What Are the Most Common Thermocouple Failures in Hot Runners & Their Root Causes?

Thermocouples are the most vulnerable components in hot runner systems and account for over 60% of temperature-related downtime. Understanding the 3 most typical failures – open circuit, short circuit and drift – are crucial to reduce unexpected downtime and ensure consistent production.

The #1 failure is thermocouple open circuit (TC break) which shows up as "OPEN", "TC ERR", or "---" on the controller. This happens when the thermocouple wire breaks inside the cable or at the juncture and breaks the temperature signal. Root causes: Mechanical stress (wire pinching during mould clamping, excessive bending radius <50 mm); thermal fatigue (repeated expansion/contraction from 20–400°C cycling); oxidation (high‑temperature corrosion of exposed connections). In high vibration situations, K-type thermocouples with thin 0.5 mm wires are particularly susceptible to damage.

The #2 failure is thermocouple short circuit resulting in "SHORT" warnings or instant breaker trips. It happens when the positive and negative wires meet and make a zero-resistance path. Root causes include: - insulation damage (e.g. melt leakage, heater overheating, or physical abrasion exposing bare wires); moisture ingress (e.g. condensation in humid settings creating electrical leakage); and inappropriate installation (e.g. loose wires contacting manifold steel). Short circuits are dangerous and can destroy the controller or manifold heater if quickly not resolved.

#3 failure is thermocouple drift (inaccuracy), where readings steadily move from actual temperature by ±2–5°C over time. Often neglected but leads to quality problems (burning, short shots, warpage) owing to wrong output of heaters. Root causes are: junction contamination (carbon buildup or melt residue on the measurement tip); thermoelement ageing (long-term exposure to >350°C causes material composition changes); and inadequate thermal contact (loose fit in the manifold bore, gaps between junction and steel). Drift is widespread in high temperature applications (LCP, PPS) and in moulds that are subject to frequent temperature cycling.

Other, less usual, faults include reverse polarity (the wires have been changed, so the readings go down when you heat) and intermittent connection (loose connectors or corroded pins, so the signal is lost from time to time).

To summarise, the causes of thermocouple failure are mechanical stress, thermal fatigue, insulation degradation and improper installation. Preventive methods are the use of mineral insulated cables, correct bending radii, good metal to metal contact and regular calibration. By reducing these root causes, downtime is minimised and dependable thermal control is provided.