How casting temp affects size

ChestnutLouie

Active Member
I was looking at Saeco bullet molds brochure that I found on Redding's website and in the instructions for use it said that if you cast at higher temperatures your bullets will shrink more as they cool. Is this your view?

When I first started casting I did not have a thermometer and was happy if my bullets were completely filled out and shiny. I then got a thermometer and heat my pot to 725 degrees and get frosty bullets. this seems good as I tumble lube them and they shoot ok.

Should I go back to a lower temp? what is that temp? to reduce that shrinkage?


Thanks
Francis
 

Rick

Moderator
Staff member
IMO that would be true yes. However, don't look for thousands of an inch, more like tenths. In addition to shrinkage there are additional reasons to not cast any hotter than you currently are. A little frosty is fine, hurts nothing.

725 should be fine for most alloys. I cast almost everything at 700 except tiny bullets in large blocks and many HP's where I'll crank it up to about 720.
 

Ian

Notorious member
Most casting instructions are written by lawyers or fools. Yes, I'm calling a lot of historically respected individuals fools, because that's what they were/are.

I think you are misunderstanding one very basic thing: Alloy temperature is independent of mould temperature. You don't get frosty bullets at 725 because the pot is at 725, you get frosty bullets because you have some antimony in your mix and your MOULD is running hot enough to give you the frosty look. You can cast shiny bullets on the large side of what the mould is capable of making with 800 degree alloy or 650-degree alloy, because degree of "frost" is exclusively a feature of MOULD temperature, not the temperature of the alloy being poured into it. Mould temperature is a function of alloy temperature, casting pace, and sprue puddle size (and a few other things like mould mass vs. cavity volume, number of cavities, ambient temperature, mould material, and pour technique). Each alloy has different needs because of different melting point and fluidity temperature curve. Also, different alloys shrink differently as they cool.

A general guideline that covers ALL bullet casting needs is this: Set the pot to maintain 100 to 150 degrees above the point where the alloy first becomes fully liquid (i.e. when the last bit of slush melts and the temperature starts to rise, indicating the end of the phase change platau). Then, adjust your casting pace and sprue puddle size until your bullets have the appearance you desire. Some people run on the minimum end to get shiny bullet but barely get them fully filled, others run on the hot frosty end of the spectrum and their bullets are a little smaller, but very sharply filled out.
 

Ian

Notorious member
This may help, too.


The largest one was on the shiny one with the rounded bands, but barely larger than the one in the middle. The super-frosty one was about two tenths smaller than the one in the middle.
 
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jsizemore

Member
Like Rick said, you'll only get a few tenths of a thousandth increase in size casting cooler. And like Ian said, at about 100degF above liquidus is the minimum temp for the alloy. And when the mold temp is right your pace will be where you cut the sprue instead of tear it out with a slight nib if any. The lowest temp I cast at is 675 with 50/50 +2% tin. Until everything is up to temp I may have to keep a propane torch at the ready. Usually I'll start at 700 and then back down to 675. Having a PID sure helps and a pot with part of the heating element beside the bottom pour nozzle.
 

JonB

Halcyon member
SNIP...

Some people run on the minimum end to get shiny bullet but barely get them fully filled, others run on the hot frosty end of the spectrum and their bullets are a little smaller, but very sharply filled out.
I have one lyman mold 410610, that is a GC style, if I cast them with my normal technique, the GC shank is a bit large and GC's are difficult to seat. I've found with that mold, if I cast fast enough to make the mold hotter, so the bullets looks as Ian described as "run on the hot frosty end of the spectrum and their bullets are a little smaller", The GC's will seat much better and easier.
 
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KeithB

Resident Half Fast Machinist
I have monitored mold and pot temperature when casting. I get the most consistent size bullets when I make sure the mold is at the same temperature for every pour. I usually cast straight COWWs at about 725F pot temp and IIRC about 375F to 400F mold temp for a four cavity iron (Lyman) or a six cavity aluminum (Lee). If I use a hotplate to preheat the mold to the proper temperature and have the pot at the right temperature I can usually get perfect bullets the first pour. Smaller molds of different materials and different alloys and you have to adjust things but once you find the sweet spot in timing and fillout casting gets to be real fun.

Never tried varying the size via temperature since most of my molds cast at a usable size when the fillout is good. and the fillout is good because I control both the mold temp and the pot temp. I use a bottom pour almost exclusively and try to cast at the lowest possible temp but there is a lower limit where nozzle issues cause flow problems.

It's just the way I think, I would rather deal with the (mechanical) mold size issues and maybe the alloy composition (chemistry) issues and be consistent with the casting process parameters as a way to get bullets the right size.
 
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Rick

Moderator
Staff member
Another consideration to this question is the percentage of antimony (Sb) in the alloy. The higher the % of Sb the "less" shrinkage there will be.
 
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Rex

Active Member
Each of my moulds is a bit different from the other. I have some that will cast fairly cool, just below 700. I have one ornery old cuss, a 2 hole Ideal 357446 that has to get hot. Both the blocks and the melt. I pre-heat on a hot plate and run the melt above 750 to get all the corners and edges filled out proper. I suspect that a bit more tin would help also. The 446 mould has two lube groves so there are a lot of corners and edges to contend with. You just have to listen to each mould and do what it tells you to do then adjust the alloy to compensate for shrink, etc.
 

Rick

Moderator
Staff member
Adding more tin if your going to cast over 750 degrees will not do any good. Tin looses it's ability to reduce surface tension of the flowing alloy approaching 750 and the higher the melt temp past that the less effect tin will have. Fortunately a higher pot temp isn't needed, a quicker casting pace is. Most molds will cast their best around 400 degrees give or take. A pot temp of 700 degrees will easily keep the mold temp at a good casting temp of about 300 degrees less.

A 446 with two lube grooves? Is that a SAECO mold? I have SAECO #446 (200 gr PBSWC) and it is a one lube groove bullet plus a crimp groove. Just looked but didn't have a picture of it.
 
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Rex

Active Member
I should have said 357446 Ideal mould. Yes, 2 lube grooves. And by adding tin I believe the temp of the melt can be lowered.
Higher temp melt is needed for a tin starved alloy.
 

Ian

Notorious member
I should have said 357446 Ideal mould. Yes, 2 lube grooves. And by adding tin I believe the temp of the melt can be lowered.
Higher temp melt is needed for a tin starved alloy.

The whole point of my "Tale of three bullets" thread is that I haven't found this to be necessarily true. In the case if ornery moulds and "tin-starved" alloy, sometimes they respond better to ladle-casting and lots of extra metal being flooded across the sprue plate to keep the mould nice and hot, and often there is a certain fill rate that the cavities prefer based on factors such as lube groove depth, mould vent arrangement, and sprue hole size. Very few mould problems can't be solved with the proper application of heat to the mould itself.
 

Ian

Notorious member
One more observation about casting quality of moulds....Meehanite. Everyone loves Meehanite moulds...but why? There are many reasons but one I have never seen mentioned specifically is the effectiveness of the vent scribes or "lines". Most moulds made of free-machining brass alloy or any of the popular machinable aluminum alloys have one annoying characteristic: Metal from cutting the cavity smears into the vent grooves following the trailing edge of the cutting tool and effectively blocks half or sometimes all of the tiny, machined vent. Meehanite doesn't have that problem because it doesn't produce swarf when machined...it produces powder.

I've examined the edges of the cavities if most of my moulds with a 20X machinists loupe and the vent blockage, or lack thereof, is striking. The first time I took a mistreated Meehanite block set and re-bored the ruined cavity to a larger-caliber design, it was an "Ah-HA!" moment for me as I saw the grey powder stacking up on the cutting edge of the tool. Even fresh from the lathe, the vent lines were clean and open to the edges of the cavity, no "Lee-Menting" required.

Even MP, NOE, and Accurate's brass and aluminum moulds need a good going-over with a sharpened needle or scribe to clear the flashing from the vent lines in order to cast their best, and after scribing, a ball of 6-ought steel wool spun in the cavities by hand may be required to polish off the burrs from scribing.
 
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Bret4207

At the casting bench in the sky. RIP Bret.
I will agree with most of the above. The only qualification I'd make is that some of our predecessors weren't "fools" so much as what they theorized turned out to be something a bit different than they imagined. And, yes, there were some things taken as near gospel that proved out as poppycock over the decades, eg- harder must be better, smoke the mould, etc.

My observations as far as temp goes lines up pretty much with what is above, especially the part about the one or 2 oddball moulds that DEMAND they get their own brand of TLC before they cast good. I also think shrinkage, what there is, varies with your specific alloy, but it's all measured in tenths of thousandths.
 

quicksylver

Well-Known Member
YEP...LOTS OF ART AND VERY LITTLE SCIENCE,,,,IMHO IF IT WERE THE OTHER WAY AROUND WE'D ALL BE DOING THE SAME THING AND GETTING THE SAME RESULTS.. IN MY FAMILY EVERYONE TRIED TO MAKE ICE TEA LIKE MY AUNT.. BUT NO ONE COULD COME CLOSE...SHE HAD THE "NACK "OR "ART"JUST SAYING.. KEEP UP THE GOOD WORK.. SORRY TOO TIRED TO HIT THE CAP LOCK BUTTON
 

Rick

Moderator
Staff member
Much of the alloy part of casting isn't speculating, figuring and guessing. It is the science of metallurgy. The effects of Pb, Sb, Sn, As etc. are well known and used in the metals industry. The guessing part comes in knowing just what your scrap alloy is. Even so with a basics in metallurgy we can come pretty close to achieving our goals. Ignore the basic science and guessing is all you have.
 

Bret4207

At the casting bench in the sky. RIP Bret.
Good point Rick. When someone says "scrap lead" or "WW" it's a guess just what they have.
 

jsizemore

Member
Rick said:
" Ignore the basic science and guessing is all you have."

There's large numbers of folks that want nothing more than to be the monkey pulling the handle. Thinking takes too much time and makes their head hurt.