Mold material vs. thermal expansion?

[badgeredd]

Just got to thinking about cutting a mold and it occurred to me, that mold material (aluminum, brass, iron) has different thermal expansion rates. I am wondering if the mold material would have a significant affect on cavity size at molding temperature. Thinking that if a cherry was cut for an aluminum mold, would a iron mold throw smaller boolits if cut with the same cherry?

In my itty bitty mind, I would think that the boolits from an iron mold would be smaller but I can't convince myself that the difference would be THAT significant. What do you guys think (or know from actual experience)?

Edd

[runfiverun]

edd contact ba bore he [i bet] could tell you right off.

[HORNET]

The perfessor was playing with a while back.
http://castboolits.gunloads.com/showthread.php?t=60129
I think the boolits from the iron mold MIGHT be slightly smaller, but probably not by much. I'd expect to see more difference in larger calibers, but differences in casting parameters could have more effect.

[machinisttx]

If I could find a better explanation of the formulas, I could tell you an exact amount. The coefficient of linear thermal expansion for lead is 29, while aluminum is 23 and iron is 11.1.

[theperfessor]

In inch-based units, here is the coefficient of linear expansion for four pertinent materials:

Aluminum - 0.0000124 in/in-deg F
Malleable Iron - 0.0000066 in/in-deg F
Brass - 0.000010 in/in-deg F
Lead - 0.0000163 in/in-deg F

The equation is as follows:

Change in length = expansion coefficient * original length * change in temperature

As an example, lets assume a mold has a 0.451" diameter cavity when measured at 75 deg F. What is the difference in size when the mold reaches a casting temperature of 350 deg F?

For an aluminum mold:
change in size = (0.0000124 in/in-deg F)*(0.451 in) * (275 deg F)
change in size = 0.00154 in

For a malleable iron mold:
change in size = (0.0000066 in/in-deg F) * (0.451 in) * (275 deg F)
change in size = 0.00082 in

For a brass mold:
change in size = (0.000010 in/in-deg F) * (0.451 in) * (275 deg F)
change in size = 0.0012 in

Note that since the change in temperature is positive (gets larger), then so does the cavity size.

What is equally important is the shrinkage of the bullet from its solidification temperature. Lets assume your mold cavity is 0.455 in at casting temperature and the lead fills this cavity completely. It freezes at 621 deg F and cools to 75 deg F. What is the change in size?

change in size = (0.0000163 in/in-deg F) * (0.455 in) * (-546 deg F)
change in size = -0.004 in

To get a 0.451 in bullet the mold cavity needs to be 0.455 in at casting temperature. You can work backwards from this size to determine the cavity size at room temperature for a mold of a particular material.

[Willbird]

For the bullet change in size did you use pure lead ??

Different alloys will have a different shrink factor.

many of the GB molds I have done typically only measure about .001 bigger than the actual as cast bullet dia in WW metal.

Also when you consider a metal object, with a hole in it....and you heat it up....does it depend on the shape of the object whether or not the hole will actually get bigger ?? many objects we heat to make holes expand are actually tubular in shape.....but what if we had a cube that was 36" x 36" x 36" that had a .500 hole bored in the center of one face of the cube....that hole would not expand the same way that a .500 hole in a piece of 1" round stock would.....would it ??

The tubing is a "hoop" the cube is not.....thinking about it the hole dia in the cube may be unchanged or actually shrink in size with the application of heat ?

Bill

[theperfessor]

As long as the mold is uniformly heated, the hole will expand no matter the shape of the mold or other object is. Non-uniform heating or warpage caused by thermally relieving internal stresses can change this. My gut feeling is that during use a mold is close enough to being uniformly heated to make deviations from this a matter of warpage. My tests bear this out.

Don't know what the expansion coefficient of WWs is.

I do know that aluminumexpands at approximately twice the rate of steel/malleable iron.

[Willbird]

Well I do know from first hand experience that hoops (bushings) grow quite a bit when heated, more I think than large blocks with holes in them, due IMHO to the fact that the linear expansion takes place around the circumference of the hoop, thus forcing the dia to grow more than a hole in a larger metal mass would.

Here is a post on Mountain Molds forum, from a guy who makes a LOT of molds...he claims more or less that what we are talking about cannot be accurately predicted. IE what size hole will cast what sized bullet.

http://www.mountainmolds.com/phpBB3/viewtopic.php?f=11&t=203

And I do agree with him that fill-out is a significant factor that is often overlooked. Pure lead and Lino as too examples....pure lead does not fill out nearly as well as lino (understatement) so that factor may overshadow any differences in thermal expansion.

Bill

[303Guy]

... the linear expansion takes place around the circumference of the hoop, thus forcing the dia to grow more than a hole in a larger metal mass would ...The circumference is diameter times Pi so the linear expansion of the circumference will be 3.1416 times the diameter. So the circumference will expand more than the diameter but always in the same proportion. But a thin loop or ring will get hotter much quicker than a solid block.

To me the biggest consideration in material choice is the thermal conductivity and thermal capacity of the material as well as the modulus of elasticity - which means nothing in real terms. (Remember that the surface of the cavity will expand rapidly as it absorbs heat from the alloy and the 'stiffer' the material, the more that expansion will be contained).