Hold Time Calculator
Estimate injection molding hold/packing time from cooling time and gate freeze-off factor.
Introduction
Hold (pack) time is the second-stage phase of injection molding where the screw maintains pressure on the cavity until the gate freezes off. Hold too short and the part suffers sink marks, voids, and short weights; hold too long and you waste cycle time pushing against a sealed gate. The Hold Time Calculator gives a disciplined starting point by scaling your computed cooling time with a gate-freeze factor, reflecting the physical reality that small gates seal well before the part core has finished cooling. From that estimate you can run a gate-seal study and converge on the true freeze-off point in a handful of trials.
How it Works
The estimate is t_pack ≈ t_cool × factor. Enter the cooling time — ideally the value from the companion Cooling Time Calculator — and a factor between 0.1 and 1 that represents how early the gate freezes relative to the part. The default pairing of a 7.4 s cooling time with a 0.4 factor yields a 2.96 s hold time, typical for a small-gated part. Larger gates and hot tips freeze later, so they warrant factors toward 0.5–0.8; pin gates and thin parts seal early, pushing the factor down.
Usage Scenarios
- Setting initial process parameters on a new tool before the first sampling run, so the gate-seal study starts near the right value instead of from zero.
- Diagnosing sink marks over thick bosses: the calculator shows the current hold time is well below the gate-freeze estimate, pointing to premature pressure release.
- Trimming cycle time on a mature job — if hold time greatly exceeds t_cool × factor, the press is packing against a frozen gate and the excess is pure waste.
- Adjusting after a gate-diameter change in a tool revision: a larger gate freezes later, so the factor and the hold time both need to rise.
- Transferring a validated process to a second press and recomputing hold time after the cooling time changes with different mold-temperature control.
FAQ
How do I choose the factor?
Start near 0.4 for conventionally gated small parts. Use lower values for pin gates and thin walls that seal early, higher values (0.5–0.8) for large direct gates and hot-tip drops that stay open longer.
Is this a substitute for a gate-seal study?
No — it is the starting point for one. Weigh parts at increasing hold times; when part weight stops climbing, the gate has sealed. The calculator gets your first trial close to that point.
What happens if hold time is too long?
Nothing improves after gate freeze: the press simply holds pressure against sealed plastic, adding cycle seconds and energy cost. Occasionally over-packing near the gate causes stress and warp.
Should hold time scale with wall thickness?
Indirectly, yes. Cooling time grows with the square of wall thickness, and since t_pack ≈ t_cool × factor, thicker parts naturally inherit longer hold-time estimates through the cooling term.