Introduction: Entwined Hearts Cast in Aluminum
This project was inspired by Kiteman's Entwined Hearts.
I had just finished building a metal casting furnace and needed to make a valentine's day present for my wife. I found his idea and decided to translate it into cast aluminum. I used a technique called lost foam casting which involves pouring molten metal directly on a pattern made out of foam. The foam vaporizes and the metal fills the void creating the casting. I hadn't tried it before but it's supposed to be fast and easy and involves fire. Who doesn't like that?
Step 1: Cutting the Pattern
First you need a pattern of what you want to cast. You will need two hearts. Mine are about 5" tall and 3/4" thick. Start with a piece of foam insulation. You want the pink or blue stuff that they sell at the hardware store. It's possible to use white beadboard but it's crumbly and won't give as good of a resolution. I cut out my heart patterns on a cnc router (I got the cnc bug from this excellent instructable: www.instructables.com/id/How-to-Make-a-Three-Axis-CNC-Machine-Cheaply-and-/ but ended up using plans from here:www.solsylva.com/ which ended up being a two year diversion) You certainly don't need to use a cnc router to make your pattern. A saw, knife, file etc. will do just fine. I cut out the heart in two pieces and glued them together. Make sure that you add two sprues off the top of the heart. This is where the aluminum will enter and exit the mold. It took about 20 minutes to cut each piece. The ridges in the pattern are because I cut it out at a low resolution. I was too impatient to wait longer for a smoother cut and I thought the they looked interesting. I uploaded the G-code if you want to use my pattern.
Step 2: Preparing to Cast
Now that you have your patterns it's time to fire up the foundry. If you don't have a metal casting set up you can find instructions for building one on this site or there is also an excellent forum/community of backyard metalcasters located here: backyardmetalcasting.com/forums/ I found them very helpful in getting me going with an inexpensive (runs off of waste vegetable oil) home-built foundry. My furnace build is here: backyardmetalcasting.com/forums/viewtopic.php
You can do lost foam casting in a number of ways. I chose to just bury my pattern in loose sand. This is the easiest and most immediate method but it gives you a grainy surface from the sand. I knew I was going to do some finishing to my castings so I didn't mind. I sifted regular play sand into an old soup pot and buried the first heart up to the top of the sprues. Then I took a short can with the top and bottom removed and put it around the entry side. The can acts as a collar to retain the metal and gives you an extra supply to help fill the mold as you are pouring. You also don't have to be quite as precise as you pour.
Step 3: Pouring
I guess it's time to mention that pouring molten metal can be exceedingly dangerous. Please take the time to research it thoroughly and use appropriate safety gear. You do not want to drop a crucible of liquid metal on your foot. I don't know what nasty chemicals are released when you vaporize foam but I would strongly recommend pouring outside and wearing a respirator with organic vapor cartridges in addition to your other safety gear. Once your furnace is heated up and glowing orange melt some aluminum in the crucible. Make sure you have plenty melted, you don't want to run out in the middle of a pour. My hearts took about 1 lb each with the sprues and excess. Have the pot with your mold on the ground near the furnace and all of your tools ready. When you pour the casting fill the metal collar and don't stop pouring until you see a puddle of metal coming out the other side. There will be some smoke and fire but you don't want to hesitate in your pour or the metal can freeze up and the mold won't fill completely. Wait about 5 minutes after the pour to pull the casting out of the sand. You want to be sure it's nice and hard.
Step 4: The Second Pour
After the first casting has cooled enough to handle, cut the sprues off with a hacksaw or sawzall. Cut the second heart and insert the casting so that they are linked like a chain. Glue the foam heart back together. As you bury the foam this time float the first casting in the sand parallel to the bottom so that it doesn't touch the pattern. Then go ahead and cast the second heart the same way you did the first.
Step 5: Finishing
Now it's just a matter of removing the sprues and finishing the surface of the hearts the way you like. I used a saw to cut them off as close to the top of the hearts as I could. Be careful not to cut too much off. Next, I used a grinder to rough shape the tops. I used a file to do the final shaping and to smooth the surface somewhat. I wanted to leave the ridges in the hearts but also have them be smooth enough that they invite you to handle them. Finally, I used 120 grit sandpaper to take out the file marks and 600 grit to get the final finish. This was my first real cnc project and my first casting. I was very pleased with the result and my wife liked her present too.
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