How much hold should I add past gate seal?

About 20% past the seal time measured in a weight study, as Paulson Training teaches. That margin keeps the gate sealed if mold temperature, melt temperature or pack pressure rises.

How do I run a gate seal study?

Mold parts at increasing hold times, stepping 0.5 sec, and weigh each shot. Weight rises, then levels off; the hold time where it stops rising is the gate seal time.

Does gate freeze matter with a valve gate?

Not for sealing. The valve pin closes the gate mechanically when hold ends, before the gate would freeze, so this estimate does not apply. Judge hold by part weight instead.

Which freeze temperature should I use for PC?

About 147 °C, its glass transition, because Moldflow takes Tg as the freeze point of amorphous resins. With an 85 °C mold that is 62 °C above mold, the default.

Injection Molding calculator

Pack & Hold Time Calculator

Estimate when the gate freezes, after which more hold adds no plastic, and check your hold timer against it. You need gate thickness, thermal diffusivity, and melt, mold and freeze temperatures.

What this calculator does

Estimated gate freeze time, the shortest pack and hold that seals the gate, checked against your hold timer.

Formula used

Gate freeze time = gate² ÷ (π² × thermal diffusivity) × ln[(4 ÷ π) × (melt − mold) ÷ (freeze − mold)]
Hold past gate freeze = pack and hold time − gate freeze time
Hold margin = hold past gate freeze ÷ gate freeze time × 100
Thickest gate your hold seals = gate × √(pack and hold time ÷ gate freeze time)

Inputs explained

Gate Thickness: Land thickness of an edge or fan gate, from the mold drawing.
Thermal Diffusivity: Moldflow or supplier data, or conductivity ÷ (density × specific heat).
Melt Minus Mold Temperature: Melt temperature less mold surface temperature, from the setup sheet.
Freeze Minus Mold Temperature: Moldflow transition temperature (Tg or crystallization) less mold temperature.
Pack and Hold Time: Hold timer you run now, or the one you plan.

How to use the result

Best suited to first hold time on a new mold, sizing a gate to seal within the cycle, checking a hold trimmed to save cycle.
Treats gate melt as still; packing flow, set partly by cavity size, can delay the real seal. Plate model for edge and fan gates; round pin and tunnel gates need a cylinder model. Not for valve gates: the pin closes the gate mechanically at the end of hold.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% 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 much hold should I add past gate seal? About 20% past the seal time measured in a weight study, as Paulson Training teaches. That margin keeps the gate sealed if mold temperature, melt temperature or pack pressure rises.
How do I run a gate seal study? Mold parts at increasing hold times, stepping 0.5 sec, and weigh each shot. Weight rises, then levels off; the hold time where it stops rising is the gate seal time.
Does gate freeze matter with a valve gate? Not for sealing. The valve pin closes the gate mechanically when hold ends, before the gate would freeze, so this estimate does not apply. Judge hold by part weight instead.
Which freeze temperature should I use for PC? About 147 °C, its glass transition, because Moldflow takes Tg as the freeze point of amorphous resins. With an 85 °C mold that is 62 °C above mold, the default.

Last reviewed 2026-10-01.