The Shop > Composites & Plastics
3d metalcasting test
tom osselton:
Ok thanks for the info I'll try again in 2 weeks I'm just having a coffee at Denny's then hitting the highway for Victoria BC should hit the ferries in 12 hours or so.
vtsteam:
--- Quote from: mattinker on August 07, 2014, 12:41:19 AM ---The Zinc actually boils at 907°C and a melting point of, it's not burning it's boiling. The melting point of brass is 900 to 940°C, so keep get to melting point and pour, hot molds will help keep the fluidity up longer. I have no experience of casting brass, it's not very attractive!
Regards, Matthew
--- End quote ---
Zinc burns, Matthew! :doh:
Ignition temperature of zinc dust is only 600C. Zinc vapor is similar. What do you think happens when you boil it? If you boil zinc in the presence of oxygen It WILL burn.
I've done it and seen it.. The flame is greenish. The voluminous white smoke is zinc oxide from burning it, not elemental zinc. Anyone else who has ever overheated galvanized metal or brass with a torch can attest to this. There are flames.
mattinker:
The Zinc burning is a respiratory problem, the boiling of zinc when melting for casting is another problem. Zinc mixed with molten copper does not have a supply of oxygen, melting brass leads to zinc loss. You will loose zinc without it necessarily burning.
Regards, Matthew
vtsteam:
--- Quote from: mattinker on August 07, 2014, 01:15:35 PM ---Zinc mixed with molten copper does not have a supply of oxygen.
--- End quote ---
It does on the surface and when vaporized!
A cup of gasoline also doesn't have much oxygen, internally, but I wouldn't suggest lowering one into a running furnace!
There's plenty of oxygen in there. Since the atmosphere inside a brass furnace exceeds zinc's ignition temperature, any free zinc vapor will oxidize instantly, ie. burn and produce the characteristic white smoke that is zinc oxide.
Visible zinc flames erupting from the surface during brass founding are called "flaring" in foundry parlance. Can we stop debating whether the flames Tom saw when melting brass was due to zinc burning? :bang:
mattinker:
I'm not debating whether the flames Tom saw when melting brass was due to zinc burning, I'm pointing out a practical difficulty of melting a substance that boils at around the melting point of the alloy that contains it. You are right about the Zinc vapors and fever. I do know from personal experience what fume fever is! I would imagine that slag on the surface of the melt which would keep the oxygen away. I wonder how brass is cast commercially.
Regards, Matthew
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