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Hot end failure

Posted by newtonrob 
Hot end failure
June 25, 2012 10:40PM
I recently had a hot end failure. I’m not going to identify the manufacturer as it might have been my fault.

I use 1.75mm ABS filament (yellow). I have a Greg’s accessible extruder with herringbone gears. I calibrated by extruding 100mm of filament prior to installing the hotend. My hotend has a .34mm nozzle.

I have set the hot end to 230c for the first layer and subsequent layers.

Fill density .4

Print speed is set to Perimeters 30, small perimeters 30, infill 60, solid infill 60, bridges 60.

Bottom layer speed ratio 0.3

I have printed excellent parts. No strings and no hollow points. I would say almost as perfect as possible. I printed a lashmaze with no errors. I did have a slicing problem and it printed the walls 2 wide. I have printed thumbwheels, again perfect. I have also printed a nickel calibration.
My hotend has a threaded brass tube 40mm long with 15mm screwed into a PTFE rod.
Originally I select “1 is 100k thermistor - best choice for EPCOS 100k (4.7k pullup)”. This was the wrong thermistor. It should have been “7 is 100k Honeywell thermistor 135-104LAG-J01 (4.7k pullup)”.

The first failure occurred with the wrong thermistor set. Since the program was set to 230c, I figure that the temperate was about 247c which is the limit for PTFE. Although, it should not have been that temperature at the PTFE.

I cleaned the parts and decided that the PTFE still had threads. Although when I put it together, it was not hard to screw in. Even though it failed again with the correct thermistor set, my assessment is that it was damaged during the first attempt.

I have another hotend on order. But I have also been given advice to increase the hotend temperature. The reasoning is that it might have been excessive pressure that caused the failure.

I really don’t want another failure. So any ideas on the likely cause?
Re: Hot end failure
June 26, 2012 02:09AM
If the brass is supported solely by screwing into the PTFE, it is almost guaranteed to fail. Higher temperatures will only make that worse, as PTFE tends to creep under pressure and heat. One option if you need to keep using that type of hot end is to put a pipe clamp around the PTFE to keep it from expanding. This is still a temporary measure, although I had success with it back in 2008 when that's how the MakerBot hot ends were made. I got rid of that hot end as soon as newer designs were available though.

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Re: Hot end failure
June 26, 2012 05:07AM
It even fails with a pipe clamp because the clamp does not stay tight. The PTFE creeps away from it by becoming longer and thinner.

Re: Hot end failure
June 26, 2012 06:56AM
For my $0.02 I say replace the PTFE with a material more resistant to heat, even if it just at the thread itself, maybe use a much larger threaded insert if you have the room, as the larger the thread the more creep is required in order for a failure to occur.

I do realise that there is unlikely to be enough room for too big an insert. but even a helicoil will help as you will have the heat resistance that comes from an extra boundary between materials as well as the poor heat conductivity of the stainless steel helicoil, might be worth a try, as helicoils do not require a vastly bigger threaded hole than was originally fitted
Best Regards
Re: Hot end failure
July 01, 2012 10:02AM
I have to agree with Nophead's comments as this is what I have found too. As a tempary measure you could try using PTFE thread seal tape around the brass barrel and a hose clamp done up really tight. After I had used the hot end for a while - perhaps 100 meters of 1.75mm PLA I pulled it all apart and was quite supprised at how deep the indentation was on the PTFE. At least you might get printing again while you wait for a replacement.
Re: Hot end failure
July 01, 2012 03:25PM
Thanks everyone. I have another hot-end on order. I have also ordered a V3 grooved MakerGear PEEK as a replacement for the PTFE rod. I think this will work better. One other issue that I will be addressing is that I had the thermistor loose in the aluminum heat block. I'm beginning to think that was a bad idea. I have some ceramic thermal compound that I will be putting in it to ensure it is getting the full temperature.
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