Sealed armored thermocouples and low-cost unsealed bare wire thermocouples form two mainstream product categories in the hot runner industry, with a huge gap in service life, applicable scenarios and long-term use cost, which directly affects the procurement decision of injection molding factories. Unsealed thermocouples only wrap a thin layer of insulation outside the J/K alloy wires without metal sheath and internal magnesium oxide filling, mainly used for temporary mold trial runs and low-temperature short-cycle packaging molds below 260℃. Under continuous 8-hour single-shift production, their average service life is only 30–60 days. High-temperature radiation from hot runner heaters quickly ages the insulation layer, causing wire short circuits; plastic volatile gas directly contacts the exposed sensing junction, accelerating oxidation and signal drift within one month. Once the workshop temperature exceeds 300℃, unsealed probes will fail completely in less than two weeks, triggering frequent TC OPEN and TC SHORT alarms.
Fully sealed armored thermocouples adopt seamless stainless steel or Inconel alloy outer sheath, filled with high-purity magnesium oxide insulation powder inside to isolate internal thermo wires from external high temperature, corrosive gas and mechanical friction. Under standard 24-hour continuous production conditions, ordinary stainless steel sealed probes can maintain stable performance for 8–12 months, while high-end Inconel sealed thermocouples used for high-temperature engineering plastic molding can reach a service life of 18–24 months, 6–10 times longer than unsealed products. The internal magnesium oxide filling completely blocks corrosive vapor from PVC, flame-retardant PA and recycled plastics, avoiding junction oxidation and signal drift. The metal sheath also resists scratch and abrasion from glass fiber melt and mold metal burrs, greatly reducing mechanical damage faults.
The service life gap is further amplified in harsh production environments. In high-humidity coastal workshops, unsealed bare wires absorb moisture quickly, leading to electrical leakage and intermittent alarms within 20 days; sealed armored sheaths form a complete waterproof barrier, effectively isolating water vapor and condensate. For medical cleanroom molds requiring strict sanitation standards, unsealed thermocouples have exposed wire gaps that accumulate plastic residue and bacteria, and must be replaced every two weeks, while sealed seamless probes only need quarterly surface cleaning without frequent replacement.
Many factories choose unsealed thermocouples to save upfront procurement costs, but ignore the cumulative hidden losses brought by short service life. Unsealed probes need frequent replacement, requiring repeated mold cooling, disassembly and wiring, consuming a large amount of labor hours and causing unplanned production downtime. Frequent temperature drift also produces batches of defective plastic parts, and the scrap loss within half a year is far higher than the price difference between sealed and unsealed sensors. In addition, unstable temperature feedback accelerates heater burnout and manifold thermal fatigue damage, increasing mold maintenance expenditure.
Application classification selection rules clarify the matching logic: unsealed thermocouples are only suitable for short-term mold testing, low-temperature intermittent small-batch production of PP/PE packaging products; all mass production molds, high-temperature engineering plastic molds, medical and automotive precision molds must adopt fully sealed armored thermocouples. Although the one-time purchase cost increases by 50%–100%, the extended service life cuts replacement frequency and downtime losses, reducing the comprehensive annual use cost by more than 60%. Regular cleaning of sealed sheath surfaces and timely replacement of aged extension cables can further extend the service life of sealed thermocouples by nearly 30%.
