Keeping tiny metal particles and other unwanted contaminants from making their way inside a finished package headed for the retail shelf is something Fortress Technology knows a lot about. Superior sensitivity has been one of the company’s key differentiators since its inception in 1996. Today, the firm manufactures more than 2,500 metal detectors a year from its Toronto-based facility, with most destined for food production facilities large and small across North America as well as overseas.
Like all metal detector manufacturers, Fortress uses spherical test samples to showcase advances in sensitivity. Spheres are used industry-wide because they are the same shape from every orientation when passing through the metal detector. Yet, visualizing what a 50% improvement in metal detection sensitivity actually equates to can be quite challenging.
In a conventional single-planar-field metal detector, a spherical-shaped metal would be relatively easy to spot. Metal contaminants, however, are more likely to be non-spherical or an irregular shape. In fact, most are likely to be swarf, flat flakes or wires than globular in shape.
Typically expressed by diameter in millimeters, test spheres provide machine suppliers and buyers with a comparative benchmark. They provide a solid and reliable gauge on machine sensitivity. So, when a supplier reports a sensitivity improvement of 0.5mm, this is a big deal. A 0.5mm sphere could equate to a wire length contaminant measuring 2.5cm.
To help illustrate this, Fortress regularly asks customers to take a piece of putty and roll out a 3.5mm sphere between their hands. This size sphere in stainless steel is currently the standard Code of Practice for a block of cheese measuring approximately 75mm high. Yet, rolled out, a sphere of this size could equate to a wire length of 30cm. This would of course also depend on the diameter of the wire.
Realistically, a wire of this length is likely to protrude from a product and would be relatively easy to spot with the naked eye. It’s when smaller wires or slivers of metal are embedded into a product where the sensitivity of your metal detector really comes into play.
Overcoming orientation effect
Another challenge for food processors is that products don’t always travel consistently in the same direction when passing through a metal detector aperture. Because size, shape and symmetry of metal contaminants cannot be controlled, operating a metal detector at the highest possible sensitivity setting is often viewed as the best method to address the challenges of product and orientation effect.
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 that’s 2mm in diameter. While it may identify and reject this contaminant, the machine may fail to detect a stainless-steel wire that is smaller in diameter, but longer than 2mm, depending on the orientation of the wire as it travels through the detector.
Often, 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.
Machinery suppliers may recommend positioning several metal detectors at various angles along the conveyor, improving the chances of identifying and rejecting a contaminated product. Additional investment and the costs incurred for maintaining multiple machines are the downside to this option.
Reducing the aperture size is another simple and effective way to increase metal detector sensitivity. Because sensitivity is measured at the geometric center of the aperture, the ratio of the aperture to the size of the product should be considered. 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 fail-safe a system is.
Know your metals
The type of metal contaminant also needs to be factored in. Most industrial metal detectors will exhibit a different level of sensitivity for the three main groups — ferrous (such as iron or steel), non-ferrous (including aluminum foil) and stainless steel. Because metal detectors work by spotting materials that create a magnetic or conductive disturbance as they pass through an electromagnetic field, stainless steel is typically the most difficult to identify.
Widely used in food preparation and production areas, stainless steel comes in various grades. Unlike ferrous and non-ferrous metals, stainless steel is a poor electrical conductor and usually non-magnetic. Consequently, a sphere of stainless steel hidden in a dry product typically needs to be 50% larger than a ferrous sphere to generate a similar signal size. This disparity can rise to 300% in wet products, such as ready meals, meat, fish, sauces, preserves and bread. The reason is moisture in the product creates a conductive signal and the metal detection can be swamped by product effect which resembles the stainless-steel signal.
Conversely, any product that is iron enriched — such as fortified cereals, supplements and breakfast bars — creates a large magnetic signal that the detector must overcome in order to detect small pieces of metal. These are referred to as “dry” products and tend to be a lot easier in terms of detection capability, because you do not have to worry about the product effect.
To identify a metal contaminant within conductive products, the detector must remove or reduce this product effect. The solution is to change the frequency of operation to minimize the effect of the product. The downside is that this can impact your ability to find different metals. When you drop frequency, you tend to enhance your ability to find ferrous metals, yet this limits performance when it comes to stainless steel, because the lower end of the frequency is more responsive to magnetic effects of the contamination.
By the same token, the reverse happens when the frequency is taken higher — it starts to limit the ferrous detection capability but enhances the stainless-steel detection.
To reduce metal contaminant risks, it is essential to identify the ideal frequency on the metal detector for any product and set it to the right level for your specific food application. Solutions, such as simultaneous multi-frequency metal detectors, are now available on the market to address these challenges.
Finding flat flakes
Dependent on how a metal flake is lodged within a product, there is also the potential for it to completely evade a metal detector’s single electromagnetic sensor. To address this challenge, Fortress has pushed the technological innovation envelope with its launch of the Interceptor DF (Divergent Field).
By adding a second electromagnetic field that scans vertically alongside a “normal” metal detector field that looks for metal contaminants in a horizontal direction, the Interceptor DF compensates for each fields’ respective weakness. The technology is especially beneficial for upstream premium applications like confectionery and chocolate, and has proved to be reliable at detecting very thin flakes and foils that could be introduced in the mixing, rolling, scoring, molding or baking processes.
Results have proved that the Interceptor DF increases the probability of identifying and rejecting products containing non-spherical metals by over 100%, particularly metal flakes.
Test sphere thresholds
The food metal detection industry has general sphere size guidelines. These are based on whether the product being inspected is wet or dry, as well as the overall size of the product. For a wet block of cheese measuring approximately 75mm high, the sphere size parameters are currently ferrous 2.0mm, non-ferrous 2.5mm and stainless steel 3.5mm.
FDA-approved and color-coded ferrous, non-ferrous and stainless-steel test samples are available to periodically test the performance of metal detection equipment. Automatic testing is also a time-saving and accurate way to check the performance of machinery. Halo, the Fortress version, works by effectively and exactly mimicking the signal disturbance that occurs during manual testing, without having to physically pass a metal contaminant through the metal detector. Popular with snack and potato chip manufacturers in North America with dozens of gravity metal detectors running side-by-side on production lines, Halo automatically generates a signal calibrated to specified sphere sizes and metal types, logging the test results to provide a reliable audit trail. It also checks the performance of the reject system.
Conclusion
In the last decade, metal detection technology has progressed significantly. Some of the latest advancements occurred around the same time the U.S. enacted its most sweeping food safety reform to-date — the 2011 Food Safety Modernization Act. Rather than just analyzing hazards, the focus shifted towards prevention.
In Fortress Technology’s early days, technology was generally limited to single fixed frequency “balanced coil” systems. However, digitalization and automation — combined with heightened awareness of food safety and more stringent rules — has seen the technology progress significantly, particularly in relation to inspection and metal detection sensitivity. Looking to the future, sustainability, health, authenticity and farm-to-fork food safety will push machine sensitivity into a whole new sphere.
Fortress Technology founder and president Steve Gidman hand-assembled his first metal detection system to help a U.S. sawmill operator detect and remove metal particles from its manufacturing process. Fortress, which was founded in 1996, now has machines installed in more than 50 countries worldwide.
