Various Single-Head Heating Tubes
I. Tube Cutting (Blank Cutting)
1. Measure tube specifications, mainly check that the outer diameter difference of each batch of tubes is ≤0.2mm; tube wall thickness difference ≤0.2mm;
2. The cut end must have no burrs caused by friction between the tool and the cut surface;
3. The cut width should not exceed 2.5mm;
4. Length tolerance is ±0.5mm;
5. Mainly self-inspection, with first-piece inspection and random inspection by inspectors;
Tools, fixtures, and equipment: High-speed steel blades, steel rulers, tape measures, calipers, instrument lathes.
II. Chamfering (Deburring)
1. Chamfer the inner holes at both ends of the tube at 0.3×45°, with no burrs;
2. Pressurize the tube with compressed air at 6-7 kgf, and immerse the tube in a water tank to observe if any bubbles appear.
Tools, fixtures, and equipment: Drill bits, bench lathe, air compressor, water tank, external pressure gauge.
III. Tube Cleaning
Use a brush to go back and forth twice. If the inner wall is very dirty, use a steel wool ball to scrub back and forth, and blow out residues with compressed air. Observe the inside of the tube under sunlight. Tools, fixtures, and equipment: Long-handled brushes of various specifications, stainless steel wool balls.
IV. Press-Fitting Solid Plugs
Press the lead rod into the solid plug. The press-in dimension shall follow the lead rod insertion length into the tube as specified in the production order. Tools, fixtures, and equipment: Steel ruler, hand-operated press.
V. Winding
1. Select the wire type, wire diameter, and mandrel according to the production order, and check the wire diameter carefully;
2. Ensure uniform winding pitch, with no "jumping coils";
3. Insert the lead rod, ensure the winding resistance value meets the production order requirements, and check that the actual tight winding length differs from the marked tight winding length by ±10mm to pass;
4. Mainly self-inspection, with random inspection by inspectors.
Tools, fixtures, and equipment: Winding machine, mandrels of various specifications, tape measure, digital resistance meter.
VI. Spot Welding (Oxy-Fuel Welding)
1. Ensure tight contact between the lead rod and the resistance wire after insertion;
2. Ensure the weld is strong, so that the resistance wire does not slip off the lead rod during tube reduction;
3. Adjust the spot welding current to the optimal level where no sparking occurs and smoke is produced. The weld point should be at least 2mm from the end face of the lead rod where the wire is inserted. There should be no fewer than 2 weld points. For the head coil wire end, spot weld it specifically and ensure the wire end does not lift;
4. For wire diameters greater than 0.8mm, use oxy-fuel welding. The weld point should be at the gap between the first and second coils of the resistance wire after it is inserted into the lead rod. The remaining parts near the flame should be wrapped with wet asbestos thread to prevent local annealing of the wire, which could cause uneven drawing;
5. Handle finished products with care, lifting and placing gently to ensure the resistance wire does not deform;
6. Only after inspection by the inspector can the product be transferred to the next process.
Tools, fixtures, and equipment: Energy storage spot welder, oxy-fuel welding equipment, welding wire.
VII. Powder Filling (Critical Process)
1. Clean the wire with high-grade gasoline and blow dry with compressed air;
2. Spread MgO powder in a tray to a thickness not exceeding 40mm, and place in an oven. If a magnesium core is required, also place it in the oven. Bake at 100°C for 1-3 hours to ensure complete drying. It is best to fill powder while still warm;
3. Ensure the lead rod insertion length into the tube meets the production order requirements;
4. For wire diameters ≥0.5mm, pre-stretch the wire to two-thirds of the tube length. When a magnesium core is required, pull gently;
5. After powder filling, the filling ratio should reach 2.4-2.6 g/cm³;
6. The finer the resistance wire diameter, the smaller the vibration amplitude should be. Note: Avoid the "wire sagging" phenomenon;
7. When installing hollow plugs, ensure the lead rod does not move or rotate, and press the plug firmly to prevent powder leakage;
8. When selecting the powder filling liner tube (double liner tube), the outer diameter of the liner tube should be at least 0.5mm smaller than the inner diameter of the tube, and the inner liner tube diameter should be no more than 0.5mm smaller than the maximum outer diameter of the wire;
9. Self-check withstand voltage and insulation. Only after passing can the product be transferred to the next process;
10. Precautions during powder filling (process description):
a. Visually inspect the uniformity of the spiral resistance wire. Stretch the wire to half the tube length and observe if the density is basically uniform;
b. Verify the material, length, and diameter of the lead rod. If bent, straighten it;
c. If the tube is bent, straighten it.
d. Formula for calculating powder weight (ignoring wire, lead rod, and plug weight):
W = d²·L·V ≈ 0.785 d²·L·V
Where: d — tube inner diameter (cm)
L — tube length (cm)
V — powder density 2.4-2.6 g/cm³
Tools, fixtures, and equipment: Powder filling machine, automatic breakdown tester, oven, tape measure, cleaning tray, gasoline.
VIII. Tube Reduction
1. Carefully wipe all parts of the tube clean;
2. Pay attention to the direction during tube reduction, ensuring the solid plug enters the rollers first (for empty-burning tubes, forward and reverse reduction is recommended);
3. After reduction, the tube diameter tolerance is ±0.08mm (including ovality tolerance);
4. After reduction, the straightness should not exceed 4mm per meter;
5. Each time the rollers are adjusted, no more than 8 test pieces should be produced;
6. Inspectors randomly check the tube diameter and straightness, and randomly check 20% of the resistance values and tube lengths.
Note: After reduction, the difference between the longest and shortest tubes should not exceed 3% of the "tube length after reduction" specified in the production order. If the tube length difference is too large to be salvaged, the scrap is attributed to the powder filling process.
Tools, fixtures, and equipment: Tube reducing machine, tape measure, calipers.
IX. End Machining
1. Ensure the lead rods are symmetrical after machining, with the exposed length difference at both ends not exceeding 2mm;
2. The tube length tolerance is ±0.5mm;
3. Use a file to chamfer the outer edge burrs;
4. Mainly self-inspection, with random inspection by inspectors.
Tools, fixtures, and equipment: Bench lathe, instrument lathe, high-speed steel blades, tape measure, steel ruler.
X. Annealing
1. Whether to anneal, the annealing method, and temperature shall follow the production order;
a. For copper tube annealing, use an oxy-fuel flame at about 400°C, with the annealing color being dark red;
b. For carbon steel tubes and stainless steel tubes, use a local annealing machine. Carbon steel tubes are annealed at about 500°C, with a medium red color. Stainless steel tubes are annealed at about 700°C, with a slightly white color.
2. When annealing in an electric furnace, place the tubes evenly in layers inside the furnace, avoiding crowding that could lead to uneven annealing.
Tools, fixtures, and equipment: Annealing furnace, local annealing furnace, oxy-fuel welding equipment.
XI. Tube Bending
1. Cut the exposed length of the lead rod according to the production order, chamfer the cut end to remove damaged threads, and chamfer at 0.5×45° to ensure the mating thread can be screwed in smoothly;
2. Bend to shape according to the drawing or sample. The tube surface must not have obvious mechanical scratches or local expansion. The bend must not have wrinkles, concavities, or other defects;
3. When the bending radius is less than 3 times the tube diameter and no annealing is performed, ensure the tube weld seam is oriented toward the inner R (to prevent breakage);
4. Mainly self-inspection, with first-piece inspection and random inspection by inspectors.
Tools, fixtures, and equipment: Tube bending fixtures, tube bending machine, hydraulic press, punch press, grinder, calipers, universal protractor, tape measure.
XII. Welding
1. When brazing flanges or quick-connect plate parts, the brazed parts must be firm and reliable;
2. For components used to heat liquids or sealed components, they must withstand a hydrostatic pressure test of 0.3-1.2 MPa for 5 minutes without leakage.
Tools, fixtures, and equipment: Oxy-fuel welding equipment, various welding rods.
XIII. Sealing
1. Whether to seal shall be determined by the production order;
2. Remove MgO powder from both ends to a depth of 3-4mm and clean thoroughly. Pay special attention that no magnesium powder remains on the inner wall of the tube or on the threaded parts;
3. Before sealing, check each piece to ensure the component withstands 2000V/1s without breakdown or flashover, and insulation is infinite;
4. Apply sealing glue, rotate the magnetic plug to press it in, then install the nut and washer;
5. For heating elements requiring two sealing operations, follow the operating procedures.
XIV. Surface Treatment
1. Determine whether surface treatment is needed and the type of treatment according to the production order;
2. The coating should be uniform, smooth, with no bubbles, peeling, or local accumulation;
3. During pickling, do not damage (corrode) the lead rods;
4. Pre-clean the surface to be treated, especially glue, oxidation spots, etc.
XV. Final Inspection
1. Insulation: Infinite, lasting 10s;
2. Withstand voltage:
For diameters ≥16mm, length within 4m: 3000V/10s;
For diameters 10-8mm, length within 4m: 2000V/10s;
For diameter 6.5mm, length within 2m: 1800V/10s;
No breakdown or flashover.
3. Cold resistance: Meets production order requirements;
4. Visual inspection: Geometric shape and dimensions should meet product requirements;
5. Leakage current: Randomly inspect 5% per batch; leakage current less than 0.5mA;
6. Hot insulation: Randomly inspect 5% per batch; simultaneously observe heating uniformity.
Within 1 minute after power-off, test hot insulation ≥2MΩ. After cooling under natural conditions for 1 hour, the insulation should recover to above 100MΩ.
Tools, fixtures, and equipment: Automatic breakdown tester, digital resistance meter, leakage current meter, calipers, tape measure, steel ruler, flat plate.
XVI. Marking: Carry out according to the production order.
XVII. Packaging: Carry out according to the production order.
