Missing layers can take an otherwise perfectly fine print and make it waste as soon as you try to remove the print from the plate, and it splits in two. These missing layers / "holes" in your print occur when the filament is not pushed through the nozzle for a while, and the problem that subsequently resolves itself and the printing then continues without problems. There are several reasons why these temporary problems occur but it can be difficult to figure out where the problem is coming from. Here are some possible solutions.
Tighten the extruder
Many extruders have a way of tightening and releasing the pressure applied to the filament, often by compressing a spring with a screw. If you have a plug as described above, your extruder will start clicking after you have tightened it up and you can now try the next solution.
Knots on 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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