Metal detection in bulk inspection

Posted November 17, 2011 in Bulk Inspection, Metal Detection
Bulk Inspection

The role of metal detection in the food and pharmaceutical industries has changed dramatically over recent years. The Food Safety Act and the increasing demands imposed by supermarket retailers has led to food and pharmaceutical manufacturers installing factory-wide systems that comply with the Hazard Analysis and Critical Points standards (HACCP). These regulations introduced a systematic and preventative approach to food and drug production that would identify physical, chemical and biological hazards during the production process rather than in the finished product itself. The aim of the new standards was to identify potential food safety hazards and eliminate them before any risk could be realised. To comply with these new standards, metal detection has changed and new machinery has been developed. Food products are now not simply checked for metal contamination at the end of the production process, but monitored throughout. This is particularly important for food that is processed and manufactured in bulk quantities, like sweets and snack foods. These are usually not packaged until the end of production, and then often packaged in metallic wrapping to keep the product protected.

To ensure that such products comply with the HACCP standards, and reach the consumer free from metal and other contaminants, the products need to be checked at all stages of the production process where metal contamination could conceivably occur. The new type of metal detector should therefore form a seamless part of the free-flowing production process. Most of the products used in these instances are gravity fed or ‘free-fall’: this allows the product to be piped directly through the aperture of the metal detector search head

How the metal detector works

As product passes through the detector aperture, any metal in the product will produce an electromagnetic field similar in operating frequency to that used in radio broadcasting.

Inside the head are three coils that surround the aperture. The central transmitter coil is driven by a powerful oscillator that generates a high frequency field within the aperture. The two transmitting coils, which are connected so their induced voltages cancel, are spaced equally on either side and receive the signals produced by the transmitter coil. If perfectly balanced, this arrangement produces a zero voltage output. However, if any metal objects move into the aperture, they will interact with the field and send out an unbalanced signal. This signal is used to trigger an automatic reject device that removes the contaminated product from the line. In the case of gravity-fed products, contaminated product is rejected by means of a high-speed mechanism, which ensures that metal contaminants are diverted out of the product flow with the absolute minimum of product wastage.

The positioning of the metal detector in the production process is critical. Ideally it should be placed at the raw product stage of the process where the bulk ingredients are stored before being added to the process. The bulk product is delivered to a silo and is gravity fed through the metal detector. In HACCP terms, this is a critical control point, as the in-coming bulk ingredient is a potential source of contamination: if the product is not checked here, the quality of the end product could be compromised. It also has the benefit of being at the earliest point in the line, so contaminated product is rejected straight away, before value is added by processing. This also lessens the chances of the contaminant dispersing through more of the product and damaging delicate machinery. The final point for the inspection of snack foods is in between the vertical form-fill-seal machine and the multi-head weigher at the end of the line. This provides the final check on the product before it is safely sealed in its final packaging. Correct compliance with HACCP demands that all metal detectors are correctly tested at regular intervals in order to ensure that the system is working at its optimum.