Mold temperature gradient source of imperfection?

[guninhand]

This observation comes from doing ladle pour in a single cavity mold. Melt temperature around 750F and mold temp around 435F. The mold temp is estimated from previous similar casting sessions with a thermocouple type probe inserted in a hole in the mold block.

When I cast I always finish up the pour with a "standing wave" at the sprue plate hole of 2 or 3 seconds duration. I find this ensures full boolit base fillout. This excess molten lead usually runs off the mold off the forward part, i.e. the part of the block opposite to the position of the handles. If I let it run anywhere else the lead builds up on the fingers of the handle.
This I think results in that part of the mold getting hotter than the rest. Though the mold is fully filled out when the lead is cast in, with subsequent cooling and solidification the hotter side of the boolit shrinks more than the side where the handles are. It's not easy to see in the fresh cast bare boolit, but when put through a lubsizer with dark lube you can see uneveness as the boolit is closely inspected.

I can't think of any other reason for this imperfection, and am wondering if others agree.

[cbrick]

To answer your question I would say absolutely possibly, maybe and could be.

I would drop both the melt temp and the mold temp slightly. If nothing else it will speed up your casting by not needing to wait as long between casts and 750 is at the ragged edge of what the tin can handle. I have noticed many times over the years that if the mold starts to get too hot, mostly with rifle boolits, that some of my molds will cause what I call "heat shrinkage" that normally shows up inside one of the lube grooves. It's an area inside and on one side of the lube groove where it appears the alloy just shrank away and the rest of the boolit is perfect. I don't know if "heat shrinkage" is the correct name but it's what I've always called it and simply allowing the mold to cool just a bit by slowing the pace cures it completely. Then should I get to casting a little to fast again it will re-appear. It's running on the edge of the mold temp just right and too hot.

Rick

[Gtek]

Trying to picture "This excess molten lead usually runs off the mold off the forward part, i.e. the part of the block opposite to the position of the handles" Mold not held close to level? Bottom or ladle? I think near perfect is a nice dome on sprue hole, that is it. Runeth over will take you to another set of problems. Have you checked with mic for real out of round, mold issue? What mold? What alloy? If I may, confirm mold dumping round boolits-lapping stickies, work on not so much on top, do not worry what something says how hot your mold is- learn to read the drops. First five or ten or so are going back in pot, these are sacraficial layed before the Pb Gods. One more if I may, I promise question is asked with no malice- engineer? Gtek

[guninhand]

Cbrick, your experience seems similar to mine, except I don't think it's all that bad to have a hotter mold, as long as the mold is kept at that temperature for the whole casting session and temperatures are uniform or at least symetric around the boolit cavity.
Gtek, I'm not an engineer but I do have a science background. The uneven bands are a consistent problem throughout the casting session, not just the firts few. I always pre-heat the mold.The puddle mound doesn't work for me, I get rounded boolit base edges. Mold is a BAC 45/70 giving a 508 grain boolit with Lyman #2 alloy. I have to let the excess lead run off the mold. This imparts excess heat to the sprue plate (in addition to the far side of the mold) which I cool off each throw with a 5 sec blast of air from the hose of an air compressor. Which just gave me an idea while I was typing this. In my next session I will give a blast of air to that far side as well. That and trying for lower overall casting temp as cbrick suggests, or even trying to direct the overflow of molten lead uniformly with respect to the mold center.

[runfiverun]

more heat on one side of a boolit will cause issues.
there is a thread in the alloy section called.. never mind it was easier to just find it.
http://castboolits.gunloads.com/showthread.php?172475-High-Copper-Alloys-Lets-discuss-this-further/page17

it starts near the bottom of the page and goes on.

[Gtek]

Another thought- possible vent line issues? Gtek

[gray wolf]

Have you tried adjusting how tight or loose the sprue plate is against the top of the mold?
I have found this can also effect how my bases come out.

[MtGun44]

Not likely to be a significant temp differential across an aluminum, iron or
brass mold. Thermal conductivity is too high to sustain much temp difference.

Bill

[runfiverun]

umm hot spots happen just from filling a mold a certain way.
uneven heat is not hard to have.

[DLCTEX]

I find the thin part of the mould where the handles attach is prone to over heating, usually shrinking the bands adjoining it in that area.

[guninhand]

My driving band shrinkage is aligned with the parting line on the boolit. If I keep my standing wave going over the sprue hole for 6 or 7 seconds instead of the usual 2-3, the base driving band will be frosted and the sizing die won't touch it yet size the other driving bands. I've got clean vent lines and the sprue plate just almost swings on its own weight.

[MtGun44]

I would be surprised if you could sustain more than a 10F or 20F difference for
more than a few seconds with the metals used in molds unless you were pouring
lead on one end and sitting the other on a wet sponge. Metals have a very high
thermal conductivity and in a few seconds will equalize within a small temp
range unless you are adding rapidly to one side and cooling dramatically on
the other.

Bill

[44man]

The worst boolits for hot spots are the TL designs. I just slow down. Going to frost on them is not good. It CAN happen more on one side if you cast fast before the temps stabilize.
Pouring half a pot over the sprue plate does not gain a thing. A decent puddle is enough. The boolit is already getting hard and you add nothing to the boolit. Tipping the mold just adds to problems.
I ladle cast and hold the ladle tight to the sprue plate long enough for the boolit to take molten lead from the ladle, it can actually be seen to draw. I tip off leaving a puddle but the size of it means nothing at all because the boolit is full and the cooling dimple is very small, just the puddle shrinkage. My boolits DO NOT NEED FED FROM THE PUDDLE.
I heat my molds to 500* in my oven and the first to last, draining a 20# pot to scraping enough to fill the ladle from the bottom will give me zero rejects.
I don't like 6 cavity molds or bottom pour.
I do not believe pouring lead on the plate will feed the boolit. Keep the mold level and just fill it. If a boolit does not fill, raise the mold heat. Your boolit shrunk and all the rest you keep pouring is just running away.
Stop reading temperatures, learn to feel and see what the mold is doing. Casting is bootstrap work, not science. You should be able to adjust from one pour to another.

[cbrick]

Bill and I are in lock step on most things but I'll disagree with him to a point on this. Yes mold metal has high thermal conductivity but the problem I described in post #2 seems to be a hot spot inside the mold or to put it a different way is where the mold begins to get too hot, the spot that gets hot first. As I mentioned it takes little more than slowing the pace a bit to solve the problem & even out the heat. It is the mold at a temp throwing good boolits and being right on the edge of being too hot.

As another poster said it is at the thinnest part of the mold where it is cut for the handles and in my instances of observing this is with rifle boolits (IE long skinny boolits) inside the lube groove, where the mirror image inside the mold is the band creating the groove that starts to get hot first.

Looking at it this way it is possible to have a hot spot inside the mold and if this is the cause of what the OP observed simply reducing his melt temp from 750 to 700-710 should tell him.

Rick

[44man]

Rick is right, you can get a hot spot.
I just leave the lead pot the same and adjust the casting rate. Both will work of course but I just relax more.
Soon, no thought is needed, it becomes natural.
My friend was up to cast his .500 boolits last week. I showed him but he tipped the mold when tipping off the ladle and spilled the sprue puddle out. He also did not hold the ladle long enough. I had to take over and make his boolits.
The problem is experience and the ability to not only listen but to put it into practice. I call it fumble casting. No matter how I talked to him, he would fail. He just got in a big hurry while I cracked jokes and ran two molds. Even stepped out to pee between pouring.
We are not all the same.
Not everyone can control the mold and not everyone can control a trigger. I get real frustrated. Mostly it is a need to HURRY. I also need to get down on guys that come to load that want to slam the press handle at a million miles per hour. Would you please feed the boolit in slow and feel what you are doing! Lead is not to be slammed in like production jacketed. We are not a factory, remove that idea.

[runfiverun]

all I really think he is doing is keeping the sprue plate hotter with the extra alloy and getting better base fill out.....

[44man]

The sprue plate only needs to be mold temps.

[runfiverun]

true...

[guninhand]

Thanks 44man. Those are very insightful comments. Luckily I think it wouldn't be hard to follow your procedure and get nice even bands; would even speed up production without even trying to.

[cbrick]

The sprue plate only needs to be mold temps.

True BUT (there's always a but isn't there :mrgreen:) the sprue plate doesn't have nearly the mass that the mold has and will cool faster than the mold, I always pour a very generous sprue puddle for this reason. If the sprue plate does get too cool round bases will result as the plate draws heat from boolit base and cools it faster.

Rick

[44man]

True BUT (there's always a but isn't there :mrgreen:) the sprue plate doesn't have nearly the mass that the mold has and will cool faster than the mold, I always pour a very generous sprue puddle for this reason. If the sprue plate does get too cool round bases will result as the plate draws heat from boolit base and cools it faster.

Rick
I hate thin plates and a larger puddle will help but the taper is so small it drives me nuts. I hate Lee plates the most.
I use thick stainless for my plates, at least 3/16" and as close to 1/4" as I can get when I mill them. I need no large puddle and only enough to fill the taper works.
My sprue does not shrink any more then the puddle shrinks, no lead is drawn by the boolit.
Yet I get the Lee to work but I will never like them. The puddle can only get so big.
I keep the propane torch handy and any problem will get some flame. I NEVER cool a plate with a wet rag. Time works better.
The plate MUST BE HOT and I will not beat around the bush. The longer it takes for the sprue to set up, the better the boolit.
I just cast several hundred .500 boolits and my friend tried so I had to toss one back in the pot. He actually poured lead out of the mold because he could not make one hand work without the other moving. There is no string connecting hands.
That is the most amazing thing I run into teaching casting. Just why does a man tip the mold when tipping off the ladle???????

[guninhand]

Quote" That is the most amazing thing I run into teaching casting. Just why does a man tip the mold when tipping off the ladle??????? nquote
Maybe they think it helps air escape the mold cavity.

[44man]

Quote" That is the most amazing thing I run into teaching casting. Just why does a man tip the mold when tipping off the ladle??????? nquote
Maybe they think it helps air escape the mold cavity.
Nope, just can't make one hand work by itself.