Casting sand suggestions: Petrobond 190 mesh or oil free 400 mesh?

I don't really know much about hard data, or mesh numbers or any of that, I just found early on when I was trying to copy the 'Myfordboy' videos, i was getting really inconsistent results until i saw a tip to pack the petrobond looser, and add a ton more venting. When I run patterns at the local foundry we rarely have any venting on them, and they just breathe through the sand.

As a beginner copying stuff you see online with greensand, I've had to add a lot more venting for the same results in petrobond. Usually at least one cold 'riser' at each difficult corner, and poke some nail holes through all the high points in the mold.

 

Petrobond is not "petrobond" I know this is confusing, I'll try to explain; every time you use a reusable sand system the heat causes some of the sand grains to crack and crumble creating "fines" dust basically, this causes sand to get less and less permeable as it is used. but unlike buying a finer sand you do not get a noticeable improvement in surface finish, that is capped by the coarsest sand in the heap, so my well used petrobond is not as permeable as a new box of the same sand, this is why its common to run virgin facing sand backed with heap, your heap slowly grows to counter this you throw away burnt sand from each pour, for a hobbiest this is wasteful for the most part unless you are stressing your system, Cast Fe in Petrobond is an example ,it will work, but its at the bitter end and hard on your sand.
also small changes can seriously affect your sand lets stick with petrobond, you double your clay, or start with a much finer sand, say 400 Vs 160 GFN (GFN is similar to the grit numbers on sand paper as a simple explanation) your sand will be FAR less permeable, if you add to much oil, it will create more gas that needs someplace to go, if you pour and mull without adding oil, it will produce less gas so it doesn't have to be as permeable or vented as well,

Green or waterbonded sand is MUCH more complicated because you have so many more variables 2 clays, a dozen or more additives for specific results, GFN , and grain shapeof sand, type of sand (Silica,Olivine, zircon) and finally the amount of water which changes rapidly , if your in arizona and mull your sand 5 rather then 3 minutes it will be noticeably drier, while here in Florida it could potentially pick up rather then loose moisture,


V/r HT1