Getting to the bottom of the Rapman hot end failure

In which your narrator gets to the bottom of why it was that his 0.3 mm Rapman hot end failed and perhaps even why it started causing resets prior to the failure.


The hot end failure that I experienced on 14 October is a relatively rare event. The only other person with a Rapman who reports the sort of ABS leakage that I experienced was Klazlo.


Once I had my Rapman repaired and usefully printing again, I turned my attention to doing forensics on the failed hot end. Once I took a hard look at the hot end and the pictures that I took immediately after the failure hints of the means and mode of the failure began to recommend themselves.


Taking a look at one of the forensic pics that I took on the 14th two items jump out at you. I've circled them in red.

If you look closely at the top red ellipse you will notice that one of the three bolts which secures the spacer on the hot end ensemble was either not tightened adequately at the factory or, more likely, worked its way loose via hundreds of heating and cooling cycles while in operation.  This threw the alignment of the hot end out opening a route for hot ABS to escape between the PEEK sleeve and the aluminum extruder barrel sleeve that fits around it.  You can clearly see the billowing flow of ABS in the picture.  ABS was only leaking on the side of the PEEK barrel where the spacer bolt was loose.  That part is straightforward.


The vertical ellipse on the right hand side of the hot end is much more ominous.  If you look closely, you can see that one of the thermistor leads has exposed above the sleeve leading down to the extruder tip where the extruder is housed.  A bit over 4 mm of exposed thermistor lead is open to the air.  You can see that happening a bit more clearly with this picture.

The larger red circle encloses the exposed lead.  The smaller one shows the recession of the purple plastic insulation on the lead that occurred once the ABS began to boil out of the gap between the PEEK and the aluminum and contacted the lead.


At first I thought that the hot ABS had actually caused the gap in the insulation.  Closer inspection, however, revealed that the lead had most likely been stripped too liberally and originally was only just barely covered by the yellow silicone sleeve.  The heat of the billowing ABS almost certainly caused the shrinking of the insulation from just being covered by the silicone sleeve to its present situation.  It's easy to see the shrinkage in the second circle.


I then removed the PEEK cylinder from the aluminum extruder head.  Originally, I had thought that the PEEK sleeve in contact with the aluminum might have shrunk.  It measured 10.09 mm outside of the aluminum sleeve and 9.9 mm inside.  That could have been either because of heat shrinkage of the PEEK over time or some sanding of the PEEK tube at the factor to get a clean fit.  Unless the parts had been measured before the hot end was put into service, it would be impossible to tell for certain.


I then removed the silicone tube from the inside of the PEEK sleeve as you can see here.

This is the most revealing forensic evidence in the whole exercise.  At first, it appears that the silicone tube has shrunk on the hot end.  While that is true to an extent, the reason for the shrinkage is actually because the inside diameter of the PEEK sleeve has shrunk because the PEEK has expanded within the confines of the aluminum sleeve.


It was fortunate that I bought several unassembled hot end kits before BfB began making premade ones because I had some disassembled PEEK and silicone tubes.  The unused PEEK sleeves had an outside diameter of 10.0-10.1 mm and a consistent inside diameter of  6.36-6.40 mm.


When I measured the failed PEEK sleeve the cold end inside diameter was 6.40 mm but the hot end had been reduced to 5.88-5.92 mm.  The silicone tube outside diameter on the hot end was more or less the same as the inside diameter of the PEEK sleeve.


What was revealing, though, was that the hot end of the silicone tube was virtually entirely closed.  It was impossible to slide a 2.9 mm ABS filament through it.  I was able to blow tiny bubbles through the silicone tube into a glass of water.  What that says is that the ABS filament was being melted in the silicone tube and squirted into the aluminum extruder barrel rather than being melted in the extruder barrel.


I don't think that that was how the hot end design was intended to work.


Getting back to that exposed thermistor lead, it is well known that blowing hot air past plastic surfaces will create a static charge.  Not only was that exposed lead within millimeters of billowing hot ABS it was also touching the bottom acrylic plate of the x-axis carriage {part 10034}.  Periodic static discharges off of this heated acrylic surface were most likely at least deranging the temperature measurements off of the thermistor and also causing the MCU board to reset.  During last winter, I walked in socks across the floor and picked up sufficient charge to where when I touched the MCU board the system reset.  


Conclusions:


The failure of the hot end had two aspects.  

• The loosening of the spacer bolt, most likely from repeated heating and cooling cycles of the hot end allowed for a hot end misalignment that created a path for leakage of hot ABS out one side of the hot end between the aluminum extruder sleeve and the PEEK sleeve for the silicone tube.
• The swelling of the PEEK sleeve over time reduced it's inside diameter almost entirely closing the silicone tube and causing the ABS to have to melt in the silicone tube rather than in the aluminum hot end.  This was certainly not the intent of the hot end designers.

It seems quite obvious that the combination of a PEEK sleeve enclosing a silicone sleeve for the plastic filament need to be rethought.

The placing of an MDF plate and an acrylic plate at the top of the hot end spacers through which the securing bolt has to pass also needs some careful.  Replacing the MDF with a PEEK plate with the bolts secured by lock washers to the PEEK plate and recessed through the acrylic with the acrylic and PEEK secured by a separate trio of short bolts might be more successful.  An aluminum plate as a replacement for the MDF might also be considered.