Even Mold Temperature

Today wasn’t the best day, so I decided to cast some boolits after my kids went to bed. I wanted things to go well, but I went against the odds by going with an RCBS mold. Up to this point, I have just not been very satisfied with the sharpness of my boolit bases from an RCBS mold. I don’t like to get my mold too hot, and I’m too cheap to use too much tin. It seems that when I get sharp bases, my mold is too hot and lead sticks in unhelpful places. If I let things cool down so that I have sharp driving bands without lead sticking anywhere, I get slightly rounded bases. Where is that sweet point in the middle?
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I might have figured out how to get to that sweet point - by keeping the mold temperature of the sprue plate the same as the bottom of the mold. The sprue plate is naturally cooler than the mold blocks. It swings open & shut every cycle, getting some forced convection cooling. When held upright, natural convection cooling is the strongest at the sprue plate. So the first thing I tried is flipping the mold over while waiting for the sprue to cool. That helps, but it is not always enough. Sometimes I hold the mold upside-down for a few seconds, and then set it down (upside-down) on my cotton drop towel or on the cover plate for my RCBS bottom pour furnace. Setting it down like that really helps to cool the sprue without effecting the mold or sprue plate so much. That’s basically it - hold the mold upside while letting the sprue cool, and set it down on the sprue if that takes too long.
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I had the best result I have ever had with an RCBS mold today. I was using the 9mm-147-FN mold. Not the best choice for .357 Sig, but it works when you have a compressed charge of Enforcer. My alloy was 96-3-1, casting at 700F.

I don't have steel molds but do you use a hot plate or a propane torch?

Hot plate, with a coffee can to make an oven.

With grooves and you're wanting nice fill at the base, I'd be casting at 740f. Yes I know, it will heat up the mold faster, but it's the price to pay.

Get a small desk fan and after each fill, let the mold sit in front of fan for 30 seconds. Once you get things hot enough to satisfy good fill, the fan will do a good job of making sure you don't overheat between casts.

Also know it makes for a slow process... get 2 or 3 of the same mold and run them in sequence. It'll give each mold time to cool, while keeping you constantly busy.

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Adding another 1% tin adds about $3/1000.

Glad you found a method that works for you but there is value in keeping things simple. Waiting for cool down does not cost $$$ but impacts productivity and that is important to some.

Most molds put out about 100 bullets per hour per cavity. That is a cycle time of 36 seconds. Adding 4 seconds takes productivity to 90/hr. Time to cast 1000 bullets with a 2 cavity mold is 5 hours normally and 5.6 hours at the slower rate. 35 minutes longer.

"Working" 35 minutes longer to save $3 is not very attractive. Also, the extra tin should result in a more consistent finished product that should group better.

There are good reasons commercial alloys have 2% tin. Folks making money at the business require quality and productivity. Saving money may be more important for many hobby casters and a bit higher quality may not be that important.

I used to work as a Process Engineer and one of the attributes we strived for was a wide "process window". WRT casting that would be an alloy that performs well with a wider range of melt and mold temperatures. Sn is the key to achieving that. Regrettably, it is the most expensive element; but fortunately, a little goes a long way.

A little extra heat will not hurt any mould ... I get my best fill out when the boolits drop right at frosty .
Trying to cast too cool is just hurting your casting ... different moulds like to be run hotter or cooler .
Jack the heat up and cast em hotter ... if you powder coat ...frosty holds onto the coating better .
Gary

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Originally Posted by gwpercle

A little extra heat will not hurt any mould ... I get my best fill out when the boolits drop right at frosty .

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That's how I do it.
Run the pot up until I get some frosting,
then ease the heat off until there is just a hint of it unless I slow down my fill/drop rate.

In years past I'd accumulated several hundred pounds of wheel weights, several hundred pounds of shot,
and a bunch of 95/5 'almost empty' rolls. I tend not to be bashful about putting in a little extra tin.

Loosen the spru plate so it swings of it's own weight, this allows better venting under the plate getting the air out faster

I cast about 730-740, alum or steel molds. I often heat molds & ingots on a hot plate. I return sprues to the pot about every 100, Set the mold aside off the heat. It cools a bit while the sprues melt, maybe 5 min & repeat.