Archive for December 26th, 2010
Having a Thing-O-Matic on order, I’d been browsing the doc and came upon the MK5 extruder specs, which uses a pair of 25 W resistors (* wrong: see bottom) similar to the 50 W resistors I’ve been building into the Hot Box Disinsector oven.
However, the head puts the two 5 Ω resistors in parallel, directly across the +12 V supply: each 25 W (* see bottom) resistor dissipates 29 W. To make matters worse, the heater block is wrapped in ceramic cloth tape thermal insulation, bundled up in Kapton, and servo-controlled to something over 200 °C.
What’s wrong with that picture?
Here’s the note I put up on the Google MakerBot Operator’s group a few days ago, with slightly cleaned-up formatting:
MK5 / Thing-O-Matic Heater Problems
My TOM is on order, halfway through its 7-week leadtime. In the meantime, I’ve been reading the mailing lists and poring over the documentation. One thing stands out: a disturbing number of “my MK5 extruder stopped heating” problems.
Right up front, I’m not slagging the folks at MakerBot. I attended Botacon Zero, toured their “factory”, and ordered a Thing-O-Matic the next day. This is my contribution to tracking down what looks like a problem, ideally before my TOM runs into it. I *want* to be shown that my analysis is dead wrong!
What follows is, admittedly, a technical read, but that’s what I *do*.
The heater uses two 5-ohm 25-watt panel-mount resistors in parallel across the 12 V supply to raise the thermal core to well over 200 C. Some folks run their extruders at 225 C, which seems to be near the top end of the heater’s range.
The resistors are standard items from several manufacturers. The datasheets can be downloaded from:
My back-of-the-envelope calculations suggest several problems with the heater, all of which combine to cause early failures.
1) Too much power
Putting 12 V across a 5 ohm resistor dissipates 28.8 W. Allowing for 0.5 V drop in the wiring, it’s still 26.5 W.
That exceeds the resistor’s 25 W rating, not by a whole lot, and might be OK at room temperature, but …
2) No temperature derating
The 25 W power rating applies only when mounted to the heatsink specified in the datasheet at 25 C ambient temperature. Above that temperature, the maximum allowed power decreases linearly to 2.5 W at 250 C: 0.1 W/C.
When the resistor is not mounted to a heatsink, its maximum free-air rating is 12.5 W. That limit declines by 0.044 W/C to the same 2.5 W limit at 250 C.
What this means: at 200 C *and* mounted on a heatsink, the resistors must not dissipate more than 4.7 W. The MK5 heater runs them at 28 W, six times their 200 C rating, and they’re not on a heatsink.
3) Excessive heat
The resistors will always be hotter than the thermal core: they are being used as heaters. The temperature difference depends on the “thermal resistance” of the gap between the resistor body and the core.
The MK5 resistors are dry mounted without thermal compound, so the gap consists largely of air.
I recently measured the thermal resistance of the 50 W version of these resistors on an aluminum heatsink using ThermalKote II compound in the gap. In round numbers, the thermal resistance is about 0.2 C/W: at 28 W the resistors will be 6 C hotter than the thermal core.
The default air-filled gap to the MK5 thermal core will make the resistors *much* hotter than that. With the core at 225 C, the resistors will probably heat beyond their 250 C absolute maximum operating temperature.
The datasheet ratings for the resistors assume mounting on a heatsink in a given ambient temperature, so that the resistors can dump heat to the heatsink (that’s why it’s called a *sink*) and to the surrounding air. The MK5 thermal core and resistors live inside ceramic insulation and Kapton tape, specifically to prevent heat loss.
The resistors operate with far too much power at too high a temperature, inside a hostile environment with too much thermal resistance to the core. They will fail at a high rate because they are being operated far beyond their specifications.
Given that, the failures I’ve read here over the last few weeks aren’t
surprising. Some links:
This picture (linked from the first message) shows a severely burned resistor slug:
I do not know what fraction of the MK5 extruders those messages represent. There are about 1000 members of this group, but not everybody has a MK5 extruder head. Assuming 250 MK5 heads, that’s a 2% failure rate.
The number of problem reports seem to be increasing in recent weeks, but that can be a fluke.
Depending on the room temperature, a MK5 thermal core can probably reach operating temperature with only one functional resistor, but it will take much longer than normal.
Indeed, I suspect some of the “my MK5 has difficulty extruding” problems may come from a thermal core that’s nominally at operating temperature, but with one dead resistor: the steel block is cooler on the side with the failed resistor. The thermistor reports the temperature at the block’s surface, not inside where the plastic actually melts.
It’s entirely possible that a resistor failure can lead to an extruder motor failure: too-cool plastic → difficult extrusion → high motor load → extruder motor failure. That’s a guess, but it seems reasonable.
The symptoms fall into two categories, with what I think are the obvious causes:
Slow heating = one resistor failed
No heat at all = both resistors failed
To discover what’s happened, disconnect the heater power cable from the extruder controller, then measure the resistance across the wires. You should find one of three situations:
1) 2.5 ohms = both resistors good = normal condition
2) 5 ohms = one failed resistor
3) Open circuit = two failed resistors
The resistance value may vary wildly if you move the wires at the extruder head, because a failed resistor element can make intermittent contact. If you measure the resistance at the extruder controller connector end of the cable, leaving the thermal core alone, you should get more stable results.
What to do
Given that the resistors operate under such hostile conditions, I think there’s not much you can do to make them happier. Some untested ideas:
1) Use the remainder of the anti-seize thread lube as thermal compound between the resistors and the thermal core. It’ll stink something awful until the oil boils off, but ought to keep the resistors significantly cooler by improving heat transfer to the core. Standard PC CPU thermal compound (Arctic Silver, et al) deteriorates well below 225 C, so it probably won’t survive in this environment.
2) Rearrange the thermal wrap to expose the ends of the resistor leads, which will cool the resistor element end plugs and reduce the deformation causing the slug to work loose inside the aluminum shell.
3) Use thicker connecting wire, without insulation, outside the thermal wrap, to dump more heat from the resistor leads.
The last two changes will cause more heat loss from the thermal core which means the controller will turn the resistors on more often. Perhaps reducing the thermal stress on the weakest part of the resistors will delay the failures, but I don’t know.
When my TOM arrives, I’ll instrument the thermal core with a handful of thermocouples, measure what’s going on inside, try some of those ideas, and report back.
If you get there first, I’d like to know what you find!
* From above
Now, as it turns out, my TOM arrived on Christmas Eve! Given the usual holiday distractions, I’m only now getting down to construction & measurements, but one thing pops right out: contrary to what I’d assumed / read somewhere, those resistors are rated for 10 W at 25 C. Everything I wrote above applies to 25 W resistors: the situation is actually much worse: a 10 W resistor dissipating 30 W while tucked inside an insulating blanket?
This is not going to have a happy outcome…
[Update: The story continues there.]