Aluminum alloys for molds

[clodhopper]

Wanted to have a discussion about aluminum alloy properties for molds and spru plates.

2024, a tough copper alloy susceptible to corrosion, can be anodized
NOE's web site says their molds are made from 2024. You can see a slight goldish tint to the metal.

6061, hardened with magnesium and silicon. Can be anodized

7075, hardened with zinc, cannot be anodized.

These seem to be the three alloys that are most available.

Price wise
1 X 1 1/2 bar from an online supplier.
6061 $0.89 per inch
2024 1.89 per inch
7075 2.29 per inch

Better prices can be had but there is a pretty fair comparison.

I plan to make s few core molds these will be multi cavity, my first blocks will be 6061 because I have some.

My first concern is spru plate material. Cavities will be close together, the mold will run hot, with the material being near pure lead, performance is far more important than cost for the spru plate.

Please comment on what other mold makers use, temperature/strength properties,
and what ever else pops up.

[ProfGAB101]

First post is all kinds of just wrong.

2xxx series aluminum is primary alloy with copper
6xxx series aluminum is primary alloy with silicon.
7xxx series aluminum is primary alloy with magnesium.

All can be anodized.

Point of notice, AR-15 receivers are 7075 forgings - and they are type III 'hard' anodized. 7075 is also the fastest to corrode in a raw state.

( sprue plates ) For what its worth I would get flat ground stock of W1 or O1 tool steel and leave them unhardened after machining, or another good choice would be 4140 pre hard in a half hard condition. Starrett flat ground low carbon would work and be cheap but won't hold a sharp edge long term if used with high copper alloys or lino.

[bangerjim]

I vote for brass! I have quit buying Al molds all together. Brass molds yield excellent performance and long-lived.

[clodhopper]

Ok ProfGAB101, thanks for pining my ears back.
I do not have a lot of information on alloy properties, and any offered to fill the gaps is welcome.

bangerjim, perhaps one day I will make molds from brass, right now I have a brand new CNC machine, I'm weak in the programming department, need a project, have some 6061 AL and two flute end mills.
Also have the desire to cast some .1875 diameter cores in the 60 grain range. I plan to make a ten or eleven cavity and want to keep the weight down.

Both Lee and NOE molds have non magnetic spru plates, Does any body know what they are made from?

Will 7075 pose a corrosion problem if uncoated and used as a bullet mold?

I will look into Starret flat ground stock, but still thinking aluminum. Cores need to be pretty soft.

[Cap'n Morgan]

I will look into Starret flat ground stock, but still thinking aluminum. Cores need to be pretty soft.

Any flat stock will do, but I would stay away from using aluminum-on-aluminum as it galls at the blink of an eye.

I've made molds from various alu alloys. As long as you stay with the free-cutting types you won't notice any difference - Still, I find brass a better mold material; Just as good, if not better swarf evacuation and the finished mold is less susceptible to get scratched or dented.

[clodhopper]

Cap'n,
Being brand new to machining, working brass will come after my skills have improved, the material is just to expensive for beginners.
But you never know what might show up at the scrap yard!

[bangerjim]

I prefer working brass to Al with my lathes and mills. I make tons of stuff out of brass of various hardnesses with great success. For me, it machines easier than any Al alloy I have, looks better, and lasts a whole lot longer for the items I make.

banger

[clodhopper]

Did some more research on aluminum alloys.

2024 .038-.049 CU, .012-.018 MAG,

6061 has .004-.008 SI, .008- .0012 MAG, .0025 ZN

7075 has .02-.29 MAG, .05-.061 ZN

Each of these alloys have other elements in them mostly less than 1/4 of 1 percent.

Still hoping to hear from someone who has some experience with aluminum spru plates. But if brass is the only subject spoken here, lets talk about the properties of different alloys.

Anybody ever melt down cartridge brass, pour it into a lump big enough to machine mold blocks?

Yeah, I fully realize how tripping over a twenty to save a penny that idea is, but some guys do the darndest stuff.

[dkf]

Alloy really doesn't matter much. I have molds in all three you mentioned and some others. The alloy Lee uses is a real soft alloy which is common for extrusions. I have quite a few brass and iron molds too, still prefer to cast with the aluminums because I can cast faster. The 6061 is the cheapest and the 7075 is the most expensive, the 6061 is easier to find and comes in more sizes. The 6061 machines the easiest. I wouldn't even consider using an aluminum sprue plate unless it has been hard anodized. Much easier to just use a hot rolled steel for a sprue plate, steel works well for that application.

[country gent]

If your planning on having these moulds anodized you need to find out what the thickness of the anodizing is and adjust for it. Some of these coatings arnt very consistant in thickness. They are hard dor added wear resistance though. Another thin to check would be their heat resistance. I would machine the blocks and cavities to the needed size and run them. any good steel will make an acceptable sprue plate 4140 pre hard at 40-45 would be long lsting and if stress relieved would machine nicely. Starret oil hard or or Air hard will work and is available in a ground finish. As above I would really recomend brass or bronze, its easy to machine and dosnt tend to load gauld up cutters like aluminums do. Also brass tends to give better finishes easier.

[bangerjim]

Did some more research on aluminum alloys.

2024 .038-.049 CU, .012-.018 MAG,

6061 has .004-.008 SI, .008- .0012 MAG, .0025 ZN

7075 has .02-.29 MAG, .05-.061 ZN

Each of these alloys have other elements in them mostly less than 1/4 of 1 percent.

Still hoping to hear from someone who has some experience with aluminum spru plates. But if brass is the only subject spoken here, lets talk about the properties of different alloys.

Anybody ever melt down cartridge brass, pour it into a lump big enough to machine mold blocks?

Yeah, I fully realize how tripping over a twenty to save a penny that idea is, but some guys do the darndest stuff.

You do NOT want to founder brass casings!!!!!!! There is a BIG WIDE world outside of just "gun stuff". Have you ever melted brass and poured it? Very tricky. Very difficult to get non-pourous large castings. VERY high heat needed. Use glass as a flux on top. Castings poured into green sand molds. Just buy your brass stock as rolled bar and machine it. Do not try casting it from shell casings!!!!!

I use whatever I find at my local scrap yards. Tons of flat. round, hex, sheet brass stock around here. Even AlBr rod for bearings and such. I have 1/8 thru 6 inch round barstock and every thickness and shape and size of flat stock you can imagine. All at scrapyard prices.....not premium on-line metal monger prices.

Look around locally.

[Cap'n Morgan]

What Bangerjim said.

I get most of my brass as end pieces from round bars. When running swiss machinery with full length bars (normally 10' or 20') the last ten inches cannot be machined and are dumped as scrap and can often be had for the scrap price (or a six-pack)

Even if you could re-melt cartridge brass, it would make for a poor mold material due to the high copper content.

[clodhopper]

I have not found any brass scrap suitable for molds yet.

Did get 165 lbs of 304 stainless in round bar stock from 3/4" to 1 1/4" for 30 cents per pound,

And have about 4 buckets of 6061 T6 aluminum.

That is why my first blocks will be aluminum.


Back at the CAD poor drawings have dogged my CAM. Hope to push "cycle start" today!

[Firebird]

The real problem, whether you want to use brass or aluminum for your molds, is what is best versus what is available. Best brass to use for molds are the hot-rolled alloys, which are very difficult to get today. Much less chance of warping compared to cold rolled brass alloys the first time you get them hot. The aluminum alloys that would be best for molds are those used to make pistons in engines, they have some nickel in them and have much more strength at high temperatures than any of the commonly available aluminium alloys already mentioned.

[longbow]

I don't know of any mould using aluminum sprue plates. All my moulds have steel sprue plates regardless of mould material. I have iron, aluminum and brass moulds from mainstream mould makers and all have steel sprue plates.

Don't use stainless steel for moulds it has poor heat transfer. I have one push out mould I had made by a machinist at the company I was working for and I guess he thought he was doing me a favour when he decided to use a piece of stainless steel round bar instead of carbon steel as I asked for. That mould works but tends to overheat. I have since made several others out of hot rolled and cold rolled steel, and bronze. All work well and cast faster and better than the stainless steel mould.

If price or availability is an issue then why not use a free machining steel? It is inexpensive, readily available and machines easily.

As for brass, it is a great mould material and machines very well but buy brass bar stock for making moulds. Why would you want to cast a blob then machine blocks out of it?

Aluminum can make a good mould but aluminum is soft and can be easily damaged if mishandled/treated roughly. My personal preference is brass or iron for mould material. Different strokes.

I do have a couple of NOE aluminum moulds though and they are very well made and cast perfect boolits. I also have a three Lee moulds and they cast just fine as well but I know the alloy is softer than what NOE uses.

Longbow

[clodhopper]

Lee's web site states that the six cavity sprue plate in "hard anodized", The six cavity sprue plates on my molds are non magnetic.
Does any body know what they are made of?

Still no "cycle start" action here, got my job assignment all tangled up.

[MtGun44]

Aluminum is what gets hard anodized, which makes it gray to black, which is what AR uppers and
lower are coated with.

I'd use 2024 and a steel sprue plate.

Bill

[clodhopper]

Thanks Bill,
I'm still making the first blocks with 6061 because I have some. 2024 is probably a good choice of materials to make production core molds with 7-10 cavities, aluminum blocks for keeping the weight down. Steel plate for the strength to shear many sprues.
This project is more about learning the ropes on my CNC mill than making the best core mold possible. Any core that falls from the mold will be sized in a squirt die any way.
My plans for this mill really do not center on bullet mold production. I just need one for making cores for 75 grain .224 swaged bullets. Most core molds available have cavities sized for 55grain bullets.

I do plan to make stuff with my mill, but looking at sights, trigger groups, and maybe nibbling around the edges of jacketed bullet forming.

[quasi]

I am only aware of one mould maker who used 7075 for his creations, and he has stopped making moulds. (he lost access to the CNC equipment he was using)

[clodhopper]

Did he use 7075 for the sprue plate too?
did you ever cast with a mold made from 7075, and was it much different from other aluminum molds?