Mendel hot-end: heatshrink on the thermistor?

Dear community,
I am assembling my first Reprappro Mendel from a kit. I understand the hot-end is one of the rapidly evolving parts of the system and Reprappro have included the latest version of their hot-end.
The instructions describe sleeving the whole 100k thermistor in PTFE heatshrink before sliding it into the generous hole in the hot-end aluminium block. This creates a snug fit but surely it insulates the thermistor and thus creates a time lag in the reaction of the thermistor? Or is inhibiting the sensitivity deliberate? Also, the heater resistor is a snug fit but does it need heatsink grease to improve thermal contact and thus faster heating of the block? Or is there a good reason why heatsink grease is not metioned in the instructions?
Regards, Extrudeboy

It will work without a thermal compond (like CPU heatsink) but it will work BETTER WITH heatsink, better meaning it will heat faster. I will go out on a limp and say that nobody has conclusivly tested if the termistor is better or worse without heatsink, so you make the choice. 5$ worth of heatsink or 2 flat light beers?

Vote Obama!

Thanks Asger,
I would vote Obama if I could but I live in the UK.
Your heatsink advise is noted but do you have a view about the heatshrink?

My reprappro Mendel is as you describe - it works fine :-)

Extrudeboy Wrote:
-------------------------------------------------------
> Thanks Asger,
> I would vote Obama if I could but I live in the
> UK.
> Your heatsink advise is noted but do you have a
> view about the heatshrink?


I can't vote either, I live in Denmark

yes, use it if you have it. The hotend will heat up faster (nobody really knows how much faster) it's like magnets, nobody knows exactly how they work. But we know they work

Edited 1 time(s). Last edit at 11/06/2012 08:40AM by Asger.

I have the same printer. I followed the instructions exactly and it works very well. I didn't add anything.

I think the critical aspect is that they cannot move and touch one of the legs to the metal block. I've heard of that causing melting of the metal block which is a bit scary.

Andy

The main function of heat sink compound is to fill in tiny imperfections in the surfaces that are otherwise in close contact. At the microscopic level, even an apparently flat surface consists of a lot of peaks and valleys, and so two such surfaces placed in direct contact are actually only truly touching in those places where the peak on one side meets a peak on the other. Heat sink compound fills in the valleys and provides a path for heat conduction which is better than air, but this benefit is only significant if the thickness of the heat sink compound is very small. This is because the compound is really not that much of a conductor, even compared to air.

If you have a cylindrical resistor that fits closely into a cylindrical hole, you would likely see an appreciable benefit from using heat sink compound. If, however, the fit is loose, the gap would probably be too wide to allow any significant conduction through the compound.

Another thing to consider is geometry. Many of the resistors I've seen are not just cylinders, but are actually cylinders with a larger diameter band around each end. Sort of like a barbell (see attached image). In a case like that, heat sink compound may actually reduce the heat transfer, since most of the transfer across the large middle gap would be by means of heat radiation rather than heat conduction. Heat sink compound is usually opaque, and so it would serve to block the radiation of heat yet add little if anything to the already quite low heat conduction. Convection, of course, would also be eliminated, but in a closed space you would see only minimal air movement anyway.

Regardless of the situation, actual data would be the best guide, so given the forgoing I would recommend that you take before and after heat transfer measurements if you choose to go the heat sink compound route.

My advice on the heatsHrink is just to cover the wires to the thermistor, I can't see any value in covering the thermistor body with heatshrink. On my hot-end I've used glass rope glue to glue the thermistor in (same on the resistor but wrapped that in one layer of aluminium foil first to make a tighter fit. The thermistor leads are covered with silicon tubing upto the solder joint, then normal heatshrink over the joint, same with the resistor joints. So far this doesn't seem to have caused any problems with the hotend and its not degraded yet (doesn't mean I can actually print anything decent ofc - other problems !)

if anyone is interested in the air gap theory talked about by DGC Partworks, it often is called contact resistance in heat transfer by conduction. just to make your searches easier and hopefully give you better results.



Morrist

The PTFE heatshrink definitely needs to cover the entire thermistor in a continuous tube, leaving about 8mm bare at the ends for the connections. Otherwise it won't fit properly.

The thermal effects of the PTFE are negligible - the entire thermistor + PTFE is enclosed in aluminium at the temperature to be measured, so in the steady state the thermistor will be at the same temperature as the aluminium. And the hot end has sufficient thermal mass that it can't change temperature quickly enough for the PTFE to cause any significant lag in measurements on transients.

Incidentally, the RepRapPro hot end is not particularly fast-evolving. It has been the same for a year. We use it in multiple machines for production 24/7, and it is one of our most reliable components. However, we are always thinking about improvements, so we may change the way the heater resistor interfaces to the aluminium block in the future. The thermistor arrangement will almost certainly stay the same though.

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best wishes

Adrian

[reprap.org]
[reprapltd.com]

HEAT SHRINK (just making sure we stay on the correct topic ) over the glass bead helps to protect the bead itself. Not just any heat shrink will do. It should be high temperature PTFE heat shrink. The thermistor lead wires should also be insulated to prevent them from shorting on the heater block, and to each other.

Dr Adrian Bowyer,
I am very grateful for a clear answer from the creator of the project. I shall continue with confidence to build your baby. I hope you may pass by one day and spot the Mendel in our Batheaston office window.
Regards
Extrudeboy