Selection Criteria of Thermocouple Extension Wire Outer Insulation Materials

The outer insulation layer of thermocouple extension wires is subject to long-term high temperature radiation, chemical corrosion, bending friction near hot runner molds, etc., the service life of the wire and the stability of the signal depend on different insulation materials. The choice of poor insulation materials will cause sleeve embrittlement, melting, volatile precipitation and short circuit failures. This study evaluates four mainstream insulation materials and appropriate hot runner temperature and environment conditions to provide unambiguous selection guidelines for procurement and wire design.

1. Teflon PTFE insulated

Temperature resistance range: Continuous operating temperature -200℃～260℃; Instantaneous peak temperature 300℃.

Advantages: Ultra low friction coefficient , anti oil , anti acid and alkali corrosion , non absorbent , high insulating performance , flexible and simple to bend , no volatile organic matter precipitation .

Disadvantages: expensive, hardens significantly on long-term low temperature, easily scratched by sharp metal edges.

Suitable for: General medium temperature hot runner wiring within 260℃, electronic clean room molds, humid coastal workshop, molds with frequent wire bending layout. Not suited for wire lines directly connected to hot runner manifold exceeding 280°C.

2. High Temperature Silicone Fiberglass Composite Insulation

Temperature resistance range: Continual 350°C, immediate 400°C.

Advantages: Ultra high temperature resistance, low cost, great flexibility, good wear resistance after braided reinforcement, resistance to short term high temperature radiation near nozzles.

Disadvantages: Easily absorbing moisture, long-term usage in high humidity environment would decrease the insulating effectiveness; Silicone base oil will volatilize and carbonize over 360℃ to generate conductive residues.

Application scenario: Extension wires in hot runner nozzle, manifold plates within 10cm, high-temperature LCP/PPS hot runner molds, general industrial molding workshops with low clean room requirements. Do not use in medical clear product cleanroom molds, to avoid oil vapor contamination.

3. Polyimide (PI) insulation with good performance

Temperature resistance range: Continuous-200℃~380℃, steady long-term high temperature without volatilization.

Advantages: Low outgassing, low halogen, smooth surface not easy to accumulate carbon dust, ultra-stable insulating performance under high temperature and humidity, no organic oil precipitation, fully comply with medical cleanroom requirements.

Cons: Highest material cost, less flexible than PTFE, prone to cracking under harsh repeated bending.

Suitable for: hot runner extension wires of medical implants & transparent products, precision molds of high temperature engineering plastics with tight VOC control, high-end automotive new energy manufacturing lines.

4. PVC common plastic insulation

Temperature resistant range: <100℃only continuous, >120℃softening deformation.

Advantages: Very low cost, silky feel.

Disadvantages: Severe high temperature volatile precipitation, poor oil and corrosion resistance, embrittlement after short-term heat radiation, insulation resistance declines significantly after absorbing moisture.

Application scenario: Only for distance wiring far away from the mold (more than 3 meters away from the hot runner high temperature components). Strictly banned for near-end wiring harness of the mold.

Complete matching selection rules.
• Near mold wiring (within 1m of hot runner manifold/nozzle) Temperature lower than 260℃ select PTFE; 260-350℃ chose silicone fiberglass; medical cleanroom all temperature ranges select polyimide PI.
• Far-distance wiring cabinet section (farther than 3m from mold) Low cost PVC can be used for non-precision general molds; high precision molds universally use PTFE to eliminate signal interference caused by PVC aging.
• Production lines for corrosive PVC and flame retardant plastic: Use PTFE or PI insulated cables with good chemical resistance, and do not use silicone fiberglass cables that can easily be affected by corrosive vapor.
• Multi-cavity stack mold wiring with frequent bending: Use flexible PTFE insulation to avoid insulation cracking after multiple bending cycles.

Insulation material grade shall be explicitly stated on the drawing during extension wire procurement, so that suppliers do not substitute with low-cost low-temperature PVC and ordinary silicone materials, which would cause early wire aging and thermocouple signal problems in production