injection_moulding — environment (humidity-rh)
Physics
Section titled “Physics”A capacitive relative-humidity (RH) sensor’s polymer dielectric absorbs ambient water vapor, changing its capacitance in proportion to %RH; a transmitter converts that capacitance to a 0-100% RH scalar. Hygroscopic resins (nylon, PET, PC, ABS blends) absorb moisture from humid storage/hopper air; if moulded above a resin-specific moisture threshold, dissolved water flashes to steam in the barrel and causes hydrolytic degradation, splay (silver streaking), and brittleness, so a sustained humidity excursion near resin storage or the hopper/dryer is a leading indicator of an upcoming defect wave. This recipe reuses the generic high/high-high plus rate-of-rise threshold detector (rw_analytics.thermal.ThermalParams) for this non-temperature scalar: the field names high_c/high_high_c/rise_c_per_min are historically named for Celsius, but that is purely cosmetic here — the underlying math (an absolute warn threshold, a higher alarm threshold, and a rate-of-rise check) is unit-agnostic and applies identically whether the signal is deg C, %RH, or mg/m3, because the registry has no dedicated environment detector; this is a deliberate, documented simplification given the current detector registry, not a physics error. Ambient humidity near a hopper/dryer changes on a minutes scale, so sampling at 0.2 Hz (once every 5 s) gives >>2x Nyquist margin over the fastest realistic RH change rate (a few %RH/min) for a slowly-varying scalar.
Sensor
Section titled “Sensor”- Spec: capacitive relative-humidity transmitter, 0-100% RH, analog or digital output
- Signal: scalar (%), range n/a
Example parts
Section titled “Example parts”- Vaisala HMP60 humidity and temperature transmitter (0-100% RH, 4-20 mA / Modbus output)
- Sensirion SHT35-DIS humidity/temperature sensor module (industrial I2C, 0-100% RH) with an industrial transmitter enclosure — pair with a DIN-rail signal conditioner for a 4-20 mA or Modbus output if the edge DAQ has no native I2C input
- E+E Elektronik EE210 humidity/temperature transmitter (0-100% RH, 4-20 mA output)
Mounting
Section titled “Mounting”Mount the transmitter’s sensing head in free air near the resin hopper or drying-hopper inlet, 0.3-1 m from the hopper wall, shielded from direct drip or spray, and away from any heater element’s direct radiant heat (which would bias a co-located temperature reading, not the RH element itself, but keep clearance for airflow representativeness). Use a perforated protective cap if dust is present to keep the capacitive element clean without blocking airflow to it.
Wiring
Section titled “Wiring”4-20 mA or Modbus RTU output (per the chosen transmitter) goes to an analog input channel or RS-485 port on the edge box’s DAQ module; use a shielded twisted pair and ground the shield at the DAQ end only to avoid a ground loop with the machine’s drive electronics.
Validation
Section titled “Validation”use a calibrated reference hygrometer to log ambient %RH at the sensor’s location for 30 minutes while cycling the dryer/HVAC on and off to create a known RH swing, and compare the recipe’s logged %RH trace against the reference
Expected: recipe-reported %RH tracks the reference hygrometer within +/-3% RH over the full excursion, and the warn/alarm events fire within one sample interval of the reference crossing the configured thresholds
Safety notes
Section titled “Safety notes”- de-energize and lock out (LOTO) any machine circuit the transmitter’s power or signal wiring passes near before routing cable through the machine’s electrical cabinet
- mount the transmitter so its cable cannot be snagged by the hopper’s material-loading mechanism or any moving auger/blower in the drying system
- use only intrinsically-safe or appropriately rated wiring if the installation is near a combustible dust environment (e.g. fine resin powder); check the site’s hazardous-area classification before wiring
180–650 USD, difficulty 2/5, ~25 min install downtime.