Air pockets and voids in bullets

[destrux]

The last two casting sessions I've done I've been having a terrible time with air pockets or voids inside the base of the bullet. Now, I don't know if they're gas bubbles or just voids, but when I drop the bullet the side of the base shrinks and sort of caves in on itself. Sometimes you can actually watch it happen. It looks like someone whacked the side of the bullet with a tiny hammer and dented it. I culled all the dented ones, which was about half of them, and shot the normal looking ones (that appeared to be completely filled out) only to find that I'm getting fliers that are all over the place (some were 12 inches away from the main group at 100yd). I collected some of the shot bullets and found the sides of the bases caved in, so there were voids in a lot of those too even though they looked fine. Last time I cast a large batch with this mold I had fill out problems with the base, but culling the bad looking ones left me with good shooting bullets.

Any ideas on what can cause these voids? Something in the melt?

I tried different casting methods, I tried pressure casting and pouring a generous sprue and neither method solved it. I'm casting at 650 degrees and using straight clip on WW lead that I smelted myself in a separate pot. The mold is a Lee 230gr "blackout" mold which I've previously gone over to make sure the vent lines are open, I'm lubing the sprue plate with a q-tip using a very light coat of two stroke synthetic oil.

I'm considering just giving up on this mold and buying an aluminum NOE 247gr mold but I don't know if it will give me the same issues.

[mongoose33]

My first guess would be your mold is not hot enough. I'd also think you may want to go hotter with your melt. You might also consider adding just a bit of tin to help w/ fill-out.

One thing I noted in my casting is that the order in which I fill the cavities matters. In a six-gang mold, the cavities nearest the handles cool faster (due to the handles drawing off heat) than the ones furthest. Solution was to fill starting at the ones on the handle-end and moving outward. This keeps those close cavities in the heat longer than if I go the other direction, and it solved my fill-out problems.

[Litl Red 3991]

........., I'm lubing the sprue plate with a q-tip using a very light coat of two stroke synthetic oil.
.......

What could be the source of the gas you feel is causing those voids?

[Toymaker]

I can't speak regarding bullets, but with roundball it is an epic problem. Some years ago there was an article in the NMLRA MuzzleBlasts magazine about this. I was unsure, so I used a sharp knife and rolled a roundball under it until it was cut in half. Sure enough, there was a void. It is a true void, not a gas bubble. According to the article it is caused when the hot lead cools and contracts. The article recommended a casting temperature between 700 and 750°F; good pre-heating of the mold; holding the mold TIGHT against the ladle spout or bottom pour spout longer than needed to fill the mold (practice and experiment) [pressure casting?]; and getting a good button. Speed in casting was also mentioned so the mold wouldn't cool off. For me it happened with 54 caliber ball more than 40 or 45 caliber. I weigh all my roundball, 5 grain variance. Using their recommendations I cut my rejection rate from 30% to about 10%. I use a single cavity mold. I wear welders gloves because, using a bottom pour pot, you can get a geyser of molten lead if the mold isn't tight against the spout or there's a bit of lead keeping it from being TIGHT.
When casting bullets (.451/535 grain, .459/405 & 500 grain) I do much the same but ladle cast only and increased my temperature to 800°F and get more consistent results and weights. Hope this helps.

[gwpercle]

More heat. start out around 750, when boolits start getting frosty back down to 725.
That "light coat" with a Q-Tip is contaminating your bases. Give it a coat about one molecule thick .
I never could get the two-stroke coat thin enough not to contaminate so I started using Liquid Wrench Dry lube on a Q-Tip . You can coat the underside of the plate, the top of the block and not get any contamination.
I started using Permatex Anti-Seize Lubricant to lube the rest of the mould because being thicker it doesn't migrate and contaminate the boolits as easily as two stroke oils . It takes so little to lube it's real hard not to over do it. Don't give up , casting has a learning curve that just takes practice.
Gary

[stubert]

try lubing with dielectric silicone grease.

[CJR]

Destrux,

On most of the sprue plates on my gang moulds, I have cut a groove connecting the pour holes. Years ago, the NRA recommended this. Likewise, I flux my newly melted lead alloys a lot if I haven't previously smelted/flux cleaned it. This removes most of the metallic inclusions in the alloy. Then when CB casting, I run my lead temperature a little hot so that the alloy stays molten a little longer in the sprue groove. This allows the alloy to gas off some, the CBs to fill-out better,and the final CB weight spread is smaller. Essentially, my casting pace slows down a little, but the quality of my CBs improves. LBT advertises their sprue plates as minimizing gas pockets.

Hope this helps.

Best regards,

CJR

[destrux]

If I go hotter (around 700) they seem to get worse, with nearly every bullet having a void. At 650 I have a molten puddle on the top of the sprue plate for about 3-4 seconds. Then I wait another second or two to cut the sprue, and drop them on a cloth.

CJR what sort of flux are you using? I use a pine 1/2" dowel to stir my lead pot instead of fluxing. It was a tip I heard on here. Maybe that's not enough to flux it? I generally get about a half teaspoon of grey crud off the top each time I sit down for a ~150 bullet casting session. I stir the pot with the dowel and scrape around the sides to get the impurities that stick to the inside of the pot.

I saw NOE recommends graphite powder to lube their aluminum molds so I guess I'll try that instead of the two stroke oil to see if it helps.

Right now I need to wait on another Lee mold because I just tried to make this one plain base on the lathe and screwed it up.

[Echo]

My dos centavos - more heat, and a sumptuous sprue puddle. I'm guessing the alloy is shrinking during solidification, and doesn't have enough excess (in the form of a sumptuous sprue) to draw from. Adding a little (2%) Sn is a good idea, too...

[Wayne Smith]

My dos centavos - more heat, and a sumptuous sprue puddle. I'm guessing the alloy is shrinking during solidification, and doesn't have enough excess (in the form of a sumptuous sprue) to draw from. Adding a little (2%) Sn is a good idea, too...

What he said. All alloy shrinks and draws from the sprue to fill in the shrinking. If the mold/alloy is too cool the sprue hardens too fast for this to happen, if the sprue is too small there is not enough alloy.

[44man]

Exactly why I ladle pour but 6 cavity would be a problem. I hold the ladle tight until the shrinking boolit pulls all it needs from the molten lead in the ladle. The sprue will freeze first and deplete the mold. 3 to 4 seconds is too soon. More heat.
Pure lead rb's need 800.

[bangerjim]

More heat in your pot.....HOT mold (heated to full casting temp on a hotplate).......more Sn. I always use 2% Sn and never have any voids.

The only time I ever saw that was with a cold mold and under-temp lead in the pot.

To REDUCE Sn back into your casting pot, use beeswax to minimize the garbage from burned sawdust and to get mirror shiny surface almost like magic.

Hope you find your solution.

[fredj338]

Could be couple of things. If you are using a lube for the spru plate, if it's too thick, you'll get voides on the base. Make sure the spru is large enough to allow some shrinking. The other thing could be venting. I had one iron mold that always gave voids. I lightly beveled the top of the molds where they come together. It creates a tiny vent under the spru plate. Now the mold produces perfect bases. Something to try. You can always try pressure casting, holding the spout tight to the spru plate for the fill, then over flow for the spru. This also works for me on certain molds where I struggle with fill out, like some of my HP molds.

[bangerjim]

Glad you brought that up! Pressure casting will/should eliminate any voids at all!

I also have a couple molds that give good drops only when p-cast.

With a good level of Pb in your pot you will have a decent head pressure forcing any air/gas out of your cavities.

banger

[mongoose33]

If you do the pour correctly, you can see a small dimple form in the sprue puddle as the shrinking boolit in the mold draws more alloy in. This only happens if it's hot enough--if the sprue hardens too fast (as others have noted), there's nothing to draw from).

To me, one of the signs it's time to cut the sprue is that dimple having formed.

[CJR]

destrux,

I've been using Marvelux for awhile and get it from Brownells. Also, if you ladle cast get in the habit of always stirring the pot before filling the mould.

Best regards,

CJR

[Toymaker]

HOT mold, fast casting. It would be ideal if the bullet solidified from the center core to the outside, but it ain't gonna happen. The outer edges are going to solidify first. You want extra molten lead available for the shrinking center core to draw from. You want the core to solidify before the button. A large button, staying liquid so the core can draw lead while it shrinks, takes practice and experimentation. Keeping the lead HOT so the ladle, pressed against the sprue plate, has free flowing molten lead available to the core is easier. Using a bottom pour is the same - HOT lead so the pour hole doesn't freeze up, mold pressed tight, wait, wait and wait for it, them back off and make a button.
It takes time to develop the knack, experiment, practice, there's some great people and information here, but we can only provide guidance. You have to do it. You're on the learning curve. Here's a promise - in six months you'll be telling someone else how to overcome this.

[williamwaco]

Classic symptom of inadequate sprue. Your sprue puddle should be as large as a quarter and half again as thick. (For each cavity) It should take three seconds minimum to solidify. This means that after filling the last cavity you should count two seconds then tilt the mold to the side and the sprue should pour off like water. If you can't, your mold is not hot enough.

when you reach this temperature, resume your normal cadence.

[44man]

I have never called it pressure casting, only lead feed. Pressure in itself does not work except to push out air. I cast two boolits with one ladle full. If half a ladle made a bad boolit I would eat it. It is bunk to keep a pot at a certain level.
You see what others say, FEED YOUR LAWN!

[popper]

That is a long skinny boolit so you need to be >700F. Maybe higher if a 6C mould. Your sprue is hardening too fast and probably (if pour rate is fast or pressure casting) a big cool sprue will stop venting at the base/plate. Drag a knife edge across the inside top of the blocks - careful around the cavities - just 'break' the edge. I use synthetic 2 cycle oil, apply with q-tip when the mould is hot then wipe with a paper towel till you can't see it anymore. Some tin may help. IIRC that is the BT mould, any voids in the base will really cause flyers even though the pressure is low.