How to Select Thermocouples for Hot Runner Systems with Very High Injection Pressures?

Injection pressures in modern hot runner systems can exceed 300 MPa, particularly in thin-wall and micro-molding applications. This extreme pressure places significant mechanical stress on thermocouples, requiring special design considerations. The first selection criterion is the probe's structural integrity. The thermocouple probe must be able to withstand the compressive and bending forces without deforming. A larger diameter probe (1.5-3.0 mm) is more rigid and less likely to buckle. The sheath material must have high yield strength; Inconel 600 has a yield strength of approximately 300 MPa at room temperature, which may be marginal; superalloys like Inconel 718 or MP35N offer higher strength. The second criterion is the mounting method. For high-pressure applications, a threaded mount is mandatory. The thread provides mechanical retention and prevents the probe from being pushed out by the pressure. Use a fine thread (e.g., M6x1) for a tight fit. The third criterion is the sealing. The thermocouple must have a robust seal at the cold end to prevent resin from being forced into the probe under pressure. A metal-to-metal compression seal or a high-pressure epoxy is necessary. O-rings alone are insufficient. The fourth criterion is the cable connection. The connector must have a strain relief that can withstand the pressure-induced forces on the cable. A locking connector (like a bayonet) prevents accidental disconnection. The fifth criterion is the position relative to the melt flow. If the thermocouple is in a side-mounted hole, ensure it is not directly in the path of the high-pressure melt flow, which could cause erosion or pressure-induced vibration. The sixth criterion is the response time. In high-pressure, thin-wall molding, the cycle time is short, and the thermocouple must respond quickly to temperature changes caused by the rapid injection. A thin probe (1.0 mm) offers fast response but may be less robust. Consider a compromise: a 1.5 mm probe with a thin-walled sheath. The seventh criterion is the testing. The thermocouple should be pressure-tested before installation. Some manufacturers offer a "pressure rating" for their thermocouples-specify a minimum rating of 300 MPa. The eighth criterion is the backup system. Consider using a redundant thermocouple in high-pressure zones. If one fails due to pressure damage, the second can take over. In summary, high-pressure applications demand thermocouples with superior mechanical strength, robust sealing, and secure mounting. The extra cost of high-pressure-rated thermocouples is justified by the prevention of catastrophic failures that could damage the mold or the injection unit. By selecting sensors designed for these extreme conditions, molders can safely and reliably produce parts at the highest pressures.