In-mold (cavity) pressure sensors: why measuring cavity pressure changes process control

The pressure on the machine screen is the pressure in the hydraulic cylinder. The pressure the material actually experiences in the cavity is what remains after losses through the runner, gate and flow resistance. The relationship between the two is not constant — it changes with viscosity, temperature and batch-to-batch variation.

An in-mold (cavity) pressure sensor measures the pressure directly inside the cavity.

What does it give you?

• Direct visibility: You see the pressure curve throughout filling, holding and cooling. The curve shows you why flash, sink marks or short shots occur.
• V/P changeover control: Triggering the changeover on cavity pressure instead of screw position compensates for material batch variation; the result is more consistent part to part.
• Gate-freeze detection: The moment pressure drops during holding is the moment the gate freezes; measuring this determines the holding time.
• Quality monitoring: Deviation from the curve enables rejection and automatic alerts (SPC).
• Balance in multi-cavity molds: Pressure differences between cavities reveal fill imbalance.

Sensor types

• Direct (behind-the-pin) sensor: Placed behind an ejector pin; leaves no mark on the part, easy to install.
• Direct (surface) sensor: Close to the cavity surface; more sensitive, but carries a risk of leaving a mark and needs more space.
• Indirect (strain-gauge) sensors: Derive pressure from mold deflection; the mold design must be suitable for this measurement.

Where to place it

• Near the gate: You see the effect of holding pressure.
• At the last-filling region: You see whether filling is complete and how far the pressure reaches.
• In a multi-cavity mold you can start with cavities representative of the balance, not every cavity.

Suggestion for getting started

1. First pick a critical mold with a high scrap rate.
2. Place the sensor behind a pin and start recording the curve; use the "good part" curve as a reference.
3. Set upper/lower limits and set up an alarm for deviation from the curve.
4. Over time, tie the V/P changeover to cavity pressure.

Note: Sensor and data-acquisition cost varies with the mold and software; the benefit should be weighed together with the gains in scrap rate and setup time.

The calculation tool estimates cavity pressure; measuring the actual pressure with a sensor and comparing is the most reliable way to adapt the estimate to your own mold.