Replacing Molten Lead with Bismuth?

[Darin McGrew]

Our derby workshops have used molten lead since "forever". We know how to take appropriate precautions with it, but given the recent recalls of lead-tainted toys from China, we're looking for an alternative.

Our kits come with an undercut channel routed in the bottom for molten weight. One of the other leaders suggested that perhaps we could replace molten lead with molten bismuth.

Does anyone have experience with molten bismuth? It's supposed to be non-toxic and have a lower melting point than lead. At about $5 per pound, it's more expensive than recycled wheel weights (which are free), but it's still reasonable.

[knavekid]

This is the first I have heard of this material. It is similar in density to lead and it has a somewhat lower melting point. If it is non-toxic or even less toxic compared to lead, I think it would make sense to use as a replacement.

If it behaves while solidifying and doesn't shrink too much, and it can be drilled (to fine tune weight), it should work just fine.

I tried to find some at a reasonable price, and could not. I can buy bismuth sinkers for about $28 per pound, or pure ingot for under $15 per pound. I think a bismuth-tin alloy would be most usable.

After reading this, however, I am concerned about its suitability for this application:

"Unlike lead and steel, bismuth is highly crystalline and extremely nonductile. Bismuth also has a very low melting temperature and expands upon cooling."

This indicates to me that pure bismuth could split open a PWD car upon cooling and being crystalline, could not be drilled for weight adjustment.

It still may be worth picking up a few ounces of bismuth alloy sinkers to see how they behave. These sinkers may be purchased here: Cabela's

[Darin McGrew]

Just from reading about bismuth, I like its property of expanding slightly as it solidifies, because we have had problems with the lead contracting and rattling around loose in the channel. The brittleness of bismuth may be a problem though, if we have to remove some of the weight at the end.

I'll have first-hand information about this in a few weeks, since our derby season is starting tomorrow night.

[chesspupil]

Bismuth is also diamagnetic... repelled by magnetism. The reason you will not find it cheap either. Still, the uneducated may cry foul if they knew this. And if your track has any iron parts esp magnetized, you might notice a slight time difference.

From Wikipedia:

Bismuth:
Density (near r.t.) 9.78  g·cm−3
Liquid density at m.p. 10.05  g·cm−3
Melting point 544.7 K
(271.5 °C, 520.7 °F)

Lead:
Density (near r.t.) 11.34  g·cm−3
Liquid density at m.p. 10.66  g·cm−3
Melting point 600.61 K
(327.46 °C, 621.43 °F)

If densities are a direct relation, and with the expansion of its crystalline structure, you would need 1.5 times bismuth by volume to make up the same weight.

I'm not a physicist so I could be wrong... I do note that Bi is one higher in atomic numbering than Pb so I could be wrong.

[GravityRacer]

While bismuth can indeed be used in diamagnetic levitation, it requires quite a strong field. Tracks are aluminum, wood or plastic, none of which can be magnetized. This is an esoteric property, and it would be pretty tough for someone to rig anything non-obvious to help their car.

For sure bismuth is non-toxic, though- it's the "Bi" in Pepto-Bismol.

edited to add: for all intents and purpose, stick with tungsten.

[Darin McGrew]

I never reported results back to this thread, so here they are:

The scrap bismuth we were given had too much junk in it. Melting it in the hotpot was tedious and smelly, and the guys in charge of the experiment decided to abandon bismuth and stick with lead.

FWIW, tungsten was also considered, but it is too expensive for the weight we provide everyone at the workshops. It's one thing if someone spends $20 for tungsten for their own car, but we couldn't charge an extra $20 per car to cover the cost of tungsten for everyone at the workshops.

[GravityRacer]

Perhaps a better answer is to buy cars that are drilled? Or drill your own? Then you could simply cast the tire weights into a cylinder that could be stuffed into the body with a little glue, and spackled shut.

[Darin McGrew]

Our kits come with an undercut slot routed into the bottom. It's great for pouring molten weight into the car.

[GravityRacer]

I understand the kit. Why not use a different one? Seems like you're locked into that molten lead deal.

[Darin McGrew]

Molten metal (lead, bismuth, or whatever alloy we end up with from recycled tire weights) is very convenient for our workshops. When you're helping 60-70 kids (plus another 20-30 adults) get their cars up to the 5oz limit, the convenience matters.

Pre-drilling holes for lead slugs is a trade-off between locations that allow flexibility in the design but require lots of slugs, and locations that require fewer slugs but limit flexibility in the design. A lead channel in the bottom allows a lot of flexibility in the design, and allows you to bring the car up to weight all at once. (Actually, we usually do a couple pours, but they're one right after the other. We're done before the first pour really cools.)

We've thought about changing though. The most likely idea is to drill 3/4" holes in the bottom that would accept pennies. Pennies aren't as dense as some other options, but right now, they're about the cheapest source of weight around.

[GravityRacer]

Pennies are mostly copper-plated zinc these days, and zinc ain't all that dense. Though the size is convenient. You do a series of pours for your Cubs? 0.o I have delusions of burnt Cubs! I'm sure you're doing it safely, but the idea seems strange to me. I've seen leaders waving hot glue guns around on Derby Day, too.

[Darin McGrew]

Pennies are mostly copper-plated zinc these days, and zinc ain't all that dense.

I didn't say pennies were the most dense weight. I said they were cheap. (They're also readily available, 100% non-toxic, etc.) You can get most cars to 5oz for $0.25-0.40. Recycled tire weights are free, but if you need to buy lead, it will cost more than the pennies.

Though the size is convenient.

Yep. That too.

You do a series of pours for your Cubs?

Well, no Cubs, but kids in our church's Pioneer Clubs and CSB Stockade programs. The molten weight is on the other side of a fence. They hand over their shaped car bodies (and any decorations they're going to add after painting the bodies), and an adult adds weight to get the total close to 142g.

By "a couple pours" I meant that the technique we've developed is to weigh the body and other bits, add weight to get the total weight to roughly 100-120g, weigh everything again, add a little more weight, and repeat until the total is about 135g. All this takes less than a minute. With just a little practice, you get pretty good at adding the right amount of weight, leaving a little room for paint, clear-coat, adhesives, and anything else. (We strongly prefer to add a little trim weight at the end, rather than drilling out excess lead.)

After the weight cools, we return the car body to its owner.

[GravityRacer]

OK, I see. It makes more sense to use pennies, I guess.