A Lab Is a System, Not a Room

Many zipper factories have a room with a tensile machine, a light box and a salt spray cabinet. Far fewer have a laboratory: a place where methods are defined, equipment is calibrated, samples are prepared consistently, results are recorded against a lot, and failures trigger action. The audit question is not whether the equipment exists, but whether the numbers it produces can be trusted and used.

Start by asking what the laboratory is for. If the answer is "to satisfy customers", the lab is a marketing function. If the answer is "to decide whether a lot ships", it is a control function.

Test Scope: What Must Be Covered

Write down the tests the laboratory claims to perform and compare them with the tests your product actually requires.

  • Physical: slider pull-off force, holding force, stop strength, tape tensile strength, elongation, width and thickness.
  • Durability: reciprocation cycles, drop test, corrosion after salt spray.
  • Colour: light box assessment, colour difference measurement, colourfastness to rubbing and washing.
  • Chemical: restricted substance screening, pH, and where relevant nickel release.
  • Ageing: heat or humidity exposure followed by functional retest.

Ask which of these are performed in-house and which are sent out. Outsourcing is acceptable; not knowing which tests are outsourced is not.

Equipment and Its Condition

Look at the equipment, not the equipment list. A tensile machine under a dust sheet is not capability.

  • Tensile or universal testing machine with the correct load cell for the force range in use.
  • Calibrated force gauge for slider pull-off, with fixtures that hold the part without damaging it.
  • Reciprocation tester with a cycle counter that can be verified.
  • Salt spray chamber with a record of solution concentration, pH and temperature.
  • Light box with standard illuminants and a lamp-hour record.
  • Spectrophotometer, if colour is judged numerically.
  • Micrometer and gauge blocks for dimensional checks.

Ask to see the last calibration certificate for the tensile machine and the force gauge. Then ask what the factory does when a certificate expires before the next calibration is booked.

Calibration and Reference Standards

Calibration is the difference between a laboratory and a workshop with instruments.

  • Calibration register listing every instrument, its interval, and its last and next date.
  • Accredited calibration provider identified on the certificate, with the scope of calibration stated.
  • Reference standards for force, dimension and colour, stored correctly and replaced on schedule.
  • Intermediate checks between calibrations, such as a daily check with a known mass or a reference slider.
  • Handling of instruments found out of calibration: which results are invalidated and how far back.

That last item is the one that separates mature laboratories from compliant ones. Ask what happened the last time a gauge was found out of tolerance, and how many lots were re-tested.

Methods and Sample Preparation

Ask for the written method for the pull-off test, then watch it performed. The gap between the document and the bench is the audit result.

  • Method reference, sample length, clamping position, test speed and number of specimens.
  • Conditioning: temperature, humidity and minimum time before testing.
  • Sample selection: whether parts come from the current lot and whether they are randomly drawn.
  • Handling of parts that fail abnormally, such as slippage in the clamp.
  • Whether operators are trained on the method and whether training is recorded.

Records and Reporting

A result that cannot be linked to a lot is an anecdote. Ask for the last ten test reports and look for the lot number on each.

  • Report includes part number, lot number, date, method, sample size and result.
  • Results are compared with a specification limit, not merely recorded.
  • Out-of-limit results trigger a documented action, with the disposition of the lot recorded.
  • Reports are retrievable for the retention period the customer requires.
  • Data is entered into a system rather than kept only on paper in a drawer.

Independence and Escalation

Ask who the laboratory reports to. A laboratory that reports to the production manager will rarely stop a shipment.

  • Reporting line, and whether the laboratory can hold a lot without production approval.
  • Written authority to reject, and a recent example of it being used.
  • Whether laboratory results are reviewed at a management meeting, and how often.
  • How the laboratory participates in complaint investigations.

A passing answer includes a specific occasion when the laboratory stopped a lot and the production team disagreed. A laboratory with no such story has never been tested, or has never been independent.

Proficiency Testing and External Checks

  • Participation in inter-laboratory comparison or proficiency schemes, with results on file.
  • Correlation with customer or third-party laboratory results, and how differences are investigated.
  • Visits or audits by external laboratories, and the findings raised.
  • Whether the factory sends a duplicate sample out periodically to confirm its own results.

The Actionable Audit Checklist

  • Written test scope exists and matches the product requirements.
  • In-house versus outsourced tests are clearly identified.
  • Every instrument carries a current calibration certificate.
  • Intermediate checks are performed between calibrations.
  • Written methods exist for the main functional tests.
  • Conditioning of samples is controlled and recorded.
  • Test reports carry lot numbers and specification limits.
  • Out-of-limit results show a documented disposition.
  • Laboratory has written authority to hold a lot.
  • At least one recent example exists of a lot being held.
  • Proficiency or correlation results are on file.
  • Records are retained for the required period and are retrievable.

What a Passing Answer Looks Like

Passing sounds like this: "The pull-off gauge was found low by four percent at calibration, so we re-tested every lot from the previous quarter, found two below limit, and downgraded both." That answer shows a laboratory that follows calibration into consequence. It is the kind of answer YCC looks for before accepting in-house test data in place of third-party reports.