From design, to plastic, to bronze!

David Hinkle and QC-Colab, a brand new hackerspace in Davenport Iowa, celebrated their grand opening with a spectacular bronze casting of their newly designed coin using a DIY blast furnace.

When the MakerBot arrived we had our first group build.   It took us 2 days to assemble the bot and about a week to get everything tuned well enough to make us happy.   I used our logo to do a basic coin design that I felt would print and cast well.

Basic casting in bronze and aluminum is cheap, fun, and very showy.   I knew it would make a great demonstration for our grand opening.   We only had a week to go, and there was no time to purchase a commercial furnace and have it shipped to us, so I decided to go the DIY route.   A furnace is nothing special.   You need a fuel source to burn, enough oxygen to burn it fast,  and some insulation to keep the heat inside.

The furnace you see pictures is our second furnace.   It’s a very practical design that operates off compressed liquid propane.   The liquid propane is fed through a “weed burner” I bought at home depot in the welding section.   The limiting factor inside a chamber like this is oxygen, so the furnace works much better if you provide some forced air.   I’m using a small industrial blower for this, but it’s really overkill.    Many people us a hair dryer.    As you can see from the photo, it’s a trash can w/ a hole in the side that’s been lined with refactory cement.   Refactory cement provides the insulation.  It is a mixture of portland cement, sand, perlite and fire clay.    We bought the portland cement, sand and perlite from Lowes.   The fire clay was harder to find, but any place that does brick or clay chimneys should have some in stock.   We got ours at a local stone yard.

Our first furnace, the one we actually demo’ed at the grand opening, ran off of charcoal using the same blower.  No liquid propane required.   This furnace produced a giant column of flame and sparks that makes for a very happy crowd, but isn’t as convenient to use.   It takes a while to get the charcoal going, and you have to continue adding it as the process goes on.   Also, the giant column of flames requires open sky to operate.

These furnaces will melt aluminum very easily.   Bronze has a much hotter melting temperature, so it’s somewhat harder….  But our second furnace goes from cold to melting bronze in 7 minutes… So, not that hard.   Iron, is of course, the gold standard.   We haven’t tried melting any of that.   As a matter of fact, the crucible (container for molten metal) in our first furnace was steel.   That was fun because it was a very bright, red hot when we pulled it from the furnace in front of the crowd.   Our current crucible is ceramic.

We pour this molten metal into a sand mold that we’ve created using the blank from the maker bot.

You can find some step by step instructions here:  http://www.foundry101.com/new_page_7.htm

As you can see, all you really need is some green sand, some wooden frames and a little parting dust.   Green sand is a fine grade silica sand mixed with a clay binder such as bentonite.   You can make your own by grinding up cheap kitty litter and mixing it with fine grade sand but it’s cheap, and we ordered ours on the internet.

All in all, the whole thing can be done for $200-$300 depending on how much you buy and how much you build out of scrap you have laying around.

It’s quite an amazing idea to be able to download a part from thingiverse.com and cast it in bronze.   The first object we did was that fish, I intend to photograph it and upload it to the thingiverse page it came from soon.  Sandcasting works great for a wide variety of parts, and for more complex builds, a person can move up to lost wax casting.  Small furnaces like this in combination with the makerbot are perfect for casting in gold and silver.