Extruder won't get hot enough to melt filament

Hi, I have an issue with my extruder (MK8 off of ebay): it just won't get hot enough inside the filament tube to melt plastic. At the same time, the nozzle itself is plenty hot as it melts the filament that touches it directly. The inside of the filament tube only shows about 82 C though (sticking thermocouple all the way down, so right about where the plastic should be melting), if my multimeter thermocouple is any good. I've tried swapping thermistors just in case, but that doesn't help. Inside of tube isn't hot enough, so no extrusion. Any thoughts as to what a solution might be? Did I just get a poorly designed extruder that dissipates heat too fast? Thanks.

... maybe you have to recalibrate the temperature curve ...

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Viktor
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I can try, but the outside of the nozzle is clearly several hundred degrees C (hot enough to melt the filament, I'm trying both ABS and PETG), while the inside of the filament tube is something like 80... If I force the temp higher, something seems liable to blow. Basically it seems as if the thermistor is showing the correct temp, it's just that the heat is dissipating or something by the time you get to the filament tube. e.g. if I calibrate so that the filament tube is 230 C, other parts of the extruder will likely be at 400.

I've never had good luck with metal extruders, but I can't easily swap it in this particular case, so advice is appreciated.

... can you make an image of your hot-end? - there's no metal with such bad heat conduction ... either not assembled/tighted properly or totally wrong design

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

If the thermocouple gives a sensible reading on the outside of the heat block then you didn't get it all the way down into the melt zone and you measured the temperature on the cold side of the heat break. Presumably the filament isn't making it down into the melt zone either, so maybe there's some sort of blockage.

Yep, a blockage it was. Thanks @JamesK for the suggestion. I took a drill to the nozzle from the filament end and got out a chunk of partially decomposed plastic that corked the extruder.

Thanks @VDX for your suggestions as well. It extrudes now, so at the very least the design is ok (regular MK8).

That's good news, glad you got it sorted. Thanks for letting us know.