Thing-O-Matic / MK5 Extruder: Protecting the Thermocouple

The stock MK5 Extruder head assembly instructions suggest wrapping the thermocouple with Kapton tape before capturing it under the washer against the Thermal Core. Alas, as I’ve found, that doesn’t work well: the tape isn’t proof against mechanical forces applied to small objects and the thermocouple bead can punch through the tape to contact the Core.

This isn’t a problem until one of the heating resistors blows out and shorts the +12 V supply to the Thermal Core. The only ground path is through the thermocouple, which leads to the MAX6675 thermocouple interface chip, which generally results in a dead Extruder Controller. The third picture in that thread is chilling, isn’t it?

I cast my thermocouple into a brick of JB Industro Weld epoxy for both mechanical and electrical protection. The epoxy is rated for 500 °F (call it 260 °C), which is barely adequate for the job, but JB Weld is cheap & readily available. Note that this isn’t your really cheap garden-variety clear epoxy, which falls apart at much lower temperatures. That discussion suggests a higher-temperature epoxy from Omega, but I haven’t gone that route yet.

Anyhow, I converted three credit-card-thickness sale coupons from Staples into a brick-shaped mold around the thermocouple. The middle card has a slot for the thermocouple wire, which means the bead is positioned in free space in the middle of the opening.

Thermocouple positioned in mold
Thermocouple positioned in mold

A close-up of the thermocouple bead:

Thermocouple positioned in mold - detail
Thermocouple positioned in mold - detail

I taped that assembly to another coupon, filled the mold with JB Weld, made sure everything was saturated, and gave it a day to cure. This view shows the brick after peeling off the top coupon, so you can see the cable slot:

Removing thermocouple from mold
Removing thermocouple from mold

A bit of filing and general cleanup made it presentable:

Finished thermocouple brick
Finished thermocouple brick

A wrap of Kapton around the brick gives the Thermal Core washer something to grab onto:

Thermocouple in place - ceramic insulation jacket
Thermocouple in place - ceramic insulation jacket

The brick could be much smaller without any penalty. There’s no issue with excessive thermal mass here, however, because the Core itself has a 10-minute time constant, so the thermocouple has plenty of time to tag along.

The red wire in the upper-left corner connects the plate above the Thermal Core directly to the static drain ground point that leads to the ATX power supply case. In the event of a resistor failure that shorts the +12 V supply to the Thermal Core, the power supply should shut down. Whether that will actually happen, I cannot tell, but now a failed resistor won’t destroy the thermistor or the Extruder Controller.

The ceramic wool insulation (from a lifetime supply of furnace chamber lining; it’s rated for direct oil burner flame impingement) may seem excessive, but I wanted measurements from a well-insulated Thermal Core at reduced power: 40 W seems to do the trick.

However, the insulation on the bottom of the Core around the Nozzle tended to catch on the ABP’s silicone wiper. The next iteration used just the original MBI ceramic cloth insulation on the bottom, protected by Kapton tape, with ceramic wool around the rest of the Core. Much better!

9 thoughts on “Thing-O-Matic / MK5 Extruder: Protecting the Thermocouple

  1. I realize you didn’t make these boards, but from that photo you linked to, why the fuck does it say “KEEP IT LOCKED / BASSDRIVE.COM”???

    1. Although this may be an unkind and unwarranted cut, I have wondered if some of the extruder’s problems stem from listening to trance techno while designing.

      Sometimes, one must concentrate really, really hard to get all the details right. Even then, I still leave loose ends untied, so it’s not like I’m pure as the driven snow.

  2. Not sure if it would be useful here, but a trick that has been used to mount T/Cs or thermistors is to pot them into the barrel of a ring lug. The lug can easily be screwed to whatever you want to measure, and the copper body of the lug does a great job of transferring heat to the sensor. Something like this photo:

    1. Now that’s tidy!

      After the epoxy tombstone turns to dust, I’ll get some real Omega high-temp adhesive and build one of those…

  3. Like your brick Ed,

    I just Kapton taped my thermocouple then bent it at 90 degrees and stuffed it into a small hole drilled near where MBI had it placed. The wire was then captured beneath the bolt and washer. But, I like yours and Johns methods. If – IF – I ever have to dismantle I’ll use either of your ideas.

    1. That’ll work… and it’s definitely having intimate contact with the block. Talk about an oven!

      If – IF – I ever have to dismantle

      *wrings hands* Bwah-hah-hah…

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