What causes localized hot spots

[Marlin Junky]

I've been casting boolits using WW metal and solder for a long, long time but decided to try only using clip-on wheel weight metal sans solder last night. I knew I was going to need more heat to promote good cavity fill-out so I cranked the MasterPot up to about 800F (and later 820F) and started filling mold cavities with my Rowell#2 as usual. I was using a pair of hot plate pre-heated RCBS30-180-FN's that have proven to be excellent casters but unfortunately I struggled to stabilize the casting conditions. Most of the boolits made early in the casting session had slightly sunken overheated spots usually on their noses just above the first driving band. The shanks were also frosted. After turning up the heat in the pot about 20F (because driving band fill-out was not great), I let the molds cool on the pot's rim and began casting more slowly which resulted in the boolits coming out nicely formed and about a grain heavier. Unfortuanely, I'm not sure all the one grain heavier boolits didn't come from the same mold because all boolits went into the same pile before being weighed.

I'm guessing that even though WW metal needs more heat to flow properly, the casting process needs to slow down in order to prevent mold over heating or am I missing something else here? And why were my boolits coming out with localized hot spots and not a complete frosting? Do we need to consider the difference in temperature between the metal and the molds when using pure WW metal opposed to WW+Sn? If there are any casters out there who are using a ladle to cast WW metal boolits in ferrous molds, I'd like to know what temperature you're setting furnace thermostats.

BTW, my RCBS-30-180FN boolits now weigh 199 grains checked and are right at BHN 12.5 to 13. There was a tad of Sn in the pot 'cause I threw a 2-3 pounds of boolits cast from my BHN 15 metal in with the 15 odd pounds of WW metal. Straight WW metal ought to hit 200 grain (checked) with this mold and BHN 12.0 to 12.5.

The next step (after figuring out how to pour these boolits) is too load 'em up with a case full of DP85 (or H414/W760) and see how they shoot.

MJ

[Ricochet]

If the mould's tilted so the stream of metal consistently hits a spot, it'll get hotter than the rest of the mould.

[Marlin Junky]

I don't know about that theory Ricochet. Have you been able to repeat that under controlled conditions? I have experimented with different pouring techniques using WW+Sn alloys and haven't been burdened by frosty, sunken defects. The frosty sunken spots (which were also occasionally created in the driving band area) are telling me that while the metal is not flowing properly, it is either too hot or there are hot spots in the mold. I don't know how the latter can actually take place but maybe if I knew something about metallurgy, it might make sense. Perhaps my two molds did not preheat evenly? Maybe that's all there is to this whole thing. Time to try it again.

MJ

[454PB]

I keep a wet cloth on the bench and touch the bottom of the mould to it when it starts to overheat, effectively using the cloth to regulate mould temperature rather than pot temperature.

[leftiye]

M.J. Have you ever seen the opposite of the hot spot from the tilted mold? It's where you see the creased outline of the stream hitting the side of a too cool mold and freezing, Kind of a c shape with the open end down.

Some hot spots are caused by crud (not saying you've got crud) or maybe mold prep coating the mold and making it so that that part of the boolit can't cool down. Causes that part of the boolit to colapse inward as the rest of the boolit cools and sucks the lead towards it.

[AZ-Stew]

Marlin,

I've often seen the effect you describe. I've never seen the effect repeat in exactly the same spot on the boolit in cast after cast, but maybe I wasn't looking closely enough. My solution was to let the mould cool a bit.

I suspect that this is caused by the interior surface of the mould reaching near the temperature of the alloy, but not uniformly. Mould metalurgy, thickness of the blocks at any given cross section and any number of other factors may contribute. Remember, the difference between water and ice is only a degree or so, and I suspect the same is true with boolit castings, alloy temperature, mould cavity surface temperature and surface frosting/sunken areas on boolits. When you're on the temperature edge these things will happen.

Solution: Cool mould, cool alloy, slow casting rythm or a combination of the above. Put some margin between the frost temperature and the filled-but-shiny temperature.

Regards,

Stew

[Marlin Junky]

I repeated everything tonight except the way I preheated the molds and everything worked as it should. I made the mistake last night of preheating both 2-cavity RCBS molds on my small hot-plate that really doesn't have enough surface area to do an even preheat to both molds. It works fine with one mold but from now on, two molds will go on the large hot plate at its medium setting.

Also, I was casting tonight with pretty close to pure WW metal at about 780F (I will try about 800-820F later) and my two shimmed (about .001") RCBS-30-180-FN's were casting 199 & 200 grain boolits (checked). Too bad there's a difference of one whole grain between them. Also, too bad this mold doesn't come in a four cavity configuration because I need to make more than about 400 boolits in an evening.

MJ

[Ricochet]

I don't know about that theory Ricochet. Have you been able to repeat that under controlled conditions?
Haven't tried. Read it in Lee's loading manual.
:-D

[beagle]

I had a mould that the steel screw for the sprue plate caused one cavity to stay hotter then the other. It was a NEI aluminum. It made great bullets but I had to eventually start running a dry cavity every other throw.

I have followed around with using aluminum heat sinks to dissipate heat and this seems to work. What I was using was cumberome and I haven't gotten around to designing and making a compact set yet./beagle

[felix]

Electronic shops will have some junk/unused power transistor or CPU heat sinks. A couple of those attatched to the sides of the mold opposite the hot cavity should do it. While there, get some thermocouple glue, and see if that works through a session. ... felix

[Marlin Junky]

Electronic shops will have some junk/unused power transistor or CPU heat sinks. A couple of those attatched to the sides of the mold opposite the hot cavity should do it. While there, get some thermocouple glue, and see if that works through a session. ... felix

Hey Felix, what a cool (pun intended) idea... cooling fins on a boolit mold! Maybe I'd be able to cast over 400 boolits in an evening with a set-up like that.

MJ

[beagle]

Felix... I have a couple of ribbed electrical heat heat sinks. I just need to get them drilled and tapped so they can be attached to the mould block bottoms. I already have CSH screwes that fit. I'll strip the paint and attach them to improve heat dissipation.

The aluminum stips worked when we were tinkering with the big 457125s but they were awkward to work with.

So, teh concept works. Just need to figure out the method./beagle