Detection domination

15 March 2012



With the might of the multiples behind it, could x-ray start to reach the parts of the market that have historically been the preserve of metal detectors? Lynda Searby reports


In the UK, several grocery retail giants are pushing their suppliers hard to deploy x-ray inspection. Whereas their codes of practice used to recommend x-ray for inspection of products packaged in foil, now some retailers, including M&S and Tesco, are being more insistent.

“In the past there was a paragraph suggesting suppliers should look at x-ray; now retailers are stipulating that suppliers should have x-ray unless there is a valid reason not to,” confirms Tony Bryant, UK sales manager with Loma Systems.

However, determining when there is a justification for using metal detectors instead of x-ray systems may not be black and white. Bryant claims that in 95% of applications, an x-ray will yield better results than a metal detector.

He says one of the few scenarios in which a metal detector will match the performance of x-ray is when inspecting bulk sacks of dried product like flour and rice. “In these applications, metal detectors and x-rays tend to achieve the same levels of sensitivity because the product is very dry,” he says.

The reason a metal detector is able to achieve high sensitivity in dry applications is because there is no ‘product effect’, a phenomenon explained by Alan Johnson, S+S Inspection’s x-ray specialist. “Metal detectors rely on either the conductive or magnetic (or both) properties of metallic foreign bodies. Therefore, if these properties exist at low levels or if the product or packaging itself is either magnetic or conductive, such as a product that contains salt and water, the performance is adversely affected.”

To illustrate how product effect might compromise a metal detector’s sensitivity, Loma’s Bryant cites the example of a chilled ready meal in a PET tray.

“The product effect means that a metal detector will only be able to detect metal pieces as small as 2.5 to 4mm in diameter, whereas an x-ray will be able to detect metallic contaminants with a diameter of 1 to 1.5mm, and ceramic and glass contaminants of 3 to 4mm.”

Another dry application where metal detectors outperform x-ray systems is on snack lines, where a throat metal detector is sandwiched between the multihead weigher and the vertical bag maker.

“A gravity-fed metal detector above the bag maker can find stainless steel fragments as small as 0.8mm in diameter – smaller than any x-ray would be able to see,” says Simon Taylor, marketing and sales manager with Lock Inspection Systems. “Several snack manufacturers have now eliminated the end-of-line detector in favour of this set-up, which avoids the problem of inspecting product packaged in metallised film and is a fraction of the cost of an x-ray.”

To an extent, metal detector designers have addressed the problem of product effect. For example, Mettler-Toledo Product Inspection says its Profile Select uses variable frequency technology and advanced electronics, combined with innovative software algorithms, to automatically select the optimum detection frequency for the product being inspected.

“This ensures the system can mask out product effect without compromising product inspection sensitivity, offering manufacturers enhanced metal detection in wet and dry products and those packed in metallised film,” says Neil Giles, marketing communications manager, Mettler-Toledo Product Inspection.

The characteristics of the product being inspected are a consideration when deciding whether metal detection or x-ray is the best technology for the job, but are by no means the only factor. Equally important is identifying the potential sources of contamination on the line. “If the most likely source of contamination is from non-metallic materials, such as glass, bone, stones or dense plastics, then x-ray is the obvious choice,” says Giles.

The decision becomes less clear-cut when metal is the potential contaminant – which it is, more often than not. “When you consider that the cutting, sieving, processing, screening and packing of food is predominantly done in a metallic environment, metal presents by far the most significant threat in terms of contamination,” says Lock’s Taylor.

It is straightforward enough where the main contaminant is aluminium: metal detectors do a better job. This is because x-ray works on a density differentiation basis, so it has difficulty finding lighter contaminants such as insects, hairs, rubber and aluminium. When it comes to non-magnetic stainless steel (the type used in the food industry), as S+S Inspection’s Johnson explains, generally speaking, an x-ray will detect contamination more easily.

“Non-magnetic stainless steel is a poor conductor of electricity so it is more difficult to detect using a metal detector, but in terms of density difference it is up to eight times denser than food products, so an x-ray usually provides better stainless steel detection than a metal detector.”

However, he adds a caveat that x-ray’s advantage can be marginal if the product and/or packaging does not adversely affect the performance of the metal detector.

Mettler-Toledo goes one step further, saying “metal detection is now as capable of detecting stainless steel contaminants as x-ray.” This is mainly due to developments in the use of higher frequencies to improve detection of stainless steels. S+S, for example, has just launched a new metal detector with three-frequency detection, including 600kHz for enhanced stainless steel detection.

The ability of the detector to find metal also depends on the shape and orientation of the contaminant; while sensitivity is usually measured using standard test balls, Taylor argues that this skews the results in favour of x-rays and doesn’t reflect real life.

“When using test balls, the sensitivity will be slightly better for x-ray because, unlike metal detectors, x-rays aren’t dependent on the size of the aperture through which the product is passed, so a typical x-ray would detect metal down to 1mm in diameter,” he says. “In practical terms though, metal contaminants don’t present themselves in perfect spheres, they are usually slivers of sieve wire, swarf or bearings that are breaking down, all of which can be difficult for an x-ray to see.”

It is worth pointing out that not all UK retailers are pushing their suppliers towards x-ray, and retailers elsewhere in Europe don’t seem to have the same obsession with x-ray.

“Outside the UK, most European retailers don’t have their own guidelines for suppliers, but instead ask that their suppliers be certified to British Retail Consortium (BRC), International Food Standard (IFS) or other Global Food Safety Initiative (GFSI) standards,” says Giles.

However, he adds that the larger European retailers are expected to introduce their own standards over the next few years. With or without pressure from retailers, there’s no question that manufacturers throughout Europe are coming round to x-ray.

“Outside the UK, it’s the multinational manufacturers who are leading adoption of x-ray. It’s not so much retailer driven as brand protection driven,” says Loma’s Bryant. “We still make more metal detectors than we do x-ray machines but the balance has shifted significantly in the last few years, and when building new lines, x-ray is the first technology companies consider.”

His observations are backed up by Mettler-Toledo’s Giles, who says: “Over the last 10 years, the use of x-ray has grown rapidly...and it is now catching up with metal detection. This is particularly true in the UK, Germany and Scandinavia, all early adopters of the technology.”

Historically, cost was one of the barriers to investment in x-ray. As economies of scale have begun to kick in, prices of x-ray systems have fallen, however. “The average cost of an x-ray system is in the region of £36,000-£42,000 at the moment,” says Bryant. “It used to be more like £48,000.”

Prices haven’t dipped that dramatically, nor have they fallen across the board, as Jim Bedford, product manager with Ishida Europe, explains. “Costs have come down in the last few years, but only for basic detector machines. If the customer is interested in detecting at the highest possible level, the prices are higher. Economy machine prices have maybe come down by 15% over the last three years.”

Chris Keenan, sales manager with Selo UK, which distributes Anritsu’s metal detection and x-ray equipment in the UK, suggests that one of the reasons prices haven’t fallen much is because x-ray designers are focusing on developments that justify added cost rather than trying to engineer cost out.

“X-ray suppliers have introduced other features that the equipment can be used for such as checkweighing, missing product detection, mass measurement and seal contamination. These help manufacturers justify the additional cost,” he says.

S+S Inspection’s Johnson also confirms that development has been in that direction “as the advance image processing x-ray uses to detect foreign bodies can be used to provide inspection of other defects, such as missing or deformed product, under- and overweight, missing content or fill.

“In addition, as more advanced and faster computers and x-ray imaging technology become available, they have been integrated into new designs, making them reliable, easier to service and less costly to maintain,” he says.

Certainly, x-ray equipment designers have been investing heavily to shake off perceptions of x-ray as unreliable, expensive to run and difficult to use, to the point where Loma’s Bryant claims glitches are old hat: “We’ve worked with our component suppliers to ensure those items deemed unreliable – the tank, detector array and power supply – are reliable.”

X-ray technology is becoming a more realistic option than it was a few years ago, but with a massive installed base and sales still strong, metal detectors are not going anywhere in the near future.


Lock says a gravity fed metal detector can deliver better sensitivity than x-ray on snack lines Lock Ishida’s IX-GA series of x-rays is positioned as a high performance option Ishida Mettler-Toledo’s Profile Select automatically selects the optimum detection frequency for the product being inspected Mettler-Toledo the IQ3+ is Loma’s latest metal detector and features variable frequency technology IQ3+ Loma’s latest x-ray system, the X4 800 XL, is designed for inspecting larger products, including Euro crates and meat boxes X4 800 XL The GHF metal detector from S+S promises enhanced stainless steel detection GHF

GHF GHF
Mettler-Toledo Mettler-Toledo
Ishida Ishida
IQ3+ IQ3+
Lock Lock
X4 800 XL X4 800 XL


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