Sand casting media?

[Joshua States 2,462]

I am currently working with Deft clay and need to buy more sand. I am looking into less costly alternatives such as Petrobond. It is about half the cost of Delft. The comparison reviews I have seen online are mixed. Has any one used the Petrobond? Is anyone currently using another sand media they would recommend?

Edited January 5 by Joshua States

[Jerrod Miller 593]

If you are interested in mixing your own, it is not that difficult.  Green sand takes a bit of work to keep the right moisture, but K-Bond is a very nice oil-bonded sand that is very stable and works well for non-ferrous stuff.  https://www.afsinc.org/sites/default/files/inline-files/k-bond sand.pdf

 

The key when making your own sand for casting is to have the right distribution of sizes.  Finer sand grains means smoother surface, but more binder agent required as well as possible venting issues.  I would recommend some where in the AFS GFN (Grain Fineness Number) of around 100.  That means most of your sand should be in the neighborhood of a 140 sieve, so about 0.088-0.125mm.  The Petrobond I see on Amazon is listed as made from 140 sieve sand (generally you want a mixture of 3 different sieves though).  With a quick search I could not find what grain sizes are used with Delft, but is has clay right in the name, so it should be much finer than that.  Which is good for surface finish, but not as easy to get everything else right.  

[Jeroen Zuiderwijk 250]

I've used petrobond sand a lot. It's great in that it it's quite stable and doesn't fall apart easily (though between petrobond sands there is quite a bit of variation) and allows very fine detail. Downside is that you get internal porosity in the cast due to the oil burning, and the gasses being dragged down with the metal. I've not found a way to completely avoid that, but sharp corners for the metal to flow over should be avoided, and it should be hammered together as compact as possible to reduce the effect. You don't notice those internal bubbles, unless you grind into the cast. 

[Jerrod Miller 593]

6 hours ago, Jeroen Zuiderwijk said:

Downside is that you get internal porosity in the cast due to the oil burning, and the gasses being dragged down with the metal.

That is exactly the problem of having too fine of sand and not proper sand grain size distribution.  Your sand should have a certain amount of permeability that allows gas to go through it.  You need enough permeability that the gas can go through the sand easier than through the metal.  Also, it is important to use no more bonding agent than absolutely necessary.  

[Joshua States 2,462]

On 1/5/2022 at 4:55 AM, Jeroen Zuiderwijk said:

I've used petrobond sand a lot. It's great in that it it's quite stable and doesn't fall apart easily (though between petrobond sands there is quite a bit of variation) and allows very fine detail. Downside is that you get internal porosity in the cast due to the oil burning, and the gasses being dragged down with the metal. I've not found a way to completely avoid that, but sharp corners for the metal to flow over should be avoided, and it should be hammered together as compact as possible to reduce the effect. You don't notice those internal bubbles, unless you grind into the cast. 

Thanks for the tips and insight. I bought 10 pounds of it from a company in NY called PMC supplies. The first thing I noticed was that it felt much finer than the Delft. I guess I'll find out if there are any gas bubbes once I put this on the lathe.

Cleaned up a bit