Investigating the cause of failing hour long running 3D-prints

Short running and small 3D-prints have never caused any problems on my Prusa Mendel i2 RepRap 3D-printer. On the other hand, bigger, hours long running 3D-prints often fail. Figure 1 shows the profile of a model that failed initially. The object is 10 cm wide and 3.5 cm high. Figure 2 shows the failed print.

Figure 1: profile of the printed object. It is about 10 cm wide and 3.5 cm high

Figure 2: failed 3D-print

As you can see, the top of the object is not closed. You can see that the printer had trouble extruding the filament. In the end, extrusion totally stopped because the hobbed bolt had eaten into the filament. My first thought was that a dirt particle had obstructed the nozzle. As the print is not finished, you can see the honeycomb infill inside the object.

Figure 3: filament of failed 3D-print

Figure 3 shows the filament used for this print. The hole is the point at which the hobbed bolt had eaten into the filament. If you look close, you can see marks of its teeth on the left side of the hole. To clean the extruder, I heated it and then tried to push some filament through the nozzle. That worked without any problem! I did not expect that to work, if there was no dirt in the nozzle, what could have caused the hobbed bolt to eat into the filament? 

Figure 4: tightening these bolts on the extruder fixed my problem

When I looking closer at the failed print at figure 2, I noticed that the print did not stop at once, but that it had trouble extruding for some time. That led me to the idea to tighten the bolts that push the filament against the hobbed bolt. See figure 4.

Figure 5: successful 3D-print after tightening the screws on the extruder

Tightening the bolts that push the filament against the hobbed bolt solved the problem my printer had for a long time.