Sunken area on bullets

[Michael]

I have a few molds that continually give me grief. Just forward of the front driving band in the same place, I get a sunken area. It is uniform from one bullet to the next, and is quite visible. Usually it is in just foward of the front driving band on the bore ride portion to about half way up from the forward driving band, sometimes it takes in the forward edge of the driving band as well.

The way I figure it either the mold or the melt is too hot or too cool, singularly or in combination. I usually cast at around 750. The main culprits when it happens are longer for caliber bullets ie 311399, 25-120-SP, 24-95-SP, 311284, etc. Most of the bullets are mildly frosty, especially the sunken area on the bullet, otherwise they look good with good fill out on bands and bases. The alloy is 4:1 COWW:Lino. When I cast I run 2 molds of similar weight, dump and fill the first one, put it down, grab the second dump and fill, grab the first, repeat. The molds sit on a hot plate while waiting and I use a RCBS bottom pour. Could be my casting cadence as well, at any rate I need to be doing something different.

Appreciate the input,

Michael.

 

[fiver]

your alloy stream is hitting the mold right there creating a super heated spot.

 

[Jeff H]

Also, consider how fast the mould is filling up.

This sounds a lot like an issue I had with the orifices in my LEE pots becoming over-sized over the years of cleaning them out. Running half a pot cleared that up by reducing the head pressure pushing alloy into the mould. A 10# pot is a lot deeper than even a big ladle.

I had originally cleaned up vent lines on the moulds, which helped a wee bit, but not nearly enough.

Putting a VERY sight bevel on the top/inside edge of the blocks helped a teensie bit more, but neither was enough.

I also had a similar problem when a thermocouple eventually gave up and started lying to me. I was running alloy at quite a bit over 800 degrees F for a short bit until I double checked it against another TC.

Sometimes, a hot spot develops at a thin spot on the mould, like near the grooves for the handles and the mould has to sit out a round or two, lying open on the bench.

I got some crazy-looking voids with both of those issues. The first one took me a while to sort out.

 

[Winelover]

Occasionally get them when casting rifle bullets. Mold telling me it's too hot.

 

[Bret4207]

Agree that if your bullets are frosty it's just a hot spot. Either slow down or forego the hotplate. You can also try letting the stream hit off center. Play around with it, you'll find something that works.

 

[JonB]

I think there is more than one cause to this problem.

In the past, I've called this "shrinkage".
That area of a long skinny bullet stays molten after the Sprue freezes. So when that center area freezes, it has no sprue puddle to "pull" alloy from, so that area just "shrinks", or sunken in as you call it.

So the solution is to pour large puddles for sprues.

* BUT, I've also had this after the casting session has gone on long enough, to where the mold isn't heated evenly. The Hot spot is just to hot and the large puddle and/or casting cadence isn't correcting the problem. I liken it, to an anomaly in the mold, where parts of the mold radiate heat at different rates. I have this with a couple 30 cal molds. I just have to stop casting and put the mold in the hotplate oven for 10 minutes or so, for the mold's heat to even out. Then I can continue casting again.

 

[JonB]

re-reading what I posted. Maybe I am incorrect... I guess it's the same problem, it's just sometimes it can be corrected with larger sprue puddle, and sometimes you just need to take a break.

 

[Dusty Bannister]

When pressure filling the mold from a ladle or bottom pour pot, the advantage of the large sprue puddle becomes less important because the weight of the alloy in the ladle or pot continues to feed the cavity as it cools.

 

[JonB]

When pressure filling the mold from a ladle or bottom pour pot, the advantage of the large sprue puddle becomes less important because the weight of the alloy in the ladle or pot continues to feed the cavity as it cools.

With a bottom pour pot, this is only true if you hold the mold to the spout with valve open, until the bullet freezes. Now, I don't pressure cast with a bottom pour myself, I find it's just too much head pressure for any mold that I've tried it with.

 

[Dusty Bannister]

Oh no, not until the bullet freezes. That will result in a lot of spurting of hot alloy with some pots that do not have the tapered nozzle. Just fill and give a slight twist as you remove the mold from the nozzle and allow a sprue puddle to form on the sprue plate. If you wait until the bullet freezes, you are still going to have that shrinkage void in the base of the bullet and weight variations.

 

[4060MAY]

Dan from Mountain Molds did an article on SBS "shrunken bullet syndrome
I had this problem with bore rider's, backed off on the tin and lowered the melt temp, problem went away, I tested this theory by starting with 1-30 with no problem, went to the harder alloy, with more tin than needed and the problem resumed, only difference was the alloy, been shooting scrap from an indoor range, BHN 1-13, haven't had any problem for 15 years or so, last time we cleaned the range we each got 1800 pounds of lead, and buckets full of jackets

I haven't bottom poured since 1978

 

[popper]

Just forward of the front driving band in the same place, I get a sunken area.
Perhaps it's vent line that is in the wrong place? Try just casting with one mould, vary cadence to see if problem goes away. I suspect it is actually a colder spot (outside or near a smaller part of the mould?) or venting. I used to get that when the mold was not up to temp. Nose pours fine, then just behind it doesn't cool right but behind it the alloy cooled fine. Didn't have liquid alloy behind it so didn't fill right. Had the same problem with a GC shank - one spot always had a crater. Breaking the top of the block solved it. Had trouble with one Accurate (al) that I had to smoke before it would cast right. After about 5 or 6 sessions it worked fine without smoking it. Venting problem.
Long skinny bullets have a problem that the mould often is 'thin' at the nose and the small dia allows uneven cooling.

 

[RicinYakima]

I too have had this issue when the tin content exceeds the antimony content. Plus it can be worse because I cast with single cavity iron moulds that can develop a "hot spot" from unequal off radiation of heat if I cast too fast.

 

[popper]

He is not in excess tin area. Try (gently, not banging) dropping the mould down on something hard at the end of the pour. I use a lee dripper, Al moulds. Pour, and drop onto workbench seems to help with good fill out. Haven't used any tin in years. My rifle alloy is 3-4% Sb, like the OPs. I don't rush casting, my shoulder wears out after an hour anyway. Last batch of aprox 250 145gr PB has 5 rejects. 3 were stutters, only half a bullet. One had a zit on the base and one rounded base. I used one for PC smash test.

 

[Tomme boy]

I have had the same problems when the heat is too high and an excess of tin in the alloy. Lower your temp and see what happens. One of my shotgun slug molds I have to tap like popper said to do to get a good fill out.

 

[popper]

How you doing now Tommy?

 

[Tomme boy]

Still on o2 but a lot better.

 

[Ben]

Hang in there Tommy !!!!!!!!!!!!!!!

Ben

 

[JonB]

Dan from Mountain Molds did an article on SBS "shrunken bullet syndrome

I thought I'd post a link to this article, as I'd never heard of it.

Shrunken Bullet Syndrome (SBS) - Mountain Molds

Shrunken Bullet Syndrome (SBS) where part of the bullet is frosty and undersized. Contributing Factors Big bore bullets make the mold too hot. No venting. Certain alloys, like half lino/half WW, and maybe WW + 2% tin Pouring too slowly Things That May Help Single cavity mold. XL blocks...

mountainmolds.com

I read that Mountain Molds closed in Dec 2020 (retirement it was said)
But the website looks active, and has this at bottom of the webpage.
"©2022 Mountain Molds"

 

[Dusty Bannister]

My experience with the small sprue hole in the plate was with a Saeco #221 mold. It did not exhibit the sunken side, but did result in rounded bands. Slightly opening the sprue hole resolved the issue with the BP pot. There does not seem to be any notice that the intimate contact with the BP nozzle and the sprue plate keeps the plate hot so the alloy flows through this choke point easily. Just opening the vent lines resulted in "whiskers" which I found undesirable. My casting was done at about 720 degrees.