Surface Scratch Inspection Standard for Zipper Sliders

Symptom

Scratch disputes are rarely about whether a mark exists. They are about whether the mark matters. A buyer's inspector rejects a carton under strong directional light; the supplier examines the same parts under diffuse light and sees nothing. Both are honest, and both are describing a real optical phenomenon, which is why a scratch standard written as "no scratches" is unusable in practice.

On a plated slider, a scratch is a breach of the corrosion barrier as well as a cosmetic defect. A shallow mark that only disturbs the lustre will polish out in service, while a deep mark that exposes substrate will corrode and spread. The inspection standard must therefore separate cosmetic from functional, and it must specify the lighting and magnification under which the distinction is made.

• Linear marks visible at arm's length under normal office light.
• Marks that catch a fingernail, indicating depth rather than lustre change.
• Scratches concentrated on one face, suggesting handling or packing contact.
• Scratch density increasing from the start to the end of a production run.
• Marks that appear only after wiping, indicating residue-filled grooves.

Root Causes

Scratches originate in handling, in tooling, or in the finishing process itself. The location and direction of the mark usually identifies which.

• Bulk handling between stations. Parts tumbled in a bin scratch one another; the marks are short, random and concentrated on high points.
• Worn or damaged mould. A polished cavity that has picked up a burr produces a directional mark on every part from that cavity.
• Plating rack contact. Contact points and spring hooks leave marks at consistent locations, often with a brighter appearance because the deposit is thinner.
• Polishing and buffing. Coarse compound or excessive pressure leaves directional micro-scratches that become visible after plating.
• Packing contact. Loose parts in a carton rub during transport; the marks appear on the outermost surfaces and match the carton's movement pattern.
• Inspection handling. Parts poured onto a hard surface for inspection are damaged by the inspection itself, a self-inflicted defect that is often overlooked.

Troubleshooting Sequence

Standardise the viewing condition before judging any part. Without that, the inspection result depends on the inspector's lamp, which makes the data incomparable between shifts and sites.

• Define the viewing distance at 300 mm, the illuminance at 800 to 1200 lux, and the light source as diffuse white with no direct specular reflection.
• Inspect at 10x for functional judgement and at 1x for cosmetic judgement; record both.
• Use a fingernail test to classify depth: a mark that catches is functional, a mark that does not is cosmetic.
• Map the mark location on a part drawing and compare across the lot to identify a pattern.
• Compare a suspect part with a retained golden sample under the same lighting.
• Check the tumbling medium, the bin condition and the rack contact points.
• Inspect parts before and after packing to isolate transport damage.

Corrective Actions

For handling damage, introduce compartmented trays, reduce bin fill level and replace worn tumbling media. For tooling damage, polish the cavity and identify the burr source rather than polishing the parts. For rack marks, move contact points to non-visible surfaces and verify that the deposit at the contact is still within thickness specification.

For cosmetic marks that do not breach the deposit, the correct action is to accept them within a defined limit rather than to rework them. Reworking a cosmetic mark by local polishing thins the deposit and creates a genuine corrosion risk, which is a worse outcome than the original defect.

Prevention

Publish a photographic limit sample for each product family, produced under the specified lighting and signed by both parties. A written standard without a photograph will be reinterpreted at every audit, and the reinterpretation always costs more than the sample did.

Donglong Zipper inspects sliders on a soft mat with compartmented trays and requires gloves at final inspection, because most scratches on finished parts are created after the last process that could have caused them. YCC buyers should confirm that the supplier's inspection bench itself does not damage parts, since a scratch standard is meaningless if the inspection station is a source of scratches.

Review scratch rejects by location monthly. A defect that moves from random to patterned is a handling change; a defect that stays in one place is a tooling change. The distinction determines whether the fix belongs to production or to maintenance, and getting it wrong wastes both.

Audit the handling path, not only the bench. Walk the part from plating to packing and count every surface it touches: bin, chute, drying tray, sorting table, inspection mat. Most scratches originate at a transfer where parts fall from one container into another, and the mark is already present before the inspector picks the part up. Photograph the transfer points, replace hard plastic chutes with lined ones, and reduce drop height at every step. Then re-measure the reject rate; a handling audit usually removes more scratch defects than a tightening of the visual limit, and it does so without rejecting acceptable parts.

Train the graders against the limit sample once per quarter and check agreement between inspectors. When two people grade the same ten parts differently on more than one part, the standard is not yet clear enough and the limit sample needs another photograph or an annotation. Grader agreement is a measurable property, and it is the cheapest way to stop a scratch standard from drifting into personal preference.