The last part of the course covered methods of metal casting, including investment and sand casting. Both techniques start with an original object to be replicated. The MIT foundry uses an electric induction furnace, which prevents combustion impurities from contaminating the metal. Common casting materials are aluminum, bronze, and brass.
For investment casting, the first step is to create an alginate or silicone rubber impression of the object, so multiple wax replicas can be created. These wax objects are attached to a “tree” of wax, creating a structure that can be encased in plaster to make the final metal mold. The trunk of the wax tree, where the metal will be poured, is called the sprue, and the branches leading to each wax replica are gates. Vents are typically also required from the top of each replica to the surface of the mold, to allow air to escape from inside the mold. After the plaster has hardened, the wax is burned off. After the metal has been poured and cooled, the plaster is broken off the outside and each replica is cut off its gate and finished. This technique requires a new set of wax replicas and plaster mold to be made for each set of metal replicas. The level of detail is relatively high, although delicate structures may not turn out well. Industrial applications sometimes modify this process by using foam instead of wax, which doesn’t need to be burned off in a separate step, but is instead removed by the molten metal as its poured.
Sand casting, in contrast, only requires one replica of the object for the entire run of casting. This replica is sliced in half and attached to a board for alignment, then placed between two halves of a rectangular metal frame. Each side is packed with sand mixed with oil, to make it sticky enough to hold together when the frame is flipped. The bottom half of the frame, or cope, is packed full, while the top half, the drag, should have a hole left to pour in the metal, and at least one vent. After the board is removed and the two halves of the mold reassembled, the metal is poured in. Subsequent replicas require repacking the sand each time. The surface resolution of these castings is limited by the size of the sand grains and the tightness of packing.
Successful casting depends not only on creating a quality mold, but on having a cavity structure that ensures quick and even metal flow without too much cooling. The sprue and gates should be wide enough to avoid clogging. Reservoirs built into the mold at junctions create hot spots and maintain a clear channel. If the object has any sections that are significantly thinner than the rest, they should be oriented towards the bottom and filled first. As mentioned above, vents to prevent gas from being trapped inside should lead from the top of the mold, or from each sub-section if applicable.
This presentation provides a brief illustrated introduction to metal casting. (PDF - 5.3MB)
