Intermittent Filament Flow
Hello all.
I’ve been having this issue with my mk3s+ where the flow of filament simply seems to stop/start, seemingly at random. It mainly seems to happen during the first layer. I’ve included a photo of a first layer calibration to show the problem. It seems to happen with various brands of PLA. The only “solution” I’ve been able to pull off is to increase the flow rate to something like 175 and slow right down to 40-50% speed and then turn the flow rate back down again as soon as the first layer finishes.
So far I have tried:
Replacing the thermistor (there was damage to the cable from a failed print).
Adjusting the “grip” pulley tension.
Checking and cleaning the nozzle and hot end, including replacing the PTFE tube. Checking the extruder pulley is not loose on the motor shaft and that the other pulley can spin properly.
The machine has about 200 days of print time and has been regularly checked, cleaned, greased etc but hasn’t had any parts replaced other than the Thermistor cable. It is running the latest firmware.
I’m honestly at a loss as to what could be causing this. Some sort of software issue somehow???
Any help and advice would be greatly appreciated.
Many thanks
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I suspect you have two problems compounding each other.
Your print-sheet is visibly smeared.
Clean the sheet with dishwashing detergent (Dawn/Fairy) and plenty of HOT water, rinse well and dry with a fresh paper towel. Handle by the edges only.
The filament in your picture looks damp; as water boils in the nozzle you may hear a popping noise, the resulting steam messes up the pressures in the extruder ... look up threads here about drying filament.
Cheerio,
RE: Intermittent Filament Flow
Thanks for your reply.
The popping sound you describe does sound familiar with what’s happening. The only thing is that the problem persists even with a spool straight out of the packaging (vacuum sealed with a silica packet) could it really get damp that quickly?
RE: Intermittent Filament Flow
could it really get damp that quickly?
Regrettably, yes. You have no idea of the storage history before you opened it. Some filaments, eg. nylon, can go from perfectly dry to unuseable in an afternoon...
But wash the bed first, that's your bigger problem.
Cheerio,
RE: Intermittent Filament Flow
Update: I have cleaned the bed and dried out the filament by heating it carefully for a few hours. Unfortunately the problem persists. What is confusing me further is that, when tried with actual prints, the issue is only really noticeable during the first layer. Also could the extruder motor be playing up? The machine has 200 days of printing time I’m not sure of the service life of the parts.
Many thanks 
RE: Intermittent Filament Flow
Just out of curiosity, what temperature do you use for your hot end?
Also, I don't know if it's the angle of the photos but it looks as though you might have the nozzle to high above the bed.
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Your print sheet is still visibly filthy. Dishwashing detergent (Dawn/Fairy) and plenty of HOT water.
Cheerio,
RE: Intermittent Filament Flow
Hi,
I've been running the hotend at 220, it's what I've found tends to work well.
RE: Intermittent Filament Flow
Whilst I appreciate the importance of a clean bed I fail to see how the bed could be making the filament stop flowing out of the nozzle. I should probably add that length of time that the flow pauses/is interrupted isn’t always the same. Anything from a split second to 2-3 seconds.
Either the filament can’t flow fast enough to keep up or the extruder motor isn’t feeding when it should be. That in mind I have just double checked that the extruder pulley/gear isn’t loose and that all the electrical connections are okay.
Should I contact Prusa support directly about this?
Cheers
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I fail to see how the bed could be making the filament stop flowing out of the nozzle.
We are addressing your adhesion problem, until the filament sticks we cannot see how well your first layer is calibrated, and until then we cannot judge your extrusion which *might* be due to too much back pressure from the nozzle - or even damp filament.
Clean the print sheet.
Cheerio,
RE: Intermittent Filament Flow
I tried again with lower offset, nozzle closer to bed, and cleaning the bed with acetone. Photographed is the PLA attempt, I also tried with some ABS and had a similar issue with the flow being intermittent. Noticeably with the ABS there was an occasional cracking/popping sound coming from the nozzle during heating. 
RE: Intermittent Filament Flow
Drying for 'a few hours' might not be long enough. My drybox with built-in drying defaults to 8 hours.
Mk3S+,SL1S
RE: Intermittent Filament Flow
It definatly looks/sounds like damp filament. Have a read of this article and see if it helps:
https://all3dp.com/2/how-to-dry-filament-pla-abs-and-nylon/
It's getting better but the bed is still not clean, there are smears visible in your picture. Use soap and hot water, reserve the acetone for once or twice a year at most.
there was an occasional cracking/popping sound coming from the nozzle during heating
Unlike PETG, nylon and even PLA, ABS doesn't usually absorb much moisture, popping sounds usually indicate water flashing to steam. Your PLA still looks damp.
The first layer Z calibration is fairly good, I think you could go a little lower, a fraction more squish, which may be beneficial.
Cheerio,