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Plaster Mold Not Drying

Discussion in 'Molding and Casting' started by tobynitch, Jul 14, 2021.

  1. tobynitch

    tobynitch Member

    I recently made a plaster mold of a rabbit head (raw). It was frozen solid when I put it in. I coated it in mold release. After pouring the second part, I went to bed, woke up the next morning and took the head out. Plaster usually cures in 30 minutes to an hour so I expected the mold to be solid by this point, but it wasn't. The inside of the mold was very chalky, and the outside was cold and over all it didn't feel very solid. I could easily scratch marks into the surface with a tool. I decided to put it in the sun for a couple of days, taking it inside at night or whenever the sun wasn't out, but after a week it was still like this. I poured the polyurethane foam into it anyways to make a cast. It kind of destroyed the mold but this was ok with me since I was just casting this head to be able to sculpt over it and make a nice change out head sculpture, so I didn't need any more than one. I just want to be able to make permanent molds of said change-out head sculptures with the plaster instead of silicone. Does anyone know what went wrong? I assumed it was the iron in the blood of the rabbit head that messed with the sulfur of the plaster, but I've seen other people carcass cast in plaster before with no issues.
     
  2. 13 point

    13 point Well-Known Member

    Not sure what happened to your plaster , next time use bondo instead. Do everything the same , but pour bond instead of plaster . To get bondo thin enough cut it with fiberglass resins
     
    msestak likes this.

  3. crablover

    crablover Well-Known Member

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    969
    You do not need any mold release at all. The fluid in the head altered the plaster from setting as did the head being frozen
     
    msestak and 3bears like this.
  4. Frank E. Kotula

    Frank E. Kotula master, judge, instructor

    Seal your head with lacquer first before you pour. Moisture, fluids caused your issues. Being frozen and getting the heat from the plaster add way to much to the plaster. Plus make sure your using a mold casting plaster and not over the counter. The mold plaster is a bit more but it is designed for casting and mold making. next time don’t freeze the head, dry it best with a hairdryer, seal, splash coat then your heaver coat. You can also try bondo resin mix yo make a mold instead of plaster.
    I’ll do the same a thin coat but as soon as the bondo kicks get the second coat on then when that kicks , use a bag if water and ice on it. That’ll cool the bondo from cooking the head. Flip, do other side use a release so bondo doesn’t attach to the other half .
     
  5. tobynitch

    tobynitch Member

    Would straight bondo with the hardener work at all? I've only ever used it for doing ears and attaching antlers and stuff like that, I've never mixed it with resin. Will any fiberglass resin work? I've got some but its 3+ years old I definitely need to get some new resin.
     
  6. 13 point

    13 point Well-Known Member

    Yes , but sometimes it’s just to thick to get into all the nocks and crannies you might say . It doesn’t take much resin to thin it , your old stuff should be fine as long as it’s still pourable. If not you can get a small can at Wally World . You can use either catalos to kick it but I like and use the bondo paste hardener
     
  7. msestak

    msestak Well-Known Member

    14,937
    14,442
    when you mix fiberglass resin with bondo, remember to use both the fiberglass hardner and the bondo hardner too :)
     
  8. 13 point

    13 point Well-Known Member

    No you don’t have to , 1 or the other will work as I said above .
     
  9. msestak

    msestak Well-Known Member

    14,937
    14,442
    i always used both. learned something new :)
     
  10. tobynitch

    tobynitch Member

    Gonna try it with my fiberglass resin, thanks for the help everyone!
     
    msestak likes this.
  11. crablover

    crablover Well-Known Member

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    Frank, why do you always refer to to much HEAT when using bondo and now plaster? Both when used are mostly thin layers or pours that disperse over the part you are molding. Unless you make a thick pour or shell, any heat is dispersed so quickly there is no cooking of anything. The reaction to final set up is very quick and doesn't last long enough to create any major problems. On the other hand, both used in bulk or thick pours will become heated only during reaction time and does take longer to cool. In the case of taxidermy, molding parts with thin layers, rarely should any issue be caused by heat