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Various methods of inspection for metal detection

Friday, 16 May, 2014, 08 : 00 AM [IST]

Amir Gull, Tajamul Rouf Shah, Pradyuman Kumar

There are three main groups of metallic contaminants - ferrous (both magnetic and conductive so easily detected), non-ferrous (non-magnetic but good or excellent conductor so relatively easily detected) and stainless steel in which detection depends on the magnetic or conductive properties of the contaminant.

Most food processing equipment are made of metal, thus there is a high probability for metal contamination. Sources of contamination may be loose screws, bolts, machine parts, metal bits (copper staples), metal tags, lead shot in meat, screen wire, blades and so on.

Various metals may contaminate the food product during its production. Hence, food processing industry has to install metal detector as online sensor for various reasons viz. product safety, equipment protection, customer requirement and regulatory compliance. The characteristics of a metal detector are accurate, reliable, easy to clean-sanitise, simple and safe to operate and matching the production line speed.

Metals
In spite of the widespread use of magnets and metal detectors on food production lines, metal fragments of wide range are still reported from food products, although they are often very small. Quick and non-destructive method, which can be used for analysis of all metals is energy-dispersive X-ray microanalysis - usually carried out in a scanning electron microscope.

Metal type involved is usually an important indication of the source of the problem. Wire pieces are particularly difficult to detect online and pieces of broken sieve, electrical wire fragments from repairs, either in the factory or the consumer’s home may be found. Metal pieces from the manufacture of can ends are a particular problem because the ends are stamped out of a flat sheet of metal, leaving behind a relatively fragile fretwork of waste metal, and a key feature in their identification is often the type and distribution of lacquer on the surface. Aluminum pieces from baking trays, scraped from the surface by mechanical contact, are sometimes found. Similarly,stainless steel fragments from food machinery are sometimes reported, and the precise composition of these steels can be helpful in identifying the source of the problem.

Modern metal detectors fall into two main categories. The first consists of systems with a general purpose search head. These are capable of detecting ferrous and non-ferrous metals as well as stainless steel, in fresh and frozen products - either unwrapped or wrapped, even in metalised films.

The other main category consists of systems, which have a ferrous-in foil search head. These are capable of detecting ferrous metals only within fresh or frozen products, which are packed in a foil wrapping. Metals are ferrous, non-ferrous or stainless steel. Ease of detection will depend on their magnetic permeability and electrical conductivity. Metal types and their ease of detection is shown in (Table 1). Orientation, size, and shape (with respect to the detector coils) of the metal particle are also important. Since shape, orientation and size of metal contaminants is not possible to control, so it is best to operate a metal detector at the highest possible sensitivity setting.

Commonly used detectors in food industries
Industrial metal detectors: During the manufacturing process contamination of food by metal shards from broken processing machinery is a major safety issue in the food industry. Metal detectors for this purpose are widely used and integrated into the production line. These type of detectors are used in food, beverage chemicals, and packaging industries. This type of metal detector was developed by Bruce Kerr and David Hiscock (1947). For common industrial metal detector the basic principle operation is based on a 3-coil design. This utilises an AM (amplitude modulated) transmitting coil and two receiving coils one on either side of the transmitter.

Metal contaminates
Physical configuration and design of the receiving coils are instrumental in the ability to detect very small metal contaminates of 1mm or smaller. Configuration of coil is such that it creates an opening whereby the product (food, plastics, pharmaceuticals, and so on) passes through the coils. The aperture or opening allows the product to enter and exit through the three-coil system producing an equal but mirrored signal on the two receiving coils. Then the resulting signals are effectively summed together and nullifying each other. When a metal contaminant is introduced into the product an unequal disturbance is created. This creates a very small electronic signal that is amplified through special electronics. The amplification produced then signals a mechanical device mounted to the conveyer system to remove the contaminated product from the production line. This process is completely automated and allows manufacturing to operate uninterrupted.

X-ray inspection technology: In food and beverage applications, metal detectors have long been used for the detection of metal contaminants. However, metal detectors have limitations in many applications that make X-ray inspection a better choice. For example, broader range of container types can be inspected by X-ray systems. This system can also detect a much broader range of foreign material than a metal detector. Finally, InspX ScanTrac (X-ray systems) offer additional features such as accurate fill-level and check weighing functions that are absent on metal detectors. The biggest advantage of this inspection system over metal detectors is its ability to inspect any type of container, e.g. (Metal cans, containers with metal lids), plastic containers with metalised foil seals and metalised foil bags cannot be inspected with a metal detector but these are easily inspected with X-ray technology.

Furthermore, such metal elements of the container have no impact on the X-ray inspection process. Generally better detection of a wide range of metal contaminants is offered by X-ray inspection process than metal detectors but there are important differences that need to be understood e.g., aluminium, is easily found by a metal detector because aluminium has a lower density than most metals - the minimum size of aluminium detectable by an X-ray system is usually larger than for most metals. This system also finds a host of contaminants that are invisible to metal detectors including: glass, rubber, denser plastics, stones and much more. For foreign material detection the accuracy of an X-ray system is also completely independent from the environment of the container. For example, the product temperature can impact the accuracy of metal detectors; temperature has no impact on X-ray technology. X-ray systems finally provide additional functions than metal detectors. X-ray inspection systems are available with fill-level monitoring and check weighing software – note that check-weighing can even be done on part of a product to validate sufficient presence of a sub-component.

CEIA THS metal detectors
This type of metal detector detects metal contaminants accidentally present in industrial products, levels of sensitivity, immunity to interference and response speeds exceeding the strictest quality control standards. CEIA THS detectors allow the interception of magnetic and non-magnetic metals, including high-resistivity stainless steel. An automatic tracking function cancels out any variation in product effect caused by the product under inspection. This type of detector allows maximum sensitivity operation at both high and low transit speeds. CEIA THS detectors have been adopted by the leading companies in the food, chemical, textile and pharmaceutical fields. CEIA’s simple Global Auto-Learn system selects and displays the optimal sensitivity setting to eliminate guesswork and inspection errors. During operation, the detector continuously tests and recalibrates itself to compensate for product effect conditions. Data for every detection and ejection action are saved for inspection, programming and functional testing, and to certify production quality. Combined with memory for over 500 products, this provides fast, error-free changeovers.

Advantages of CEIA THS detectors Detection

High sensitivity to ferrous, non-ferrous and stainless steel metals
Sensitivity is independent of the product speed

Automatic product effect compensation

For smallest metal fragment detection this procedure automatically acquires product characteristics and compensates for product effect variations

Cleanability

Smooth surfaces
Detection head in AISI 316L

Reliability

Extended mechanical life due to robust construction and careful material selection
Protection of the electronic boards via high thickness conformal coating

Other technologies use optical methods to differentiate a good product from a bad one. Raw agricultural commodities are more commonly sorted by this method. Image processing modules and optical sensors use a mono or dichromatic system and high, low, or band bass filter systems. Microwave reflectance and electrical impedance, newer technologies, which are intermediate between conventional metal detection systems and microwave-based approaches, have yet to be commercialised. These work on the principle that portions of food will impede microwaves in a different manner than foreign matter. This discrepancy helps to determine the spatial, three-dimensional location of the foreign matter. Nuclear Magnetic Resonance (NMR) techniques, like magnetic resonance imaging used for humans, can detect and identify foreign matter, but it has practical limitations. The spatial resolution and three-dimensional images need to be improved, so this technique is not ready for prime time in the food industry. When all is said and done and all the technologies are considered, nothing compares with simply looking at the foreign matter, a technique employed in all food companies. Observation plays a major role in identifying foreign matter, but while this time-tested method is effective, its drawbacks, including employee fatigue and excessive time required, outweigh its advantages.

(Gull and Shah are research scholars and Dr Kumar is associate professor, department of food engineering & technology, Sant Longowal Institute of Engineering & Technology, Sangrur, Punjab. They can be contacted at pradyuman2002@hotmail.com)