Injection molding is a technique of achieved an object of a desired shape by melting the base material, and then forcing that material into a mold, often under tremendous pressure. This can invovle molten metals, such as iron and aluminum, but also invovles melted and moldable plastics.
Plastics, which are made either from petroleum or increasingly from plant oil, are polymers. They are typically flexible and constant in their shape when cool, and cannot be deformed by bending as a metal can. Many plastics can be repeatedly melted, and then poured or injected into a mold. The oldest and simplest molds are open, and the molten fluid is simply poured into. This process is useful with simple designs, but has the drawback of flaws such as bubbles. Delicate shapes, such as the human face on an action figure, are often difficult to achieve with simple pouring. At the very least, it takes much more time to achieve an excellent mold.
An injection molding machines speeds the process by using a strong vaccum to fill every niche of an intricate mold. The mold itself is two plates, which are held together under at least several tones of force. The stiffer the material, the more forefully it must be molded. The process can include pouring the content into the loosened plates, then the plates pressed together to finish the job.
A plastic injection molding machine typically only needs five to ten tonnes of force to achieve most shapes, since plastic melts at a lower temperature than other medium, and is more liquid once melted. A metal mold, on the other hand, must deal with an extremely hot material that has the consistency of thin clay even at temperatures over a thousand degrees. The plates are forced together with pressure at hundreds and even thousands of tonnes.
The first plastic was derived from cellulose and could be heated and molded just like later petroleum plastics. The Hyatt brothers improved on the formula and invented the first of the plastic injection molding machines, and they built others. The technology slowly progressed from manufacturing very simple shapes to increasingly more subtle.