Cartridge Heaters: Mounting Blocks

Drilling SHCS head clearance
Drilling SHCS head clearance

MBI sent me a selection of 1/4-inch cartridge heaters to evaluate, seeing as how I’ve been such a pest on the subject of those poor aluminum-case power resistor heaters. Thanks, Zach!

I initially thought I could punch the cores out of the resistors and slip the cartridge heaters into the holes, but it turns out the resistor bodies aren’t quite the right size: slightly too short with slightly too large holes. So it goes. Some earlier thoughts live there.

This is a first pass at building mounting blocks to attach cartridge heaters to a stock MK5 Thermal Core. Ideally, you want a solid Thermal Core with a hole or two for the heaters next to the filament extrusion nozzle, but that requires fancier machining that I’m ready for right now. The fabled nophead shows how that looks for a ceramic power resistor.

The obvious question is whether you want a single high-wattage cartridge heater or a pair of low(er)-wattage units. I think a core-with-hole can get away with a single heater, which is also the lower-cost option. My thermal measurements suggest the Core is pretty much isothermal, so there’s no problem with distributing the heat evenly from one side to the other.

However, adding two lower-wattage heaters to a stock MK5 Thermal Core makes more sense, because the interface between the blocks and the Core seems to run a bit under 1 °C/W. A single 40 W heater would thus run 30-40 °C higher than the Core: call it 260 °C. IMO, that’s much too high for something an inch away from a plywood frame and an acrylic support structure.

A pair of 25 W heaters would run at 245 °C-ish. That’s still pretty hot, but every little bit helps. I’ll start with that arrangement and see how it works.

Block top and bottom
Block top and bottom

The blocks are ordinary steel from the Scrap Box: a convenient length of 1×1-inch bar stock that somebody else had made into something else a long time ago. I bandsawed off four 1×1-inch slabs, each about 5/8″ thick. A second bandsaw cut turned the square slabs into rectangles. I finished two blocks; the other two slabs await more experience with how these work.

I squared up the blocks with a flycutter in the Sherline, then sanded down the bottom surface a bit. The thermal tests suggest the contact is Good Enough with a reasonably flat surface, so I settled for a used-car finish: high shine and deep scratches. They’re actually smoother than the pictures would have you believe.

The Thermal Core has hard inch dimensions (minus cleanup cuts): 1 inch front-to-back and 13/16 inch tall. I generally work in metric, so the sketch at the bottom has everything in millimeters.

The mounting blocks have holes matching the resistor footprint. I drilled clearance holes for the heads of the original M2 socket head cap screws, ran an end mill down the hole to flatten the bottom, then drilled clearance holes for the threads. Those holes are perilously close to the edge, but the blocks really don’t want to be any taller. Perhaps use a less-generous clearance?

The alternative would be to mill a flange along the edge to match the resistor mounts and put the SHCS heads in free air, but that seemed like more work and it would cramp the thermal path from cartridge to block.

I also thought about chamfering the edges to make the blocks look less, well, blocky, but that’s in the nature of fine tuning.

The cartridge heaters slip-fit into a nominal 0.250 hole; the samples are 0.247 to 0.248 and (from what I read) the diameter tolerance stays on the minus side of 0.250. I don’t have a 0.250 reamer, which is how you get a precise hole ID, so I’ll go with drilled holes. Fortunately, I have a set of letter-size drills in nearly new condition:

  • A drill = 0.234 to poke a hole in the block
  • E drill = 0.250 to get the final diameter

The final holes worked out to be exactly 0.250 inch, to the limits of my measurement ability, which I will declare to be Good Enough. The cartridges have a loose slip fit with no side-to-side play.

The cartridges expand when heated and squeeze against the hole to make good thermal contact. While cool, however, they can slide out without much urging, so I added a 4-40 setscrew. It’s on the butt end of the cartridge heater shell, away from the leads, so if a cartridge becomes one with the block I can drive it out with a pin punch. Putting the setscrew at the end with the wire leads makes more sense (it’s cooler there), but then you’d be beating the entire length of the cartridge out past the setscrew hole.

The setscrew and the M2 SHCSs get a liberal dose of anti-seize grease before assembly.

Here’s what the holders looked like, just before bolting them in place:

Cartridge heaters in blocks
Cartridge heaters in blocks

Doodles with the more-or-less as-built dimensions:

Heater block dimensions
Heater block dimensions

23 thoughts on “Cartridge Heaters: Mounting Blocks

  1. Hi Ed,
    Thanks for your continued work on this problem.
    On your advice I finally got around to stripping down my Plastruder MK 5 to make permanent solder connections of the electrical contacts rather than use a mechanical solution (which I selected for quick resistor replacement).
    Boy am I glad I did. To my horror, I found that both my resistors have all but failed, having expanded length-ways and pushed out of their casings. I am reluctant to “repair” my Thing-O-Matic with the stock solution, and am very keen for an unofficial fix.
    I need to clarify my understanding of your solution. Is the intent to simply wire these cartridge heaters into the Thing-O-Matic without any additional components? If so, it seems to have become a practical mechanical problem rather than an electronic problem. That I can manage. I’d be brave enough to have a crack at something like this immediately if I can find a source for the cartridge heaters. Suggestions?

    [Edit: Here’s his picture of those resistors, snatched from the discussion there.]

    Overstressed MK5 Resistors

    1. having expanded length-ways and pushed out of their casings

      Oooooh! Send pictures! The autopsy results for mine (up today) aren’t nearly so dramatic…

      In truth, though, you can replace those resistors and get another few months of extrusion fun madness. I’m astonished they work as well as they do, but they’re obviously not the right hammer for the job.

      wire these cartridge heaters into the Thing-O-Matic without any additional components

      Cartridge heaters can be electrical drop-in replacements, with just a bit of mechanical adaptation, and I’ll have some “first light” results up tomorrow.

      I’ve been burning a pair of 2 ohm resistors at 36 W for a couple of weeks, which can keep the head at extruding temperature with a rather high duty cycle. The pair of 25 W cartridge heaters I’ve been testing have a lower duty cycle (no surprise there), so anything around 40-50 W should work fine; there’s no need to duplicate the 60 W from the present resistors.

      However, cartridge heaters have one critical consideration: they won’t burn out when an electrical flaw leaves them turned on. The maximum steady-state temperatures are scary-high, the Thing-O-Matic has plenty of plastic and wood within the high-heat zone, and IMO popping a pair of heaters into the existing design is an Exceedingly Bad Idea.

      I have a rant on that tomorrow, too…

      a source for the cartridge heaters

      That’s the real problem. I haven’t found a retail source for the low-voltage, relatively low wattage heaters required for a MK5 extruder, but that surely means I haven’t looked in the right spots.

      The folks at Makerbot are obviously on the case and I’d be surprised if Makergear didn’t add cartridge heaters to their inventory at some point, but I have no inside information.

      1. OhMyGosh! I just realized, THERE IS NO SAFETY THERMOSTAT on the extruder! I’m kicking myself for not noticing, but it’s always a Bad Idea ™ to have a heater loop without one. While I have not spent too much time looking for high temperature ones, Digikey P/N: 317-1142-ND is a non-resettable one that is suppose to melt at 240 deg. C (data sheet says 200 deg. holding temperature). Data sheet has two that are hotter, one says working (melting) temperature of 260 deg. C and a holding temperature of 220 deg C, and the other one must be a typo :-) working temperature of 280 deg. C and holding temperature of a chilly 20 deg. C. While only a buck in singles, the hottest one Digikey seems to cary is the 240/200 deg. C one where I listed the digikey part number.

        PLEASE everyone, don’t leave your Plastruder unattended without one!

        – Steven Ciciora

        1. Well, based on the evidence to date, those resistors will burn out long before things get too far along. [weak grin]

          Extended rant coming up tomorrow, honest…

          The real question is where to put the thermal cutout. The Thermal Riser Tube seems like a good candidate, which would also benefit from some additional heatsinking to reduce the heat below the Extruder Filament Drive. I’m thinking of a combination temperature sensor mount / heatsink at the top end, but it needs more numbers to convince me it’d catch an overtemperature.

          I put a much thicker insulation blanket around the Core that (as you’ll see later) holds the external temperature around 130 C. Putting a 240 C fuse outside that would be a Good Idea, indeed, although it should tie into a complete thermal lockout: leaving the Extruder motor running while the plastic solidifies causes other Bad Things to happen.

  2. Hmm, another safety feature might be to select a heater with about twice as much power as you would ever want and limit it’s max duty cycle in firmware to one half of that. Then fuse it for this half power. If the controlling device (fet, relay, whatever) sticks on, or the firmware locks up and keeps the output full on, then the fuse should blow before too long. You might have guessed that I have not looked at the schematic yet; I’ve not had the time to start on my Makerbot…
    – Steven Ciciora

    1. Although the average power would be half the maximum, the power supply / FET switches / wiring must handle the peak current and PC ATX power supplies really don’t like high transient currents. The supply seems to be barely within spec right now; I must add a dummy +12 V load at some point to see whether that stabilizes things.

  3. Hello

    We are a manufacturer of 1/8″ diameter cartridge heaters. While it appears to me that a 1/4″ diameter high density cartridge heater is an adequate solution, you may want to investigate mounting a 1/8″ diameter heater directly into the extruder barrel. We offer 1/8″ diamater heaters as short as 1/2″ long, so finding a spot for a 1/8″ x 1/2″ deep bore would allow direct mounting for heating the extruder body from the inside out. Of course one must be careful not to weaken the extruder body if there is significant pressure.

    You can see more information about our products at

    John Churchill

    1. A small DC motor stuffs a 3 mm filament into the top of the extruder head using a pinch wheel, so there’s not much pressure involved: I can grab the filament, push, and extrude some plastic! The entire head could be smaller with an integral heater; I think that’s what Makerbot will eventually use.

      However, the head requires about 40 W to maintain extruding temperature and the Thing-O-Matic’s ATX power supply poses the real constraint: the highest voltage is only +12 V. Those small-and-short heaters simply don’t dissipate enough power at that voltage!

      The next generation of low-cost 3D printers will require a higher supply voltage for better stepper motor performance. At 24 V, a 40 W 1×1/8 inch heater would be adequate, but then another constraint kicks in: that unit costs nearly a factor of two more than equivalent 1×1/4 inch heaters with higher wattages. Although I’m sure you could come up with a custom unit, the price difference would be daunting for the folks actually trying to sell the printers.

      I looked at Sun heaters and really liked the package, but they’re optimized for a different set of problems than found in the MK5 extruder.

  4. Ed,
    That looks great man and very promising too! The mendel I was adapting the MK5 for is slowly coming along so I have no news to report with the 240VAC heaters. If MBI stocks a 12VDC version I would eagerly use those instead. One less power source to deal with the better. Since MBI sent you the cartridge heaters I imagine they are taking serious consideration into correcting the design flaw. Very encouraging.

    The extra thermal mass in the extruder head, I thought, would be a good thing. Lends to a more stable operating temp.

    I cant agree with you more about the thermal cutout failsafe. A simple latching relay circuit would do the trick. If you ask me, it would be a good addition to any extruder hot end.

    Looking forward to see how this develops.

    Best Regards,
    Jason [CodeRage]

    1. The heaters have been working fine and the slow warm-up isn’t a dealbreaker for me.

      I just built the thermal cutout control box and must now mutilate the Motherboard to get access to all the ATX power supply signals. It’s a simple circuit, but wow it touches a bunch of wires!

      Thanks for the original suggestion; I think this will work out very well.

  5. Thanks Ed for your response. It is certainly true that 1/8″ heaters are more expensive than 1/4″ heaters. One should note however, that one 1/8″ heater mounted in the extruder body would replace two 1/4″ units. Additionally, the cost of two mounting blocks for the 1/4″ heaters would also be saved.

    Of course, the foregoing assumes that there is a space within the extruder body for a 1/8″ heater.

    As to operating voltages, our heaters can supply 40 watts at 12 volts. Sun stocks only 24 volt heaters, but 12 volt units can readily be manufactured.

    1. Sounds good to me!

      The existing block is about 1 inch front-to-back and 3/4 inch left-to-right, with a 3/8-24 threaded hole down the middle. I think that doesn’t leave quite enough meat for a 1/8 inch hole on either side of the nozzle.

      But, of course, the existing block is the way it is only because it was designed to hold those two resistors on either side. There’s no reason it can’t be a smidge wider on one side and somewhat shorter top-to-bottom; I’d want maybe 1/8 inch of steel on both sides of the heater bore to ensure the heat doesn’t concentrate on one side of the nozzle.

      Actually, having the heater lower on the head would be an improvement, as the only part that needs serious heating is the extrusion nozzle itself. The rest of the block just softens the filament, but you don’t want any more heat than absolutely necessary traveling up the Riser Tube.

      If I understand the specs correctly, a 1 inch long heater has a 0.75 hot zone, so it runs at 135 W/in2. I don’t know enough about cartridge heaters: does that power density call for a reamed-hole fit, rather than the rough-and-ready double-drilling I used, to get a sufficiently low thermal coefficient?

  6. Ed –

    A 1″ by 1/8″ 40 watt heater would have a power density of 135 watts per square inch, as you have calculated. If you can create a heater bore of .125 to .126 (by 2 step drilling or by drilling an dreaming) that would fit the heater closely enough for a 400F operating temperature. Heater life would well exceed the 2000 hour warrantee.

    While a single heater can provide 40 watts output, if you want better heat distribution you can go to two 1/8″ heaters at 20 watts each. The power density would be 65 watts per square inch, very conservative, very long life.

    1. It seems like the 1/8 inch heaters really aren’t the right hammer for this job.

      The extruder head must operate at 200-230 C (400-450 F). From your description, those temperatures push the heater outside its expected operating conditions, at least in a hole with practical tolerances.

      Worse, if reducing that power density requires two 1/8 inch heaters, then the savings over a pair of 1/4 inch heaters evaporates. They’d certainly make for a smaller extruder head, but that’s not a compelling advantage in this situation.

      Because a single 1/4 inch heater has half the power density, it should last pretty much forever. The larger diameter would spread the heat out a bit more, although I think that’s largely irrelevant given the geometry of the extruder head. I’d be tempted to go with 50 W, rather than 40 W, but that wouldn’t change the decision.

      I think one 1/4 inch cartridge heater is the way to go, even though it requires a somewhat larger extruder head. The nozzle must remain centered below the existing filament drive, so the heater hole must be off-center.

      My chubby two-heater retrofit kludge to the existing MK5 head has the sole advantage of reducing the external temperature of the mounting blocks. I’ll probably get around to milling a two-hole head, just to see how it works, but …

      Thanks for all the detailed information: somebody will certainly have an application perfectly matched to those 1/8 inch heaters. Might even be me, now that I know more about ’em!

  7. Ed –

    I think I have comfused you. (A real sin for me since I handle tech support.)

    SImply put, 40 watts (with it’s attendant 135 watts per square inch loading) is well within the capacity of a single 1/8″ by 1″ cartridge heater operating at 450F.

    1. Given that I’m not in the MBI engineering/purchasing chain-of-command, I should bow out at this point. The thermal data I’ve collected so far puts some reasonable bounds on the problem, so what’s left is designing & manufacturing a new core to fit whatever cartridge heater makes the most sense overall.

      I’ll drop a note to the folks at MBI, who get to make the decisions, and they can take it from here.

      Me, I’ll continue fiddling with the 1/4 inch heaters I have on hand, as they suit my simple needs. I want to try somewhat higher power, but only after I wrap a thermal cutout circuit around the whole affair…

      Again, thanks for all the information: I’ve certainly learned a lot!

  8. Ed,
    On the thermal cut out. A simple solution would be a thermal fuse. Its a one shot deal so it would be wise to have a couple on hand. They could be wired in series with the heater cartridges in close proximity. The question is, can one be found at the proper temperature.

    Click to access Thermalcutofffuses.pdf


    1. Yup, except that you really must turn off the filament drive motor at the same time, which requires some circuitry around the fuse or switch to yank the power.

      The discussions for the First Light post were edifying…

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