Quality

Sock Metal Contamination Control in Production

Published: 2026-07-05By ZheSock TeamReading time: 8 min

Metal contamination in socks is rare, but one broken needle tip can stop a shipment. It can also cause a chargeback, a warehouse hold, or a product safety complaint. Sock metal detection is not just one pass through a packing line machine. It needs source control at knitting, trimming, boarding, packing, and final carton release. A procurement manager should write the rule into the RFQ, the purchase order, and the approved sample file. At ZheSock in Datang, Zhejiang, we produce custom export socks from a 100 pair MOQ for trial orders. Typical sampling takes 7 to 10 days after the artwork, size, yarn, and logo method are confirmed. Bulk production usually takes 20 to 35 days after approval. Common unit prices run from USD 0.35 to 1.80 per pair, based on yarn, needle count, construction, and packing.

Table of Contents

1. Where metal contamination usually starts in sock production
2. What buyers should put in the purchase order
3. How sock metal detection is run on the factory floor
4. Product types that need stricter metal control
5. Records importers should review before shipment
6. Cost, lead time, and what is realistic to ask

Where metal contamination usually starts in sock production

The knitting room is the main risk point. A sock machine may run 96, 108, 120, 144, 168, or 200 needles, depending on the sock type. Coarse thermal socks often use 96N or 108N machines. School socks and sports crew socks often use 144N or 168N. Fine dress socks often use 200N. Each machine also has sinkers, selector parts, yarn guides, screws, and small tools close to moving yarn.

When a needle breaks, the missing piece may be 2 to 8 mm long. It can hide inside a terry loop, sit near the toe seam, or fall into a work basket. Thick socks carry more risk because the pile can cover a small fragment. A heavy terry sport sock may be around 320 to 520 GSM by finished fabric estimate. A thin cotton dress sock may be around 180 to 260 GSM. Risk is not theory. One missing part should stop the related machine output until the factory finds the full broken piece or isolates the affected bundles.

Other sources are plain but costly. Staples from yarn cartons, trimming blade tips, broken hand sewing needles, loose screws, and metal clips from packing tables can enter work in progress. The fix starts with discipline. No staples in open production areas. No open blade boxes at trimming tables. Blades are issued and returned by count. Scissors and hand tools are tied to a station. Broken needles are logged by machine number, operator, time, size, color, and bundle number.

High risk source: knitting needles, sinkers, and hand sewing needles used for toe closing repair.
Medium risk source: trimming blades, brushing machine parts, boarding pins, and loose machine screws.
Low but real risk source: carton staples, packing table clips, damaged hang tag pins, and metal rulers.

For an RFQ, ask the factory how it controls each source. A useful answer gives check points and records. A weak answer only says the factory has a detector.

What buyers should put in the purchase order

Do not ask only, "Do you have a metal detector?" Ask for a written metal control plan. The plan should state the check points, sensitivity targets, reject handling, record format, packing release rule, and sign off duty. Put the rule on the purchase order before yarn booking. Changing the rule after packing can add USD 0.01 to 0.05 per pair and 2 to 5 working days if cartons must be opened.

MOQ: from 100 pairs for a trial style, with practical color MOQ often 300 to 500 pairs for dyed yarn or special Pantone matching.
Normal sampling lead time: 7 to 10 days after artwork, size, yarn, and logo method are confirmed.
Normal bulk lead time: 20 to 35 days after sample approval and deposit, longer for dyed yarn or retail packaging.
Common FOB price range: USD 0.35 to 0.80 for basic ankle socks, USD 0.70 to 1.30 for terry sport socks, USD 0.90 to 1.80 for wool blend or anti-slip socks.
Inspection level: AQL 2.5 for major defects and AQL 4.0 for minor defects is common for export socks, unless the buyer sets a stricter rule.
Metal detection rule: define 100 percent checking or batch checking, with the exact stage and records required.

For baby socks, anti-slip socks, thick terry socks, brushed socks, and chain store orders, require 100 percent metal detection before carton sealing. For lower risk promotional orders, the buyer may accept batch detection plus final AQL inspection. The rule still needs to be written.

A clear purchase order should include acceptance criteria. State that no broken needle fragment, blade piece, staple, loose screw, or other metal foreign matter is acceptable. State that any metal alarm must be investigated. The affected goods must be isolated, searched, rechecked, and released by a supervisor. If a broken needle cannot be fully recovered, all bundles from the last confirmed good point should be held for 100 percent detection and visual review.

Add sample approval steps too. The pre-production sample should be approved for yarn, size, logo, grip print if used, packing, and metal detection feasibility. If the sock includes lurex, metallic yarn effect, foil print, metal clips, or a gift box magnet, the packed format must be tested before bulk approval. A sample can look good and still fail the detector. That is a commercial risk.

How sock metal detection is run on the factory floor

A textile metal detector uses a tunnel and conveyor belt. Socks pass through the tunnel. The machine alarms when the signal changes beyond the set limit. Common tunnel widths for socks are 400 to 600 mm. Belt speed is usually 20 to 30 meters per minute. For thick terry bundles or packed sets, speed should be reduced so the product is not stacked too high inside the tunnel. One thin layer is better than a fat bundle.

Detector checks should be made with test pieces, not by guesswork. A practical target for socks is 1.0 mm ferrous, 1.5 mm non-ferrous, and 2.0 mm stainless steel when the product is dry and not packed with metal trims. Some thick or damp socks create more product signal, so the real setting must be verified on the production style. Wet dyed socks should be dried and conditioned before final checking. The acceptance rule should be based on the approved product, not only the empty belt.

The test routine should be simple. Check the detector before the first batch, every 2 hours during running, after any machine stop, after product change, and at shift change. Place the test piece at the left, center, and right side of the belt. Put the test piece on top of the socks, inside the socks, and under the socks during validation for thick terry or baby styles. Record pass or fail with the operator name and time. If the detector fails a test, stop the line. Then recheck all socks passed since the last valid test.

Reject handling needs a locked process. When the alarm sounds, the operator should stop the belt, remove the bundle, attach a red hold tag, and place it in a reject box. The bundle should not go back to normal packing by hand decision. A supervisor opens the bundle, searches for the cause, records the result, and sends the goods through the detector again. If metal is found, the record should show the part type, size estimate, and where it was found. If no cause is found, the whole bundle should be checked again at least two times, then held for quality manager release or rework.

Normal pass rule: product passes detector at the agreed setting with no alarm.
Alarm rule: affected bundle is isolated and recorded, then searched and rechecked.
Machine fail rule: stop detection, repair or recalibrate, then recheck goods passed after the last good test.
Moisture rule: do not use final detection on damp socks unless the setting has been validated for that condition.

Product types that need stricter metal control

Risk changes by construction. A 144N flat knit promotional ankle sock has less space to hide a fragment than a 168N terry crew sock. A 200N dress sock uses finer needles, so small damage may show as a hole or ladder earlier. A 96N thermal sock has larger loops, which can trap a larger piece if source control is weak.

Thicker socks need slower handling and clearer bundle control. For brushed socks, the brushing machine adds another metal contact point. For anti-slip socks, the silicone printing area needs control because socks move through extra trays and curing racks. For socks with lurex, metallic yarn effect, or metal-look accessories, the supplier must test the finished construction before production. Some trims may trigger alarms and make standard detection impractical.

For orders above 3,000 pairs per color, add a metal control line to the tech pack or order sheet. For small runs from 100 to 500 pairs, keep the same safety rule, but agree on the record depth. A one-day trial batch may only need a needle log, detector test log, and final release note. A 10,000 pair order should have daily records tied to carton numbers.

Baby socks and toddler socks deserve the strictest rule. Many buyers require no staples in inner packaging, no metal clips, no loose hang tag pins, and 100 percent detection before carton sealing. Anti-slip socks need the same care because extra handling happens after knitting. Slipper socks with heavy terry and silicone grip print often need a slower belt speed and smaller bundle height. Do not rush this stage.

Commercial trade-offs should be discussed before quotation. Metallic yarn effect may limit detector sensitivity. Gift packing with magnets can make final packed detection impossible. Retail hangers with metal hooks may need detection before hanger assembly, then a separate packing audit. If the buyer wants detection after every retail pack is complete, the factory may need different accessories, higher labor cost, or a longer schedule.

Records importers should review before shipment

A useful record file connects knitting, inspection, packing, and cartons. A photo of the detector is not enough. The buyer should be able to trace a rejected bundle back to a machine section and production date. If that link is missing, the record will not help when a claim appears. Ask for the file before shipment booking, not after the vessel closes.

Needle breakage log showing date, shift, machine number, operator, missing part status, and affected bundle.
Broken needle recovery record showing whether all parts were found, with supervisor signature.
Detector sensitivity log showing test piece size, time, belt result, and product style.
Reject log showing alarm time, bundle number, reason found, recheck result, and carton action.
Final inspection report using the agreed AQL level, usually AQL 2.5 major and AQL 4.0 minor for standard export socks.
Packing list showing carton number, color, size ratio, pairs per carton, gross weight, and net weight.
Carton release sheet showing which cartons passed final detection or packing audit.

For example, a 10,000 pair order packed 12 pairs per polybag and 240 pairs per carton will ship in about 42 cartons. If one carton is held, carton-level traceability limits the recheck. Without carton numbers, the factory may need to reopen the full lot.

Acceptance criteria should be stated in the inspection booking. Metal contamination is a critical defect. One confirmed metal fragment should trigger lot isolation and a written corrective action. The buyer and factory then decide the recheck scope based on traceability. If the affected machine, time, and bundle range are clear, the hold may cover only a defined group. If the record is weak, the safe choice is 100 percent recheck of the full lot.

For third party inspection, add simple packing checks. Inspectors should confirm carton numbers, shipping marks, size ratio, barcode labels if used, polybag warning text for children's items, inner pack count, and carton sealing method. They should also check that no staples are used on inner bags or retail packs. If master cartons use staples, they must not be loose, raised, or broken. Tape sealing is safer for many sock programs.

Cost, lead time, and what is realistic to ask

Metal control adds labor, but it should not double the cost. For standard cotton, polyester, or wool blend socks, routine detector use with proper logs often adds about USD 0.01 to 0.03 per pair. If the buyer requires pair-by-pair checking before retail carding and another pass after retail packing, the added cost can reach USD 0.04 to 0.08 per pair because handling time increases.

Lead time impact depends on when the rule is set. If sock metal detection is included from the start, it usually fits inside a normal 20 to 35 day bulk schedule. If the rule is added after goods are packed, add 2 to 5 working days for carton opening, rechecking, repacking, and carton relabeling. Rework takes longer than detection.

For private label buyers, the best request is short and clear. State the detection point, sensitivity target, record requirement, reject rule, packing check, and AQL level on the purchase order. Ask for records before shipment booking. If the product is for babies or a major retailer, request 100 percent detection before carton sealing and keep the logs with the shipment file.

There are also price choices. A basic promotional sock with 500 pairs per color may not support heavy paperwork at the same unit price as a 20,000 pair repeat order. A retailer program with carton traceability, pair-by-pair detection, barcode checks, and photo records needs more labor. Pay for the control you need. Do not pay for vague promises.

A practical RFQ can ask for four items: unit price with standard metal control, unit price with 100 percent detection, added cost for retail pack detection, and added days if final detection must be done after packing. This lets procurement compare suppliers fairly. It also avoids arguments after production starts.

Frequently Asked Questions

What is sock metal detection?

Sock metal detection is the process used to find broken needles, blade pieces, staples, screws, and other metal fragments in socks before shipment. It includes knitting room needle logs, broken part recovery, detector checks, reject records, packing controls, and final carton release. The machine matters. The records matter too.

Can a metal detector find every needle fragment in socks?

No. Detection depends on metal type, fragment size, sock thickness, moisture level, packing format, and belt speed. A 1.0 mm ferrous piece is easier to detect than a small stainless steel piece inside a thick terry sock. Buyers should require needle control at knitting and final detector checks before release.

When should socks be checked for metal?

For many export socks, checking after boarding and before packing is practical. For baby socks, anti-slip socks, brushed socks, and thick terry socks, use 100 percent checking before carton sealing. If retail packing uses hang tags, cards, magnets, or clips, test the packed format before mass packing.

What documents should I ask a sock factory for?

Ask for the needle breakage log, broken needle recovery record, detector sensitivity log, reject log, final AQL inspection report, carton release sheet, and packing list. Records should show dates, machine numbers or lot numbers, operator names, and carton references. For a 7 day production run, expect daily records, not one generic form.

Does sock metal detection increase the unit price?

Usually yes, but the increase is small when planned early. Routine detection and record keeping often adds about USD 0.01 to 0.03 per pair. Pair-by-pair rechecking after retail packing may add USD 0.04 to 0.08 per pair and 2 to 5 working days if cartons must be reopened.

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