I was starting to get intermittent extrusion with my J-Head. It wasn’t completely jammed, but I could see the idler bearing stop turning every now and then while the gear was still rotating. If the bearing isn’t turning, the filament isn’t feeding. It was obvious in the print of course, with missing lines in the solid fill and weak walls.
I ran a violin string into the nozzle to try and break up whatever was getting in the way and it seemed to work for one or two prints, but then go back to getting jammed. I thought it was a little odd because I was using a .5mm nozzle which should be a lot less likely to clog. I took off the nozzle and put on a .35 nozzle that I knew was fine, expecting to go back to printing as normal, but it still jammed. That meant the problem wasn’t the nozzle, but something with the PEEK.
Clogged nozzles are common enough, but it isn’t always the nozzle opening that is the problem when it comes to a hot end that won’t extrude. The jam can be happening higher up. The J-Head and similar hot ends have a PEEK barrel with a teflon tube inside. There are threads at the bottom that hold a brass barrel which butts up against the the teflon, and on the J-Head there is a hollow set screw at at the other end which keeps the teflon from sliding out the top.
It is very important that the brass barrel stay pressed firmly against the teflon liner. The filament will start softening and melting at the junction between the two. If there is any gap, it will fill with melted plastic and some of the force needed to push plastic out of the nozzle will instead push it sideways into the gap and attempt to force the barrel down.
After I pulled the filament out of the hot end and took off the nozzle/barrel I found this bit of plastic left behind at the bottom of the teflon liner:
The cone shape at the top is from the filament stretching as I pulled it out, and the plug at the bottom is a small amount that was in the barrel. The disc is the plastic that filled the gap between the barrel and the teflon.
I don’t know what caused the gap to form in the first place, maybe the teflon managed to compress a little bit. Just putting another nozzle on didn’t work because the bottom of the teflon maybe had moved up a little bit, or maybe I didn’t get it screwed on as tight as the previous barrel. When I put the hot end back together, I first took out the set screw at the top and pushed the liner down a little with the back of a drill bit. Then I screwed on the barrel which pushed the liner back up a little, insuring that they were in contact with each other. Then I put the set screw back in and tightened it down so it would keep the teflon pressed against the barrel.
On the 1.75mm J-Heads there is a short piece of PTFE tubing in the set screw to guide the filament through the set screw into the liner. You have to get it out before you can remove or tighten the set screw. I did it by sticking a sharp point into the inner wall and levering it out. Be careful when doing this because it might suddenly fly across the room never to be seen again.
There is an all-metal E3D hot end waiting at the post office, so hopefully this is the last time I will have a problem like this.