An overview of detection, inspection and validation

Metal detectors are both effective and inexpensive protectors of processing equipment, products people consume and companies’ brand image with limited liability due to contaminated product. Metal detector usage can be divided into three main categories:
1)    Packaging – where higher sensitivity (the smallest piece of metal) is desired,
2)    Bulk processing – where gravity drop or pipeline metal detectors are used to protect equipment (e.g. blenders, mixers, cutters, choppers, etc),
3)    Industrial – where industries have lower sensitivity requirements (mining, aggregate, gravel, plastic, lumber etc.

Metal detectors measure electrical conductivity and magnetic permeability. However the sensitivity of detectors depends on how easily metals are magnetized (the magnetic permeability), and the electrical conductivity of the metal. Metal detectors are calibrated for sensitivity to ferrous metals, non-ferrous metals and stainless steel.

• Ferrous materials, the most common contaminant, are any metal easily attracted to a magnet (e.g. steel, iron, etc.)
• Non-ferrous materials are highly conductive non-magnetic metals (e.g. copper,aluminum, brass, phosphor bronze etc) and produce almost the same size signal as ferrous metals because they are all good conductors. When inspecting conductive products, increasing the test sphere size by at least 50 per cent is a good practice.
• High quality 300 series stainless steels (e.g. Type 304, 316, etc.), most commonly used metals in food processing and pharmaceutical industries are arduous metals to detect due to their poor electrical conductive qualities due to low magnetic permeability. When inspecting non-conductive products, a stainless steel test sphere typically needs to be 50 per cent larger than a ferrous sphere to produce the same size signal. And for inspecting conductive products, a stainless steel test sphere needs to be 200 – 300 per cent larger than a ferrous sphere to produce the same size signal.
• Typical non-conductive products like wood products, plastics and rubber, textiles, paper products etc. create a large magnetic signal that hampers the detector’s ability to detect the magnetic signal of small pieces of metal. Conversely, products like metalized films, plastic and rubber products having high carbon black content are electrically conductive and produce a conductive error signal.

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

Metal detectors are used at different stages of the production process, from checking of bulk raw material, through the manufacturing process onto finished products. Inspecting a product prior to the packaging process brings in economic gains, as contaminated products can be rejected without wasting packaging material. It is also a key to complying with Hazard Analysis and Critical Control Points (HACCP) and Critical Control Points (CCPs) on the production line for inspection. The position of points can vary depending on the type of products and its processing technique. The beginning of the production line is a common potential CCP. It is important to identify unwanted matter early in the process to prevent further problems down the line, as contaminants can fragment into smaller particles, irregular shapes and changing properties of the process product, thus making detection more difficult.

Metal detector is an important milestone in quality control management system and one cannot imagine preventive quality control management without installing the metal detector system. Metal detection forms a key part of product quality assurance in the pharmaceuticals industry, helping to improve the quality of manufacturing process and compliance with regulations. They are must for quality check of products for audits like WHO-GMP, HACCP, US FDA, FDA, MHRA and other international audits.

The four “Ps” of metal detection are:
Preventing contamination
Metal in a product can be a cause of consumer complaints even with metal detection systems in operation. The complaints (which usually do not include visual items such as bolts, washers, and pieces of blades or screens found in product) are usually due to lack of effective controls, poor working methods, and incorrect specification (and therefore design) of the system rather than actual failure of the metal detector. Effective metal detection programs focus on minimizing contamination through good manufacturing practices (“GMP”), proper equipment selection, effective testing, and ever-improving knowledge of how industry standards, customer requirements, and legislation affect manufacturers.

Profitable production
The cost to implement and maintain an effective metal detection program is significantly lower than the potential cost of metal contaminates being found in the product. Metal contaminated product found before shipment may result in product and packaging waste, possible machinery damage, and loss of output. Metal contaminated product found after shipment can affect customers, cause product recalls, create adverse publicity towards a company or brand, create financial liabilities, and potential litigation. An appropriately implanted metal detection program will lead to reduced failure costs, improved customer and consumer satisfaction, protects profitability as well as a company or brand.

Protecting customers
Manufacturers have a legal and moral obligation to their customers to minimize contamination of their product and ensure consistent quality. They are responsible for taking steps to protect their product’s quality and the welfare of the consumer.Today’s manufacturers incessantly look for ways to eliminate contamination in their products, yet processes or procedures, or compliance with them, break down and product becomes contaminated. Failure to do this can be costly.

Promoting brand confidence
Branding not only helps maximize sales, but also generates customer perception of a product safety and quality, and drives repeat customer purchases. Product brands are valuable assets which must be protected. Contaminated products reaching a consumer can have financial consequences by product recall, from potential litigation, and can damage a brand or reputation which has been built and consumed significant company resources. Accurate documentation of compliance with production standards can help minimize damage from customer complaints.

Metal detectors are sensitive and need to be calibrated periodically. The goal of this calibration is to locate the extremely narrow sensitivity setting that will allow anyone to locate metal without the metal detector producing excess chatter. Here is the general procedure followed for the routine calibration of the metal detectors:

• Turn the metal detector on, adjust the sensitivity to the maximum setting and adjust the discrimination to the minimum setting.
• Pass the piece of iron in front of the coil at a range of 3/4 inch to three inches. A highly intermittent or "spitty" signal indicates that the metal detector may be too sensitive. A very short chirp or no signal at all may mean that the detector is not sensitive enough.
• Locate the ground adjustment trimmer. This control adjusts the amount of signal that the metal detector produces when metal passes in front of the coil. Change the ground adjustment trimmer setting until a single clear signal is obtained on passing the iron piece one inch to 1.5 inches away from the coil.

Once a product goes through a properly functioning metal detector, it either has metal in it or it doesn't. Is there actual metal in the product that has passed through the machine? If the machine is assumed to be working properly, you won't know. There are a couple of ways to know - customer complaints and product testing. It is necessary to have reliable, robust and effective monitoring and reporting capabilities from single inspection system that will enhance any organizations TQM system. Monitoring activities are necessary to ensure that the process is under control at each critical control point. When monitoring indicates a deviation from an established critical limit a necessary corrective action must be taken to ensure that no product is injurious to health or otherwise adulterated.

The metal detectors must offer integral Performance Validation Routines (PVR) to be aware of the suitability of performance testing programme. They can be configured based upon a package changeover, a change in a critical metal detector parameter like frequency, or upon power up. Regular checking of the metal detector’s sensitivity should be a normal part of any quality assurance procedure. This is particularly important should it become necessary to demonstrate due diligence. One approach is to make regular sensitivity checks and keep handwritten records of the results.

Verification includes the ability of detector system to alarm and/or kick off product suspect for metal contamination and to determine its functionality of the metal detector while validation includes how effective it is. Validation ensures that the detectors do what they were designed to do; that is, they are successful in ensuring the production of a safe product. Metal detector validations are usually performed with a test with various materials (304 SS, nickel, brass, chrome steel etc).Evaluation is performed in order to assess if the device and the rejecting system is capable of deflect metal particles of different characteristics.

Monitoring is usually performed with SS and ferrous metal and is performed in order to challenge the device and the system surrounds them. Metal detectors are usually monitored with standard spheres every hour or at least twice a shift. Verification is usually performed with internal audits, inspections, supervision etc. The validation usually includes such activities as:

• Reference to validations carried out by others, to scientific literature or to historical knowledge,
• Experimental trials to simulate process conditions,
• Biological, chemical and physical hazard data collected during normal operating conditions,
• Statistically designed surveys,
• Mathematical modeling and
• Use of a guide approved by competent authorities.