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Cast iron porosity.

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airmodel:
I know that air vents are used in sand molds to prevent porosity so in this video plenty of air vents were used in the sand molds. The sand molds were made in a backyard setting and sent to a professional iron foundry to be poured. Watch from 47:39 to 49:34, this is a hour long video. The castings are the worst I have ever seen! Clearly the air vents never prevented the swiss cheese castings from happening so it looks like a metal-mold reaction where excess gas bubbled through the casting.
http://www.youtube.com/watch?v=L3gzFhcXMWo

CI:
I use identical sand and resin binder that she uses.

I see several things that could cause problems.

Those type molds should be flamed with a soft propane flame to burn off an uncured resin that is on the surface.
I did not see any mold flaming in the video.

I spray on an alchol-based ceramic mold coat, light that to let it burn off, and then flame that too with propane.
I did not see any mold flaming after they applied the graphite.
Not flaming the graphite can cause terrible gassing/bubbles in the casting (don't ask me how I know that, but I have photos that look like the surface of the moon, from the early days).

She mentions the foundry doing a terrible casting job, but to be fair, she is the one who laid out the spure, runners and gates, and from experience, using a sprue/runner/gate system like that can cause those problems.
I use a sprue that has a smooth transition into the runner(s), with spin traps at the ends of the runners, and thin rectangular gates at the top of the runner.

When the sprue is initially filling, there is a point where both air and iron are flowing down the spure.
Once the spure is full, and if the pour is not interrupted, you have only iron flowing down the spure.
If you don't use spin traps at the ends of the runners, then that initial mix of air and iron can flow anywhere, and really cause a lot of damage.

With the spin traps open to the top of the cope, the initial flow will go into the spin trap along with the initial aspirated air, and any loose sand.
When the level of metal rises to the top of the runner, then and only then should the clean iron start flowing into the mold cavity.
If you use dead-end non-vented runners, the initial air/iron mix will strike the end of the runner, and bounce back into the runner system, churning more air into the iron.

The thin rectangular gates on top the runner also help skim an slag that may be floating on top of the iron, as iron flows into the mold cavity.
She does not use thin rectangular gates.

I use a small small vent hole at the highpoints of each mold cavity, with the vent extending out the top of the mold.
I forgot these vents on one mold, and had two very large bubbles in the casting at the top of the mold.
Resin bound sand is not very permeable, and the high points of the mold need small vent holes.

I think with spin traps at the end of the runners, these defects could be avoided.
I don't have defects like this, using the exact same sand and resin systems.

I don't think this was a venting problem, I think it was a runner with no spin trap problem.
You can also cause a lot of defects if you even slightly interrupt the pour, since that will aspirate air, which can cause defects like this.

That molding sand and resin are great to use, and make very accurate molds that are durable when handled.
I use a slide hammer just like the one they use, and that is why resin-bound molds are so accurate, because the pattern pulls straight out of the sand, and there is no gap between the pattern and the sand.

That is my slant on what went wrong with these otherwise very nice molds.

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vtsteam:
Thanks airmodel!  :cheers:

airmodel:

--- Quote ---I spray on an alchol-based ceramic mold coat, light that to let it burn off, and then flame that too with propane.
--- End quote ---

For many years I used S.S. sand on thin sectioned castings, an alcohol based mold paint called "MOLDCOTE" by foseco, it was a zircon flour with a binder. Never had any problems with it and all I did as the instuctions said was use a match to burn the alcohol off and the mold is ready to use. Some time later I had to cast in iron a headstock for a small lathe in greensand. It needed two cores where the casting is very thick (30-35mm). Cast iron does burn on in thick sections so I used some of that mold paint so the cores could be removed a lot easier. Machining those bores revealed plenty of blowholes close to where the paint was. If more was machined the blowholes disappeared. To me it was puzzling as to why it worked for thin castings but on thick castings it did not. Thick walled castings take a lot time to solidify than thin castings and has plenty of time for the metal mold reaction to happen.

As to why cast iron gypsy castings failed I will give my two cents worth.

Most of those molds she made were thick castings and needed a coarser sand so the gases can escape through the sand instead of bubbling through the casting. It could also be that the pouring temperature was too hot making the thick sectioned castings take a lot longer to solidify and make the problem worse. In hindsight this is what should have been done using a coarser sand for the cores in my headstock casting.

CI:
They use that same sand and resin at the local Metal Museum, and when they have a big event, they literally pour a LOT of iron.
The sand is OK85; not sure if that means mesh 85 or what, but it is pretty fine and apparently kiln dried sand, all round grain, and all the same grain diameter.
Very dry sand is required for the resin to work correctly (per Ask Chemical, the company that makes the resin Linocure).

I have been to a number of iron pours at the Metal Museum, and seen perhaps 100's of their resin-bound molds, which are OK85/Ask Chemical molds, including Laura's molds.
I have never seen a problem with having the sand too fine, reflected in the castings they make, at least it not obvious to me that it is an issue.

You can get a pinhole in a bag of OK85, and the entire bag will have emptied itself onto the floor by the next morning.

It sure would have been nice to see a video of the pour for those castings.
That would perhaps shed some light on things, with something obvious like an interrupted pour, or a pour that had slag floating on top, etc.

Laura casts some fantastic stuff in iron; she has talent.
She has a very large cupola tatooed on her upper leg in front, which is quite impressive, but my wife does not like me to notice things like that.
The art iron group people are very passionate about what they do, perhaps because many of them sell it as artwork.
And the art-iron folks are the nicest people I have ever met, and will stop what they are doing to answer questions or help you with what you are doing.
Those big multi-cupola iron pours are wild.  If you have not attended one of those, you should try to find one.

Our member "airmodel", aka "Luckygen" has done a lot of cupola iron pours.
He is the king of hobby iron in my opinion.

Edit:
The most impressive cupola tat shown below.
LOL, don't tell Laura I posted this; it can be our little secret.

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