How does head pressure affect porosity? e.g. extended gates/risers

Discussion in 'Sand Casting' started by solarguy, Jul 3, 2019.

  1. solarguy

    solarguy Lead

    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
     
  2. DavidF

    DavidF Administrator Staff Member Banner Member

    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...
     
  3. Jason

    Jason Silver Banner Member

    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.
     
  4. DavidF

    DavidF Administrator Staff Member Banner Member

    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.
     
    Jason likes this.
  5. Jason

    Jason Silver Banner Member

    Thanks.. Good example for others. You're better than google.:p:D
     
  6. HT1

    HT1 Silver Banner Member

    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

    [​IMG]


    V/r HT1
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    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
     

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