How do you calculate pack and hold time?

Start from the gate freeze time, apply a base factor, and add a safety extension so the gate is fully sealed. With a 4.5 second freeze estimate, a 1.0 base factor, and a 1.10 safety extension you get a recommended hold time of 4.95 seconds.

What is gate freeze time?

It is the time for the plastic in the gate to solidify enough that no more melt can flow in or back out. Once the gate freezes, additional hold pressure does nothing, so hold time only needs to slightly exceed gate freeze.

How do you run a gate seal study?

Mold parts at increasing hold times and weigh each one. As hold time increases, part weight climbs and then plateaus; the hold time where weight stops increasing is the gate seal point. Set hold just past that plateau.

What is a good pack and hold time?

There is no universal value because it tracks gate size and wall thickness. The right hold time is the shortest one that fully seals the gate, typically a few seconds for small gates; the 4.95 second example reflects a 4.5 second freeze plus a 10 percent margin.

What happens if hold time is too short?

The gate hasn't sealed, so melt flows back out of the cavity as pressure drops, causing sink marks, voids, low part weight, and dimensional shrinkage. Increasing hold time until part weight stabilizes fixes it.

Can hold time be too long?

Yes. Once the gate freezes, extra hold time adds nothing but cycle time and can over-stress the gate area. Holding far past gate seal wastes seconds on every shot and inflates cost per part.

Injection Molding calculator

Pack & Hold Time Calculator

Pack and hold time is the duration the screw maintains pressure after fill to push extra melt into the cavity and seal the gate before it freezes, compensating for the volumetric shrinkage that causes sink marks and voids. Process engineers set it from a gate seal study, where parts are weighed at increasing hold times until weight stops rising. It matters because too little hold leaves sink and dimensional drift, while too much hold wastes cycle time and can over-pack the part near the gate. This calculator gives you a defensible starting hold time anchored to the gate freeze estimate before you fine-tune on the press.

What this calculator does

Estimate pack and hold phase duration from gate freeze time, part thickness, and a safety factor for dimensional stability.
Use this when setting initial pack/hold time for a new mold, optimizing cycle time by finding the minimum effective hold time, or troubleshooting sink marks and voids.
It estimates the recommended pack and hold time by scaling the gate freeze time with a base factor and a safety extension.

Formula used

Hold time = Gate freeze time x Base factor x Safety extension
Validate with gate seal study (part weight vs. hold time curve)

Inputs explained

Gate freeze time estimate:
Hold time base factor:
Safety extension factor:

How to use the result

Use it when setting initial hold time for a new mold, trimming cycle time by finding the minimum effective hold, or troubleshooting sink marks and voids near the gate.
It is a starting estimate; only a gate seal study (part weight versus hold time) confirms the true freeze point for your specific gate, resin, and melt temperature.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 319.371 (BLS, May 2026), up 19.5% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate pack and hold time? Start from the gate freeze time, apply a base factor, and add a safety extension so the gate is fully sealed. With a 4.5 second freeze estimate, a 1.0 base factor, and a 1.10 safety extension you get a recommended hold time of 4.95 seconds.
What is gate freeze time? It is the time for the plastic in the gate to solidify enough that no more melt can flow in or back out. Once the gate freezes, additional hold pressure does nothing, so hold time only needs to slightly exceed gate freeze.
How do you run a gate seal study? Mold parts at increasing hold times and weigh each one. As hold time increases, part weight climbs and then plateaus; the hold time where weight stops increasing is the gate seal point. Set hold just past that plateau.
What is a good pack and hold time? There is no universal value because it tracks gate size and wall thickness. The right hold time is the shortest one that fully seals the gate, typically a few seconds for small gates; the 4.95 second example reflects a 4.5 second freeze plus a 10 percent margin.
What happens if hold time is too short? The gate hasn't sealed, so melt flows back out of the cavity as pressure drops, causing sink marks, voids, low part weight, and dimensional shrinkage. Increasing hold time until part weight stabilizes fixes it.

Last reviewed 2026-05-12.