Last updated: 2026-09-05 · 5 min read
Packing Pressure Optimization in Injection Molding — How Do You Find the Right Hold Pressure to Eliminate Sink, Flash and Dimensional Drift?
Packing pressure (hold pressure) is applied after the initial injection fill — typically 50–80% of injection pressure, for 3–15 seconds. It forces additional material into the cavity to compensate for shrinkage as the melt cools. Too little → sink marks, voids, undersized parts. Too much → flash, over-packing, stuck parts, dimensional oversize. The optimization is systematic: weight study → dimensional check → cavity pressure verification → packing time set to gate freeze + 1–2 s.
Key Facts
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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.
What Is the Step-by-Step Procedure for Optimizing Packing Pressure?
- Weight study (30 minutes). Run 5 shots at each packing pressure level (40%, 50%, 60%, 70%, 80% of injection pressure), weighing each part to 0.01g precision. Plot part weight vs. packing pressure. The curve rises linearly until the gate freezes — at that point, additional pressure cannot push more material in, and the weight plateaus. The optimal packing pressure is at the knee of the curve.
- Dimensional check (2 hours). At the optimal packing pressure from the weight study, run 20 consecutive shots and measure 5 critical dimensions on each part. Check for dimensional consistency (Cpk above 1.33) and for sink marks on thick sections. If dimensions are within tolerance and no sink is visible, the packing pressure is correct.
- Cavity pressure verification (if sensors installed). Monitor the cavity pressure curve during packing. The ideal curve shows a smooth decay from peak packing pressure to zero as the gate freezes. If the curve drops abruptly (pressure collapse), packing is too high and flash risk increases. If the curve decays too quickly, packing is too low and sink or voids may appear.
- Packing time optimization (1 hour). With the packing pressure set, vary the packing time from 2 to 15 seconds in 2-second increments. Weigh parts at each time. The optimal packing time is where part weight no longer increases — this is the gate freeze time. Set packing time to gate freeze time plus 1–2 seconds as safety margin.
Longer packing times beyond the gate freeze waste cycle time without improving part quality. The weight study identifies the gate freeze time in 30 minutes; the full optimization takes about 3.5 hours.
What Defects Result from Incorrect Packing Pressure?
Five defect categories comparing too little vs. too much packing:
| Defect | Too Little Packing | Too Much Packing | Diagnostic |
|---|---|---|---|
| Part weight | Below target (under-packed) | Above target (over-packed) | Weigh 5 parts; compare to target from weight study |
| Dimension | Undersize (shrinkage not compensated) | Oversize (over-packed cavity) | CMM measurement of critical dimensions |
| Sink marks | Visible sink on thick sections | No sink (but may have flash) | Visual inspection; profilometer for sub-visible sink |
| Flash | No flash | Flash at parting line, ejector pins or sliders | Visual inspection; feeler gauge for flash thickness |
| Ejection | Part sticks on core (shrinkage lock) | Part sticks on cavity (over-packing) | Observe which half the part stays in after mold open |
The diagnostic sequence: weigh the part (under or over target?), check for flash (present or absent?), check for sink (visible or absent?), measure the dimension (under or over nominal?). The combination of these four checks identifies whether the packing pressure is too high, too low or correct.
Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.
Packing pressure is 50–80% of injection pressure, applied for 3–15 seconds. The optimization takes 3.5 hours: weight study (30 min), dimensional check (2 hr), cavity pressure verification and packing time set to gate freeze + 1–2 s. Longer packing times waste cycle time without improving quality.
Frequently Asked Questions
How does packing pressure affect cycle time?
Packing pressure does not directly affect cycle time — cooling time is determined by wall thickness and mold temperature, not packing pressure. However, packing time does affect cycle time: if packing time is set longer than the gate freeze time, the additional time is wasted because no more material can enter the cavity.
The weight study identifies the gate freeze time. For a thin-walled part with a small gate, freeze time may be 2–3 seconds; for a thick-walled part with a large gate, 10–15 seconds. Cycle time savings from optimizing packing time can be 3–10 seconds per shot — 5–15% productivity improvement on high-volume programs.
What is the difference between packing pressure and injection pressure?
Injection pressure fills the cavity (high speed, high pressure, short time — 0.5–2 seconds). Packing pressure compensates for shrinkage (low speed, moderate pressure, longer time — 3–15 seconds). If packing equals injection pressure, the cavity is over-packed — material is compressed beyond equilibrium density, and the part sticks, flashes or develops internal stresses causing warpage.
Packing pressure must be lower than injection pressure — typically 50–80% — to allow the cavity to equilibrium without over-compression.
Does packing pressure need re-optimization when material lot changes?
Yes — different lots of the same resin can have different melt flow rates (MFR), which affect how the material packs in the cavity. A lot with higher MFR flows more easily and may require less packing pressure; lower MFR may require more. Variation is typically ±5–10% in packing pressure between lots.
For critical programs, run a quick weight study (5 shots) when a new lot is loaded. The check takes 15 minutes and prevents dimensional drift or sink marks from the lot change.
Key Takeaways
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Too much → flash, over-packing, stuck parts.
If your mold is producing sink marks, flash or dimensional drift, packing pressure is the first parameter to check. Send the process sheet and 5 weighed parts for a packing pressure review.
Sources: Society of Plastics Engineers (SPE) — Injection Molding Process Optimization Papers · ASTM International — Injection Molding Process Standards · Beaumont, Runner and Gating Design Handbook (2nd Ed.) · ENGEL — Process Optimization Guide for Injection Molding · Arburg — Practical Guide to Packing Pressure Optimization
— Arthur, Tooling Engineer at aCatMould