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Aluminum molds for ZA-8 casting?

Posted: Sat Jun 16, 2018 7:04 am
by dorin
Would it be possible do you think to use aluminum for a mold to cast ZA-8?
The melting temps are 2 or 3 hundred degrees apart.

Or would the ZA-8 bond to the aluminum like that aluminum solder we see sold on late night TV??

Thank you,
MIke

Re: Aluminum molds for ZA-8 casting?

Posted: Sat Jun 16, 2018 8:50 am
by John Hasler
Aluminum dissolves in zinc.

Re: Aluminum molds for ZA-8 casting?

Posted: Sat Jun 16, 2018 3:33 pm
by Harold_V
While aluminum dissolves in zinc, it also oxidizes readily, and rapidly, and I suspect aluminum oxide won't dissolve in zinc. It might be possible that it would work just fine, assuming it is allowed to oxidize adequately before being put in service.

To that end, it might be worth the effort of making a simply mold form of aluminum, heating to several hundred degrees with an oxidizing flame, then allowing the mold to cool. Once cool, try pouring some zinc to see if it bonds. If not, you should be good to go.

Not speaking from experience.

H

Re: Aluminum molds for ZA-8 casting?

Posted: Sat Jun 16, 2018 9:16 pm
by DavidF
It works for short runs, but cold shuts can be troublesome if just gravity casting.

Re: Aluminum molds for ZA-8 casting?

Posted: Sun Jun 17, 2018 1:29 am
by Harold_V
Aluminum molds are common in bullet casting. So long as you pour a few pieces to warm the mold, cold shuts aren't a problem, and I suspect that would be the case for zinc, too. Fact is, any metallic mold will require proper pre-heat to avoid cold shuts. Once the mold reaches a desirable temperature, one paces themselves in pouring, so the mold doesn't overheat, or cool too much. That's true for bullets, may not be for zinc.

H

Re: Aluminum molds for ZA-8 casting?

Posted: Sun Jun 17, 2018 5:17 am
by dorin
Thank you!
Oxidizing the aluminum seems like an interesting idea. One of my friends needs some journal boxes.
We got the idea of cutting out a mold using my little, working CNC machine...but it is so little
I don't think it will handle much more than aluminum...and getting a thick enough graphic block
is a bit expensive. (Or perhaps I am a bit cheap!)

Re: Aluminum molds for ZA-8 casting?

Posted: Sun Jun 17, 2018 8:30 am
by John Hasler
I suggest hard anodizing it.

Re: Aluminum molds for ZA-8 casting?

Posted: Sun Jun 17, 2018 9:04 am
by DavidF
Harold_V wrote:
Sun Jun 17, 2018 1:29 am
Aluminum molds are common in bullet casting. So long as you pour a few pieces to warm the mold, cold shuts aren't a problem, and I suspect that would be the case for zinc, too. Fact is, any metallic mold will require proper pre-heat to avoid cold shuts. Once the mold reaches a desirable temperature, one paces themselves in pouring, so the mold doesn't overheat, or cool too much. That's true for bullets, may not be for zinc.

H
More to it then just pre heating the mold. Yes getting the mold hotter than Hades helps get the mold to fill, but you have to keep in mind the thermal expansion of the mold and its contraction upon cooling which can really make it difficult to remove parts from the mold if not well drafted and adequate use of ejectors also depending on shape of the part.
I have only experiment with the feasibility and yes you can get it to work. I wouldn't bother oxidizing the mold, it will do that itself after a few pours.
What I was most happy with in my experiments was the surface finish quality of the cast part which is what I need for most of the castings I need to do in small production runs. Once i get moved into my new home I plan on building a small and simple bench top pressure die casting machine for zinc castings.


http://forums.thehomefoundry.org/index. ... sting.130/

Re: Aluminum molds for ZA-8 casting?

Posted: Sun Jun 17, 2018 5:20 pm
by Harold_V
DavidF wrote:
Sun Jun 17, 2018 9:04 am
I wouldn't bother oxidizing the mold, it will do that itself after a few pours.
You missed the point. If aluminum dissolves in zinc (and it does), pouring to a freshly machined mold will most likely result in some dissolution, and, most likely, some soldering of the part to the mold, especially near the gate(s). The entire point of oxidizing is to avoid those problems.

Most folks aren't set up for anodizing, but John's comment is spot on--although ANY anodizing will work just fine. Hard anodizing changes size considerably more than a simple anodizing process, which would serve perfectly well, too. Difference would be if wear resistance would be an issue. Hard anodizing is extremely hard, and can be deep (thick). As an example, folks who own an I-22 Gorton mill can likely tell you that the spindle pulley has a brake band at the bottom, which is hard anodized. It takes years of full time use to wear through the anodizing.

H

Re: Aluminum molds for ZA-8 casting?

Posted: Sun Jun 17, 2018 6:28 pm
by DavidF
No Harold, I did not miss the point. What I said was I wouldn't bother oxidizing the surface and that it would do that itself after a couple of pours.
You sir are seriously misjudging the amount of time that the zinc will stay molten when in contact with the aluminum mold.
We are not talking about a production mold that will be cycled thousands of times, if we were then we certainly would not be considering using aluminum for the mold material as the mold will be far more likely to suffer from warpage long before its eaten away from the zinc. Powdered graphite rubbed on the molds surface works great to keep it from sticking.

Re: Aluminum molds for ZA-8 casting?

Posted: Mon Jun 18, 2018 1:49 am
by Harold_V
DavidF wrote:
Sun Jun 17, 2018 6:28 pm
No Harold, I did not miss the point. What I said was I wouldn't bother oxidizing the surface and that it would do that itself after a couple of pours.
Yeah, you did miss the point, which was that the molten zinc had the possibility of soldering to the mold without a little oxidation to shield it.

I've had silver fuse to a properly blackened (graphite) cast iron mold before, and gold, too, so that it can happen isn't beyond reason, and that was my point. I don't consider any of this excessive---I consider it as taking measures to prevent a problem that can happen. I'd certainly not skip the oxidation for that reason. That simple procedure could prevent ruining a mold that might be labor intensive to create. Or not. There are no guarantees.
You sir are seriously misjudging the amount of time that the zinc will stay molten when in contact with the aluminum mold.
Not at all. While the metal will chill rapidly at the far reaches of a mold, at the sprue and runner that may not be the case, as it sees hot metal until the mold is full, and it has the possibility to fuse to the aluminum, especially if the zinc is very clean, free of oxides itself. The washing action of the metal being poured can easily clean the aluminum enough for some to fuse. Been there, done that, just not with zinc.
We are not talking about a production mold that will be cycled thousands of times,
That may or may not be the case, but what is the case is that what ever method is chosen, I'm sure I speak for Mike when I suggest that he wants it to work. Tempting failure by ignoring a simple process isn't exactly what I'd call a good idea. From my vantage point, the minute it takes to expose the mold to the required heat isn't wasted--it's cheap insurance.
Powdered graphite rubbed on the molds surface works great to keep it from sticking.
I've experienced a proper graphite wash (purchased from a foundry supply) being displaced by molten metal (see my comments, above) in areas where the metal is introduced. If that can happen to a wash that's well adhered to the mold, how well do you think a film of graphite is going to withstand the metal being poured where it's introduced to the mold?

I think you've let enough controversy in to this discussion, simply for the sake of controversy, don't you?

H

Re: Aluminum molds for ZA-8 casting?

Posted: Mon Jun 18, 2018 3:33 am
by steamin10
Gents: related to open mold and die casting is the problem of short run casting with aluminum or iron molds. Metals have a tendency to dissolve others as mentioned, but in this case you will quickly see erosion of the mold along any sharp edge and picking out the grain structure of the mold itself. Exotics like beryllium copper and such have less tendancy to erode but are not only extreme on expense but poisonous to handle and machine.

I have blown aluminum onto the ceiling with a bit of rust(oxide) in my pig molds from being clean overnight. The aluminum pops as it reduces the oxides of iron in a violent reaction. Adding aluminum to brass can do the same thing, given the copper is usually gassy, and it reacts violently until reduced. Aluminum shot is used in steel making to de-gas and deoxidize steels poured in foundries during handling. The weights are calculated according to a table, and added as ladles are poured from furnace to casting ladle. The garbage floats off in the made slag. It is also why the steel standard was opened up for more points of tramp alloys in the base steel to assimilate more scrap into the recycle stream. It is all chemistry.

i would look for a more compatable material to permanent mold aluminum. Zinc alloys are diecast all the time, from BBq grills to auto parts and housings. I am not sure of the best ( easy) way to set this up for a home shop. I use sand castings and repeat, repeat, repeat. Once the method is established it goes very easy.