Metal detection, x-ray – Or both?

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

By Ian Robertshaw, global key account manager at Mettler-Toledo

A layman might consider it an easy choice: if you are looking for the best inspection technology designed to detect metal contaminants, then opt for a metal detection system. On the other hand, if you are looking for a system to detect both metal and non-metal contaminants, then choose the x-ray inspection system.

There is a kernel of accuracy in this supposition because very often, such as with aluminium and wire, a metal detection system will be better at detecting metal than an x-ray system. However, the decision is not necessarily so simple. For example, what if you do need to identify metal contaminants, but the product is packaged in aluminium foil? The foil will be seen as a detectable contaminant by the metal detection system, thus rendering the system unusable. An x-ray system, meanwhile, sees straight through the foil to get a better view of a contaminant inside.

The choice of technology is not simply about the potential type of contaminant that you are looking for; part of the consideration is where in the production process should the product be inspected. For example, if your raw product needs to be inspected before other valuable ingredients are added then maybe a metal detection system is best. However, for end-of-line inspection when packaging integrity checks are required, as well as contaminant detection, then x-ray technology will be more suitable.

What is certain is that product inspection is a critical part of the food production process, ensuring that consumers are confident in the quality and safety of the products they buy. Food manufacturers should therefore consider carefully when choosing whether to equip their production lines with metal detection or x-ray technology, or perhaps with both.

There are fundamental differences in the ways that metal detection and x-ray inspection technologies work. It is important for food manufacturers to understand what these differences are and how they can impact the ability to perform optimally on certain product inspection applications.

Application is key here: the nature of the product, the vertical form fill seal (VFFS) filling process, the potential contaminant types, and factors such as the physical packaging, must all be brought into the equation when selecting the right contaminant detection technology. In addition, constraints on finance and physical space, and the range of additional quality control checks should be considered.

Metal detection

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

A well-designed metal detector for use in the food industry can detect a pinhead in a loaf of bread, while a detector for pharmaceutical applications can detect metal contaminants less than 0.3mm in diameter. To meet industrial demands, the detector construction must be stable and rigid enough to eliminate any movement of the coil system, as even tiny vibrations can cause the rejection of perfectly good products. Airborne electrical noise can also be problematic, so it is essential for the metal detector to operate reliably in a factory environment.

The product effect: When it comes to inspection capabilities, 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”. Products with high moisture content, or those that are salty or acidic, are conductive; as they pass through the metal detector, they will emit a signal, which is the product effect, that disturbs the detection field.

Product effect is a major consideration that can lead to high false reject rates. Besides the moisture or salt content of the product, other factors that contribute to product effect are product temperature, format, consistency, size and shape, and orientation on the production line.

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 enables the system to have the right level of sensitivity – picking up signals from very small metal contaminants regardless of the application and providing the highest level of brand protection.

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. In these applications, however, an inspection would need to take place before a metal cap or closure is applied.

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

A further aspect to consider – both with metal detection and x-ray – is product size, and we will come to this later.

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 products 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 beams are 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. The product is accepted or rejected based on this comparison. When rejected, the system sends a signal to an automatic reject system, removing the offending product from the line.

However, while x-ray can easily detect these dense contaminants, with low-density contaminants such as 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.

Choosing the right technology

As shown, both metal detection systems and x-ray inspection technologies have strengths and weaknesses in the field of product inspection. The process of choosing the right system means going back to the application and carrying out hazard analysis and critical control points (HACCP) or hazard analysis and risk-based preventive controls (HARPC) audit. This will identify the risks of contamination with the application and possible types of contamination, and a greater understanding of the requirements of any customer or compliance-related issues.

Critical control points (CCPs) should be established to mitigate the risks, and product inspection equipment needs to be installed at these points. If the HACCP/HARPC audit determines that metal is the only likely contaminant, then a metal detection system is probably going to be the best solution. If metal or other contaminants, such as glass, stone or dense plastics, are likely to be encountered, then x-ray systems will be the best solution. However, there are many applications where the choice is less clear and others where 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 the 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 strength 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 inspection.

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 and variable line speeds; situations where there is limited space: With a speed of 400m/min, metal detection is able to inspect at faster speeds than x-ray which typically performs around 120m/min. This possesses an advantage if other aspects of the application suit metal detection better. Metal detectors are also less space-consuming than x-ray detectors, 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 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. As the chart illustrates, in these scenarios a more complex evaluation of options is needed.

There may also be situations in which more than one type of product inspection system is desirable at different CCPs 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 carry out further quality control checks. 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 summary, the first step in choosing a metal detector or an x-ray system for product inspection is to consider its application – the type of product, the type of likely contaminant, and the location of CCPs. Metal detection offers many advantages for raw product inspection whereas x-ray inspection provides multiple products and packaging integrity checks, in addition to contaminant detection. While more factors will influence the decision, including space limitations, the total cost of ownership and productivity targets, the application is where the assessment begins.