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Pouring Lead Mold

01/04/2017 2:03 PM

We are working on our molded lead part of a device we sell. I am designing the new mold and incorporating some improvements. I watched them pour a part last week and observed that they have to melt in a significant amount of extra lead as it cools. The operator stated that the bubbles (voids?) must be filled in and sometimes he has to add more lead than others. If we made enough of these i would have him track how much lead he has to add on enough to get a trend.

By my calculation (based upon the information in the link below) we are shrinking about 2.6 in3 as the lead freezes but I saw him melt in over 8 in of a lead rod about square inch in profile.

One problem with the mold is that the center is a hollow core of steel about 4 in∅ which seems to be causing the part to cool from the center out and the lead to shrink away from the steel core. This is where the operator used a torch to re-melt the surface of the lead and then melted in more lead to fill in the gap.

We currently pre-heat the mold with a torch but are considering adding an induction heater to pre-heat the mold and already have an oven that the mold could be placed in to cool slower.

One question is how bad does lead 'flash freeze and cause voids in a potentially unevenly heated mold, where else could the voids be coming from and does anyone have any suggested reading for learning more about lead molding.

I found the linked book below which has good information but is incomplete digitally (unless someone else can find a full digital version).

https://books.google.com/books?id=TtGmjOv9CUAC&pg=PA320&lpg=PA320&dq=volume+change+of+freezing+lead&source=bl&ots=yUqHxnQLM5&sig=iI8Y2MvV4rrWdEU-jo4AI3RGWbM&hl=en&sa=X&ved=0ahUKEwjI86_E6ZfRAhWEwVQKHYHSD7gQ6AEIJzAC#v=onepage&q=volume%20change%20of%20freezing%20lead&f=false

Drew K

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#1

Re: Pouring Lead Mold

01/04/2017 2:07 PM

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.

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#2
In reply to #1

Re: Pouring Lead Mold

01/04/2017 2:23 PM
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#3

Re: Pouring Lead Mold

01/04/2017 4:13 PM

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.

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#4

Re: Pouring Lead Mold

01/04/2017 4:29 PM

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.)

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#5

Re: Pouring Lead Mold

01/04/2017 5:21 PM

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

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#6
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Re: Pouring Lead Mold

01/04/2017 5:42 PM

Good luck with that.

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#7
In reply to #5

Re: Pouring Lead Mold

01/04/2017 7:36 PM

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?

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#12
In reply to #7

Re: Pouring Lead Mold

01/05/2017 11:31 AM

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

Drew K

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#8

Re: Pouring Lead Mold

01/04/2017 10:44 PM

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.

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#9

Re: Pouring Lead Mold

01/05/2017 5:51 AM

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)

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#14
In reply to #9

Re: Pouring Lead Mold

01/05/2017 11:43 AM

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

Kiddos too. and Hippos.

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#15
In reply to #14

Re: Pouring Lead Mold

01/05/2017 11:46 AM

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

Drew K

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#16
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Re: Pouring Lead Mold

01/05/2017 1:43 PM

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.

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#17
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Re: Pouring Lead Mold

01/05/2017 2:38 PM

Way out of my knowledge range, I am still reading the basics on detection! I know they use ion gas chambers and photo luminescent materials with photo multiplier tubes...but not much on the details yet.

Drew K

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#20
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Re: Pouring Lead Mold

01/05/2017 3:10 PM

Brings back fond memories of SiLi detectors used in Idaho. During winter we were warned not to wear polyester pants as the Radon daughter products would stick to cuffs due to static charge and set off the gate sniffers. Radcon would come, frisk you and then leave you in the waiting area less pants. The pants would go to the SiLi detector to verify that this was natural organic Idaho radiation and not that nasty government stuff. Then they would give you back your pants and send you on your way.

The SiLi detector gave the energy level of radiation emitted from the particles and tell you what the specific emitting isotope was.

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#22
In reply to #20

Re: Pouring Lead Mold

01/05/2017 3:26 PM

SiLi must be a pretty good neutron detector. Will it detect Beta, seems like it might.

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#24
In reply to #22

Re: Pouring Lead Mold

01/05/2017 5:49 PM

SiLi absorbs and generates a pulse proportional to the energy level of gamma radiation.

It was basically a big lead box with the detector and the test material. It wasn't near any neutron sources and the working distance pretty well filtered out alpha and beta radiation.

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#23
In reply to #20

Re: Pouring Lead Mold

01/05/2017 5:38 PM

I have heard conversations about energy levels to detect types of radiation but again, still above my present knowledge level. Sounded a bit like solving for variables with multiple equations and multiple unknowns where a good matrix comes in handy.

Drew K

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#21
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Re: Pouring Lead Mold

01/05/2017 3:23 PM

Sounds like we are both in the slow lane of life, my friend. Keep chugging along, and sooner or later we will arrive at the finish line.

I am just glad it is not the Finnish line. I don't even speak Norwegian on purpose.

I would go raiding coastal villages and plundering, but there just isn't anywhere that has not all been plundered, and the women in the ones that have not been are ugly as a mud fence that has been through a hurricane.

Besides that, the U.S. Coast Guard would kick my butt. Hey, here I am waving my sword around with my hair (both of them) in the wind, and my round wooden shield!

Coast Guard Captain: "Put this guy out of my misery." "Open up with the 25 Mike Mike on that little boat." Just like me to show up to a naval gun battle with a freaking sword.

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#18
In reply to #15

Re: Pouring Lead Mold

01/05/2017 2:47 PM

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

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#25
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Re: Pouring Lead Mold

01/06/2017 10:03 PM

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

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#10

Re: Pouring Lead Mold

01/05/2017 7:15 AM

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.

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#11

Re: Pouring Lead Mold

01/05/2017 10:52 AM

What you are looking for is called linear solidification. To pour the part shown, you want to cool the base of the mold and heat the upper end. The result is that the part begins solidifying at the bottom and always has a molten reserve to draw from sitting on top. I have experience in casting design, foundry, patternmaking and in casting lead shielding for nuclear reactors. In casting the reactor shielding, the fabricators spend a lot of time with rosebud torches keeping the top surface liquid while the material solidifies from the bottom up.

The heater on the center pin is a good idea, but I would temperature control it to match the lead temperature around the pin by putting a thermocouple on the outer wall of the mold at the same elevation, so the pin temperature will keep the lead melted at the pin surface until the lead surrounding the pin is starting to solidify.

I'd put cooling on the base of the mold and heater jackets around the upper end of the mold and preheat the mold to melting temperature at the upper end before adding melted lead. A series of thermocouples up the side would indicate progress of solidification to ramp down the heater jacket temperature.

I realize it's probably overkill, but it would be relatively poka yoka and would lend itself to some level of semi automation.

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#13
In reply to #11

Re: Pouring Lead Mold

01/05/2017 11:42 AM

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

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#19
In reply to #13

Re: Pouring Lead Mold

01/05/2017 3:03 PM

Permanent and metal injection molds typically have some water ports drilled through a block of metal that is part of the mold. The rate of water flow and temperature of the water control the cooling rate.

For the jackets, consider an array of calrod type heaters in a bronze or steel block that clips on to the outside of the mold. They are pretty durable and set in a block would be easily moved and attached without damaging the heaters. I've used these as barrel heaters on small prototype plastic injection molders and as bolt heaters without problems.

Concerning adding alloying elements to the lead, as long as the mass is present, the shielding is equivalent for gamma radiation. The only issue would be activation products if neutrons are present. Your design might tolerate a little bismuth or indium which would lower melt temp and reduce shrinkage if the final weight comes out the same and the ambient temp is within range of the alloy.

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#26

Re: Pouring Lead Mold

01/08/2017 1:33 AM

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.

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#27
In reply to #26

Re: Pouring Lead Mold

01/09/2017 9:38 AM

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

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