A stack mold is a group of two or more mold parting surfaces (or mold split lines) that have been stacked together in order to increase the output of an injection molding machine. There is usually an equal number of cavities in each parting surface; for example, 4 exact copies per cycle could be created using a 2 + 2 cavity grid mold with an 8 + hot runner. A stack mold doesn’t require any more clamp force than a single phase mold because the cavities on both sides of the center block counteract each other’s force. The general formula for estimating clamp force is to take the projected part surface area times the melt pressure and multiply it by a factor of 1.1. Flat parts (as opposed to deep core parts) tend to be more fitting for stack mold production due to the machine opening stroke and expanding mold height.
Basic Advantages of Using a Stack Mold
There are several impressive reasons to opt for using a stack mold. You can double the output without adding more cavities or increasing the mold or machine size simply by adding layers of cavities parallel to the first layer. This means that the fill, pack, and cooling times stay the same and only the mold open and close times are added to the cycle. Cost cutting is another advantage because part price is calculated by the machine hour rate which is related to the machine clamp tonnage. A stack mold only needs approximately half the clamp tonnage of a single-face mold with the same number of cavities because the cavities of either side of the center block cancel out one another’s force.
The Versatility Offered by Stack Molding
A stack mold can also be used to create objects made from a group of parts with each part varying in shape and size. Some stack molds can even work with multi-material injection, including hard and soft combinations as well as varying color combinations. Where a single-face mold would need production synchronization between many machines, a stack mold can create multi-component assemblies in a single shot, using one machine with the same parameters.
Tandem molding is another type of stack molding that can be used by almost any type of injection molding machine, wherein parts are injected in alternating cycles. When the molding machine opens to demold one parting line, the other is held together by a locking system. This system gives us the advantages of stack molding with longer cooling times.
Sprue-Less Gate-to-Gate Transfer
Another aspect of stack molds that has proven to be useful is the difference with the hot runner. Plastic melt is usually transferred into the mold center block by a heated sprue bar that directs the plastic melt from the machine nozzle through the fixed mold part, then the first mold parting surface, into the hot runner manifold. The issue with using the sprue bar in this way is that when the mold opens for part injection, the sprue bar gets separated from the machine nozzle and can remain in the initial mold parting surface. With stack molds you can use optimum ways of transferring plastic melt into the center clock, using sprue-less gate-to-gate transfer methods.
Initially, stack molds were used for throw-away plastic packaging and smaller items but post-millennium plastic injection mold technology has found stack molding being used more and more for bigger parts. Today, a stack mold can often be the creative solution one needs to bring a concept to life.