Rich Alloy cast ingots distorted

[RogerDat]

I have some fairly rich alloy 7.5 Sn 17.3 Sb and when I cast them into ingots there is of course a tendency for the surface to cool with a depression from contraction I presume. What I'm seeing now is actual gaps forming on the sides from I believe contraction.

With really hot mold and melt it seems like I can reduce the voids seen in the sides from actual voids to stretch marks.

Wondering if anyone else has had this experience, and if they had a solution to prevent it.

This also made me start thinking about contraction from a bullet mold, wonder how much additional size contraction a richer bullet alloy gets. Could it cause undersized bullets?

[David2011]

There are many discussions here that cover using various alloys to manipulate the as-cast size of boolits. Some will cast bigger; others smaller depending (iirc) antimony content. With that much antimony I would expect the shrinkage you describe. Even pure lead will shrink some in the middle of the ingots.

[Mitch]

There is a bullet shinkage chart here.it kinda give you an idea of what is happening.
http://www.lasc.us/CastBulletNotes.htm

Another thing think about the sprue when you are casting the alloy and the size of the bullet makes a differance in how much lead sucks into the mold.I dont blieve there is any cure just that way things work.the bigger the piece of lead the more it will shrink.

[Conditor22]

The hotter you cast the lead the more it will shrink.

[RogerDat]

Since I am dealing with an alloy near the monotype listed The .66 percent linear contraction seems likely. I could try lower temperature melt and see if that helps. I still have another batch left that is 6/16 alloy. I expect it will have a tendency to do the same thing.

Since this is essentially intended to sweeten rather than cast with having it in ingots just makes it easier to weigh a more exact amount than having it in 10# slabs. Not like it is a diaster if the ingots are a bit ugly. I would rather they were pretty but it isn't a requirement.

Using Lyman #2 it isn't uncommon to see the sprue puddle get a dimple from sucking in lead as casting contracts.

Thanks I think that explains why and gives me cooler melt to try.

[RogerDat]

Well cooler melt didn't help, I also tried filling the ingot mold a little less. I was thinking contraction of smaller volume would be less. Didn't seem to make a difference.

I think essentially the exposed surface hardens and can't follow the contracting lead down. The mold sides are tapered so as soon as the top is hard at all it won't be able to drop lower in the mold. lead would be too wide I had the ingot mold pretty hot from a hot plate. You couldn't hold onto the mold with welding gloves on.

The voids all appear near the bottom of the ingot / top of mold and look like the exposed surface formed a skin and the void formed just under that. If I get around to this again I will try a hotter melt that may not start to set up as quickly. Meanwhile job is done for now.