Thermocouple extension wires laid inside mold high-temperature zones rely on fiberglass or PTFE insulation layers to isolate positive and negative alloy wires and metal mold plates. Long-term continuous exposure to radiant heat above 300℃ will trigger irreversible aging of insulation materials, forming hidden short-circuit and signal drift faults that seriously affect hot runner temperature control stability. This article sorts aging mechanisms, failure manifestations and complete protection schemes for extension wire insulation layers.
Two mainstream insulation materials have different high-temperature aging limits. Fiberglass braided insulation is the most widely used low-cost material, with a long-term stable working temperature limit of 420℃. When the ambient temperature exceeds 450℃ for a long time, the glass fiber will gradually become brittle, pulverize and peel off layer by layer, losing insulation isolation performance. PTFE insulated wires have better low-outgassing and anti-corrosion performance, continuous temperature resistance up to 380℃; if used near ultra-high temperature PEEK hot runners above 400℃, the PTFE layer will soften, deform and generate organic volatile substances, causing pollution in cleanroom molds and reducing insulation resistance sharply.
Progressive aging process of insulation layers. In the early aging stage, the insulation material slowly loses toughness under long-term high-temperature baking, surface micro-cracks appear, and insulation resistance drops slightly without obvious production abnormalities. Entering the medium aging stage, micro-cracks expand, mold water vapor and plastic corrosive volatiles penetrate into the wire core, alloy wires oxidize, and temperature readings begin to drift slowly with daily production time. In the late aging stage, the insulation layer pulverizes and falls off completely, positive and negative alloy wires contact each other or touch the mold steel plate, triggering continuous short-circuit faults, uncontrollable heater full-power output and mass material burning defects.
Typical on-site phenomena of insulation aging failure. The temperature value of the corresponding zone jumps randomly when the mold heats up for more than one hour; after shutting down and cooling the mold overnight, the temperature reading returns to normal temporarily after restarting heating, and the disorder reappears after constant temperature for a long time; the megohmmeter test shows that the insulation resistance of the extension wire drops below 50MΩ after heating, while the resistance returns to normal at room temperature. These periodic abnormal fluctuations are typical signs of high-temperature aging of wire insulation, which are easily misjudged as thermocouple probe damage by maintenance personnel.
Targeted anti-aging protection configuration for extension wires. For manifolds and nozzles working above 360℃, wrap extension wires with double-layer thickened fiberglass thermal insulation sleeves to isolate direct radiant heat from hot runner metal surfaces, reducing the actual ambient temperature borne by the insulation layer by more than 80℃. Avoid binding wires tightly on the outer wall of the manifold; reserve a certain gap between the wire harness and the hot runner to form heat dissipation air layer. For PEEK, PPS ultra-high temperature mold wiring, prioritize thickened high-temperature fiberglass insulated extension wires instead of PTFE wires to prevent softening aging. For corrosive plastic production lines, select insulation materials added with anti-volatile filler to slow down chemical erosion aging.
Regular inspection and replacement cycle standards. Conduct insulation resistance testing of all extension wires every quarter; if the insulation value drops below 80MΩ under heating state, replace the whole bundle of wires in advance to avoid late-stage short-circuit faults. Molds running 24-hour continuous high-temperature production replace all internal extension wires every 10 months; single-shift intermittent production molds implement annual wire batch replacement. When customizing new extension wires, inform suppliers of the maximum ambient temperature near the wiring position to select matched insulation materials and double-layer protection structures, fundamentally slowing insulation aging speed and extending the service life of thermocouple wiring systems.
