Milk jugs are usually made from high density polyethylene, or HDPE. In the milk jug manufacturing process, HDPE pellets are usually mixed with recycled HDPE flakes. Because HDPE loses its durability after it has been used, only a limited amount of the recycled flakes can be used.
After the HDPE pellets and flakes have been mixed, they slide down a tube and drop into a plastic injection machine. There the material is heated to 599° Fahrenheit. When the material becomes a thick, gooey plastic it's injected into molds that create "preforms." These look like relatively large, thick test tubes. They harden almost instantly due to a built-in cooling machine.
The preforms are sent down a line, where they enter a machine called a reheat stretch blow molder. Within a few seconds, each preform is heated enough to make the plastic flexible again. A rod is placed inside the preform to stretch it out, while at the same time blowing hot air into it. The preform is stuck inside a mold while it's being heated and stretched, until it looks like a plastic milk jug. Cold water running through the mold cools the jug almost instantaneously, hardening the plastic. All of this happens within a matter of a few seconds.