trying to understand casting temperature

[happy7]

It seems to me that as I start casting with a mold, first the bullets have wrinkles, and so are rejects. Then as the mold warms I get perfectly formed shinny really beautiful bullets. Then, if I let the mold get warmer, (and it is hard to keep it from doing so) the sharp edges on the bands frost and when they frost they also round off, and are no longer sharp. Then if I continue to get warmer, the whole bullet frosts and everything gets sharp again.

So what is the frosting of the bands, which rounds them off? Why does the mold go from giving my well filled out bullets to rounded ones with an increase in temp? I thought hotter was supposed to mean better filled out.

Is this observation consistent with what all you experienced guys know, or am I observing it wrong? I tell you this thing of getting nice bullets and then getting hotter and getting rejects again, before getting frosted bullets had me confused. Maybe it still does. Any comments are appreciated.

It is easier to just get everything real hot and frosted and I seem to get less rejects that way I think, but the shinny ones sure are pretty, and on a cold day outside, it seems like sometimes I can't get my iron molds hot enough to frost the bullets.

[montana_charlie]

So what is the frosting of the bands, which rounds them off? Why does the mold go from giving my well filled out bullets to rounded ones with an increase in temp?
happy7,
I'm going to give my 'theory'...then others can tell you the real story.

If a mould is designed to throw a bullet of (say) .460" with a particular alloy (that's what you get with custom moulds) then it will do that in a particular temperature range.

I think that if you allow the temperature to climb so high that the bullet comes out completely frosted, it will not be .460".
I don't know for sure which way it will be 'off', but I think it will shrink more...and be 'small' when it cools.

When your mould is in the range where just the edges are frosting, I think they round off as the those 'hotspots' shrink too much during cooling.

To me, the 'fully-formed and shiney' stage is the right place to be...the range the mould was designed to operate in... and it can be maintained by turning the pot down, slowing your rythmn, or both.
CM

[beagle]

It's a hard thing to tell you. It's more of a "feel". You get wrinkles, then good fillout, then frosting and then whiskers. The temp between frosting and whiskers is where you get incomplete fillout.

A set of mould blocks will dissipate so much heat between casts dependent on the cubic inches. Small volume blocks get hot quicker than do large volume blocks. Large volume blocks having more surface area dissipate heat slightly more.

Supposedly the optimum mould block temp is 400 degrees according to the NRA.

Your objective is to attain and maintain this temp. It will vary dependent on cavity size and between moulds so this is not ahard and fast temp.

We're talking steel moulds here now. Attain casting temp which will give good fillout. Then, if frosting starts, you have to lower block temp somewhat. This can be done in several ways. Lower the therostat setting which drives you nuts, slow the casting tempo, induce a cooling method (fan or wet sponge) or lower the mould blocks resulting in a longer and thus cooling drop.

Probably the best method is through the use of a small fan directed on the mould combined with a slowing of the casting tempo.

You just have to play with it./beagle

[Idaho Sharpshooter]

me, I alternate casting with two moulds and get about a thirty second rhythm going at 725-750 degrees with 375 and smaller boolits. I find I need a little more heat for my 416 and 458 and 505 diameter stuff. The three biggies, 780gr .550's, 900gr .577's and 510gr HB12 gauge Lyman Sabot Shocker slugs need about 800 degrees to keep the moulds up to casting heat range.

That's just me, and about a dozen casts will get me 95% keepers.

Rich

[shotstring]

Back in the days when I was doing a whole lot of commercial casting everyday, I learned that "feel" has a lot to do with casting good boolits. It is here where casting becomes more "art" than "science". By doing enough repetitions of pouring the lead, waiting for the sprue to harden, break the mold open, etc, you get a feel for the timing. If the mold wasn't quite hot enough, I sped up my rhythm slightly.....if bullets started frosting, I slowed down just a tad. If I still couldn't get it right, I added another mold to the rotation to give me more control of speed and rhythm as well as more time for the mold to cool between castings.

Once I had found the best temperature to leave the pot at by trial and error, the only thing to adjust was the above mentioned technique. If you are patient and just play with it a bit, eventually you will find the sweet spot where everthing just works. :-D