Metal detectors recognize all metals

Guide to metal detection

LOMA SYSTEMS® has compiled this metal detection guide to help food and packaging companies design an effective metal detection program that complies with government regulations.

How a metal detector works

In the food industry, a metal detector is usually used that works with the so-called principle of "balanced coils". The patent for this functional principle was registered in the 19th century, but the first metal detector was not manufactured until 1948.

Technical progress has produced metal detectors with valves, transistors, and even integrated circuits, and recently also with microprocessors. This, of course, has resulted in an increase in performance that provides greater sensitivity, stability, and flexibility, as well as expanding the bandwidth of the output signals and information they can provide.

However, modern metal detectors are still unable to detect every piece of metal that enters through their opening. The physical laws applied in technology limit the absolute performance of the system. As with all measuring systems, the accuracy of metal detectors is therefore limited. These restrictions vary depending on the application, but they mainly affect the size of the detectable metal particles. Nevertheless, metal detectors play a valuable and important role in the quality monitoring of processes.

Two main categories for checking "normal" and film-wrapped products

Modern metal detectors can be divided into two categories. The first category includes systems with a universal detector head. These systems recognize ferrous and non-ferrous metals as well as stainless steel in fresh and frozen products. The products can be packaged, unpacked and even packaged in metallized foils. The second category includes systems that have a detector head to search for ferrous metals in products packed in foil. These systems can find ferrous metals in fresh or frozen products that are wrapped in foil.

The system of "balanced coils": how it works

All universal metal detectors use the same principle of operation, although for optimal performance a metal detector should be selected that has been specifically designed for the respective application.
Structural measures ensure that an independent mechanical movement of the individual components of the seeker head and the penetration of water and dirt are not possible. For best results, choose a metal detector that is specifically designed for your application. As can be seen in Figure 1, the typical detector is enclosed in a housing. It contains the components of the coil, which are protected and shielded in this way. The opening, the tunnel through which the products run, is lined with a non-ferrous material (usually plastic), which hygienically seals the internal components.

The relationship between the size of the opening and that of the product is very important for optimal performance. The sensitivity of the detector is measured in the geometric center of the opening, as this is where the most insensitive point is located. It is inversely proportional to the size of the opening; H. to the smaller of the two sides.

The device is equipped with a total of three coils. The transmitter coil creates a magnetic field similar to a radio transmitter. This process, which is used to make a metal particle identifiable, is known as “lighting” the metal particle. The second and third coils serve as receivers, are connected to one another and recognize a "glowing" metal particle. The answer depends on the conductivity and magnetic properties of the metal.

Control and the signal processor

The controls can be mounted on the seeker head or remotely. The decision is made depending on the structure and application of your system. The location where the controls are mounted does not affect the performance of the
Systems. The signal processor is highly complex. When a metal particle "lights up", the receiver coil receives a signal of a millionth of a volt. This is first amplified by an HF high-power amplifier and then converted into a low frequency. In this way information about the amplitude and the phase is obtained. Then the signals are digitized and digitally processed to optimize sensitivity.

Magnetic field systems for products packed in foil

These devices work with a completely different functional principle. The product passes through a tunnel or passageway in which a strong magnetic field is applied, which magnetizes all magnetic materials (e.g. a metal fragment with an iron content) as it passes through it. Several coils are built into the tunnel. As the magnetized particle passes beneath them, electricity is generated which is then amplified by the electronics of the detection system and used to trigger the detection signal output.

Secondary effects caused by the movement of any conductive material in a magnetic field also generate signals for non-magnetic metals. However, these are low compared to materials with a magnetic component. Therefore, only large non-metal iron and stainless steel parts can be recognized. For most applications, this technology can only
used to detect ferrous metals.

The user interface

The user interface offers the possibility to communicate with the system and to set it up and optimize it so that it works together with the application, the environment and the mechanical handling systems.
The use of microprocessors provides a multitude of communication options, statistical analyzes and system information.

Network modules (Loma eNet) (up to 40 metal detectors) can be connected to each metal detector from LOMA SYSTEMS, each of them can in turn be connected to a printer or PC and deliver coordinated operational and administrative information within seconds. The networking of the information can also be coupled with the PVS, LOMA's Performance Validation System for metal detectors, which was developed taking into account the strictest quality controls of the world's leading retailers. The resulting set of multiple user information not only provides information about the metal detection data, but also about the performance quality of the metal detectors.

Search heads: configuration

Seeker heads can be used in various arrangements. As can be seen in Figure 2, the most common installation is on a conveyor belt with a fixed or adjustable speed.

If a contaminated product is detected, it is automatically sorted out. The metal detector seekers can also be surrounded by a flat housing that is mounted on the conveyor belt of a checkweigher. This creates a compact and space-saving combination system.

A seeker head can also be designed for the free fall mode, here the product moves through a system based on gravity (see Figure 3).

Other options include integration into a pipeline system for pumped products (e.g. meat) or a gravity system with a small opening (e.g. for tablets).

Optimization options for your metal detector

Loma Systems: competence and experience

Since LOMA SYSTEMS was founded in 1969, the company has worked very closely with food manufacturers and retailers around the world. Therefore, Loma has unmatched experience and extensive know-how in the field of efficient metal detection in production lines in the food industry. The following guidelines are based on LOMA's extensive experience with so-called "best practices" and are intended to help you meet the strictest quality control requirements.

The following basic conditions should be met

Your metal detection system must be integrated into the main product flow, either at the end of the packaging process or downstream. The system will not be affected by excessive amounts of water or steam at this point either.

In order to achieve optimal performance with a conveyor belt metal detector, the following are important:

  • An efficient, automatic ejection system
  • A lockable container in which the rejected products are collected
  • A device that confirms that the rejected products have been successfully fed into the container
  • A fail-safe, automatic conveyor belt shutdown device that triggers in the event of a lack of compressed air, detector malfunctions, malfunctions in the ejection system and a full ejection container
  • Pipeline systems must be equipped with an acoustic and visual indicator that indicates that a product is being rejected, and free-fall systems must be able to generate double packages if an automatic ejection system is not possible.

Products packed in foil

Ideally, products packed in foil are checked for metal contamination BEFORE they are packed with the foil. If this is not possible, products packed in aluminum trays or in aluminum foil must be sent through a so-called "Ferrous-in-Foil" detector for products packed in ferrous foils. One possibility here is the IQ3 Ferrous-In-Foil-System from Loma. On the other hand, the advantages offered by an X-ray scanner can also be taken into account in this case.

For products that are packaged in metallized foil, “compensated” conventional detectors or free-fall detectors should be used to detect both ferrous and non-ferrous metals.


For optimal sensitivity, the size of the seeker head must be matched to the specific food product. It is important that the best possible sensitivity is determined and set for each product size, type and packaging. This process should only be carried out in consultation with the manufacturer of your metal detector.

If you want to use your detection system in a different location in your plant or if you want to introduce new products, your system will have to be reset. Again, this should only be done in consultation with the manufacturer.
Many of the leading retailers will insist that their private label suppliers explain and communicate in writing any changes to metal detection sensitivity. If you are a private label manufacturer
LOMA SYSTEMS strongly recommends that you coordinate the preferred procedure with your retailer.

The sensitivity settings must not be accessible to personnel who have not been appropriately trained. Access should only be granted to qualified, fully trained personnel and, for security reasons, the settings should be password protected or locked. Of course, you will want to maximize the sensitivity of your detection system. In doing so, however, you have to watch out for possible instabilities, since product or environmental influences can lead to incorrectly sorted out products.

Types of contamination

There are essentially three different groups of metal contaminants:

  • Ferrous metals
  • Non-ferrous metals
  • stainless steel

Ferrous metals are both magnetic and conductive and can therefore be easily identified. Non-ferrous metals are not magnetic but are good or excellent conductors and can therefore be easily recognized. Stainless steel is the most difficult to find because it is typically non-magnetic and a poor conductor.

Stainless steel comes in different grades, with some being magnetic and others not at all. Their conductivity fluctuates, but is generally low. These two factors make stainless steel difficult to find.

The two best-known grades, 304 (L) and 316, are used in processing plants in the food, packaging and pharmaceutical industries. Recognizing these quality classes is made even more difficult if the product is wet or has a high salt content or both factors that contribute to a high product intrinsic signal apply.

Since the properties of stainless steel can be changed by its processing (improvement of the magnetic properties), it is difficult to specify specific sensitivity values. In general, it can be specified in relation to iron, at best with 1: 1.5 to 1: 2.5.

Further complications arise from the orientation of impurities such as screen wire and thin splinters (e.g. chips) when the smallest dimension is smaller than the smallest detectable sphere.

Test procedures for the devices

The procedure for checking the metal detection must be documented in detail and communicated to the responsible personnel. The inspection should be carried out at the beginning of each shift, with each product change and under all circumstances at least every hour.

The periods between the individual tests must be short enough so that in the event of an error, all possibly affected products have not yet left your factory premises and can be identified, recalled and tested again. We would like to point out once again that if you manufacture your own brands, you should agree in writing with your retail customer on all changes to test procedures.

As already mentioned, all IQ3 metal detectors from LOMA have an integrated PVS (Performance Validation System). This function automatically advises the operator to perform the required test with the preset frequency.

Check routines

When testing a traditional metal detection system, both ferrous and non-ferrous metal test packaging must be used. These samples must be demonstrably metal-free and clearly marked and labeled so that they are not inadvertently packed for shipping. The test packaging for the test procedures must be produced at intervals that correspond to the type and shelf life of the product in question. If you use “expired” test packaging, it does not reflect the actual properties of the products that the metal detector is testing.

All LOMA SYSTEMS metal detectors are supplied with a set of “plastic sticks” containing pieces of different metals, the size of which corresponds to the specified test size. They are practical, can simply be placed in your test packaging and make your test procedure even more efficient. When testing pre-packaged products on a conveyor belt system, please place the test stick at the far end of the pack, if possible. Should this not be practical, e.g. B. when testing individual small packages or sandwich corners, place the test stick in the middle of the product.

Then let the ferrous and non-ferrous packaging run through the seeker head twice. For the first run, place the packaging on the belt so that the metal test body is at the front; for the second, turn the packaging so that it is at the back. In both cases it must be observed whether the test packaging is successfully sorted out into the ejection container.

If the products are not wrapped, be sure to place the metal detectors on the production line AFTER packaging the products. If this is not possible and you manufacture your own brands, it is advisable to agree on the inspection routine in writing with the retailer concerned.

In the case of free-fall systems, introduce the ferrous and non-ferrous test specimens independently of one another into the product flow and observe whether they are sorted out as desired. The same principle also applies to pipeline systems. If this is not possible within a pipeline system, place the test specimen between the pipe and the seeker head and then observe whether it is properly sorted out. Should any part of your test procedure fail, discard all products made after the last satisfactory test and repeat the test procedure with a different detector with the same settings as the original system.

Handling rejected products

Of course, rejected products must never be returned to the production line. However, this does not apply to the products that were rejected as part of the normal test procedures. If these products are in perfect condition, they should be reintroduced into the product flow so that they can be recognized again.

The rejected packaging must be examined by a suitably trained person within one hour of the time at which it was ejected. Frozen products must still be frozen or have been re-frozen. The inspection should be carried out with the metal detection system that had previously sorted out the products, but not while it is being used for actual production.If the production line cannot be stopped, use an external detector that has at least the same or a higher sensitivity.

Let the rejected products run through the detector in the same direction as they ran through the search head of the production line. Then run the same products through the seeker head twice, each time having to lay them differently. If the products are sorted out again during one of the runs, the contamination must be found and identified. Then take appropriate action to prevent similar contamination from recurring. If more than one metal-contaminated product is rejected on the same production line within a shift, it is very worrying. In this case, every possible measure must be taken to find out the cause and eliminate it. If you are a private label manufacturer, you should write to your retail customer to inform them of the incident.

Maintenance of your metal detection equipment

As with all machines, the accuracy of which is essential, a consistently high performance of your metal detector is only guaranteed if it is regularly and properly maintained. It is therefore always worthwhile to draw up a preventive maintenance plan for your systems, in which the intervals between the various maintenance work are specified in accordance with the manufacturer's recommendations. The maintenance work must be carried out by the original manufacturer. They can also be taken over by your own technicians or engineers if they have been trained appropriately by the original manufacturer. After all repairs, maintenance and adjustment work, the metal detector must be subjected to a complete test before it can be used again.

Training your staff

In order to ensure the highest level of efficiency and safety, all affected personnel should be trained in the principles and the use and operation of metal detectors and the implementation of the test routines.

Retention of documents and records

It is important that you carefully keep and archive all relevant documents and records on the various aspects. Among other things, this applies to:

  • Commissioning and sensitivity tests and protocols for new equipment, but also for those that are carried out due to the transport or relocation of equipment
  • Results of routine tests indicating the time, results, sensitivity, product and actions taken
  • Number of packages sorted out in the individual layers
  • Number of and details of the contaminants found
  • Measures taken to find the source of the contamination
  • Preventive maintenance plan and maintenance interventions
  • Staff training

Avoidance of metal contamination through maintenance and cleaning

All maintenance and cleaning personnel in your company should be appropriately informed of the importance of avoiding metal contamination. The maintenance of your operating systems should be planned in such a way that signs of wear can be identified and rectified before malfunctions occur. Try to carry out the maintenance work or the assembly of new system components outside of production times. If this is not possible, the area must be shielded from adjacent raw material production or packaging areas.

  • Repairs to production lines should be carried out by employees who carry their tools in a closed box. A small brush with dust extraction and a magnet can be helpful for subsequent cleaning.
  • Under no circumstances may welding, riveting, drilling or soldering be carried out on a system that is used for production or on an immediately adjacent system.
  • Cutting or meat grinder knives, wire mesh conveyors and sieves must be checked daily for damage. These checks must be carefully documented.
  • Maintenance and cleaning personnel who dismantle machines should have a suitable, clearly labeled container for the safe storage of nuts, bolts, washers, etc.
  • Avoid using tape or wire to make makeshift repairs to equipment. Missing or loose screws and damaged fastenings / connections must be promptly and permanently replaced or repaired and chips, wire scraps and other possible impurities must be disposed of safely and quickly. All welds must be continuous and smooth.

It is important that all equipment or machinery repaired in workshops or factories is cleaned and vacuumed (not blown with compressed air) before being returned to the production area. The workshop floor must be cleaned and vacuumed at least once a day. If the workshops are located within your factory building, an appropriate lock should be provided at the exit of the workshop. In addition, notices must be attached to the staff that they must clean their shoes before leaving the workshop.

When repairs, maintenance, and installation are complete, a quality control member should inspect the facility and the surrounding area BEFORE production resumes.

Ever since LOMA SYSTEMS was founded in 1969, the company has worked very closely with both food manufacturers and retailers to deliver advances in technology. As a result, LOMA has an unrivaled level of experience and expertise, in the practice of effective metal detection within food industry production lines.

Our latest introduction of the IQ4 series of Metal Detectors is the result bringing together the combined experience of LOMA, LOCK Inspection, CINTEX and BRAPENTA, and nearly 50 years of customer-back innovation. LOMA’s engineering teams have employed our Designed to Survive philosophy to deliver great metal detection sensitivity in a package that is easy to install; easy to use; easy to clean and maintain; can withstand the arduous food production environments; and ultimately brings a brilliant cost of ownership.

Metal detection and X-ray inspection offer different capabilities - to assess which you should choose, you should carry out a Hazard Analysis and Critical Control Points audit. If the audit determines that metal is likely to be the only contaminant found, then a metal detector is likely to be the most cost-effective solution, however, if other issues such as packaging and product effect are raised by the audit then it is recommended you perform a full product test to establish the most suitable technology.

If, the audit determines that metal and other contaminants such as glass, mineral stone, calcified bone or high-density plastics and rubber are likely to be encountered, then X-ray is the only suitable solution. In many cases, there’s only one suitable solution. However, there may be occasions when it could be helpful to install both metal detection and X-ray inspection systems on the same production line.

LOMA are unable to state that metal detectors are safe for someone fitted with a heart pace-maker or any other kind of electronic implant, as we have no knowledge of heart pace-makers or other types of implant nor the particular limits that the individual person is fitted with.

However, we can confirm that our equipment meets all relevant safety standards, and the magnetic field strengths emitted into the environment by our equipment are at no higher a level than other electrical machinery that would be encountered in a factory environment. - i.e. if the person is permitted to work near electrical machinery then our equipment will not pose a risk.

We would clearly not recommend passing any person’s body through a metal detector.