Coefficient of themal expansion

[Topper]

I've been reading to much and getting more confused.
I have never purchased a custom mold, and want to do everything I can to get it right the first time.
Looking at the materials chart on Mountain Molds web, aluminum has the highest degree of "Coefficient themal expansion" and ductile iron the lowest.
If I'm understanding this correctly, aluminum will expand and contract more at varing temps than iron.
So, I presume that means iron mold will cast truer in size and weight than an aluminum mold?
Am I on the right track, or rationalizing too much :???:
Thanks

[David R]

I think it doesn't matter, all molds will be hot when you are casting good boolits. I am sure Dan will cut the mold so it makes the size you want.

I took physics 25 some years ago in college. I don't remember the exact formula. Delta L/L?

David

[fourarmed]

Delta L over L equals alpha times delta T, where alpha is the coefficient of thermal expansion. For aluminum it is about 25E-6 per Kelvin. For Iron, it varies in the range of 9 to 12E-6. Interestingly, lead is close to aluminum at 29E-6. Yellow brass is about 19E-6.

[David R]

Delta L over L equals alpha times delta T, where alpha is the coefficient of thermal expansion. For aluminum it is about 25E-6 per Kelvin. For Iron, it varies in the range of 9 to 12E-6. Interestingly, lead is close to aluminum at 29E-6. Yellow brass is about 19E-6.

Doesn't make much sense to me, should have paid more attention.

Thanks Fourarmed

[fourarmed]

Sorry. The change in length (or width, or diameter) of the object, divided by the original length (or width, etc.) equals the coefficient times the change in temperature (in Celsius degrees.) The larger the coefficient, the more the expansion for a given temperature change.

A lead bullet cast at 800*F., and cooling to .458" at 70*F. would shrink about .005" or a little more. Dimensions of the mold cavities would change less than that by the ratios of the coefficients.

[Topper]

Thank you forearmed for the insight
I do appreciate it.

[trk]

Sorry. The change in length (or width, or diameter) of the object, divided by the original length (or width, etc.) equals the coefficient times the change in temperature (in Celsius degrees.) The larger the coefficient, the more the expansion for a given temperature change.

A lead bullet cast at 800*F., and cooling to .458" at 70*F. would shrink about .005" or a little more. Dimensions of the mold cavities would change less than that by the ratios of the coefficients.


Thanks for spelling out the details (your previous post). It explains instantly why the bullet drops out of the mould - it shrinks more than the mould - as the bullet is coming down in temp from very hot as the mould is absorbing the heat and coming up in temp a little.