A few weeks ago I decided to learn something about casting, thinking I might learn something about making molds and forms such that I could cast resin models. So I signed up for a lost wax casting course. Can you imagine my surprise when I discovered that lost wax casting was all about casting jewellery in molten metal?
Oh well. So be it. Not being particularly interested in making rings I figured – why not cast a resin model in bronze? This can be an interesting experiment.
Jewellers cast things that are small. The first thing I learned was that I can’t cast a fully formed model because it will require far too much metal at once. I can only melt and cast less than 80-90 grams of metal in one go. I have to cast the model in pieces, and for bronze this means that each cast can only be for pieces that weigh less than about 6-7 grams. A model at 1/8 scale was much too big for the equipment. Fortunately, a few months ago I bought an original Wonderfest kit (WSC#025) on Yahoo Japan and when it arrived I could see that the figures were quite small, standing only about 100mm high. This was ideal for my bronze casting experiment.
So, I’m going to do Mana Mizuki (White Album) in bronze! Mitzuki is the one bent over holding her case.
The first thing I wanted to do was to pin my model so that I knew how it would fit. If I had to do any fixing of the pieces I wanted to do this now.
The jewellery folks make their rings and other things in wax. Wax is good because it can be carved and will melt at a fairly low temperature. Wax residue will also vapourize when it gets really hot. Jewellers put their wax piece into a metal flask and pour a plaster compound around it which hardens. This plaster compound is called investment. Next, they heat the flask up or burn it out in an oven that runs up to about 1000 degrees C or 1800 degrees F. The wax melts and runs out and anything left will vapourize in the heat leaving a nice vacant place for the metal to occupy.
Plastic sometimes works and will burn out cleanly. Plastic is a thermoplastic compound which means that it becomes soft and melts with heat. Resin? Who knows? Resin is a two part compound that is thermosetting. This means that it cures by chemical reaction. It doesn’t come apart with heat. How resin will fare is all part of the experiment.
Being unsure of the results I thought to try an experiment using my barbeque. I took some resin sprues from the model parts and cooked them in a tin plate on the barbeque grill. I also cooked some Tamiya two part epoxy putty that I had mixed up as a filler for my resin joints to see how well this would fare, too?
It turned out that the resin for this model first got really hard and brittle but then it did seem to melt around 500 degrees Fahrenheit. Puddles of a thin clear liquid showed up. Then it carbonized and burned. The Tamiya epoxy was just a lump that got brittle and black and carbonized at 600 to 700 degrees. Knowing this I decided to chip all the Tamiya filler off my joints that I had used. It would be a bad thing if not everything burned out in the oven as I would have contaminants in my bronze casting.
Oh, by the way, they call this a lost wax process because once the wax model is burned out, it’s gone or lost. So I am going to completely destroy my resin figure model when I cast this in bronze.
The first problem that I have is to stick little sprues onto the pieces that I will cast. The sprues are wax and they are the channels by which molten metal will run into the model to cast the piece. I use 5 minute quick set epoxy glue to attach the wax sprues to the resin. Jewellers just melt the wax sprue onto their wax model, so it is easy for them.
When you attach sprues you have to think about how the metal is going to flow when you pour it or sling it into the mold. I will be using centrifugal casting so there is a lot of force pushing the metal into the mold. I can’t expect the metal to flow backwards or bend easily. Some of my sprues like the ones on the pigtails and the briefcase are wrong according to my teacher.
There is a process to follow. See? Notes in a classroom. You may not be able to read them very well because I didn’t have the camera set quite right.
Here are a couple of pictures of the ‘studio’, or the classroom where I and other students will work. There are workstations with flexible lamps and special tools like our Dremel tool that jewellers use to carve and polish their work. It all seems just as messy and confused as our workshops, yes?
The first thing that we need to do is put our pieces into the rubber bottom of a metal flask that is where our plaster investment is going to be poured. Plasticine or modelling clay is put into the bottom and we stick our sprues into the plasticine.
The next thing we need to do is mix up the investment. Once you start this process you have about 13 minutes to complete this before the investment hardens. Investment is a powder that is mixed with water in a very specific ratio. You want to mix up just enough to fill your flasks.
Once the powder is added to the water it needs to be mixed well for at least three minutes. Then the mixture is put into a vacuum bell to suck air out of it. You don’t want air bubbles near your piece or you might end up with metal bubbles when you cast. After you have taken the air out of the mixture you half fill the flask and then swish it around. You then fill the flask completely, and then place the flask in the vacuum bell to suck more air out. Once this is done the flask should sit for three or four minutes to harden. Then, it can be moved.
The pictures below show a few steps in the process. One shows a student measuring the water and the other shows scales for weighing the investment powder, the powder being added to the water in a bowl, and the vacuum bell. It’s really quite stressful doing this as a student because you are watching the clock and you really don’t know quite how this all works yet.
Now, you did weigh your model pieces and sprues before you set them in the plasticine and in the flask, didn’t you? You need to know the weight of your model before you cast it, because this determines the amount of metal that you need to use. As I am casting bronze the formula is 10 times the weight of the model plus 20 grams for the ‘button’.
What is the ‘button’? This is the extra metal that sits outside and must be available for shrinkage as the cast cools. Metal shrinks as it freezes or solidifies and a pool of additional molten metal needs to be available to move into the cast as it solidifies. This metal has to flow into the cast through the sprues or elsewhere, so hopefully the sprues don’t freeze up first.
So, accurate weighing of your model and the metal when you come to cast is critical. In the baggies to the right is the sterling silver metal that is being used by the jewellers to cast their rings.
If we know what we are doing we can fill our flasks with the investment. You can see three of my flasks with resin model pieces and a flask or two with wax models by the other students.
Once the flasks are solid they are put into an high temperature oven overnight. The oven needs to cook things at about 1800 to 2000 degrees Fahrenheit for 14 hours for my resin model. This burn the wax sprue and resin model out of the investment leaving behind a cavity where the model once was. Or, that is the plan. Hopefully it works.
We are now ready to cast. We are going to use centrifugal force to fling the molten metal into the flask with the hopes that it goes up the sprue and into the hole left by the model when it was melted and burned out.
The picture below shows the centrifugal caster. There is a big spring in the bottom of this device. The hot flask from the oven is put in the cradle at the end of the arm. The arm is then wound up three times. The red crucible is pushed up tight to the flask, heated, the metal is placed in the crucible, and melted with an oxygen and acetylene welding torch. For my flask with the head and front hair I needed to melt 80 grams of bronze. This took about 8-10 minutes. When the bronze is molten the centrifugal arm is released, it spins violently, and the molten metal is flung or forced up the sprue holes and into the flask.
Hopefully it works?
Once the centrifugal arm comes to a stop the hot flask can be removed from the machine. If things worked a nice button of metal will be glowing red hot at the end of the flask.
You can see that there is some black junk in my button. I don’t know what this is? The bronze that I was using didn’t seem to melt very well and I almost wonder if there was some slag or something in it? In fact, I think this metal messed up the crucible so much that it might not be able to be cleaned and re-used. But, I did get a button and it looked like something was cast?
The fun part is to quench the hot flask in water which cools the metal, dissolves the investment, and lets you see the result?
After cleaning things up, this is what Mitzuki’s head looks like. It’s not perfect, but it is something that can be worked with. It’s not finished yet as I want to see how all the rest of the pieces come out.
Unfortunately, I was unable to cast the body, arms, and legs today. The flasks never made it into the oven last night for the burnout, so it will be at least another month or so before I can see if I can produce a full anime figure in bronze!