How We Test Magnetic Closure Strength

How We Test Magnetic Closure Strength: Inside Our QC Process

Factory-side strength testing answers a different question from buyer-side inspection — it asks whether the process is staying in control, not whether one batch passed. This guide walks through our four testing stages: incoming magnet verification, in-process assembly checks, finished-box measurement with the real product loaded, and simulated transit where the channel justifies it — plus the six questions to ask any supplier.

Factory Testing vs Buyer Testing

There are two sides to strength verification, and they answer different questions. Buyer-side testing asks “did this batch meet my threshold?” Factory-side testing asks “is my process staying where it should be?”

Table 1: Differences between factory-side and buyer-side testing — the two are complementary and cannot replace each other.

We have published the buyer-side method separately in how to test magnetic box closure strength. This article covers what happens on our side before shipment.

Stage 1: Incoming Magnet Verification

Testing the finished box cannot recover from the wrong magnet. Incoming verification catches problems before they become waste.

Stage 2: In-Process Assembly Checks

Most closure failures trace to assembly rather than to the magnet. These are the points we check during the run.

Table 2: Five checkpoints during assembly. Most "insufficient magnetism" complaints ultimately trace to one of them.

Stage 3: Finished Box Testing

Finished testing measures the assembled result rather than its parts. Three measurements, taken with the real product loaded wherever the customer supplies it:

1. Rest gap — measured at several points across the closing width. Any measurable gap indicates spring-back that will worsen in transit.
2. Pull force — force gauge on a fixtured base, applied perpendicular to the closing faces, recording peak release force.
3. Cycle test — repeated opening and closing followed by a repeat of the two measurements above, to detect degradation rather than only initial state.

Stage 4: Transit Verification

Where the shipping channel justifies it — fragile contents, new distribution route, or a documented requirement — finished boxes go through a simulated transit sequence rather than relying on static measurement alone.

• Sequence: per the applicable ISTA or ASTM procedure for the parcel class.
• Configuration: inside the actual outer shipper, with the real product and the real void fill.
• Measurement: before and after each stage, so degradation can be attributed rather than merely detected.

The before-and-after structure is what makes the result useful. A box that passes but loses a third of its retention during vibration will fail in the field; a simple pass/fail report hides that. For detail on reading these results, see magnetic box shipping tests.

What Gets Recorded

Records exist so that a question six months later can be answered with evidence rather than recollection. Per production batch we retain:

• Incoming magnet lot identification and verification result.
• In-process check results at defined sampling intervals.
• Finished-box measurements — rest gap and pull force — with the method noted.
• Approved reference samples, retained for comparison against future batches.
• Any transit test report where the customer requested one.

What to Ask Your Supplier For

These six questions separate a supplier with real process control from one relying on final inspection alone. They cost nothing to ask.

1. “How do you verify incoming magnets?” Looking for functional comparison, not just a certificate.
2. “How do you check magnet seating depth?” This is the highest-frequency assembly defect.
3. “What do you measure on the finished box?” Should be numbers with a stated method.
4. “Do you test loaded or empty?” Empty tells you almost nothing useful.
5. “What records do you keep per batch?” Retained references enable long-term consistency.
6. “Can I see the actual data from the last batch like mine?” The answer is informative either way.

Conclusion

Factory-side strength testing is process control, not a certificate. Verifying magnets on arrival, checking seating depth through the run, measuring the loaded finished box before and after cycling, and keeping retained reference samples is what keeps a third order matching the first one — which is the part that actually matters once a design is in production.