Bronze for Molds

[Greg5278]

Does anybody have any thoughts on using Bronze or Aluminum Bronze for molds? They should transfer heat well, and the aluminum Broze will release the bullets well. Any thoughts. Yes I know Aluminum Bronze can be a PITA to machine.
Greg

[Buckshot]

................I'd suggest you start here:

http://www.anchorbronze.com/alloydatabase.htm

If you have an understanding for the ideal mechanical requirements you want. There are 2 lists for various copper and bronze alloys, with mini 'pull down' menus for common alloys, both wrought and cast.

There are several bronzes which reach some very substantial and impressive tensile strengths if that is a requirement. Very possibly an e-mail to them would be the best bet for a high strength/high temp alloy for working at cast lead temps.

The only thing I see as a negative is the relatively high price of the constituent parts (tin, lead, zinc, antimony, aluminum, copper, etc) comparted to the more common cast iron, aluminum, or steel.

..................Buckshot

[Linstrum]

What Buckshot pointed out certainly needs to be taken into account when choosing a mold material because some the aluminum bronze, bronze, and brass alloys that show promise for that are pretty pricey if you get them new. If you want to make an aluminum bronze, brass, or bronze mold and have the patience, after you find an alloy that you are interested in go check out eBay because stuff like that shows up there regularly as surplus. I have found lots of the hard to find metals and alloys I use over on eBay. Last year for my steam car project I got 500 feet of new-in-the-box Inconnel-600 1/2" od x 3/8" id tubing that cost $15,000 new for only 15 cents on the dollar!

It sounds like you have had some experience with aluminum bronze before, you are quite right about its properties. With the exception of the aluminum bronzes, the problems with the various bronze and brass alloys are twofold, first is that most brass and bronze will “tin” extremely easy and once a surface has been "tinned" it will take a lot of work to remedy so the boolits won’t stick. This is because the “tinning” metal actually dissolves down into the substrate metal and alloys with it. The other trouble, which you already know about, is that many of the aluminum bronze, bronze, and brass alloys are difficult to machine because they are either gummy or quite slippery, which makes machining the close tolerances necessary for molds next to impossible unless the cutting tools are kept absolutely razor sharp.

I have used aluminum for making my own molds since 1964 and I found that 2024 T-6 aluminum machines very nicely and makes excellent molds. It contains a substantial amount of copper and although it is not bronze colored it is pretty darned close to being an aluminum bronze if not actually so. Other aircraft aluminum alloys work nicely but 2024 with a T-4 or T-6 tempering is the best that I have found that is also very easy to buy, which can’t be said for other bronze and aluminum bronze alloys. Cast iron is another good mold material, but its very low thermal conductivity causes problems that can be avoided by using aluminum-aluminum bronze, bronze, or brass. Because the brass and bronze alloys are such good conductors of heat they are just as good as aluminum-aluminum bronze in that respect, some of them like the phosphor bronzes may be better, and if a brass or bronze mold is seasoned correctly to prevent it from becoming tinned with boolit alloy, and then properly taken care of, it will last a long time. There are antique brass and bronze molds around that have given centuries of service!