Cavity Pressure Sensing: The Only Measurement Inside the Mould

Why nozzle pressure is not enough

The pressure the machine reports is measured in the hydraulic system or at the nozzle. Between that point and the cavity there is a sprue, a runner, a gate and a flow path, and each of them drops pressure in a way that depends on viscosity, temperature and flow rate.

So two cycles with identical machine readings can deliver different cavity pressures, because the melt viscosity changed with a lot of resin or a barrel temperature drift. The machine cannot see the difference and the parts differ.

A cavity pressure sensor sits inside the mould, usually behind an ejector pin or flush with the cavity wall, and measures what the polymer actually experiences. It is the only measurement that is inside the part-forming event.

That makes it the most useful signal available for both control and diagnosis, and it is why scientific moulding is built around it.

Reading the curve

The chart on the left is a cavity pressure trace for a PA66-GF30 bracket with a 4 mm wall over a 6 second window. The pressure rises from zero to 462 bar between 0.4 and 1.6 seconds as the cavity fills, holds near 450 during packing, and falls away as the gate freezes and the part cools.

The shape is diagnostic. A slow rise at the start indicates a restricted gate or a cold runner. A peak that arrives late indicates a fill that is too slow. A hold that does not hold indicates a gate that has frozen before packing finished.

The peak value itself is the most useful single number. It tells you the maximum pressure the polymer saw, which correlates with the internal stress locked into the part.

And the area under the curve between the start of fill and gate freeze is the pressure integral, which is the single best predictor of part weight and dimension.

The integral predicts the part

The right hand panel plots part weight against the pressure integral for the same mould. At an integral of 180 bar seconds the weight is 8.42 grams, at 260 it is 8.60, at 340 it is 8.77 and at 380 it is 8.84.

The relationship is almost linear and it is far tighter than the relationship between weight and any machine setting. That is because the integral captures both the pressure and the time it was applied, which is exactly what determines how much material is packed into the cavity.

The practical use is as a control variable. Instead of holding a hydraulic pressure and hoping, you hold the integral by adjusting the hold pressure and time within the cycle. The part weight then becomes a controlled output rather than a measured surprise.

The spread across the window is 0.5 grams on a 8.4 gram part, which is about 6 percent. That is the range of parts a conventional process produces without knowing it.

Table 1 — Weight follows the integral almost linearly.

Sensor types and where to put them

Piezo-electric sensors are the most common. They are fast, they handle high pressure and high temperature, and they need a charge amplifier. They are direct pressure sensors and they are the reference for scientific moulding.

Piezo-resistive sensors are cheaper and need simpler electronics, and they are less stable at high temperature. They suit monitoring rather than control.

Strain gauges mounted on the mould or on a tie bar measure the force rather than the cavity pressure. They are useful for overall clamping but they cannot see individual cavities.

Placement matters. A sensor behind an ejector pin measures the pressure at that point in the cavity, which is not necessarily the pressure at the gate. For control, place the sensor at the last point to fill, where the pressure history is most representative of the whole part.

Table 2 — Sensor types and what each is good for.

From monitoring to control

Most installations monitor and do not control. The sensor is fitted, the curve is displayed, and the machine still runs on hydraulic pressure. That captures perhaps a third of the value.

Control means using the sensor to switch from velocity to pressure at a defined cavity pressure rather than at a screw position. That makes the transfer point independent of viscosity, which removes the largest single source of part-to-part variation.

The second control level holds the integral by adjusting hold pressure. That makes the part weight independent of resin lot and barrel temperature drift.

The third level rejects parts automatically when the curve falls outside an envelope. That is the point at which the sensor starts paying for itself, because it catches the marginal parts that pass a visual inspection and fail in the field.

Installing without creating problems

The sensor has to be flush with the cavity surface. A sensor that protrudes leaves a mark on every part; a sensor that is recessed fills with flash and stops reading.

The cable route has to avoid the parting line and the ejection path. A cable that is pinched once will fail intermittently, and an intermittent sensor is worse than no sensor because it is trusted.

The connector has to be accessible without removing the mould, and it has to be rated for the mould temperature and the wash-down. A connector that fails after three months is a maintenance item nobody budgeted for.

And the calibration has to be recorded. A sensor that has drifted by 5 percent will still produce a plausible curve, and the process will be controlled to a wrong number.