wierd blockage in top of extruder

I have a mega prusa that has been printing ok (sort of) for 4 months now but yesterday the filament ran out mid way through a print while I was out. Now I can't load more filament. I preheat the nozzle but I cant push the filament down the barrel. The nozzle is clear. It seems to be happening near the top of the barrel but I cant see anything. I tried pushing a 1mm rod down but it cant move whatever it is.
Has any body else experienced this? Any ideas what to do?

What temp are you extruding the filament at regularly? I find that my hotend runs PLA fine at 210° C. Turning the extruder gear will feed it through easily, but I can barely push it through with the hobbled bolt disengaged. What material / filament diameter / extruder setup are you using?

Edited 1 time(s). Last edit at 07/04/2015 09:53AM by Montiey.

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Master Tinkerer

Take the heatsink off the heatbreak, leaving the heatbreak in the heater block. Remove the nozzle if you think the blockage is not filament. Heat up to 210 or so (assuming PLA), wait a while for the heat to soak to the top of the heatbreak, and then push whatever it is out.

Montiey, I usually print at 195c and everything is good. I'm using 1.75mm PLA and a .7mm nozzle. Usually when I change filament I'll heat the extruder and then feed the filament until I feel it softening in the heated section then I'll engage the hob and extrude until it runs out the nozzle. But now the filament goes about 5mm into the top of the barrel and stops like its hit a wall.
thanks.

Thanks Frankvdh.
I tried heating to 200c and leaving it for a while to try and soften the plug but when I tried to push it out it would not move. It seems like the block is above the heated section.
I'm pretty much resigned now to dismantling the whole extruder assembly to clear the block. I thought I may be able to do it without going that far.

It's fixed and i'm printing again. I had to disassemble the whole x axis carriage so I could take apart the extruder to find the problem. When the filament ran out mid print, the end was pushed through the hob and then stopped. Because it had a little kink right at the end I could not push it down the barrel of the extruder. The whole assembly is designed in such a way that the kink was impossible to reach without taking apart the whole unit.
I took some photos too show what I mean.
Thanks Guys.

Stuffmaker....
Stil using outdated hotends on new machines.
I had similar issues because the teflon liner they used was
a) cut a bit too short and it moved up and down
b) it was sliding into the hotend and made it impossible for the filament to get trough the bottle neck

Due to the use of PEEK and the heatsink being mounted quite high, the teflon liner overheats quite easy.
Once hot enough the plastic inside will wxpand and jam - especially if the hotend was turned off and cooled dow it takes quite some time to get this area soft again.
They don't really like retraction either
I am not sure what board the never models use now but if it is still a GEN6 board and your temp is fluctating by 10 or more degrees there is a simple fix available.

Great, another problem to look out for on this crappy printer. I only bought it cause it was the largest build size I could afford.
I'm not sure wether my board is a GEN6 or not but the temp does vary wildly when heating the extruder and then will drop at least ten degrees when printing.
Could you point me in the direction of this simple fix please?

If your firmware has a PID auto tuning feature, I highly recommend using it. If not, you should look for some online guides and they will help you. Getting your temperature PIDs tuned is pretty important for an accurate temperature reading.

P.S
If your not sure what control board you have, you should post a picture and someone around here will tell you what it is.

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Master Tinkerer

This is the page for the Gen6 board: Gen6 Reprap.
If your board looks like this than there is a resistor marked R12 on the board it is near the ethernet port.
Solder a 10nF ceramic capacitor on top of it and the temp will be much more stable.
It also removes a lot of the noise the stepper motors make when in hold position.

The original firmware has no PID tuning or EEPROM support - at least mine did not.
You can try by sending M303 e0 s200 c8 to the printer.
It will perform the autotune for the hotend.
I upgraded mine to a Wades Extruder and modified a Marlin Firmware to fit the printer, quite happy now and getting print speeds of over 100mm/s if I have to.

I checked out my board and it is a Gen6. I dont feel comfortable with trying to solder onto surface mount so I tried the PID tuning. It seemed to help, I just need to fine tune it a bit more.
Thanks everybody for your help and suggestions.

I used a wired capacitor and a pair of tweezer, works just fine and I have the hotend stable within less than 1°C.
Just noticed there is also a thread about the problem: http://forums.reprap.org/read.php?181,93663