I did some searching and reading, but didn't find anything definitive. I have heard that additional head pressure reduces porosity problems, but I have not tested it myself. It's the same effect that causes the bends for divers. At depth (greater head pressure) the solubility of gasses goes up substantially. If they surface too fast, those extra dissolved gasses come out of solution and make bubbles. Bubbles = troubles
Some but not a whole lot. If you use the s/g of the metal to figure out the actual psi in the mold you'll see it would take a pretty tall riser to get much pressure. Now if you can get the mold with the metal in it into a pressure chamber before it solidifies, that would be something...
Hey David.... What's the math to figure out what kind of pressure I have on a 28" drop? I haven't poured that thing yet. just curious.
Seriously?? 1 inch of water is .04 psi, water has a specific gravity of 1.o Bronze has a specific gravity of 7.4 - 8.9 So on the mid range s/g of 8.15 bronze would be .326 psi per inch or 9.128 psi at the the full extent of from your pour height.
why are you under the impression that head pressure directly effects porosity ? porosity is gas in-trained in the hardening metal. gating design can cause this but normally by causing turbulence . also wet sand but wet sand will normally cause alot of other defects along with porosity are you getting a defect like this ? this is a cold shut , and more head pressure, or heating your metal hotter is the cures for cold shuts (edit) venting can cure Cold shuts also and Cold shuts are occasionally call mis-runs V/r HT1
As HT1 notes, there are various sources of casting defects that often get lumped into the generalization of porosity. If you are talking aluminum casting and our purposes, virtually nothing is soluble in molten aluminum except hydrogen. It comes out of solution as the aluminum changes phase and freezes to solid creating porosity. The solubility of hydrogen in aluminum increases with temperature and pressure. Defects cause by entrained or captured air is not the same mechanism because air are not soluble in aluminum. Oxides are big contributor to casting defects and can affect many reactive casting alloys. That subject is discussed at length in the Bifilm thread. Best, Kelly
