injection_moulding — pressure_hydraulics (pressure-transducer-hydraulic)
Physics
Section titled “Physics”An injection-moulding machine’s hydraulic circuit drives both the clamp unit (which holds the mould shut against injection pressure) and, on hydraulic/hybrid machines, the injection ram itself. Clamp tonnage and injection pressure are both set by hydraulic line pressure; a worn pump, a drifting relief valve, or a leaking cylinder seal lowers the pressure the circuit can hold, which first shows up as flash (clamp-tonnage loss) or short-shots/inconsistent part weight (injection-pressure drift) well before an operator notices a scrap trend. A pressure transducer tee’d into the clamp or injection pressure port reads that line pressure continuously. As in the generic hydraulic-pressure recipe, this binds to the generic run/idle/off/fault threshold-state detector repurposed for pressure-level classification: holding above run_above is the normal charged state, falling below off_below is depleted/fault-risk, and staying there past fault_after_s is a confirmed fault. The hydraulic pump’s own pressure ripple settles on the order of tens of milliseconds, but the state machine only needs to track the much slower trend of seal/valve wear over minutes to shifts, so sampling at 5 Hz gives wide margin over that trend’s bandwidth while a 1 Hz low-pass ahead of it removes pump ripple.
Sensor
Section titled “Sensor”- Spec: hydraulic pressure transducer, threaded port, 0-5000 psi range, 4-20 mA
- Signal: scalar ([psi]), range n/a
Example parts
Section titled “Example parts”- ifm electronic PT5404 pressure transmitter (0-580 bar / 0-8410 psi, 4-20 mA, G1/4 port)
- Gems Sensors 3100 series pressure transmitter (0-5000 psi, 4-20 mA, 1/4 NPT)
- Wika A-10 pressure transmitter (0-5000 psi, 4-20 mA, 1/4 NPT)
Mounting
Section titled “Mounting”Install the transducer in a tee fitting on the clamp or injection hydraulic circuit’s existing gauge port, or a spare SAE port rated for the circuit’s working pressure, placed as close to the clamp cylinder or injection ram’s hydraulic actuator as practical so the reading reflects actual holding/driving pressure rather than pump-side drop across hose runs. Torque to the fitting manufacturer’s spec and pressure-test the new joint at system pressure before resuming production.
Wiring
Section titled “Wiring”4-20 mA two-wire loop to a 24 V DC loop-powered analog input on the edge box’s DAQ module; route the shielded twisted-pair cable away from the machine’s servo-drive and heater-band wiring, and ground the shield at the DAQ end only to avoid a ground loop.
Validation
Section titled “Validation”during a normal clamp-closed/injection cycle, read the transducer’s live psi value on the edge box dashboard against a calibrated test gauge tee’d into the same port, then briefly crack the relief valve to confirm the detector transitions to depleted/fault within fault_after_s
Expected: transducer reading within +/-2% of the test gauge, and the detector reports depleted/fault state within one debounce window of the induced pressure drop
Safety notes
Section titled “Safety notes”- hydraulic fluid under system pressure (often 1500-5000+ psi on the clamp/injection circuit) can penetrate skin through a pinhole leak and cause a serious fluid-injection injury with no visible external wound; never attempt to feel for a leak with a bare hand
- de-pressurize and lock out the hydraulic circuit (LOTO on the pump motor, bleed accumulated pressure via the machine’s rated bleed-down procedure) before opening any fitting or installing the tee for the transducer
- verify the transducer’s pressure rating and port thread exceed the circuit’s maximum working pressure before installation; a transducer rated below system pressure can rupture
90–320 USD, difficulty 3/5, ~60 min install downtime.