An auditor will not ask whether you have a metal detector. They will ask you to show that it works, every shift, and what you did the last time it did not. A reliable metal detector test routine is what turns a machine on the line into a food safety control you can defend. This guide sets out a practical routine for QA leads and production supervisors in food factories.
Your own HACCP plan, customer requirements and certification standard (for example BRCGS or FSSC 22000) set the exact rules for your site. Use this as a framework to build or check your procedure against, not a replacement for it.
Why a metal detector test matters
A metal detector’s sensitivity can drift. Settings get changed, products change, a reject device sticks, a belt is replaced. None of this is visible from the outside: the machine keeps running and the light stays green. Regular testing is the only way to know the detector still finds what it should, and that the reject system actually removes the contaminated pack.
If a test fails, the records also tell you which product was made since the last good test, which is the product you need to hold and re-check.
Test pieces: one for each metal type
Metal detectors respond differently to different metals, so you test with one of each:
- Ferrous (iron and most steels): the easiest to detect.
- Non-ferrous (such as brass, aluminium and copper).
- Stainless steel: usually the hardest to detect, especially grade 316, and the most common metal in food factories because the equipment is made of it.
Test pieces are certified spheres of known diameter, usually set in a plastic card, rod or capsule and colour-coded by metal. Their sizes should match the sensitivity your detector has been validated to achieve with that product. A conveyor metal detector machine such as the one Welpack supplies is specified at ferrous 0.8 mm, non-ferrous 1.0 mm and stainless steel 1.5 mm, but the real figure always depends on the product, the pack and the aperture, so each product is validated on its own.
Keep test pieces clean, identified and controlled. A lost or damaged test piece is itself a foreign-body risk.
When to test
Most procedures test at these points:
- at the start of production, before any product is released;
- at regular intervals during the run, set in your HACCP plan;
- at every product changeover, because sensitivity depends on the product;
- after any maintenance, adjustment or stoppage of the detector;
- at the end of production.
The interval during a run is a risk decision. The longer the gap, the more product you may have to hold if a test fails. Many food sites test at least every hour; your customers or certification scheme may set a specific frequency.
How to pass the test pieces through
- Test with product. Place the test piece in or on a real pack, because the product itself affects detection. Wet, salty or frozen products are harder to inspect than dry ones.
- Use the least sensitive position. Sensitivity is lowest at the centre of the aperture, so the test pack should carry the test piece through the middle, not along the edge.
- Test each metal type separately, one pass at a time.
- Check the reject, not just the alarm. The test is only passed if the contaminated pack is actually removed into the reject bin. A detector that beeps while the pack carries on down the line has failed.
- Test at normal line speed, in the normal product flow.
Where the detector feeds a lockable reject bin, check that the bin is locked and that only authorised people hold the key. Rejected product should be investigated, not quietly put back.
What to record
Each check should leave a record that shows:
- date, time and line;
- product and pack size being run;
- each test piece used (metal type and size) and the result: detected and rejected, yes or no;
- the name or initials of the person who tested;
- any corrective action taken.
Records should be reviewed and signed off by a supervisor or QA. Modern detectors store product settings (the Welpack detector holds up to 100) and many log events electronically, which makes records easier to keep complete.
What to do when a test fails
- Stop the line or divert product away from release.
- Hold all product made since the last successful test. This is why the testing interval matters.
- Find the cause: wrong product setting, changed sensitivity, a faulty reject device, the wrong test piece, or a hardware fault.
- Fix it and re-test before restarting production.
- Re-inspect the held product through the working detector.
- Record everything: the failure, the cause, the fix and what happened to the held product.
Combining weight checks and metal detection
Many lines check pack weight at the same point. A checkweigher and metal detector combination unit does both on one conveyor, saving floor space. Welpack’s combi unit weighs from 12 g to 3,000 g in 0.1 g steps at up to 320 products per minute. The checkweigher needs its own routine checks with calibrated test weights, recorded the same way.
Frequently asked questions
Who should carry out the test?
Trained line operators usually do the routine checks, because they are at the line when the checks fall due. QA sets the procedure, trains the operators, reviews the records and investigates failures. Whoever tests must know what a pass looks like, including the reject.
What sensitivity should we validate to?
The smallest test piece sizes the detector reliably finds and rejects with that product, in that pack, through that aperture. Your customers or certification scheme may set targets. Validate each product when it is introduced and whenever the product, pack or line changes.
Can a detector inspect products in foil or metallised film?
Metallised film makes conventional detection difficult and may need multi-frequency detection or a different inspection method. Tell your supplier about the packaging before you choose a detector.
Keep the detector performing
Testing shows whether the detector works; maintenance keeps it that way. Keep the aperture and belt clean, check the reject device and its air supply, and have sensitivity and calibration checked periodically by a technician. Welpack supplies, installs and calibrates metal detectors and checkweighers across Malaysia from Puchong, Selangor. Ask us about inspection equipment or calibration.