I. Raw Material Inspection
1. Metal Shell
Preferred is SUS304 seamless stainless steel tube; no sand holes, cracks, or uneven wall thickness. The pipe must undergo eddy current testing to prevent tiny pinholes from causing water ingress later.
2. Heating Wire: Nickel-chromium wire (Cr20Ni80) or iron-chromium-aluminum; check the wire diameter, resistance tolerance, and ensure no burrs, oxidation, or broken wires on the surface. The power density should be designed according to the refrigerator defrosting requirements; if it is too high, it is prone to dry burning and wire breakage.
3. Magnesium Oxide Powder (MgO): High-purity electro-fused magnesium oxide; moisture resistance is of utmost importance. Moisture-damaged magnesium oxide directly leads to insulation degradation and leakage. The moisture content of the powder entering the factory must be controlled; long-term storage requires drying.
4. Lead-Out Rod/Ceramic Base: The ceramic insulator must be dense without air holes or cracks; air holes will absorb condensate water, causing creepage and arcing.
5. Lead-Out Wire: High-temperature silicone wire, resistant to both low and high temperatures and moisture; ordinary PVC wire must not be used.
6. Sealing Materials: High-temperature epoxy resin/silicone rubber, resistant to both high and low temperatures and low water absorption; the refrigerator’s cold and hot cycles + condensation are high-risk failure points for sealing.
II. Winding, Pipe Insertion, and Powder Filling Process
1. The resistance wire should be wound evenly, with consistent spacing, and must not touch the pipe wall; local contact with the pipe wall will cause local overheating and wire burning.
2. Insert the lead-out rod to ensure a secure weld between the resistance wire and the lead-out rod; the weld points must be free of burrs.
3. Powder Filling + Tube Shrinking (Compression)
- The MgO powder must be filled densely; no holes are allowed; gaps will cause the heating wire to shake and overheat locally.
- The reduction in diameter during shrinking must be controlled to ensure the MgO is compacted; excessive pressure is likely to break the internal resistance wire; insufficient pressure will cause the powder to be loose.
- The environmental humidity must be controlled throughout the process; the powder filling workshop should be low in humidity to prevent moisture absorption by the MgO.
III. Both Ends Sealing (The Most Critical Process, Failure Point of the Heating Tube Head in the Refrigerator)
Refrigerator internal condensation dripping water will occur repeatedly. Once the sealing is compromised, it will directly cause an electrical leakage.
1. Clean the tube head, ensuring no powder remains in the sealing area; residual powder will form an electrical leakage path.
2. The ceramic base must be properly assembled, with the sealing glue filling without bubbles; the glue must fully cure and not be semi-cured.
3. After curing, check: there should be no cracks or shrinkage gaps in the glue layer.
> Common problems: the sealing glue has a tiny gap; due to alternating cold and hot temperatures, condensate water slowly seeps into the tube, the MgO absorbs moisture, and the insulation resistance drops rapidly, causing the leakage protection to trip.
IV. Bending and Forming
1. The bending radius should not be too small to prevent pipe wall wrinkling or cracking; the wrinkled position will have concentrated stress, which is prone to corrosion and perforation.
2. Bending should not pull on the internal resistance wire to avoid hidden breaks (the resistance wire is qualified during factory testing, but may break after use for a period of time).
3. Check the pipe mouth sealing after bending; the bending external force must not tear the sealing.
V. 100% Line Full Inspection (Must be done at the factory, in accordance with NB/T10307)
1. Resistance Test: Verify the cold-state resistance, control the power deviation within the standard allowable range, and eliminate open circuits, short circuits, and out-of-tolerance parts.
2. Cold-state Insulation Resistance: Steel tube type ≥ 100 MΩ;
3. Withstand Voltage (Electrical Strength): AC 1500V/1 minute, no breakdown or flashover.
4. Appearance Inspection: The pipe wall should have no scratches, cracks, and the sealing glue should have no bubbles; the ceramic should have no chipping.
VI. Batch Reliability Sampling (Simulating the actual working conditions of the refrigerator)
1. Cold and Hot Cycling: From -40°C to 150°C, cycle and retest insulation and resistance after the cycle; the drift should not exceed the standard limit.
2. Wet Heat Aging: 85°C/85%RH, evaluate the waterproof sealing of the sealing and the ceramic insulation.
3. Immersion Test: Measure insulation and withstand voltage after immersion to verify the sealing’s ability to prevent condensation.
4. Dry Burn Overload Test: Simulate the failure of the defrosting temperature control, long-term dry burning to evaluate lifespan and safety.
5. Terminal pull force test: The lead terminal should withstand a pull force of ≥ 50N to prevent the wire from being pulled and broken during assembly.
6. Leakage current test: The leakage current under working heat condition should comply with the limit values specified in NB/T10307.
VII. Key Points of Structure, Assembly and Safety Design
1. The creepage distance/electrical clearance meet the requirements of GB4706. The ceramic base size should be left sufficient, and no creepage or arcing should occur in the condensation state.
2. Stress release is performed at the root of the lead. During the transportation of the refrigerator, vibrations may cause the root to break. Therefore, the root should be protected.
3. The accompanying defrosting temperature fuse (thermal fuse): It should be assembled in conjunction with the heating tube as an overheat protection device to prevent runaway dry burning and fire.
4. Fixed clips/supports: The heating tube should not be directly attached to the metal fins of the evaporator to avoid local overheating.
VIII. Storage, Packaging
1. Semi-finished products/finished products should be stored in a dry environment. Magnesium oxide is afraid of moisture; the insulation resistance will decrease after long-term storage. It is necessary to re-heat and re-inspect before shipment.
2. Prepare moisture-proof bags for packaging to avoid moisture during transportation.
Post time: Sep-29-2026