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First brass casting :-(


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Hi guys,

i try my first brass cast and this coms out very poor.

 

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I used Ultra Vest as investment. I mixed it how tere say. I think the liquid was a little to stiff. The next time i give a litte bit more water.

I miexed, then vacuum, filling in, vacuum, lat stand 2 hours.

 

2 hours wax meltig by 150° . next 5 hours fro to 730°. 2 hours by 730°.

 

Cast the brass ( with a bit borax ) The brass have it the right temperature.

 

I not clean the melted brass befor casting. I think this was a mistake.

 

At the wax model i give to casting chanels but not air chanels.

 

 

Someone have a idea ?

 

Ruggero

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It looks a bit like very extreme shrinkage, but I've not seen it as severe as that. So I don't have a direct cause and solution for you. It could be trapped air/gass/wax fumes. At any rate, do add air vents, it definitely makes a big difference in completely enclosed moulds. How was the brass behaving when you poored? Was it a nice liquid all the way, or where there lumps or gooy metal in the crucible? I know brasses can sometimes act funny, depending on alloy. And was the mould filling nicely, or spitting, bubbling?

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Assuming this is a gravity cast, I think Jeroen is right on about the air vents. You need to run a couple 3mm or so sprues out the bottom if the object and up to the top of the flask outside of the pour cup. It looks like the air in the mold had nowhere to go when the metal came in. Do you know what alloy you were using, or is this cast from scrap?

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I am bout to try my first attempt at investment casting, so I am not qualified to say this. However, from what I have read, I think the air vents need to exit at the top of the mold.

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Brian is right. The vents start in the right place, but they need to go to the rubber base outside the pour cup. If you connect them to the sprues like you have then metal will go down them. you want to pour into the cup until you see metal in the vent holes. Here's what the vents should look like:

Vents.jpeg

The vents I drew are meant to hit the rubber base outside the pour cup.

 

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Also, with mystery metal you should cover it with some broken beer bottles (1/8 of a bottle or so). Most mystery metal has zinc in it which vaporizes much lower than the casting temp. The beer bottle forms a liquid glass cap which helps hold the zinc in. Remove the glass cap with a rod just before you pour.

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Also with the air vents, they particularly need to go where the air rises towards. The ones at the bottom don't do much, because as soon as metal starts filling up the mould they will be blocked off. Put them where the last amount of air will remain when the shape is filling up, and make sure the channels can remain free of metal until the shape is filled all the way.

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Also with the air vents, they particularly need to go where the air rises towards. The ones at the bottom don't do much, because as soon as metal starts filling up the mould they will be blocked off. Put them where the last amount of air will remain when the shape is filling up, and make sure the channels can remain free of metal until the shape is filled all the way.

 

Yes, exactly. If you want to get fancy make it so your sprue comes in at the bottom of the part and the vents come off the top of the part. This will give the smoothest fill and highest quality results. For gravity pouring at least.

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Yeah, Jerrod & Jeroen are right. I haven't done gravity casting in a long time. The best method might be a bottom-fill mould, where the metal goes down a J-shaped sprue and enters from the bottom of the mold, and the vents go straight up off the high spots of the object. Vacuum casting has made me stupid when it comes to spruing <_<

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I did a lot of bronze casting in my time at college. One other thing that helps is adding some new brass or bronze to the mix. Sometime the scrap has all ready been through the ringer. Pretty much what MathewBerry said about the venting!

-Gabriel

Edited by grpaavola
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Thanks all for your help. The cast is now in the klin to melt out the wax..

The brass i us are rests of cuted bars and sheats. There are clean. But is not special brass for casting.

Tomorow i try with the green glas.

Can i take the glas out simply ?

 

Ruggero

Edited by ruggero
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Here's a rough sketch of what I think will work:

scan022.jpg

The biggest problem with this is that cutting off that main sprue will be very hard. A bottom fill mould would be best, but it's not really going to fit in that flask and rubber bottom you have. Those are designed for spin or vacuum casting. Have you thought about either? They make most of these problems go away and you'll get much better results. A spin caster shouldn't be too much money, and it sounds like you have all the other equipment you need.

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Let us see how the next one comes out I want to try this for a dagger............ some day

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Glad it worked out! Still a bit puzzled about the surface. I think it's still insufficient firing of the moulds, either remaining wax soaked into the matrix, or chemically bonded water. Perhaps heating the moulds to a bit higher temperature might give better results in the future.

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They look great! The bubbles might be water vapor, but they also might be zinc vapor. If you can, get yourself some bronze that has no zinc. Silicon bronze is easiest because it doesn't need flux.

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I kind of late to the conversation but one quick question. How are you melting the brass? Overheating the metal when melting can also cause this kind of porosity. I also agree that part of it looks like an incomplete burnout. Depending on the diameter of your flask you might need to go several hours longer to get the whole flask up to temp and vaporize the wax.

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