My first choice would be fine-grained cast iron. My second would be bronze. After that brass is too prone to possible damage and aluminum isn’t even heavy enough for a boat anchor.

40 Rod

you have to look at the requirements of a mold to function well and then weigh your requirement.

AL is less expensive and it is easy to machine, so you can get less expensive molds  (i.e.  Lee)  But while some work well they are soft and susceptible to accidental damage.  Also AL heats and cools faster. If you have developed an ingrained casting rhythm you may run into quality control issues unless you alter the rhythm.

However I have several custom double cavity Al molds I ordered from Accurate Molds.  My idea was to test the bullets and reorder in iron  (Tom offers his molds in all three materials)  His blocks are larger than Lee and of a much better Al alloy. Lee seems  to use soft alloy extrusion stock. AA seem to be machined from harder denser AL billet stock.  They don't have as near much temp variation as Lee and they are working out very well for me.  I'm in no hurry to reorder in iron yet.  I have a few more designs I want to try out first.

Iron seems to be the most common, traditional mold material (though brass was very common in the M/L era probably for cost/tech reasons). 
Assuming the block size and the bullet(s) cavities are well proportioned they hold the heat well, are durable, etc etc.   The main downside is that they rust and require special care for storeage.  ( I have had a couple Metal-Lifed and really ought to have a few additional favorites so treated) 
The gross weight of iron molds which is a factor in their heat dynamics can be a problem.  Large block multicavity molds weigh a lot when lead is added; and big bore bullet runs can create hand wrist strain for aging joints.   Extensive casting could result in RSI injuries.
I have little experience with brass/bronze mold but it seems to fall between the other two
Assuming you are comparing the same bullet design machined to equally tight QC standards form the same shop, most of us would probably find iron would be the best bang for the buck in the long run.

This is from Accurate Moulds site;

Quote:


I have one of his aluminum moulds for a tapered .40cal and I'm happy with it. It heats up quick and casts great bullets. I get few rejects after warm up.

Bob

My favorite molds are brass from Old West.  Second favorites are cast iron from good custom makers.  Last in all regards are aluminum molds.

Brass molds that I have come to temperature better and cast more uniform bullets.  They also drop the bullets nicely.  Iron is harder to keep at a uniform temp than brass, but is more durable and once to temperature casts good bullets.  I have more problems with lead sticking to them and them not closing well than I do with brass.

Aluminum varies temperature way too much, is susceptible to damage, and generally doesn't seem to work as well in any respect.  Most of mine are NEI, and some LBT, not cheap Lee, so workmanship shouldn't be the problem.

Walt Melander used to tell customers that he liked aluminum molds:  They were easy to machine and the customers that used them a lot soon came back and bought a mehanite mold instead.

dave

What about unobtanium? Anyone?

Aluminium molds suck. 

    Joe.   

Quote:

Accurate aluminum mold ...
(You need to Login or Register to view media files and links)
Bullets cast with this Accurate mold ...
(You need to Login or Register to view media files and links)
Ordered the mold in 1:20.  Heated the pot to 780 degrees - cast with a 5 second pour - sprue puddle frost in 7 seconds.  Results:
* 04 bullets = 209.8 - 209.9 
* 96 bullets = 210.4 - 210.8 
The mold was spec'd as a 210gr
(You need to Login or Register to view media files and links)

No complaints from me!

John,

The results I'm getting with my Accurate aluminum mould mirrors yours. I had some reservations before ordering mine based on my experience with others. After casting about a thousand or so bullets with mine I have no complaints. 

Bob