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THE GUIDE TO REDUCING METAL CONTAMINATION IN THE FOOD PROCESSING INDUSTRY Since

THE GUIDE TO REDUCING METAL CONTAMINATION IN THE FOOD PROCESSING INDUSTRY Since its formation in 1988, Safeline has become the largest metal detection manufacturer in the world with operations in the U.K., U.S.A. and Europe. Design and manufacturing experience from over 30 years, supports a worldwide distribution network to advise on minimizing the risk of metal contamination. Safeline engineers provide additional customer support in the form of training seminars in your facility, to discuss all aspects of “How to Develop an Effective Metal Detection Program.” If you would like more information about the Safeline Microprocessor Metal Detector or to discuss a seminar, please contact one of the Safeline offices. THE METAL DETECTION GUIDE Copyright © 1990 Safeline Ltd. First Printing 1990 Second Printing 1994 Revised Printing 1996 THE GUIDE TO REDUCING METAL CONTAMINATION IN THE FOOD PROCESSING INDUSTRY by Andrew Lock METAL DETECTION I THE METAL DETECTION GUIDE I CONTENTS Chapter Subject Page 1. Defining the Problem ...................................................................... 1 EQUIPMENT 2. Basic Principles ................................................................................ 2 3. Amplitude Detection - Zero Crossover............................................. 4 4. Sensitivity ........................................................................................ 6 5. Inspecting Wet or Conductive Products .......................................... 9 6. Conveyor and Reject Systems ......................................................... 11 7. Reject Timing .................................................................................. 16 MANAGEMENT OF QUALITY 8. Establishing An Effective Metal Detection Program ....................... 18 9. Reasons Why Your Program May Fail ............................................. 21 10. Developing a Foreign Material Control System .............................. 23 11. Comparing Metal Detector Brands ................................................. 26 12. Final Questions ............................................................................... 28 I THE METAL DETECTION GUIDE I I 1 M M M M M etal detectors are now accepted as essential equipment by most food and pharmaceutical processors. Many companies lay down strict inspection standards in terms of detector sensitivity. However, installing metal detectors will not necessarily guarantee a metal-free product unless they form part of an effective overall metal detection program. This handbook has been written to help companies set up this program and “prevent metal from getting through.” The regulatory bodies in both the U.S.A. and U.K., are making increasingly strong recommendations for the universal inspection of all food and allied products by metal detection equipment. For example, the United States Department of Agriculture (USDA) comments: “The extensive exposure of some products to metal equipment such as grinders, choppers, mixers, shovels, etc., causes the possibility of metal contamination...therefore the use of electronic metal detectors is highly recommended....” (USDA Technical Services) Revised guidelines were also issued by the Food and Drug Administration (FDA) to update the regulations for Good Manufacturing Practice and to establish new updated, or more detailed provisions for the food industry to help ensure a safe and sanitary food supply. “Effective measures shall be taken to protect against the inclusion of metal or other extraneous material in food. Compliance with this requirement may be accomplished by using sieves, traps, electronic metal detectors, or other suitable effective means.” (Federal Register Vol. 51 No 118) 1.1 Sources of Contamination The sources of contamination are numerous and even the most stringent controls cannot prevent the occasional incident. Working practices described in Chapters 8, 9 and 10, will minimize the likelihood of metal particles entering the production flow and maximize the likelihood of reliably detecting and rejecting any that do. Contamination normally comes from one of four sources: Raw Materials Raw Materials Raw Materials Raw Materials Raw Materials Typical examples include metal tags and lead shot in meat, wire and rust in wheat, screen wire in powder material, tractor parts in vegetables, hooks in fish, staples and wire strapping from material containers. Personal Effects Personal Effects Personal Effects Personal Effects Personal Effects Buttons, pens, jewelry, coins, keys, hair clips, thumb tacks, paper clips. Maintenance Maintenance Maintenance Maintenance Maintenance Screwdrivers and similar tools, welding slag and swarf following repairs, copper wire offcuts following electrical repairs, miscellaneous items resulting from inefficient cleanup or carelessness and metal shavings from pipe repair. In-Plant Processing In-Plant Processing In-Plant Processing In-Plant Processing In-Plant Processing The danger exists every time the product is handled or passes through a process. Crushers, mixers, blenders, slicers and transport systems all contribute. Examples include broken screens, metal slivers from milling machines and foil from reclaimed products. Identifying the likely source of contamination is an important stage in developing an overall foreign material reduction plan. Inspecting raw materials will eliminate many large, easily detected pieces before being broken into numerous, difficult to detect pieces. 1.2 Why Metal Detectors are Installed To prevent damage to processing equipment. To comply with stringent quality standards by major customers. These include high volume retailers, fast food chains, food service and vendor certification programs. To avoid the cost and implications of consumer complaints, adverse publicity, product recall and litigation. To win new markets and customers with high quality products. To comply with legislation such as “Due Diligence” and FDA/USDA Directives. 1. DEFINING THE PROBLEM I 2 I THE METAL DETECTION GUIDE I T T T T T he most common types of metallic contamination in a broad range of industries include ferrous (iron), copper, aluminum, lead and various types of stainless steel. Of these, ferrous metal is the easiest to detect and relatively simple detectors, or even magnetic separators, can perform this task well. Stainless steel alloys are extensively used, particularly in the food industry, and they are the most difficult to detect, especially the common non-magnetic grades such as 316 (EN58J) and 304L (EN58E). The non-ferrous metals such as copper and lead fall between these two extremes. Only metal detectors using a balanced three-coil system have the capabilities to detect small particles of non- ferrous and stainless steel. The three coils are wound on a non-metallic frame or former, each exactly parallel with the other. (Figure 1). The center coil is connected to a high frequency radio transmitter. The two coils each side of the center coil act as radio receivers or aerials. As these two coils are identical and the same distance from the transmitter, they pick up the same signal and an identical voltage is induced in each. When the coils are connected in opposition, they cancel out resulting in zero output. When a particle of metal passes through the coil arrangement, the high frequency field is disturbed under one coil, changing the voltage generated by a few microvolts. The state of perfect balance is lost and the output no longer zero. The resulting signal is processed and amplified. It is this phenomenon which is used to detect the presence of unwanted metal. To prevent airborne electrical signals, or nearby metal items and machinery disturbing the detector, the complete coil arrangement is mounted inside a metal case with a hole in the center to allow the passage of product. Aluminum is normally used for the case but on some applications, where frequent washdown is required, stainless steel may be used. In addition to creating a screen, the metal case adds strength and rigidity to the assembly. This is crucial for satisfactory operation of the detector. Other special mechanical and electrical techniques are essential to overcome many practical difficulties. 2.1 Mechanical Techniques The metal case, itself, will have an effect on the state of balance. In addition, microscopic movements of the coils relative to each other, as small as 1 micron, can cause an out-of-balance voltage and a false detect signal. One of the major design problems for metal detector manufacturers is to design a totally rigid and stable system, unaffected by vibration from motors, pulleys, auto-reject devices, temperature changes, transportation and close-by machinery. The selection of former material, coil specifications, and case design, are crucial. To increase mechanical rigidity further, some manufacturers pot the detector with a material to prevent relative movement of the metal case to the coils. This helps produce a unit which is able to operate at maximum sensitivity under normal factory conditions. 2. BASIC PRINCIPLES I THE METAL DETECTION GUIDE I I 3 2.2 Electronic Techniques Mechanical construction methods will minimize false signals from coil and case movements. However, temperature changes, buildup of product in the aperture, aging of electric components and slow changes in the mechanical structure, will also contribute to an out-of-balance voltage. These can be eliminated by various electronic techniques. Automatic Balance Control continuously monitors this out of balance voltage and automatically corrects it to zero. This eliminates the need for periodic fine tuning by an operator and ensures the detector is permanently set to its optimum operating condition. In a similar way, by controlling the frequency of the oscillator with great accuracy using quartz crystal control, frequency drift is eliminated. Automatic Balance Control and Quartz Crystal Control will not, themselves, enable the detector to detect smaller pieces of metal. They will, however, enable the detector to permanently maintain this sensitivity with no operator attention and without false reject signals. For high performance over an extended period, Automatic Balance Control, Quartz Control and potted heads are essential. 2.3 Ferrous-in-Foil Detection When the product to be inspected is packaged inside an aluminum foil pack or plate, a metal detector using the balanced coil system cannot be used. An alternative detector design uploads/Geographie/ red-guide.pdf