Metal Detection, X-ray — Or Both?

Both metal detection and x-ray inspection systems play key roles in product inspection for food and pharmaceutical companies. Choosing the right technology requires consideration of many factors — starting with the application.

Deciding whether a metal detector or an x-ray system is the best choice for your product inspection needs is not as straightforward as it might seem. The application is the starting point, but several factors can complicate matters. Consider this scenario, for example: you need to identify metal contaminants, but the product is packaged in aluminium foil. A metal detector will see the foil itself as a detectable contaminant, while an x-ray system sees straight through the aluminium because it is a low-density metal, allowing it to get a better view of any contaminants inside. X-ray will be the better choice, despite the potential contaminants being metal.

Both technologies have their sweet spots, as we will see, and a range of factors must be considered, including, the nature of the product, product size, the fill process, the potential contaminant types, the packaging, constraints on finance and physical space, and the range of additional quality control checks needed.

Metal detection

Modern metal detection systems can identify all metals, including ferrous (such as chrome and steel) and non-ferrous (brass and aluminium, for example), as well as both magnetic and non-magnetic stainless steels. They work through a system of coils, charged with an electrical current, to create a balanced electro-magnetic field. If a product passing through this field contains a metal contaminant, the magnetic field is disturbed, and this disturbance is interpreted by sophisticated electronic circuitry and software algorithms.

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In order to perform as required, the detector must be stable and rigid enough to eliminate any movement of the coil system, as even tiny vibrations can cause rejection of perfectly good products. Airborne electrical noise can also be a problem, so it is essential the metal detector can operate reliably in a factory environment.

The product effect

Product effect is a major consideration, which can lead to high false reject rates. Products with a high moisture content, or those that are salty or acidic, are conductive, and as they pass through the metal detector will emit a signal (i.e. the ‘product effect’) that disturbs the detection field. Other factors that contribute to product effect are product temperature, format, consistency, size and shape and orientation on the production line. Metal detection is especially suited to dry products, where the lack of moisture means the product is non-conductive, and therefore does not generate a significant ‘product effect’.

Manufacturers can eliminate the impact of product effect by installing a high-quality metal detection system that uses a combination of multi-simultaneous frequency operation and software algorithms to optimise performance and reduce the possibility of costly false rejections. This technology will also result in the system having the right level of sensitivity to pick up signals from very small metal contaminants irrespective of the application.

In addition to packaged products, other applications where metal detection can be used include loose, unpackaged products, pumped products such as liquids, pastes and slurries, bulk powders or free-flowing solids under gravity-fall conditions. In addition, tall, rigid containers such as bottles, jars and composite containers can also be inspected, although in these applications, inspection would need to take place before a metal cap or closure was applied.

Type of packaging

Metal detectors using multiple frequencies simultaneously or operating at a single low frequency can usually be used with products packed in metallised film packaging, depending on the film thickness. If aluminium foil packaging, such as foil wraps or products trays, is used, then standard balanced coil metal detectors will not be suitable.

X-ray Inspection

X-ray inspection systems have the capability to detect a wider range of contaminants than metal detectors, including metal, glass, stone, calcified bone, high-density plastics and rubber. They can also perform a range of additional in-line quality checks on food and pharmaceutical products, including measuring mass, counting components, identifying missing or broken products, monitoring fill levels, detecting product trapped within the seal and checking for damaged product and packaging.

The technology works by generating an x-ray beam that passes through a product for inspection and onto a detector. Some of the x-ray beam is absorbed by the product and any contaminant present, and because most contaminants are denser than the food and pharmaceutical products that are being inspected, the contaminants usually absorb more of the x-ray energy. This difference in absorption becomes apparent in an image generated by the x-ray system, which is then compared to a pre-determined acceptance standard for acceptance or rejection.

However, while X-ray can easily detect these dense contaminants, with low density contaminants such as aluminium, insects, wood and polyethylene film, detection by x-ray is not possible.

Nevertheless, x-ray systems are able to inspect a wide range of different product types, including pumped products such as slurries, fluids and semi-solids, bulk, loose products, jars, bottles and cans, and packaged products, including those packaged in foil or metallised film.

Which technology?

The process of choosing the right technology for product inspection means going back to the application and carrying out a Hazard Analysis and Critical Control Points (HACCP) or Hazard Analysis and Risk-Based Preventive Controls (HARPC) audit. This will identify what risks of contamination there are with your application, and what types of contamination are likely, as well as providing a greater understanding of the requirements of any customer or compliance related issues.

It is rarely a clear-cut decision, sometimes the right answer might be to deploy both.

Consider these examples:

Aluminium contaminants in non-metal packaging: as a lightweight, low-density metal, aluminium is hard for x-ray to detect as a contaminant; metal detection is generally the better solution.

Metal contaminants in aluminium foil packaging: metal detection will be unable to spot the contaminants amidst the foil packaging unless it is a metallised film; x-ray is generally the better solution.

Metal contaminants in gravity-fed products: x-ray does not work well with falling, accelerating objects that do not have a uniform direction of travel; metal detection is the only viable solution.

Metal contaminants in non-metal packaging: this can be complicated. Metal detection systems are more cost-effective but if the product is very large, a bigger detector aperture will be needed, which can reduce the sensitivity of the detector. Multi- and high-frequency technology can help, but a bigger metal detection system will be required. X-ray power can be increased for larger products, but the cost of installation increases as size increases. If there is a need to protect against non-metallic contamination, the choice will swing towards x-ray.

Non-metal contaminants in any packaging; performing additional quality control issues: x-ray inspection is the only solution, and the additional QC checks can justify the additional cost of the technology.

Fast/variable line speeds; situations where there is limited space: metal detection (at 400m/min) is able to inspect at faster speeds than x-ray (120m/min), so may have the advantage if other aspects of the application suit metal detection better. Metal detectors are also less space-hungry than x-ray, so depending upon the application, might be more suitable in factories with limited space.

Making it simple

Metal detection or x-ray? The flow chart below is a good starting point for identifying the right answer. However, there is an area of indecision where the application is not packaged in foil, and metals other than aluminium are potential contaminants. Here, a more complex evaluation of options is needed.

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There may also be situations in which more than one type of product inspection system is desirable at different Critical Control Points on the production line. For example, it may be wise to install a metal detector early in the processing line to remove large metal contaminants that could, if left present, cause damage to machinery downstream, or fragment into smaller and less easily detectable pieces. Further down the production line, an x-ray machine could then check for non-metal contaminants, as well as carrying out further quality control checks, while a second, and more sensitive, metal detection system at the end-of-the-line could be used to make a final inspection for smaller metal contaminants.

In closing, while factors such as space limitations, total cost of ownership and productivity targets are important, it is worth reiterating that the first step in choosing a metal detector or an x-ray system for product inspection is to consider the application — this is where the assessment begins.

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