How to Cast Bronze Butterflies – Good and Bad Sprues and Gates
When designing a piece for investment or lost wax casting a lot of time is spent creating the original pattern and subsequent mold. When the pattern is created its shape and undercuts will determine whether one or more molds needs to be developed so that the detail of the original is captured and so that subsequent waxes can easily be released from the mold.
When the mold is being made it is important to anticipate how the molten metal will subsequently flow into the void of the ceramic shell. The shelling process and the function of the autoclave are discussed in earlier posts. The mold we made for the large butterfly was a two-part rubber mold and the pattern was carved in wood. We added wax runners and one large wax sprue to the butterfly pattern and made a hollow box to hold the butterfly. The area where the wax runner attaches to the pattern is often referred to as a gate. The number of runners and their size and location play a key role in determining whether the bronze will pour easily, and the casting will be successful.
The rubber mold is created by pouring a cold rubber solution into the box containing the butterfly with its runners attached. The rubber encases the butterfly and its attachments, and when set, the 2- part mold will be opened, the pattern will be removed, and a hollow mold will be left. Hot wax will then be poured in through the center hole or sprue and helped along by gravity and some gentle rotation will travel down the central sprue and into the runners to fill the void created around the butterfly pattern.
In this post I am including 2 examples of a well gated wax and a poorly gated wax together with shots showing the resulting bronze castings.
While it may be tempting to assume that more gates will ensure a successful casting, having more gates will impact the speed, pressure and temperature of the flowing bronze and the success of the casting. Having more gates will also increase the labor intensity of cleaning the casting as all the gates need to be ground off.
Unlike the butterfly on the left the bronze did not flow to fill the wing tip of the right butterfly so the casting was rejected and the bronze was melted down and re-cycled
A beautiful butterfly pull can only emerge from a perfect casting.
