Arctic MX-4 in the hot end?

[oeginc]

So, long story short - I am rebuilding my hot-ends. While re-assembling them, I realized that the thermistor basically sits in an air pocket and I'm sure doesn't have good thermal connectivity.

While reading another thread on here, someone had mentioned putting Arctic silver 5 in the hot-end, but it was the general consensus that the maximum temperature for Arctic silver 5 was 150C - which obviously isn't high enough for the hot-end.

Does anyone know what the max temperature of Arctic MX-4 is? I have some laying around, and it is technically supposed to be better than Arctic silver 5 - but I could not for the life of me find a data sheet that listed the max operating temperature.

P.S. If it's not > 250C - does anyone have any other recommendations for a thermal compound that could fill the air gap? I still plan to use RTV over the hole to hold it in place, just wanted to increase my temperature accuracy a bit.

Thanks!

[lordbinky]

I thought along similar lines, the conclusion I came to was it was more trouble than I found it to be immediately worth since you need a thermal interface material that is something akin to Feser H-Bridge Thermal Compound (max: 240C ) or compounds in the carbon/Diamond category of TIM such as the Antec Formula 6 Nano Diamond Thermal Compound (max: 250C)

[Eric]

Also, you have to worry about electrical conductivity for fear of shorting out the thermistor leads. That's generally not an issue when you use thermal compounds on heat sinks.

[lordbinky]

Most thermal compounds are generally good electrical insulators even the silver compounds.* Foreign particles as always may change this attribute though. The compounds DO add capacitance to connections so that is the way they will interfere with proper operation of components like processors. For the application of our mere heating resistors the capacitance is negligible.

*Which is why I also put thought into this wanting to ditch the air filled conductive aluminum foil wrapping. Aluminum being stable over any of our working temperatures made finding a quick and simple substitute method less appealing.

[JohnStack]

As an alternative, you might try snipping off a bit of the PFTE insulation and jamming it into the hole after you've gotten the RTV in. It holds the thermistors in more tightly as the RTV sets. I cut off just enough to have the end of the tube open - instead of creating an air pocket...

I just did that yesterday and it seemed to work. I'm hopeful that it also pushed more RTV down into where it counts.

As a side note, many of the newer hot ends are coming out with tiny screw-in thermistors.

[lordbinky]

The idea is that where aluminum comes in direct contact thermal conductance is 220 W/m°C if they were two perfectly molecularly flat surfaces, but if there's any air between and it's not perfectly flat that air filled area has a 0.024W/m°C barrier in it. If it was the H-Bridge TIM instead that area would be 9.24W/m°C, so you'd have a 385x better transfer of heat where there wasn't direct metal on metal contact. Really though this is just affecting the rate at which your resistors heat up the hot end so it's only a nitpick unless you're cooling your hot end faster than the resistors can heat it up.

[Gr8Scott]

I bought a small syringe of Red Ice HTC. It's good to go up to 360c and is pretty much ideal for the thermal junction of the sensor on the hot end. I also use it in my heated bed sensor and to surround my heater cartridge and the threads on my brass nozzle. I want maximum heat transfer rates for all these spots.