Mold Cavitation

Setting Mold Cavitation From Demand, Not Habit

Cavitation is a once-per-program decision that sets press hours for years. How to size it from grossed-up demand, verify tonnage, and review it annually.

Cavitation is the most expensive number you pick before a program exists, and you only pick it once. Stepping a tool from 4 to 8 cavities adds $30,000 to $80,000 in steel. Under-building costs more: put 2 cavities where demand needed 4 and a 2 million part year takes roughly 8,300 press hours instead of 4,200. At $65 per press hour that is $270,000 a year, every year of the program, to save $40,000 once. The cavitation decision sets your press hours, your tonnage requirement, and your quoted piece price for 3 to 7 years, so it deserves real math, not the habit of building what you built last time.

The base math is simple. Required cavities equal annual demand divided by the shots one cavity can deliver. Take 2,000,000 parts a year on a 30 second cycle with 5,000 available press hours: each cavity delivers 120 shots an hour, or 600,000 parts a year, so you need 3.33 cavities and you round up to 4. The Mold Cavitation calculator runs this from demand, cycle time, and production hours in seconds. The discipline is in the inputs, because every one of those three numbers is usually quoted optimistically, and the errors all stack in the same direction.

Now gross the inputs up to reality. If the press fleet runs 85 percent uptime, your 5,000 hours are really 4,250. If the tool scraps 2 percent, demand is really 2,041,000 good-part equivalents. Rerun it: 2,041,000 divided by 4,250 hours times 120 shots gives 4.0 cavities exactly, with zero headroom. The clean math said 3.33 and felt comfortable at 4; the honest math says 4 is the floor. Any shop quoting cavitation without uptime and yield adjustments is systematically under-building, and the gap shows up two years in as weekend overtime on one press.

Stick to natural cavitation numbers: 1, 2, 4, 8, 16, 32. Those layouts balance geometrically, so every cavity sees the same runner length and pressure drop. A 6 cavity tool needs an artificially balanced runner, and artificial balance drifts: expect 5 to 10 percent fill imbalance across the life of the tool, which becomes dimensional spread and cavity-specific scrap. If the math says 5, build 8 and bank the capacity, or build 4 and plan a second tool at volume. The odd layout saves steel on paper and pays for it in process windows forever.

Check the press before you cut the layout. Clamp tonnage runs 2 to 4 tons per square inch of projected area for polyolefins and 3 to 5 for engineering resins. Eight cavities of a part with 6 square inches of projected area at 3 tons needs 144 tons plus 10 to 15 percent for the runner, so a 175 to 200 ton press. Then verify shot size lands between 20 and 80 percent of barrel capacity. Building an 8 cavity tool your biggest press cannot clamp is a $60,000 lesson, and it happens more often than anyone admits.

Cycle time and cavitation trade against each other, so work both. Cutting a 30 second cycle to 24 through better cooling raises each cavity from 600,000 to 750,000 parts a year, and a requirement of 3.33 cavities drops to 2.67. You still build 4, but now you carry 33 percent headroom for demand growth instead of none. Size for year 3 volume on programs with a ramp, since retooling from 4 to 8 cavities mid-program costs more than the incremental steel would have, and requalification adds 8 to 16 weeks of timeline risk.

The failure modes are predictable. Sizing to a sales forecast that runs 20 to 30 percent optimistic, then carrying idle cavities that pushed you into a bigger press. Ignoring changeover and maintenance in available hours, so the 6,000 hours in the model are 4,800 on the floor. Family molds that chain two parts' demand curves together, guaranteeing one cavity set is always wrong. And skipping the tonnage check, then running a 200 ton tool in a 150 ton press with flash, tie bar stretch, and a rebuilt platen bearing 18 months later.

Deploy cavitation as a living decision. Monthly, compare demand against demonstrated capacity by tool and flag anything loaded over 85 percent or under 40. Annually, rerun the cavitation math on the top 20 tools by press hours; demand moved, cycles improved, and some 4 cavity tools now justify 8. World class shops load tools at 70 to 85 percent of capacity, hold a documented tonnage and shot size check for every tool and press pairing, and approve second tools with the same demand math that sized the first one, not with a panic order after the second missed shipment.

Published 2026-07-02.