How to Perform a Gate Seal Study

After fill speed is understood, the next question is:

When does the gate stop delivering material during pack and hold?

That moment — gate seal (also called gate freeze in much of the industry) — defines how long second-stage pressure can influence the part, and when hold time can be shortened without affecting weight or dimensions.

PolyTakt uses gate seal as the preferred term because it describes the functional outcome: no further packing through the gate. A gate seal study is the structured way to find that point.

Without it, hold time is often set by default, copied from a similar tool, or extended "to be safe" — masking whether second-stage pressure is actually doing useful work.

Most tools are characterised with a single-phase second stage: one Hold Pressure and one Hold Time. That is the default path in this article and in PolyTakt.

For some large or thick-walled parts, trainers and process texts also teach a dual-phase Pack and Hold profile — a higher-pressure pack step while the screw is still moving forward, then a lower hold step until gate seal. That option is documented later in this article. It has a clear physical basis; use it when geometry or quality evidence justifies a split.

What Gate Seal Tells You

During the pressure-controlled second stage, material continues to enter the cavity until the gate solidifies (or, on valve gates, until the valve closes the path).

Before gate seal:

• Second-stage pressure affects shot weight and dimensions
• Time at pressure matters

After gate seal:

• Further hold time adds nothing to part weight
• Melt cannot flow back through a sealed gate — that is the definition of seal
• Machine pressure may still be applied, but mass transfer through the gate has stopped

The study identifies the shortest second-stage time at which shot weight stabilises — the point beyond which additional time does not increase part weight.

That is gate seal time. Production time is seal time plus a small variance cushion — not an arbitrary long “safety” hold.

Prerequisites

Complete or advance fill development first:

• Injection speed selected from viscosity study or equivalent
• V/P transfer approach defined (position, time, or pressure)
• Fill-only short shots confirm consistent fill pattern before pack / hold optimisation
• Hold Pressure characterised (minimum stable pressure for weight and cosmetics) — PolyTakt’s Hold Pressure study, or equivalent

Hold constant during the study:

• Melt and mould temperatures
• Injection speed and transfer settings
• Second-stage pressure profile (single Hold Pressure, or the Pack/Hold pressures if you are running the dual-phase option)
• Cooling time long enough that parts can be weighed without distortion

Use pressures that represent intended production — not zero, not extremes that will not be used.

Default Method: Single-Phase Hold Time

This is the standard gate seal study. One Hold Pressure. Vary Hold Time. Find the weight plateau.

Step 1: Establish Baseline Pack Shots

Run three to five full shots at a hold time clearly longer than expected gate seal — for example, twice the estimated hold time or a conservative default from similar tools.

Confirm:

• Shot weight is stable across consecutive shots
• Parts are acceptable for weight measurement (no short shots, no flash)

Record average shot weight at the long hold time.

This is your reference that second-stage pressure has fully acted.

Step 2: Reduce Hold Time in Steps

Reduce hold time in equal decrements — for example, 1 second or 2 seconds per step depending on part size and gate type.

At each hold time:

1. Run a minimum of three consecutive shots
2. Weigh each shot (or use cavity pressure plateau if that is your validated method)
3. Record average weight and variation
4. Proceed to the next shorter hold time only when results are stable

Continue until shot weight drops consistently below the baseline established in Step 1.

Step 3: Identify Gate Seal Time

Gate seal time is the shortest hold time at which shot weight matches the baseline within normal variation.

Practical interpretation:

• If weight drops when hold time is reduced from 8 s to 6 s, gate seal is not yet reached at 6 s
• If weight at 4 s matches weight at 10 s within measurement noise, gate seal occurs at or before 4 s

Add a small margin for production — typically about +0.5 s on many tools — enough to absorb normal variation.

Document:

• Observed gate seal time
• Selected production Hold Time
• Margin rationale

Step 4: Verify Under Production Conditions

Before approving hold time for production:

• Confirm at the selected hold time across multiple consecutive cycles
• Check critical dimensions if weight alone is not sufficient for the application
• Verify cosmetic and functional requirements on packed parts

For some applications — especially thin gates, hot runner systems, or semi-crystalline materials with continued crystallisation — weight stabilisation may not capture all dimensional effects.

Use weight as the primary gate seal indicator, but apply engineering judgement for the part.

Optional Method: Dual-Phase Pack and Hold

Why split Pack and Hold?

Industry teaching is not unanimous. Many processes run a single second-stage pressure successfully. Others — especially for large or thick-walled parts — split the second stage into two pressure steps:

The physical basis:

• While the screw is still moving forward under pressure, melt is still entering the cavity (mass transfer).
• Once forward velocity approaches zero, further high pressure mainly loads the gate region rather than adding useful volume.
• A lower hold pressure can still keep the cavity pressurised until the gate seals, without the same overpacking stress near the gate.

Important: once the gate has sealed, melt cannot backflow through it. Dual-phase studies that use a reduced hold pressure are probing the period before seal. If hold is too short while the gate is still open, cavity pressure can push melt back past the lower machine pressure and weight falls. When weight plateaus, the gate path is closed.

PolyTakt documents this as an optional Gate Seal study mode. Prefer single-phase Hold unless geometry or quality evidence (sinks, voids, gate-area stress, thick sections) justifies a split.

Dual-phase procedure

1. Characterise Pack Pressure
Use your Hold Pressure study (or equivalent) at a deliberately long second-stage time so pressure is the only variable. That characterised high pressure becomes the Pack Pressure when you split the profile.

2. Isolate Pack Time from screw position
At Pack Pressure, observe Screw Position vs Time on the machine. Record the time at which the screw’s forward velocity approaches zero (the position curve flattens). That duration is Pack Time — the mass-transfer step.

Pack Time should finish before gate seal. Confirm on the weight curve that Pack Time is shorter than the observed seal time; if it is not, revisit the screw graph or collect more weight trials.

3. Set Hold Pressure
Program a second pressure step lower than Pack Pressure. A common starting point in dual-phase teaching is about half of Pack Pressure — enough to confine, low enough that an unsealed gate still shows backflow as weight loss. Adjust the ratio for your tool and material.

4. Weight study vs total second-stage time
Each trial uses a total Pack + Hold duration:

• Apply Pack Pressure for Pack Time
• Then Hold Pressure for the remainder of that total time
• Weigh parts; increase total second-stage time until weight plateaus

Gate seal is that absolute second-stage time — the elapsed Pack + Hold duration at which weight stabilises.

5. Apply a cushion and split the profile

Example:

• Gate seals at 4.0 s total second-stage time
• Thick-wall-style cushion: max(seal × 1.15, seal + 1.5 s) → 5.5 s total
• Pack Time from screw graph → 3.0 s
• Hold Time → 5.5 − 3.0 = 2.5 s

Production profile:

• Step 1: Pack Pressure × Pack Time
• Step 2: Hold Pressure × Hold Time
• Total second stage = Pack Time + Hold Time (= seal + cushion)

When dual-phase is worth trying

• Thick sections that sink or void at a single moderate pressure but flash or stress at a single high pressure
• Parts where gate-area overpack appears before thick sections are densified
• Tools where screw position clearly shows a long forward-creep under pack before velocity reaches zero

If a single Hold Pressure already gives stable weight, cosmetics, and dimensions, stay on the default single-phase method.

Gate Type Considerations

Edge gates
Often show a clear weight plateau. Studies are usually straightforward.

Submarine or tunnel gates
Seal behaviour may be less obvious. Weight steps can be smaller — use precise measurement and more shots per step.

Hot runner valve gates
Gate seal may be controlled by valve timing as well as thermal solidification. The study must reflect the actual production gate sequence. Dual-phase pressure steps do not replace valve timing — document both.

Hot runners (open tips)
Classical cold-gate freeze behaviour differs, but Hold Time (and Pack Time if you split) still needs characterisation for your process — do not skip gate seal because the runner is hot.

Multi-gate parts
Gate seal may not occur simultaneously. Weight may stabilise gradually. Consider cavity-by-cavity evaluation where imbalance is suspected.

When Weight Never Plateaus

The hold-time stepping method assumes shot weight eventually stabilises — that additional hold time stops adding measurable mass because the gate path has closed.

On some materials, that plateau never appears clearly.

Common situations include:

• High-shrink materials — shrinkage during hold and early cooling continues to pull material through the gate while hold pressure is applied. Weight keeps climbing even though classical gate seal is not the right mental model.
• Slow-crystallising or off-intent grades — materials not designed for injection moulding may not show the seal behaviour IM grades typically do. The gate may remain open longer, or densification may continue in ways weight alone does not capture cleanly.
• Pack compensating for shrink rather than filling — hold is doing useful work, but the study looks inconclusive because the endpoint is dimensional stability, not weight levelling off.

If hold time is reduced and weight drops only slightly — or weight continues to rise across the whole range tested — do not force a gate seal time from a curve that has no plateau.

Instead:

• Step hold time against critical dimensions, sink, and functional requirements — not weight alone
• Define hold time where dimensions stabilise within acceptable limits, subject to overpack
• Document explicitly that classical gate seal was not observed and that hold time was set by dimensional response
• Record material grade and rationale, especially where an off-label or non-IM grade was specified for application reasons

That record protects the process team if the approach is questioned later. The study method failed to find a plateau — not because the work was skipped, but because the material did not behave as the standard model assumes.

Common Mistakes

Studying before transfer is stable
If fill varies, pack study results vary. Fix fill first.

Using inconsistent second-stage pressure
Pressure must remain fixed during a single-phase study. In dual-phase, keep the Pack/Hold pressures fixed and vary only total second-stage time.

Too few shots per time step
Shot weight variation from fill noise can hide or fake a plateau. Three minimum; five is better for tight applications.

Confusing gate seal with part ejection readiness
The part may still need time in the mould after the gate seals before it can be ejected without distortion.

Setting hold time to "be safe" without data
Excessive hold time can overpack — especially near thin sections or delicate cores.

Assuming a plateau must appear
High-shrink or slow-sealing materials may never show a clear weight levelling point. Switch to dimensional hold optimisation and document why.

Ignoring material lot variation
Gate seal time can shift with material viscosity or thermal behaviour. Revisit when material conditions change significantly.

Relationship to V/P Transfer

Gate seal and V/P transfer are related but distinct.

• Transfer defines where fill ends and the pressure-controlled second stage begins
• Gate seal defines where effective packing through the gate ends

A process can have a well-established transfer point and still run hold time that is too long or too short.

Both studies are needed for a decoupled process that is efficient and stable.

Medical Production Considerations

In validated moulding, hold time is often a documented process parameter with approved limits.

Gate seal study data supports:

• Initial OQ parameter selection
• Rationale for hold time (and dual-phase pack/hold times, if used) in the validation protocol
• Investigation when hold-related deviation occurs

If hold time is changed after validation, treat that as a change requiring assessment per your change control procedure — not an informal optimisation.

The study provides the evidence base.

The quality system decides whether the change is allowable.

What to Record

Include a simple plot of time vs average shot weight where possible.

The plateau is easier to defend visually than a table alone.

Conclusion

A gate seal study converts second-stage time from a guess into a measured parameter.

Fix fill and transfer first. Characterise Hold Pressure. For most tools, hold pressure constant and step Hold Time until weight stabilises. Verify. Document.

Where thick sections or quality evidence justify it, split Pack and Hold: find Pack Time from screw velocity ≈ 0, find absolute gate seal from weight vs total second-stage time, then set Hold Time as (seal + cushion) − Pack Time.

The reward is confidence that second-stage time is doing useful work — not running on habit.

That is scientific moulding at its most practical.