--- title: "Proximity sensor of an injection molding machine" url: "https://www.imspares.com/injection-molding-machine-parts/electrical/proximity-sensor/" --- # Proximity sensor of an injection molding machine How inductive proximity sensors detect platen and ejector positions on an injection molding machine, PNP and NPN types, the sensing gap, faults and replacement. A proximity sensor on an injection molding machine is an inductive sensor that switches when a steel target comes within its sensing distance, without touching it. It is used to detect positions such as the mold stroke, the clamping position and ejector home, and for mold protection. Because nothing touches, there is no lever or contact to wear, which is why proximity sensors have replaced many mechanical limit switches. ## How an inductive proximity sensor works A coil behind the sensing face produces an alternating magnetic field. When metal enters the field, eddy currents in the metal take energy from the sensor's oscillator, and the electronics switch the output. An LED on the sensor body usually lights when it switches. The drawing shows a typical installation: - the **proximity sensor**, a threaded cylindrical body; - the **lock nuts**, which hold it in its bracket and set the gap to the target; - the **target**, a steel plate on the moving platen that the sensor detects as the platen passes. The gap must stay within the sensor's rated sensing distance, which depends on its size and type and is printed on the sensor or its data sheet. Mild steel gives the full distance; stainless steel, aluminum and brass are detected only at a shorter distance. ## Types of proximity sensors - **Output, PNP or NPN.** A PNP sensor switches the positive supply to the input; an NPN sensor switches the input to 0 V. The controller's inputs are built for one or the other, so a replacement must have the same output type. Our [inductive proximity sensor](https://www.imspares.com/parts/ec-sf-03/) runs on 10–30 V DC and is listed with PNP or NPN output. - **Function, normally open or normally closed.** An NO sensor switches on when the target is present; an NC sensor switches off. - **Shielded (flush) or unshielded.** Shielded sensors can be mounted flush in metal and have a shorter range; unshielded sensors reach further but need free space around the face. - **Size and connection.** Metric threaded bodies in several diameters, with a fixed cable or a plug connector. ## Signs of a faulty proximity sensor - The LED does not light when the target is in front of the sensor. - The signal flickers or drops out as the platen passes, causing position alarms or a stopped cycle. - The sensor stays on with no target present. - The cycle stops at the same step every time, waiting for a signal. ## Causes - The gap has grown because the lock nuts or the bracket have worked loose. - The sensing face was struck by the target or the platen and damaged. - Metal chips stuck to the face keep the sensor switched on. - The cable is crushed or cut where it runs near moving parts, or the connector is loose or oily. - A replacement of the wrong output type (PNP for NPN, or the reverse) or the wrong function (NO for NC). - Heat, for example from a hot mold close to the sensor. ## Checking, adjusting and replacing a proximity sensor Wiring work is for qualified electricians. Replace a sensor only with the machine switched off at the main isolator and locked out. 1. With the machine in manual mode and nobody in the mold area, move the platen or the ejector slowly and watch the sensor LED and the controller's input page. 2. Check the gap to the target and the condition of the face, and clean off any chips. 3. Isolate and lock out the machine, note the wire colors, and remove the sensor with its lock nuts. 4. Fit a sensor with the same output type, function, sensing distance, thread size and connection. 5. Set the gap within the sensing distance given in its data sheet, tighten the lock nuts, and make sure the target cannot strike the face. 6. Test in manual mode at low speed, then in the cycle. ## What to send for a replacement quote - A photo of the sensor label, which usually shows the type, output, function and sensing distance. - The thread size and body length, and whether it has a cable or a connector. - What the sensor detects, and the machine brand, model and serial number. The [limit switch guide](https://www.imspares.com/injection-molding-machine-parts/electrical/limit-switch/) covers the mechanical switches used alongside proximity sensors, the [linear transducer guide](https://www.imspares.com/injection-molding-machine-parts/electrical/linear-transducer/) covers continuous position measurement, and the [electrical system guide](https://www.imspares.com/injection-molding-machine-parts/electrical/) gives the overview. See also the [safety switches and sensors](https://www.imspares.com/categories/electrical/safety/) we supply. ## Questions ### What does a proximity sensor do on an injection molding machine? It detects a steel target without touching it, for positions such as the mold stroke, the clamping position and ejector home, and for mold protection. ### What is the difference between PNP and NPN proximity sensors? A PNP sensor switches the positive supply to the controller input; an NPN sensor switches the input to 0 V. They are not interchangeable: the replacement must match what the controller input expects. ### Why does my proximity sensor not detect the target? Usually the gap has grown because the bracket or the lock nuts have moved, the sensing face is damaged, or the cable is broken. Check the sensor's LED and the gap against its rated sensing distance. ### Can a proximity sensor detect aluminum or stainless steel? An inductive sensor detects them, but at a shorter distance than mild steel. Use the correction factors in the sensor's data sheet, or a steel target. ## Replacement parts - [EC-SF-03 Proximity Sensor / Position Sensor](https://www.imspares.com/parts/ec-sf-03/): Inductive proximity sensor for injection molding machine position detection.