Snapping PLA filament when left overnight

I have a Prusa i3, which is working nicely. One problem I've experienced lately is after a long session of printing, I'll switch it off and leave it overnight, with the PLA still in the machine. When I come down in the morning the filament is often broken a few centimetres above the extruder (which is on the X carriage).

Currently I'm using 3mm e-sun PLA filament from HobbyKing, but I have seen this with other brands. It doesn't matter if it's at the end of the reel (when the filament tends to be tightly curled) or at the beginning - it keeps happening.

The work-around is simple - just pull the filament out at the end of the day (a good idea anyway) - but I was wondering why this should happen overnight and not when the printer is pulling the filament off the reel.

Any ideas?

I have this problem on my Ormerod, with 1.75mm e-sun PLA (which I find otherwise very good, e.g. I can print it on to plain glass). The Ormerod has a 90 degree turn on the extruder drive inlet, and by reprinting the inlet with a larger radius of turn, I managed to reduce the incidence of the problem, but not eliminate it. I notice that the filament appears to be more brittle around the breaking points than it is down by the reel on the floor. So I suspect that either the filament is somehow drying out, or it is being affected by UV light - the room does get quite a lot of sun in the mornings.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

My best guess is stress, PLA won't flex (specially 3mm), if you leave it in the hotend, it will dissipate the heat into the filament which will become soft and once cooled will become brittle and brake at any stress point.

Most likely at that distance from extruder is where the filament curbs and creates a stress point. Do you support the filament in a tube? If not, you might consider use one.

I forgot to mention that my filament spool sits high on the frame above the extruder, so it has a short and easy path. It does have to straighten out to get into the extruder, but otherwise it's easy.

I'd be surprised if the residual heat got 150mm up the filament - I use an E3Dv6 hot end with its fan, and everything above the head stays quite cool. Obviously the fan is off once the power is switched off, but that is a long way for the heat to travel. I'll have to feel the filament an hour or so after switching off to see if it is warm.

Otherwise - I'll just wind the filament out when I've finished! That's probably good practice anyway.

Quote
David J
I have a Prusa i3, which is working nicely. One problem I've experienced lately is after a long session of printing, I'll switch it off and leave it overnight, with the PLA still in the machine. When I come down in the morning the filament is often broken a few centimetres above the extruder (which is on the X carriage).

Currently I'm using 3mm e-sun PLA filament from HobbyKing, but I have seen this with other brands. It doesn't matter if it's at the end of the reel (when the filament tends to be tightly curled) or at the beginning - it keeps happening.

The work-around is simple - just pull the filament out at the end of the day (a good idea anyway) - but I was wondering why this should happen overnight and not when the printer is pulling the filament off the reel.

Any ideas?

I've used a couple of different brands of PLA and this has happened with only 1 roll of filament.
It was about 2/3 way through the roll when this started happening.
When not in use I do keep the filaments in seal-able bags with silica desiccant.
Could be an older roll from the suppliers stock and the PLA has gotten brittle.

Strange to see - since then I tend to remove the filament from the hotend/extruder when I am finished printing.

Steve

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My updated Instructable on our Prusa i3 Build
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