The defining characteristic of a stack mold is the two 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 other’s force.

A rule of thumb for a clamp force estimate is to take the projected part surface area times the melt pressure and multiply it, 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 hot runner 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, such as the collapsible crate
Stack molds can also handle multi-material injection—different materials such as hard/soft combination or multi-color molding. Three-level stack molds are not as common as two-level stack molds.

When the stack mold opens for the ejection of parts, both mold parting surfaces open (in most cases simultaneously) by means of a mechanical connection between the mold center block, the fixed halves of the mold and the moving half of the mold.

The mechanical connection, which moves the center block of the mold, can be:
a rack and gear operated motion system

a harmonic lever system

a hydraulic push-pull system

a helical gear system