Last updated: 2026-09-01 · 5 min read
What Is a Gate Freeze Study — and Why Does It Set the Holding Time?
Gate freeze is the moment during the cooling phase when the resin at the gate solidifies enough to stop flow. After gate freeze, additional holding pressure does not pack more material into the cavity — which means the holding time should be set to match the gate freeze time, not longer. Holding pressure beyond gate freeze wastes cycle time and energy without improving part quality.
Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.
Key Facts
Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. The related guide covers adjacent topics.
How to Run the Study
The gate freeze study is one of the simplest and most valuable process-development tests. It takes 15–20 minutes at T1 and eliminates the guesswork from holding-time setup for the entire production run.
Establish a stable fill
Set the fill to 95–98 % of the cavity volume (short of full pack). Set holding pressure to the normal production value. Set holding time to 1 second. Run five shots to stabilize the process and discard them.
Weigh the parts at 1-second increments
Produce one shot at 1 second holding time, weigh the part. Increase to 2 seconds, weigh. Continue in 1-second increments up to 15–20 seconds or until the weight stabilizes. Use a scale with 0.01 g resolution.
Plot weight vs holding time
The graph shows a rising curve that levels off. The point where the curve flattens (weight increase less than 0.1 g between consecutive increments) is the gate freeze time.
Set production holding time
Set the production holding time to the measured gate freeze time plus 0.5–1.0 seconds of safety margin. This ensures full packing without overextending the hold phase.
Document and verify at each start-up
Record the gate freeze study data in the process sheet. Re-verify at each production start-up if the resin lot, melt temperature, or mold temperature has changed — gate freeze time shifts with any of these variables.
What Happens When Holding Time Is Wrong
| Condition | Symptom | Root Cause |
|---|---|---|
| Holding time too short (before gate freeze) | sink marks, voids, low part weight | insufficient material packed into cavity |
| Holding time too long (far past gate freeze) | overpacking, flash, longer cycle | excess pressure after gate is sealed |
| Holding time at gate freeze + margin | optimal weight, minimal cycle | packing stops when gate seals |
Why Gate Size Changes Everything
A 0.5 mm pin-point gate in a thin-wall part may freeze in 1–2 seconds — if the holding time is set to 5 seconds, only the first 2 seconds of holding pressure actually pack material. The remaining 3 seconds are wasted energy and extend the cycle.
This is necessary for thick-wall parts that need extended packing to compensate for volumetric shrinkage during cooling.
If the gate is too small for the required packing, the only fix is to re-machine the gate — which is why gate sizing belongs in the mold-flow simulation, not in the T1 adjustment.
If one cavity’s gate freezes 2 seconds earlier than another’s, the holding time must be set for the earlier freeze — otherwise that cavity overpacks while the later-freeze cavity is still being packed.
Frequently Asked Questions
Does the gate freeze study apply to hot-runner molds?
Yes, but the interpretation is different.
A thermal hot-runner tip does not freeze the same way a cold-runner gate does — the tip stays molten until the machine nozzle stops injecting. For valve-gated hot runners, the valve pin close time replaces the gate freeze concept: holding pressure is effective until the valve pin closes.
How does resin type affect gate freeze time?
Amorphous resins (ABS, PC, PMMA) freeze faster than semi-crystalline resins (PA66, POM, PBT) at the same gate size because amorphous resins solidify over a broader temperature range. Semi-crystalline resins hold their melt viscosity longer and then freeze abruptly at the crystallization temperature, producing a sharper freeze point.
Can I skip the gate freeze study if I use mold-flow simulation?
The simulation predicts gate freeze time, but the prediction assumes exact melt temperature, mold temperature, and resin properties. The physical study at T1 confirms the simulation and catches discrepancies from actual processing conditions. The 15-minute study is the verification that the simulation was correct.
Key Takeaways
- The gate freeze study takes 15 minutes and eliminates the most common cause of both under-packing (sinks, voids) and over-packing (flash, long cycles).
- Holding time should be gate freeze time plus 0.5–1.0 seconds — not a round number, not a machine default, and not carried over from a different resin or a different mold.
- Gate size is a design decision that locks in the holding-time window — too small a gate limits effective packing; too large a gate wastes cycle time. The gate freeze study at T1 verifies that the gate design matches the packing requirement.
A process engineer who runs the gate freeze study at T1 and documents the result has a production process that starts correctly on day one. A process that relies on the machine default holding time starts with a guess — and the first 500 shots of every production run pay the cost of that guess in scrap and adjustment.
Sources
Plastics Technology — process development
Beaumont Technologies — mold filling analysis
RJG — scientific molding training
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet