Last updated: 2026-09-04 · 5 min read

Packing Pressure Optimization — How Do You Set the Packing Pressure and Hold Time to Eliminate Sink Marks Without Overpacking the Mold?

Packing pressure is applied after initial injection — it pushes additional melt into the cavity to compensate for shrinkage as the melt cools. Too little causes sink marks, voids and undersized dimensions. Too much causes flash, stuck parts and residual stress leading to warpage. The optimization starts with one number: the injection-to-packing switchover at 90–95% cavity fill. Get the switchover wrong and nothing else matters. The weight study confirms when packing is correct.

Key Facts

Injection phase fills the cavity 90–95% at high speed (800–1,500 bar, 1–3 seconds). Packing phase pushes in remaining melt to compensate for shrinkage (400–800 bar, 50–80% of injection pressure, 3–15 seconds).

Under-packing: sink marks, voids, undersized dimensions. Over-packing: flash, stuck parts, warpage from residual stress. The diagnosis is straightforward — sink → increase packing; flash → decrease packing.

Optimize with a weight study: run shots at increasing packing pressure. Part weight stabilizes at correct packing (±0.02g across 5 shots). If weight keeps rising, the hold time is too short — the gate freezes before packing completes.

Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.

What Is the Difference Between Injection Pressure and Packing Pressure?

Two phases, two purposes:

Phase	Purpose	Pressure	Duration
Injection (1st stage)	Fill cavity 90–95% at high speed	High: 800–1,500 bar	1–3 seconds
Packing (2nd stage)	Push additional melt in to compensate for shrinkage	Medium: 400–800 bar (50–80% of injection)	3–15 seconds (depends on wall thickness)

The injection pressure must be high enough to fill the cavity before the melt freezes — it overcomes the flow resistance of gates, runners and thin walls. The packing pressure must be lower: it maintains steady pressure on the cavity to push in additional melt as the part shrinks. If packing pressure equals injection pressure, the mold is overpacked — excess material is forced in, creating flash, stuck parts and high residual stress.

The switchover point — where the machine transitions from injection to packing — is the single most important setting. Set it at 90–95% fill: the part is almost full, and the packing pressure gently pushes in the remaining 5–10% to compensate for shrinkage. Too early (80% fill) and the packing pressure must fill the remaining 20% — causing a pressure spike and overpacking.
Too late (98% fill) and the cavity is already full — the gate freezes before packing can push in additional material, making the packing pressure ineffective.

How Do You Optimize the Packing Pressure Step by Step?

Four steps, in order:

Set the switchover at 90–95% cavity fill. This is the position or time at which the machine transitions from injection to packing. The switchover determines whether the packing phase has room to work — too early causes overpacking, too late makes packing ineffective.
Start with packing pressure at 50–60% of injection pressure. If injection is 1,000 bar, start at 500–600 bar. Adjust in 50-bar increments: sink marks present → increase by 50 bar; flash present → decrease by 50 bar.
Set the hold time based on gate freeze-off. The hold time must be long enough to keep the gate open so additional melt can enter, but not so long that the gate freezes before packing completes. A 1mm pin-point gate freezes in 3–5 seconds; a 3mm fan gate freezes in 8–12 seconds. Set hold time to match the gate freeze-off time.
Verify with a weight study. Run shots at increasing packing pressure (constant hold time). Weigh each shot. Weight increases with packing pressure until the gate freezes — at that point, additional pressure cannot push in more material and the weight stabilizes. The correct packing pressure is where weight stabilizes at ±0.02g across 5 consecutive shots. If weight keeps rising, increase the hold time.

Symptom	Under-Packing	Over-Packing
Sink marks	Yes — on thick sections, ribs, bosses	No
Voids	Yes — in thick sections	No
Flash	No	Yes — at parting line, ejector pins
Stuck parts	No	Yes — overpacked parts shrink onto cores
Dimensions	Undersized (shrinkage not compensated)	Oversized (excess material in cavity)
Warpage	Uniform shrinkage (less warpage)	Uneven shrinkage (residual stress → warpage)

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

The switchover point is the single most important setting. Get it wrong and nothing else matters — the packing pressure has no room to work or no time to act.

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Frequently Asked Questions

Should you use a single packing pressure or a multi-stage profile?

For most parts, a single packing pressure (constant throughout the hold time) is sufficient. For complex parts with varying wall thickness, a multi-stage profile is more effective: start at high pressure (80% of injection) for 2–3 seconds to pack the thick sections, then reduce to 40–50% for the remaining hold time to avoid overpacking the thin sections.

The multi-stage profile matches the shrinkage pattern: thick sections need more packing initially (they shrink the most), thin sections need less (they freeze quickly). Most modern machines support 3–5 packing stages.

How does the material affect the packing pressure?

Different materials have different shrinkage rates. Semi-crystalline materials (PP, PE, PA, POM) shrink 1.5–2.5% — they require higher packing pressure (70–80% of injection) and longer hold time. Amorphous materials (ABS, PC, PS, PMMA) shrink 0.5–0.7% — starting point is 50–60%. Glass-filled materials shrink 0.2–0.5% — starting point is 40–50%.

The material supplier’s data sheet provides a starting point, but the actual packing pressure depends on the part geometry, gate size and mold temperature. Always verify with a weight study — the data sheet is a starting point, not the final answer.

Are cavity pressure sensors worth the investment?

Cavity pressure sensors (piezoelectric transducers installed in the cavity) measure actual pressure during injection and packing. The curve shows exactly when the gate freezes (pressure drops to zero), when the cavity is full (pressure peaks) and whether packing is effective (steady pressure during hold time).

Sensors are the most accurate method for optimizing the packing profile — they eliminate the trial-and-error of the weight study and provide real-time monitoring. The investment is $5,000–15,000 per sensor plus data acquisition — justified for high-volume production (millions of shots per year) where a 1% scrap reduction or 5% cycle time reduction pays for the system within months.

Key Takeaways

Packing pressure: 50–80% of injection; hold time: match gate freeze-off.

Switchover at 90–95% fill — not too early, not too late.

Under-packing: sinks, voids, undersized. Over-packing: flash, stuck, warpage.

Weight study: part weight stabilizes at correct packing (±0.02g).

Packing pressure optimization is the most impactful process adjustment for dimensional accuracy and surface quality.
A well-optimized packing profile eliminates sink marks, minimizes warpage and holds tight tolerances — without overpacking the mold. The weight study is the simplest and most reliable method for finding the correct packing pressure. Start with the switchover point, verify with the weight study, and adjust in 50-bar increments until the weight stabilizes.