lost foam casting problems

[shoeswith]

Hi

I have been messing around with a bit of lost polystyrene casting. I have done a couple of simple ones and moved onto something a bit more ambitious. I made a form from thin sheet polystyrene and glued it together with a hot glue gun. The pics may be a bit unclear but there is one pourer that splits into four feeding into each corner, the thin rods coming up were risers made from drinking straws. The thickness of the walls of the casting is 4mm. The cad image is what it should look like.

When I poured the casting aluminum came up the drinking straw risers very quickly with a surprising amount of force and the final casting is missing a sizable chunk from the bottom. I am guessing I should have made the wall thickness a bit thicker maybe 6mm and that it should have some risers from the bottom of the casting. I am not sure why it didn't complete. maybe it was starved of metal, maybe gasses couldn't escape or simply there wasn't the pressure to push it down the narrow 4mm gap. I am not sure but have you any suggestions. I want to continue to use lost polystyrene and I don't mind the odd small fault. Thanks for reading.

[awerby]

If you're using solid polystyrene, it won't get out of the way fast enough to make a solid casting. This only works with light foams. Even then, 4mm is very thin to expect a void-free casting. A porous sand-bed is also critical, or the fumes will resist the metal, since they can't escape quickly enough.

Andrew Werby
United Artworks- Sculpture, Jewelry, Furniture,and other Art Objects, plus Art Resources, Techniques of Sculpture, Jewelry and Lapidary, an Online Bookstore, and a Custom Art Referral Network

[lukewarm]

You can try coating the foam in drywall mud, drying it well in the oven, and removing the foam with acetone giving you a shell.

Thin wall casting is tricky, maybe your metal cooled too much before it reached the bottom. Try orientating it sideways and putting more sprues.

[shoeswith]

Thanks for your replies

Sorry I should have added more about my process. The model to be burnt out was made from very light white expanded polystyrene and was coated with drywall mud. I had it in a bucket of dry loose sand so it should be porous.

From what you have said with lost polystyrene most of the fumes generated by the burning polystyrene defuse into the sand. I had given my models two coats of drywall mud. I wonder if too much mud will actually be detrimental and stop the escape of the gasses into the sand.

Also is venting even needed. My vents were at the top of the casting I am guessing that it was easier for the aluminium to escape up the vents (with the escaping gasses) than to descend into the polystyrene. Having the vents here also must have reduced the internal pressure.

I will try making the wall thickness thicker and putting it sideways. I am not sure whether to vent perhaps fewer with some of them at the bottom

Thanks

[awerby]

Yes, it seems like you may have put too much mud on for the venting action to work through the sand. And a vent at the top won't work too well if the gasses have to bubble up through a mass of aluminum. Try running a vent from the bottom straight up to the top next time.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[RomanLini]

How hot did you pre-heat the mold?

If the mold was too cold and your alloy was not hot enough it will solidify before it reaches the bottom, compounded by having thin sections (as they cool faster and carry less heat to the bottom).