extruder problem: barrel too tight/narrow?

Hi

I'm reaching the final stages with my reprap project and have a problem with the extruder. I will try to describe what happens, since I am not sure of the fault (I don't know how it should behave).

- The barrel heats up to chosen temperature. This is measured by the thermistor and conversion tables. i.e. I don't know the actual temp. in degrees, but it seems plausible up to 100 deg. C. measured by dipping into boiling water, room temp. water etc. I don't know if ALL the barrel is at this temp, or only the bit near the thermistor (which is close to the resistance wire)

- The pinch wheels drive the plastic feed rod into the PTFE insulator successfully, with speed control

- The plastic goes into the first cm of the barrel and then gets stuck. It can be pushed through manually, but this required some force (my guess, say, 70 Newtons ie about the effort required to lift 7kg. Not sure easy way to measure)

- The reason it is stiff is that the plastic is not quite straight nor is the cross-section perfectly circular.

- I operated the extruder heater for about 10 minutes
- the plastic feed is ABS

- When I dismantled the extruder after testing, I observed that the plastic had become molten within the PTFE and the first cm of the barrel, where it's threaded, but had not flowed down into the actual heated part of the barrel or as far as the nozzle.

So with these observations in mind, my list of possible errors is:
- Didn't leave it long enough to fill the barrel/ melt enough plastic
- Wasn't hot enough to fully melt the plastic
- My temperature readings are not correct (e.g. wrong conversion table)
- I read on the wiki that the plastic feed rod should become molten throughout the barrel, in which case, the tight fit shouldn't matter?
- The barrel is too tight and needs reaming or drilling out to a larger size
- My driver isn't pushing the plastic hard enough (I did take over and push it manually, needs a LOT of effort to push it in, not sure I can manually push it far enough)

Could someone with a working extruder please describe how tight the barrel should be, the extent of molten plastic within the barrel/insulator, and whether I am operating the extruder wrong in my tests?

NB: Since I have Darwin electronics and 64 bit ubuntu, I am using a hacked together version of the original Darwin host software which isn't versioned. I notice that the temp. reading on the GUI oscillates between 2 temp readings, one about 30 deg. C higher than the other. The period of oscillation is about 0.5 Hz and the temp. diff is consistent throughout the temp range. I wondered if the thermistor reads higher when the PWM controller has the heater on, compared to off?

Many thanks
dave

Edited 1 time(s). Last edit at 06/03/2012 10:10PM by davidra.

The hole through the PTFE has to be a little bigger than the worst case filament cross section. The hole in the brass is less critical because the plastic should melt when it touches it.

70N does seem high, probably about twice what it should be to extrude at a reasonable speed.

The most likely cause of it jamming is a small gap between the end of the barrel and the PTFE. See this video and the blog article it links to: [www.youtube.com]

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[www.hydraraptor.blogspot.com]