This is one of the most common methods used to mass produce objects with complex shapes. The most frequently molded materials are thermoplastic plastics, but some thermosetting plastics, metals, ceramics, and composites, can be formed that way as well.
In order to prevent objects from sticking to the mold that they are formed in, a mold release is often applied to the inside of the mold. This is often a low molecular weight PTFE, aka Teflon, spray. Mold releases usually only need to be applied once in a while.
Common types of Molding
This is possibly the most common type of molding. It is most frequently used to mass produce plastic objects, often with complicated shapes. A thermoplastic resin is melted and then squirted into a mold, where it rapidly cools. The molds for injection molding tend to be very expensive, and they are often made outside of the U.S. Cheap plastic toys are usually made with this method, so are most of the plastic parts found inside of cars. Some Zoos have a machine that you can put four quarters into, then watch as it makes an injection molded toy animal right in front of you.
Reaction Injection Molding
Polyurethane is the main material that is formed by this process. Two different precursor chemicals are mixed by impingement before they are squirted into the mold. They react in the mold to form a solid object.
Thermoplastic resins are often formed by this method. In general, a resin powder is loaded into a heated mold and a great deal of pressure is then applied.
A balloon shaped piece of plastic is inserted into a mold and filled with air or water under high pressure until it stretches out to fill the entire mold. This is used to form hollow objects like bottles. Polyethylene and polyethylene terepthalate are probably the two most frequently blow molded materials. They are also the two most common resins used for the production of beverage bottles.
Resin Transfer Molding
A method for producing composites. Reinforcement material is placed in the mold before it is closed, then the resin is injected into the mold. The resin surrounds the reinforcement to make a composite.
Molten plastic or metal are squirted or poured into a rapidly spinning mold. This spreads the molten material out within the entire mold. It is often used to make jewelery from metals, like pewter, that have a low melting point.