Problems casting large 45/70 bullets

[4296]

I am a moderately experienced (ladle) caster with a low rejection rate-so I believe my casting technique is fairly sound. My biggest issue is frosted bullets which I see as a non issue as the bullets are well filled out and weight variance is minimal. My latest mold is the Lyman 457124 385 grain RN which will be shot in my Ruger No 1 in 45/70. Using my normal procedures the bullets are not to the same standard as others I have cast (all with iron molds). The forward (nose) section of the bullets look good-shiny and filled out- but the multiple lube groove area of bullet is very rough (almost porous)  and is not completely filled out. When sized only the front driving band is contacted by the push through sizing die. I believe the mold is plenty hot when I am casting, and this condition continued to occur throughout my casting session-whether the mold was very hot with quick casting or a cooler mold when allowed to cool somewhat between casts. I hope You can help me with this very frustrating problem. THANKS VERAL.

[stubshaft]

I had the same problem with a 457125.  The metal around the grooves in the mould is thinner and was creating a hot spot.  I had to lower the alloy temp and cast slower, with more time between pours.

[Veral]

  The simplest solution is to purchase an LBT mold.  I've designed all this trouble out of them.

  The problem is caused by the cavities being too close together for a metal with relitively slow thermal  conductivity. Cast iron molds are worse than steel, and steel about twice as slow as aluminum, if all three metals have the same space between cavities.  (I use aluminum and give them a lot more space, which combined with several other features I won't spend time explaining, is why LBT molds cast perfect bullets consistently.)
  If you cast in just one cavity at a time you will probably get perfect bullets.  -- When the second cavity is filled the thin web between cavities gets extremely hot, which prevents it from solidifying before the sprue.  IF the sprue solidifies first, you'll have undersize in the middle and shrink pockets in the bullet sides, especially in the area where the thin web is.

[Veral]

  I forgot to mention one more solution which works very well and allows use of all cavities in a mold.

  Run a stream of lead into the sprue holes for a few seconds until the bullet has time to harden.  Keeping a stream running into the sprue will supply lead to fill the shrinking.  If using a ladle, let the excess run back into the pot, or if a bottom pour, set an ingot mold under to catch the runoff.  Tip the mold slightly just as the sprue fills so it runs off of one corner.

[4296]

The mold in question is the Lyman 457124- a 385 grain RN- which is a single cavity mold. Will I have to cool the mold while casting? If so what are techniques used to cool iron mold blocks other than simply allowing  time for radiant cooling?

[Veral]

  The tiny blocks are a real problem with such a heavy bullet, as they don't absorb very much heat.  Running the sprue over will be your best option in turning out quality bullets.  Also, keep your lead temperature just a little over the melt point so you don't have so many calories going into the mold.

  Set up a small fan so you can hold the mold up close to cool the mold between casts.  Fans move the most air about 1/3 of the way in from the outer edge of the blade,  Air in the center moves much slower as it does at the perimeter.  In fact much of the air at the perimeters outer 1/3 actually swirls around and goes back into the back of the blades.  --  If you are wondering why I gave ALL that information you probably won't be lonesome!