Underextrusion and jams - typically after retractions.
This has been driving me crackers for a couple of months now.
The Mini starts to underextrude, or jams completely, at alarmingly frequent intervals. For 12+ months it was fine, but for the last two months I’ve barely been able to ring a darn thing. I’ve stripped, cleaned and rebuilt the hot end about twenty times. Swapped the PTFE tubing again and again. Stripped and cleaned the extruder. Changes the nozzle. And it still flaming does it.
Problems are always triggered by retractions; it'll print a plain square, or something very simple, without trouble. But the moment it starts to perform retractions, things go to hell in a hand basket.
Odd thing is, most of my filament is fine - doesn’t happen. But the seller no longer stocks the line, and every flaming brand I’ve tried since (including Prusament) has the same ruddy problem. Filaments have been dried to within an inch of their lives; makes no different. Altered the tension on the idler so much it’s a wonder that there’s any thread left makes no difference. Changed retraction distances and speeds - no difference. About the only thing I haven’t tried is throwing the machine out of the window - that’ll be the next thing I try. 
RE: Underextrusion and jams - typically after retractions.
Can you give more info about how you dried your filament, what temp, how long the filament is being dried, and how much is relative humidity in your room?
Perhaps you could describe the symptoms of the issue...
How do you clean your print sheet?
Cheerio,
RE: Underextrusion and jams - typically after retractions.
With 99.9% isopropyl, or washing up liquid, or both. This is NOT a bed adhesion issue - what you see in the pictures with a failing first layer is underextrusion/clogs. Which does not happen with large, flat surfaces, only with prints (like these) with lots of first layer retractions.
Nor is it a filament problem; the black pictured has spent the last fortnight in a dryer, heated to 55 degrees, and surrounded by silica gel. It’s bone dry.
I’ve been using the bondtech heartbreak for about twelve months without problems, but I’m wondering if a firmware update a few months ago has done something like thrown the retraction distance and speeds out of whack - because the more I look at this, the more I am convinced that this is the real source of the problems. And it was fine at the start of the summer.
I'll rebuild it tonight with yet another new PTFE tube, might swap the bondtech out for the stock heartbreak (though that never worked reliably for me either), and see if I can get anywhere by tweaking some settings. Another night wasted won't matter - I’ve barely been able to print anything for quite a few weeks now.
There are a few visible smears on the print surface ...
What were the symptoms that caused you to change the heatbreak and what are the symptoms that cause you to suspect retraction on upper layers?
Rather than tweak settings return them to defaults. While you are working on the machine check through this page in case there's anything you've missed:
https://help.prusa3d.com/en/article/regular-maintenance-mini_133222
Cheerio,
RE: Underextrusion and jams - typically after retractions.
The smears are there because I'd had to use my fingernails to scrape off 20+ failed parts; I cleared just enough space on the print bed to print a 75m square first layer calibration test (which printed fine - save for the first few mm of the first corner - again, suggesting to me that something is up with the retraction), then printed the Benchy which failed on the wheelhouse (the bit with lots of retractions), and took a picture. Normally the print plate is much cleaner than that.
And yes, I do the usual routine maintenance from time to time. This weekend I'd gone several steps further - such as cleaning the filament-gripping "teeth" of the extruder gear with a dental pick, as some material was not coming out with a light brush. But that was a symptom of the jams (the gear grinding hollow sports on the filaments) rather than the cause (jams in the hot/cold end).
I'd swapped to the Bondtech heatbreak over a year ago - largely to deal with "ordinary" clogging and jamming that was plaguing me! It has been fine until relatively recently.
I think that it's returning things to their defaults that might have thrown things off!
RE: Underextrusion and jams - typically after retractions.
Ok, so if you think this is a retraction issue, you might want to reduce it. Many prusa mini user have retraction problem that causing many issue, and fixed by reduce retraction length from default 3.2mm to 2.0mm.
Honestly i don't want to suggest you this at first because you mention at earlier that you change the nozzle. And this retraction tweak is to compensate when people don't want to change the nozzle. But if you curious, you can try it.
RE: Underextrusion and jams - typically after retractions.
I remember having a massive problem with stringing early in my Mini ownership journey, so I'd never tried going BELOW 3.2mm. But that's certainly something I'm going to try.
The nozzle change is unrelated, but something I thought that I'd do as I mostly print plain geometric shapes (board game inserts), so I'd be printing faster and stronger. Someone suggested less clogging, too - but that has NOT proved to be correct in teh few hours I've had the machine operating recently.
I need to do a whole load of calibrations now (such as flow rate), but I get scared off by the idea that the sodding thing will just jam almost straight away 🙁
RE: Underextrusion and jams - typically after retractions.
Some days this 3D printing malarky can really hack me off.
I put everything back together, including the Bondtech heatbreak, changed a few entirely inconsequential settings, loaded my worst, most finicky PLA filament, and printed a few calibrations and tests. Which whilst not perfect were good enough, and exhibited none of the clogging and underextrusion I've been getting for weeks.
I'm currently printing a complete git of an object to print - overhangs (the 0.6mm nozzle should not like them), small details, and retractions every second or two.
And the ruddy thing seems to be printing fine.
I wish I knew how/why.
A fix is a fix, sometimes it is unwise to question the fates...
Cheerio,
RE: Underextrusion and jams - typically after retractions.
Hi, I have the exact same problem. I have taken apart the hotend and extruder and cleaned and put back together. If I print the z axis calibration it works just fine (no retractions), however when I try to print something with retractions it works initially then the extruder starts to click and a few moments later everything jams up and nothing comes out of the nozzle. I've gone through this process multiple times and the same problem occurs every time. Driving me crazy.
@john-13
Which filament?
Have you tried tweaking the extruder idler pressure?
Cheerio,
RE: Underextrusion and jams - typically after retractions.
PLA, multiple vendors. I have adjusted the extruder. I'm going to take it all apart today and see if I can find any faulty bits. After watching it for hours I think the problem may be in the hotend as it seems the extruder can't push the filament after some number of extractions.
RE: Underextrusion and jams - typically after retractions.
Well after disassembling the hot end and remove all the ground bits in the extruder it's working. The only problem I could find is that the hot end heatbreak may have been slightly loose. Went thru the remove/replace procedure and it seems to be okay now.
Wooooa, slow down. IF you have *exactly* the same problem as the OP, which I doubt, then his published 'fix' is not going to help you.
On that point, please show us a typical failed print then run a first layer Z calibration and show us the result *on the print sheet*
Do you get clicking on the first layer?
Does the clicking speed vary along with the speed the head is moving?
Cheerio,
RE: Underextrusion and jams - typically after retractions.
Well Richard's "symptoms" were exactly what I experienced, we only had problems when the print had retractions. I could print first layer calibration all day without any problems. I noticed that while there was no conclusive fix, the last thing Richard did involved the heartbreak which got me thinking that was the problem - maybe just the nozzle, heartbreak, PTFE tube adjustment, after cleaning it carefully. It did work for me and Richard it seems. I think the loose heatsink, nozzle connection would catch some melted filament on retraction and then cool in the gap between the nozzle and heatbreak. causing the ultimate jamming. My hotend is totally prusa. Anyway it is back to printing perfectly now.
RE: Underextrusion and jams - typically after retractions.
As John says. I’m not sure quite what it was, but it went away for a bit. It’s crept back into my prints slowly, though, with some filaments behaving better than others.
To be exactly the same @john-13 would have had to change the heatbreak and nozzle as you did and so on.
We see quite a few issues that magically go away when some fundamental change is tried - often not because that was the cause but because in the process the general maintenance, screw tightening, alignment checking or whatever actually was the cause is done as a part of the process, if you like it's a side effect.
with some filaments behaving better than others.
We usually talk first about humidity but there is some evidence that some filaments degrade with age in other ways.
Cherio,