Dewaxing is a process of introducing a solvent into a petroleum stream to remove wax impurities from the desired product. It is also a term used in precision and metal casting for the removal of waxes or binding agents from molds. A third definition is any solvent system used to remove mold release agents from injection molds and parts.
Petroleum dewaxing is used to extract high molecular weight components, often referred to as waxes, from a petroleum stream. These impurities cause petroleum products to gel or solidify at higher temperatures. They also can affect the combustion performance of diesel and gasoline fuels. Different solvents can be used, but the desired effect is to separate the waxes into the solvent for later extraction, or to cause the waxes to separate and float to the top for removal.
Gases such as propane are useful as dewaxing agents. When propane is injected into the fluid stream it flashes or becomes a gas; this effect, called auto refrigeration, causes the fluid to cool rapidly. The waxy materials will turn from liquid to solid and can be filtered out or skimmed from the top of the tank.
In precision or investment casting, wax molds are first made of the desired part. The wax mold is covered with a plaster shell by repeatedly dipping the mold in liquid plaster. When the mold is heated slowly, the wax melts and is removed, leaving an interior void in the plaster of the part shape. This type of casting is sometimes referred to as lost wax casting. The plaster mold is then fired at a very high temperature to form a ceramic, which can hold hot metal used to make the part.
Sintered mold casting is a slightly different type of dewaxing process. Molds are made from a mixture of sand or ceramic powder with a waxy binding agent and compressed at high pressures to form a solid mold. The mold is then heated in an oven to burn off the wax and sinter, or bind, the powder together. It can then be used for metal casting in the same way as a plaster mold.
Injection molding can also use the sintered mold mixture in a different way. The mixture is heated until the wax component softens, and the mixture is injected into a prepared mold. When the part is removed from the mold, it is dewaxed in the same was as a sintered mold.
Plastic parts are often made in injection molds, using thermoplastic resins that melt at reasonably low temperatures. A mold release agent is sprayed onto the mold surfaces to prevent the plastic parts from sticking. Dewaxing refers to removing the mold release agent, many of which can be removed with a soap and water rinse.
