The Lost Wax Process
Part 2: Duplicating the Master
Once master wax model has been created, it can be “invested” in plaster and burned out to create the void into which metal can be injected. The wax master will be gone forever, replaced by silver or gold or bronze. But what if you wanted to make a few of the same piece?
A new master doesn’t have to be carved for every identical piece you want to create. Instead, a rubber mold is made from the master. This rubber mold can be filled with wax over and over to create hundreds of duplicates of the same piece.
What follows is the method preferred at Whirling Sun: RTV silicone.
First, a wax sprue is attached to the master. Sprues are attached by heating a tool over a flame and “welding” the sprue to the wax master model with it. The sprue should be thick enough to form an adequate channel through which the hot wax can be injected. It should also be placed strategically so that no detail is obliterated by it. This sprue will probably also be used when it is time to inject molten metal into the casting flask, so it is good to make sure the metal will have a path that is clear of turbulence or right angles. It is bad for metal to be injected through a small opening into a big one, like an hour glass. This causes the metal to “spray” into the cavity like putting your thumb over a garden hose, and can cause a “blowout”. More sprues should be added to avoid this.
The sprued master is then attached to a cone in the bottom of a mold frame.
Once the mold frame is ready, a batch of RTV silicone is prepared for it. RTV stands for “Room Temperature Vulcanizing”. As the name implies, it does not require heat to vulcanize, such as natural rubber would. Such heat would melt and destroy a wax model. RTV comes in 2 parts, the rubber and the hardener.
RTV is usually mixed in a 10/1 ratio by weight -ten parts rubber to one part hardener. It is thick like warm taffy and is a bit tiring to mix, but you should mix it with a spatula or putty knife until it is all a uniform color.
Once mixed, it is placed under a bell jar and vacuumed for 5 minutes to release all the entrained air bubbles. After this it is slowly poured into the mold frames, being careful not to knock the wax model off its sprue. Next, the RTV filled mold frame is placed under the bell jar and vacuumed for 5 more minutes. It will most likely boil over during this operation, so it is advised to wrap masking tape around the top of the mold frame to contain as much RTV as possible. a plastic container lid under placed under the mold frames during vacuuming can catch the spilled rubber so you can pour it back in.
After the vacuuming is complete, the pressure is released and the overflowed rubber scooped back into the frame. The Rubber takes around 24 hours to vulcanize. The process can be accelerated with heat, but remember, there is a wax model in there!
After the rubber has cured, it is time to remove the mold from the frame and cut the model out of it. Working slowly and deliberately, an incision is made around the entire perimeter of the mold using a hobby knife or a scalpel. Next, zig-zag cuts are made so that the 2 mold halves will line up again later. When the wax model is reached, special care is taken to ensure it is not cut. The rubber is pulled apart to gain access while cutting out the model.
Finally, the mold is held between two metal plates and the opening is pressed onto the nozzle of a wax injector. The wax injector is pressurized and filled with molten wax. The best results are achieved when the temperature and pressure are at the lowest possible settings where the wax still flows and fills the mold. Too hot and the wax will shrink as it cools. Repeat this over and over to create duplicates of your wax masterpiece.