Archive for January 19th, 2011
Based on the thermal coefficients found there, it looks like the MK5 Extruder head can reach operating temperature with only 15-ish watts of power, rather than the usual 60 W. While a factor-of-four more power certainly reduces the delay from power-on to building parts, the stress it puts on the resistors causes early failures.
What I’m trying to find here is the minimum power required to heat the head to 225 °C, in order to guide some future tweaks. The resistors will be operating outside their specification at anything more than 1 W each, but reducing the maximum power dissipation can’t possibly be a Bad Thing.
With that in mind, I wrapped the ceramic cloth insulation around the Thermal Core. Because the Core-to-ambient thermal coefficient was 12 °C/W without insulation and 16 °C/W with cotton cloth insulation, I didn’t do a thorough job of taping the gaps. Any insulation is better than none, but I knew I was going to dismantle the poor thing several times over the next few days.
The MK5 thermocouple is now in its intended position, clamped under the washer at the rear of the Thermal Core. I added a pocket of Kapton tape to electrically insulate it from the Core to prevent a resistor failure from shorting +12 V to the MAX6675 thermocouple interface chip, but that didn’t actually work: the bead punched through the tape. I think a small epoxy blob is in order.
I used just the Fluke 52 dual-thermocouple meter, in addition to the MK5 thermocouple:
|TOM||MK5 t-couple||Standard location, Kapton wrap|
|T2||Fluke 52||Core edge adjacent to resistor|
|CA||Craftsman A||not used|
|CB||Craftsman B||not used|
|MPJA||MPJA meter||not used|
For the previous test, I read the temperature as the head warmed up at a specific power level. In this test, I picked the power level, read the temperatures until they sort of stabilized, then increased the power. As a result, the numbers aren’t quite comparable to what you’ve seen before: the head is not at a stable temperature.
The adjusted temperature readings, taken every 10 minutes:
The Core was still heating with 25 W applied, but I couldn’t resist sticking an ABS filament into the Extruder, at which point my data-taking went downhill. Suffice it to say that 25 W heats the core well beyond 225 °C; I found a power level of 22 W (a current of 1.5 A) maintained the Core temperature at 225 °C.
One thing popped right out: the adjusted values for the MK5 thermocouple seem completely out of line, which isn’t surprising given what I saw in the isothermal block during calibration. I’ll have more to say about that in a bit, but the calculations you’ll see here use the raw MK5 thermocouple reading.
The last two lines show that the MK5 head, even with my crappy insulation job, can reach operating temperature with a total power under 25 W. That’s far less than the 58 W; it looks like running at about half power will be feasible.
The temperature differences:
|Power||R – Edge||Edge – TOM||R – Ambient||Edge – Amb||TOM – Amb|
And, from those, the thermal coefficients for the boldified lines (which are as stable as you’re going to get for this dataset):
|R – Edge||Edge – TOM||R – Ambient||Edge – Amb||TOM – Amb|
You can’t compare the Resistor-to-Edge coefficient to the previous numbers, as the Core is getting heated from both resistors. Indeed, I think that column is totally bogus; using half the power gives a number comparable to the previous measurements, but I’m not certain that’s valid.
The rate of heat loss increases with higher temperatures: the Core-to-Ambient thermal coefficient is half of its previous value. The crappy insulation wrapper contributes to that, but the decline tracks the temperature for both types of insulation.
In round numbers:
- Heating the core to 225 °C requires maybe 25 W
- The resistors run 10 °C higher, with thermal grease
- This operation really stinks up your Living Room