Thursday, December 27, 2007

Things Get Affected By Things

The new extruder firmware was doing odd things. I was using the heat profiler and noticed that the temperature leveled off at about 77C. I looked at the heater LED and noticed that it was alternating between flickering and being off. About 6 seconds flickering; about 6 seconds off.

Troubleshooting mode...
- Maybe it's the heat profiler. Try the extruder exerciser. Same symptoms.
- Remove the thermistor from the heater barrel and let it cool down. Symptoms go away.
- OK, so, it's an interaction between the temperature sensor and the heater control.
- Replace the thermistor with a variable resistor.
- Start at 10k and decrease. Heater cuts out at 8.8k (temperature reads 77C).
- I've got C3 at 20nF. Try 10nF. Heater cuts out at 17.5k (temperature reads 89C).
- Try 100nF. Up to 130C.
- 200nF? 156C. Close enough.

I'm not entirely sure why it's cutting out like this. I'm guessing something in the extruder firmware is acting up, causing the safety cutoff to kick in at a lower than intended temperature. Whatever the cause, increasing the value of C3 seems to be a viable workaround.

After bumping the C3 to 200nF, my heater profile came out like this:


That gave me some numbers so I could adjust the hm value. It was pretty close at 1.1 but I dropped it to .9, just to make sure I got a decent heater temperature. I cranked up the heat, loaded some filament and tried extruding some HDPE.

I managed to extrude 2770mm (109") of .8mm (1/32") plastic. That works out to about 1.5cc.


Near the end, I pushed the extruder speed to maximum. The JB Weld joint between the drive screw and the wire cable snapped. That's a lot of torque. To be fair, I probably messed up that joint. I think that was the first thing I glued using JB Weld and I'm not sure I mixed it properly.

I'm now on thermistor number four, having fried or mangled the previous three. This one came from a cheap indoor outdoor thermometer, similar to this one. There are two thermistors in it. I used the outdoor one, setting aside the other for later.

5 comments:

nophead said...

Hi Steve,
Did you bake the JBWeld?

Beware the thermistors out of the thermometer. They may not be rated for 300C like the original ones are. I started off with a random thermistor and it soon went intermittent on me causing much confusion.

What temp did you extrude at? I get jams with HDPE below 180C.

Steve DeGroof said...

No, I don't think I baked that connection.

And I extruded at 160C, which was probably a bit on the low side.

RoundSparrow said...

Thanks for the post, getting close to trying myself.

nophead said...

It took me a few attempts to get the JBWeld to stick. The succesful techinque was to cut a cross in the end of the thread about 2mm deep. Rough up the inside with a small ball shaped grinder. JBWeld and let it cure 15 hours and then bake it at 200C for 2 hours. After I did that it has held up since August and is still going strong. In fact I think the steel cable will wear out before the JBWeld goes, a couple of strands have broken already.

Vik Olliver said...

Steve, look out for thermistors that go below 1K at operating temperature. One of mine did that, and I had to stick a 560R in series with it to prevent exactly the kind of cutout you're experiencing.

Maybe this is a capacitor charge/discharge issue?

Vik :v)