Stack Molding
Double Your Machine Output
During the concept phase of a new project, the option of selecting a stack mold is often overlooked because many mold designers are unfamiliar with the technology. Compared to a regular, single-face injection mold, 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.
 
3600 kg (8000 lbs) hot half for a stack mold 160 x 152 cm (63 x 60” ) 1+1 cavities for HDPE flame-retardant pallet with a part weight of 7.7 kg (17.5 lbs)
What is a Stack Mold?
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 phase mold because the projected part surface areas of the cavities on both sides of the center block cancel out each others 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 by a factor of 1.1.
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 (such as this collapsible crate).
The floor and four side walls of this collapsible crate are produced in one shot. Fill, pack, cool and warp analysis accurately simulate the flow of the melt during the design phase of a project. This optimizes plastic part quality and the production output by improving cycle time.
Some 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) a rack and gear operated motion system (see above)
b) a harmonic lever system
c) a hydraulic push-pull system or
d) a helical gear system
Aside from the center block motion system, another key element of a stack mold is the hotrunner system. The transfer of plastic melt from the machine barrel into the mold center block has traditionally been achieved by a heated sprue bar which leads the melt from the machine nozzle through the fixed mold half and through the first mold parting surface into the hot runner manifold, which is located in the center block of the stack mold.
The relatively simple sprue bar has the disadvantage of getting disengaged from the machine nozzle at each cycle when the mold opens for part ejection. The sprue bar may also become an obstacle at part removal, especially with robot handling equipment because it always remains inside the first mold parting surface.
Mold cavities which by design cover the center surface of the molding area do not allow a center sprue bar so the hot runner has to by-pass the melt by leading it with an off-set manifold to the outside of the mold and from the side position through the first mold parting line into the center block for further melt distribution through the nozzles to the cavities.
 
Off-set manifold with melt transfer through two opposing valve gate shut off nozzles that meet in the first mold parting surface (part size: 102 cm/ 40" diameter, center gated)
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