Testing the insulation performance of thermocouples after diameter reduction process mainly involves measuring insulation resistance to ensure signal stability and eliminate the risk of leakage or short circuits under high temperature and high pressure environments.
1. Insulation Resistance Measurement Method
① Room Temperature Insulation Resistance Test
Test Conditions: Ambient temperature 15–35℃, relative humidity ≤45%, atmospheric pressure 86–106 kPa.
Test Voltage: DC 500V (±50V).
Test Steps:
Short-circuit the two electrodes of the thermocouple;
Connect one end of a megohmmeter (or high resistance meter) to the electrode, and the other end to a metal protective tube (or grounded casing);
After applying the voltage for 60 seconds, read the insulation resistance value. Take the smaller value of the two measurements (positive and negative polarities) as the final result.
Acceptance Criteria:
Length > 1m: Insulation resistance × length ≥ 100 MΩ·km;
Length ≤ 1m: Insulation resistance ≥ 100 MΩ.
Note: Before testing, the thermocouple must be placed under standard atmospheric conditions for at least 2 hours to avoid the influence of moisture.
② High Temperature Insulation Resistance Test (Upper Limit Temperature)
Applicable Scenarios: Used to simulate insulation performance under high-temperature conditions such as hot runners.
Test Voltage: DC 10V (±1V).
Heating Requirements:
Heating Length: 300mm or 50% of the total length (whichever is smaller);
Temperature Deviation: Within ±10℃;
Holding Time: Not less than 5 times the thermal response time T₀.₅.
Acceptance Criteria: Depending on the operating temperature, the insulation resistance should meet a specific threshold (e.g., ≥0.02 MΩ at 1300℃).
2. On-site Rapid Testing Methods
|
Method |
Operation |
Judgment Basis |
|
Multimeter Preliminary Judgment |
Measure the resistance between the electrodes and the casing using the high-resistance setting. |
>100MΩ: Good; <1MΩ: Severe leakage. |
|
Megohmmeter Actual Measurement |
Measure using a 500V megohmmeter. |
Meeting the above standards indicates normal operation. |
|
Comparison Method |
Compare the reading with normal products from the same batch. |
A significantly lower reading indicates insulation defects. |
3. Common Problems and Risk Warnings
Insulation Degradation Manifestations: Temperature drift, signal fluctuations, PID control oscillation;
Main Causes: Magnesium oxide moisture absorption, micro-cracks, processing contamination (oil, sweat, lint);
Preventive Measures: Immediately vacuum seal or dry after diameter reduction;
Avoid contact with insulating materials with bare hands;
Use moisture-proof packaging for storage.
Industry Practice: High-end hot runner armored thermocouples require an insulation resistance ≥1000MΩ, far exceeding general industrial standards, to ensure long-term stability.
