metals and heat/ cold expansion?

Question

Here's the deal, a few years ago I decided to do some custom shifter knobs. The goal was to make them durable enough to last a life time structuraly so I put a steel core on the inside as the main way to mount it.

I've had one in my truck for four years now and just noticed last week it has a big crack in the back. It's not loose on the core or anything but it troubles me and I don't want to have this problem. My thought is the steel core is expanding and contracting too much with the weather changes and produced the crack. (there's only one other I did for now to test up north, I'm in the south.) What kind of metal can I use as a core that expands minimally during extreme weather changes to help stop this?

The last thing I want is to start doing these things and have them fall apart in someone's hand. (That's why I've been running a five year test.)

I did think that maybe it was what I used on the outside but I checked on it and it doesn't shrink once baked.

Thanks for any help!

I'm using sculpy on the outside. When it's built up thick enough you have to hit it with the claw end of a hammer to get it to break. (And then only in small chuncks.) It doesn't flex.

Yes it is the sculpy that cracked. I live in the south and during the summer the inside if the truck can easily hit 130 degrees so what was said makes sense.

All four answers do have their valid points and the common thing that comes up is the sculpy. I know if it got too thin it probably would crack so I went for a minimum 1/4 in thick. But I think that the shrinkage thing is what's going on the and continuous baking in the summer (It makes sense). I never was going to mass produce them (If I did I would have used something else.)but as custom one offs which is why I tried this route first. Make a core, add skuply and skulpt it, paint it and done. If some one wanted something done cool if not also cool. And I have e mailed the company to pump them for info with no answer so that's why I came to you guys.

I can't tell you who gave the best answer simply because they were all a big help so it's now up to a vote.

Thanks much guys!

(White pants in a shop!LOL! Dumbass!)

Answer 1

To clairify, it is the sculpty that has broken, correct? I wonder if temps in your truck are sometimes high enough to "bake" it. They might not be often, but if they were close for a long time, I suspect that would cause the further shrinkage. Sculpty is a form of PVC and the "plasticizers that are in it are quite volitile so a little bit of heat over a long time might do it. Cracking due to differing expansion rates is possible, but I bet it is baking in the summer.

I wonder if you could use a urathane clear coat to limit the escape of the plastizer? Probly wouldn't solve the long term issue that is important to you though. Epoxy is a better material, but most epoxies have poor uv resistance, so the colors would fade. Casting them in Aluminum might be the way to go and then having them anodized might be very cool. Doing two color anodizing is not cheap, but aparently if you do the easter egg method (light color first, then wax areas that you want to remain light, then next darker color ect....and ect..) it's not heartbraking. Some small foundry guys are around on the internet you might get one done by one of them? I checked your profile, I hear ya about the pansies and dirty hands, I have been an artist blacksmth metal fab guy for 10 years and you should see the "artists" who have fled my shop with stains on their white pants.

Answer 2

I don't think your problem is expansion and contraction, if that were so then the knob wouldn't remain tight with the internal steel core. Expansion and contraction creates problems dealing with looseness. If it were the inner core then the expansion would push the outer material away, the outer material would not recede at the same rate or return to the same fit as the inner core. The problem maybe malleability. I am not familiar with the material you are using on the outer core, but it sounds like it maybe a chemical epoxy material such as "instant" steel or aluminum. The type that come in stick form were you kneed the parts "A" and "B" together which starts the chemical process. I have used this stuff extensively. I have only had cracking problems were the material is thinnest. It is designed for use more as a filler rather than a base material. Your only answer may be to figure out a way to cast the outer part in aluminum or possibly even a hard plastic. You can use the pieces you currently have to make the casting form. Just an idea...good luck

Answer 3

There is a lot of information out there about Sculpy including how it behaves when it is thick and how it behaves under various conditions of baking.
I would suspect that the problem is shrinkage in the Sculpy over time. The tension builds up until the material can't take it any more and a crack forms.

Answer 4

I'm no ME but my feelings are that I'd probably make the core out of steel also, since the knob screws in fairly tight and you definitely don't want it to start moving around. Anything with plastic threads seems cheap and less durable. Especially in the hot oven environment of a car, that can't be good for long term life.

Checking on a list of thermal expansion it seems like steel actually has a relatively low coef of thermal expansion on the list of commonly-available metals.

What you haven't mentioned, though, is what the material on the outside is made of. I think that the outer body would have to be made of something flexible (like leather), or precisely matched, because otherwise thermal expansion would cause the inner core and outer layers to separate or impinge on each other when heated.

Even though the material on the outside doesn't shrink when baked, if its coef of thermal expansion is lower than that of the inner steel, the inner steel would expand more than the outer layer, and that would cause pressure on the steel. It's odd that the steel would crack and not your outer layer (since I assume it's not as hard as steel).