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I have a homemade 3D printer running on ramps 1.4 When i start a new print and the hotend reaches melting point of PLA, the PLA start coming out of the hotend.
This goes on for as long as the temperature is kept above melting point, without moving the extrusion gear.

The extruder is a bowden type.
Hotend is a J-head.
I am currently using simplify if that makes any difference regarding configuration.

Any ideas what to do to prevent this from happening?

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  • $\begingroup$ Is filament advancing, or is it just expanding with the temperature change? $\endgroup$
    – Davo
    Commented Feb 6, 2018 at 13:52
  • $\begingroup$ I will have to look into whether its advancing on a micro scale, I had not considered that. I am fairly certain that it is not advancing, not easily visible atleast. $\endgroup$
    – Claudi
    Commented Feb 6, 2018 at 13:58
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    $\begingroup$ @Claudi Is it a lot or a little? With my Anet A6 some filament will extrude as the filament heats up and expands. It does stop shortly after for me. That is my thought, the filament is expanding. $\endgroup$
    – Ljk2000
    Commented Feb 6, 2018 at 14:27
  • $\begingroup$ I will say its a lot, it goes on for at least a couple of minutes, the oozing ends up as a fair amount of filament on the bed before print starts. $\endgroup$
    – Claudi
    Commented Feb 7, 2018 at 7:46

2 Answers 2

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Any ideas what to do to prevent this from happening?

You cannot prevent it entirely, but you can probably mitigate the problem by depressing the lever that squashes the filament against the hobbed gear of the extruder before starting to heat the nozzle.

In bowden extruders, the long portion of filament between the stepper motor and the nozzle is subject to compression during the print. Because of the hysteresis in the filament, and of the slack between filament and PTFE tube, this filament acts like a slow-releasing compression spring. When the nozzle cools down, the potential energy stored in the filament is "frozen" in place.

By depressing the lever, you allow the spring to extend "backward" towards the spool, rather than "forward" through the nozzle.

Some oozing is still bound to happen because of gravity and - as highlighted by others - thermal expansion, but it should be significantly less.

If you adopt the lever trick, remember to print with a skirt, as you will want the printer to recreate that "compression" in the filament before the model proper begins.

Another way to address the issue would be to add a little bit of retraction in the closing stanza of your GCODE (the part where you also tell the printer to unpower the steppers and stop heating). This will prevent any "compression" to be "frozen" in the first place.

This anwer is based on the assumption that the stepper motor is not actively spinning (i.e.: yours is not a hardware/firmware issue).

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  • $\begingroup$ This will probably end up being the solution i'm seeking, as i am in the testing/calibration part of my project i often stop prints before they are done, thus rendering any added added end G-Code useless. If i understand you correctly i just need to release the mechanical lever before heating up. $\endgroup$
    – Claudi
    Commented Feb 7, 2018 at 7:45
  • $\begingroup$ @Claudi - yes. Beware it will only improve the situation, not eliminate completely the problem. If you are there when the print stops, you can release the lever immediately (before cooldown) for even better improvements. $\endgroup$
    – mac
    Commented Feb 7, 2018 at 8:44
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Does it really go on for more than a minute or so? You can't get filament from nowhere, so if the feed gear isn't moving, sooner or later all the material in the reservoir inside the nozzle & hotend will be melted and gone. Leakage like this is normal, and probably a lot more noticeable if you have a larger diameter nozzle.

I would recommend checking to see if your gcode includes a large retraction at the end of the print. That would reduce the pressure and the amount of material left inside the hotend.

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