Experiments in Inert Gas Casting by Steven F. Hurst, Tucker, GA was published in the #163 May/June 2003 issue of the Fouling Shot.
The research lab I was working in had an assortment of controlled atmosphere hoods for working with materials sensitive to air and moisture. Having been a bullet caster since high school, I was well aware of the effects of dross on bullet casting. I wanted to see what life was like without this bother.
We had three choices of atmosphere: argon, very inert and very expensive; nitrogen, inert and much cheaper; and carbon dioxide, not used much because it reacted with many of our compounds, but it was dirt-cheap. I determined from reference sources all three would work for bullet casting, as there was no chemical reaction between lead, tin or antimony and any of the three gasses at casting temperatures. One weekend I brought in my casting equipment. The hood was set up for nitrogen, so I passed the electric pot, ladle and moulds through the airlock.
For those of you who have not had the pleasure of casting in an inert gas atmosphere, it is a unique experience. Once you have skimmed off the initial dross, the pot stays clean and bright forever. This was ladle caster’s heaven! A perfectly clean pot, with no fluxing ever. Sprues were uncommonly bright, with no oxidation; I put them right back in the pot and the surface stayed clean and clear. Even the moulds were safe from rust, you could have left them sit for 1000 years, or at least until the gas ran out.
So now I do all my casting in my own hood under inert gas protection, right? Wrong! You see there is a small problem. You have to work with your arms and hands in gloves. Not just gloves, but thick rubber gloves attached to the wall of the hood. The gloves are about ten sizes too big so everyone can easily get their hands into them. You stick your arms in and you feel like the guy that was inside Robby the Robot. Setting up experiments in a hood is bad enough, trying to cast bullets is another mat-ter, manual dexterity is nonexistent, the gloves are hot and your arms sweat. After 30 minutes, this is about as comfortable as a root canal.
Ok, there had to be a better way, I considered a home cabinet, with Mylar lining so it would be oxygen proof, petal ports so I could put bare arms in, and positive pressure so 99.9% of the air was excluded. A card-board mock-up showed this was not the way to go, finger dexterity was good, but very limited arm mobility, just like the cabinet, made casting difficult. Just try to cast with your elbows held in one place, you will get the picture. I considered other options, like filling the garage with inert gas and wearing a scuba regulator with an outside exhaust, but this was a lot of work and I am a lazy guy.
The research lab I was working in had an assortment of controlled atmosphere hoods for working with materials sensitive to air and moisture. Having been a bullet caster since high school, I was well aware of the effects of dross on bullet casting. I wanted to see what life was like without this bother.
We had three choices of atmosphere: argon, very inert and very expensive; nitrogen, inert and much cheaper; and carbon dioxide, not used much because it reacted with many of our compounds, but it was dirt-cheap. I determined from reference sources all three would work for bullet casting, as there was no chemical reaction between lead, tin or antimony and any of the three gasses at casting temperatures. One weekend I brought in my casting equipment. The hood was set up for nitrogen, so I passed the electric pot, ladle and moulds through the airlock.
For those of you who have not had the pleasure of casting in an inert gas atmosphere, it is a unique experience. Once you have skimmed off the initial dross, the pot stays clean and bright forever. This was ladle caster’s heaven! A perfectly clean pot, with no fluxing ever. Sprues were uncommonly bright, with no oxidation; I put them right back in the pot and the surface stayed clean and clear. Even the moulds were safe from rust, you could have left them sit for 1000 years, or at least until the gas ran out.
So now I do all my casting in my own hood under inert gas protection, right? Wrong! You see there is a small problem. You have to work with your arms and hands in gloves. Not just gloves, but thick rubber gloves attached to the wall of the hood. The gloves are about ten sizes too big so everyone can easily get their hands into them. You stick your arms in and you feel like the guy that was inside Robby the Robot. Setting up experiments in a hood is bad enough, trying to cast bullets is another mat-ter, manual dexterity is nonexistent, the gloves are hot and your arms sweat. After 30 minutes, this is about as comfortable as a root canal.
Ok, there had to be a better way, I considered a home cabinet, with Mylar lining so it would be oxygen proof, petal ports so I could put bare arms in, and positive pressure so 99.9% of the air was excluded. A card-board mock-up showed this was not the way to go, finger dexterity was good, but very limited arm mobility, just like the cabinet, made casting difficult. Just try to cast with your elbows held in one place, you will get the picture. I considered other options, like filling the garage with inert gas and wearing a scuba regulator with an outside exhaust, but this was a lot of work and I am a lazy guy.
It seemed like fill-out was very easy in the hood, but the small sample of bullets and the lack of dexterity made statistical evaluation impossible. I decided to see if I could provide inert gas protection at the casting table. This idea proved to be more tractable. I decided to use carbon dioxide, since I could buy dry ice and not have to rent a tank. Ladle casting was impossible,
so I purchased a Lee bottom pour pot. The gas generator was a gallon can on a hot plate. The pressure regulation was a bubble tank, giving zero to 12 inches of water head pressure. (Twelve inches of water is about 0.5 psi.) Two to four inches of water pres-sure provided a smooth flow of gas. One-eighth inch copper tubes delivered the gas to the pot.
One tube supplied gas to the top of the pot, which was sealed with aluminum foil. One tube sup-plied a fitting I made for the spout, surrounding the stream of metal with a column of gas. This is just like the nozzle on a Metal Inert Gas (MIG) welder that covers the arc with an inert gas shield. Another tube to the rear of the spout provided a flow of gas to fill the mould cavity just before fill. This is not perfect, but carbon dioxide is heavier than air so it tends to stay in the cavities.
I selected two Lyman designs. The 311467U is a .30 caliber Loverin design with fine multiple grooves and the 358093 is a 125 grain .38 caliber spire point with thin deep grooves and a thin wadcutter front band. Both bullets are tough to cast. Ready at last for casting perfection, I proceed to make bullets using linotype alloy. I ran the test alternating with the gas on and the gas off and threw the bullets onto separate towels.
Yes, there was less dross, but oxygen did get in, so there was some dross formation. The spout did not plug up as often, but with the gas shield installed, the spout was harder to clean. The gas system was a bother to adjust, but if you had carbon dioxide in a tank it would be simple. Overall convenience was good, just like normal casting. I gathered the bullets for quality control and defect analysis.
Statistical analysis of the number of defects showed no difference with the gas on vs. gas off. Ouch! There it was in the numbers; all that work for nothing. The experiment would have ended right there if I had not decided to try some range lead plus tin with poor casting quality. Behold, a statistically significant 7% reduction in rejects! This was more like it; the inert gas protection was doing something good.
One tube supplied gas to the top of the pot, which was sealed with aluminum foil. One tube sup-plied a fitting I made for the spout, surrounding the stream of metal with a column of gas. This is just like the nozzle on a Metal Inert Gas (MIG) welder that covers the arc with an inert gas shield. Another tube to the rear of the spout provided a flow of gas to fill the mould cavity just before fill. This is not perfect, but carbon dioxide is heavier than air so it tends to stay in the cavities.
I selected two Lyman designs. The 311467U is a .30 caliber Loverin design with fine multiple grooves and the 358093 is a 125 grain .38 caliber spire point with thin deep grooves and a thin wadcutter front band. Both bullets are tough to cast. Ready at last for casting perfection, I proceed to make bullets using linotype alloy. I ran the test alternating with the gas on and the gas off and threw the bullets onto separate towels.
Yes, there was less dross, but oxygen did get in, so there was some dross formation. The spout did not plug up as often, but with the gas shield installed, the spout was harder to clean. The gas system was a bother to adjust, but if you had carbon dioxide in a tank it would be simple. Overall convenience was good, just like normal casting. I gathered the bullets for quality control and defect analysis.
Statistical analysis of the number of defects showed no difference with the gas on vs. gas off. Ouch! There it was in the numbers; all that work for nothing. The experiment would have ended right there if I had not decided to try some range lead plus tin with poor casting quality. Behold, a statistically significant 7% reduction in rejects! This was more like it; the inert gas protection was doing something good.
So now I do all my casting with a state of the art, inert gas injection casting rig, right? Wrong again; it is too much trouble for too little return. The range alloy I used produced 40% rejects using these moulds with no inert gas protection. Yes, inert gas did reduce rejects, but 40% minus 7% improvement still leaves 33% rejects. This is not something to write home to Mom about. This means that for every three hours spent casting, you waste one. So what do I do?
First: I keep the pot temperature low. Chemists know for every 15 degrees F increase in temperature, chemical reactions double in speed. The difference between 625° F and 700° F is 32 times the oxidation and dross. In reality, the increase it is not this high, because oxidation becomes limited by the oxygen diffusion rate.
First: I keep the pot temperature low. Chemists know for every 15 degrees F increase in temperature, chemical reactions double in speed. The difference between 625° F and 700° F is 32 times the oxidation and dross. In reality, the increase it is not this high, because oxidation becomes limited by the oxygen diffusion rate.
Examples of difficult to cast styles (top) Lyman 358093 and 311467U and easy to cast designs (bottom) SAECO #377 and RCBS 30-150-FN Above 750° F the protection of the tin oxide on the metal surface starts to break down and oxidation becomes really rapid. I cast linotype at 550° F, other alloys at 600° to 650° F. If I have to exceed 700° F, either the alloy is bad or I am doing something wrong. Please note these temperatures are measured with a thermometer, not the pot thermostat.
Second: Ladle casting constantly stirs air into the alloy, making oxidation more rapid. Ladle casting at high temperatures is double trouble, if the oxidation rate is limited by diffusion, stirring the pot gives it a big boost. I now use a bottom pour pot almost exclusively. When I use a ladle, I watch the temperature closely.
Third: There is no use in fluxing a bottom pour pot every few minutes. The ingots and sprues you are adding should be clean, why are you fluxing? All you are doing is stirring in more oxygen. The dross floating on the surface does no harm - for-get it. When I shut down I flux heavily to clean the walls and empty the pot to clean the bot-tom. If you are adding scrap directly to your pot while casting, I cannot help you; you have not yet learned one of the basic rules of how to cast good bullets.
Fourth: You can cast good bullets at low pot temperatures if your mould is clean and the mold temperature is high. This means a high casting rate. It also may require supplemental heat if you are casting a light-weight bullet. You get fewer rejects by setting the mold on a small hotplate to “cool” than by casting with 800° F alloy.
Fifth: Avoid complicated bullet designs. Avoid sharp corners and thin driving bands. If you want a plain base bullet, buy a bevel base design if possible. Unless you need sharp wadcutter shoulders, avoid them. Mould makers are machinists, and most machinists take great pride in sharp, 90-degree corners. Some mould makers understand sharp corners are unnecessary. A mould designed with easy casting in mind is a joy. The easy casting of a well-executed simple design lasts a lifetime.
Sixth: Use quality alloys. If you purchase enough metal from the scrap yard, you will eventually get a bad batch of alloy. Some “typemetal” I purchased from a local scrap yard was so contaminated it destroyed casting quality when only 10% was added to good alloy; there was no way to make bullets out of it. It is important to check a small amount of each batch of scrap before you waste a lot of metal. If adding 1 or 2% tin to a small sample does not result in good casting, take the batch back to the junkyard and sell it. Life is too short to waste casting with poor alloy.
Now if NASA will build a Moon Base, and I can get a ticket, then I can try vacuum casting…
Second: Ladle casting constantly stirs air into the alloy, making oxidation more rapid. Ladle casting at high temperatures is double trouble, if the oxidation rate is limited by diffusion, stirring the pot gives it a big boost. I now use a bottom pour pot almost exclusively. When I use a ladle, I watch the temperature closely.
Third: There is no use in fluxing a bottom pour pot every few minutes. The ingots and sprues you are adding should be clean, why are you fluxing? All you are doing is stirring in more oxygen. The dross floating on the surface does no harm - for-get it. When I shut down I flux heavily to clean the walls and empty the pot to clean the bot-tom. If you are adding scrap directly to your pot while casting, I cannot help you; you have not yet learned one of the basic rules of how to cast good bullets.
Fourth: You can cast good bullets at low pot temperatures if your mould is clean and the mold temperature is high. This means a high casting rate. It also may require supplemental heat if you are casting a light-weight bullet. You get fewer rejects by setting the mold on a small hotplate to “cool” than by casting with 800° F alloy.
Fifth: Avoid complicated bullet designs. Avoid sharp corners and thin driving bands. If you want a plain base bullet, buy a bevel base design if possible. Unless you need sharp wadcutter shoulders, avoid them. Mould makers are machinists, and most machinists take great pride in sharp, 90-degree corners. Some mould makers understand sharp corners are unnecessary. A mould designed with easy casting in mind is a joy. The easy casting of a well-executed simple design lasts a lifetime.
Sixth: Use quality alloys. If you purchase enough metal from the scrap yard, you will eventually get a bad batch of alloy. Some “typemetal” I purchased from a local scrap yard was so contaminated it destroyed casting quality when only 10% was added to good alloy; there was no way to make bullets out of it. It is important to check a small amount of each batch of scrap before you waste a lot of metal. If adding 1 or 2% tin to a small sample does not result in good casting, take the batch back to the junkyard and sell it. Life is too short to waste casting with poor alloy.
Now if NASA will build a Moon Base, and I can get a ticket, then I can try vacuum casting…
