The Casting Process Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. The following steps are for the indirect process, which, in total, can take two days to one week to complete. 1. Produce a master pattern: An artist or mold-maker creates an original pattern from wax, clay, wood, plastic, steel, or another material. A master pattern can also made generated in the form of a CAD model. This CAD geometry can then be used to machine the aluminium or steel die directly 2. Moldmaking: A mold, known as the master die, is made of the master pattern. The master pattern may be made from a low-melting-point metal, steel or wood. If a steel pattern was created then a low-melting-point metal may be cast directly from the master pattern. Rubber molds can also be cast directly from the master pattern. The first step may also be skipped if the master die is machined directly into steel or aluminium. 3. Produce the wax patterns: Although called a wax pattern pattern materials also include plastic and frozen mercury. Wax patterns may be produced in one of two ways. In one process the wax is poured into the mold and swished around until an even coating, usually about 3 mm (0.12 in) thick, covers the inner surface of the mold. This is repeated until the desired thickness is reached. Another method is filling the entire mold with molten wax, and let it cool, until a desired thickness has set on the surface of the mold. After this the rest of the wax is poured out again, the mold is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer. If a core is required, there are two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern, whereas ceramic cores remain part of the wax pattern and are removed after the workpiece is cast. 4. Assemble the wax patterns: The wax pattern is then removed from the mold. Depending on the application multiple wax patterns may be created so that they can all be cast at once. In other applications, multiple different wax patterns may be created and then assembled into one complex pattern. In the first case the multiple patterns are attached to a wax sprue, with the result known as a pattern cluster, or tree; as many as several hundred patterns may be assembled into a tree. Foundries often use registration marks to indicate exactly where they go. The wax patterns are attached to the sprue or each other by means of a heated metal tool.The wax pattern may also be chased, which means the parting line or flashing are rubbed out using the heated metal tool. Any other imperfections are also addressed at this point so that the wax now looks like the finished piece. 5. Investment: The ceramic mold, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into a slurry of fine refractory material and then letting any excess drain off, so a uniform surface is produced. This fine material is used first to give a smooth surface finish and reproduce fine details. In the second step, the cluster is stuccoed with a coarse ceramic particle, by dipping it into a fluidised bed, placing it in a rain tower sander, or by applying by hand. Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness, which is usually 5 to 15 mm (0.2 to 0.6 in). Note that the first coatings are known as prime coats. An alternative to multiple dips is to place the cluster upside-down in a flask and then liquid investment material is poured into the flask. The flask is then vibrated to allow entrapped air to escape and help the investment material fill in all of the details. 6. Dewax: The investment is then allowed to completely dry, which can take 16 to 48 hours. Drying can be enhanced by applying a vacuum or minimizing the environmental humidity. It is then turned upside-down and placed in a furnace or autoclave to melt out and/or vaporized the wax. Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it, so as the wax is heated it expands and induces great stresses. In order to minimize these stresses the wax is heated as rapidly as possible so that the surface of the wax can melt into the surface of the investment or run out of the mold, which makes room for the rest of the wax to expand. In certain situations holes may be drilled into the mold beforehand to help reduce these stresses. Any wax that runs out of the mold is usually recovered and reused. 7. Burnout & preheating: The mold is then subjected to a burnout, which heats the mold between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mold. Sometimes this heating is also as the preheat, but other times the mold is allowed to cool so that it can be tested. If any cracks are found they can be repaired with ceramic slurry or special cements. The mold is preheated to allow the metal to stay liquid longer to fill any details and to increase dimensional accuracy, because the mold and casting cool together. 8. Pouring: The investment mold is then placed cup-upwards into a tub filled with sand. The metal may be gravity poured, but if there are thin sections in the mold it may be filled by applying positive air pressure, vacuum cast, tilt cast, pressure assisted pouring, or centrifugal cast. 9. Removal: The shell is hammered, media blasted, vibrated, waterjeted, or chemically dissolved (sometimes with liquid nitrogen) to release the casting. The sprue is cut off and recycled. The casting may then be cleaned up to remove signs of the casting process, usually by grinding.