Q: What metal fragment size should needle detection reject?
Sensitivity is expressed as the smallest ferrous sphere the machine will detect at a given aperture and belt speed. A common production setting on a zipper line is around 1.0 to 1.5 millimetres for ferrous metal, with a slightly larger figure for stainless steel, which is less magnetic. The correct figure is the one that satisfies your customer's specification and the risk assessment for the product.
The claimed sensitivity only holds for a defined aperture and a defined position in the aperture; a fragment near the edge or a thick folded assembly can escape detection. Test at the centre and near the edges of the aperture with certified test pieces, and confirm that the reject fires in each position. Non-ferrous fragments, including aluminium and brass, are not reliably caught by this technology, so the process must also control needles, blades and tools.
Acceptance means the machine detects every test piece at the agreed size at every position, at the production speed, at the start and end of the run. Record the size tested and the result, not just a tick in a box.