What Is Stack Mold?
A stack mold is basically two or more molds stacked vertically and mounted on one injection machine. Sounds straightforward, but anyone who has actually built one knows the headaches involved.
Our shop took on our first stack mold project back in 2019. Client made food containers, needed around 80 million pieces per year. Running two machines was out-floor space was tight. Building a massive single-level tool meant jumping to a 2000T press, way beyond their existing 1250T Haitian. The solution was a two-level stack, 32+32 cavities. Actual clamp force only went up about 100 tons compared to a single-level 32-cavity tool, but output doubled. The math made sense to everyone.
Synchronization Is Everything
The number one failure point on stack molds is the opening mechanism. Both parting lines have to open and close at exactly the same rate. Even a tiny mismatch-we are talking fractions of a millimeter-can drag parts, damage hot runner nozzles, or worse.
Most builders use rack and pinion. A large gearbox sits in the center, with racks running to the moving platen and center section. Looks simple enough, but gear machining tolerances, rack alignment, and long-term wear compensation all matter. Cut corners here and problems show up fast.
We learned this the hard way. One tool ran about 500,000 cycles before the customer reported flash on parts. Opened it up and found the racks had worn roughly 0.15mm, throwing off synchronization. The center plate was shifting slightly at clamp-up. We switched rack material from 40Cr to carburized 20CrMnTi, surface hardness above HRC58. That fixed it for good.
Some customers ask about hydraulic cylinders for synchronization. Technically possible, but oil temperature swings affect flow rates, and precision suffers during fast cycling. Unless the tool is too big for a gearbox, stick with mechanical.
Hot Runner Gets Complicated
Single-level molds have short flow paths-maybe 200 to 300mm from machine nozzle to gate. Stack molds double that or more because melt has to travel through the entire center section. Longer paths mean higher pressure drop, tougher temperature control, and harder balancing between levels.
Our standard practice now is running the full runner system through Moldflow before cutting steel. Key metrics are fill time difference and packing pressure distribution between levels. Under 3% variation is good. Over 5% means reworking runner diameters or nozzle specs.
One detail that gets overlooked is sprue bushing guidance. The main nozzle moves with the center section during mold open. If bushing clearance is too loose, thousands of cycles will shift nozzle position and sealing surfaces start leaking. We run H7/g6 fits on guide bushings with anti-rotation pins.
Cold runners do not work here. Every shot generates waste on both levels, volumes add up, and cold slugs mess with fill consistency. Stack molds need hot runners. That cost is unavoidable.
Supporting the Center Section
When the mold opens, the center plate hangs in mid-air. Leader pins alone cannot hold it-plates weigh several hundred kilos minimum, sometimes over a ton. Without proper support, the plate sags, leader pins wear fast, and parting line fit goes bad.
The proven approach uses four guide blocks riding on the machine tie bars, moving along with the center section. Blocks have rollers or bronze bushings underneath to reduce friction. Some shops use spring-loaded supports that pop out at a certain opening distance, but reliability is questionable. Springs fatigue over time. We stopped using that design years ago.
Where Stack Molds Make Sense
Not every part justifies a stack tool. Based on what we have seen over the years, good candidates are flat parts, shallow draw, high annual volumes, and short cycles. Think container lids, disposable food trays, cosmetic caps. Thin walls cool fast, cycles drop under ten seconds, and the output boost from stacking really pays off.
Deep parts, long core pulls, or complex side actions are poor fits. Center section thickness is limited-there is only so much you can pack in there. And if volumes are not high enough, the price premium over a standard tool takes too long to recover.
One more thing worth checking before quoting: machine daylight. A two-level stack needs roughly double the opening stroke of a single-level tool, three-level needs triple. Confirm the customer's press can handle it. Building a tool that does not fit the machine is an expensive mistake.
Maintenance Is Higher
No way around it. More complexity means more upkeep. Synchronizing gears need regular inspection and grease. Hot runner heater bands and thermocouples fail more often because wiring runs longer with more connections. Center section supports need periodic gap adjustment.
We tell customers to schedule full inspections every 200,000 to 300,000 cycles. Waiting until something breaks costs more in downtime than preventive maintenance ever will.
Bottom line: stack molds trade higher tooling complexity for higher output. For customers with big volume demands, simple part geometry, and limited appetite for adding machines, this approach deserves serious consideration. The keys are solid upfront engineering-synchronization, hot runners, and support systems all done right.
Technical notes compiled by ABIS MOLD engineering team, drawing on project experience in multi-cavity mold and hot runner mold development. For detailed discussion on high-volume injection molding tooling or custom plastic mold solutions, our engineers are available to talk through specifics.