Manufacturing Process Specification for Heating Elements

I. Tube Cutting (Blank Preparation)

1. Measure tube specifications: Focus on checking each batch for outer diameter deviation ≤ 0.2mm and wall thickness deviation ≤ 0.2mm.

2. Cut quality: No burrs should be generated due to friction between the cutting tool and the cut surface.

3. Kerf width: Should not exceed 2.5mm.

4. Length tolerance: ±0.5mm.

5. Quality control: Primarily based on operator self-inspection, with inspector performing first-article inspection and periodic sampling.

• Tools, Fixtures, Measuring Instruments & Equipment: High-speed steel turning tool, steel ruler, tape measure, caliper, instrument lathe.

II. Chamfering (Deburring)

1. Chamfer the inner holes at both ends of the tube to 0.3×45°, ensuring no burrs.

2. Air pressure test: Apply 6-7 kgf/cm² of compressed air pressure to the tube and immerse it in a water tank to observe for bubble formation.

• Tools, Fixtures, Measuring Instruments & Equipment: Drill bit, bench lathe, air compressor, water tank, external pressure gauge.

III. Tube Cleaning
Use a long-handled brush to scrub the inside of the tube back and forth twice. If the inner wall is heavily soiled, use a stainless steel scrubbing pad, then blow out any residue with compressed air. Inspect the tube interior under sunlight.

• Tools, Fixtures, Measuring Instruments & Equipment: Various sizes of long-handled brushes, stainless steel scrubbing pads.

IV. Press-fitting Solid Plugs
Press the lead rod into the solid plug. The press-in depth must strictly follow the "lead rod insertion length into tube" specified in the production order.

• Tools, Fixtures, Measuring Instruments & Equipment: Steel ruler, manual press.

V. Resistance Wire Winding

1. Select resistance wire material, wire diameter, and winding mandrel according to the production order, paying attention to verifying the wire diameter.

2. Ensure uniform winding pitch; no "skipped turns" are allowed.

3. Insert the lead rod. The final resistance value must meet production order requirements. The allowable deviation between the actual close-wound length and the specified length on the order is ±10mm.

4. Quality control: Primarily based on operator self-inspection, with inspector sampling.

• Tools, Fixtures, Measuring Instruments & Equipment: Winding machine, various sizes of winding mandrels, tape measure, digital ohmmeter.

VI. Spot Welding (Oxy-acetylene Welding)

1. Ensure tight contact between the lead rod and the inserted resistance wire.

2. Welding must be secure, guaranteeing the resistance wire does not slip off the lead rod during subsequent tube reduction.

3. Spot welding procedure: Adjust the welding current to the optimal level where no sparking occurs, only smoke. Weld points should be located more than 2mm from the wire entry end face of the lead rod, with no fewer than 2 points. The starting end of the resistance wire requires dedicated spot welding to prevent it from lifting.

4. Oxy-acetylene welding procedure: Required for wire diameters greater than 0.8mm. The weld point should be located at the gap between the first and second turns of the resistance wire tail after it is inserted into the lead rod. Protect other parts near the flame by wrapping them with wet asbestos cord to prevent localized annealing of the wire, which could cause uneven stretching.

5. Handle welded semi-finished products with care during movement to prevent wire deformation.

6. Proceed to the next process only after passing inspector approval.

• Tools, Fixtures, Measuring Instruments & Equipment: Capacitor discharge spot welder, oxy-acetylene welding equipment, welding wire.

VII. Magnesium Oxide (MgO) Powder Filling (Critical & Important Process)

1. Clean the resistance wire with high-grade gasoline and dry it with compressed air.

2. Spread the MgO powder on a tray no more than 40mm thick and place it in an oven. If a magnesium core is required, place it in the oven as well. Dry thoroughly at 100°C for 1-3 hours. It is best to fill the powder while the components are still warm.

3. Ensure the lead rod insertion length into the tube meets the production order requirement.

4. For wire diameters ≥ 0.5mm, pre-stretch the resistance wire to two-thirds of the tube length. If a magnesium core is inserted, stretch gently.

5. Post-filling bulk density requirement: 2.4 - 2.6 g/cm³.

6. Vibration parameter: The finer the resistance wire, the smaller the vibration amplitude should be. Caution: Watch for "wire sagging."

7. When installing hollow plugs, ensure the lead rods do not shift axially or rotate. Press the plugs firmly to prevent powder leakage.

8. When selecting filling liner tubes (double liner): The liner's outer diameter should be at least 0.5mm smaller than the tube's inner diameter. The inner liner's inner diameter should be at least 0.5mm larger than the maximum outer diameter of the resistance wire coil.

9. Operator self-inspects for withstand voltage and insulation. Proceed only after passing.

10. Filling Precautions (Process Notes):
    a. Visually inspect the uniformity of the helical resistance wire: Stretch the wire to half the tube length and observe if the coil density is basically uniform.
    b. Verify the material, length, and diameter of the lead rods. Straighten if bent.
    c. Straighten the tube if it is bent.
    d. Powder weight calculation formula (neglecting wire, lead rod, and plug weight):
    W = 0.785 * d² * L * V
    Where: d — Tube inner diameter (cm), L — Tube length (cm), V — Powder bulk density (2.4-2.6 g/cm³)

• Tools, Fixtures, Measuring Instruments & Equipment: Powder filling machine, automatic dielectric withstand tester, oven, tape measure, cleaning tray, gasoline.

VIII. Tube Reduction (Swaging)

1. Carefully wipe the entire exterior surface of the tube.

2. Pay attention to the reduction direction: ensure the end with the solid plug enters the rollers first. (For open coil air heaters, reduction from both ends is recommended).

3. Post-reduction tube diameter tolerance: ±0.08mm (this tolerance includes ovality).

4. Post-reduction straightness requirement: Not more than 4mm per meter.

5. After each roller adjustment, process no more than 8 trial pieces.

6. Inspectors sample-check tube diameter and straightness, and sample 20% for resistance value and tube length.
   Note: After reduction, the length difference between the longest and shortest tubes should not exceed 3% of the "tube length after reduction" specified in the production order. If the length discrepancy after reduction is too great to remedy, the scrap is attributed to the filling process.

• Tools, Fixtures, Measuring Instruments & Equipment: Tube reducing machine (swager), tape measure, caliper.

IX. Machining Both Ends

1. After machining, ensure the lead rods are symmetrical, with the exposed length difference not exceeding 2mm.

2. Final tube length tolerance: ±0.5mm.

3. Remove external burrs using a file.

4. Quality control: Primarily based on operator self-inspection, with inspector sampling.

• Tools, Fixtures, Measuring Instruments & Equipment: Bench-type instrument lathe, high-speed steel turning tool, tape measure, steel ruler.

X. Annealing

1. Whether to anneal, the annealing method, and temperature are strictly according to the production order:
   a. Copper tube annealing: Use oxy-acetylene flame, temperature approx. 400°C, annealed color is dark red.
   b. Carbon steel / Stainless steel tube annealing: Use a local annealing machine. Carbon steel: temp. approx. 500°C, color medium red. Stainless steel: temp. approx. 700°C, color faint white.

2. When using a furnace for annealing, place tubes in layers evenly inside the furnace to avoid uneven heating due to overcrowding.

• Tools, Fixtures, Measuring Instruments & Equipment: Annealing furnace, local annealing machine, oxy-acetylene welding equipment.

XI. Tube Bending

1. Cut the exposed portion of the lead rods per the production order. Remove burrs and damaged threads from the cut, and chamfer 0.5×45° to ensure mating threads can be screwed on smoothly.

2. Bend to shape according to the drawing or sample. The tube body must not show obvious mechanical damage or local swelling. The bent area must not have wrinkles, dents, or unevenness.

3. When the bend radius is less than 3 times the tube diameter and no annealing is performed, ensure the tube's weld seam direction faces the inside of the bend (to reduce fracture risk).

4. Quality control: Primarily based on operator self-inspection, with inspector performing first-article inspection and periodic sampling.

• Tools, Fixtures, Measuring Instruments & Equipment: Tube bending jigs, tube bender, hydraulic press, punch press, grinder, caliper, universal bevel protractor, tape measure.

XII. Welding

1. When brazing components like flanges or connection plates is required, ensure the joints are firm and reliable.

2. For elements used in heating liquids or as sealed components, they must withstand a hydrostatic pressure test of 0.3-1.2 MPa for 5 minutes without leakage.

• Tools, Fixtures, Measuring Instruments & Equipment: Oxy-acetylene welding equipment, various welding rods.

XIII. Port Sealing

1. Whether to perform port sealing is per the production order.

2. Remove MgO powder from inside both ends of the tube to a depth of 3-4mm and clean thoroughly. Pay special attention to ensure no magnesium powder remains on the inner tube wall or threaded areas.

3. Before sealing, inspect each unit: it must withstand a 2000V voltage for 1 second without breakdown or flashover, and insulation resistance must be infinite.

4. Apply sealing adhesive, press and rotate the magnetic plug into place, then install the nut and washer.

5. For heating elements requiring two-stage sealing, follow the corresponding operating procedure.

XIV. Surface Treatment

1. Whether to perform surface treatment and the type of treatment are determined by the production order.

2. Coating must be uniform, smooth, free of bubbles, peeling, or localized accumulation.

3. During pickling, the lead rods must not be damaged (corroded).

4. Pre-clean the surfaces to be treated, especially residues like adhesive and oxidation spots.

XV. Final Inspection

1. Insulation Resistance: Infinite, test duration 10 seconds.

2. Dielectric Withstand Voltage Test:

• Tube diameter > φ16, length ≤ 4m: 3000V for 10 seconds.

• Tube diameter φ10 ~ φ8, length ≤ 4m: 2000V for 10 seconds.

• Tube diameter φ6.5, length ≤ 2m: 1800V for 10 seconds.

• Requirement: No breakdown or flashover.

3. Cold Resistance: Must comply with production order requirements.

4. Visual Inspection: Geometric shape and dimensions must conform to product drawing requirements.

5. Leakage Current: Sample 5% per batch. Leakage current must be < 0.5mA.

6. Hot Insulation & Heating Uniformity: Sample 5% per batch. Measure hot insulation resistance within 1 minute after power-off; it should be ≥ 2 MΩ. Re-measure after natural cooling for 1 hour; insulation resistance should recover to ≥ 100 MΩ. Also observe heating uniformity.

• Tools, Fixtures, Measuring Instruments & Equipment: Automatic dielectric withstand tester, digital ohmmeter, leakage current tester, caliper, tape measure, steel ruler, inspection surface plate.

XVI. Marking
Perform according to production order requirements.

XVII. Packaging
Perform according to production order requirements.

XVIII. Warehousing
After completing all above processes and passing inspection, products are warehoused.