Air bubbles in cast balls?

[Salty]

I have a group of cast balls that are 2-3 gns lighter than the rest. I’ve noticed that there are several flyers from this group. A fellow MLer suggested that I may have an air bubble in those balls. So I cut one in half and sure enough there is a small air bubble which I have been told throws off the trajectory. I target shoot for accuracy so this is unacceptable.
What causes the air bubble to occur? What can I do to prevent it?

 

[Phil Coffins]

It come from how you fill the mold, I found that a bottom pore pot creates the most problems and using a dipper the least. My 525 grain bullets vary by one grain using the dipper but using the bottom pore the best I can get is always much greater. How you use the dipper also is very important.

 

[RobertIN]

Keep your mold hot and pour as smoothly as you can. This looks like a case of a cool spot on the mold to me. Lead solidified on the side/top and fell into the center creating the void.

 

[waksupi]

Molds do have preferences on how the lead is poured in, also temperature. Some want shiny balls and bullets. Not me, I want them to come out frosted. That tells me I am running the lead hot enough to more compleletly fill the mold.
If shooting serious competition or working up a load, I weigh the bullets to be sure I have the most consistant projectiles. Other than that, I don't worry about it, as none of us shoot well enough to know the difference when shooting off hand.

 

[Salty]

I heat the Lee mold on a hot plate before casting. I heat the pure lead to 760. I weigh every ball I plan to shoot.
I shoot for accuracy so I try to eliminate all variables off the rifle rest. Then practice offhand knowing flyers are the shooters fault. Have not been able to eliminate that variable yet.

 

[Salty]

Do you all find steel molds or aluminum molds cast better balls?

 

[fleener]

Do you assume that you do not have air bubbles in your other balls?

 

[Salty]

Of the same diameter and similar weight (174)I do assume they do have bubbles.
I’ve cast some that are same diameter but 177-178 grains. I’m assuming they probably do not have bubbles. I’m going to test them.

 

[aussie pete]

Of the same diameter and similar weight (174)I do assume they do have bubbles.
I’ve cast some that are same diameter but 177-178 grains. I’m assuming they probably do not have bubbles. I’m going to test them.

@Salty. I'll bring my metal casting experience to the table here, spending two decades as a dental technician in the army, casting chrome cobalt based partial denture frames, gold inlays and precious metal crowns etc. I spent five years teaching casting techniques at Dental School too. All that experience helped me for sure, when I took up casting my pure lead .

Porosity, (bubbles) in a casting, can be caused by four key issues: uneven flow into the mold that incorporates air (turbulence), the mold not hot enough causing the metal to solidify too quickly, the metal not hot enough causing the metal to solidify too quickly, and, not allowing enough metal to fill the sprue where the cooling metal can draw (suck) from as the casting cools (it is a combination of the poured molten metal and the reservoir in the sprue that creates good fill out). All metals shrink when they cool, naturally drawing from the pool in the molten button, the last part of any pour to solidify. I like a healthy chubby button when I cast - tells me that I have enough metal to feed and fill the mold.

A hint too - if a few postmortem cuts of your poured balls show a consistent single void (air bubble) just below where you cut the sprue off, then change your technique to pour consistently leaving a good amount of metal in the button. I admire guys and girls who cast with great results using gang molds - two or more. I can only concentrate on a single cavity!!

@RobertIN did mention another notion too, and he's dead right - an inclusion of a foreign object or spur of already cooled metal can fall into the mix causing porosity; though, this would be a rarer event than the four key areas that I explained above.

Hope this helps. Best of luck with your endeavours! Casting is fun huh?

Cheers, Pete

 

[Salty]

@Salty. I'll bring my metal casting experience to the table here, spending two decades as a dental technician in the army, casting chrome cobalt based partial denture frames, gold inlays and precious metal crowns etc. I spent five years teaching casting techniques at Dental School too. All that experience helped me for sure, when I took up casting my pure lead .

Porosity, (bubbles) in a casting, can be caused by four key issues: uneven flow into the mold that incorporates air (turbulence), the mold not hot enough causing the metal to solidify too quickly, the metal not hot enough causing the metal to solidify too quickly, and, not allowing enough metal to fill the sprue where the cooling metal can draw (suck) from as the casting cools (it is a combination of the poured molten metal and the reservoir in the sprue that creates good fill out). All metals shrink when they cool, naturally drawing from the pool in the molten button, the last part of any pour to solidify. I like a healthy chubby button when I cast - tells me that I have enough metal to feed and fill the mold.

A hint too - if a few postmortem cuts of your poured balls show a consistent single void (air bubble) just below where you cut the sprue off, then change your technique to pour consistently leaving a good amount of metal in the button. I admire guys and girls who cast with great results using gang molds - two or more. I can only concentrate on a single cavity!!

@RobertIN did mention another notion too, and he's dead right - an inclusion of a foreign object or spur of already cooled metal can fall into the mix causing porosity; though, this would be a rarer event than the four key areas that I explained above.

Hope this helps. Best of luck with your endeavours! Casting is fun huh?

Cheers, Pete

Pete, thank you for the educated reply. This is the first time I’ve actually read such a thorough analysis.
I will increase the temp of both the mold and the lead. I already pour a button on top so I think I have that covered. I’ve been using a mold with 6 cavities so the pour might not be an even consistent flow. I have a two cavity mold so I’ll see if they come out better with more attention to the flow.

BTW, do you feel that a Lee aluminum mold might not cast as well as a steel mold?

 

[fleener]

When I switched to a ladle vs bottom pour my casting improved. I leave the full laddle on the mold after it is full for a 5 second count. This has helped my casting weight consistency. I like the head pressure of the full ladle pushing on the lead in the mold.

I also use a hot plate to get my mold up to temp and my pot is Waage that keeps a fairly consistent temperature for the lead. All of these help with my casting. I typically cast conicals for long distance shooting and a good bullet is even more critical IMO.

You dont have to pre heat the mold and a steel mold will certainly take longer to get at a good temp.

Fleener

 

[Grenadier1758]

Lead shrinks as it cools. If the sprue is not completely filled with enough molten lead to flow back into the mold, then a void is formed under the sprue. The hot mold and the flow of lead from a dipper help to prevent those voids. Weighing the balls after casting will identify those balls or bullets with casting voids.

 

[#BPBReloadingFFL]

I drill out my bottom pour pot spouts as well as open up the hole in the top of the sprue plate.
Then when I cast to fill the mold I don't have any flow problems, and I flood the mold.

When you're shooting & every round ball you pick up or projectile. You're wondering if there's an inclusion in it or it might be a grain off. That's a definite distraction when you're shooting.

 

[oldramrod]

I use a bottom pour pot and after the mold is filled I hold it up against the pour spout for about ten seconds to allow hot lead to fill any shrinkage due to rapid cooling and have no problems.

 

[45man]

Some of you have it right. Let the mold take molten lead from the ladle and not the sprue. I cast very large bullets and if you watch inside the ladle you will see the lead go down, stop for a second and then go down again as the bullet shrinks. I gave up weighing them as they only vary by a few tenths of a gr. I will never use a bottom pour.

 

[HighUintas]

I tried the pressure pouring method with my ladle last time I cast balls. I noticed that I would get little strings and somewhat if a parting seam on lots of them due to the lead entering the vent channels.

Any ideas on how to deal with that?

 

[aussie pete]

Pete, thank you for the educated reply. This is the first time I’ve actually read such a thorough analysis.
I will increase the temp of both the mold and the lead. I already pour a button on top so I think I have that covered. I’ve been using a mold with 6 cavities so the pour might not be an even consistent flow. I have a two cavity mold so I’ll see if they come out better with more attention to the flow.

BTW, do you feel that a Lee aluminum mold might not cast as well as a steel mold?

@Salty, I feel that molds have their own idiosyncrasies, and some are made better than others. I like the lee mold for round balls, the Lyman for minies. I had a monstrous Pedersoli mold for minies - we have never become close friends. The important thing with molds is to keep them clean, and heat them to a good temperature to receive the molten metal.

A couple of posters here have made a valid observation too - the weight of the metal going into the mold helps for a complete fill-out. I'm reminded that most industrial casting machines use centrifugal force or vacuum to assist, but us simple folk don't need that luxury .

Pete