How do Boolits harden in the mold

[klutz347]

I was out in the garage casting some boolits this afternoon and while I was sitting there I began to think (a bad thing sometimes).

I started wondering how the melt solidified in the mold. Nose to base, outside to inside, inside to outside?

After the melt gets solid on the sprue plate I cut it off and there inside the cavity is a solidified boolit so I had no idea.

Anyone? Bueller?

[madcaster]

I always think that since the sprue at top is molten iF you open too soon,bottom to top cooling.Outside to inside probably,but I won't stick my finger in there to verify it!:kidding:

[ETG]

Next time your casting pour the lead in and before it hardenes turn the mold upside down and pour the melted lead out. You should get some hard evidence to answer your question.

[SP101GUY]

Done it. I was playing around with the same thought. I poured the lead in and tipped it over before it cooled. I didn't take any pics, as I was just testing the theory. But from the looks it cooled bottom to top and outside in, as the top poured off leaving a sort of hollow base effect with what was left in the bottom. Like a hollow point from the base forward. Does that make sense to all?

AJ

[montana_charlie]

Does that make sense to all?
The first lead to solidify will be the first drop that falls out of the spout.
The outside will cool faster than the inside because it is in contact with the cooler-than-melting-temperature surface of the mould.

What you said makes perfect sense to me because it's a logical progression.
CM

[redneckdan]

Ya'll are on the right track. I took several classes about this stuff. If you want a slower picture of what happens, watch how water freezes into ice cubes in the freezer, pretty much the same process except that water expands instead of contracting during solidification. Solidification starts at the bottom outside and moves inside and up. Some casting processes actually make a full pour and then dump out the liquid alloy to leave a sort of "shell".

[rhead]

Hardening from the outside to the inside the volume of the lead would decrease and leave a less dense area in the center. this would cause the still molten lead in the sprue to flow downward into the mold. I have observed a small dimple in the sprue at times. If this flow was less than 100% efficient then the boolit would have a small void or at least less dense area near the center. No big problem if it is well centered. Consider a double cavity mold. One side of the boolit will be cooling slightly slower than the other. The less dense area would be off center. Having a center of gravity that is not located at the geographic center cannot be good for accuracy. It may not cause an easily detectable difference.

Has anyone ever checked boolits from a single cavity mold with identical boolits from a double cavity mold.

[Shiloh]

It hardens from bottom of the mold to the top.

As it cools, it draws metal from the sprue. You want this.
There has to be a source for the cooling, shrinking metal to draw from. that is why there is a dimple on some of the more skimpy sprues.

If there is no metal to draw from, the bases of the boolit would have porosity and be poorly formed.

Shiloh

[Leftoverdj]

Once in a long while, I've dropped bullets that were still molten inside an outer shell. You can only see it if the shell is so thn and soft that it cracks open when it hits.

[AZ-Stew]

The molten alloy cools from the outside in, with respect to it's contact with the mould surfaces. Think of it as the heat energy running away from the melted metal of the boolit into the cooler metal of the mould. Pure physics. Energy wants to go to a place where there is less of it.

And yes, the sprue metal is sucked down into the mould as the boolit cools, thereby filling what would otherwise be a base void caused by the cooling metal shrinking in the middle of the casting.

Regards,

Stew

[ETG]

The molten alloy cools from the outside in, with respect to it's contact with the mould surfaces. Think of it as the heat energy running away from the melted metal of the boolit into the cooler metal of the mould. Pure physics. Energy wants to go to a place where there is less of it.

And yes, the sprue metal is sucked down into the mould as the boolit cools, thereby filling what would otherwise be a base void caused by the cooling metal shrinking in the middle of the casting.

Regards,

Stew

Bingo! It appears the bottom solidifies first, but, that is because it is narrower at the nose (and more aluminum). The mold acts as a heat sink. If the mold has a hot spot that area will solidify slower. Aluminum transfers heat faster than steel and that is why you can cast faster in aluminum molds. The cooling is from the outside in.