TL;DR
A thermocouple that reads persistently above or below actual melt temperature will cause out-of-specification parts. Verify with a reference thermocouple before replacing the suspect sensor.
What you might see
- zone temperature displays a value inconsistent with part quality
- one zone runs visually hotter or cooler than adjacent zones at same setpoint
- gate drool or freeze-off on one drop at parameters that worked previously
- thermocouple reading shifts gradually over days or weeks
Likely causes
Thermocouple junction degraded from thermal cycling and partial oxidation
Extension wire polarity reversed at the mold junction box introducing a consistent offset
Thermocouple contamination from plastic ingress through a cracked thermowell
Controller input card calibration drift
Required tools
- Calibrated reference thermocouple probe
- Multimeter for thermocouple continuity check
- Replacement thermocouple (Type J or K, mold-specific configuration)
- LOTO kit
Safety first
- Lock out the TempMaster and the injection press before opening the mold junction box.
- The mold surface remains hot long after the TempMaster is shut off. Wear heat-resistant gloves.
Procedure
- 1
Identify the suspect zone by comparing its setpoint and actual reading to adjacent zones running the same material.
- 2
Lock out the press and TempMaster before accessing the mold junction box.
Warning: Hot runner circuits carry line voltage. Lock out both the TempMaster and the injection press before opening the junction box. - 3
Insert a calibrated reference thermocouple probe near the suspect thermocouple location on the mold surface. Compare the two readings.[1]
- 4
A significant offset between the reference and the installed thermocouple confirms sensor drift or incorrect polarity.
- 5
Check the thermocouple extension wire connections in the junction box for correct polarity. Type J and Type K extension wires are colored differently. Reversed polarity causes a predictable offset.[1]
- 6
If polarity is correct and drift is confirmed, replace the thermocouple.
- 7
After replacement, run 20 production cycles and verify the gate area part quality matches specification.
Sources
Mold-Masters (Milacron) Mold-Masters TempMaster Hot Runner Controller / System general technical documentation, Mold-Masters (Milacron)
Mold-Masters TempMaster controller documentation, thermocouple inspection and calibration verification (general)
More guides for Mold-Masters (Milacron) Mold-Masters TempMaster
How to fix gate drool or stringing on a Mold-Masters TempMaster hot runner system
Gate drool and stringing are nearly always caused by a gate temperature too high or insufficient decompression at end of shot. Lower the gate zone temperature by 5 C increments before adjusting decompression.
How to diagnose a hot runner zone not heating on a Mold-Masters TempMaster controller
A zone that fails to heat is usually a failed heater element, open thermocouple, or blown fuse on the TempMaster output card. Check the fuse first before testing heater resistance.
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