Pressure transducer of an injection molding machine

In injection molding, a pressure transducer (pressure sensor) turns the hydraulic pressure into an electrical signal that the controller can read. On a hydraulic machine it usually sits in the main pressure line, on or near the valve manifold, or in the line to the injection cylinder. Its signal shows the pressure on the screen, triggers the pressure alarms and, on many machines, closes the control loop for injection pressure, hold pressure, back pressure and clamping pressure. On machines with a servo pump, the servo drive uses the transducer signal to run the pump at the pressure the cycle asks for. A faulty transducer makes the machine control the wrong pressure, so its symptoms look like a hydraulic fault.

Where the pressure transducer sits

The transducer is not drawn separately in the hydraulic system drawing. It is fitted in the pressure line at a point like the pressure gauge's, or on the valve manifold, and its cable runs to the controller or the servo drive:

• Main system pressure: near the pump outlet or on the manifold. On a servo pump system this transducer is wired to the servo drive, which uses it as its pressure feedback.
• Injection pressure: some machines measure the pressure in the injection cylinder line, close to where it acts on the screw.

Melt pressure sensors in the nozzle or the mold measure the plastic, not the oil. They are a different, high-temperature type of sensor.

How a pressure transducer works

In most industrial transducers, the oil presses on a thin steel diaphragm carrying strain gauges. As the diaphragm flexes, their electrical resistance changes, and built-in electronics turn that change into a standard analog signal, for example 4 to 20 mA or 0 to 10 V, typically from a 24 V DC supply.

• With a 4 to 20 mA output, zero pressure gives 4 mA, so a broken wire (0 mA) can be told apart from zero pressure.
• The controller converts the signal back to pressure using the range set for that transducer. If the range or the signal type changes, the scaling in the controller must change too.

The controller compares the measured pressure with the set value and drives the proportional valve or the servo pump to close the gap. Positions, such as those of the screw and the moving platen, are measured by a different sensor, the linear transducer.

Signs of a faulty pressure transducer

• The screen shows pressure when the pump is stopped and all pressure has been released.
• The reading jumps, drops to zero or shows a sensor fault or signal out of range alarm.
• The pressure hunts or oscillates, or overshoots its set value.
• The machine builds more or less pressure than set, compared with a calibrated gauge.
• On a servo pump machine, the pump runs unsteadily, gets noisy or gives pressure alarms.
• The reading changes when you move the cable or the connector.

Common causes

• Pressure peaks above the transducer's overload rating, from fast valve switching.
• A damaged cable or a connector with oil, water or loose pins.
• Wrong supply voltage or a wiring mistake after a repair.
• Electrical noise from poor shielding or earthing of the signal cable.
• Drift with age and heat.

Testing a pressure transducer

Electrical checks must be done by qualified staff, following the machine's wiring diagram.

1. Compare. Fit a calibrated pressure gauge at the same point and compare it with the controller reading at zero and at several pressures in manual mode.
2. Check the zero. With the pump stopped and all pressure released, including the accumulator, the reading should be zero.
3. Measure the supply at the transducer connector.
4. Measure the signal. For a current output, measure in series in the signal line; for a voltage output, between the signal and 0 V. At zero pressure it should read the zero value, such as 4 mA, and rise in proportion to the pressure.
5. Move the cable and the connector while watching the reading.
6. Locate the fault. If the signal at the transducer is right but the controller reading is wrong, look at the cable, the I/O module and the scaling in the controller.

Replacing a pressure transducer

1. Stop the pump, lock out the power and release all hydraulic pressure, including the accumulator.
2. Unplug the connector and unscrew the transducer by its hexagon, not by its body.
3. Fit the new transducer with a new seal and tighten it as its maker specifies.
4. Check the zero and, if the machine manual describes it, calibrate the zero and span in the controller.
5. Compare the reading with a calibrated gauge before returning to production.

What to send for a replacement quote

1. Everything on the transducer's label: maker, model code, pressure range, output signal and supply voltage.
2. The thread size and type, and the seal.
3. The connector type and the pin assignment.
4. Where it is fitted, and whether it is wired to the controller or the servo drive.
5. The machine brand and model, and photos of the transducer and its label.

Request a quote. For the rest of the circuit, see the hydraulic system guide and our article on troubleshooting hydraulic pressure loss.