Casting the Medallions

Now that we have some wax models from the injection molds, it's time to turn them into metal. I do investment casting, which uses a gypsum "investment" to make the mold around wax models, which are then melted out. This is opposed to other forms like sand casting, where green sand (not literally green, but a fine sand that has been mixed with a binder, like water and bentonite clay for traditional green sand or oil to make petrobond) is packed into a frame around a model, then the mold is separated and the model gets removed by hand.

At a high level, there are four phases of investment casting:
  1. Mold making
  2. Burnout
  3. Casting
  4. Cleanup

Phase 1: Mold Making

The first step in mold making is to create a "sprue tree." Essentially, you're creating a system of pipes to deliver molten metal to your models. An incorrectly sprued model can get casting defects. 

If the sprue connected to the model is too small, the metal in the tube will solidify before the model does. This is a problem because of the high temperature; as the metal cools, it shrinks. You want the sprue to stay molten longer than the model does so as the model cools and shrinks, extra metal can flow from the sprue into the model. In general, the sprue should be the same thickness as the part you're casting.

If you make a Y in the sprue tree, you need to melt some additional wax into the crook of the Y so the intersection is smooth instead of sharp. This is because when you pour the investment around it and melt out the wax, if you have a sharp angle, you are left with a thin piece of plaster sticking out into the flow of metal which can get broken off and included in your final piece.

You also generally want to mount the models at a angle so the metal can flow into the cavity, instead of dropping in. Dropping directly in creates turbulence, which can cause tiny bubbles of air to get mixed in with the metal and create a porous finish. This is less of an issue with vacuum casting, since you have a machine pulling the air out of the mold.

Back to the sprue tree. For the medallions, I'm using 3.5"x4" flasks (metal tubes that hold the mold). These will fit about four medallions each. So, I've got my workbench set up with four piles of four models each, a small pile of blue wax rings I'm attaching to the medallions as I go, a rod of "sticky wax" at the bottom, which essentially acts like hot glue and is easier to attach wax to other wax with it rather than directly melting it, and an alcohol lamp to melt stuff with. Not pictured is some extra red sprue wax, which is used to form the base of the tree.
Each sprue tree is mounted on a rubber base. These are the same diameter as the flasks so the flask can slip into the base when it's time to pour the investment. They also have a dome at the bottom that creates what is called the "button." The button is an extra reservoir of molten metal that feeds the sprue system as everything downstream solidifies and shrinks.  
At this point, it is very important to weigh each base with the sprue tree and record the weights. I have the weight of each rubber base alone written on the bottom of it, so I can subtract this weight from the rubber base's weight to get the wax weight. This is needed to figure out how much metal to use later.

Afterwards, I spray the models with a vacuum debubbler solution. It's a chemical that, once it dries, leaves a film on the models that reduces the surface tension and makes it less likely that an air bubble will stick to some part of the wax during the mold making. Some people don't use it and say they have fine results; this was part of the process during college, so I'm being stubborn. I also view it as insurance, since a bubble can cause a lot of work or destroy some detail, making this whole process for nothing.

Now that we've got the trees made, we slip the metal flasks onto the base and get ready to make the investment. You can also see below that a sleeve has been put on the top of the flask. This will come into play later. Other materials are the giant tub of investment powder, a rubber mixing bowl and spatula, a scale, and the vacuum machine.
The investment I have right now is Kerr Satin Cast 20, and they have a handy chart for amounts of investment and water to use based on the flask's size. For a 3.5x4 flask, I'm using 2 pounds of investment and 364 ml (or grams) of water.

For this process, it's good to have a pitcher of water sitting around a while before hand to come to room temperature. Once water touches investment, there is a very specific time table of how things happen, and it is made assuming the water is about 70 degrees F. If it's warmer, the investment sets up faster. If it's colder, it goes slower. If you finish too soon, you have your investment slurry sitting around longer than intended and the water and plaster can start to separate, creating water bubbles, which turn into casting defects after all the water evaporates during burnout. If you finish too late, well... Nothing good happens.

The timetable for Satin Cast looks like this:
So, first step: we have our measured, room temperature water. Add our two pounds of investment and mix for about three minutes.
The whole bowl gets transferred to the vacuum table, placed under the dome, and vacuumed until the six minute mark.
At that point, the vacuum is released and the plaster is poured down the side of the flask, slowly filling it, so you don't disturb the sprue tree. I don't have pictures of that part because I only have two hands.

The whole flask is transferred back to the vacuum table and vacuumed some more. This pulls any air that got mixed in or trapped against the models out. This is where that rubber sleeve on the top comes in. When the mixture is vacuumed, it starts boiling. The sleeve keeps the plaster from going all over the table.
After the second round of vacuuming, if the flask is a little low, some more investment can be poured in to top it off. The last stage called "gloss off" refers to then the mold sets. Once it hits that point, in a matter of seconds, it goes from a shiny, liquid looking top to a dull, solid surface.

Now, I had three more molds to make, and I don't want to do everything one at a time. This calls for a larger bucket, a paint mixing attachment, and more power. *insert Tim Allen grunts here*
Once all the flasks are poured, they need to sit for at least two hours to finish setting. They generally should start the burnout process within 24 hours to prevent too much moisture from evaporating, which can cause cracks during burnout.

While they're setting, the plaster should have set up enough to clean up the equipment. This is where having a rubber mixing bowl makes things really satisfying.



Phase 2: Burnout

Once the molds have set, it's time to get the wax out of them. The first thing to do is flip them over and pull the rubber bases off, since we want to reuse those.
The flasks are then loaded into a kiln. You can see below they are all facing down, and sitting on a grate with a tray beneath it. This is so the wax can all flow out in a way that we can remove it from the kiln before ramping up the temperatures.
The kiln gets programmed with a burnout cycle. Investment makers have a recommended burnout cycle; Kerr's chart is below.
Basically, we hold at 300 degrees F for a while to melt the wax out, remove the tray with the wax, then start the rest of the cycle to finish burning out any remnants of wax and bring the flask to the final casting temperature. This whole process is fairly hands off, unless your smoke detector starts going off.

While that's going, now is a good time to get the metal weighed out. We need to figure out how much volume the wax is taking up and how much metal is needed to fill it. Conveniently, wax has a density close enough to 1 g/cm^3 that we can treat it as 1 at this scale. So, we take the wax weight multiplied by the density of whatever metal we're casting to get the number of grams of the metal needed. In this case, I was casting three flasks in white bronze, which has a density of 8.1. The wax weight for those three flasks was about 57g. 57g * 1 g/cm^3 = 57 cm^3 of metal. 57 cm^3 * the density of 8.1 g/cm^3 = 461.7 g. That is how much metal we need to fill up the models and sprue system. If you remember, there is also a button we want to have some metal in to account for shrinkage, so we add an extra 10% on to this number, giving us a little under 508 grams of white bronze. Being a little over that number isn't going to kill you though, if your scrap metal isn't cooperating.

Phase 3: Casting

Something like 12-16 hours later depending on the flask size, it's time to cast. Going from left to right, there's the kiln, a heat resistant pad with a pair of flask tongs on top, the vacuum machine, a pair of welding gloves, a set of crucible tongs, two melting furnaces (one for brass and one for white bronze), a graphite rod, and a thermocouple thermometer. Not pictured is a leather apron and a full face respirator (zinc fumes from brass are nasty).
About thirty minutes out, the melting furnaces get turned on, the metal is added, and a pinch of borax flux is added to prevent oxidation. These furnaces get set to 1200 degrees F until the temp settles, then to 1700, and finally to the casting temp of 1850 for white bronze or 1900 for brass. When the metal is molten, I wait about five minutes for it to come up to temp, verify it's correct with the thermocouple, then give the surface of the metal a quick stir with the graphite rod to skim off any scum from the metal.

Right after the stir, a flask gets removed from the kiln and flipped over on the pad so the sprue opening is facing up. It gets transferred to the right side of the vacuum machine, where there is a silicone pad with a small hole in the middle. The vacuum gets turned on, and once the pressure starts rising, showing the vacuum is set, the furnace is opened back up and the metal poured.

Once the pour is complete, the vacuum is left running until the button solidifies, which indicates that the rest of the metal has also cooled and no gas pockets can form. The vacuum is then released, the machine turned off, and you have your medallion.

There's no pictures here, since I don't want to try to take them while pouring molten metal with the other hand. I was able to prop the phone up on a shelf though, so there's video.

Phase 4: Cleanup

First, we need to get the metal out of the plaster. We could wait for it to cool then break apart the plaster. Or we could dunk it in a bucket of water and dissolve the plaster. That's more fun. Which route you takes depends on whether the metal you were casting can handle being quenched.
It's important to note that with this method, the resulting slurry can not go down the drain. The plaster dust settles out and would clog up everything. Instead, once it settles, the water on top gets poured off and the plaster is left to dry until it can be thrown out.

Once the sprue tree is free, the models get cut free with a pair of sprue cutters (heavy duty wire cutters) if you're lucky, or a jeweler's saw if the sprues are too thick.
There's still some remnants of investment on the models at this point. I used to get to work with a brass wire brush and water to finish cleaning them up. But, I got a new toy, and now I can throw them in the bead blaster.
And finally, you have medallions. They still need to be sanded, polished, and patina'd, but that's a different post.




Comments