Car exhaust putty for hotend resistor - hardening prematurely (in tub)

I have been using car exhaust putty to secure the power resistors in the hot-ends I have built - it's similar to this stuff from Halfords: [www.halfords.com] .

Over the 18 months or so that I have had it, however, the putty has hardened in its plastic tub. So, I have tried to 'revive' it with water - and it seems to be working to a certain extent (i.e., it seems to be a soft putty again).

I assembled a hot-end using this last night (after mixing it with water, etc.), and am leaving plenty of time to cure. Am just wondering though ...

Even if this looks like it was originally, will the integrity of the bond be weakened?

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Semi-commercial engineering | Reprap diary

How does the car exhaust putty hold up other than this problem you are having?

Does it conduct heat?

There is a product available in the US called Lab Metal from www.eastwood.com which is used primarily for a finish filler for parts that are to be powder coated. It comes an a number of sizes. Your material, if it is more than say a year old, as long as it is thoroughly mixed, and as long as it dries properly, it should be ok, unless it is solvent based, in which case adding water will affect it. Since the website advertizes no delivery on the product, I would wonder if this were not the case.

@ShadowRam: this car exhaust putty has worked without problem so far. In fact is one of the methods recommended by both Adrian Bowyer (for his Universal Mini Extruder) and reifsnyderb (the designer of the J-head hotend) for securing the resistor in the hot-end.

Other people have mentioned some sort of high-temperature silicone sealant, on the one hand, or just an interference fit if the resistor is tight enough in the mounting hole.

P.S. reifsnyderb refers to the stuff as 'muffler repair putty' rather than 'car exhaust putty' (I presume he's referring to the same thing).

Edited 1 time(s). Last edit at 05/06/2013 02:02PM by dslc.

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Semi-commercial engineering | Reprap diary

Don't use silicone or an interference fit. Both will cause the resistor to fail due to not having good thermal conductivity.

Exhaust putty works best but you need to put a heat cycle through it before it becomes strong. I glue it, wait for it to set, trim any excess with a knife while it is soft and then heat it to 220C for a few minutes. Before that it is very fragile.

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[www.hydraraptor.blogspot.com]

Good to know,

I put a bit of thermal compound in the center of the resistor and the end of the thermistor to help with thermal conductivity.

But then I used Muffler Putty on the outside to adhere the two into place.

Hopefully it works.

I've only brought the hotend so far up to 50degC with a 9V battery, and the thermistor appears to react immediately

I was measuring the resistance of the thermistor with a multimeter. My only worry is the resistance seemed to drop really low for 50degC compared to every 100k table I've seen.

@Maxx: thanks for your input.

@nophead: I had overlooked the necessity of heat-cycling. Thank you!

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Semi-commercial engineering | Reprap diary

Most exhaust putties that I am aware of are based on sodium silicate. Also, many of the wood stove gasket cements and other high temp "adhesives". It is *not* silicone. It takes high temps but is very brittle.

[en.wikipedia.org]

In my experience, it does have limited shelf life and once dryed, it really doesn't seem to come back if you try to reconstitute it with water. It does need to be "baked" for full, let's call it "curing".

Be aware that it is best to dry in before the bake cycle, otherwise the water in the paste will boil and create bubbles and voids. It believe it is reasonably thermally conductive, not an thermal insulator, not a metal. I use it to pot the thermistor in an aluminum tube for my hot end and it seems to work well.

I am getting problem in the car putty when applied on the metal body and after finishing the surface, base coat of any colour applied it appears to be ok but as soon as lacquer sprayed after few minutes there is a clear spot of putty visible on the surface. This problem occurs only on basecoat after lacquring.

kindly tell me why this spot occurs on the surface.
Pls discuss with me on my mail
faiz@matrix.pk

@mars: I appreciate your advice. I think I will at least try this 'reconstituted' putty and see how it goes. If it doesn't work a new tub isn't prohibitively expensive anyway (it would just seem a shame to waste the old tub).

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Semi-commercial engineering | Reprap diary