Introduction to Metal Detection

Sources of Metal Contamination:

Metal contamination sources are numerous – and even the most stringent controls cannot prevent the occasional incident in which small pieces of metal find their way into products destined for consumer consumption.

Good working practices will minimise the likelihood of metal contaminants entering the production flow; furthermore, correct equipment design and appropriate selection will maximise the likelihood of reliably detecting and rejecting any metal particles that have found their way into products.

Contamination normally comes from the following sources:

Raw Materials

Typical examples include metal tags and lead shot in meat; wire in wheat; screen wire in powder material; tractor parts in vegetables; hooks in fish; staples; wire strapping from material containers.

Personal Effects

Buttons; pens; jewellery; coins; keys; hair-clips; thumb-tacks; pins; paper clips, etc.

Maintenance

Screwdrivers and similar tools; swarf and welding slag (following repairs); copper wire off-cuts (following electrical repairs); miscellaneous items resulting from inefficient clean-up or carelessness; metal shavings from pipe repair.

In-plant Processing

The danger of contamination exists every time the product is handled or passes through a process. Crushers, mixers, blenders, slicers and transport systems can all act as sources of metal contamination. Examples of metal contamination from these sources include broken screens, metal slivers from milling machines, and foil from reclaimed products.

Identifying the likely source of contamination is a vitally important stage in developing a successful overall metal detection programme.

What is a Metal Detection System?

An industrial metal detection system is a sophisticated piece of equipment used to detect and reject unwanted metal contamination. When properly installed and operated, it helps to reduce metal contamination and improve food safety. A typical metal detection system consists of four main parts:

Detector Coil or ‘Detector Head’

Most modern metal detectors fall into one of two main categories, with respect to the ‘Detector Head’, which is that part of the metal detector system that identifies the presence of metal contamination:

• The first type of metal detector utilises a ‘balanced coil’ Detector Head. Detectors of this design are capable of detecting all metal contaminant types, including ferrous, non-ferrous and stainless steels, in fresh and frozen products. The products being inspected can be either unwrapped or wrapped, and can include products wrapped in metallised films.
• The second detector type utilises permanent magnets in a ‘Ferrous-In-Foil’ Detector Head. These Detector Heads are capable of detecting ferrous metals and magnetic stainless steels only within fresh or frozen products which are packed in an aluminium foil wrapping.

Whilst it is recognised that other technologies exist, this guide concentrates mainly on the ‘balanced coil’ detector type − and (to a much lesser extent) on Ferrous-In-Foil (FIF) technologies.

Detector Heads can be manufactured in virtually any size, in order to suit the product being inspected. They may be rectangular or round, and may be mounted horizontally, vertically or on an incline.

Each Detector Head has an opening (known as an ‘aperture’) through which product passes. When a metal

contaminant is detected by the Detector Head, a signal is sent to the electronic control system.

User Interface/Control Panel:

The user interface is the front-end of the electronic control system, and is often mounted directly on the Detector Head. However, the user interface can be mounted remotely (with connecting cables) if the Detector Head is too small, or if the Detector Head is installed in an inconvenient or inaccessible location.

Transport System

The transport system is used to pass the product to be inspected through the aperture of the metal detector. The most common type of transport system is a conveyor. Alternatives include:

• A plastic chute with the detector mounted on an incline
• A non-metallic pipe, mounted horizontally or vertically. This type of transport system is commonly used in the inspection of powders and liquids

Automatic Rejection System 

An automatic reject device is frequently fitted to the transport system in order to remove any contaminated product from the production line. There are many different styles available including ‘air blasts’, ‘push arms’, ‘drop flaps’, etc. The style of the reject device installed will depend on the type of product being inspected.

In addition to the four main parts of a metal detection system, other important items may include:

• A lockable container fixed to the side of the conveyor, to collect and hold rejected product
• A full-length cover between detector and reject device
• A fail-safe alarm which operates if the metal detector develops a fault
• A reject confirmation device, with sensors and timers, to confirm that contaminated product is actually rejected from the line
• A beacon and/or audible alarm that alerts operators to various other events, such as an automated warning that a detector is due to be tested or that the reject bin is full
• Numerous optional fail-safe systems to raise level of “Due Diligence”

Where Can a Metal Detection System Be Used?

Metal detectors may be used at various stages of a production process:

Bulk ‘In-Process’ Inspection 

• Eliminates metal before it can be broken into smaller pieces
• Protects processing machinery from damage
• Avoids product and packaging waste by subsequently rejecting a finished higher-value product

Typical examples include bulk inspection of meat blocks prior to grinding, ingredients for pizza toppings and grain products.

Finished Product Inspection 

• No danger of subsequent contamination
• Ensures compliance with retailer and consumer brand quality standards

A combination of bulk and finished product inspection will provide optimum protection.

The most common types of metallic contamination include:

• Ferrous (iron)
• Non-ferrous (brass, copper, aluminium, lead)
• Various types of stainless steel (magnetic and non-magnetic

Of the three types listed above, ferrous metal is generally the easiest to detect − and relatively simple detectors (or even magnetic separators) can perform this task well.

Stainless steel alloys are extensively used in the food industry, but are often the most difficult to detect, especially common non-magnetic grades such as 316 and 304.

Non-ferrous metals, such as brass, copper, aluminium and lead, usually fall between these two extremes, although in larger metal detectors operated at higher frequencies, non-ferrous metal may be harder to find than non-magnetic stainless steel.

Only metal detectors using an alternating current ‘balanced coil’ system have the capability to detect small particles of non-ferrous and non-magnetic stainless steel.

Why Should you Choose the Correct Metal Detector?

Compliance 

When installed at critical control points (CCP’s) in your processes, metal detectors enable your business to comply with the requirements of Hazard Analysis and Critical Control Points (HACCP) and the broader needs of food safety regulations and standards.

However, it is not enough to simply install a metal detector at critical control points. Regular testing of the performance of metal detection equipment is an essential part of any well designed quality management system in the food and pharmaceutical industry.

Cost Reduction 

Selecting the most stable and reliable metal detector, and installing it at the most appropriate point(s) within the manufacturing process enables overall lifetime costs to be managed and kept to an absolute minimum through:

• Eliminating false rejects and product waste
• Reducing performance testing requirements

Productivity Improvements 

Research suggests that on average, plants waste between 28% up to 40% of their capacity through stops, speed losses, interruptions and defects. Choosing a metal detector that is simple to set up and operate, offers reliable and consistent performance, with low maintenance requirements and easy-clean system design can enable productivity to be optimised by ensuring costly downtime is all but eliminated.

Overall Equipment Effectiveness (OEE) is an important tool in the pharmaceutical, packaging and food processing industries. In any capital-intensive business OEE improvement is a critical methodology to drive improved efficiency, higher quality and reduced cost.

Adopting OEE methodology can provide benefits in the following areas:

• Equipment: Reduced equipment down-time and maintenance costs, and better management of the equipment life cycle
• Personnel: Labour efficiencies and increased productivity by improving visibility into operations and empowering operators
• Process: Increased productivity by identifying bottlenecks
• Quality: Increased rate of quality and reduced scrap.

A number of software tools exist to capture manufacturing performance data and display OEE performance graphically. Selection of an appropriate OEE software tool is critical to the success of any OEE initiative. A mistake to be avoided is the belief that this tool will drive OEE improvement − remember that any OEE software application is just a tool, and if not harnessed correctly will merely measure OEE, not improve it. Refer to Chapter 15 for more detailed information regarding measuring and calculating OEE, and guidance on how it can be improved.

Improved Competitiveness

Ultimately, by complying with regulatory, industry and retailer standards, reducing your costs and increasingly your productivity you can improve your competitiveness. This in turn enables you to protect your brand and your reputation and be in a better position to win more customers.

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