Voids/Sinking Bases

A few months ago I was having trouble getting sharp edges at the base of the RCBS 45-230-CM, which was resolved by filling each cavity without connecting the sprue puddles for each cavity. I’m using an RCBS bottom pour pot, and the valve would open and close for each cavity. After the two cavities were filled, I would pour a swath of lead over both sprues to keep things hot.
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Using the method of pouring each cavity separately, I found a surprisingly high rate of boolits with excellent fill out except for voids and sinking on the bases. The edges of the bases are sharp, but the flat midsection has voids & sinking. I thought that perhaps my COWW +1%Sn alloy was a bit dirty, but today I observed that perhaps it was the pouring technique.
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I got low on lead and added SOWW ingots to the tune of 4:3 COWW:SOWW without adding more tin. Then I cast up some NOE 454-237-RN boolits starting with the “pour each cavity independently” method described above, and I saw the sinking & voids again. Then I went to my old method of keeping the valve open while pouring all cavities (i.e. connecting the sprues) and the voids were eliminated. It was the same alloy, same mold, same casting session. Maybe a chunk of oxides were stuck in the valve and broke free at the same time I changed the pouring method, or maybe connecting the sprues gives less porosity.
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Pouring each cavity helped with the RCBS mold, because the bottom driving band was thin. I may have to increase the tin for that boolit and pour both cavities at once.

I wonder if your sprue plate temperature is too low.

I gave up on bottom pour and use a ladle using 4 cavity molds.

In general, I find that having a large sprue that does not overrun into adjacent cavities or off the edge gives me the best fill out. When I have a small sprue puddle I get things like sunken bases and voids. When I have sprue puddles that run into adjacent cavities, I get rounded bands. Of course there is always the exception where a mould does something it's own way. I'm ladle casting though.

With a bottom drop furnace, you can cast fast enough that when you connect the cavities (at least on a 2 cavity mould) they are essentially the same sprue and do not have the issues of the second cavity pulling metal from the first.

Im thinking mould sprue plate temp is borderline. the sprue needs to be the last to cool so as the bullet shrinks it can draw from the sprue and fill. as a casting cools and air / gasses dissipate the moulds fill from the sprue or risers. Ive seen aluminum sand casting where the risers drop over an inch as the mould cools.

I would also vote for sprue plate temp being low. That's what I see on mine.

I usually pour all four continuous (or two). I get a larger puddle and better fillout. When the plate starts getting too cold the last cavity filled will start to show issues.

I have been tempted to use a Lyman ladle, just not made the leap yet :)

I use a RCBS Pro Melt, that is old, rebuilt and repainted with grill paint, in other words, old, old. Never gets switched off, just unplugged.

The trick to getting 100% fill with all sharp edges and a flat square base, on every bullet, is the rest height setting, adjusted to pour center of cavity, the sprue plate flat should be just below the pour nozzle, so there is no mould to nozzle seal, allowing the hot air or gas to escape as the alloy is added to the mould cavity, bottom to top, always pour bottom to top. The sprue puddle on the sprue plate should be about half again larger than the bullet diameter and poured in one pour. There should not be a multiple cavity overrun, just pour one cavity at a time.
The alloy used is very important for consistant fill, consistant weight, consistant diameter and thus accuracy.
I set four iron moulds directly on the alloy, then plug the pot in, usually requires two bullets to be cast to make 100% fill out. Then cast with four moulds at one time. Refill with muffin tin ingots at 1/2 pot, flux at ever refill, leave full mould at refill, two bullets in sprue box, back to casting 100% filled out bullets.

Opinion and experience: accuracy is way more important that expansion, style, etc. If the bullet does not shoot well, it is a waste of time, energy, primers and powder. I cast very hard bullets for accuracy, first and foremost, and in order to hit what I shoot at. I was trained to shoot with a single shot, the only thing that ever matters with a sinle shot is the first shot, put exactly where you want it.

I also think it's the sprue plate as well. Sprue plates need to be preheated, the same as the rest of the mold.

I used to use the bottom pour mechanisms in my 20 lb. Lee pots and found that, although a bit harder to do correctly, filling one cavity at a time with plenty of extra sprue metal worked the best for bottom fill out as well as groove fill outs. Plenty of flow, but exact cut off was critical to make certain that each cavity was done the same for best consistency without overflow into the next cavity.

Due to the Lee pot's consistent dripping and valves hanging open for runaways, even with really clean alloy, I removed all the mechanisms and plugged the bottom holes. I now only ladle or dipper pour all my molds and actually have done much better as it is so much easier to please each individual mold as to it's particular pouring needs. I started out dipper pouring with a 10 lb. cast iron pot heated on a Coleman stove back in the middle 70's anyway, so I guess I never really missed any of the convenience of a bottom pour as I just did not get all that used to that method anyway.

Try Pressure Casting.

My general method is when using the Lee 6 cavity molds. I do start and stop at each cavity and that seems to avoid rounded bases because the flow is directly into the sprue hole and not splashing off the edge and sometimes trapping air in the cavity. Perhaps my difference is that I try to pour a large puddle at each cavity and then proceed on to the next cavity as quickly as possible and when that puddle is poured, it connects to the previous puddle. This eventually forms a long single sprue that sort of looks like a decent stick weld bead. Being connected, they transfer the heat to the sprue plate and might provide a more uniform mold block temperature. Yes, the mold is slightly tilted so the over flow drains toward the previous puddle. We all have our own preferences, and then there is also the need to consider what the individual mold likes.

It seems every mold needs a little different technique (distance to spout, pour one, or pour all with a continuous stream, ect ect).
BUT, the thing they all need is Heat. The sprue plate should be a little hotter than the mold, how that is typically done is to pour a large sprue puddle, because pouring lead is pouring heat.
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So, how hot is hot enough(mold/sprue plate)? Watch the sprue puddle freeze, it shouldn't freeze right away(too cold), and if it's still molten 10 seconds later, it's too hot.
These are things I think about when I have issues with a specific mold.

I think a lot just has to do with every component being the correct temp - between the mold, the sprue plate and the lead if one thing is not hot enough you will have problems. I try to have excessively large sprue puddles when starting in order to get the sprue plate temp up. Even with preheating the first few drops aren't perfect but after a minute or two I'm good to go.

FWIW I use 1 continuous pour for any mold whether it is 2 cavity steel up to 8 cavity aluminum. It works well as long as you can efficiently jump from cavity to cavity without making a mess. I have used other techniques but this is the fastest and easiest for me. I think a WIDE variety of techniques can work as long as temps are correct. It's just a matter of finding out what works for you.

I just move from hole to hole. Lee pot. Large sprue keeps plate hot. Dropping the mold on a hard surface will force sprue alloy into the cavity - don't slam it.