What is the formula for injection molding cycle time?

Cycle Time = Fill + Pack and Hold + max(Cooling, Screw Recovery) + Open, Eject and Close. With 1.5 s fill, 4 s pack and hold, a 10 s cooling timer, 7 s recovery and 5 s to open, eject and close, the cycle is 20.5 s.

How do I calculate parts per hour from cycle time?

Parts per hour = 3,600 / cycle time in seconds x cavities running. A 20.5 s cycle gives 175.6 shots an hour; on an 8-cavity mold that is 1,405 parts an hour, before downtime and scrap.

Does screw recovery add to the cycle time?

Only when it takes longer than the cooling timer. Recovery happens during cooling, so with a 10 s timer and 7 s recovery it adds nothing; with 12 s recovery the mold waits 2 s for the screw and the cycle grows by 2 s.

Which phase of the cycle is usually the longest?

Cooling, in most parts, because the plastic must solidify enough to eject without warping, and cooling time grows with the square of wall thickness. Check your own process sheet: thin parts with long mold movements can be dominated by open, eject and close.

How do I compare my cycle with the quoted cycle?

Subtract the measured cycle from the quoted one. A 20.5 s cycle against a 22 s quote leaves 1.5 s of headroom, about 12 more shots an hour than the quote assumed; a negative result means every hour makes fewer parts than the price was based on.

Engineering and Process

Injection Molding Cycle Time Formula

Injection molding cycle time is fill + pack and hold + cooling (or screw recovery, if it takes longer) + mold open, eject and close: 1.5 + 4 + 10 + 5 = 20.5 seconds, or 175.6 shots an hour. Use it to check a quoted cycle, see which phase to work on, and turn cycle time into parts per hour.

Formula

Cycle Time = Fill + Pack and Hold + max(Cooling, Screw Recovery) + Open, Eject and Close

Variables

Fill: Injection time to fill the cavities, in seconds
Pack and Hold: Time the screw holds pressure while the gates freeze, in seconds
Cooling: Cooling timer after pack and hold ends, in seconds
Screw Recovery: Time the screw takes to plasticize the next shot; it runs during cooling, so only the longer of the two counts
Open, Eject and Close: Mold open, part ejection or robot takeoff, and mold close, in seconds

Understanding the Injection Molding Cycle Time Formula

A molding cycle is a sequence of phases that mostly cannot overlap: the mold fills, the screw holds pressure while the gates freeze, the part cools, and the mold opens, ejects and closes. The one overlap is screw recovery, which plasticizes the next shot during cooling, so the cooling phase is the longer of the cooling timer and the recovery time.

Cooling is often the longest phase; in the example it is 10 of 20.5 seconds, nearly half the cycle. That makes wall thickness, mold temperature and cooling-channel design the biggest levers, while robot takeoff and faster mold movements trim the open, eject and close time.

Cycle time sets output: shots per hour = 3,600 / cycle time, and parts per hour = shots per hour x cavities running. A 20.5 s cycle on an 8-cavity mold is 175.6 shots and 1,405 parts an hour, which is why a second shaved off the cycle shows straight up in the part cost.

Worked Example

A PC housing mold fills in 1.5 s and packs and holds for 4 s. The cooling timer is 10 s while the screw recovers in 7 s, and opening, ejecting with a robot and closing take 5 s. The quote assumed a 22 s cycle.

Cooling phase = max(10, 7) = 10 s
Cycle time = 1.5 + 4 + 10 + 5 = 20.5 s
Shots per hour = 3,600 / 20.5 = 175.6
Headroom to the quote = 22 - 20.5 = 1.5 s

Result: 20.5 s cycle, 175.6 shots per hour, 1.5 s inside the 22 s quote

Common Mistake

Adding screw recovery to the cooling time. Recovery runs while the part cools, so only the longer of the two counts; adding both overstates the cycle. When recovery outlasts the cooling timer, the mold waits for the screw and recovery sets the cycle.

Frequently Asked Questions

What is the formula for injection molding cycle time?: Cycle Time = Fill + Pack and Hold + max(Cooling, Screw Recovery) + Open, Eject and Close. With 1.5 s fill, 4 s pack and hold, a 10 s cooling timer, 7 s recovery and 5 s to open, eject and close, the cycle is 20.5 s.
How do I calculate parts per hour from cycle time?: Parts per hour = 3,600 / cycle time in seconds x cavities running. A 20.5 s cycle gives 175.6 shots an hour; on an 8-cavity mold that is 1,405 parts an hour, before downtime and scrap.
Does screw recovery add to the cycle time?: Only when it takes longer than the cooling timer. Recovery happens during cooling, so with a 10 s timer and 7 s recovery it adds nothing; with 12 s recovery the mold waits 2 s for the screw and the cycle grows by 2 s.
Which phase of the cycle is usually the longest?: Cooling, in most parts, because the plastic must solidify enough to eject without warping, and cooling time grows with the square of wall thickness. Check your own process sheet: thin parts with long mold movements can be dominated by open, eject and close.
How do I compare my cycle with the quoted cycle?: Subtract the measured cycle from the quoted one. A 20.5 s cycle against a 22 s quote leaves 1.5 s of headroom, about 12 more shots an hour than the quote assumed; a negative result means every hour makes fewer parts than the price was based on.