Extruder failure is bad. Intermittent 3D printer extruder failure is worse.
I have had to put my signal generator project on hold. While trying to 3D print the enclosure, a plastic extruder failure cropped up. It seems to be some sort of intermittent blockage taking place in the nozzle.
As you can see in the picture above, the extruder failure is causing the printer to produce a stringy surface rather than a solid surface. This problem started unexpectedly, as I had checked the calibration and printed some perfect test cubes before starting on the enclosure for the signal generator.
Since the enclosure print will take five hours and consume 33 meters of plastic filament, it made sense to stop printing once this extruder failure occurred.
Following the aborted print, I did two test pads, shown in the inset lower right. These are small round pads with a diameter of 10 mm and a height of 0.6 mm. As described previously, it is a good idea to put pads on the corners of a large 3D printed object. Otherwise, the corners will lift and the sides will probably warp, due to the stresses of different temperatures as the plastic cools.
The first test pad (left side, lower inset) printed perfectly. The second test pad (right) was a disaster. No changes were made to the 3D printer settings between these prints. So, go figure.
Fixing the extruder failure
I will make another attempt to clean and re-build the hot end, but I have no confidence that the extruder failure will not happen again. That is the nature of an intermittent problem. In the meantime, I have ordered a replacement extruder assembly from China. But that will take a few weeks to arrive.
To the left is a picture of the parts in my E3D-style all metal hot end. The heating block contains the heating element and thermistor (electronic thermometer), which are not shown in this picture.
The heat break and nozzle screw into the heating block, and they need to be cleaned occasionally. The normal way to clean these pieces is a combination of heating with a propane torch (to burn off any junk plastic) and to soak them in an acetone bath (which causes little pieces of plastic to break off the surface and float away.)
The inside diameter of the heat break is the same size as the plastic filament (1.75 or 3.0 mm) and is easy to clean out with a paper clip. The nozzle has a much smaller opening (such as 0.4 mm or less) and you need to use a very fine steel or brass wire.
When you are printing, the plastic filament is not supposed to melt until it gets to the heating block and nozzle. That is why there is a heat break and sink, to prevent the plastic from heating earlier along the way.
As you can see, there are various places where an extruder failure can happen. The most likely failures are blockages in the nozzle and heat break, followed by improper mounting or breakage of the parts located in the heating block.