injection_moulding — count_cycles (photo-eye)
Physics
Section titled “Physics”An injection moulding machine’s platen travels through a fixed, repeatable open stroke once per shot to allow part ejection, and the ejected part (or sprue) drops through the machine’s ejection chute once per shot as well. A through-beam photoelectric sensor positioned across the mould-open travel path, or across the ejection chute beneath the mould, is broken once per shot as the platen (or part) passes through the beam, giving a clean digital pulse per cycle independent of clamp tonnage or part weight. Shot times on production injection moulding machines range from a few seconds (small thin-wall parts) up to tens of seconds for larger parts, so the fastest realistic shot rate is on the order of 0.3-0.5 Hz; sampling the sensor’s digital output at 200 Hz gives >400x margin over Nyquist for that rate and fully resolves the beam-break pulse width (typically tens of milliseconds for a fast-moving platen edge or falling part).
Sensor
Section titled “Sensor”- Spec: through-beam photoelectric sensor pair, digital, NPN/PNP, NO
- Signal: counter ({count}), range n/a
Example parts
Section titled “Example parts”- Banner Engineering Q45VP6LV through-beam photoelectric sensor pair (visible red, PNP, 10 m range)
- SICK WT4-3P Thru-beam photoelectric sensor pair
- Keyence PZ-G41CP through-beam photoelectric sensor
Mounting
Section titled “Mounting”Mount the emitter and receiver on a rigid bracket on either side of the mould-open travel path (clear of all moving platens and tie bars at every point of the stroke) or across the part-ejection chute directly beneath the mould, aligned so the beam is broken once per shot by the platen edge or the falling part/sprue, and unbroken at all other times during the cycle.
Wiring
Section titled “Wiring”3-wire DC (24 V, 0 V, signal) from the receiver to a digital input / counter channel on the edge box’s I/O module; run the cable along the machine’s existing guard wireway, away from the barrel heater bands and their high-current switching to avoid noise pickup.
Validation
Section titled “Validation”run 30 manually-counted shots at normal production cycle time and compare the detector’s counter delta over the same window
Expected: counted shots exactly equal to the manual count (a correctly aligned through-beam sensor gives an exact 1:1 count since the beam-break is the ejection event itself)
Safety notes
Section titled “Safety notes”- de-energize and lock out the machine (LOTO) before working inside the mould-open area or ejection chute to install the bracket and sensors
- mount the bracket and sensor bodies clear of the platen’s full stroke and the mould’s clamping tonnage path at every point of travel, including the fully open position
- verify the sensor’s voltage rating matches the 24 V DC supply before wiring, and route cabling away from the barrel heater bands’ line-voltage wiring
60–220 USD, difficulty 2/5, ~35 min install downtime.