10 December 2012
Of course, the thermodynamics of casting is a juggling of temperatures and times to get a method working satisfactorily for each individual. You mention your cooling with a damp pad in a jar lid to cool your cutter plate. I have tried that also, and have also tried a small fan. The puddle and plate cooling does have a drawback for my juggling thermodynamics that works for me.
What sticks out to me is that when I rapid cool like that, I get what appears like more Antimony migrating and resting in patterns on the surface of my sprue puddles. The Antimony on the surfaces of the puddles increases the effort needed for the sprue metal to re-alloy with the metal in the pot, no matter when I return the sprue metal to the pot. This makes successful fluxing a bigger job for me.
My alternative to a cooling like you use is a different timing schedule that leaves my sprue puddles more clear on the surface without the Antimony patterning. And, I don't re-flux for a whole potful when bottom pouring .
I get the 6 cavity mold up to to casting temperature on the pot top and then micro torch the cutter plate evenly about 10 seconds so I won't have cold short pouring or too rapid solidification of the sprue puddle on the first pour. I pour a continuous sprue filling all 6 cavities. I watch the sprue metal carefully and define a timing that I can easily cut sprue leaving a smooth cut without any smear or chinking out of metal from the bullet bases.
Now is the significant change in my timing from yours. I leave the mold closed with the cutter open and select a timing to cool the metal in the cavities. This timing allows 2 things for me. The puddle metal cools slower and doesn't pattern up with Antimony, additionally the the bullet metal cools longer for an easier release when I finally open the mold to drop bullets.
This whole scenario works for me, yields nicely filled bullets and sprues without antimony patterning so that I can drop each sprue directly back into the pot immediately before the next pour. What amazes me casting this way is that my rhythm actually does maintain sufficient thermodynamics to just keep going and cast a whole potful of about 18 pounds of metal without stopping!!!!
Then I take a break and start another whole new potful if I want.
I first got this working with my RanchDog 460-350 FNGC, 6 cavity mold and now I can also do it with slightly less open plate cooling time with my RanchDog 311-165 FNGC 6 cavity mold. Now I have way too many bullets and share them with my grandson! I have been casting over 50 years but only 2 years ago just began with 6 cavity molds and now I want more.
My pot temperature stabilizes about 620-635 F. after about a 25 minute melt of a new potful of #2 clone alloy with the mold sitting on top and the dial set at 6 and a half on my Lee 4-20. I flux once, ignore the thermometer, and cast till I run out of easy metal flow. This usually is when there is about 2 pounds left in the pot....a lot of bullets!
My first 2 potfuls with each new mold was all about finding the temperature and rhythm, it is working now.