The defining characteristic of a stack mold are the two (or more) mold parting surfaces or mold split lines. A stack mold does not require much more clamp force than a single face mold because the projected part surface areas of the cavities on both sides of the center block cancel out each others force.
Without increasing the machine platen size you can double the amount of cavities producing parts. Most stack molds have an equal number of the same cavities in each parting surface. For example, a 2+2 cavity grid mold with an 8+8 hotrunner produces four identical parts per cycle.
Some stack molds have different cavities in each mold parting surface and they produce a family of parts per shot, each different in shape and size.
Double Your Machine Output
Compared to conventional single-face molds, stack molds can virtually double the output of an injection molding machine by distributing the plastic melt into two or more separate mold parting surfaces