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My first hotend jamb

Posted by plankton 
My first hotend jamb
December 04, 2015 05:07PM
Not bad, I've been using my Mendel90 for 10 months and this is the first hotend jamb, however having no experience with jambs I'm trying to work out the cause.

I hadn't used the printer for a couple of weeks, but wanted to print a Raspberry Pi case for an Owl nest box web camera I'm making my Dad for Christmas. I did a quick calibration test with a 20mm cube, that printed without problems, so I moved on to the Pi case (in PLA). The print started without problems, but when I next checked the head wasn't extruding plastic.

I did a complete strip down and found plastic jambed the whole length of the hotend (a genuine J-head from hotends.com), there was no obvious cause, however this is the first time I've had the J-head apart so I'm not sure if I would spot all problems. I removed all but a smear of plastic inside the hot end, when it was as clear as I could make it, I reassembled

After rebuilding the hotend and extruder I tried again. Another calibration cube printed without problems, but the Pi case resulted in exactly the same failure:

When I next checked on the print, there should have been several layers completed, instead I found this, so I cancelled.
Removing the hotend you can see the plastic has melted the whole length of the heat break...
...and even back into the extruder.
This time (rather than completely dismantle the hotend), I put it in the oven at 80C for 15 mins, then was able to tug the blockage out of the hotend with small pliers. Now I have no experience with hotend jambs, but that looks to me as though the heat break has failed somehow, either that or it is normal for a jamb to cause heat to propagate back up the heat break?

So not having any previous experience, can anyone tell me if that heat creep is normal when the nozzle jambs, or should I be looking for a problem with the heat break?


Prusa i3 (plywood box frame)
RAMPS 1.4 and Arduino Mega

Mendel 90 (sturdy)

My RepRap Blog
Re: My first hotend jamb
December 04, 2015 06:34PM
What temperature are you using? What speed is the first layer?

If you print too slowly with PLA the heat can creep up it, particularly on the first layer because the fan is off.

Re: My first hotend jamb
December 05, 2015 06:01AM
I'm pretty sure you've nailed it!

First layer temperature is quite warm (200C) but I've been trying a new slicer and had noticed the first layer speed was slow. I'd been planning to tweak those speed settings, but it wasn't high on my to-do list because it never occurred to me it was a potential problem. Most of my recent prints have been small (like the test cubes) or in ABS, so this must be the first large PLA part I've tried with these settings.

Just checked, with a base print speed of 60mm/s it had these modifiers:
  • first layer 35%
  • outline underspeed 50%
  • solid infill underspeed 50%
(I don't know if those are cumulative)

I'll increase those and try again.
Re: My first hotend jamb
December 05, 2015 08:56AM
Also check the warm up order in the start code does not heat the hot end sooner than necessary.

Re: My first hotend jamb
December 05, 2015 12:11PM
Yep, increased first layer speed to 50% and no more jambs (I've printed both halves of the Pi case this afternoon).

Thanks for your help, I had no idea that slow print speeds could cause a problem like this.
Re: My first hotend jamb
December 06, 2015 05:50AM

I do not allow the hot-end to hold at temperature for long before printing. For PLA, printed at 185C and bed temperature of 70C, the following gets the bed to temperature a little before the hotend:
M190 S40 ; pre-heat bed
M104 S185 ; set extruder temp
M190 S70 ; set bed temp & wait
M109 S185 ; wait for extruder temp
I am careful about not abusing my hotend and have not suffered a single jam in over two years of printing on my Mendel90.

Neil Darlow

Edited 1 time(s). Last edit at 12/06/2015 05:52AM by neildarlow.

I try to write with consideration for all nationalities. Please let me know if something is unclear.
Printing with Mendel90 from fedora 25 using Cura, FreeCAD, MeshLab, OpenSCAD, Skeinforge and Slic3r tools.
Re: My first hotend jamb
December 06, 2015 09:53AM
Yes, I do something similar, my hotend doesn't start to heat until the bed has reached target temperature, the problem here was purely caused by an accidentally slow first layer print speed (something I hadn't previously considered as a potential problem).

Edited 1 time(s). Last edit at 12/06/2015 09:53AM by plankton.
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