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.


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)