New Hollow Base Mold Being a Challenge to Cast

[Loren]

I have the new mold from MP. Nice tool! .457 257gr, casting in pure lead for a big bore airgun.

I have the mold running hot (350 -400deg) and using a Lyman Mag 25 bottom pour furnace and running a quick cadence to keep the mold nice and hot. Leaving a large sprue puddle and keeping it fluid for a LONG time, but I am consistently getting a shrinkage void above the hollow base core pin (see marked up photo). I even torch heated the pins to try and get this to stop but had not luck. Pressure cast a few and still getting that stinking void, but start getting flash around the base pin, so I know I'm putting the lead to it.

Any ideas? Tricks?

 

[S Mac]

Not familiar with the Lyman furnace, can you slow the flow a bit? I think maybe it's filling the mould too fast, trapping air above the pin.

 

[Ian]

I agree with SMac. Slow down the pour. The lead gushes up the sides of the pins and surges over the tip, trapping an air bubble. Also, pure lead shrinks more than alloyed lead as it cools so it can have a tendency to "core", but I don't think that's your main problem.

 

[Loren]

I did slow the pour flow until the point I wasn't getting good fill out on the exterior. I will try slowing the flow again and see if I can find a sweet spot.
Here are a few photos of the bullet bases and what they are typically looking like. The hole is .125-188 deep It probably is trapping a cone of air as the lead is flowing up the sides of the base pin and closing in on the nose being a nose pour.

Thanks I appreciate the input!

 

[S Mac]

I am a ladle caster, it's easy enough to regulate.

 

[Ian]

Pins might be too cold too. Try three full pour cycles a minute with 800⁰ lead for five minutes and see if that helps. You've done everything right so far but just doesn't seem to be quite enough. Mould should run 420-440⁰ with pure lead. As a last ditch try adding 2% tin to the lead, this will improve the fluidity of the metal and also turn the freeze point into a range so the metal can still move as it is solidifying instead of going solid instantly.

 

[fiver]

3?
I'd run full tilt fill open close fill until I seen smoke coming out of it.

 

[Bill]

My 500 gr lee was doing the same thing, the only fix was to keep pouring on the coals, I think I had the dial maxed out

Bill

 

[Dusty Bannister]

How much distance between the nozzle and the sprue plate? Are you pouring directly down the hole or splash off the side into the hole? Pure lead in a normal mold would be about 800 degrees, but perhaps the HB mold likes even more heat to get correct fill. The sides of the casting should be smooth and well filled out, and of the three I see one is badly formed suggesting cold mold. The lip of the skirt looks pretty ragged, but that might just be a bit of flashing. That appears to be a brass mold, and I have no experience with those. There are some comments about mold break in, but that would not cause your void. Are you sure it is a shrinkage void? It appears very smooth on the inside surface as if you have oil off gassing from the pin lube. With an extended casting session that should no longer be a factor as it would burn off eventually.

 

[Ian]

3?
I'd run full tilt fill open close fill until I seen smoke coming out of it.

Ideally....but have you cast with one of those 4-C MP contraptions? I can manage four cycles a minute if I really hustle and use cup point pins. Longer pins and taking some care not to slam things around gets me 3-3.5.

 

[fiver]

I wouldn't be looking for good bullets for the first 2 maybe 3 sessions anyway.
just burning everything in and getting a patina going on the mold faces.

the pins themselves might be outgassing something like those non stick muffin pans do when guys use them for ingot molds.

 

[Rally]

How did you clean it when you got it, and how many heat cycles? I soot all my brass moulds with wooden matches and preheat pin side down on a hotplate to get them running good, but usually have to clean them after each heat cycle a few times. Brass moulds with HP pins was a real learning curve for me, and the reason I started using a hotplate.

 

[Loren]

Cleaned with dish soap and hot water same as with all new molds. This was the first casting session, so first heat cycle. Bullets were a little sticky on the core pins at first and I did smoke them, which took care of that. I do use a hot plate to preheat mold.

I know first run always look rough, and I dropped plenty back into the pot before the exteriors started looking good. The pins look like blued steel, and I put a propane torch to them to heat them up even more to insure they were HOT.

I was casting fast. Three drops a minute is no problem.

I had the PID set at 850, I know that is hot, and if the mold had settled in and started dropping good bullets I would drop that to 800-810. The sprue plate is 3/8" to 1/2" off the sprue plate and I pour right down the center of the sprue as best I can with nice smooth laminar flow coming from the pot.

No, I am not sure it is shrinkage. It just has that look to me. Like it is sucking in while it cools. It could very well be trapped air with it being a nose pour and the upward funnel shape to the sprue opening. Just completely opposite of your typical base pour HP mold.

If slowing the pour flow doesn't help I will add a little tin to the melt.

Thanks for the ideas!!

 

[fiver]

there's always room for more ideas.
or pouring techniques.
venting probably needs to be looked at as the main culprit first, and then how much alloy the bullet has to draw from.
I see there is already some extra vent lines between each cavity, but that's no guarantee those scribe lines are allowing the air to get to them at the fill rate.
also the air needs to escape the top under the sprue plate, so a little edge breaking there could be in order.
tilting the mold slightly could help.
or using a ladle and pressure pouring.

or swirling the alloy in down the sprue hole might be what this mold wants, you could be hitting the pins with the hot alloy and creating a hot spot on top of the pin where the alloy is hitting it, this will cause voids and shrinkage just as much as not enough heat.
it usually affects a spot on the mold more to the side but with that wide pin sitting right there under the opening it could be taking the brunt of the heat.

 

[Ian]

All MP moilds will have the vent lines blocked by flashing on the trailing edge of the cavities.

Another idea is to fabricobble a vibrating mould rest under the furnace. Wahl hair clippers can be used for the source of vibration.

 

[popper]

venting probably needs to be looked at as the main culprit first The 'new' method of vents bothers me, yea , easy to make but have little effect. Might try pour the base first to just over the pins, then fill the rest. Kinda like bruceB soft nose in reverse?

 

[Ian]

Goid idea. Or just pouring some partials (just covering the HB pins) and examining them for voids might prove beneficial to diagnosis.

 

[358156 hp]

I've had problems with MP hollowpoint mould pins that finally went away after I soaked the pins in acetone overnight, then I reinstalled them and smoked them and preheated with a propane torch. They worked fine until the smoke sloughed off after a while. I then tried a spray graphite on them and that lasts longer than smoking. If you use graphite, use the stuff without an oil base. I think the stuff I have is the "Blaster" brand dry graphite. I have also decided that MP moulds work best for me when they are miserably hot. I think you're probably already covering this angle by now.

 

[Rally]

Looking at the wall thickness of that HB design, I'm betting it's going to take some tin, some heat, and several heat cycles to get good fill out, and a good scrubbing between heat cycles. My NOE 432-265-WFN with HP pins took some real patients to get right for the same reasons.

 

[358156 hp]

Some hollowbase designs aren't intended to enable base band expansion. Their sole intent is to lighten the bullet. Keith hollowbase designs are that way. You can compare a Keith hollowbase to that of a hollowbase wadcutter and see the difference in the designs.