So I’m trying my luck at lost foam, casting I’m having an issue with trying to cast a Styrofoam skull life-size. I caught the phone with plaster of Paris. That’s working fine. Put the mold in the sand but when I pour my metal in it seems to boil out I’ve tried pouring it from the top down and from the bottom up. I do have a vent. I don’t know if it’s a placement of my vent. If my metals too hot I’ve tried a few different ways and I can’t seem to figure it out. Suggestions would be highly appreciated. Thank you.
I assume you meant to say, "I coat the foam with plaster of Paris" Anyways, how thick is your layer of plaster and what type of sand are you using? A lost foam mold vents through the sand, to ensure the sand is sufficiently permeable it must be dry and free of clay or any other binders also, the plaster coating must be thin enough that it doesn't prevent the mold from venting either.
It's recommended you coat the foam with a thin coating of drywall mud, not plaster of Pairs. Just make a thin liquid, dip or pour over. It needs to be thin for the gas to escape through it. Some casters even tap on the side of the metal container with a mallet. Helps to settle the sand around the foam. There's a lot of foam Halloween items that can be cast using the lost foam prosses. I sometimes look the day after for cheap things.
In addition to the comments above, if the life size skull is a solid ploystyrene pattern the primary reason is likely the sheer mass of the skull and molten metal. That would not cast well with any casting process and likely have horrible shrink defects without incorporating an aboslutely massive riser. It should be hollowed out to to produce a relatively unfiorm wall thickness. If it is life size I'd recommend, <1/2" wall thickness should be practical. It will then have to be position in the sand mold with pen cavity facing up so the sand can pack well. Best, Kelly
Thanks for your reply’s. I have been putting a very thick coat of plaster a pairs.. at least 3/8” thick to 1/2 “. And as far as the thickness of the foam skull I did gut a lot of foam out of the center of the skull but still had spots that approached 1 1/2 “ thick trying to have the smallest hole at the bottom to dig foam out of… as far as filling the mold from the top or have it fill from the bottom up. Which is better ?
That is waayyyyy too thick. If you were applying it to colored foam, it doesnt need to be any thicker than necessary to cover the color. As mentioned earlier, a thinned, lightweight drywall joint compound is better than just PoP, because the adiditives that make it lightweight and apply more easily also make it more permeable. Many people assum the coating is tsructural mold so they apply it thick. It is not a structural component of the mold and a thick layer is counterproductive. Then you should remove more of the pattern than you have. Just for clarification, when you say fill the mold, you actually mean where you attach the sprue/feed system for pouring the casting correct? The mold is the sand that surrounds the pattern. The pattern becomes the casting after it is evaporated during the pouring of molten metal. My prior comment was about positioning the pattern in the mold so the internal cavity of the skull faced (mostly) upward, and you then filled the mold from top with sand. I would also say yes, it is better to fill/feed the pattern with molten metal from thtop as well. In addition to talk texting verbal posts, if you wan't better quality advice, you'll eventually need to post pictures of what you are doing so people can better understand what you are doing. In addition to what has already been covered, how you make and pack the mold, attach a sprue to the pattern, position the pattern in the mold, whether or not you use a pouring cup, how you are contolling the melt and it's temperature, will all be important to your results and advice you receive. If all you want to do is pour some molten metal on a polystyrene skull and see what happens, that's fine. But if you want to use it as a learning experience that actually allows you to progess and succeed with good castings, and all you can tell us is you poured and it boiled out, you are leaving out a enormous amount of information relevant to your result. Best, Kelly
