What Outgoing Inspection Is For

Outgoing inspection verifies that a finished lot meets the agreed acceptance criteria before it leaves the building. It is a confirmation step, not an improvement step. No outgoing inspection has ever made a zipper better; it has only prevented a bad lot from shipping and, when it is well designed, produced evidence about which process needs attention.

Confusing the two purposes leads to inspection being used as a substitute for process control. The symptom is a growing outgoing inspection team and a flat complaint rate.

Classify Defects Before Choosing a Level

An AQL is meaningless until defects are classified, because the level is chosen per class, not per product. A workable classification for zippers:

  • Critical: a defect that could cause injury, an unsafe condition or a regulatory failure. Sharp edges on a puller, a slider that releases under load on a child's garment, or a restricted substance above a limit all belong here. The acceptance number for critical defects is normally zero, regardless of lot size.
  • Major: a defect that makes the product unusable or unacceptable to the end user. A non-functioning slider, a chain that splits, a missing stop, a wrong length or a wrong colour against the approved standard.
  • Minor: a defect that does not affect function or sale, but deviates from the standard. Slight plating discolouration in a hidden area, a minor tape edge irregularity, a packing label imperfection.

The classification must be written and illustrated with physical or photographic limit samples. Without limit samples, two inspectors will classify the same zipper differently and the sampling result will carry the disagreement rather than the lot quality.

Choosing the Level

Common practice assigns tighter levels to more severe classes and looser levels to cosmetic ones. A starting structure that many zipper suppliers use, subject to customer agreement, is:

  1. Critical defects: zero acceptance, verified by specific testing rather than by attribute sampling.
  2. Major defects: AQL 0.65 to 1.5, tighter for functional characteristics and for customers with automated assembly lines.
  3. Minor defects: AQL 2.5 to 4.0, looser for hidden areas and tighter for visible branded surfaces.
  4. Inspection level: general level II for routine lots, level III for a new supplier, a new product or a lot under tightened inspection.

The choice should reflect the consequence of escape, not the negotiating position of the customer. A zipper that stops an automatic sewing line is far more expensive to the buyer than its price, and the AQL should reflect that asymmetry.

What AQL Actually Means

AQL is the quality level that the sampling plan is designed to accept most of the time. Under the standard single sampling plans, a lot whose true defect level equals the AQL has a high probability of acceptance, by design close to ninety-five percent. It is not a target defect rate, and it is not a guarantee that shipped lots contain no more than that level.

Two consequences follow, and both surprise people new to the topic. First, at the AQL, roughly one lot in twenty is still rejected, which is expected behaviour rather than a supplier failure. Second, a lot materially worse than the AQL will still be accepted sometimes, because sampling cannot discriminate perfectly. The probability of accepting a lot at a stated worse level is read from the operating characteristic curve of the chosen plan.

Average Outgoing Quality

The average outgoing quality is the defect level in shipped material after inspection has removed rejected lots. It is calculated as the probability of acceptance multiplied by the incoming defect proportion, adjusted for the fraction of the lot inspected. The formula for a lot of size N with sample n is: acceptance probability, multiplied by the defect proportion, multiplied by the ratio of N minus n to N.

Plotting average outgoing quality against incoming quality produces a curve with a maximum. That maximum is the average outgoing quality limit, and it is the worst average quality that the plan can allow to reach the customer. The limit is a property of the plan, not of the process, which is a useful thing to know: no sampling plan can protect a customer from a process that is out of control, because rejected lots are assumed to be corrected before resubmission.

The Practical Design Steps

Designing outgoing inspection is a sequence, and skipping a step produces a plan nobody follows:

  1. Agree the defect classification with limit samples, signed by both sides.
  2. Agree the inspection level and AQL per class, and record them in the specification.
  3. Define the sampling method: random across cartons and layers, not from the top of one carton.
  4. Define the disposition rule for rejected lots: one hundred percent sort, rework, or hold pending a concession.
  5. Define the record: lot number, sample size, findings by class, decision, inspector and date.
  6. Define the feedback loop: any rejected lot triggers a documented internal notification to the responsible process step.

Verifying the Inspector, Not Only the Product

An inspection result is only as good as the inspector's judgement. Two practices protect it. The first is periodic agreement studies, where several inspectors classify the same set of sample zippers and the disagreement rate is measured. The second is a gauge or limit sample audit, confirming that the physical standards in use match the approved version and have not drifted through handling.

Where a customer operates a supplier scorecard, alignment of classification with that customer matters as much as internal consistency. A disagreement about whether a plating mark is major or minor can turn a passing lot into a complaint.

Publishing the Plan With the Specification

YCC publishes the outgoing inspection plan on the same document as the product specification, so that the levels, the limit samples and the disposition rules travel with the order rather than with the inspector. When a customer requests a tighter level, the change is visible to production before the run starts, which is the only point at which it can be acted on cheaply.