Hot runner thermocouples are divided into armored and non-armored types according to external protective structure, with obvious gaps in mechanical performance, temperature measurement stability and service life. Mold designers often struggle to select appropriate structure type in early mold development; clear comparison of structural characteristics, applicable scenarios, advantages and defects of two types can simplify model selection work for different hot runner molds.
Armored thermocouple takes seamless stainless steel or alloy thin tube as outer sheath, filling compact magnesium oxide insulation powder inside to wrap positive and negative alloy wires integrally. The whole armored tube can be bent at small radian without breaking internal wires, possessing strong compression, scratch and anti-extrusion capacity. The metal sheath completely isolates internal alloy wires from external mold environment, effectively blocking erosion of plastic volatile corrosive gas, oil mist and carbon deposits. Armored structure supports three core hot runner matching products: flat-head manifold type, deep probe nozzle type and miniature ultra-thin bending type, covering almost all high-precision and long-cycle production molds. Its outstanding advantages include long service life, stable temperature measurement under complex mold layout, strong mechanical fatigue resistance and wide temperature adaptation range. The main shortcoming is higher unit cost compared with non-armored sensors, and customized bent armored thermocouples need longer delivery cycle for small batch orders. Armored thermocouples are mandatory matching choice for automotive plastic molds, medical transparent product molds, electronic micro multi-cavity molds and hot runners processing high-temperature engineering plastics.
Non-armored thermocouple, also known as bare wire thermocouple, only wraps alloy wires with mica tape and fiberglass protective sleeve without metal armored tube outside. The biggest merit lies in low manufacturing cost and fast delivery speed, suitable for low-demand temporary molds and small batch trial production tools. The exposed wire structure brings ultra-fast heat conduction speed, so non-armored exposed bead sensors are occasionally used on packaging thin-wall hot runners requiring ultra-quick temperature response. However, its inherent defects restrict large-scale application in mass production workshops. Without metal sheath protection, internal alloy wires are extremely vulnerable to scratch, extrusion and high-temperature corrosion. After dozens of mold disassembly operations, fiberglass sleeve will embrittle and fall off, exposing alloy wires to air, leading to oxidation breakage and open-circuit faults. Internal insulation mica tape cannot resist permeation of plastic oil vapor, and conductive carbon layer forms easily under long-term heating, triggering short-circuit alarm frequently. Non-armored wires cannot be bent at small angles; sharp bending will directly crack mica insulation layer and cause signal disorder. In addition, non-armored thermocouples lack electromagnetic shielding performance, generating severe temperature jump interference in electronic injection workshops with dense automation equipment. They are only recommended for simple low-temperature open hot runner molds with short production cycle and low precision requirement, such as disposable toy plastic parts and ordinary bottle cap molds.
In mixed matching scenarios, many mold factories adopt combined configuration: install armored thermocouples on core valve gate nozzles and manifold main temperature measuring points to guarantee stable production, while arranging low-cost non-armored sensors on auxiliary low-precision sub-channels to control overall spare parts cost. This collocation balances production stability and procurement expenditure. But for molds running uninterrupted mass production over three months, full armored thermocouple configuration is still the most cost-effective long-term solution, cutting downtime loss caused by frequent non-armored sensor replacement.
To sum up, armored thermocouples dominate mainstream high-standard hot runner supporting market relying on comprehensive mechanical and temperature control performance, while non-armored types only serve temporary low-precision short-cycle molding demands. Mold engineers shall confirm production cycle, product precision requirement and plastic processing temperature before choosing thermocouple protective structure.
