Small and medium-sized injection molding factories with limited budgets cannot afford high-precision portable thermocouple calibration furnaces, nor can they send all probes to third-party testing institutions for regular calibration due to long delivery cycles and expensive testing fees. A set of operable comparative calibration schemes relying on verified standard reference thermocouples, existing hot runner mold heating systems and portable multimeters completes accurate error testing of batch thermocouples on-site, judges drift qualification standards and effectively screens severely drifted probes without large professional equipment, meeting daily production quality control requirements.
Pre-calibration preparation work includes three core steps. First, prepare a verified standard reference thermocouple with valid third-party calibration reports, stored separately and exclusively used for calibration testing without installation on mass production molds to avoid self-drift interfering with comparison accuracy. Classify all thermocouples to be tested by K-type and J-type to prevent mixed comparison of different wire types generating calculation errors. Clean all test probe sensing tips with anhydrous alcohol cloth to remove carbon deposits and oil dirt; residual carbon layers form heat isolation barriers and distort comparison data. Second, prepare two independent channels on the factory's existing temperature controller, high-temperature metal binding fixtures, thermal insulation cotton and a portable multimeter for auxiliary resistance testing. Third, cool all molds to room temperature before testing to eliminate residual thermal interference affecting temperature reading stability.
Step one: 200°C static low-temperature comparative test utilizing idle intact hot runner molds. Select an idle production-free hot runner manifold heating zone, install the standard reference thermocouple and test probe side by side, tightly bind two sensing tips together with high-temperature metal strips to ensure complete gap-free contact, and wrap binding positions with thick thermal insulation cotton to isolate external air heat loss. Set controller heating temperature to 200°C, hold constant temperature for 20 minutes after readings stabilize, then record displayed temperature values of standard and test probes respectively to calculate temperature deviation between test samples and standard benchmarks. This low-temperature test point covers working temperature ranges of PP and PE low-temperature packaging molds, screening probes with severe low-temperature drift defects.
Step two: 450°C medium-temperature comparative test simulating conventional engineering plastic production conditions. Adjust controller set temperature to 450°C and maintain constant temperature for 25 minutes after full reading stabilization to record the second group of deviation data. This temperature point corresponds to mainstream ABS, PC and PA66 engineering plastic molding temperatures, the most widely used working interval for hot runner thermocouples where most drift faults concentrate. If deviation between test probes and standard references exceeds ±2.5°C at this temperature point, preliminary judgment classifies probes as severely drifted and marked for elimination.
Step three: 600°C high-temperature comparative test targeting high-temperature reinforced plastic molds. Raise heating set value to 600°C and hold constant temperature for 30 minutes to record the third set of deviation data. This test point targets glass fiber reinforced plastics, PPS and other high-temperature molding resins, testing thermocouple alloy wire high-temperature anti-oxidation stability. J-type thermocouples generate extremely large deviations at 600°C due to iron wire oxidation, consistent with their inherent high-temperature performance limitations and serving as a distinguishing marker for misclassified mixed K/J probes.
Step four: Multimeter auxiliary resistance hidden fault screening after temperature comparison testing. Cool molds down to room temperature, switch the multimeter to resistance mode to measure internal resistance of each test probe, shaking cables back and forth during measurement. Drastic resistance fluctuation during shaking indicates hidden broken strands inside wires that evolve into intermittent open-circuit faults after heating expansion; such probes are directly scrapped regardless of temperature comparison deviation data.
Unified comparative calibration result judgment standards. K-type thermocouples with deviation ≤±1.5°C at all three temperature points qualify as Class 1, suitable for automotive and medical precision molds; deviation ranging ±1.5°C to ±2.5°C qualifies as Class 2, permitted for ordinary packaging and household appliance molds; any deviation exceeding ±2.5°C at single temperature point classifies probes as severely drifted and prohibited for mass production. J-type thermocouples allow maximum deviation of ±3°C below 450°C, with any deviation over ±4°C at 600°C triggering direct elimination.
Post-calibration data archiving and classified probe management. Fill unified comparative calibration record forms documenting probe serial numbers, wire types, three-point deviation data and qualification judgment results, storing electronic records in workshop MES systems for full traceability. Affix waterproof classification labels on qualified probes marking calibration expiration dates; severely drifted unqualified probes are placed in dedicated waste storage boxes with clear marking to avoid accidental reuse by maintenance staff.
This mold heating comparative calibration method fully utilizes factory existing hot runner equipment resources without purchasing additional high-cost calibration furnaces, with test error controlled within ±0.8°C to meet daily regular inspection demands of small and medium injection molding factories, effectively reducing batch defective product risks induced by uncalibrated drifted thermocouples.
