Yes, hot-state testing exceeding limits is relatively common in industrial rework. A typical example is a medical device heating element whose hot-state insulation resistance approached 0 after rework. This problem was caused by incomplete sintering and sealing failure, and the root cause was ultimately located through process tracing and structural analysis.
Case Background: Failed Rework of Heating Element for Medical Injection Molding Machine
Equipment Type: U-shaped heating element for medical plastic molding machine
Fault Phenomenon: After refilling with magnesium oxide, the cold-state insulation resistance was 510 MΩ (qualified), but after running the hot-state test for 30 minutes and then powering off, the insulation resistance plummeted to 5 MΩ, rendering the equipment unusable.
Preliminary Judgment: No surface damage, pressure resistance test passed, suspected internal moisture or sintering defects.
Problem Investigation Process and Findings
X-ray Inspection
Local voids and off-center filling were found in the filled area, indicating uneven filling density; Micron-sized air bubbles were found inside the sealing adhesive, invisible to the naked eye.
Metallographic Analysis
Magnesium oxide grains were not fully recrystallized and remained in a loose powdery state, indicating a sintering temperature below 800℃. This was determined to be due to a temperature control deviation in the sintering furnace; the actual temperature was approximately 120℃ lower than the set value.
Moisture Test Reproduction
After 48 hours at 40℃ and 95%RH, the cold insulation resistance dropped from 510 MΩ to 180 MΩ, verifying insufficient sealing.
Final Confirmation: Insufficient sintering temperature + poor curing of the sealing adhesive allowed moisture to seep in through micropores during operation, vaporizing at high temperatures and forming conductive paths.
Solution and Results
Rework: 900℃ holding time for 1.5 hours + high-purity nitrogen protection + furnace cooling; Changed sealing process: Used a composite structure of ceramic adhesive and metal sealing rings; Post-repair testing: Hot insulation resistance stabilized at 210 MΩ, and the equipment operated continuously for over 12,000 hours without failure.
This case was included as a typical warning case in the implementation of the HG/T 6445-2025 standard, emphasizing the necessity of process traceability and seal verification.
