Two neighbouring heating zones on a large press platen, Zone 3 and Zone 4, are commanded to the exact same setpoint. Yet a thermal camera and the controller's own display show that Zone 3 is persistently 8°C hotter than Zone 4. The controller is following the programmed instruction, but the two zones are operating in very different thermal conditions. This persistent offset is a diagnostic puzzle with three primary suspects: an uneven thermal load, an inaccurate sensor, or a weakening heater element.
Understanding the Thermal Imbalance
A persistent temperature difference adjacent zones identical setpoint platen condition indicates that one zone is either losing more heat, reporting incorrect temperature data, or producing less heat than expected.
The two zones are like two siblings, told to do the same chore, but one is secretly carrying a heavier load, has a broken watch, or is simply weaker.
The troubleshooting process therefore becomes a structured elimination of these three possibilities.
Check for Uneven Thermal Load
The first suspect is a difference in thermal loading between the two zones. One section of the mould may be thicker, contain more metal mass, or be positioned near a cooling channel with higher flow.
A thermal survey should be performed while the platen is idle and under no active process load. If the temperature difference disappears during idle conditions, the imbalance is likely caused by the tooling or cooling arrangement rather than the platen hardware itself.
Additional checks should include:
Variations in mould thickness
Uneven contact pressure
Differences in airflow or cooling water distribution
Nearby structural heat sinks or mounting hardware
Verify Thermocouple Calibration
The second suspect is inaccurate temperature feedback. A poorly calibrated or drifting thermocouple can cause the controller to regulate incorrectly.
A calibrated surface temperature probe should be placed directly on each zone surface and insulated from ambient air to provide a reliable comparison reference. If accessible, the thermocouples can also be physically swapped between zones.
If the temperature offset follows the sensor, the thermocouple is the source of the error. Many controllers allow a simple offset correction to compensate for minor calibration drift.
Diagnostic Tip
A surface probe provides an independent temperature reference and helps distinguish between a real heating imbalance and a false measurement artifact.
Inspect Heater Output
The third suspect is a weakening heater element in the cooler zone. Aging cartridge heaters can develop increased internal resistance, reducing heat output even when receiving the same power command.
A clamp meter provides a fast, non-invasive method for comparing current draw between adjacent zones. With both zones commanded to 100% output:
Equal current suggests healthy heaters
Lower current in one zone indicates reduced heater output
Significant imbalance may indicate partial heater failure or wiring issues
If the cooler zone consistently draws less current, replacement of the heater cartridge is typically required.
Additional Factors That Influence Zone Balance
Several secondary factors can also contribute to persistent temperature differences:
Poor thermal contact between heater and platen bore
Oxidized electrical connections creating voltage drop
Variations in platen thermal mass
Improper PID tuning between adjacent zones
Heat losses caused by external airflow or insulation gaps
These factors should be evaluated if the primary diagnostic checks do not identify the root cause.
Conclusion
A persistent temperature difference adjacent zones identical setpoint platen condition is a solvable diagnostic problem. The root cause is typically uncovered by systematically evaluating the thermal load, verifying sensor calibration, and comparing heater performance. A perfectly uniform platen depends on balanced heat flow, accurate temperature feedback, and healthy heating elements operating together as a coordinated thermal system.
