Injection Molding is the ultimate mass production technique. From a single mold, millions of objects can be produced.
The machines use high pressure to force liquid plastic into the mold. The plastic then cools and solidifies in a perfect shape of the mold. The mold opens up, object comes out, and the process repeats.
This is ideal for large run, mass production jobs. The molds can be created from computer models and used over and over again (within reason) without any noticeable degradation. One unique mold per one unique object is required.