The detection aperture of a needle detector consists of tunnel height and tunnel width, and selection should start from the actual dimensions of the product as it passes through, not from nominal specifications. Measure the longest, widest and tallest parts of the product, and allow for packaging bags, trays, hang tags and other attachments that add to the size. An aperture that is too small prevents smooth passage or causes frequent scraping, while an oversized aperture spreads the magnetic field over a larger volume and can reduce the ability to detect small metal contaminants, so a balance between throughput and detection sensitivity is needed.
When determining aperture size, clarify the orientation of the product as it passes the detection zone. Lying flat, standing upright or passing at an angle changes the height and width occupied by the product. Boxed goods usually pass with the base flat, while soft products such as garments and plush toys may stack higher. It is advisable to leave a modest margin so the aperture is slightly larger than the product outline while avoiding excessive clearance. For lines where several items pass together, calculate the total width and height of the batch and discuss custom tunnel sizes with the equipment manufacturer when necessary.
The conveying method also affects aperture choice. On belt-type needle detectors, belt width corresponds to tunnel width, and a safe gap should remain on both sides when products pass centered. For drop-through or pipeline structures, the size of the discharge opening should be considered. Where product height varies considerably between batches, an adjustable-height tunnel may be useful. Before purchase, provide actual product dimensions, pass-through orientation and daily output so the manufacturer can assess the match between aperture size and detection capability, and verify actual detection with standard test pieces of appropriate size during acceptance.