certain situations, holes may be drilled into the mould before heating to help reduce these stresses. Any wax that runs out of the mould is usually recovered and reused. Burnout preheating: The mould is then subjected to a burnout, which heats the mould to between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mould. Sometimes this heating is also used to preheat the mould before pouring, but other times the mould is allowed to cool so that it can be tested. Preheating allows the metal to stay liquid longer so that it can better fill all mould details and increase dimensional accuracy. If the mould is left to cool, any cracks found can be repaired with ceramic slurry or special cements. Pouring: The investment mould is then placed open-side up into a tub filled with sand. The metal may be gravity poured or forced by applying positive air pressure or other forces. Vacuum casting, tilt casting, pressure assisted pouring and centrifugal casting are methods that use additional forces and are especially useful when moulds contain thin sections that would be otherwise be difficult to fill. Divesting: 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. Finishing: After grinding, the completed casting is then subject to finishing. This usually goes further than grinding, with impurities and negatives being removed via hand tooling and welding. In the case that the part needs additional straightening, this process is usually carried out by hydraulic straightening presses, which bring the product in line with its tolerances. Advantages Excellent surface finish High dimensional accuracy Extremely intricate parts are castable Almost any metal can be cast No flash or parting lines Effective utilization of metal Fewer environmental hazards from the foundry process Disadvantages The main disadvantage is the overall cost, especially for short-run productions. Some of the reasons for the high cost include specialized equipment, costly refractories, and binders, many operations to make a mould, a lot of labor is needed and occasional minute defects occur. However, the cost is still less than producing the same part by machining from bar stock; for example, gun manufacturing has moved to investment casting to lower costs of