Metal detectors
A metal detector is an inspection machine that finds pieces of metal in food or packs as they pass through it and triggers their rejection from the line. Metal can enter food from raw materials, worn or broken machine parts, screens, blades, wires, tools and maintenance work. Metal detectors are usually placed after packaging, so that nothing can enter the pack after inspection, and often also earlier in the line to protect expensive machines such as slicers and weighers or to catch contamination before value is added. In most food safety plans metal detection is a critical control point or an equivalent control, with regular testing and records. Most food metal detectors use a balanced-coil system. A transmitter coil creates a high-frequency field and two receiver coils sit either side of it in balance. When a metal particle passes through, it disturbs the field and the imbalance is detected. The machine can find ferrous metals most easily, non-ferrous metals such as aluminium and copper less easily, and stainless steel least easily, particularly in wet or salty products. Wet, salty, warm or frozen product can itself produce a signal, known as product effect, which makes detection harder. Modern detectors use several frequencies at once and digital processing to separate product effect from real contamination. Metal detectors come in several forms. Conveyor systems pass packs through an aperture on a belt and reject them with an air blast, pusher or retracting band. Gravity-fall systems inspect free-falling products such as powders, grains and snacks before packing, and pipeline systems inspect pumped products such as sauces, meat emulsions and liquids. Many detectors combine with a checkweigher on one frame. Where product effect or metallised film makes metal detection difficult, or where other foreign bodies such as glass and stone must be found, plants use X-ray inspection instead or in addition. Cost and performance depend on the sensitivity achievable in your product, aperture size, product effect, packaging material, reject system, fail-safe features, hygiene rating and the testing and data functions supplied.
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Buying criteria
Test sensitivity in your own product. Detection depends on product effect, pack size, orientation and aperture size, so ask for trials with test pieces in each metal type. Keep the aperture as small as the pack allows, since smaller apertures detect smaller particles. Check fail-safe design: reject confirmation, a locked reject bin, belt stop on faults, and alarms if the detector, air supply or reject device fails. Many retailer codes of practice specify these features. Look at testing routines, automatic test reminders, data logging and audit reports. Consider packaging. Metallised films usually rule out standard metal detection and point to X-ray inspection. Review hygiene rating, cleaning access and local service support. Ask whether the detector can be combined with a checkweigher to save floor space, and how its settings are protected from unauthorised changes.
Buyers ask
How much does a food metal detector cost?
It depends on the aperture size, conveyor or system type, sensitivity required, multi-frequency technology, hygiene rating and fail-safe features. Conveyor systems with full retailer-compliant fail-safes cost more than basic units. Pipeline and gravity systems, combination checkweigher units and data software also change the price.
What is product effect in metal detection?
Product effect is the signal a food produces in a metal detector because it is wet, salty, warm or conductive. It can look like metal to the detector. Operators and machines must compensate for it, which can reduce the size of metal that can be reliably detected.
Why is stainless steel hard to detect?
Stainless steel is non-magnetic and a poor electrical conductor compared with other metals, so it produces a weaker signal in a balanced-coil detector, especially in wet or salty products. Detectors therefore usually need a larger stainless steel particle than ferrous to trigger a reject.
Should I choose a metal detector or X-ray inspection?
Metal detectors are well suited to most unwrapped or plastic-wrapped products and cost less. X-ray inspection is preferred when packaging is metallised, when product effect limits metal detection, or when you also need to find glass, stone, bone or dense plastic. Some plants use both.