J-Head Jam (premature melting)

My extruder jammed and I was unable to reverse the filament or pull it out by hand. I had to take my j-head hot end off of the extruder and pull the PLA filament out with a pair of pliers while the unit was heated to 240C to remove it. I was running it at 185C the last time I used it. The filament appears to have softened along the length of the PTFE tube and melted in the extruder assembly. The only obvious deformation is the area that mushroomed and turned brown, but I measured the diameter of the filament on the reel and the part that entered the PTFE with a caliper. The filament in the PTFE was from 3.30mm to 3.40mm and the filament on the reel was between 2.95mm and 3.10mm. Does anyone have any suggestions to prevent this kind of jam? I've read that fans can be used, but I don't know of a good way to mount one to cool the PTFE without also cooling the brass.

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I have one mounted by inserting it 10mm into the base of a Wades, i.e. the old fashioned way, not groove mount. I can print PLA at 200C without any problems.

The heat at the top is caused by convection, not conduction. The outside of the insulator gets too hot to touch where it meets the extruder base but it must be cooler inside or it would jam. If you run the extruder on its side you will find the top stays cool. That implies insulating the brass heater block would keep it cool, or a tiny fan to disrupt the convection.

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[www.hydraraptor.blogspot.com]

Okay, thanks. The simplest thing for me to do seems to be to wrap the brass in a few layers of fiberglass tape.

Do you have a link to a page describing the method you used to mount the hot end?

Edited 1 time(s). Last edit at 06/25/2012 12:42PM by MaFraL.

I just insert it into the base and drill through one of the holes with a 2.5mm drill, tap it 3mm and insert a M3x30 bolt. Then I do the same in the other hole. I also put nuts on the back. Long self tappers can also be used.

The J-head is tricky because it usually comes with a groove that doesn't quite line up with the holes. That tends to make the drill wander and can snap the tap. It is better to get one without the groove.

I have yet to try groove mount. It seems to need a precisely machined metal plate because the thickness of most sheet materials would not be accurate enough, or am I missing something? I would love a better mounting method.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> I have yet to try groove mount. It seems to need a
> precisely machined metal plate because the
> thickness of most sheet materials would not be
> accurate enough, or am I missing something? I
> would love a better mounting method.

Tantillus uses a groove mount but it does not rely on the groove as much as the lip above the groove. This is also how the Gregs groove mount works, you are simply trapping the lip above the groove between the extruder body and the two bolts.

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It doesn't need precisely machined metal. My j-head is mounted with a piece of acrylic. What Sublime mentioned sounds right. It traps the lip above the groove between the mounting plate and the bottom of the extruder, which has a circular depression to accept the top of the nozzle holder.

Edited 1 time(s). Last edit at 06/25/2012 03:12PM by MaFraL.

Hi there!
In my experience of extrudes, you do need active cooling on the insulator (including the J-head) to remove the risk of jamming. I don't think this is emphasised enough to people. It doesn't have to be a strong fan, just a gentle breeze across it will prevent heat creep upwards. The fan may cool the heater block as you suspect, and you do have to take some action to prevent this. You can insulate the block, fit a ducting to the fan, and so on. I have both. I use some Teflon sheet (oven liner) and some kapton to wrap the heater block. I use some teflon sheet simply taped below the fan/above the heater block to keep the airflow away from the print and heater block.

Have you a blog or photos of your extruder? Do you need any more advice on reattaching the j-head to the extruder?

I agree with Nophead. The j-head slot doesn't quite align with the holes provided to pin it in, and can be a challenge if you are trying to drill it through. I pinned it with some coat-hanger wire! I may move to a grove mount once I can find some aluminium of the matching thickness, or get a friend to mill a slot!
Regards,
NumberSix

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[numbersixreprap.blogspot.com]

pictures would help

What kind of fan do you actually use? My first thought was to use a fan from an old PC, but the smallest of those (maybe 50mm) still seems like overkill for what you're describing. Although I looked on Newegg and found them as small as 25mm. Also, like aplavins said, pictures would help. I'm not sure of the best way to mount a fan to point at the insulator. I've seen some printable fan mounts on Thingiverse, but printing is imperfect and a little difficult at the moment.

Now, you don't say what printer you are using, but for mine (original mendel) I have mounted a 40mm fan next to the wade extruder - pointing downward.
At the bottom of the carriage I have just put a piece of ducttape, which points the airflow (sort of) through the vents.

I have had similar problems with my j-head before I added the fan. After that I haven't had a single jam.

Okay. I still haven't had a chance to play with my printer since I posted this, but I have a Prusa Mendel.

I've done a few more prints now and haven't had the problem again. I'm letting my gcode control the extruder temperature now and just keeping the heater turned off when I'm not printing.