The thermocouple cable carries the signal from the temperature sensors inside the mold to the control unit. An ordinary cable cannot be used for this job: the thermocouple voltage is very small and the cable material directly affects the measurement.
For this reason Fe/CuNi type J compensating cable is used; the conductor pairs are 0.5 mm² in cross-section and polarized as black (+) and white (−). The cable conforms to the DIN IEC 584 / DIN 43713 E standard.
Wrong polarity or an unsuitable cable causes the measured temperature to deviate from the real one — and hence the zone temperature to be set wrongly.
The black PVC sheath is resistant to oil and chemicals.
Why an ordinary cable cannot be used
A thermocouple produces a very small voltage, proportional to temperature, at the junction of two different metals. For this signal to be read correctly, the cable must also be made of the same pair of metals. If a different metal comes in between, a new thermocouple junction forms and adds its own error to the measurement.
That is why Fe/CuNi (iron/copper-nickel) cable is used for a type J thermocouple; its standards are defined by DIN IEC 584 and DIN 43713.
The polarity is critical
If the plus and minus cores of a thermocouple cable are mixed up, the temperature is read in the opposite direction: as the mold heats up, the control unit thinks it is cooling down and increases the power. The result is rapid overheating. The color codes are therefore tied to the standard and must always be followed during installation.
The interference problem
The thermocouple signal is at millivolt level; when it runs alongside power cables it picks up electrical interference. The symptom is unexplained fluctuation on the temperature display. The solution is to route the signal and power cables separately, or to use the combined cable that brings the two together in a single body.
Before checking
If there is a problem with the temperature reading, the connection at the cable end should be checked first; a loose or oxidized terminal is the most frequently encountered cause.
