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Beginner needs bronze advice


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I would guess it has to do with lack of venting.  As the metal enters into the mold, the air that is in there needs a path out to make room for the metal.  

ETA: It could also be that your mold pre-heat is not adequate or that your pour temp is too low to begin with.  Or a few other things.  

Edited by Jerrod Miller
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Not sure, it was a gravity pour , 2lbs of bronze. The mold/flask size was 8inch x 12inch. Never heard of venting on larger pours.

maybe it is something with burn out process, like you said.

 

just find it odd the sprue poured well, but none of the objects.

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General rule for determining vents:  Vent, vent, and vent some more.  When you think you have enough, double it.  Then maybe you really do have enough.  

 

Granted, my expertise is generally with sand casting, but I have poured a fair few investment castings, too.  Always vent as much as is practically possible.  

 

Also, your pour cup looks extremely rough.  You want that to be smooth to reduce turbulence.  It also looks like you have cold lap in your pour cup.  

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Also worth keeping in mind, anything thinner then 3mm, the bronze will have difficulty entering. Below 3mm the number of failures grows exponentially. Below 2mm, you can pretty much forget. Then the surface tension of the bronze is too great to be overcome with gravity and you'll either need centrifugal casting or vaccuum casting.

 

And in addition to that, bronze needs time to fill the mould. And to have that time, the mould should be hot enough to allow the metal to fill the mould before it solidifies. That doesn't mean hotter is better. The hotter the mould, the slower the metal cools after it's cast, leading to coarser grain structure. This means that the surface will have much less detail. For good detail, you want the mould to be as cold as possible. But to get to fill it, it needs to be hot enough. 

Jeroen Zuiderwijk

Facebook page: https://www.facebook.com/barbarianmetalworking

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10 minutes ago, John OBrien said:

So what is cold enough and hot enough, 800 degrees for 2 hours? After the initial 1350 cure hold.

I thought was 1000 degrees less then melting temp, which would put the hold at 1100 degrees??

That depends entirely on the cast. The more solid, the colder the mould and the thinner, the hotter it needs to be. Some objects can be cast in cold moulds, and for others (thin walled casts in particular) it can be near the melting point of bronze to have any chance of the casting being complete. But the gate and vent system also plays a large role in this. The more and bigger the gates and vents are, the easier it casts and the colder you can allow the mould to be. However, the size and placement of the gates and vents need to be such that the air can escape correctly, and the gates such that you don't get shrinkage porosity and that you get optimal use of gravity. You want the bronze to flow downwards as much as possible, and as little sideways as possible and certainly not upwards, and also not against the flow of the air that needs to escape. It can be a lot of trial and error for each different casting. 

Jeroen Zuiderwijk

Facebook page: https://www.facebook.com/barbarianmetalworking

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Also, the actual specific chemistry of the metal will play into its solidification properties and viscosity.  Pour temp can be played with a lot, too.  Casting has a lot of variables. 

 

6 hours ago, Jeroen Zuiderwijk said:

You want the bronze to flow downwards as much as possible, and as little sideways as possible and certainly not upwards,

This is exactly the opposite of what is done in many industrial applications.  The reason for wanting to flow up is to keep a uniform metal front (no splashing or spreading).   

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1 hour ago, Jerrod Miller said:

Also, the actual specific chemistry of the metal will play into its solidification properties and viscosity.  Pour temp can be played with a lot, too.  Casting has a lot of variables. 

 

This is exactly the opposite of what is done in many industrial applications.  The reason for wanting to flow up is to keep a uniform metal front (no splashing or spreading).   

 

Yes, but you two are approaching the issue with a separation of about 2700 years of technology.  ;) 

 

I do agree on the vents.  Small parts at the bottom of the sprue tree should have so many vents coming off the thinnest sections that they look like big spiders.  The vents hook back into the trunk, unless you add a riser, which in some cases may not be a bad idea.  

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15 hours ago, Alan Longmire said:

 

Yes, but you two are approaching the issue with a separation of about 2700 years of technology.  ;) 

 

I do agree on the vents.  Small parts at the bottom of the sprue tree should have so many vents coming off the thinnest sections that they look like big spiders.  The vents hook back into the trunk, unless you add a riser, which in some cases may not be a bad idea.  

I’m lost on the vent topic. Vent gases to where. My usual casting size (cylinder) 8” diameter by 12” high.

the tree is made and items attached, the investment is mixed, and the tree is put carefully into the investment.

 

venting to me, was a thought on small casting. On a large cylinder your not making holes that go right through the investment walls so gas can escape properly out to atmosphere. 
 

you mentioned the vent going from the object back to the main trunk?

so then where is the gas escaping to if the main trunk is being filled with molten metal? Not understanding that.

the gravity pour was 1.7lbs of bronze, which I felt was heavy enough for a gravity pour. There were 4 items on that tree that didn’t even cast.

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15 hours ago, Alan Longmire said:

...Small parts at the bottom of the sprue tree should have so many vents coming off the thinnest sections that they look like big spiders.  The vents hook back into the trunk, unless you add a riser, which in some cases may not be a bad idea.  

So the vents don't need to go all the way to the top of the mold?  I've been worried that if I hook back to the sprue that the vent would just get blocked by the incoming metal.  (This is why I built the vacuum casting setup!)

 

oops, John and I posted at the same time :)

Edited by Brian Dougherty

-Brian

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On 10/19/2021 at 5:41 PM, John OBrien said:

Hello,

I’ve been tinkering with lost wax and bronze. I can’t figure out why my sprue tree casted but not any of the pieces that were attached??

photo attached for reference.

thanks...Johnny9C88D8B6-A0E6-4248-8A7C-CB8FDE0EFD0C.jpeg

What are the objects you are trying to cast?The sprue is very wrong and it does need "vents". The base for the pour appears flat and should have a large "funnel" shape.....see example.

sprue.jpg

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Like that, or this, from Matthew Berry's blog:

IMG_1756.JPG

 

And here:

The vents connecting back to the tree mean you'll most likely get porosity in the sprue, but who cares?  It might also help if you re-oriented those little flat parts to be vertical, with vents coming off the bottom end and looping back to the sprue way high in the system.

 

All these pictures are from vacuum casting, and as such they don't have nearly the amount of venting a gravity cast needs.  Have you looked into centrifugal or even steam casting?  Both kind of scary, and you're limited as to the size of what you can do in one cast, but if you're good with doing single objects it can work a lot better than a gravity pour. 

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Ideally the vents should go out of the mold well above the parts (typically out the top) and not be connected to the sprue at all.  It is even possible (but not at all best practice) for the vents to not go all the way out.  What you end up doing there is pushing the air out of your sprue and part cavity, and forcing it into the vent, which will build up pressure (in steel this would be ferro-static pressure, in water hydrostatic, in copper cuprostatic, but I am not sure if there is a proper term for bronze but I would guess cuprostatic).  This requires a certain amount of head pressure (density*gravity*head height), which will change based on how much volume your sprue and parts take up.  You can see why this isn't the recommended practice.  Best practice is to have essentially a second sprue that the vents connect to, and keep it away from the actual sprue so you don't accidentally get any metal poured into it and thus block the air venting out.  

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^ they're correct about venting. 

 

I used to work in an aluminum foundry where we cast large aluminum intake manifolds. And other parts.

 

V8 manifolds had a vent tube on each runner end on the intake. Usually these were just 1/4" to 3/8" diameter or so.

 

Even with a large pour hole- once the molten metal fills the hole- there has to be a place for it to push the air and steam  out of the mold, or it can/will vacuum lock the metal in the mold- and cool... causing incomplete pours.

 

Our hunter sand casting machine molds had rods coming out to make the vents as it compressed the sand.

 

A longer cone shaped pour hole- caused the aluminum to swirl more as it poured in... keeping it flowing, and  from just filling the pour neck while pouring.

 

Ours were 3" deep normally, going to a 1" sprue tree.

 

 

Edited by Welsh joel
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