Ian
Notorious member
Here's my experience based on three pounds of metal and two casting sessions, so there's no way I've mastered this stuff but I did learn a few things which might help the next person.
The alloy in question is the new 87.25Bi/12%Sn/.75%Sb alloy we developed to mitigate brittleness as much as we can figure out at the moment.
First and foremost, this metal does NOT like a hot mould. It flows and fills out pretty well with a room-temperature mould, and the mould temperature I maintained during extended casting (30 minutes or more) was cool enough that I could hold the mould a half inch from my face and just barely feel the warmth. Hot enough to burn skin if touched, but not instant blisters like a 400⁰F mould will. If I had to guess I'd say around 200⁰.
A pot temperature of around 500 to 550⁰ is plenty. My .22 aluminum mould liked 550 but my steel 300-grain .45 mould liked 490.
Another very important thing I learned is just when to cut the sprue. This timing varied greatly between mould block material, mass, number of cavities, caliber, and bullet weight, but regardless is based one one critical factor which is an unique feature of this alloy: Some component (unsure which at this point) that is the last to freeze, takes an unusually long time to do so. If the sprue is cutt too soon, portions of it will smear a tin-like substance across the block faces, and if the bullets are dropped from the mould before this final constituent freezes, the bullets will "sweat" liquid metal from pinholes throughout the surface and be unusable.
Part of the reason the final liquid phase sweats out of the bullet is because the whole structure of the bullet is actually expanding as it cools, unlike our usual lead-based alloy which always shrinks. This expansion of the lead-free alloy upon freezing means that the sprue does not draw in as the bullet cools. It also makes the bullets a little mire difficult to free from the mould blocks, but not prohibitively so; a small rawhide mallet will be your friend.
So the timing of the sprue cut can be determined for each mould by simply watching the sprues as they cool. They will at first be shiny and glass-smooth like pure tin. In a few seconds they will appear to flash over and be solid.....but wait. After they mostly freeze, liquid will begin to sweat out in beads and stay molten for what can be a long time, from 30 seconds up to several minutes depending on how quickly the heat of the poured metal is dissipating. What I found best is to cut the sprues just as the little sweat beads become firm, but not completely solid. Cutting the sprues worked best for me to do with a gloved hand, but takes some effort. Deliberately forcing the sprue plate against the block faces as I cut them helps form a good bullet base with no nib and minimizes tear-out. If you cut the sprue too late, it simply breaks out a little crater; no big deal if it isn't too big, but there is a sweet spot to try and find fhat will give perfect bullet bases.
I found that filling the sprue well and forming a "blob" is all that is necessary. If too little sprue is poured, the bases can become rounded. Any more sprue volume than necessary only adds unwanted heat and extends the cooling time.
Freshly poured:
After the first "flash over", the sprue becomes very hard, but it isn't ready to cut yet:
Now the sweat beads have formed, and after a much longer interval of time have set up:
Better picture from a different mould:
The alloy in question is the new 87.25Bi/12%Sn/.75%Sb alloy we developed to mitigate brittleness as much as we can figure out at the moment.
First and foremost, this metal does NOT like a hot mould. It flows and fills out pretty well with a room-temperature mould, and the mould temperature I maintained during extended casting (30 minutes or more) was cool enough that I could hold the mould a half inch from my face and just barely feel the warmth. Hot enough to burn skin if touched, but not instant blisters like a 400⁰F mould will. If I had to guess I'd say around 200⁰.
A pot temperature of around 500 to 550⁰ is plenty. My .22 aluminum mould liked 550 but my steel 300-grain .45 mould liked 490.
Another very important thing I learned is just when to cut the sprue. This timing varied greatly between mould block material, mass, number of cavities, caliber, and bullet weight, but regardless is based one one critical factor which is an unique feature of this alloy: Some component (unsure which at this point) that is the last to freeze, takes an unusually long time to do so. If the sprue is cutt too soon, portions of it will smear a tin-like substance across the block faces, and if the bullets are dropped from the mould before this final constituent freezes, the bullets will "sweat" liquid metal from pinholes throughout the surface and be unusable.
Part of the reason the final liquid phase sweats out of the bullet is because the whole structure of the bullet is actually expanding as it cools, unlike our usual lead-based alloy which always shrinks. This expansion of the lead-free alloy upon freezing means that the sprue does not draw in as the bullet cools. It also makes the bullets a little mire difficult to free from the mould blocks, but not prohibitively so; a small rawhide mallet will be your friend.
So the timing of the sprue cut can be determined for each mould by simply watching the sprues as they cool. They will at first be shiny and glass-smooth like pure tin. In a few seconds they will appear to flash over and be solid.....but wait. After they mostly freeze, liquid will begin to sweat out in beads and stay molten for what can be a long time, from 30 seconds up to several minutes depending on how quickly the heat of the poured metal is dissipating. What I found best is to cut the sprues just as the little sweat beads become firm, but not completely solid. Cutting the sprues worked best for me to do with a gloved hand, but takes some effort. Deliberately forcing the sprue plate against the block faces as I cut them helps form a good bullet base with no nib and minimizes tear-out. If you cut the sprue too late, it simply breaks out a little crater; no big deal if it isn't too big, but there is a sweet spot to try and find fhat will give perfect bullet bases.
I found that filling the sprue well and forming a "blob" is all that is necessary. If too little sprue is poured, the bases can become rounded. Any more sprue volume than necessary only adds unwanted heat and extends the cooling time.
Freshly poured:
After the first "flash over", the sprue becomes very hard, but it isn't ready to cut yet:
Now the sweat beads have formed, and after a much longer interval of time have set up:
Better picture from a different mould: