The internal insulation layer of hot runner thermocouples isolates the positive and negative thermoelectric wires to prevent short-circuit failure, and the outer sheath insulation resists high temperature, melt corrosion, oil pollution and mechanical friction in the mold working environment. Different insulation materials have distinct temperature resistance, chemical stability and mechanical toughness, which become the core factor determining the service cycle of thermocouples in different hot runner application environments. The mainstream insulation materials used in the industry include magnesium oxide mineral insulation, PTFE high-temperature plastic insulation, glass fiber braided insulation and mica winding insulation, each with exclusive applicable hot runner scenarios.
Magnesium oxide (MI mineral insulation) is the standard filling material for mainstream high-performance hot runner thermocouples. It is filled and compacted inside the stainless steel sheath, wrapping the two core thermoelectric wires separately. Magnesium oxide has excellent high-temperature insulation performance, stable insulation resistance above 1000°C, and good thermal conductivity, which can quickly transfer the temperature of the sheath to the internal alloy wires to ensure real-time sensing. At the same time, the solid filling structure can fix the internal wires tightly, avoiding internal wire displacement and fracture caused by repeated bending, mold vibration and thermal expansion and contraction. The limitation of magnesium oxide insulation is that it cannot be exposed to moisture for a long time; damp magnesium oxide will lead to a sharp decline in insulation resistance and trigger intermittent short circuits. Therefore, MI thermocouples must be equipped with sealed connectors, and spare parts need to be stored in a dry environment. All spring bayonet thermocouples used in high-end automotive and medical hot runners adopt full magnesium oxide mineral insulation structure.
PTFE (polytetrafluoroethylene) insulation is widely used in clean-grade and low-temperature hot runner thermocouples. PTFE does not produce toxic volatile substances under high temperature, has strong corrosion resistance to acid-base gas volatiles precipitated by plastic melting, and will not shed debris to pollute medical and food-grade melts. Its continuous working temperature range is -20°C to 260°C, so it is mostly used for the outer wire insulation of thermocouple outgoing lines, and matched with micro nozzles of medical disposable product molds. The disadvantage of PTFE is poor high-temperature resistance; when the local temperature exceeds 300°C for a long time, the material will soften and age, resulting in insulation layer melting and wire short circuit. Therefore, PTFE cannot be used as the internal filling insulation of the probe near the hot runner heating zone, and can only be applied to the cold end wiring section outside the mold plate.
Glass fiber braided insulation is the most cost-effective general insulation material for entry-level hot runner thermocouples. It has low raw material cost, certain bending flexibility, and can withstand long-term working temperature below 550°C, which is suitable for J-type thermocouples of ordinary packaging hot runner molds. The defects of glass fiber are obvious: the braided structure has gaps, which is easy to absorb workshop oil, dust and plastic carbon deposits, resulting in decreased insulation performance after long-term use; the fiber is brittle, and it is easy to break and fall off when repeatedly bent, forming conductive debris inside the cable to induce short circuit. In addition, glass fiber will release tiny fiber dust when worn, which cannot meet the hygiene requirements of medical production workshops, so it is completely excluded from medical hot runner supporting sensors.
Mica winding insulation is mostly used for special ultra-high temperature hot runner thermocouples matched with PEEK, LCP and other high-temperature engineering plastics. Mica can withstand instantaneous temperature above 1300°C, and has stable insulation performance under long-term high-temperature radiation. It is often wound on the surface of thick thermocouple alloy wires inside high-temperature nozzles. However, mica has poor flexibility and is easy to crack when bent, so it cannot be used for micro thin sheath probes that need frequent bending and disassembly, and is only limited to fixed straight probe structures of large manifold plates.
In the process of thermocouple model selection, technicians must first judge the highest continuous temperature of the hot runner heating zone and the production environmental standards to select matching insulation materials. Many premature thermocouple failures in the workshop are caused by mismatched insulation materials: for example, using glass fiber insulated probes for medical molds leads to product pollution; using PTFE insulated wires to extend into the high-temperature nozzle zone leads to insulation melting and short circuit. Reasonable matching of insulation materials can extend the average service life of hot runner thermocouples by more than 50%, and effectively reduce the frequency of sudden shutdown maintenance caused by insulation damage.
