Apparently systematic nozzle clogging
The following observations can be made.
The nozzle clog appears at different heights/elapsed times, so it does not seem to be because of heat creep, which ought to take the same time to take effect, each time.
All the clogs so far have been easy to clear manually, once the hotend has been made accessible.
The filament (1.75 mm PLA) is quite new, has only been sitting on the spool at the top of the printer, seems regular in density, is not visibly damaged etc.
In the original configuration, before the upgrading, the printer worked fine for some months.
One possible explanation might be inconsistent temperature in the extruder, but the display does not show any great variation.
Any other theories? These latest prints were made at 210 °C.
The nozzle clogging was verified, each time, by trying to extrude using the printer controls; and then cleaned manually by taking off the "cold end" part, heating the extruder, and pressing the remaining filament out by hand.
Not so long ago the nozzle was cleaned with ethyl acetate and a needle, so I do not believe carbon build-up is the explanation.
Did you by any chance disassembled nozzle from heat block? There is a chance that the nozzle is not (or was not) adhering to the throat and that creates deposit which blocks filament. The best way will be to clean the block, nozzle and throat, especially thread.
Possibly the same question as "Extruder prints fine up until further down the print"
Now you have your print settings added, I've updated the answer. Basically, you print too hot, too slow and have a far too high retraction length. This causes heat to affect the filament before it enters the nozzle, with high retractions (retracting hot/flexible of heat filament) you then quickly cause a blockage to develop.
The Micro Swiss hotend uses an all metal hotend. These type of hotends are more difficult to operate considering they do not have a Teflon liner that shield the filament of heat creep. From this article:
Jams and clogs are often from a combination of excessive heat and
non-optimal material flow. This effect is worsened by poorly cooled
all-metal hot ends, high torque extruder gears, small nozzles/layers,
slow printing speeds, too thin first layer, and excessive retraction.
The bold faced text in the quote sums up what is causing this. A smaller gear requires more force/torque as the arm i.e. the radius is smaller.
The article describes what steps you could do to alleviate the problem. Of all the suggestions, "Minimize retraction", seems a possible candidate for you to look into considering the posted print settings. As this is a heat related problem it is advised to also increase your printing speeds, these are pretty low (30 mm/s for slow and 60 mm/s for normal printing) and also check the cooling of the "cold end" (the fan that cools the radiator fins). Also reduce the printing temperature, 210 °C is pretty high for PLA filament, personally I don't go over 200 °C (note that this depends on your filament, but most PLA brands can be printed in the 185 - 195 °C range).
You have a pretty large retraction specified. The Ultimaker default is 6.5 mm is considered to be large, but works perfectly for Ultimaker machines (read Bowden tube setup). In my Ultimaker 3E which uses all metal hotends, or, in my custom HyperCube Evolution, also Bowden, but with a lined hotend, 6.5 mm retraction works perfectly.
Please look into this answer and this answer. Both describe that the retraction performance is worse with all metal hotends. My experiences are exactly the same with metal hotends, at least the cheaper production ones (I tested cheap all metal hotends, but also ran into problems because of production and design errors, I have not tried the better quality heat breaks/throats yet).
Please lower the retraction setting considerably to see if it has an effect. The Monoprice Maker Select uses a direct drive. Direct drive extruders do not need a large retraction length setting. If the filament is hot in the throat (as there is no PTFE lining that in fact acts as an insulator), too large of a retraction may not be reversed when the filament cooles during the retraction.
To comment on your statement in comments above, I am not suggesting you should use a liner in your current extruder. I'm pointing out the differences. Metal hotends are just more tricky to operate regarding retraction and heat management.
@TomasBy You should increase the speeds, current values are way too low. Only for the first layer you could do a 20/30 mm/s (don't go under 20). If it was my problem I would revert to default Ultimaker Cura settings and start over by first reducing retraction.
@Oscar: there are quite a few differences between the default settings in the sw and the suggested values in the manual. I have now tried several different combinations of speeds, with low retraction and 200 deg. print temp., and am getting the same results all the time, with some minor variations. Under-extrusion apparently in the form of some temporary clogging (the filament spool does not move) but no actual clogging of the nozzle. I can extrude fine using the control panel.
@TomasBy That is the pain with those all-metal hotends (please confirm in the question that it is an all-metal hotend, so no PTFE lining in the heat break). It could be that when you extrude from the panel, the extrusion rate is higher than when you actually print, too low print speed cause heat to creep up.
There are several reasons why the nozzles can clog.
If you set the temperature too low, the force to push the filament through the nozzle gets up. This can lead to filament grinding and this can lead to clogging. If you set the temperature too high while not extruding, the filament can degrade, which may lead to clogging. So always test the temperature and viscosity by manual pushing filament through the nozzle.
Many users do not like to change the nozzle but for many filaments 0.4 mm is not advisable. Try 0.6 mm. The print quality is nearly the same @ similar layer sizes, print time goes down a bit and the likelyhood for clogging is close to zero. Reason is that the flow resistance is proportional to r^4.
Grind dust/traction force -> Retraction:
I suggest a max of 2 mm to 2.5 mm for direct extruders. Higher values bring no benefit and lead to filament grinding. Bowden Extruders can use values > 5 mm due to bending sometimes.
Sometimes the spring of the feeder loses tension over time, thus the filement is not pushed forward and instead, grinded down.
Another problem can be related with the pushing gear of the feeder (which you mount on the motor). I use gears with fewer theeth. They produce traction force and stay clean from grinding dust. Gears with many small teeth become dusty and produce less traction force.