Making a Mold for Casting Aluminum
I sort of skipped over this part in other posts, but now I am going to go more in depth into it. I make a mold for casting whenever I want to duplicate anything. Generally I use it to turn things that I make out of wood into aluminum. Making a mold requires a lot more work than simply carving something out of foam, but it yields beautiful results (if you do it right).
The first step when making a mold is getting molding sand. There are two main types of molding sand. Oil based sands, and water based sands. I use Petrobond, which is an oil based sand. Petrobond has the ability to provide a very detailed finish. The only problem with Petrobond is that it gets burnt when molten aluminum touches it. Because I am too cheap to throw out the burnt stuff, I have to mix it back in with the unburnt sand to make it usable again. I do this by putting a little at a time into a bucket, and then mixing it with my cordless drill.
The second step is to get out your flask. The flask is a box, usually wooden (at least for hobbyists), that can come apart in two pieces called the cope and drag. When the mold is ready for the pour, the cope is the top half, and the drag is the bottom. They are built in such a way that they can be put together the same way each time. If you have an adjustable flask like mine, then adjust the size to properly fit the pattern that you are making a mold for. I happen to be attempting to make a mold for a Ping Pong paddle.
You can see in the above picture that my Ping Pong paddle is covered in a white powder of some sort. This is parting dust. Parting dust is supposed to keep the sand from sticking to things you don't want it sticking to. You can buy actual parting dust online, or you can just do what I do and use baby powder. It works just fine in my experience.
Because the ping pong paddle doesn't have any flat sides, we can't just lay it down on the ground and pack sand on top of it. If you had a pattern with a flat side, you could do just that. Because of the irregular shape of the paddle, it was necessary to place a layer of sand about 1/2 inch thick on the bottom. The paddle could then be placed on top of that, and pressed down to compress the sand underneath. I made sure the paddle was firmly pressed into the sand by picking it up, putting more sand in where it looked less compressed, and then pushing the paddle back in again. It's important that the pattern doesn't bend when sand is packed around it.
The sand that will be touching the side that is facing up at this point will be a part of the finished mold, so it is important that it captures all the details. The best way to ensure this is to put fine particles of sand on it before larger pieces of sand are packed on top. I use a cheap metal strainer to sift (riddle is probably the more correct term) sand on top of the pattern. I push the sand through the mesh with my fingers until the entire pattern is covered. I then pack the fine sand down gently with my fingers.
From here on out, it's all just pouring sand in and packing it down. First, put in enough sand to fill the drag up about halfway. Then push it down firmly with your fingers, making sure to especially get the edges. Then pack it down with your chosen tamping implement. I use the head of a maul. Anything heavy with a large base will work. You don't need to hit the sand very hard. Just hit it firmly. First pack around the edges, and then go back and forth across the rest of it. Keep packing until you feel that more packing isn't really changing anything.Then add more sand and repeat.
Once you get to the top, add more sand on top until it is above the sides of the drag. Keep packing it down. After you have a nice firm mound of sand, take a long straight and rigid object, like a board or metal bar, and scrape off the sand that is above the sides. This will make a nice smooth surface to rest on the ground.
Now carefully flip the drag over. The bottom side should be nice and flat. However, you might notice that in the picture below the sand looks like it is broken a little. Remember when I said that it is important to push the pattern down firmly in the sand before you start packing? This is why. Since there wasn't enough sand supporting part of the paddle, it bent and broke out of the sand a little. It might also be because I didn't put enough sand underneath it.
Unfortunately I forgot to take pictures while I was doing the next step, but I'll show the result. In order to make the second half of the mold, the second side needs to be exposed. To do this, take any implement you like and start carving the sand away from the top of the pattern. Usually I use a spoon, but you can also use finer implements like a screwdriver or razor blade to get the fine details.
Your goal is to carve down to the parting line. The parting line is the imaginary line on the pattern that the mold will part at. For a symmetrical pattern like a Ping Pong paddle, the parting line is simply halfway down the side. On more complex shapes the parting line might not be perfectly level.
You may notice that there is a white powder covering the entire surface in the previous image. That's because the next step is to apply parting dust to the entire top surface. Be liberal with it, but not enough that it piles up. You don't want the sand to stick. Also, put the top half of the mold (the cope) into place.
Once again, riddle sand over the top of the of the pattern until it is covered. Since I had just received new sand, I decided to use it on the top half of the mold. It starts out very orange.
Add sand, and compact like before until either the cope is full, or you run out of sand. In my case, because I never have enough sand, I ended up with the cope being about halfway full. If you don't fill the cope full enough, it could break. My general rule of thumb is that if the cope is halfway full, it's probably safe. If you do have enough sand to fill up the entire cope, then scrape off the top like before.
Now it's time to add a hole to pour the aluminum into. I do this by pushing a thin, hollow metal tube into a place where I know I am not going to hit the pattern. I only push it just far enough to make a small hole in the sand on the other part of the mold.
Next, slowly and carefully lift the cope off the drag. Just lift straight up. After you get it off, place it down somewhere. Remember that there will be sand sticking out past the bottom of the cope, so be careful how you place it down. After you have it off, look at the bottom half of the mold. In the picture below, you can see that there is some new sand from the top that stuck to the bottom! I even applied parting dust! I suspect that the main reason this happened is because my new sand was extra sticky. One important thing to look for is sand from the top half that broke off on the edges of the pattern. If you see some, blow it away. There is no way to put it back where they are supposed to be.
The next step is to remove the pattern. There are many different ways to go about this. The first thing I do is tap the pattern in different directions until the pattern seems a little looser in the sand. If the pattern is allowed to be damaged, then the easiest way to get it out is to put screws in it and lift it out by holding onto those. Otherwise it gets more difficult. Try to find somewhere on the pattern that you can hold onto with your fingers. Then try to lift it straight up. Once you get it out, blow away any loose bits of sand from the edges of the cavity. It's easy to remove too much aluminum, but it's not possible to add aluminum where some is supposed to be.
The last thing you have to do on the inside is make a path for the aluminum to flow from the spout to the pattern. This is called a gate. It is important to make the gate pretty large. The gate that I made in this picture is probably too small. You want the gate to be one of the last things to cool. This will help prevent deformities in the casting. I just use the metal pipe that I used to make the spout to carve it out. You can then place the top half of the mold back on. Make sure that it is put on with the same orientation that it was taken off with. The last thing you want is to ruin a perfectly good mold because you put it on backwards.
The final step is to get a regular empty steel can, and cut off the other end. This will be used as a sort of funnel to hold the extra aluminum that doesn't fit into the mold. The extra aluminum in it will help to increase the pressure in the mold, hopefully resulting in a better casting. I just place it around the spout, and kind of twist it into the sand a little so that it doesn't move.
And that's it! If you want some more clarification, or you have any suggestions, please contact me. But Simon, why don't you show us how the casting turned out? Let's just say, a Ping Ping pong paddle requires a surprising volume of aluminum.