A metal detector finds metal only; X-ray finds metal plus glass, stone, bone, plastic and other density differences. Most food lines end up with both, but not for the reason people assume — the metal detector is the cheap, fast first pass, and the X-ray catches what it structurally cannot see.
| Contaminant | Metal detector | X-ray |
|---|---|---|
| Ferrous metal | Yes | Yes |
| Non-ferrous metal (copper, aluminium) | Yes, with reduced sensitivity | Yes |
| Stainless steel | Often marginal | Yes |
| Glass, stone, bone | No | Yes, by density |
| Hard plastic, rubber | No | Sometimes |
| Low-density or density-matched items | No | No |
Dry, free-flowing products with a well-defined metal risk and no glass or bone risk: flour, sugar, grain, some snacks. The metal detector is cheaper, faster and simpler to validate. If the HACCP study identifies metal as the only significant hazard, adding X-ray is cost without benefit.
Put the X-ray first where the product is thinnest and least packaged, and the metal detector after packaging — or the reverse if the packaging itself contains metal. The rule is: whatever cannot see through the package must sit before the packaging step.
Both systems need a defined reject mechanism (air blast, pusher, flap) and a record of every rejection. A common audit failure is a detector that rejects correctly but cannot show what happened to the rejected unit.
Can X-ray replace the metal detector? Usually not — X-ray is slower per unit and more expensive, and metal detectors are better at detecting small ferrous fragments in a thin product stream.
Do both need annual validation? Yes, with test pieces at the start and end of each production run, plus a documented annual check.
What about dual-energy X-ray? It adds material classification, which helps separate glass from dense product, but it does not change what is physically detectable.
X-ray inspection: food X-ray system. Metal detection: food metal detector.
