Injection Molding

Mold Parts Per Hour: Cavity Count Versus Cycle Time

Adding cavities and cutting cycle time both raise output, and they have completely different cost structures. One is a capital decision you cannot undo, the other is a process decision you can.

Output from an injection molding cell is cavities divided by cycle time, and there are only two ways to raise it. Both work. They differ in that cavity count is a capital decision made once, at tooling, and effectively permanent, while cycle time is a process variable you can keep attacking for the life of the part. Understanding which one you are actually constrained by determines whether the next investment is a tool or a chiller.

The worked comparison

Take a 30-second cycle in a 4-cavity tool at 85% uptime: roughly 408 parts an hour. Move to 8 cavities and the cycle typically lengthens somewhat, because a larger tool carries more thermal mass and needs more clamp and fill time; assume 34 seconds. Output becomes about 720 parts an hour, roughly 76% more rather than the doubling the cavity count suggests. That shortfall against the intuitive answer is the single most common surprise in tooling decisions.

• 4-cavity at 30s cycle: 408
• 8-cavity at 34s cycle: 720
• Real gain from doubling cavities: +76%

Cooling is where cycle time actually lives

Cooling commonly dominates a molding cycle, and it scales with the square of wall thickness, which makes it the highest-leverage variable in the whole equation. A part redesigned from three millimetres to two and a half in its thickest section can cut cooling substantially without any change to the tool count or the press. That is why the sequence matters: examine wall thickness and cooling circuit design before committing capital to more cavities, because a cycle reduction improves every tool you own while a new tool improves one part number.

More cavities buy output for one part forever. A shorter cycle buys output on everything you run, and it does not need a purchase order.

Where more cavities stop paying

Cavity count runs into three ceilings. Clamp tonnage caps projected area, and exceeding it produces flash rather than parts. Fill balance degrades as runner systems grow, so a high-cavity tool can produce cavity-to-cavity variation that consumes the yield the extra cavities gained. And tool cost rises faster than cavity count while tool maintenance and the consequence of a single cavity failing both rise with it. A sixteen-cavity tool down for a damaged cavity is a bigger event than a four-cavity tool in the same state.

Use the mold parts per hour calculator to compare cavity and cycle scenarios on your own tool. Model the output

Published 2026-08-08.