(Editor’s Note: This is an online-only article attributed to the June/July 2018 issue.)
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With over 23 years’ experience in food manufacturing, including holding various roles in quality assurance for several Fortune 500 companies, the one thing that always surprises me is that many manufacturing facilities still get customer complaints for metal in their products even though the companies have systems in place to prevent foreign material contamination. It begs the question, how can metal get through their process with metal detection equipment in place?
How Do Metal Detectors Miss Metal?
There are many ways that a piece of metal can make it through a manufacturing system if the proper systems are not put in place. Below are just a few examples of the most common system failures that may have caused metal to make it to a customer.
One of the most preventable failures tends to be the timing being off on the rejection system, which would allow product to pass or not get rejected. This can happen due to many factors. For instance, in air reject systems, the air pressure could be too low or the air blast not effective for the product to fully reject. Also, the timing for the rejecting system could be set up incorrectly. Be sure to set up a testing protocol to have one of your test spheres placed at the leading edge of the product and one product with the test sphere placed at the trailing edge of the product. This will assure your timing covers the whole range of the product length depending on where the metal is located in the product. Many manufacturers, such as Anritsu Infivis, have systems in place to assure contaminated products do not pass. For example, photo eyes assure that product is actually rejected in the rejection bin if the metal detector triggers a contaminated product. Or an alarm sounds if the air pressure gets low in the reject system. These features are ideal but there are many systems and options available to reject product from the metal detector so examine your own setup to determine the best method and ask your metal detector suppliers what they would recommend for your specific application.
Another failure could occur from a large piece of metal, such as a large bolt, making it past the reject mechanism. This occurs because a large piece of metal may be sensed by the machine at a different time than a small piece of metal and this would affect the timing of the machine for rejection. When a large piece of metal approaches the aperture, it could be detected earlier than your standard metal test sphere and if the timing delay is not long enough for the reject mechanism, it could reject other product before the actual metal piece gets there. This is how it could be missed and make it to the customer. The only way to assure this does not happen is to test a large piece of metal as part of your protocol. Many companies use a larger bolt or metal washer as part of their test procedure to replicate potential risk factors. Walking through a manufacturing line, you may find a spot where a nut or bolt is missing. It may have been removed intentionally but if not, how would you know where it will be found? Hopefully, not in transit to a customer. This is why it is important to review your lines daily for missing parts before product leaves your facility.