Last updated: 2026-09-02 · 5 min read

How Do You Read a Packing Pressure Curve — and What Does It Tell You About Part Quality?

The packing pressure curve plots cavity pressure against time during the hold phase — and its shape reveals whether the gate freezes at the right moment, whether the material is adequately packed and whether the process is stable shot to shot. A flat curve after peak means the gate sealed correctly; a continuing rise means the gate is still open and overpacking; a drop means the material is cooling too fast or the check ring is leaking.

Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

Key Facts

  • Cavity pressure sensors — placed behind the flow front, not at the gate — measure what the plastic actually experiences, unlike hydraulic pressure which only shows what the machine is doing.
  • The packing curve has three phases: fill (rapid pressure rise), pack (pressure held to compensate shrinkage), and cool (pressure decays as the gate freezes).
  • Shot-to-shot consistency of the curve is the process stability indicator — if the curves overlap within ±5%, the process is in control; if they diverge, the material, machine or mold is drifting.

    • Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. The related guide covers adjacent topics.

    What Each Phase of the Curve Reveals

    Phase Curve Shape What It Tells You
    Fill Rapid linear rise Flow front reaching the sensor; slope indicates fill speed and melt viscosity
    Pack Plateau at set pressure Hold pressure compensating shrinkage; flat plateau = stable process
    Cool / Gate freeze Gradual decay to zero Gate seals; decay rate indicates cooling efficiency and material thermal properties

    How to Use the Curve for Process Decisions

  • Gate seal study: reduce hold time shot by shot until the part weight stops changing — that is the minimum hold time needed, and the curve shows when cavity pressure reaches zero (gate frozen).
  • Overpacking detection: if cavity pressure continues rising after the set hold pressure, the gate has not sealed and the process is adding material to a closed cavity — this causes sink, flash and high ejection force.
  • Check ring wear: if peak pressure varies shot to shot with the same machine settings, the non-return valve (check ring) is leaking — melt flows backward during injection, and the curve shows inconsistent fill.
  • Material lot variation: running the same mold with different resin lots produces different curves if viscosity differs — the curve catches lot-to-lot drift before it shows up as dimensional variation.

    • What This Means for Your Quotation

    A mold quotation that includes cavity pressure monitoring specifies the sensor location, the expected curve shape and the acceptance criteria for shot-to-shot consistency. The trial report includes the actual curves — not just the machine’s hydraulic pressure log — so the process window is documented with what the cavity saw, not what the machine attempted. This is the difference between a process that is stable at the mold maker’s bench and one that transfers reliably to the production floor.

    Frequently Asked Questions

    Why not just use hydraulic pressure instead of cavity pressure?

    Hydraulic pressure shows what the machine is doing; cavity pressure shows what the plastic is experiencing. The hydraulic system has losses — friction, check valve leakage, seal wear — that do not transfer to the melt. Cavity pressure is the direct measurement of the packing force at the point that matters, and it catches problems the hydraulic system cannot see.

    How many cavity pressure sensors does a mold need?

    At minimum, one per cavity — placed behind the flow front, not at the gate. For multi-cavity molds with family parts or imbalanced runners, one per cavity reveals filling differences. For critical dimensions, a second sensor near the critical feature shows whether that zone is adequately packed. Each sensor adds cost to the mold, so the number is driven by the part’s quality requirements.

    Can the curve predict sink marks before they appear?

    Yes — if the packing curve shows premature pressure decay (gate freezes too early), the thick sections will not receive enough material to compensate shrinkage, and sink marks will appear. The curve catches this at trial, before the first production run, so the hold time or pressure can be adjusted while the mold is still on the bench.

    Key Takeaways

    1. Cavity pressure curves show what the plastic actually experiences — fill, pack and gate freeze — not just what the machine attempts.
    2. Shot-to-shot curve consistency within ±5% is the process stability benchmark; divergence signals material, machine or mold drift.
    3. The trial report should include cavity pressure curves, not just hydraulic logs — this documents the process window at the cavity, where quality is made.

    Send aCatMould the part geometry and quality requirements, and the trial plan includes cavity pressure monitoring — with sensor locations, expected curve shape and shot-to-shot acceptance criteria documented in the trial report, so the process is proven at the cavity, not just at the machine panel.

    Sources
    Plastics Technology — cavity pressure monitoring in injection molding
    SPE — process monitoring and control guidance
    Priamus — cavity pressure technology and application notes

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet