Hotter mold = smaller boolits? I don't understand!

[fatelvis]

In this thread by Swede, he states that when the mold gets hotter, the mold drops a smaller diameter boolit. I would have thought it would be the opposite. Please help me understand! Thanks-
http://noebulletmolds.com/smf/index.php/topic,82.0.html

[500MAG]

Here is a good thread on the subject:
http://castboolits.gunloads.com/showthread.php?69608-Does-a-hotter-melt-smaller-boolits

[44man]

Not only is lead expanded more but the mold blocks are different then a hole in one piece of metal. The cavities expand towards the edges of the inner sides. Think of the inside of a half cavity as another side of a block.
I mess up now and then, get a cherry too small, I have to heat the mold very hot, drop the cherry back in and remove a bunch more metal. The cherry will also expand some as it heats, aids cutting more too. But if I work quick, I still get a lot more cutting. I have removed the cherry and it is just warm. The cavity shrinks if too hot so keeping it at the right heat will make the best boolits.
Notice boolits will be more out of round if very hot because the inside expands outward but so do the parting line portions. Parting lines will be larger. When I re-cut a hot mold, I get rounder boolits.

[44man]

I have learned a lot since making my own molds, had it wrong once when I froze a mold, Cherry just wobbled in the large hole.

[shredder]

Hmm. So much for me to learn! Great info, though a bit counter intuitive as the OP noticed. Thanks for the hands on info 44man! Once you explained it it made sense.

[Hickory]

It's not that hard to figure out, when anything is heated the molecules that
make up the structure expands or becomes more active.
The hotter the object gets the more it expands- up to a point.

When it cools the molecules slow down and contract to a smaller object.

[Hickory]

A very good example of the above is when you cook pasta.
How many of you have had a pot of pasta boil over?

Lead and its alloy are different, in that their atomic structure is on the opposite side of the periodical scale.

[44man]

It is hard to explain a hole in a solid piece is different as it expands away from the center. But a mold is not a hole in a solid piece of metal.

[chutestrate]

You guys have completely lost me. What's a cherry? And if your mold is too hot how does that contribute to a smaller boolit? To me the mold would experience some expansion, the lead added to the mold would fill in the slight expansion, then when dropped from the mold the boolit would be slightly larger.

[dondiego]

The cherry is the hardened cutting tool shaped like the boolit you want to cast.

[7br]

You guys have completely lost me. What's a cherry? And if your mold is too hot how does that contribute to a smaller boolit? To me the mold would experience some expansion, the lead added to the mold would fill in the slight expansion, then when dropped from the mold the boolit would be slightly larger.

Two things are happening to make the bullet smaller. The lead will have expanded more at a higher temperature, so when it cools, it will shrink more. The second thing is the mould cavity actually decreases in diameter instead of increasing. Think of arrows at a right angle to the edge of the block. This is the direction of expansion of the mould block. When you get to the cavity, the arrows are pointing to the middle of the cavity, so the diameter has decreased.

[Beagle333]

Lemme take a crack at it... I sketched this up real quick....

http://i613.photobucket.com/albums/tt214/shutupandjump/BLOCKEXPANSION_zps858c923f.jpg?t=1403284446

[44man]

Lemme take a crack at it... I sketched this up real quick....

http://i613.photobucket.com/albums/tt214/shutupandjump/BLOCKEXPANSION_zps858c923f.jpg?t=1403284446
Now you have it, wish I would have made it that easy.

[Bagdadjoe]

a picture is worth more than 864 words... :)

[theperfessor]

Well, my measurements don't show a smaller cavity when the mold is evenly and uniformly heated. I even posted a thread about this a couple of years ago, where I detailed my test protocol and results.

The problem is that molds are not evenly and uniformly heated during use, and the cavity can shrink a little if the outside of the mold is a lot cooler than the inside. The outside doesn't expand as much and in effect restrains the expansion in a direction of least resistance, which is into the mold cavity. And the hotter the melt, the hotter the cavity, and the greater the difference in temperature and expansion from inside to outside.

All of which is a good reason to get a PID and run your melt as cool as possible and still get good fillout.

[Beagle333]

All of which is a good reason to get a PID and run your melt as cool as possible and still get good fillout.

+1 on getting a PID! I highly recommend it. They completely change the game, in a good way. :cool:

[fatelvis]

Wow, the diagram helped immensely. Now I get it!

[Larry Gibson]

I don't think the problem is so much with the mould as with the alloy itself. It's cold solid state remains equal in size. However the hotter the alloy the more the alloy expands. The mould volume also remains constant. Thus you get less volume of the hotter alloy into the mould. When in cools back to the cold solid state it shrinks more. This is why the bullets also weigh less. Could be a combination of both but based on the fact the bullets are still smaller diameter and weigh less with the hot alloy in a mould that hasn't gotten above normal casting temp I'd say the majority of the effect is alloy caused.

Larry Gibson

[Beagle333]

In most cases, probably.
In this example, the alloy was at 700 for both pours. So the volume of alloy would be the same. The volume of the hole must have changed with mold temp.

[44man]

I heat my molds in my little furnace to 500* and soak it there until the lead is ready. If I paint the inside with a marker before I re-cut, you can plainly see metal is removed from the center of the cavities. There will still be black at the parting line.
All the molds I re-cut cast almost perfectly round boolits but one right off the mill doesn't. That metal came from somewhere fellas. You won't believe the pile of shavings.
When I mill, I deburr constantly and flush chips. For the final cutting when I get the blocks closed all the way, I can spin the cherry until it does not cut anymore but get the mold hot in the oven I will get a pile of chips. I work fast, turn the cherry with a wrench so I don't remove the temper.
Like I said, I tried freezing the mold to find the cavities were much larger then the cherry.

[9.3X62AL]

This explains my results with cooler vs. hotter alloy very succinctly. I had guessed at this, it is good having others confirm it for me.

[Elkins45]

I havent really thought about it in terms of one piece vs the two pieces of a mold, but if it is indeed true that a hotter mold has a smaller cavity then post #12 surely explains it, but see post #26 below. I can promise you that it isn't the case in a solid piece of metal because I have experienced it firsthand. I have done this demonstration with my students dozens of times:

http://www.youtube.com/watch?v=V0ETKRz2UCA

Notice that the hole inside the solid ring does expand.

[fatelvis]

I heat my molds in my little furnace to 500* and soak it there until the lead is ready. If I paint the inside with a marker before I re-cut, you can plainly see metal is removed from the center of the cavities. There will still be black at the parting line.
All the molds I re-cut cast almost perfectly round boolits but one right off the mill doesn't. That metal came from somewhere fellas. You won't believe the pile of shavings.
When I mill, I deburr constantly and flush chips. For the final cutting when I get the blocks closed all the way, I can spin the cherry until it does not cut anymore but get the mold hot in the oven I will get a pile of chips. I work fast, turn the cherry with a wrench so I don't remove the temper.
Like I said, I tried freezing the mold to find the cavities were much larger then the cherry.
Could the first milling leave mold material that should have been removed behind due to a springing back of material after the cutter is withdrawn? And maybe the second milling trims and trues the springback material? I'm not a machinist, I'm just wondering.....

[fatelvis]

Watch this video and then revisit what has been said in some of these replies:

http://www.youtube.com/watch?v=V0ETKRz2UCA
And we have a winner! Thank you for that link!

[44man]

Could the first milling leave mold material that should have been removed behind due to a springing back of material after the cutter is withdrawn? And maybe the second milling trims and trues the springback material? I'm not a machinist, I'm just wondering.....
No, no spring back when cutting. You don't withdraw a cutter, you open the blocks off it. A cherry is different from lathe boring.

[Elkins45]

And we have a winner! Thank you for that link!

Ha-you quoted me before I expanded my response! Now I will expand this one.

A metal that is heated will expand, but it will expand in such a way that each atom increases its distance from its neighbors. That's why the hole in the ring can't get smaller: the atoms on the inner edge of the ring must expand outward because if they expanded inward they would be packing closer to each other. The only way the hole can get smaller is if the atoms along the radius of the hole get closer, and getting closer is contraction not expansion.

After thinking it over I'm starting to suspect the system may be more complex than just the blocks alone. The behavior of the alloy may play a part as well.

[M-Tecs]

Hole expansion (material) is constant. One piece, two or four makes no difference. The easiest way to visualize this is to take ten dimes and lay them in a circle. Now replace the dimes with quarters. Dimes, quarters or molucules it's all the same.

[Elkins45]

In the case of the open cavity, there is no compression of the cavity wall if the block closes the gap slightly, and since it is not a closed circle (a hole), there is not the rigidity (the "other side" of the cavity and its associated metal pushing back) to prevent the cavity from shrinking.

I don't think it matters if the ring is closed. The atoms along the radius of the cavity would still have to be crowded against each other more tightly to decrease the circumference of the cavity, and that is contrary to what their increased level of kinetic energy wants them to do.

Do we have any physics professors on this forum? I think we need to call in a consult. :)

[Elkins45]

Hole expansion (material) is constant. One piece, two or four makes no difference. The easiest way to visualize this is to take ten dimes and lay them in a circle. Now replace the dimes with quarters. Dimes, quarters or molucules it's all the same.

Wow--great model! That totally works for me. Thanks.

[M-Tecs]

Part two - hotter blocks will produces larger boolits, however, hotter alloys will produce smaller boolits (if the block temp doesnt change) since the alloy going into the mold has expanded more.

The third leg is when you increase the block temp you are also changing the cooling rate in the mold as it freezes. The sprue freezes first and so it doesn't fully replace the molten material in the block as it cools.

antimony (http://www.chemicool.com/elements/antimony.html) also has the unusual property that (like water) it expands as it freezes. Four other elements expand when they freeze; silicon (http://www.chemicool.com/elements/silicon.html), bismuth (http://www.chemicool.com/elements/bismuth.html), gallium (http://www.chemicool.com/elements/gallium.html) and germanium (http://www.chemicool.com/elements/germanium.html).

[fatelvis]

Part two - hotter blocks will produces larger boolits, however, hotter alloys will produce smaller boolits (if the block temp doesnt change) since the alloy going into the mold has expanded more.
So this begs the question:
If you bring the mold up to a hot temp (about 350*) with a hot alloy (about 800*), will the shrinkage of the cooling boolit overtake the advantage of the expanded mold cavitiy, or would it depend on boolit size?

[9.3X62AL]

Being the social science major that I am......I am among the LAST people whose info I would rely upon as "gospel" in a physical sciences venue. My background in such matters is not deep at all. My own ideas are that the alloy temperature in the context of the block temperature is the determining factor in a bullet's finished/as-cast diameter and resulting weight. I strive to keep casting rate as constant as possible in order to keep the block temp at a "status quo" condition during a casting session. My "default" alloy temp for most iron or steel moulds is 675*, aluminum moulds generally act better for me at 725*. This applies to 92/6/2, Lyman #2, and Linotype alloys. Pure lead has a higher melt temp than the tri-metal alloys, so I bump the temps up about 150* from the moulds' usual "settings". 90% of my casting involves use of 92/6/2, it's kind of my "default" metal, doing good work in rifles and magnum revolvers with gas checks but hard enough for use in plain-base bullets used in high pressure autopistols like the 9mm and 40 S&W. Loverin bullets like #2 alloy very much, and Loverin 22 and 24 caliber bullets thrive on Linotype. A lot of #225438 I cast a few months back showed EXTREME consistency when scaled for use in the 22 Hornet. Using 1/2 of 1% variance for rejection, I only had to toss about 10% of the castings from the determined mean of 42.1 grains. Subsequent shooting of these through my Ruger #3 gave 2500 FPS from 9.0 grains of 2400/Rem 6-1/2 primers/W-W cases, and 1.4" to 1.7" five-shotters at 100 yards. Even a blind squirrel finds a nut once in a while.

[M-Tecs]

Info is here if you want to run the numbers

http://inspectapedia.com/exterior/Coefficients_of_Expansion.htm (http://inspectapedia.com/exterior/Coefficients_of_Expansion.htm)


http://www.muller.net/mullermachine/docs/temperature.html (http://www.muller.net/mullermachine/docs/temperature.html)

[Beagle333]

I don't think it matters if the ring is closed. The atoms along the radius of the cavity would still have to be crowded against each other more tightly to decrease the circumference of the cavity, and that is contrary to what their increased level of kinetic energy wants them to do.

Good point. Perhaps the cavity only gets smaller along the parting lines, as the open sides of the cavity grow toward each other?

The math behind it might be hard for me to grasp, but the test of it is pretty easy. A lot of us have PIDs and micrometers, and would be able to keep a pot and a mold at exact and known temperatures and duplicate Swede's experiment and compare.

[Elkins45]

So this begs the question:
If you bring the mold up to a hot temp (about 350*) with a hot alloy (about 800*), will the shrinkage of the cooling boolit overtake the advantage of the expanded mold cavitiy, or would it depend on boolit size?

Pb has a higher coefficient of expansion than Al, so as temperature of each increases the volume increase of the Pb will be occurring faster than the expansion of the cavity. So the shrinkage problem will continue to be worse as melt temp increases for almost any reasonable temperature...or at least that's how my limited math skills make me see it. I don't think the two ever intersect.

So that means the recipe for fatter boolets is to cast with hotter molds and cooler alloys.

[fatelvis]

So if I want a boolit to be at max diameter, I should keep my mold preheated to 350-400*(?) and keep my PID for my furnace at about 700*?

[SwedeNelson]

fatelvis, others

What I have got out of all this is that if you're not getting what you want
you have some options to get there. Work with your mould and you can
make it do some very interesting things.
Would be inclined to agree with Larry Gibson and M-Tecs - melt temp. has to
do as much with it as block temp.

Now try it with brass!

Not trying to explain it all just observations.
Had more fun with a thermometer and a hot plate than I ever thought I would!

Try it and see what you get.

Bullet maker, maker
Swede Nelson

[M-Tecs]

Good point. Perhaps the cavity only gets smaller along the parting lines, as the open sides of the cavity grow toward each other?


Nope, the sketch of the block with hole expanding is correct. The laws of Physics don't change with a parting line. The sketch of the mold block expanding is not correct.

[Beagle333]

The sketch of the mold block expanding is not correct.


Well.... there goes that physics job. 'Guess I'll stick with bus drivin'. :violin:

[44man]

Easy to convince you if you want to test it. Clamp two blocks of any metal together and bore a hole for a perfect fit to a pin or shaft, dead center of the parting line. Heat the blocks and see if you can put the pin back in.

[mrbill2]

Just how much difference in bullet size are we talking about with say a 30 cal bullet? Is it enough to make a difference that we will notice in the groups we shoot. Can you take a mold that cast .299 at the nose and by changing mold and alloy temp to increase it dia. of the nose and by how much? All with the same alloy.

[M-Tecs]

Easy to convince you if you want to test it. Clamp two blocks of any metal together and bore a hole for a perfect fit to a pin or shaft, dead center of the parting line. Heat the blocks and see if you can put the pin back in.

As a Toolmaker and precision machine builder I have done this several thousand plus times in the past 36 years. At one time I worked for a company that did very high tolerance work. 50 millionths was not considered a tight tolerance. We monitored the ambient air temperature (standard is 68 degrees of measurement, but normally we held 70 to 73 degrees), the part temperature, the machine spindle temperature, the machine leadscrew/table temperature and the coolant temperature to achieve these tolerance. Expansion and contraction calculations were apart of daily life for me.

In both my plastic injection mold and machine rebuilding days I worked a lot with precision holes with parting lines.

I currently manager an aerospace machine shop. We use heat and/or liquid nitrogen daily for component installation.

In all these years (36) I have never seen an example of material expansion or contraction that did not follow the laws of psychics.

Your claim goes against the laws of psychics and my 36 years of experience.

[243winxb]

Higher temperatures will result in greater shrinkage as the bullet cools, thereby producing a slightly smaller and lighter bullet than one cast of the same alloy at a lower temperature.

http://www.redding-reloading.com/online-catalog/88-bullet-moulds-charts (http://www.redding-reloading.com/online-catalog/88-bullet-moulds-charts) All i know is , a frosted bullet will be smaller in diameter by a tiny amount. [smilie=s:

[44man]

The law says a material expands outward. Am I breaking the law? Since half a cavity is on a side that is expanding, how can it expand towards the center of the block?
I have to press a bearing today so the shaft part will go in a freezer for a while so the bearing is easier to press on. If I could heat the bearing races, I could drop the bearing on. Cold contraction will be towards center and race expansion will be out from center. Can't heat the bearing because of grease and rubber.
I see no comparison with lead and plastic, much heat differences.
If I made a precision mold I would compensate for uneven expansion.

[Cap'n Morgan]

44man.

Expansion is just that - expansion. It is not related to the center of the part in any way. When you mold halves expand, they expand evenly on all sides - and the cavity is also a "side" even if it is actually a half circle. If the circumference (length) of our half circle increases then the radius must increase too or it is no longer a half circle. Think of the cavities as the cuts across a loaf of bread set aside for rising; as the bread rises the cuts grow larger...

[44man]

But "sides" get larger don't they? Measure a cold block, then a hot one, isn't the hot one larger? There is no way a half hole can move to the center of the block. It expands outward.
Even a hole off center in a piece of metal will get larger in one direction then another, depends on the mass on each side of the hole. Only a hole centered in a piece of metal will expand evenly.
I tried to explain to the drawing that the parting line might get larger but the other direction will get smaller.
Explain how a half hole on the edge will expand towards the center of a block that is expanding outwards.
Your bread thing does not hold as the whole loaf expands away from the cuts, like a center hole. Ever notice the cuts at the end of the loaf are never as large as those in the center?
I am not the sharpest tack in the world, but see things for myself.

[M-Tecs]

You are looking at expansion as one dimensional. The expansion is three dimensional. This is somewhat of an over simplification but to understand it correctly think about the molecules individually. If you have a single molecule it will expand all directions. It is the same of all the molecules in a group. When you have a hole the expanding molecules does want to expand to the center of the hole but it is not allowed to since the molecule next to it is also expanding and forcing it away from the hole.

The easiest way to visualize this is with a two dimensional model. As I stated earlier take ten dimes and lay them in a circle. Now replace the dimes with quarters. The changing size of the dimes and quarter represents the expanded molecules. If you do this simple example it is easy to see and understand why holes expand and why it doesn’t make a difference if it is a complete hole or a half of a hole like in a mold half.

The key to understanding this issue is to visualize each molecule getting larger and how it effects the molecules next to itself. Molecules don’t actually get larger when heated. They just get more excited and move away from each other. The effect is the same and calling them larger is simplified way to explain expansion.

[Tom Myers]

You are looking at expansion as one dimensional. The expansion is three dimensional. This is somewhat of an over simplification but to understand it correctly think about the molecules individually. If you have a single molecule it will expand all directions. It is the same of all the molecules in a group. When you have a hole the expanding molecules does want to expand to the center of the hole but it is not allowed to since the molecule next to it is also expanding and forcing it away from the hole.

The easiest way to visualize this is with a two dimensional model. As I stated earlier take ten dimes and lay them in a circle. Now replace the dimes with quarters. The changing size of the dimes and quarter represents the expanded molecules. If you do this simple example it is easy to see and understand why holes expand and why it doesn’t make a difference if it is a complete hole or a half of a hole like in a mold half.

The key to understanding this issue is to visualize each molecule getting larger and how it effects the molecules next to itself. Molecules don’t actually get larger when heated. They just get more excited and move away from each other. The effect is the same and calling them larger is simplified way to explain expansion.



Wonderfully stated.

[303Guy]

Gents, 44man has told us that when he cuts a mold, he heats the cut blocks then finish cuts while hot. The cherry removes more material from the centre region of the cavity and does not touch the area near the edges. The atoms are not packing closer together on the circumference of the hole because the hole is going out of round. He has told us that a freshly cut new mold will cast out of round but if he heats the mold and finish cuts them they will cast round boolits. It's not about science or laws of physics, it's what happens.

The idea that hotter alloy should cast larger because it expands doesn't hold because the alloy freezes at the same temperature regardless of the casting temperature or mold temperature. However, the rate of freezing will change and I would expect that to effect the casting size but I don't know in what way. I have found in one of my one piece moulds that if cast cool the boolit will drop out easily but if cast hot it will jamb in the mold (due to a defect). This shows that the relative shrinkage is less at higher temperatures. No doubt alloy has an effect on it too. I'm not too sure now but I think cooler alloy in a hot mould jambs while a hotter alloy in a cool mould does not - I could have it the wrong way round or totally mixed up.

[Cap'n Morgan]

303Guy.

I had the same experience as you regarding one-piece molds. When the mold gets too hot the boolit tends to stick in the cavity. In my case I'm pretty sure it's due to a very large sprue cavity on the mold (the weight of the sprue alone varies between 300-400 grains). When casting, shrinkage cause a dimple to form in the sprue as it is solidifying from the outside in, indicating that the semi-molten core of the boolit is still drawing lead from the sprue. As a result the shrinkage is almost zero - in fact, I can't measure any shrinkage at all. See the "boolit" in question here: (#27)

http://castboolits.gunloads.com/showthread.php?243178-June-2014-Photo-Submission-Topic-a-single-cast-boolit/page2

[1bluehorse]

Sure are a lot of smart guys here.....enjoyed reading all the responses of the "how things work", (still a bit mystified by some of it), why it works/doesn't work, experiences from mold making (wish I could do that) the pictures and all the effort put into the responses......now, would someone tell me/others what it takes to make a mold increase a bullets diameter....ie; hot mold, cooler melt---cooler mold, hotter melt.....neither of the above...all of the above.....I know, it depends.....:bigsmyl2:

[mrbill2]

I know, it depends.....:bigsmyl2:

Now you got it, welcome to the organized mass confusion!

[303Guy]

... shrinkage cause a dimple to form in the sprue as it is solidifying from the outside in, indicating that the semi-molten core of the boolit is still drawing lead from the sprue. As a result the shrinkage is almost zeroThe light just came one! Of course. Get the alloy mould and sprue temperature just right and the skin freezes, setting the diameter while the core remains molten and draws in more alloy. My one mould locks in the boolit because it has a defect in it so when the boolit is a close fit in the mould it sticks there and has to be melted out.

I'm with 1bluehorse (http://castboolits.gunloads.com/member.php?11415-1bluehorse) in wanting to know that relationship.

[44man]

It is a mystery since the sides expand so you would think everything would even out. Since blocks are fatter, why is a half cavity still smaller? I can only surmise the cavity is hotter then the rest of the blocks.
About the sprue supplying lead to the boolit. Should not do that, should let the boolit take molten lead from the ladle. You should only see shrinkage from the sprue alone, should be very small. Cast right, there will be no divot at the sprue surface, maybe a tiny one. The sprue sets faster then the boolit.

[243winxb]

would someone tell me/others what it takes to make a mold increase a bullets diameter Add Antimony See my link above.

[303Guy]

About the sprue supplying lead to the boolit. Should not do that, should let the boolit take molten lead from the ladle.That is true of normal moulds and sprue plate. With solid moulds the game changes somewhat. I doesn't have to I suppose but with my set up it is different and the idea is to get the casting to freeze progressively bottom up toward the sprue but of course the core will remain molten till last. My molds are nose pour and can produce a hollow nose or flat nose, depending on the sprue. I even managed to get one alloy and mould combo to produce a boolit with a softer nose section and harder base shank by cooling the base section faster (which was at the bottom). Sadly, I have no idea what the alloy mix was.

[44man]

Must be difficult to use a one piece mold.

[Elkins45]

Must be difficult to use a one piece mold.

Only for the second bullet. :p

Actually, aren't there some cylindrical core molds with variable length ejector plugs that are just straight thru holes with plugs in the bottom? I seem to remember seeing pictures somewhere.

[M-Tecs]

The light just came one! Of course. Get the alloy mould and sprue temperature just right and the skin freezes, setting the diameter while the core remains molten and draws in more alloy. My one mould locks in the boolit because it has a defect in it so when the boolit is a close fit in the mould it sticks there and has to be melted out.

I'm with 1bluehorse (http://castboolits.gunloads.com/member.php?11415-1bluehorse) in wanting to know that relationship.

Came across this looking for something else but it does address some of the questions in this thread very well.

http://castboolits.gunloads.com/showthread.php?144116-How-Much-Shrink-Inside-a-Cast-Bullet


Having spent many, many years in the commercial casting business with 3 different casting processes, they all share this common issue, shrink. Not dimensional shrink but volumetric shrink.

Most elements expand as they melt and shrink when they solidify. Take a 1 inch cube of solid aluminum and make a simple sand mold with it. Melt the 1" cube and pour it into the mold, all the molten metal will not fit.

For example, in steel casting, we poured 50 pounds to produce a sound 25 pound part.

As metal begins to solidify, it starts with the surfaces of the mold it contacts first and draws to the middle of the section. While solidifying, it is pulling molten metal to the solid metal. If there is none or not enough liquid metal to pull from, internal pockets develop called “shrink”. It shows up easily in X-ray and the only simple means for us Lead Casters to detect the shrink, well, most likely is weight. Casting tubes and cylindrical parts are not easy in industry or in Cast Boolits!

Before I forget, casting metal too hot usually does nothing but create more shrink. Cast as cold as you can and keep the mold temp constant.

This is also why the pool of metal on top of the sprue plate draws down and in, called "suck in" in some foundries. The center is solidifying last and looking for more metal to draw from.
One "Rule of thumb" in the casting industry is that a ratio of 1:1 is preferred when designing a casting. In other words, larger diameter, shorter boolits will be easier to cast provided you can fill the cavity fast enough. Long, narrow designs of boolits are difficult to cast. Most shrink will be in the base of the bullet most likely from the midsection to the base.

Shrink will impact accuracy as the tiny shrink pockets will throw the weight off and as the boolit spins, it will spin off center and more in an orbit rather than in a line.

One thing that would help reduce shrink is to provide more metal than what the mold can hold. One simple fix is to make a sprue plate thicker and hold a small reseviour (called a “riser” in the industry) for the base to have more metal as needed. I have thrown around a few designs and it can be done but haven’t cut chips yet.

So, has anyone ever wondered how sound/solid the cast bullet is? A sound/solid bullet is an accurate bullet!!! Would like to hear what you have experienced. If this seems a little technical, this is the engineer in me trying to figure out how to do it better.

[10mmShooter]

OP fatevlis,

I was a little confused also on how a question about the effects mold temperature and bullet size.....turned into a machining discussion about mold cherries(they are like broachs or end mills, the tool that is used to cut our mold blocks, the tools spins and the blank blocks are closed on the tool head(cherry) cutting the molds, the trick is to close the halves equally so that the cavities are cut without being off center. And that where the discussion was going.

[DrCaveman]

I think the discussion drifted toward the effects that heating the mold blocks had upon the cherry's ability to continue cutting material from the blocks after closing the halves together, after they had already been fully cut and cooled

My attempts at mold and alloy reglation since the start of this thread have resulted in a consistent observation that the seams are 2-3 thou under the perpendicular measurement. Every mold ive tried. Im probably keeping the mold too hot, according to the overall take from this thread, but when i try to let it cool, the rounded corners show up real quick.

I dont believe there is any way that the expansion/contraction activities of a functional pair of bullet mould halves would be the same as a laboratory-created situation which tested the outcome of a similar, yet perfect, set of mould halves, in a highly regulated environment

Too many variables

I do like m-tecs quote

[303Guy]

Must be difficult to use a one piece mold.Actually no. Slow maybe. I did manage to cast at a reasonable speed but still way slower than a split mould. My way is particularly slow because I am relying on the sprue 'header' to fill the shrink cavity as it forms so cooling takes time. For my hollow nose boolits I have to 'pluck' the sprue off then open the base and let the boolit drop out.

I made a mould or two with the adjustable nose as per longbow's directions. They worked pretty well actually. I made the mould body with two diameters with the adjustable nose in the smaller bore riding nose section as would be expected and a taper section between (to match the throat). It was a paper patch mould.

[leftiye]

Well, my measurements don't show a smaller cavity when the mold is evenly and uniformly heated. I even posted a thread about this a couple of years ago, where I detailed my test protocol and results.

The problem is that molds are not evenly and uniformly heated during use, and the cavity can shrink a little if the outside of the mold is a lot cooler than the inside. The outside doesn't expand as much and in effect restrains the expansion in a direction of least resistance, which is into the mold cavity. And the hotter the melt, the hotter the cavity, and the greater the difference in temperature and expansion from inside to outside.

All of which is a good reason to get a PID and run your melt as cool as possible and still get good fillout.

AND use a mold heater to keep the mold heated from the outside instead of the inside. Also allows cooler melt with still good fillout.

[Cap'n Morgan]

Since this thread refuses to die, here's a drawing where a mold-half is scaled an exaggerated 10%. The blue profile is before scaling, and the center of the cavity is also the center of the scaling. Notice how the radius increases as well as the side lengths.

http://i1370.photobucket.com/albums/ag248/driftwood4/836a58e6-063b-4825-8bd1-e0b569e7153b_zpsqd82kypv.png (http://s1370.photobucket.com/user/driftwood4/media/836a58e6-063b-4825-8bd1-e0b569e7153b_zpsqd82kypv.png.html)

[Tom Myers]

Since this thread refuses to die, here's a drawing where a "mold" is scaled an exaggerated 5%. The red profile is before scaling, and the center of the mold is also the center of the scaling. Notice how the radius increases as well as the sides.

At last!! The light shines through!!!!

[44man]

Good illustration.
I still get confused with my blocks when I cut again because you would think because the sides expand where blocks touch, the cavity would expand the same amount without a change.
But the amount of chips I get is amazing. Never cuts at the parting line.
My .476 boolit cast out of round and a little small so I recut when hot and just measured an unsized boolit. It is .4765" and at the parting line I get .4769" So I am now just .0004" out of round.

[303Guy]

Could it be that the heated mold half cuts deeper than when cold so you actually cutting it true once hot while not quite true when cold? I can't imagine why that would be though. You've suggested that the mold halves do not heat evenly and so become distorted when hot and you are correcting that distortion. One often hears of moulds casting out of round boolits and how beagling casts larger boolits. Does beagling result in out of round boolits? Remember where the heat is coming from and where it goes. The centre region of the mould might be expected to be stressed be the cooler outer regions.

[DrCaveman]

Hmm. Im confused even more now, since the small measurement on my boolits as-dropped is always at the parting line (that is the seam, right?) which suggests that:

1. Either my molds suck, or
2. They are contracting when heated in the direction parallel to the seam
3. They are expanding perpendicular to that in excess of the seam diameter expansion

I found this to be true with steel lyman & rcbs molds, and aluminum lee mold. The brass mold from accurate is just sweet with no noticable irregularities, so i retract my previous statement about "all my molds"

This thread wont die because molds do not respond the same as CAD objects within a computer.

[Cap'n Morgan]

44man.

You cut your molds closing the mold over the cherry in a double-acting vise, right?
It is not easy to cut a truly round hole this way. The cutting forces are pushing the mold halves in each direction making the hole slightly oval. At the same time the cherry will rise a small burr when the teeth come out of contact with the cavity. Brushing the chips away is not enough. Unless the mold sides are stoned flat before the final cut, the mold will not fully close. This will happen even when the mold are cut from a fully closed block as I do myself. The burr is tiny, tiny, but it's there and it's matched by a just as tiny, tiny dent in the opposite mold half.

Next time you cut a mold, try stoning, then closing the mold, several times before taking it down - it just might do the trick.


Today's boring history lesson: (may safely be skipped)

When I started making molds some 35 years ago I had no idea of how it was done. I cut each mold half separately, making sure the cavities was cut slightly deeper than necessary (luckily, we did have DRO on the mill) I then used a flat grinder to trim the mold faces to the correct cavity depth.
Then the mold halves was then clamped together over two guide pins in the cavities and the holes for alignment pins were drilled & reamed. Nowadays, thanks to CNC, I can mill the mold face, integrated alignment pins/holes and venting lines all in one setup and then cut the cavities to whatever diameter I prefer using helical interpolation - and the cavities will be round within .0003... Life is good!

[44man]

44man.

You cut your molds closing the mold over the cherry in a double-acting vise, right?
It is not easy to cut a truly round hole this way. The cutting forces are pushing the mold halves in each direction making the hole slightly oval. At the same time the cherry will rise a small burr when the teeth come out of contact with the cavity. Brushing the chips away is not enough. Unless the mold sides are stoned flat before the final cut, the mold will not fully close. This will happen even when the mold are cut from a fully closed block as I do myself. The burr is tiny, tiny, but it's there and it's matched by a just as tiny, tiny dent in the opposite mold half.

Next time you cut a mold, try stoning, then closing the mold, several times before taking it down - it just might do the trick.


Today's boring history lesson: (may safely be skipped)

When I started making molds some 35 years ago I had no idea of how it was done. I cut each mold half separately, making sure the cavities was cut slightly deeper than necessary (luckily, we did have DRO on the mill) I then used a flat grinder to trim the mold faces to the correct cavity depth.
Then the mold halves was then clamped together over two guide pins in the cavities and the holes for alignment pins were drilled & reamed. Nowadays, thanks to CNC, I can mill the mold face, integrated alignment pins/holes and venting lines all in one setup and then cut the cavities to whatever diameter I prefer using helical interpolation - and the cavities will be round within .0003... Life is good!
My blocks do not move, they are locked in very tight but I am aware of the lead-lag at edges and thought that was the problem. I de-burr constantly as I cut, easy to just open the jaws and smooth the blocks. Once the cherry stops cutting I remove burrs again, close the blocks and let the cherry free spin a little. I flush with coolant all the duration and keep chips out of the cutter.
Blocks have pins installed. I also barely mill the tops flat after they are in the vice. The hardest job is to get the cherry dead center.
Using a CNC would be Heaven compared to my cheap Smithy.