Pouring Lead Mold

I take it, its sand castings.

In college, we always had enough risers to make sure there aren't any possibility of air pockets trapped when you pour.

See earlier discussion.

So, nothing I say is because I've ever cast lead in anything larger than 158 grain lead bullets.

But ignorance has never stopped me from voicing my opinion before.

Any molten material will shrink as it cools.

Improperly VENTED tools will trap air as they fill so some of your voids may be due to no venting.

Lead is introduced into the mold and fills it by gravity, top to bottom, I assume.

So, proper venting of any features which might trap air is essential.

Materials harden and shrink as they cool, so the coolest part (usually the part in contact with the tool will solidify first. As the material continues to shrink, it will pull the solid parts in (with respect to the tool surfaces) and create sinks in the outer surfaces. Are these the "voids" your operator sees?

Voids in the interior will be caused by the vacuum created when the walls can no longer move inward to accommodate the shrinking material. (This happens with injection molded plastic also even though lots of pressure is being applied)

Without knowing your part geometry, tool material and heating zones, I've exhausted my lack of knowledge.

You can find the book in hardcover but it is pretty pricey.

https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=Engineering+Properties+and+applications+of+lead+alloys

You can find out if there are any voids in the finished parts by immersing in a full container of water and catching the spillage. Weigh the spilled water, weigh the part, divide the part's weight by the water's weight and compare with the density of lead.

(In addition to heating the mold, it might help if there were some way to vibrate it to encourage the molten lead to fill all voids.)

Not a complete drawing, missing the cap that holds the steel core and the base that bolts on the bottom and makes the pointy end of part.

Drew K

Good luck with that.

OK, I won't ask WHY.

Can you put a bunch of ∼3/4 dia core pins in the lid to reduce the volume of lead? Say whatever would give you a consistent lead thickness between each pin and each pin and the inside and outside of the part?

The mold is for radiation shielding so reducing the lead content wouldn't help us.

Drew K

When pouring steel, slag and bubbles rise to the top so for ingots they pour down ceramic tubes that then flow under the ingot mould and the steel slowly fills from the bottom up without splashing or air entrapment. The top of the mould has an narrower neck where the slag, bubbles and other contaminants collects. When cooled the neck is cut off leaving a void free ingot.

I have used this method when pouring some Thermosets, perhaps it may help you.

Also do you add any flux to lower surface tension, though that will not help when pouring into a cold mould.

Last time I poured lead was to make toy soldiers some 60 years ago so I am no expert but pouring hot into cold gives disastrous results, fill slowly.

You say you have an oven. Can you not heat the mould in there for some time? I used to work at a foundry that cast propellers and they always made the mould with an extra 20-30cm on the boss. This provided a reservoir of hot metal to sag down as the casting cooled. This was cut off when cooled. All casting that i saw had this sprue. I made my wife's wedding ring and preheated the mould to 1100deg C. Mostly to burn out the wax and the resultant carbon, it worked well. You are using a low temperature melt so it should be easy to preheat the mould, even in a domestic oven. I would prefer an oven to a torch as it is more even. I guess that you are trying for a full mould with no, or very little, post work. You will most likely find that the top will have dross inclusions. Another reason foundrys use a sprue and post work the casting. (fettling)

Never, never, ever heat lead in a GD domestic oven, mate! It is bad for the budgies.

Kiddos too. and Hippos.

No worries, our oven is much bigger than a domestic...you could fit a small hippo in it!

Drew K

that makes me feel a lot better about the situation.

I heard the other day that silicon drift detectors can also detect Beta particles, if all the plastic surrounding the chip is removed so the particles can impact directly on the silicon. In your opinion is there anything to this possibility? Been also hearing about new silicon photomultipliers, too.

I'll bet the Hippo wouldn't like that.

I really didn't think that a domestic oven in an industrial setting would be used for food. But your point is valid. I use an old domestic oven to heat plastics for forming and for drying and... Clearly i was thinking from within my own experience. Anyhow, a person taking advice from a forum should check the definition of 'caveat emptor'. Jim

I'll throw in my two cents. To reiterate what others have said; "pouring hot into cold is a bad idea [sic]". Heaters aren't that badly priced, depending on production levels needed. A core heater and an exterior band heater would help greatly. Don't heat it up to melt lead, but just keep it hot until the pour is over.

Again, venting is not only useful, it is needed if you have any hard to fill spots. It does look and sound like you just have cold spot issues though. A handheld torch can not heat a mold evenly enough to get consistency. Band heaters will greatly enhance being able to heat evenly. Add a PLC, and you'd be golden. Lead mold - golden...get it? HA! Alchemy... Ok, moving on.

Once you get repeatable results from shrinkage, then you know how to modify the cavity. A heated mold and consistent 'pouring' will help greatly with repeatably.

If you're making a true production mold and, with the right budget, you can get fancy, add heating and cooling lines to help with pouring and releasing.

Also, to repeat others, an ideal situation is to have equal cavity thickness whenever possible. Unfortunately, this doesn't happen often.

We are considering induction heating for the mold. We did consider using heater jackets but were concerned they wouldn't put up with being removed and replaced every time the mold is used.

What method would you use to cool it from the bottom?

Drew K

If allowable, adding some bismuth to the lead will help casting via decreased shrinkage, lower solidification temp and (very important) reduced surface tension.

Increasing the pour temp can help....of course this is lead, so there are other considerations.

Lead and lead alloys can be fairly easy to machine. It might be much easier to cast a larger blank and then machine to final shape.

We are doing some machining, and the new mold design has tapered sides to ease releasing from the mold. I will look into the bismuth trick, i know we use an alloy, but didn't think to get the mix to give in my initial details...plus i should make sure it isn't proprietary.

Drew K