The instructable teaches basic pewter metal casting skills.
Metal casting is a very old art that dates back to antiquity. In this Instructable the old technology of metal casting is getting a a little help from the very new technology of LASER Etching/Cutting.
Historically detailed models of art object to be cast in metal had to be hand made. Making the models are a slow process and one minor mistake can cause the model to be ruined and the crafts-person making the model would need to start over. With the advance of LASER cutter/etchers it is possible to create detailed models from designs created in CAD or graphic-art software packages.
Step 1: Making the Aztec Coin Model
The model is created by gluing the laser etched Aztec design to a 1/4" piece of birch plywood which was cut and sanded to match the laser cut Aztec design.
Next two dowel rods are glued as shown to the outer surface of the coin model. These dowel rods are used as 'risers'. One will be used to during casting process for pouring in the melted pewter while the other will work as exhaust port for air to escape. (This will be discussed more in later steps.)
Step 2: Making the Mold Form From Legos
Lego's work great for creating mold forms. Various shapes can be constructed with Lego's and during the process of pouring the molding compound if the mold needs to be a bit deeper your simply add an extra layer of Legos to the form.
Although not always needed, I lightly spray the inter surface of the mold form with a mold release agent.
Step 3: Making RTV Mold of Aztec Coin Model
A RTV silicone rubber that can handle melted pewter is required. Lead free pewter typically melts between 462-474 degrees F.
The mold is made in two steps.
The mold form is filled about 1/2 full with the mixed RTV silicone rubber.
The wood model is sprayed with a mold release agent.
The wood model is then carefully placed into the liquid RTV rubber till is about half covered. The wood model was suspended at this level via several strings taped to the model and to the sides of the Lego form.
The RTV silicon was next allowed to cure for 24 hours.
Next the top part of the mold was poured, but before the top portion is poured into the Lego form the wood model and the cured RTV Rubber is lightly sprayed with a release agent. (This is needed to enable the mold to be split apart in later steps.)
Special instructions when mixing the RTV silicon rubber:
Mix the RTV silicon rubber with its catalyst per the product's instructions. It is best to mix slowly to limit air bubbles from becoming trapped in the mix.
Use a vacuum chamber to degas the liquid RTV silicon if have one available. (In this example, I did not do this and several imperfection occurred due to air bubbles.)
Step 4: Removing Lego Blocks From Rubber Mold of Aztec Coin
Step 5: Seperate the Two Halfs of the Rubber Mold
Start at a corner of the mold and located the seam line between the top and bottom half of the mold. Apply a separation force and carefully pull apart the two halves of the mold. ( I have needed to use a razor blade to lightly cut along the seam line on past molds in order to get the mold halves to separate.)
The images show the two halves of the mold.
Notice the mold half that has the Aztec Art work. The molding material picked up very fine details. Unfortunately, in this case an air bubble that was in the RTV silicon rubber mix floated up and laid against the surface of the design. (This a good example why mixed RTV silicon rubber needed to be degassed via a simple vacuum chamber. In a future Instructable I'll describe the vacuum chamber that I have made.)
Once the mold halves were separated I also carved out the port that will be used for pouring the pewter. Notice one port is slightly larger, this was done with a sharp razor blade.
Step 6: Preparing the Mold for Casting Pewter
Using Talc powder (not cornstarch) lightly dust the mold surfaces. (The talc assists in the pewter not sticking to the mold.) A fine haired paint brush is used to lightly spread around the talc to all surfaces of the mold.
Turn over the mold and lightly tap to shake off any excess talc.
The mold is now ready to be assembled for casting.
Step 7: Assemble the Rubber Mold for Casting
Clamping can be accomplished in a number of ways, but what I have found to work well is shown. Thin flat wood strips as shown on either side of the rubber mold are lightly clamped.
Enough clamping force is needed to insure the two halves of the mold are tightly fixed to each other, but not so much force as to distort the mold's shape.
Caution: If the molds are not clamped tight enough the melted pewter can leak from the mold.
In the image, I have the mold setting on a small stack of bricks. This may not be needed depending on your clamping arrangement.
Step 8: Tools for Petwer Casting
Depending on the alloy of Pewter, the Pewter will melt in the range of 462 to 474 degrees F.
You will need:
Heat Source (Propane torch)
(I purchased a number of the items from rotometals.com )
Step 9: Casting the Pewter
The pewter was then carefully poured into the mold via the enlarged port.
Once the pewter is poured allow several minutes to pass before moving the mold.
Step 10: Pewter Coin
Remove the clamps and carefully separate the mold haves as shown. The Pewter Aztec Coin can now be removed from the mold by using pliers. The pewter is still very hot at this stage.
Notice in this example the flaw that is on the surface of the Aztec Coin. The flaw looks like a bubble of pewter on the surface of the coin. In fact that is exactly what it is and it was caused by the cavity created by the air bubble discussed in earlier steps.
A perfect casting needs good mold with no flaws.
Several other things to note with this casting. Most of the fine detail came out well. But an area close to the 'risers' was a bit blurred and lacked detail. This blurred area was likely due to the pewter being poured a little to close to its melting temperature and the pewter solidified too quickly and did not pick up the mold details. Pouring the Pewter slightly hotter would have likely helped with this. (Companies that sell the ladle also sell special thermometers for measuring melted pewter temperatures. Insuring the pewter is hot enough, but not too hot to damage the rubber mold, is the only way to have castings come out consistently.)
Step 11: Comparing the Cast Pewter Coin to the Wood Model
The metal risers on the pewter coin were trimmed off with a hack saw and then the rough areas lightly filed smooth.
The Aztec coin was next buffed using 000 grade steel wool to remove discolorations from the casting process.
The pewter coin is shown with the original laser etched wood design.
Using this basic technique much more complex shapes can be made from laser cut parts or from handcrafted designs.