Last updated: 2026-09-02 · 5 min read

How Do You Read a Cavity Pressure Curve to Catch Process Drift?

A cavity pressure curve records the pressure inside the mold cavity through the cycle — fill, switchover, packing, and gate seal — and it catches process drift earlier than part weight because it measures what the melt actually experienced, not what came out afterward. Learning the curve’s four segments turns a mysterious waveform into a daily diagnostic: each segment has a signature, and each signature points at a specific cause.

Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

Key Facts

  • The four segments: a steep fill ramp as the cavity fills; a switchover point (a sharp peak or V) where the machine transfers from velocity to pressure control; a packing plateau where holding pressure compensates shrinkage; and a decay to the seal point where the gate freezes and the cavity is isolated. Shape changes in any segment are process information.
  • Cavity sensors see the part; machine sensors see the hydraulics. A piezoelectric sensor flush-mounted behind the cavity surface reports the pressure the melt delivered inside the cavity — which is why it responds to resin viscosity changes that a machine-side transducer partially masks. Machine curves remain useful, but the cavity signal is the closer witness.
  • Drift announces itself in the curve before it reaches the dimension report: peak pressure creeping up across a shift usually means viscosity is rising (moisture uptake, more regrind, or barrel heat loss); inconsistent switchover peaks point at the non-return (check) valve; a moving seal point points at gate wear or temperature drift.

    • Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.

    The Curve, Segment by Segment

    Segment What It Reflects Typical Drift Signature First Check
    Fill ramp Flow resistance during filling Ramp steepens shot to shot Resin moisture, regrind ratio, melt temperature
    Switchover point Transfer from velocity to pressure control Peak height or position varies shot to shot Check ring (non-return valve) condition
    Packing plateau Holding pressure delivered to the cavity Plateau level or shape shifts Holding profile settings, gate condition
    Seal point Gate freeze-off and cavity isolation Seal time moves earlier or later Gate wear, mold temperature drift

    A Baseline-and-Alarm Routine That Runs Itself

    1

    Record the healthy baseline

    At process acceptance, save a set of curves from stable production: overlay several consecutive shots and record peak pressure, switchover pressure, integral (pressure over time), and seal time. This baseline is the reference every future shot gets compared against — without it, you only see drift when it is already a defect.

    2

    Set alarm windows on signature numbers

    Put limits on the numbers, not the picture: for example, a peak pressure window of a few percent around baseline and a switchover consistency limit. Alarms on the curve’s signature numbers fire when the process moves — long before parts fall out of tolerance downstream.

    3

    Correlate with weight and dimensions

    Check that the alarm thresholds make sense against part weight and critical dimensions: a shot whose curve sits outside the window should also show a weight shift. If the curve moves but weight does not, adjust the window; if weight moves but the curve did not, the sensor position may need review.

    4

    Review by lot, not by shot

    At each resin lot change and each shift start, compare a fresh curve overlay against the baseline. Lot-to-lot viscosity changes appear as a small uniform shift — which you can compensate deliberately, instead of discovering as a morning of short shots.

    What Each Drift Pattern Is Telling You

  • Peak pressure climbing steadily through a shift — the melt is getting harder to move: check drying (moisture), regrind percentage, and barrel heater performance before blaming the machine.
  • Switchover peaks scattered shot to shot — the check ring is not sealing consistently; expect shot weight scatter on the scale to confirm it.
    Wear in the non-return valve is the classic cause.
  • Seal point moving earlier — the gate is wearing open less predictably or the gate area is cooling differently; check gate land condition and mold temperature uniformity.
  • Sudden step change between lots — resin lot viscosity difference; recalibrate the process window against the new lot and record the change on the setup sheet.
  • Curve noisy on one cavity only in a multi-cavity tool — a sensor or a single-cavity local issue; check that cavity’s gate, vent, and sensor mounting before assuming a global process problem.

    • Frequently Asked Questions

    Does every cavity need a sensor?

    Not necessarily.
    Common practice instruments selected positions — the hardest-to-fill cavity, the cosmetic-critical cavity, or one per quadrant in a large tool. You gain most of the drift protection at a fraction of the instrumentation cost; high-cavity precision tools justify more sensors, prototype work often justifies one.

    Can I catch drift without cavity sensors?

    Partially — machine-side pressure curves plus a strict shot weight check will catch gross changes. What you lose is sensitivity: the cavity signal reacts to viscosity shifts earlier and distinguishes inside-the-cavity events (gate seal, pack delivery) from machine-side ones. For tight-tolerance parts, the cavity sensor pays for itself in avoided sorting.

    How does this relate to scientific molding?

    Cavity pressure is one of its standard instruments. Scientific molding builds the process on recorded evidence — viscosity curves, pressure curves, gate seal studies — and the cavity pressure curve is the record that keeps running after setup day. It converts a one-time qualification into a continuous check.

    Key Takeaways

    1. Learn the four segments — fill ramp, switchover, packing plateau, seal point — and each carries its own drift signature pointing at a specific cause.
    2. Run baseline-and-alarm: save healthy curves at acceptance, put windows on peak pressure and switchover consistency, and review against the baseline at every lot change.
    3. Cavity sensors see what machine sensors mask; instrument the cavities that matter most and correlate the curve with weight and dimensions to keep the alarms honest.

    A cavity pressure curve turns drift detection from a downstream quality event into an upstream process event. aCatMould qualifies critical tools with a recorded cavity pressure baseline and hands the alarm windows over with the setup sheet — so the curve keeps guarding the process on your floor, with the same numbers it was qualified against.

    Sources
    Plastics Technology — cavity pressure monitoring
    Kistler — cavity pressure sensing basics
    MoldMaking Technology — process monitoring practice

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