Metal detection advances offer productivity benefits

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Modern metal detectors should have the ability to monitor their own performance and functionality. The difference now being that if a change in detection performance is identified the system is able to pre-warn the relevant operator of a potential problem while still being within the production line contamination specification. 

Traditionally, metal detection systems have been tested to provide confirmation that it is operating in accordance with the specified sensitivity for the product it is inspecting; that it accurately rejects products that contain metal contaminants; and that it confirms that any sensors employed as part of the detection/rejection circuit are set up and functioning correctly. 

The latest developments in detector systems are now so much more sensitive than in the past that the frequency of performance monitoring tests can potentially be reduced by as much as 80% according to Jonathan Richards, head of marketing at Mettler-Toledo Safeline Metal Detection.

Mettler Toledo’s Reduced Test Mode, for example, is an analytical monitoring tool that has been developed within a Predictive Analytics framework of its latest metal detection systems. Along with additional increases in sensitivity of up to 20%, the tool has the ability to monitor the metal detectors’ performance on the production line to a level of certainty that replaces the need for frequent manual checking and monitoring. This can lead to significant uptime advantages, improved overall equipment effectiveness (OEE) and cost savings in many applications.

Critically, this same Reduced Test Mode within Predictive Analytics technology allows for a greater time interval in between monitoring tests. Typically, a test interval of somewhere between one and four hours can be used in some snack food factories. As an example, one Mettler Toledo customer was testing every two hours, with an operator climbing up to a mezzanine floor and dropping a test piece into the production flow, which in turn would be delivered to the metal detector. This, having detected the piece, would then stop the line. A second operator on the ground floor would remove the bag containing the test piece, reset the bag-maker and restart production – a process that was carried out three times at every two-hour test interval.

It was calculated that the process took approximately three minutes (six man minutes due to the multiple operators involved) plus the time it took to document the results which, if you do the calculations across the 24 metal detectors in operation, means the customer was employing two operators all day, every day to do nothing but test the systems. 

By employing metal detectors equipped with the latest technologies, such as Reduced Test Mode functionality, it is possible to significantly reduce testing frequency – as unless the warning alarm sounds to indicate a potential issue with the system there is simply no need to halt production to carry out checks. Having seen a demonstration of this in action, the Mettler Toledo customer changed to the new format and was immediately able to reduce its testing regime from every two hours to every 12 hours, representing an 80% reduction in terms of testing – saving time, money and lost productivity.

Food products come in all shapes, sizes and density and, when passing through a metal detector they are not always travelling in the same direction. Since size, shape and symmetry of metal contaminants cannot be controlled, operating a metal detector at the highest possible sensitivity setting has, traditionally, been viewed as the best method to tackle product and orientation effect said Phil Brown, sales director at Fortress Technology.

There are many variables that determine the theoretical sensitivity of a metal detector. Among them the aperture size (the smaller the aperture, the smaller the piece of metal that can be detected), the type of metal (ferrous, non-ferrous or stainless steel), product effect, and the orientation of the contaminant as it passes through the detector. Environmental conditions, such as airborne electrical interference - static, earth loops - vibration and temperature fluctuation may also affect performance.

Orientation effect is a result of asymmetrical metal contaminant shards being more easily detected if they pass through the metal inspection system in one direction rather than another. A typical scenario occurs when equipment is calibrated to detect a stainless steel sphere 2mm in diameter. While it could identify and reject this contaminant, the machine may fail to detect a stainless steel wire that is smaller in diameter but up to 24 mm long, depending on the orientation of the wire as it travels through the detector.

Typically, it is easier to detect stainless steel and non-ferrous wires when they pass through the aperture space sideways or upright, rather than in alignment with the conveyor. The reason for this is down to the magnetic permeability of the metal, which for stainless steel is much lower than other metals.

“Most test pieces use a spherical object, so the signal they give off is the same in any orientation,” said Brown. “Yet for a sliver of metal, wire or pins, the product needs to be tested in different orientation to ensure maximum effectiveness. It’s also critical to remember that locating a metal contaminant in one orientation doesn’t automatically mean another won’t slip through at a different orientation.” 

Some suppliers recommend positioning several metal detectors at various angles along the conveyor, improving the chances of picking up and rejecting a contaminated product. Additional investment and the costs incurred for maintaining multiple machines are the downside to this option. “You would also have to ensure that spherical sensitivity is not compromised and the benefits of having several machines on a line are not offset,” notes Brown.

  

Reducing the aperture size is considered to be the simplest way to increase metal detector sensitivity. “Because sensitivity is measured at the geometric centre of the aperture, the ratio of the aperture to the size of the product is an essential consideration. Maximum sensitivity occurs when the belt and food item is closest to the edge of the metal detector portal, so it makes sense that the smaller the aperture, the more failsafe your system is,” said Brown.

Frequency settings must also be factored in. While dropping the frequency can enhance the ability to find ferrous metals, it can limit performance when it comes to non-ferrous metals, because the lower end of the frequency is more responsive to magnetic effects of the contamination. By the same token, the reverse happens when taking the frequency higher.

To help combat product and orientation effect, Fortress introduced the Interceptor range in 2015. Using multi-frequency technology, the higher frequency signal can improve detection levels for stainless steel by as much as 100%, as well as performing better on lower density metals. The high and low frequencies emitted by the Interceptor machine also help to remove the product effect signals caused by wet and conductive products. 

During the regular testing of metal detectors, manufacturers are advised to insert test pieces in various locations and orientations within products which provides assurance that metal detectors are picking up contaminants regardless of metal type, size or orientation.