, I’d have been a bit stuck It’s hard to tell from the photos but the pins weren’t melted they simply weren’t printed at all; the mess is what remains of half-extruded perimeters and infill rather than overheated blobs.

[Perry_1]

So glad to hear its been fixed. I was going to try to jump in and print your model (just the top part) on my printer to see if I could figure it out! I was just imagining you needed these 10 hour prints, and the fact a tiny part on top was preventing the print job from being completed was making me lose sleep!!!