Barrel temperature control on an injection molding machine is the closed loop that holds each heating zone at its set point. A thermocouple measures the zone, the temperature controller compares the reading with the set point, and an output (a solid-state relay or a contactor) switches the zone's heater bands on and off. On modern machines the temperature controller is part of the machine's main controller; older machines may have a separate controller for each zone.
The parts of the control loop
- Thermocouple: measures the zone temperature. See the thermocouple guide.
- Controller input: reads the thermocouple voltage and applies cold junction compensation. It must be set for the thermocouple type.
- Control algorithm: decides how much heat the zone needs.
- Output switch: a solid-state relay (SSR) is the usual choice because it switches often without wearing; contactors are used on some machines.
- Protection: fuses or breakers for each heater circuit, and an earth leakage device.
- Heater bands: the heater band types page covers them.
The diagram beside this page shows the heating parts on the barrel; the controller sits in the electrical cabinet. For the controller hardware and its I/O, see the machine controller guide.
How the controller holds a zone
Simple on/off control switches the heater fully on below the set point and off above it. Barrels have a lot of steel, so heat keeps arriving after the band switches off, and on/off control overshoots and swings.
Most injection molding machine controllers use PID control with a time-proportioning output. The controller switches the heater on for a share of each short time period, shown on screen as the output percentage, and adjusts that share continuously:
- The proportional part sets the output according to how far the zone is from the set point.
- The integral part removes a steady offset that would otherwise remain.
- The derivative part reacts to how fast the temperature is changing and reduces overshoot.
The right settings depend on the zone's mass, heater power and thermocouple position. Many controllers have auto-tuning, which heats the zone, observes how it responds and calculates the settings. Run it as the machine manual describes when a zone behaves badly after a band or thermocouple change.
Functions that protect the machine
Most controllers have some or all of these functions; the machine manual says which.
- Deviation alarms. Each zone has a tolerance band. The controller raises an alarm when a zone runs above or below it, and many machines stop the cycle.
- Cold start protection. Screw rotation and injection are blocked until every zone is within tolerance and, on many controllers, a soak time has passed. Never override it to save time: turning the screw on cold plastic can break the screw tip or the screw.
- Sensor break detection. An open thermocouple raises an alarm instead of leaving the zone heating without limit.
- Standby temperature. During a stop, the zones drop to a lower set temperature to reduce material degradation, then return to their set points before production restarts.
- Fixed percentage mode. Some controllers can run a zone at a fixed output without a thermocouple. It is an emergency measure until a new thermocouple is fitted, not a way to run.
Diagnosing zones that run hot or cold
Read the actual temperature and the output percentage together:
| What you see | Most likely causes |
|---|---|
| Below set point, output high or 100% | Band burnt open or loose, broken cable, tripped fuse, relay not switching, wrong voltage |
| Above set point, output 0% | Shear heat from the screw or back pressure, relay stuck closed, thermocouple swapped with another zone |
| Display at set point, but a handheld probe reads differently | Wrong thermocouple type or extension cable, thermocouple not seated in its hole, aged thermocouple |
| Swinging around the set point | Poor control settings, thermocouple with poor contact, loose band |
| Reading jumps or drops suddenly | Damaged thermocouple cable or connector |
Confirm what the controller shows:
- With a clamp meter on the heater cable, check that the band draws current when the output is on, and none when it is off. Current when the output is off points to a relay stuck closed.
- Compare the reading with a calibrated handheld probe at the thermocouple position.
- With the power isolated and locked off, measure the band's resistance and compare it with its rating (R = V² / P).
If a front zone regularly runs above its set point from shear heat, the cause is in the process or the screw, not the heaters: see back pressure and barrel temperature zones. Zones can get mixed up after maintenance; labeling each band cable and thermocouple with its zone prevents this.
Replacing parts of the control loop
- Thermocouples are a common replacement. Use the same type and mounting; see our thermocouple sensors.
- Solid-state relays must match the voltage and current of the heater circuit and need their heat sink and thermal paste to stay cool.
- Controller inputs and output cards are replaced by board; send the board's part number and photos.
What to send for a quote
- For thermocouples: the type, mounting style, thread, probe length, cable length and plug.
- For heater bands: the size, power, voltage and connection.
- The machine brand and model, and the controller make if you know it.
Request a quote with these details. The heaters guide covers the whole system.
Updated by the IMSPARES technical team.