MK5 / Thing-O-Matic Heater Problems
Ed Nisley
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*.
Background:
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:
KAL (Stackpole) http://www.seielect.com/Catalog/SEI-kal.pdf
Dale (Vishay) http://www.vishay.com/doc?30201 (will download a PDF)
Ohmite http://www.ohmite.com/catalog/pdf/89_series.pdf
Possible Problems:
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 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.
4) Insulation
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.
Conclusion:
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:
http://groups.google.com/group/makerbot/msg/6a2a49bb02f0702f
http://groups.google.com/group/makerbot/msg/aaa3ee724177fe15
http://groups.google.com/group/makerbot/msg/b28f1524e36055eb
http://groups.google.com/group/makerbot/msg/764f4c7196feb5cb
http://groups.google.com/group/makerbot/msg/a92cf3e8ab7e235c
This picture (linked from the first message) shows a severely burned
resistor slug:
http://img.skitch.com/20101108-nhrj8rjx68ffxrdq6p2fgwjcqx.jpg
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.
Observations:
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.
Diagnosis:
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!
Onward...
Andrew Plumb
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Ed Nisley
> Welcome to the MakerBot community.
Good to be here; I'll finish moving in after my TOM arrives!
I just sent off a column involving those 50 W resistors on heatsinks,
which is why I'm sorta spring-loaded on the subject...
> the duty cycle is well below 50% (at least on the relay-driven Cupcake
> variant) when the extruder head is properly insulated.
Ah, now that's interesting. The overload spec for the resistors is 5x
rated power for 5 seconds, with the assumption you won't whack it again
for a while. Even if the duty cycle is under 50%, I'm sure the relay is
ON for more than a few seconds at a time and, of course, "rated power"
means "derated power at the current temperature".
That also confirms my guess that a single resistor can get the core up
to operating temperature, albeit slowly and with a high duty cycle.
Alas, that poor resistor will cook itself in short order: full throttle
at liftoff, all the way to orbit.
> bad batches of motors
I saw that saga and was amazed; wonderful detective work! What's
puzzling is repeated motor failures, even with tested and known-good
motors. The most recent:
http://groups.google.com/group/makerbot/msg/11e6f14a5571c85f
Something is killing those poor motors, whether it's over-temperature at
the motor, over-stress at the feed roller, or under-temperature in the
extruder. Another puzzle!
> the total system load approaches the 11A max on the 12V channel of the
> original PSU
Good catch; I just checked a pair of ATX supplies around here. One
supports 12 A, the other has two 14 A channels, and I suspect those
ratings are, mmmm, optimistic. If the TOM supply seems a little short,
I'll swap in the dual-channel box.
You'd think the supply's -Power Good output would go *ding* and shut it
down, but that may be magic thinking.
Nothing like a good new problem to take your mind off all your old
problems, that's for sure...
Andrew Plumb
On Wed, 2010-12-22 at 18:58 -0500, Andrew Plumb wrote:
the duty cycle is well below 50% (at least on the relay-driven Cupcakevariant) when the extruder head is properly insulated.
Ah, now that's interesting. The overload spec for the resistors is 5x
rated power for 5 seconds, with the assumption you won't whack it again
for a while. Even if the duty cycle is under 50%, I'm sure the relay is
ON for more than a few seconds at a time and, of course, "rated power"
means "derated power at the current temperature".
That also confirms my guess that a single resistor can get the core up
to operating temperature, albeit slowly and with a high duty cycle.
Alas, that poor resistor will cook itself in short order: full throttle
at liftoff, all the way to orbit.
the total system load approaches the 11A max on the 12V channel of theoriginal PSU
Good catch; I just checked a pair of ATX supplies around here. One
supports 12 A, the other has two 14 A channels, and I suspect those
ratings are, mmmm, optimistic. If the TOM supply seems a little short,
I'll swap in the dual-channel box.
You'd think the supply's -Power Good output would go *ding* and shut it
down, but that may be magic thinking.
Nothing like a good new problem to take your mind off all your old
problems, that's for sure...
makeme
> datasheets can be downloaded from:
>
> KAL (Stackpole)http://www.seielect.com/Catalog/SEI-kal.pdf
> Dale (Vishay)http://www.vishay.com/doc?30201(will download a PDF)
> Ohmitehttp://www.ohmite.com/catalog/pdf/89_series.pdf
about releasing the plans and parts list for their open-source bot,
but less-good about providing manufacturer's specs on the parts they
choose to use. That might be fine for you EE guys who know how to look
it up...but electricity is black magic to me. For example, it would be
a good idea to compile the various estimates of how long each part
will last. This resistor seems to be rated to 1000 hours, and it's
being used at the top end of its capabilities.
> 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,
stay hot? If it's exposed to air, or if it's paired with a core and
insulated, it's still going to be at a certain temperature. As long as
the temperature it's at is within its operating range why would it
matter how fast it bleeds off the heat? It seems like insulating it
would just mean that you use LESS power to keep it hot, since you can
turn the juice off for longer and it maintains the desired
temperature. It seems, intuitively, that keeping it at a high (but
stable) temperature would stress it less than constantly cycling it
across a larger range of temperatures. Basically, wouldn't insulating
it actually lower the duty cycle?
> 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 at 28
> W, six times their 200 C rating, and they're not on a heatsink.
as you don't apply high power to keep them at that temperature? Like,
you could use them low and slow at that temp?
Also, isn't the hot end itself a heat sink? It's obviously dumping its
heat or we wouldn't have to use so much juice to keep it hot, right?
I thought when the documentation referred to "without a heat sink"
they meant that they had the resistor itself exposed; that it hadn't
yet been installed in the aluminum heat sink housing. In that case it
would just be a cylinder, the second worst shape for getting rid of
excess heat. Mounting the resistor in the aluminum allows the heat to
be pulled out of it. So, in the same way, doesn't mounting the aluminum
+resistor combo on a steel block allow the heat to be pulled out too?
I mean, if you just had the aluminum+resistor bolted to any random
piece of steel wouldn't it count as an extension of the aluminum heat
sink?
Is the resistor itself limited to 4.7 W at 200C when it's not attached
to any kind of heat sink? Or is it limited to 4.7 W as soon as it
reaches 200C, even if it is attached to metal that's drawing away the
heat?
Or am I asking the wrong question?
> 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.
paste when mounting the resistors, which seemed strange. But there are
those two bolts going straight into the hot end core.
It seems like that wouldn't be too hard to test. The thermometer can
just be sandwiched between one of the resistors instead of as far away
from both as possible.
However, wouldn't the temperature of the resistors be limited by how
much power is applied to them? As long as the applied power doesn't
raise them above a safe temperature you can just sit and wait for the
temperature of the whole thing to rise.
> 4) Insulation
>
> 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.
rises above the 250C limit, right?
I mean, it seems like these resistors are supposed to be used as you
describe, as resistors where the heat is an unfortunate byproduct.
But, if they get really hot in use, then they're still really hot. The
steepness of the thermal gradient in the aluminum seems irrelevant.
The important thing is the highest temperature. Isn't the point of
installing a thermometer to control the environment inside the
insulation so that it stays in a certain range? And if that range is
within the operating temperature of all the components they should all
be fine, since that's what operating temperature means. It does seem
like the amount of time the resistors spend at such a high temperature
would be relevant, but I can't tell from the documentation whether or
not their life time is limited by how long they spend cooking or how
long they spend powered. Like, if you just stuck one in an oven would
it break after a 1000 hours? Or is it pushing the juice through it
that wears it out?
> 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.
diagnose since the temperature wouldn't increase, or at least not
enough, and even if you got a false reading that claimed the hot end
was at the right temperature you wouldn't get any plastic oozing out
of the nozzle. Trust me, I've been staring at that thing intensely
ever since I got it. A total lack of plastic migrating towards the
center of the earth as the hot end approaches operating temperature
would stand out as an anomaly.
> 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
troubleshooting wiki.
Too much of the problem solving stuff is dispursed around a dozen
websites and blogs. What we need is a "You have [X] problem so look at
[1] then [2] then [3]" sort of format. It seems like inevitable
changes in the design will only require alterations to some
troubleshooting variables, like what resistance to look for. The basic
idea (Is the motor slipping? Yes? Maybe there's nowhere for the
plastic to go, check the heater.) is applicable across a variety of
designs. We just need some of you EE guys to help explain what numbers
to look for on the multimeter :)
> 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.
antiseize closed cuz I couldn't think of anything to do with it.
It can't hurt. Even if it doesn't help the resistors live longer it
will at least help the hot end reach temperature faster. That's not
nothing.
> 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.
procedure closely and on mine the actual connections to the resistor
are only insulated by a layer of kapton tape.
> 3) Use thicker connecting wire, without insulation, outside the thermal
> wrap, to dump more heat from the resistor leads.
Like this? They've still got insulation on them, but it's thin.
> 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.
the insulation.
Ed Nisley
> This resistor seems to be rated to 1000 hours, and it's
> being used at the top end of its capabilities.
The resistors are actually 10 W units, not the 25 W I'd assumed before
my TOM arrived. The derating curve puts them at 15.6% of 10 W @ 225C =
1.6 W, but they're running at 28 W: call it a factor of 18 *beyond*
their ratings.
> This resistor seems to be rated to 1000 hours
When used according to the datasheet.
There is no combination of circumstances where dissipating 28 W in a 10
W resistor will result in a 1000 hour lifetime; the fact that we've seen
only a few percent early failures is truly remarkable.
In normal use, resistors *never* fail. They may age out, but they don't
burn out.
> Isn't the point of a heater to get hot and stay hot?
These are not heaters. They're power resistors.
That's a vital difference: resistors operate under specific conditions
that, incidentally, involve high ambient temperatures. They're not
intended to operate under extreme heat beyond their ratings.
> So, insulating them to keep them above 200C for longer is fine as long
> as you don't apply high power to keep them at that temperature?
That's correct: the maximum allowed power dissipation is a function of
the ambient temperature. The data-sheet curve shows that the resistors
can handle a decreasing fraction of their maximum power at higher
temperatures.
> Or is it limited to 4.7 W as soon as it reaches 200C, even if it is
> attached to metal that's drawing away the heat?
There are two curves in the datasheet for each resistor size. One (the
higher one) applies to a resistor mounted to a specified heatsink, the
lower one applies to an unmounted resistor.
> I thought when the documentation referred to "without a heat sink"
> they meant that they had the resistor itself exposed
Nope, the "resistor" is the packaged unit in the aluminum shell. It's
intended for panel mounting on some flat surface that normally acts as a
heatsink. By "unmounted" they mean, by and large, dangling in mid-air,
which is the worst-case condition for getting rid of heat.
While we can quibble about just how much power goes out in the melted
plastic, the simple fact is that the *maximum* power rating on the
specified *heatsink* is 20% at 225 C: 2.0 W.
> But that doesn't necessarily mean that any point inside the package
> rises above the 250C limit, right?
Nope. By definition of heat, it flows from hotter objects to cooler
objects: in this case, resistor to thermal core to plastic. In the
simplest case, there's a temperature difference across each interface
that's measured in degrees C / watt of power: more power = higher
temperature drop across the gap.
The air gap below the resistors acts as a fine insulator, with a
relatively high C/W value. That's something I want to measure, using a
low voltage to keep the dissipation within reason, so we have some
useful numbers. The bolts don't count, because they have such small
cross-sectional area and *two* air gaps.
The extruder controller maintains the thermal core at a specific
temperature that's somewhat above that of the molten plastic by
adjusting the duty cycle of the resistors. It does not control the
resistor temperature, which will rise to whatever value is required to
maintain the commanded core temperature.
You'd like to measure & control the temperature of the plastic, but
that's awkward. The steel block is massive enough and has intimate
contact with the brass nozzle (with anti-seize grease) to make that
temperature drop relatively low.
> Any results?
I'm just now starting the extruder assembly. I want to finish the build,
then make some measurements, then do some tweaking, then see how it
works... which will take a while.
So many projects, so little time...
Zip Zap
From: Ed Nisley <ed.n...@gmail.com>
To: MakerBot Operators <make...@googlegroups.com>
Sent: Wed, December 22, 2010 1:35:47 PM
Subject: [MakerBot] MK5 / Thing-O-Matic Heater Problems
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