Filament getting clogged at top of feed tube

I'm having some issues with my extruder,
When I try and extrude filament, be it during a print or by extruding X amount of filament in pronterface, the extruder will only extrude about 10mm of filament before it gets clogged around the top of the feed tube, I've changed the tube (for another unrelated issue) but i'm still having the same problem.
I have a feeling it may be that the filament if melting at the top of tube instead of in it, even though i have a cooling fan and a heatsink on it.

Originally I thought it was that I had it too hot and the fan couldn't keep the temperature down low enough at the top of the tube, however, I've tried changing the temperature in my slicer (Cura) and it still does nothing

I'm using a Prusa i3 from Sintron with a Mk8 extruder

Edited 2 time(s). Last edit at 03/27/2016 01:01PM by Mumblemore.

So is the extruder a bowden type with a long Teflon tube between the extruder and hot end or direct type with the hot end immediately below the extruder? What type of filament are you using and at what temperature? Can you take a photo and show us where the filament is getting stuck?

Maybe you have mistaken PLA for ABS. A 40mm fan blowing on the inlet has worked for me.

the mk8 is direct drive the cooling fan is mounted on a heatsink with the fins facing the fan which blows inward and should always be running. If you make the fins run horizontal instead of vertical then drill a 5/16 hole in the lower left quarter of the heatsink it allows air directly to the top area of the heat break where the filament goes in below the feed gear. The teflon inside may be deformed as well filament should run freely inside the heat break tube when cool i.e. without the nozzle. you can run a filament size drill bit inside the teflon otherwise replace it. The fan should always be on whether printing or not and wired directly to the low v power. Melzi boards have two fan plugs one to cool the hot end and the other to cool the print if they are reversed then the extruder cooler would only run during a print by then too late as filament would melt at the top. Extruder probably making a clicking noise as well

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rwt

thanks for the help, i figured out the jamming issue was because I was heating the hotend before inserting the filament, so the top was just that little bit hot enough to melt it and get built up around the top. However, that appears to only be half the problem, it now isn't pulling the filament down thorugh the extruder. Here is a video of the extruder running, but not pulling the filament through.
Any Suggestions?

The top of the throat in a mk8 should never get hot enough to melt filament, not even close. You have the fan on the extruder connected directly to the psu so that it runs all the time?

In the video, the throat is slightly too high - it's touching the idler and preventing adequate pressure being transmitted to the filament. It also looks like you don't have enough pre-tension on the spring.

Edited 1 time(s). Last edit at 04/02/2016 04:33PM by JamesK.

I adjusted the throat and now it extrudes when i press the extruder button in pronterface, but it doesn't extrude during a print, the head moves like it should, and from what i can tell the extruder motor is still moving, but it isn't extruding any filament, i tried with several different gcode files that were created using cura,
Any Suggestions?

When I last used a mk8 I was surprised how tight I had to make the idler to get it to work. You want it tight enough to grip but not so tight it strips the filament if it jams, at the right tension it will click if it's jammed which is a useful indicator.

Also what temperature is your hot end? Can you verify it's the temperature you have set it to? Thermistors are not generally that accurate and very dependant on software being set right. Too cold and you'll have trouble getting any filament through. Try pushing filament through by hand with the idler wide open. It should flow through reasonably easily if the temperature is high enough. Min 185 for pla and 235 for abs but you can go 15 deg c higher on both depending on the filament.

Try a cooking thermometer or an IR thermometer but paint a bit of black high temp paint on one side or permanent black marker if you go for the ir they don't read well on shiny surfaces.

Edited 1 time(s). Last edit at 04/09/2016 11:27AM by DjDemonD.

I highly doubt that it's the temperature being too low because i've tried everything from 185 all the way up to 215 (i'm using PLA) and the same issue happens regardless,

Fair enough but can you be even slightly certain that your 215 deg C indicated is actually 215 deg C? I say this as I bought a kit printer but the default firmware had the wrong thermistor table and 210 was only 180 actual temp?

Can you easily hand extrude filament?

Edited 1 time(s). Last edit at 04/09/2016 01:15PM by DjDemonD.

Yep, it's really easy to do it by hand

Try printing in mid-air. Disable the auto-levelling (if you have that), and set the nozzle about 1cm above the bed, and try printing. It'll just make a spaghetti mess, but will tell you if the problem is that the nozzle is too close to the bed.

Also check for retraction problems. Push a cm or so of filament, pull back 5mm, push again. Does it jam? Try reducing/disabling the retraction in your slicer.

Agree about tightening up the idler... if the stepper doesn't click when the filament is jammed, the idler isn't tight enough.