The double injection molding process requires a machine with two independent injection units, each of which shoots different material. The first material is injected through a primary runner system, as in a normal injection molding cycle. During the injection, the mold volume to be occupied by the second material is shut off from the primary runner system. The mold is then opened and the core plate rotated 180 degrees. The mold is again closed and the secondary runner system is connected to the volume to be filled. After sufficient part cooling, the mold is opened and the part is ejected.
The double injection molding process can also be accomplished with an indexing system (a round table rotating around a horizontal axis) with a primary and secondary station. While the first injection is accomplished at station #1, the second injection is accomplished at station #2. Each injection station is run by an independent injection unit. This allows injection speeds and pressures to be controlled for each material being utilized.