High pressure die casting is a process in which molten metal is forced under pressure into a sealed mold cavity. It's held in place by strong compression (the real mold is set in a hydraulic press) until the metal solidifies.

After solidification, the mold is released, opening and releasing the metal.

Once removed, the cavity is resealed for the next cycle.

Molten metal is injected into the mold cavity in fractions of a second (typically less than 100 milliseconds). Once the cavity is filled, extremely high pressure (often over 1000 bar) is applied to the actual injection plunger of the molten metal. This phase is called reinforcement.

This pressure compresses any gas in the metal (during extremely fast and turbulent cavity fill) and sends more metal into the mold, partially compensating for the shrinkage of the metal as it solidifies.

In addition to the traditional high pressure die casting method, the process has also seen some improvements in recent years. E.g:

Vacuum Die Casting
semi-solid casting
squeeze casting