Mould temp and cast size?

Just out of interest Al what was the alloy temperature?

Al,

That’s really good info, and very helpful too. Thanks for posting it.

Dave

I have noticed some of my bullets being shiney at the nose but frosted near the base. Is this possibly a function of the sprue plate being too hot or casting too fast in general?

I had a certain, other, Utah-based custom mold maker cut me a 1-cavity brass mold for my own .44 cal. 437 gr bore-rider bullet.
It took failure of three molds to produce the desired diameters, for him to realize that all of his mold expansion formulas had only been tested with small caliber bullets of moderate or short lengths.  He had never actually tested large, long, heavy bullets; and that oversight bit him.  He had alloy shrinkage nailed down for a wide range of alloys and temperatures, but forgot to test brass molds with big bullets.

But, even with the final mold that he cut after some more testing and the development of a formula specifically for large bullets in brass molds, he missed the target by 0.0015″ (out of tolerance); because he also never factored in the stresses in the brass for large bullets.  Straight out of the lathe, the mold blocks were warped.  (Something he, to this day, blames on customers “overheating” his brass molds…)

The mold gets the job done, but I have to run it hot (lightly frosting bullets) to keep the bore-riding nose at a usable diameter. 

And, I have almost half a dozen different variations of NOE 314316 molds.  They don’t have problems, but I use temperature to my advantage with them.  One of the molds has a GCFP cavity that drops at only 0.313″.  That’s perfectly usable in my revolvers, but if I run the mold hot (lightly frosted bases), the bullets don’t have to be sized down quite as far for use in .30-30 plinking loads.
And, there’s another 314316 that drops on the high side.  If I run the mold as cold as possible, while still getting good fill-out, I can get the bullets to drop at 0.3163″ for use as-cast in my .315″-grooved Mosin-Nagant.

Bottom line…
There are a lot of factors that come into play.  For some bullets, mold temperature doesn’t affect the finished product by enough to matter to most casters.  But when you have critical dimensions, achieving the proper mold temperature can be the difference between a usable bullet and a bunch of wasted time.
And, knowing how your mold reacts to different temperatures can let you ‘stretch’ it into doing things it otherwise may not be capable of achieving.

I’ve got to figure the cavities are shrinking as the metal of the mold heats up and expands.

Which leads me to guess that the largest possible diameters would come from cooler molds (larger cavities) and hotter pots (best fillout due to longer period to returning to solid)

I’ve got to figure the cavities are shrinking as the metal of the mold heats up and expands.

Which leads me to guess that the largest possible diameters would come from cooler molds (larger cavities) and hotter pots (best fillout due to longer period to returning to solid)

Back in reply #6  I posted the testing result and temp from aluminum molds.  I also tested iron (ran slightly cooler), but no brass molds.  For both alum and iron, the largest bullets come from a mold that is just shy of frosting.  The pot temp didn’t seem to matter much except that it, along with cadence, helps keep the mold temp even.

The bullets actually start with a larger diameter along the seam compared to across it, and become more round as they heat up.  Once you cross that point where they drop frosted they start shrinking again.

I would assume brass would work the same way.  But that’s pure speculation on my part.

Below are charts of the mold temp and the diameter for the aluminum molds.  This is with BHN 8, which requires a hotter mold to cast full size bullets, so you may need a cooler mold.  I found that 400-410 was a good temp for my casting with my alloy:

The diameters measured on the seam (big is best):

The diameters measured across the seam (big is best):

The amount “out of round” (small is best):

Those measurements came from a Lee 6 cavity.  The temps were different with NOE, Lyman and RCBS but the overall end result was the same.  The bullet gets more round and drops at it’s largest size with the mold temp just shy of frosting. 

I did find that casting at a full cadence worked best with slight lower temp, again just shy of frosting.  It’s the difference of doing a test vs how things work in production mode.