Sunday, August 2, 2009

Casting failure on the way to success

I did a lot of work in July and took lots of photos with the idea of writing about my adventures with lost foam casting. There is a lot of good material in the backyard metalcasting forum however I seem to have gone beyond what others have posted about -- although I am sure others have succeeded at what I am attempting.

Now that the hook is baited, what am I attempting? To make bigger castings. My goal is to be able to make a piece with as much aluminum as I can handle myself - about ten to twelve pounds.

With small pieces, lost foam is a piece of cake. You make the foam piece, hot glue a sprue and a vent, plonk it in some loose sand, and pour away.

When the piece gets bigger, everything changes. There is too much thermal mass for the molten aluminum to vaporize the foam. Acetone dissolves foam but then the loose sand collapses into the void. The first technique I tried was to put the piece in petrobond sand, ram it up, and then drip the acetone. Acetone and the vapors are very flammable - probably explosive - so I do this the day before pouring so the vapors have a good time to escape. . There is a flask with petrobond in the photo above sitting in the wagon and you can just see the red color of the sand. No explosions so far

When the piece gets bigger than the flasks on hand the options are to build a bigger flask or do something else. My pieces are often odd shapes, my petrobond is limited, my space for flasks is limited, so I opted for the something else - investment casting. This is simply investing, i.e. coating, the piece with a liquid material that becomes solid. Contemporary investment materials are expensive and require firing in a kiln. The process is designed for lost wax rather than lost foam so it will not work. As a result I have been trying a material that worked for sculptors for centuries - sand and plaster. In the photo above I am pouring a small piece in this investment.

So far the investment casts have mostly failed, however I am learning from each failure so it has been a positive experience. More to come.

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