It's a shame to come back to your print to find out that the print head is going around on its own without building on the print. It requires a restart of the print and it is annoying. There are some simple solutions to how to prevent this from happening again.
The run out of filament
Before you start your print, you should check if there is enough filament on the roll to complete your print. If it runs dry in the middle of your print, the plastic will stop coming out of the nozzle and your print will quickly come to a dull end.
Tighten the extruder
If your printer's extruder is not tensioned well enough, it can quickly end up slipping on the filament instead of pushing it down into the hotend. In this case, no more than a small temperature change or the weight of the spool having to be rotated can make it loose grip. Many extruders have a way of tightening and releasing the pressure that is applied to the filament towards the gear that drives it forward, often by compressing a spring with a screw.
Knot on in the filament
The very long string of filament can be difficult to keep track of over time, unless you mount a brand new roll on your printer and leave it there until it is empty. Unfortunately, it happens very easily that the end of the string gets crossed over itself and forms a knot. They can be difficult to spot as they can constantly move further down the spool. The problem arises when at some point it binds up and keeps the filament from being rolled off the roll. This can result in the print not being finished printing and you are left with half the print. But it can also happen that a number of retractions and vibrations can loosen the knot and send it on its way further down the roll again and therefore resume printing as per usual.
The knot will probobly show itself again at some point. To prevent this, you can solve the problem by putting a hand on the filament to stop it from moving on the spool and then pulling on the end till you find the place it crossed over. You should be able to pull the loose filament through now and restore a perfecly good spool of filament.
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