Hi,
Are you leaving the extruder heater enabled for long periods between prints with the cooling fan disabled?
The only explanation I can think of is excessive heat-creep occurring under those conditions. Your end GCode should disable both heated bed and extruder heater at the completion of the print.
Another possibility would be printing at very low speed with lots of retract operations again without the cooling fan enabled.
Operating the printer under recommended conditions should not allow this to happen. It is something I have never experienced.
Regards,
Neil Darlow
I try to write with consideration for all nationalities. Please let me know if something is unclear.
Printing with Mendel90 from fedora 25 using Cura, FreeCAD, MeshLab, OpenSCAD, Skeinforge and Slic3r tools.