This method allows two bottom mold halves to be used with one top half. While one bottom half is molding with the top half, the other bottom is free to be loaded with inserts. When the press opens, the bottom half that was just loaded with inserts shuttles into location with the top half, while the bottom that was just molding shuttles out for part ejection and insert reloading.

With a single top and bottom half, the operator waits when the mold is closed, and the mold waits when the inserts are being loaded. Since multiple bottoms allow the inserts to be loaded in one bottom while the other is molding, press time is reduced.

Diagram of the Shuttle Injection molding process