What Are the Differences Between Single-Pair and Multi-Strand Thermocouple Wires for Hot Runners?

Hot runner thermocouple internal alloy wires are classified into single solid core construction and multi-strand twisted wire structure, which have obvious differences in bending resistance, signal stability, thermal shock resistance and relevant mold circumstances. Many consumers just pay attention to the classification of K/J type and the diameter of thermocouple sheath, but ignore the matching of wire core structure, which results in the frequent wire breakage faults after short-term mold use.

The single solid core thermocouple wire has the advantages of stable thermoelectric performance and low intrinsic temperature drift, because it uses one full piece of alloy metal without any separate strands. The integrated metal core has a consistent coefficient of thermal expansion and no internal strand friction during heat cycles, and the factory calibration tolerance can stably achieve Class 1 precision for a long period. Single core wire is cheap to make and is often used in straight fixed manifold probes that are rarely disconnected and bent. The fatal flaw is inadequate bending fatigue resistance . The wire will develop metal fatigue cracks if bent over 5 times at the same place , and eventually break the circuit . Single core probes are therefore not allowed for nozzle assemblies that require frequent disassembly and arrangements with bent wiring.

Multi-strand twisted thermocouple wires twist dozens of ultra-fine alloy filaments together to create a composite conductor. When bending force is applied to the wire, the stress is distributed uniformly among several thin strands rather than concentrated on a single solid core, considerably enhancing the bending cycle resistance. Qualified multi-strand probes can resist more than 500 repeated bends without breakage, and are well suited to valve gate nozzles, micro molds, and stacked molds with complex bent wiring paths. Strong anti-vibration performance is another advantage, injection machine vibration and mold opening/closing impact will not readily fracture multi-strand cores, suited for high-speed continuous production equipment with severe mechanical vibration.

The disadvantages of multi-strand wires are a slightly increased chance of long term drift. Long-term high-temperature operation causes moisture and corrosive plastic volatile gas to be trapped in the tiny crevices between twisted strands. This eventually oxidizes the surfaces of the filaments and increases the overall thermoelectric potential deviation. Multi-strand goods have also higher production costs than single-core probes, which increases spare parts procurement prices of manufacturers with large volumes of molds.

Scene-based selection rules are intelligible and feasible . Fixed straight manifold thermocouples without frequent disassembly and bending choose only single solid core k-type wires to control cost and keep ultra low drift. All nozzle integrated probes, bent customized probes, micro mold thin-sheath sensors and stacked multi-layer mold thermocouples must use multi-strand twisted alloy wires to avoid bending fracture failure. Multi-strand wires with passivation coating on each filament surface decelerate inter-strand oxidation drift in corrosive plastics manufacturing molds.

Difference between two wire structures in installation process. The minimum bending radius for single-core probes is 5 times the sheath diameter, and the same place should not be bent repeatedly during mold assembly. The bending radius standard of the multi-strand probe is 3 times the sheath diameter, however it is still banned to twist the cable too much during installation, violent torsion will separate the twisted strands and generate internal disconnection concealed flaws.

In the daily spare parts inventory management, single-core and multi-strand thermocouples should be classified separately to prevent mis-installation. Matching the wire core structure to the frequency of mold disassembly and the complexity of wiring bending can reduce the open-circuit failure rate of the thermocouple caused by bending fatigue by more than 70%.