What Stamping and Assembly Do
Metal teeth begin as a flat wire or strip, usually brass, german silver or aluminium alloy, in coils of 0.6 to 1.2 mm thickness. A progressive die blanks, bends and pierces the strip into a Y-shaped element with two legs and a head, and the element is then clamped or swaged onto the tape edge in the same machine or in a dedicated assembly unit. The result is the traditional metal zipper: strong, dimensionally stable and able to take a plating finish.
The two operations are usually described together because they share a tolerance chain. A tooth stamped 0.03 mm wide will clamp loosely no matter how well the closing dies are set, and a tooth clamped perfectly will still sit crooked if the stamped head is off-centre. Buyers who audit only the plating line often miss that the geometry was decided at the press.
Equipment and Key Parameters
A progressive stamping press of 15 to 60 tonnes runs the strip through 8 to 15 stations at 200 to 600 strokes per minute. The strip is fed by a servo feeder with a pitch accuracy of plus or minus 0.01 mm. Assembly is done by closing dies that fold the legs around the tape bead and crush them to a controlled thickness.
- Strip thickness: typically 0.6 to 1.0 mm for sizes 3 to 5, and 1.0 to 1.2 mm for sizes 8 to 10.
- Die clearance: typically 5 to 8 percent of strip thickness per side. Too much clearance tears the edge; too little causes die wear and burrs.
- Press speed: 200 to 600 strokes per minute; above this, slug pulling and burr height both increase.
- Tooth height tolerance: typically plus or minus 0.05 mm, tightened to 0.03 mm for fine-gauge fashion chains.
- Clamping force: set so that the crushed leg thickness is 1.15 to 1.30 times the tape thickness.
- Closing die wear limit: typically 0.02 mm before replacement; a worn die produces loose teeth that pass a visual check.
- Strip hardness: typically 90 to 130 HV for brass, so that the tooth deforms rather than cracks at the bend.
Common Defects
- Burrs. Raised metal on the tooth edge from die wear or excessive clearance. Burrs cut thread, scratch skin and interfere with plating adhesion.
- Off-centre heads. The head sits to one side of the legs, so the tooth meshes at an angle and the chain runs stiff.
- Tooth height variation. Caused by strip thickness variation or insufficient press rigidity. It shows as a stepped profile along the chain.
- Loose teeth. Insufficient clamping force or a worn closing die. The tooth holds by friction alone and releases under load.
- Tape cut-through. Excess clamping force severs the warp yarns, leaving the tooth anchored to almost nothing.
- Cracking at the bend. Strip hardness too high, or a bend radius too tight for the material.
- Contamination and slugs. Punched slugs carried into the strip, which later appear as foreign matter in the finished product.
How the Defect Shows Up Downstream
Slug pulling deserves separate mention. When a punch re-enters the hole it has just cut, friction can drag the slug upward instead of clearing it downward. The slug then travels with the strip and appears inside a finished zipper, sometimes inside a closed slider body. The defect is not dimensional, so no gauge finds it; only die design, vacuum extraction and a scheduled punch check control it.
A cracked bend is the slowest failure of all. The crack starts on the outside of the bend where the strip was work-hardened, grows with each slider pass, and separates the head from the legs after a few hundred cycles. The user reports a zipper that will not close, and the tooth that caused it is usually lost by then.
Burrs are the most underrated defect in metal zippers. They survive polishing, they survive plating, and they appear in the finished garment as a rough edge that snags lining fabric or scratches the wearer. In children's wear they are a safety issue, and in technical outerwear they abrade the seam tape from the inside.
Loose teeth pass a visual inspection because the tooth looks correct. They fail in the laundry, where agitation works the tooth off the tape, and they fail at the garment factory incoming inspection, which is the first place the problem becomes visible to the buyer. By then the plating line has processed the whole order.
Off-centre heads and height variation show up as slider drag and as uneven closing force. A slider that has to be forced wears its channel, and a worn slider is the complaint the consumer actually makes. The root cause is two operations upstream.
How Buyers Can Judge
- Feel the chain edge with a cotton glove along its full length. A burr will catch the fibre. This simple test finds more than a magnification check on a short sample.
- Measure tooth height at twenty consecutive teeth and calculate the range; a spread above 0.08 mm indicates press or strip problems.
- Pull teeth to destruction and record where they fail: tape, clamp or tooth body. The failure location tells you which operation is out of control.
- Check for tape cut-through by opening the clamped leg with a probe and inspecting the warp yarns underneath.
- Ask for die maintenance records with the replacement interval and the measured wear at change.
- Confirm slug control: closed dies, air blow-off and a documented check for loose punches. YCC treats punched slugs as a foreign-matter risk and controls them at the press rather than at final inspection.