Sunday, June 22, 2008

 

Up in Smoke

Let me recount a little episode involving my fledgling RepRap extruder and its self destruction last weekend.

I had just finished assembling my extruder and was ready to test it. I followed the Thermoplast Extruder instructions, slowly increasing the set temperature to the melting point of HDPE.


( I found that changing the Desired temperature didn't have an effect on the "current temperature" until the heater was deactivated and reactivated. That was a little confusing at first, and could maybe be improved upon. Ideally, the set temperature would take effect after changing focus out of that field or after a delay of a second or two.)

With a set point of 50C, my 'current temperature' wouldn't budge above 38C. My extruder started smoking, and by the time I had shut it down (a minute or so later), all of my JB weld was charred and my 8 ohm winding of nichrome wire had transformed into 32 ohms, rendering it useless. What happened? Apparently, the temperature of the extruder barrel was not interpreted correctly by the reprap-host software. When it thought the barrel was at 38C, it was actually around 260C, the point at which both JB weld and the PTFE (Teflon) used in the extruder break down. I figured out the 260C figure by looking at the displayed temperature when submersed in ice water and (near) boiling water and extrapolating.

Here is a picture of my extruder (with insulation removed).

My barrel's charred remains (sad. :-[ ).


With help from the kind and generous Ian Eagland in the RepRap forums, the culprit was found.

These three settings in reprap_host were my problem:
Extruder0_Beta(K) 550.0
Extruder0_Capacitor(F) 0.000003
Extruder0_Rz(ohms) 4837

I had set these to the actual values used in my hardware, which was a mistake. These constants are left-over from the pic-based electronics, and in the Arduino-based systems no longer represent real physical entities. Setting these values to what I was explicitly told to do in the Setup Host Software part of the Generation2Electronics instructions fixed my temperature readings.

I have since tore off all of the weakened JB weld and nichrome wire and replaced it with new stuff, using both a thermocouple and thermistor in case I ever want a second opinion on the temperature.

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