Checking Hot End

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How to check your hot end construction

On the original design of the Rigidbot hot end (the part that heats the filament) there were issues with filament leaking (oozing) out of various places that it wasn't supposed to. This was initially attributed to poor machining/design in the thermistor location as the threading broke through to the filament pathway. Later contributions by the G+ community suggested that if the design of the PTFE filament guide were correct, there shouldn't be any gaps for the filament to leak out of in the first place. This page is to show how to check your hot end for this defect.

Note: At the time of this writing (July 1st 2014), InventAPart (IAP) has indicated that they have a redesigned the hot end block (IAP Part# 1063) to correct for these defects. While one beta tester has indicated it has fixed the problem by reducing the bore depth in the hot end block by 0.020", only time will tell in the field. Also, the measurements here are insightful for all hot end designs of this type.

Data Measurement

(see Analysis further down for assembly suggestions to avoid leaks if you have this problem)

Rigidbot Hot End Measurements.PNG


Analysis

In essence, it looks like IAP's design was caught by cumulative tolerances (each one being off in the same direction). Ignoring the breakthough issue with the thermistor threads, I looked at the supplied length of the PTFE tube and compared that the the cumulative length of the bore it would fit into. The result (for my two hot ends) was that they would both result in a gap once I assemble them. This is the "gap" in the spreadsheet above. I would encourage anyone else to check theirs and see if their parts are significantly different.

Given this data, what are some ways to avoid leakage? One possibility is to shim the Cold Block side of the PTFE tube with either some aluminum (soda can or foil in layers) to achieve a few thousandths compression based on measurements. Another possible way is to use a small amount of high temp RTV gasket material on the cold block end to create a thin but elastic gasket. This would serve like a washer to ensure pressure once the hot end is tightened.

One big suggestion during assembly. Because the IAP supplied PTFE tubing is slightly oversize (but again, likely in spec), it will bind a bit when inserted into the PEEK (mine do). Position it with about 0.5mm more on the hot end side so during assembly any gap will be on the cold end side. Tighten the blocks WITHOUT the nozzle screwed in. Tighten the nozzle last (and I'd suggest anti-seize compound on it and the thermistor). Note this applies even if you receive the new design hot end.


Suggested Build Method for Original Block

Do you have the original "milled block" with threaded hole for the thermister breaks through to the adjacent area? Assemble it as follows and it should be just fine (I used this technique on both of mine). You'll only need some RTV compound and a short 3mm screw.

  1. Assemble the stack (heater block, PEEK insert, cold block) according to the "directions". Make sure your PTFE tube is the correct length (see the Wiki above for how to measure, http://rigidtalk.com/wiki/index.php?title=Checking_Hot_End). Adjust by trimming or shimming if needed. Make sure you have the nozzle in place as well.
  2. Thread in the thermister. I recommend using a small amount of anti-seize compound on the threads for easy removal later (I also recommend this on the nozzle threads). As an alternative to anti-seize, you could also use some high quality non-hardening thermal paste. Note that most thermal pastes are NOT rated for the max temp of your heater which means they may harden over time.
  3. Add some RTV compound to the hole on the opposite side of the thermister. I used Permatex 81878 Ultra Copper (http://www.amazon.com/gp/product/B0002UEOPA) although many other RTV compounds should be sufficient for the temperature (read the label). Ultra Copper is rated for 600F continuous duty (315C).
  4. While the RTV is still wet, thread in a 3mm screw into the opposite side. Do not force, you're just using this to fill the rest of the hole and ensure the RTV fills all the gaps. Think of the screw being the plunger of a syringe and it's just squeezing the RTV ahead of it. At some point all the gaps will be filled and further pressure is not needed. If you want to be more precise, measure your thermister threaded length, the block width, and subtract to get your required max 3mm screw length.
  5. Set aside and let cure overnight.

I did this with both of mine and have had 0% leakage, excellent temperature control, and no other issues. I have had to "re-hone" my PTFE tube once after about 100 hours of use with a ~1.85mm drill bit but that is a separate issue of creep (I probably shimmed the PTFE tube a bit too tightly). No problems since.