Thermocouple of an injection molding machine

A thermocouple on an injection molding machine is the temperature sensor of one heating zone. It is a probe set into the barrel wall or the nozzle body, made of two wires of different metals joined at the tip. The pair of wires produces a small voltage that depends on the temperature at the tip, and the temperature controller reads that voltage to switch the zone's heater bands. Every barrel zone and the nozzle has its own thermocouple, so a single wrong reading puts one zone at the wrong temperature.

Where the thermocouples sit

On the barrel, each thermocouple sits in a hole in the barrel wall, usually in the gap between two bands, and sometimes through a hole in a band. The drawing highlights them between the ceramic heater bands. The nozzle thermocouple sits in the nozzle body under or beside the nozzle heater. Other thermocouples on the machine measure the hydraulic oil temperature and, on some machines, the feed throat. The heaters guide shows how they fit into the whole heating system.

How a thermocouple works

Two wires of different alloys are welded together at the measuring tip, the hot junction. Where the wires connect to the controller, the cold junction, the circuit produces a voltage that depends on the temperature difference between the two ends. The controller measures the temperature at its own terminals and adds it back (cold junction compensation) to work out the temperature at the tip.

Three practical consequences follow:

• The type must match. Each type (K, J and others) gives a different voltage at the same temperature. The controller input is set for one type.
• The cable matters. Between the probe and the controller, the wires must be thermocouple or matching extension cable of the same type. A joint made with ordinary copper wire, or with cable of another type, adds an error wherever that joint is warm.
• Polarity matters. If the two wires are swapped at a terminal, the reading moves the wrong way or shows a large error.

The thermocouple measures the temperature where its tip is. A tip deep in the barrel wall reads close to the bore; a tip near the surface and close to a band reads closer to the heater. That is why a thermocouple must go back into the same hole, fully seated.

Thermocouple types and mounting styles

K-type and J-type are the common types on injection molding machines, in spring-loaded (bayonet), threaded and needle versions, and an oil-temperature version for the hydraulic tank. Our article on thermocouple types for injection molding compares them and shows which suits each location. The thermocouple sensors we supply come in K-type and J-type, with spring-loaded, threaded M6 and M8, needle and oil-temperature versions and a range of cable lengths.

Faults and what they cause

• Open circuit (broken wire or junction). Most controllers detect it, raise a sensor break alarm and stop heating that zone; check what yours does in the machine manual.
• Probe out of its hole, or not pushed home. The reading is too low, the controller keeps heating, and the zone overheats without an alarm. This is one of the most damaging faults because nothing looks wrong on the screen.
• Damaged cable shorting between the wires. The short becomes a new junction, so the controller reads the temperature at the damage point, usually lower than the barrel.
• Wrong type or wrong extension cable. A steady offset: the zone runs consistently hotter or colder than displayed.
• Thermocouples swapped between zones. Zone 2 heats when zone 3 reads low. Symptoms are one zone overshooting while its neighbor never reaches temperature.
• Aging. Long use at temperature changes the wire slightly, and the reading drifts.

Signs on the machine are a zone that will not settle, a reading that jumps when the cable is moved, burnt or degraded material from one zone, or a zone whose displayed temperature does not match a handheld probe.

How to test a thermocouple

1. Compare the controller's reading with a calibrated handheld contact probe or needle probe on the barrel at the same spot, after the zone has been stable for a while.
2. Check that the probe is fully seated in its hole and that the cable is intact along its length, especially where it passes near the bands and moving parts.
3. With the cable disconnected from the controller, check for continuity with a multimeter. An open circuit means the thermocouple is broken.
4. Swap the suspect thermocouple with a known good one at the controller input. If the fault moves with the sensor, the sensor is at fault; if not, check the controller input.

Replacing a thermocouple

1. Note the zone and the controller input, and make sure the new thermocouple is the same type.
2. Remove the old probe. Clean any plastic or scale out of the hole so the tip touches the bottom.
3. Fit the new probe to the same depth and lock it (bayonet cap or thread). Do not overtighten a threaded probe into a hot barrel.
4. Route the cable away from the heater bands and their terminals, and fix it so it cannot be pulled.
5. Connect it with the correct polarity, then heat up and check the reading against a handheld probe.

What to send for a replacement quote

1. The thermocouple type (K or J), from the old sensor or the controller setting.
2. The mounting style: spring-loaded bayonet, threaded (and the thread size), needle or oil probe.
3. The probe length and diameter, and the cable length.
4. The connection at the controller end: plug type or bare wires.
5. The machine brand and model, and a photo of the old sensor.

Request a quote with these details. To understand how the controller uses the reading, see temperature control.