Cassel's Compendium

The task of metal detectors is to detect metallic foreign bodies in non-metallic products, and they are thus used in many industrial production processes, particularly in the food production sector, where great demands are made on product quality and hygiene.

Pure Raw Materials - Incoming Inspection
Producers who can precisely verify raw materials are able to reduce non-conformance costs by making early diagnoses and also by assessing the quality of their suppliers.

Pure Finished Products - Final Inspection
The customer shall receive only pure products having the highest degree of purity which can be realized on a technical basis.

Protection of Production Facilities - Process Control
Metal pieces having managed to enter the production chain can severely disturb or even destroy production facilities. Down-times of machines and plants can negatively affect the constancy of product quality and quantity.

HACCP - Critical Check Point for Metallic Foreign Bodies
A critical control point refers to a point within the production chain where lacking supervision can mean a non-acceptable risk to human health.

Function

The metal detector operates on the basis of an inductive measuring principle. A high-frequency electromagnetic alternating field is generated by a transmitter coil. If a metal piece passes through the metal detector, the field experiences a change corresponding to the magnetic and electrical properties of the metal piece.

The change in the field is measured by a balanced pair of receiver coils, subsequently reprocessed and digitally analysed by the control unit

This measuring principle reacts to electrical conductivity and magnetism and is therefore able to recognise all types of metals, i. e. non-ferrous metals and stainless steel. However, magnetic metals can be slightly better recognised than non-magnetic metals. The measuring field penetrates the food products and recognises metal pieces even in the interior of the product or in packages.

Technical limits

The maximum sensitivity is limited by the physical laws upon which the measuring principle is based. Food often has a more or less strong electrical conductivity due to small contents of salt, sugar, minerals and water which cause the metal detectors to react. This reaction is referred to as product effect. In most cases, the sensitivity attainable in practice primarily depends on how well the metal detector is able to compensate the product effect.

Classic Metal Detecting Systems

Conveyors: The metal detector is integrated into a conveyor belt either with the belt passing through the frame search coil or being installed just below the belt as flat search coil. There are several devices available to reject the detected metal pieces (e. g. pusher).

Gravity Feed Applications: Bulk material falls into a pipe through the metal detector. Metal pieces are rejected by means of a separation flap.

Pipeline Metal Detectors: This in-line system serves to detect metal pieces in paste-like or liquid products and is integrated into the conveyor pipes (e. g. with milk pipe fittings). The metal pieces can be transferred out of the system by means of three-way-special-valves.

Installation Site

Both packed and unpacked products can be monitored for metal impurities.

The verification of already packed goods is mainly preferred as it is almost impossible that the product may be polluted again with metal.

Unpacked goods are sometimes much smaller and so a ' smaller and more sensitive detector can therefore be used.

Unpacked goods should be supervised to ensure additional protection, or if it is not possible to supervise the packed goods (e. g. due to aluminium packages or too large packing dimensions).

If the metal detector serves as a machine protection, it should be installed as close as possible to the machine which is to be protected.

Sensitivity and Reliability

The right choice of the search coil dimension is of prime importance to achieve the highest possible metal sensitivity. The size of the coils has to suit the product properties. In this case, most applications are subject to the following rule: The smaller the metal detector, the higher the degree of sensitivity.

Optimum sensitivity is attained if the metal detector recognizes the smallest possible metallic test ball without leading to random metal trips due to external influences or fluctuations in the food product properties.

It is of top priority that the metal detector reliably operates without causing random alarms.

Setting Modifications

Modifications to the detector settings must only be carried out by specially trained staff.

Each new setting should be documented so that the current operating state is always clear.

Testing Procedures and Test Intervals

With the help of a metallic test ball of a defined size, the function of the metal detector can be tested in operation. A test ball is added to the product and is subsequently passed through the metal detector. If the detector recognizes the ball, the test is considered passed.

The test intervals shall be short enough: All products having passed the metal detector since the last test procedure must be able to be re-tested once again. In addition, tests should be carried out at the beginning of working shift and when changing products.

Reviews/Documentation

In all production lines working in accordance with HACCP and ISO 9000, the metal detection process has to be documented in written form thus making it comprehensible, verifiable and safer. By means of a logging printer which is connected to the metal detector via a data interface, it is possible to effect complete, automatic documentation of metal detection, tests and all set parameters. On request, a suitable printer can be supplied together with the metal detector.

Search for the Source

If a metal piece is detected in the production process, it is absolutely necessary to search for its source. The source of metal contamination has to be rigorously eliminated in order to exclude further pollution from the same source.

Maintenance

Modern metal detectors are equipped with an automatic self-control system and are, therefore, maintenance-free.

Preventive maintenance by the manufacturer of metal detectors include consulting the customer and verifying the system on site if it is still correctly installed after a longer operation period. It is checked if no external interference affects the system's performance and if the detecting sensitivity for the current product and site is still optimally set. This includes, in addition, the training and introduction of new staff and the integration into quality management.

Remove contaminated Products

It is absolutely necessary to make sure that metal-contaminated products are not re-used or delivered to customers by mistake. These products have to be removed from production and given an appropriate clear marking, e. g. a red sticker saying "Not for release/Metal".

Regulation on Food Hygiene

The European Community Guideline on Food Hygiene 93/94/EU obliges all food manufacturers to install a self control system based on HACCP. Metal detectors are not explicitly demanded by this regulation, but however, the HACCP includes the demand to control physical dangers as metallic foreign bodies.

HACCP

HACCP (Hazard Analysis and Critical Control Points) is a program for the prevention of non-conformances in the Food Processing Industry. With respect to metal detection, the HACCP program requires the following:

• The metal detector is a critical physical control point of the danger of metal pollution. If no control is carried out it is likely to result in a health risk.

• Here, detect ability defines the critical limiting value for the calibration to the smallest recognizable metal ball. A product has to be kept free from metal pollution as made possible by "Good Manufacturing Practice" (GMP) and according to the latest state-of-the-art. With reference to metal detectors, this means that all metal pieces which a modern metal detector reliably detects on optimum installation have to be removed from the product.

• Setting of the critical limiting value, however, must in any case enable the metal detector to operate in a stable and reliable manner without triggering random or false alarms.

• Endurance is needed that all products are systematically and continuously tested by the metal detector.

• Complete documentation must be made on the operation and results of the metal detection.

DIN ISO 9000

It is recommended to write detailed job instructions for the metal detector where e. g. the measures to be taken after metal alarm and testing procedures are laid down.

On request, a sample for job instructions can be obtained from CASSEL Messtechnik. Furthermore; reference is made to all the items specified under HACCP.