J-head pops out when pritning PLA

Hi,
I used to use MakerGear hotend, but it recently get badly clogged so I decided to give J-head a shot. I like it's simple design better, and first prints were great, but when I tried to print bigger peaces I got hot end simply pop out of holder. I got them from hotends.com preassembled and contacted the maker, he said that most probably that I overheated the hot end and get holder melted. That could be the issue but what I found strange is that when it happened second time with brand new holder I was watching the process, and as I printed PLA I think I would notice if hotend went as high as 248C. Another thing that against that theory when I put hotend back there it tights strong, so looks like thread is still there.
I think it is possible that pressure is too high there, as I have 1.75mm filament and 0.35mm orifice, plus for better adhesion I lowered hotend on first layer little bit too close to the glass. But it would mean that this holder is way too weak, I never had such issue with MakerGear hotend at all. Even when it clogged filament would just stuck, but could not pop hotend out. Another thing that worries me with J-head design that 248C (that the temperature when holder starts to melt according to Brian) is very close to working temperature for ABS. Why some material with higher melt temperature not used for holder?
Did anybody else had the same issue with J-head? May I miss something?
Thanks,
Slava

do you have a fan blowing acros the top of your j-head? If not, try that. PLA can soften too early and cause a lot of resistance inside.

From the j-head wiki page

When using a hot-end, with a vented PEEK nozzle holder (i.e. Mk IV-, it is critical that the milled vents are not obstructed in any way. These vents are used to cool the hot-end by both removing as much PEEK as possible, to prevent the liner from being insulated, and to allow air to blow through and cool the hot-end. If these vents are obstructed (i.e. taped over), it is quite possible that PLA will jam in the hot-end.

Although Brian says PEEK melts at 248C, I run ABS at up to 255C, so it definitely doesn't. I have also accidentally heated a J-heat to about 300C, and it did not melt, but I wasn't extruding with it. It worked fine afterwards.

The thread does seem a bit weak though. A couple of my customers have blown the end off with PLA at 220C by sending g-code with absolute extrusion when the firmware is set up for relative. Normally I would expect the hobbed bolt to slip or the motor to stall before damaging the hot end.

I notice the MK5 seems to be sealed with PTFE tape, which I don't remember seeing on MK4. I wonder if that makes the thread more slippery and thus easier to slip out.

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[www.hydraraptor.blogspot.com]

Hello,

For some reason, I've had a couple other reports of the brass nozzle popping out. In one case, it was 100% confirmed that it was not due to temperature. As this has only started in the last month, I have found it quite odd.

I think that nophead's theory may be correct about the PTFE tape. If there are too many layers, maybe it is causing it to slide out somehow? I can't think of any other reason that this could be happening as nothing else has changed.

I have started putting less tape in the threads and would welcome anybody who had this problem, and is 100% sure it wasn't due to melting, to contact me at reifsnyderb@hotmail.com.

Regards,

Brian

Too many layers is indeed bad with PTFE tape.

I dont really know for high temp, but in hydraulic use, the rule of thumb is to never exceed 2 1/2 turns, preferably do 1 1/2. Also, you must apply the tape in the reverse direction of the thread, so that the tape is stretched when locking it.

If you want more PTFE there you can use compound, but that is even more sliperry.

Brian,
Unfortunately your replacement where you used less tape has exactly the same problem - it worked for a day and now I have exactly the same problem.

Just read another topic below: [forums.reprap.org]
"Iv been running a few machines pretty hard with the typical J heads running ABS

Having problems with reliability. They either come apart and leak or the threads strip is suspect this is because running ABS at 240C is very close the melting temp of the insulator."

Looks like this is pretty common.

Edited 1 time(s). Last edit at 04/15/2013 09:38AM by rGlory.

Hello,

Lately, there have been some failures, unfortunately. I have been working on the problem and think that I have found it.

As the problem has, to the best of my knowledge, only occurred lately I was trying to figure out what was going on. The 5/16-24 thread is quite similar to an m6x1 (it is almost like m8x1) and shouldn't be a problem. Upon measuring the SAE threads, I found out that the OD is too small. Looking through my scrap, I found out that the OD had been too small for quite some time. So, while it contributes to the problem, I don't think it is the complete answer as there should have been failures, like this, back as far as September.

I then looked at the PEEK nozzle holder and the threads inside them. I believe that the failure is a combination of how I've been cleaning up the PEEK and the brass 5/16-24 threads. I have modified the way I clean up the PEEK and the nozzles seem to be able to take a lot of force before the threads give way. In the near future, the brass threads will be fixed. I have contacted the machine shop, that has been doing the first 4 operations, and they were embarrassed to discover that their thread gauge was not accurate.

Please e-mail me your order number or name (my e-mail is [email protected]) and I'll send you another hot-end.

Best Regards,

Brian



rGlory Wrote:
-------------------------------------------------------
> Brian,
> Unfortunately your replacement where you used less
> tape has exactly the same problem - it worked for
> a day and now I have exactly the same problem.
>
> Just read another topic below:
> [forums.reprap.org]
> "Iv been running a few machines pretty hard with
> the typical J heads running ABS
>
> Having problems with reliability. They either come
> apart and leak or the threads strip is suspect
> this is because running ABS at 240C is very close
> the melting temp of the insulator."
>
> Looks like this is pretty common.