What Detection Does

Needle detection is the last control before packing. Every finished zipper passes through a metal detector or an X-ray unit, and any piece that triggers the alarm is quarantined and inspected. The purpose is not to find defective zippers; it is to prove that no fragment of needle, blade, staple or tool has travelled through the process inside a product.

Foreign matter is a safety issue rather than a quality issue. A three millimetre fragment of needle in a children's jacket is a recall, a regulatory notification and a reputational event, and no amount of downstream inspection recovers the cost.

Equipment and Key Parameters

Conveyor-type metal detectors with a tunnel aperture are standard. X-ray is used where product geometry or metal mass defeats a detector, or where the buyer requires it for plastic components.

  • Sensitivity: typically 1.0 mm ferrous, 1.2 mm non-ferrous and 1.5 mm stainless steel for a garment zipper.
  • Aperture size: matched to the product. A larger aperture reduces sensitivity, so the smallest workable tunnel is used.
  • Belt speed: typically 15 to 30 metres per minute, slow enough that every piece passes through the centre of the field.
  • Test frequency: certified test pieces at the start, middle and end of every shift, and after any adjustment.
  • Rejection handling: a locked bin, individual re-scan and a signed record for each rejected piece.
  • Needle control: numbered needles, issue and return logs, and a documented search for every break.
  • Record retention: typically three years, open to buyer audit.

Detector validation is product specific. A unit that reliably finds a 1.0 mm ferrous sphere in free air may not find the same fragment inside a bundle of metal zippers, because the product itself saturates the field. The only meaningful validation is a challenge test with the actual product in the actual packing configuration, and the result should be recorded as a sensitivity for that item rather than for the machine.

Common Defects

  • Missed fragments. A detector that is out of calibration, or a fragment oriented in the least sensitive direction.
  • False rejects. Plated sliders and metal teeth legitimately trigger the detector, so the unit must be set to ignore the product and detect only anomalies.
  • Unrecorded needle breaks. The most serious control failure. A break without a search means the fragment is somewhere.
  • Contaminated bins. Rejected pieces returned to production without individual re-scan.
  • Test piece drift. Test pieces that have been lost, damaged or replaced with unverified ones.
  • Bypass during maintenance. The detector switched off and not restarted, or an alarm silenced to keep the line running.

How the Defect Shows Up Downstream

Handling after detection is the weakest link in most factories. A piece that is scanned, then carried across an open floor, then packed at a bench that also handles returned goods, is no longer controlled. The rule that makes detection work is simple: the last operation before the carton is sealed is the scan, and nothing is added to the carton afterwards without a record.

False rejects carry a second cost that is rarely counted. When a line rejects 2 percent of output for no reason, the inspection team stops investigating each reject and starts releasing them in batches, and a real fragment can leave the factory inside a batch that was waved through. The control fails not at the detector but in the response to nuisance alarms.

A missed fragment shows up at the consumer, which is the worst possible place. It can injure a wearer, it triggers a recall that costs more than the entire order, and it usually reveals that the detector was working but the test pieces were not being used.

An unrecorded needle break shows up in an audit. A brand compliance team asks for the needle log, finds a gap, and the factory loses its approval, not because a fragment was found but because the control cannot be demonstrated.

False rejects show up as cost. If plated zippers trigger the detector continuously, operators start waving pieces through or turning down sensitivity, and the control degrades quietly until the next audit.

How Buyers Can Judge

  1. Watch a challenge test during the audit: the operator passes the test pieces and you see the alarm and the record.
  2. Read the detector log for the last month and look for gaps in shift coverage.
  3. Check the needle log against the issue records; every break must have a search entry and a signature.
  4. Inspect the reject bin and confirm it is locked and that each piece has a re-scan record.
  5. Verify the test pieces are certified and within their validity date.
  6. Ask for the detector or X-ray validation report for the specific product, and confirm the sensitivity claim was demonstrated on that product rather than on a generic sample. YCC runs detection after packing preparation so that the last handling step cannot reintroduce a fragment.
  7. Confirm the aperture is the smallest that fits the product; a tunnel sized for cartons will not hold garment-level sensitivity.
  8. Check the quarantine route: a rejected piece should never travel back through the production line without a signed release.

Ask for the detection record of a specific carton and follow it back to a shift, a line and an operator. If that trace takes more than a few minutes, the records exist but the system does not.