How Sheet Metal Is Manufactured
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● Raw Materials and Initial Processing
Raw Materials and Initial Processing
Sheet metal begins as ore or scrap. Iron ore feeds blast furnaces, while aluminum comes from bauxite through electrolysis. Steel mills melt scrap in electric arc furnaces or refine pig iron in basic oxygen converters. The goal is consistent chemistry—carbon below 0.2 % for mild steel, chromium above 10.5 % for stainless grades.
Molten metal pours into continuous casters to form slabs 200–250 mm thick. These slabs cool and move to hot rolling mills. Rolls reduce thickness in multiple passes, typically to 2–6 mm. Temperature stays above 900 °C to keep the metal soft. Surface scale forms during hot rolling and requires removal later.
Cold rolling follows for tighter tolerances. The strip enters a series of stands at room temperature. Each stand reduces thickness by 1–2 %. Work hardening increases strength but reduces ductility. Annealing in a continuous furnace restores formability. Hydrogen or nitrogen atmospheres prevent oxidation on stainless coils.
Coil weight reaches 20–30 tons. Slitting lines cut wide coils into narrower strips. Edge trimming removes cracks from the coil sides. Tension leveling flattens the strip and removes coil set. Oil or paper interleaving protects the surface during storage.
Primary Cutting Operations
Large sheets or coils arrive at the fabrication shop. The first task is to cut manageable blanks. Guillotine shears handle straight cuts up to 12 mm thick. Hydraulic cylinders drive the upper blade while hold-downs clamp the sheet. Back gauges position the cut within 0.1 mm.
Turret punch presses create holes and internal cutouts. A 30-ton machine with 58 stations runs at 300 hits per minute. Tools include round punches, rectangles, and clusters. Thick turret guides keep alignment. Stripper plates prevent the sheet from lifting with the punch.
Laser cutting dominates for complex contours. A 6 kW fiber source melts 3 mm stainless at 8 m/min. Nitrogen assist gas produces oxide-free edges. Nesting software arranges parts to use 85–90 % of the sheet. Piercing takes 0.5 seconds; lead-in tabs prevent burn marks at the start.
Waterjet cutting avoids heat. Abrasive garnet mixes with 60,000 psi water. The stream cuts 25 mm aluminum without a heat-affected zone. Taper compensation tilts the head 2–3° to keep vertical edges. Stacking multiple sheets increases throughput but requires clamping to prevent separation.
Forming Processes
Flat blanks become three-dimensional parts in forming. Press brakes bend along straight lines. A 175-ton machine with 3 m bed handles 4 mm mild steel. V-dies range from 6 to 100 mm opening. Air bending leaves a gap under the punch; bottoming closes it for sharp corners. CNC controls compensate springback using material tables.
Stamping uses progressive dies for high volume. Coil feeds into a 400-ton press. Each station performs a different operation—piercing, blanking, forming, trimming. Transfer fingers move the part between stations. Sensors detect misfeeds and stop the press. Automotive door inners run at 600 strokes per minute.
Deep drawing forms cups and boxes. A blank holder controls metal flow into the die cavity. Draw beads restrict material to prevent wrinkles. Lubricants reduce friction by 30 %. Triple-action presses allow independent control of punch, blank holder, and cushion. Battery housings for electric vehicles use 1.5 mm aluminum in 800-ton presses.
Roll forming produces long profiles. A coil feeds through 10–20 stands. Each stand bends the strip a few degrees. Welded tubes start as flat strip, form into circles, and close with high-frequency induction. C-purlins for steel buildings run at 30 m/min.
Joining Methods
Parts assemble through welding, riveting, or adhesive bonding. Resistance spot welding joins overlapping sheets. Electrodes apply 8–12 kN force and 10 kA current for 200 ms. Nugget diameter reaches 5√t mm, where t is sheet thickness. Robots with servo guns maintain consistent force in automotive lines.
MIG welding fills gaps and joins thicker sections. A 350 A power source feeds 1.0 mm wire at 8 m/min. Pulse waveforms reduce spatter on galvanized steel. Fume extraction keeps air clean in enclosed cells.
Self-piercing rivets join aluminum to steel without pre-drilled holes. A 70 kN hydraulic tool drives a semi-tubular rivet through the top sheet and flares it in the bottom sheet. Joint strength exceeds 4 kN in shear. Electric vehicle battery trays use 500 rivets per assembly.
Structural adhesives complement mechanical fasteners. Epoxy films cure at 180 °C in 20 minutes. Bond lines of 0.2 mm transfer load evenly. Aerospace panels combine rivets and adhesive for 30 % weight savings.
Surface Treatments
Bare steel rusts quickly. Pickling in hydrochloric acid removes mill scale. Rinse tanks prevent acid carryover. Zinc phosphate conversion coating provides a base for paint. Seven-stage spray washers clean, rinse, and seal automotive parts.
Electrogalvanizing deposits 7–15 μm of zinc. Continuous lines run at 150 m/min. Alloy coatings with nickel or iron improve corrosion resistance. Appliance makers specify 720-hour salt spray performance.
Powder coating applies dry resin electrostatically. Parts preheat to 200 °C, then enter a spray booth. Overspray collects for reuse—transfer efficiency hits 95 %. Cure ovens cross-link the polymer in 15 minutes. Colors match RAL standards within ΔE 1.0.
Anodizing thickens the natural oxide on aluminum. Type II sulfuric acid process yields 5–18 μm. Sealing in hot water closes pores. Hardcoat Type III reaches 50 μm for wear parts. Dye tanks add color before sealing.
Quality Assurance
Dimensional checks start with coordinate measuring machines. A 1.2 m arm probes 50 points on a door panel in 3 minutes. Tolerance bands of ±0.3 mm are common. Go/no-go gauges verify hole positions on high-volume lines.
Surface roughness uses portable profilometers. Cold-rolled steel for visible panels targets Ra 0.8 μm. Paint defects appear under 50× magnification. Automated vision systems scan 100 % of parts at 1 m/s.
Non-destructive testing includes ultrasonic thickness measurement. Handheld units check drawn shells for thinning below 80 % of nominal. Magnetic particle inspection finds surface cracks in welded frames.
Process capability tracks Cp and Cpk. Laser cutting width holds Cpk 1.67 at ±0.05 mm. Spot weld strength samples every 500 welds. Control charts trigger tool changes when trends approach limits.
Emerging Developments
Industry 4.0 integrates sensors across the line. Coil tracking uses RFID tags from the mill. Press force monitors detect die wear in real time. Cloud dashboards show OEE for every machine.
Lightweight materials drive change. Advanced high-strength steels reach 1,500 MPa tensile strength. Tailored tempering heats specific zones for ductility where needed. Hot forming lines quench parts in the die to achieve martensite structure.
Additive manufacturing complements sheet processes. Hybrid cells laser-deposit features onto stamped blanks. Conformal cooling channels in dies reduce cycle time by 20 %.
Recycling closes the loop. Shredders separate steel and aluminum. Remelted scrap feeds the same furnaces. Closed-loop systems return edge trim directly to the caster.
Sheet metal manufacturing combines heavy machinery with fine control. Each step affects the next, so upstream decisions ripple through the line. Operators who understand the full sequence spot problems early and suggest improvements that save material and time.
Frequently Asked Questions
Q1: Which steel grade works best for deep drawing appliance panels?
A: DC04 or SPCE offers low carbon and high elongation—r-value above 1.8 ensures uniform thinning.
Q2: How thick can fiber lasers cut before switching to plasma?
A: Up to 25 mm stainless at reasonable speed; beyond that, plasma or oxyfuel becomes faster.
Q3: What causes orange peel in formed aluminum, and how to fix it?
A: Coarse grain from thick starting stock. Use finer gauge or intermediate anneal.
Q4: Why does galvanized sheet sometimes crack in bending?
A: Zinc embrittlement at the bend line. Use vent holes or lower punch speed.
Q5: How often should press brake tools be inspected for wear?
A: Every 50,000 cycles or when radius tolerance exceeds 0.1 mm.