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Hot-end thermistor: cement and construction

Posted by xiando 
Hot-end thermistor: cement and construction
August 15, 2012 08:20PM
Hi all,

Building up my first Prusa. I just received my hot-end, a 3mm filament x 0.35 nozzle, J-head.

So I'm wondering this. IF I choose to cement the thermistor into the body (there's a pre-drilled hole for the sensor that's a bit oversized for the thermistor body), what kind of cement is recommended?

I realize that many may say "just tape it in..." but I can't bring myself to that. Too many years as a professional engineer.

Refractory cement? If so, isn't that also an insulator (which would cause a deviation between the observed and actual temperature of the brass) ? Or do you inject a bit of thermal compound it to the bottom of the hole, insert the thermistor, then spooge cement in behind to secure the sensor and thermal transfer compound?

Re: Hot-end thermistor: cement and construction
August 15, 2012 09:08PM
I use glass rope glue. I have also wrapped the resistor in aluminium foil to make it a snug fit. Both methods work well. The glue has to be dried out slowly after it sets to get rid of any water before taking it about 100C otherwise it tends to blister.

If you just tape it in it will fail as any air gap has too much thermal resistance to keep it cool.

Re: Hot-end thermistor: cement and construction
August 15, 2012 09:17PM
Thanks for the advise, the tin foil trick seems like a pretty good solution for maintaining thermal contact.

I don't have any glass-rope cement, but I do have a bucket of refractory cement. I'll keep in mind the recommendations about allowing it to cure for a few days.
Re: Hot-end thermistor: cement and construction
August 16, 2012 02:31AM
i just use 300c high heat silicon easy to remove then if you ever need to.

i used to use artic cooling thermal paste. (MX-2) problem was when heated up to 200c it goes stupid hard.

had a problem with my j-head and needed to clear it out and was very very very dificult to get both resistor and thermistor out. endedup using a hammer and a drill
Re: Hot-end thermistor: cement and construction
August 16, 2012 11:47AM
I use foil and silicone, but be warned using any cement if there is a significant amount of air trapped in the hole, it expands when it heats and though I've never seen it happen, I know of at least one person who had the silicone plug blown out of the hole by the pressure.
Re: Hot-end thermistor: cement and construction
August 16, 2012 02:42PM
Enlight: I noticed that about the power resistor. It's a very tight fit, so I'll be buying a few extras in case I have to rebuild the head. I doubt they'll hold up to being removed and reinserted, or maybe better said I would not trust in it. Same goes for the thermistors, due to the "gluing", and I've already ordered a few extras of them as replacements.

Polyonhell, thanks for point about air entrapment.
Re: Hot-end thermistor: cement and construction
August 16, 2012 03:12PM
I use muffler sealant, the stuff that comes in a tube. I pack it with cement, smear some around the perimeter of the resistor, put it in, then remove the excess from the opposite end and use it to cap any gaps at the end. I let it set for an hour or so, then hook it up and run it at ~120C for another hour. After that it should be good to go.

I also use the sealant in the thermistor cavity as I wasn't getting consistent readings without any sort of interface material.

The one time I had to replace a resistor, I clipped off the leads and used a spring loaded center punch and the resistor came out cleanly. The remaining sealant can be picked off with a fingernail. Muffler sealant is brittle though it holds the resistor in well enough for its purpose.
Re: Hot-end thermistor: cement and construction
August 16, 2012 08:56PM
I believe products just as stove rope cement/glue and muffler patch and many (most?) common refractory cements are silicate based, often sodium silicate. Another old name for this stuff is "water glass" (if memory serves). I have had the same experience as described in one of the posts...basically, let the stuff dry out before "baking". High temp is required to make it harden to full hardness. When it is wet, heating too fast tends to make the water boil and what was referred to as blistering. I use muffler cement to encase the thermistor in an aluminum tube. Yes, the cement is not as thermally conductive as metal so the thermistor is insulated to some degree and therefore won't read as high due to heat losses out of the connecting wires. I also suspect that these cements have a somewhat limited shelf life and may not harden if they get too old (anyone know if I am correct?). By old, I mean something, say, over a couple of years.

I looked at various silicone compounds that claim to withstand the temperatures involved: 500 to 600 degrees F, depending on the additives. As best I can figure, the basic silicone chemistry is pretty much the same for these compounds...the art (and the temperature durability) is a function of the additives such as iron oxide. Bottom line for me (YMMV) was to go with the silicate/muffler cement...the silicone just didn't have enough margin for my taste. As pointed out, the silicate cement is brittle and it get be chipped out if you need to repair. Another interesting factoid: I found that the wet muffler cement is mildly *electrically* conductive until it is fully set. The resistance kept going higher and higher until it fully hardened, then it was effectively infinite.

In general, I find it very interesting and challenging that the temperatures involved in printing ABS are pretty much the limits of virtually all polymers, save for the exotics...even PEEK doesn't have a great margin. So polymers up to about 550 to 600 F, most metals (no problem), ceramics (no problem) silicates (no problem), glass fiber (no problem).
Re: Hot-end thermistor: cement and construction
August 17, 2012 12:58AM
Yes, all cement begins to absorb moisture, if the container has been opened or is otherwise in contact with the air (unless I suppose, if someone lives in Death Valley or somewhere else with really low humidity). And moisture absorption kills it. I have a bag of mortar-mix in the basement that got left in the basement about a year ago that's worthless due to soaking in the humidity. It's not a completely solid block, but close enough. And it doesn't take very long to compromise it either. a few months and it's junk. a year...well then you have what I have...basic a crumbly boat anchor. I still need to figure out how to get rid of that thing.

Fortunately, my tub of refractory cement has never been opened. It was intended to repair my fireplace but I just never got to it. The "tub" is actually a paint can. Not sure why it came in a paint can, but maybe that's why. Sealed tight and never cracked even once, although I guess I'd better cross my fingers, since it's about 5 years old now. I think it'll be fine...ok I hope it'll be ok. I guess a small test batch might be in order before I commit to using it on the hot-end...

At this point, I intend to bake the hot-end assembly in the oven for ~12 hours or so, on the lowest temp, to bake out the moisture, then let it air cure for another day or two before installation. Still days away though. I'm not going to rush this, and truth is, I have other things I need to get done before I start assembling and playing with my new toy ;-) (i know, it's not a toy, but I think you get my drift). Still waiting on my motors and a handful of connectors anyway.
Re: Hot-end thermistor: cement and construction
August 17, 2012 08:42AM
I use high temperature silicone that is typically used as a gasket in car motors: engine block, carburator or even evacuation pipes, rated like 560 degC, and typically should be available at any gas station or automobile service station supplies e.g. where you buy car oil or auto parts. Typically color red, orange or black.

Sometimes makes a little fume when first heated but that clears out pretty soon. Can be easily cutted with a cutter even after long time use, to dismantle the thermistor or such. Otherwise fairly rugged. I prefer to give it a porous surface finish considering it would rather trap air molecules or slow their convection speed rather than a plain lustruous finish which i believe would do the opposite. I cover the entire heater block in it, and also used this to cover the entire back side of heated bed. Believing that anyway it can not possibly make a better thermal transfer than aluminium direct to air, so it anything it is, it necesarily has to be worse than that - although i have no real data for thermal coef so thats just my impression.
Re: Hot-end thermistor: cement and construction
August 18, 2012 04:57AM
nophead Wrote:
> I use glass rope glue.

Ok, I admit it. I have never heard of this: What is "glass rope glue", and who sells it in the UK?

Re: Hot-end thermistor: cement and construction
August 18, 2012 05:15AM
Re: Hot-end thermistor: cement and construction
August 20, 2012 02:50PM
I have been using Car Exhaust repair putty to do my hot ends since I started selling them.

Low Moisture absorbtion.
Great Thermal properties.
Great electrical insulation.
Low cost and easy to get hold of at any car spares place.

Is My way the bestd ? I have no idea but what I do know is that it works really well.

Muffler sealant seams to have very good thermal conductivity (especially compared to air). A muffler sealant such as Holts Firegum is sodium silicate based

Sodium silicate has a thermal conductivity of around 14 W/mK
Compare this to air which has a thermal conductivity of ~0.025W/mK
High performance thermal greases for computers have thermal conductivity of ~<10W/mK
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