3D printer has a filament blob over the hot end

Issue

You've returned to your unattended 3D printer to find that it appears to have leaked filament from the nozzle and made a filament blob engulfing the hot end. 

This is known as a "Blob of death" and is something that you need to fix yourself - it is not a warrantable fault with your printer. It can be avoided if you check on your prints every so often - many printers even have a camera to let you do this remotely via the slicing software.

Why this happens

Most commonly it is due to incorrect Z-offset, lack of calibration, or poor build plate adhesion - the bed height was not calibrated correctly or the build plate is worn-out, or dirty and not sticky enough, causing the filament to fail to stick the first layer down properly. The result is that the print can lift off the plate at any time - it can then stick to the hot end, and if left unattended, forms a big blob that sticks to the hot end. A worn or blocked nozzle can also cause the filament to bend back as it extrudes and stick to the nozzle.

How to fix

Removing the toolhead from the gantry may make it easier to access the blob. Manually pre-heat the nozzle to 200 degrees or more, and leave it for a while to see if that loosens it up a bit (Key word here is "Soften" - not "Molten"). Higher temp filaments like ABS and PETG will need a higher manual temp setting. Don't touch the metal parts with your fingers as they will be very hot. Once you soften the blob, use tools to gently work at the blob to pry it away from the nozzle and hot end so as not to damage the wires of the heater and thermostat. Paper towel can clean up the residue. 

For printer-specific help, search Google using <your printer model> blob of death removal

How to stop this from happening again

It is good practice to do the following in order:

• Clean the print surface before starting your prints.
• Use a gluestick or spray contact glue on your build plate to promote proper adhesion.
• Calibrate the Z-offset - this option should appear in the slicer at the last step of the print dialog.
• Watch your print and make sure that the first layer is sticking properly to the entire print surface.
• Check on the print occasionally - do not leave printers working unattended for hours at a time.