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Prusa MINI back from the problem zone.  

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fjkraan
(@fjkraan)
Eminent Member
Prusa MINI back from the problem zone.

Just recently I fixed my Prusa MINI which had all the usual symptoms; skipping, damaging filament and above all, serious under-extrusion. At first I thought the problem was in the extruder gears, then the stepper motor driver and finally the the hot-end assembly. At the end disassembly, cleaning and reassembly of the hotend using mostly original components solved the problem. I did communicate with Prusa support, which led to focussing on the hot-end and this thread: https://forum.prusaprinters.org/forum/general-discussion-announcements-and-releases/under-extrusion-over-extrusion-clicking-pilling-inconsistent-layers-clogging-unloading-problems-its-probably-all-the-same-core-issue/.

 

But first how the problems started. In my case, there is a distinct moment which started the problems. After the Prusa MINI arrived, it worked very well and I switched to a 0.25mm nozzele for printing smaller, more detailed objects. There is also a MK3S so switching one printer to a different nozzle for a longer period made sense. With the smaller nozzle, the MINI also worked very well, printing both PLA and PETG.

 

When switching back to a 0.40mm nozzle, I accidentially replaced ot with a different 0.25mm one and started printing with gcode expecting the bigger nozzle. This led predictably to problems; skipping and under-extrusion. After finding and fixing the problem however, the problems remained. Small objects might print almost without problems, bigger ones were sure to fail. For testing I reverted to Prusament and the example gcode that came on the USB stick. Higher hotend temperatures did seem to work a bit, but only temporary.

 

Cleaning and tightening the extruder gear tension did not help, but at least I managed to eliminate the filament skips, having those being replaced by stepper motor skips. My conclusion here was that the coupling between filament and extruder gear can be made solid. The filament gets serious gear marks on one side and is pressed flat on the other (PLA). So this construction can be resist more force than the stepper motor can deliver than. Which could mean the BondTech gear solution is overkill here.

 

So far I never heard about problems with the driver chip on the Einsy board or with driver current settings. This and the input from Prusa support led to the hot-end. After disassembly but before cleaning I made an image peeking down the heatbreak tube. The dark ring (black/dark-blue) is the filament I used just arter the failed nozzle switch, the red was used during the last experiments before disassembly. The PTFE tube was only very slightly deformed, being somewhat narrower at the bottom end. The amount of filament at the bottom of the tube looks and actually was, very small.

After cleanup, the parts looked good enough to reuse* until the ordered spare parts (heatbreak tube and PTFE tubes) arrived. Since then the MINI worked as expected. Just once the extruder gears failed, but this was with known under-dimensioned filament. I threw that away. But all and all it prints reliable again (just 0.40mm nozzle and PLA for now).

 

My sincere thanks to Šimon from Prusa Customer support and all on this forum.

 

Greetings,

Fred Jan

 

*) The only exception are the three M3 heatbreak grub screws that were replaced by normal Hex-head screws. I think it is more the tolerances in hex-tool sizes than the screws themselves that lead  to problems here.

Posted : 16/05/2020 11:33 am
bobc
 bobc
(@bobc)
Reputable Member
RE: Prusa MINI back from the problem zone.

I think this issue has been well documented here, it comes up at least once a week.

I guess it would help people to find if people used more descriptive thread titles.

Posted : 16/05/2020 3:40 pm
kevman
(@kevman)
Estimable Member
RE: Prusa MINI back from the problem zone.

I'm glad that your issues have been resolved, unfortunately my luck has not been as good. I thought for sure my issue was the hot end, ptfe tube etc. I've rebuilt my hot end seveal times, even have not upgraded to the bond tech heat break, but still have the same issues. Prints go on great for the first few layers and then 20-30 mins in I get skipping and ultimately clogging. I beleie that there is perpetual heat creep that that cannot be solved with all the steps I have take, maybe some additional cooling might help but I am close to give up.

Posted : 04/06/2020 12:50 am
Crawlerin
(@crawlerin)
Prominent Member
RE: Prusa MINI back from the problem zone.

Glad to hear your MINI is working now. Yes, the Bondtech motor upgrade is overkill, but to be fair it was stated as such when they presented it in Facebook group. They said it's overkill and not for everyone, it's a "nice to have" for those who want to have the best parts possible, not an essential upgrade.

Posted : 04/06/2020 12:26 pm
fjkraan
(@fjkraan)
Eminent Member
Topic starter answered:
RE: Prusa MINI back from the problem zone.

@kevman: Before I found the apparant cause, I tried to make testing fast and cheap by designing and printing 1cm cubes. This was less frustrating than the larger sample objects. I noticed the problems were considerable less and came later. Initial printing went well. So the problem is a dynamic build of filament pressure, which somehow restricts the flow of pressure.

@crawlerin: It speaks for BondTech that they consider it overkill too. The standard extruder can overload the hotend and cause problems there without problem.

Posted : 04/06/2020 1:07 pm
bobc
 bobc
(@bobc)
Reputable Member
RE: Prusa MINI back from the problem zone.

Initially I had problems with extrusion, I moved the heatbreak up and the problem went away. However, I subsequently had the problem recur, but it turned out the cooling fan wasn't running 100%. One of the wires to the hotend was rubbing slightly on the fan reducing it's speed. After rerouting the wires it worked again.

All the evidence points to a quite marginal design. When everything is dialled in, and good quality filament like Prusament, then you can find yourself on the good side, but if you are unlucky it's easy to stray into the non-working area, and workarounds like increasing print temp are required.

As an engineer, I would say Prusa have optimised the design to work well in ideal conditions, but it is not robust enough to work well for "worst case" conditions.

Anyway, I am building a Mini "clone" with a standard E3D v6, which should eliminate hotend issues.

Posted : 04/06/2020 1:36 pm
Oxygen liked
kevman
(@kevman)
Estimable Member
RE: Prusa MINI back from the problem zone.
Posted by: @bobcousins

Initially I had problems with extrusion, I moved the heatbreak up and the problem went away. However, I subsequently had the problem recur, but it turned out the cooling fan wasn't running 100%. One of the wires to the hotend was rubbing slightly on the fan reducing it's speed. After rerouting the wires it worked again.

All the evidence points to a quite marginal design. When everything is dialled in, and good quality filament like Prusament, then you can find yourself on the good side, but if you are unlucky it's easy to stray into the non-working area, and workarounds like increasing print temp are required.

As an engineer, I would say Prusa have optimised the design to work well in ideal conditions, but it is not robust enough to work well for "worst case" conditions.

Anyway, I am building a Mini "clone" with a standard E3D v6, which should eliminate hotend issues.

I'd love to see how that hot end setup works, I may have to move to something like that.

Posted : 04/06/2020 4:56 pm
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